Electrical oral
Question 1:- MSB Maintenance
Answer: 1. Checking contacts of the circuit breaker for damage and dirt.
2. Clean by electro-cleaner for contacts.
3. Dirt and dust are to be removed vacuum cleaner.
4. Circuit breaker contact is never to be filed and scrapped.
5. Defective or burnt contact to be replaced.
Question 2:- Safeties of MSB?
Answer: -1. Minimum 3 mm thick rubber mat on front and back.
2. Insulated handgrip and rail ebonite handle.
3. Dead front is required for the switchboard for AC.
4. No pipeline to pass in the vicinity.
5. Earth fault indicator.
6. If the breaker is on the top cover cannot be opened.
7. Ventilation grid.
8. Manual tripping device.
9. under voltage relay, 0.8 x normal voltage (3-5 seconds).
10. Reverse power trip (5 - 15 % below normal power).
11. over voltage trip- 1.3 X normal voltage.
12. Preferential trips- 110% full load current.
Question 3: How sound is transmitted through the telephone from the engine room to Bridge?
Answer: There is a diaphragm from where we speak due to the sound wave diaphragm vibrating. A coil is there, due to the vibration of the diaphragm the coil rotates in a magnetic field, which means the sound wave is converted into an electrical wave.
The diaphragm is vibrated by the sound wave and moves the driving rod and armature rack out forth through the stationary magnetic field existing between the magnet and the armature.
The moving armature Alters the magnetic field making it move and cutting the conductor in the coil thereby inducing a small AC voltage that produces a fluctuating magnetic field about the armature. The armature is repelled and attracted by the field of the magnet and moves back and forth. The diagram of the receiver vibrated in and out by the driving rod, thus creating sound waves, which correspond with the sound waves striking the transmitter diaphragm.
Question 4: As a second engineer how will you order a battery for emergency lightning?
Answer: voltage, ampere hour, make and model number, dimension and type of battery.
Question 5: What is an ampere-hour?
Answer: The power rating for a battery is given in Amp-hr, which means how many amps can a battery deliver to be completely discharged over an hour.
Example of required amperage
Emergency load (navigational equipment + lighting) if the load 100 Amps need a minimum of 8 hours so the required capacity is a 24-volt 800-ampere hour.
Question 6: if your ship is designed for 60 hertz at 440 volts. shore supply volt 380 Volts is operating at 50 Hertz reasons?
Answer: supply voltage should be 380 volts. Has the frequency been reduced by 16.7% proportionally the voltage should be reduced by 16.7% ratio of voltage and frequency can be maintained a speed of the motor is directly proportional to the frequency and torque current is directly proportional to the voltage so the increase in torque current is controlled by the proportional reduction in voltage?
Question 7: difference between a switch and a circuit breaker?
Answer: a circuit breaker is an automatically operated electrical switch designed to protect a circuit from damage caused by circuit faults.
Its basic function is to detect a fault condition while maintaining continuity and immediately disconnect the circuit upon the fault detection.
A Switch is manually operated by using a contactor to open and close the circuit.
Question 8: current transformer secondary winding is earth why?
Answer: the reason is to reduce the insulation costing. When you earth one secondary terminal you can insulate the current transformer to phase voltage else you have to insulate for line voltage, which might increase the cost as well as size of the current transformer. By earthing one end of the current transformer, the circuit is anchored to zero voltage and any fault can be detected by Earth fault meter.
Question 9: checks when taking shore supply?
Answer: connection box at entrance of accommodation or emergency generator room.
The connection box must have suitable terminals to accept the Shore supply cable including an earthing terminal to earth the ship’s hull to shore earth. The connection box must have circuit breaker or an insulator switch and fuses to protect cable linking to connection box to main switch board.
A voltmeter is fitted to indicate polarity of DC shore connection.
For an AC shore supply, phase sequence indicator is used to indicate correct supply phase sequence.
Shore supply switch on MSB is interlocked with generator circuit breaker so that it cannot be closed generator is still connected
Reverse phase sequence will produce reverse shaft rotation because the rotating magnetic field will be reversed with disastrous result.
Phase sequence can be corrected by interchanging any two conductor of shore supply.
Higher shore supply frequency will cause motor to run faster, overloaded and overheated. A higher voltage will generally cause equipments to take excessive current cause overheating. Lower voltage is generally not so serious but may cause motor to run slower and overheat may cause motor to stall.
Voltage, frequency and phase sequence should be same. Metre reading should be taken. Earthing, insulation of shore supply cable and shore connection box to be tested.
Question 10: what is slip?
Answer: the difference between synchronous speed (Ns) and the actual speed (N) of the motor. Expressed as percentage of the synchronous speed.
Question 11: undervoltage trip?
Answer: It is intended to open the brakes at loss of excitation for any reason, has a time delay. The solenoid is wired across the main leads on the generator side of circuit breaker and when not energized leave the plunger in trip position and breaker cannot be closed. Normal voltage output with the machine running energizes the coil and the plunger is pulled down against loading spring to release the trip. This allow closure of breaker and means the device is set.
Question 12: Measurement of air gap in alternator?
Answer: It is measured at 4 location 90 degree apart on both ends using feeler gauge rotate rotor by 90 degree and repeat the readings shall not be beyond 10% on average at each end. Values of air gap 1.5 mm maximum.
Question 13: What is SCR?
Answer: A silicon controlled rectifier is a four layer pnpn semiconductor device that does not allow the current to flow until it is triggered. The flow of current is in one direction. It has three terminals gate - input terminal. Anode - output terminal. Cathode which is common to both the gate and anode application SCR is used in alarm circuit.
Question 14: Circuit breaker types?
Answer: Air Circuit breaker, Oil circuit breaker, Gas circuit breaker with sf6 (Sulphur hexafluoride), vacuum circuit breaker - The high dielectric strength of a vacuum allow a very short contact separation and a rapid restrike free of arc intruption is achieved.
Question 15: what is single phasing and method of protection against it?
Answer: single phasing is usually caused when one of the three backup fuse blows if one of the contactor contact is open circuited the effect of single phasing is to increase the current in the remaining two lines and cause the motor for become noisy due to uneven torque produced in the Rotor.
The loss of current through 1 phase in 3 phase supply is single phasing prevents a motor from starting but running motor may continue to operate with fault.
Method of protection:
Bi metallic single phase protection. Three bimetals cold or three bimetals hot is balanced. 2 bimetals hot and one cold is differential.
Electromagnetic overload relay.
Short circuit protection fuses.
Question 16: Factors on which stresses losses depends on?
Answer: Applied voltage, material of the core and frequency.
Eddy current losses is proportional to the square of the applied voltage and is independent of frequency.
Transformer losses = iron losses + copper losses.
Iron losses is equal to hysteresis losses and Eddy current losses.
Question 17: How to reduce flux leakage in the Transformer?
Answer: To reduce flux leakage primary and secondary are wound on the same Core. page 496 BL theraja.
Question 18: What is zener diode? Uses?
Answer: It is a diode across which the voltage drop is control over the range of current.
It is used as a means of providing a stabilized voltage in same circuit. It acts as a electrical relief valve.
Uses: Voltage stabilizer and battery charging circuit.
Another definition of zener diode: A properly doped crystal diode which has a sharp breakdown voltage is known as zener diode.
Question 19: Function of Zener barrier and use and diagram?
Answer: Zener barrier prevents the transfer of unacceptably high energy from safe area to the hazardous area.
The Zener diodes in the zener barrier are connected in the reverse Biased direction. The breakdown voltage of the diode is not exceeded in normal condition.
If this voltage is exceeded due to fault in safe area, the diode start to conduct use to blow.
Application of Zener barrier:
Alarms, proximity sensors, fire and smoke detectors.
Question 20: Cable is wet how to bring back insulation resistance?
Answer: Cable drawing out by injecting heating current from: -
Current injection set
Welding transformer
The cable should be disconnected at both ends from equipment. The injection cable must have good connection at each end.
Current flow and cable temperature should be carefully monitored.
Criteria during heating:
The injected heating current must never exceed the rated current for the cable (connect an ameter).
The voltage should be in the region of 30 - 55 volts depending upon the current setting.
The temperature should not be allowed to exceed 30 degree Celsius.
Temperature and insulation resistance should be measured and recorded every hour.
When Insulation resistance become study the heating should be carried out for a further 4 hours before switching off.
When satisfactory insulation resistance value have been restored a final check should be made with the cable at normal ambient temperature.
Insulation resistance final values reading:-
Between Core and earth is approximately 20 mega ohms.
Between Core and core is approximately 100 Mega ohms.
Question 21: Cycloconverter, thyristor and transistor?
Answer: The cycloconverter method of controlling speed also relies on the ability of the converter to accept current from the switch board at constant frequency and voltage but to pass this current to the AC motor at a reduced frequency and with voltage adjusted.
The cycloconverter is different in that it operates without an intermediate stage of conversion.
Question 22: - Fuses
Answer: - tinned copper in re-wirable fuse melts at 1080 degree Celsius.
Silver in Cartridge fuses at 90 degree Celsius.
Current rating gives the normal current that may be carried.
Minimum fusing current is the smallest current that will cause melting.
HRC fuses have silver wire enclosed in quartz powder filled ceramic tubes with metal end caps.
They have accurate time/ current characteristic and reliability for discrimination.
Question 23:- semi enclosed fuses
Answer: - Re-wirable fuse has an insulated carrier for safe handling and containment of the wire in an asbestos lined tube.
Fault: - is that of premature failure of wire oxidation on contact with air or being stretched when fitted in.
Current characteristics are found by testing few of the same type of fuses at different current and plotting the result. The bottom current testing is not more than 1.05 % minimum fusing current. Top current is the one that will melt wire is not for more than 0.5 of a second. The other test currents are equally spaced between these.
Question 24: - how you restore residual magnetism in an alternator?
Answer: - to restore residual magnetism connect a 12 volt battery to the exciter field generator is at rest. Remove exciter field lead F+ & F- from the AVR. Measure the exciter field resistance from the F+ and F- lead. You should be able to read some resistance as you are measuring the continuous winding. An infinite resistance reading would indicate and open in the exciter field. Also check to be sure there is no path to ground.
connect F+ to positive pole of the battery.
Hold the F- led by the insulated portion on the lead wire, touch F- in the negative pole of the battery for 5 to 10 seconds, then remove.
Reconnect F+ and F- to the AVR regulator. Repeat the procedure if the generator fails to build voltage.
Question 25:- principle of a tachometer.
Answer:- types of tachometers are AC and DC.
It works on the principle of generation of EMF when a Rotary conductor cuts the magnetic flux. When magnetic flux is constant then EMF produced is directly proportional to the speed of armature that is coupled to the shaft of motor.
Question 26: - parts of MSB
Answer: - 1. Synchronoscope.
2. Air circuit breaker
3. Frequency meter.
4. Synchronising panel
5. Earth fault indicator.
6. Feeders
Question 27:- what is a diode and how it is tested?
Answer: - it is a semiconductor device that is formed by joining p-type and n-type materials and allow the current to flow only in one direction.
- Act as a non-return valve for the current.
- Use to convert AC to DC.
Testing: -
- Check the multi-meter for proper functioning. Short the probes and check resistance and/or continuity. Switch the instrument to diode test mode.
- Switch off the power of circuit. Discharge the capacitors and remove the fuses.
- Connect the two probes of multi-meter across the diode.
- In forward biased measurement, a healthy should be showing 0.5-0.8volts
- If display is over range in both the directions, the diode has open circuit fault.
- If displays less than 1volts in both the directions, the diode may be short circuit fault.
Question 28: - Alternator maintenance.
Answer: - before maintenance, safety should be taken.
1. Shut down and lock off generator prime mover.
2. Generator circuit breaker is locked off.
3. Electric heaters are switched off and isolated.
4. Put the tag. “MAN AT WORK”.
Checks: -
All wiring of the generator should be inspected for damage.
Check oil and water contamination of cable insulation within terminal box.
Check cooling air filter and exhaust opening are not blocked and free from dirt and dust.
Remove dust and dirt from winding by vacuum cleaner.
Oil deposited on insulation will reduce the insulation resistance the oil deposition can remove by washing with special detergent liquid.
Winding insulation can be improved by suitable air drying and varnishing.
Measure the insulation of stator and motor winding to earth and between stator phase minimum insulation resistance should be 1 Mega Ohm.
Check pedestal bearing lubrication.
The area between rotor and stator( 1.8- 3 mm).
Check the space heater.
Question 29:- alternator safeties
Answer:- under voltage protection, reverse power protection, preferential tripping of non essential load, over-current protection, overload trip, short circuit trip, Prime mover over speed trip and emergency stop.
Question 30:- an induction motor has been flooded with seawater insulation resistance is down to zero ohm. What is the procedure for putting back into the service?
Answer: - isolated electrically discharge all Static Current (by earthing). Dismantel rotor and stator. clean all parts through fresh water, allow stator and rotor to dry completely with low power electrical heater or lamp. Provide sufficient ventilation to allow the moisture to escape. Once dried check insulation resistance. if it is zero continue heating till it get good value of insulation resistance is achieved.
If it is not improving then send motor ashore for rewinding. if insulation resistance improves then varnish can be applied. Varnish has to impregnate through the winding. Commutator separated to be cleaned thoroughly. Commutator required to be cleaned by appropriate contact electro cleaner and soft brush. Resurface the commentator by sanding to remove pitting arcing marks and embedded dirt. Check brush contact by pressing. If the pressing is not ok change the spring. Change both the bearings. Assemble and check insulation resistance again. Run on no load if everything is ok. After 2 hours is temperature rise and noise are normal then connect the motor to the load.
Question 31:- what kind of stator winding in generator? Why so is required? Hard reduces cable size? Why you wish to reduce cable size?
Answer:- in case of star connection phase and line current are same. For same number of turns in star connection voltage generation is more but insulation of winding is less. it provide neutral for earthing. Size, cost and maintenance are reduced. In case of delta connection line current is equal to the √3 x phase current. For same voltage we need larger cross sectional area of cable to carry more current.
We want to reduce cable size for lowering the cost of installation as well as maintenance
Question 32:- AVR
Answer:- it is a automatic voltage regulator which senses the generator output voltage and alters the field current to maintain the voltage at its set value.
Question 33: - ensure the fair current sharing when generators continues to run.
Answer: - ensure the overvoltage and undervoltage alarm & trips are operational.
Question 34:- how capacity of battery is decided?
Answer- by passing fixed amount of current till the voltage drops to set value. The time for discharge is multiplied to current and amp-hr is found out.
Question 35:- what are motor protections?
Answer:- overload trip, fuse, thermistors and bimetallic strips
Question 36:- difference between welding Transformer and normal/lighting transformer?
Answer:- welding transformer is a step down transformer that reduces the voltage from the source voltage, that is suitable for welding usually between 15 to 45 volts. Secondary current is quite high 200 to 600 amps. The secondary may have several steps to adjust the secondary voltage to control the welding current. For welding with DC a rectifier is connected to the secondary of the transformer.
Question 37:- purpose of motor varnish
Answer:- it refers to the secondary layer of insulation that is applied to the coils on the rotor and stator winding in a motor. It immobilizes windings so that coils do not move. If the coils are allowed to move coating of inner primary insulation would quickly wear out as the rotating magnetic field cause the winding to vibrate. Motor varnish also increases the dielectric strength of the winding which is more important for transformer winding then motor winding. Dielectric strength is the maximum voltage that can be applied to a material without causing it to break down.
Question 38:- purpose of interpole in DC machine?
Answer:- interpoles or auxiliary poles are placed in between main poles. Their function is to assist commutation by producing auxiliary flux which thereafter helps in eliminating brush sparking. Interpole winding are connected in series with the armature windings.
Question 39: - MSB trips and method of testing?
Answer:- mSB trips
Overcurrent undervoltage reverse power
Testing
Reverse power can be tested by lowering the power of DG.
Question 40:- difference between induction and synchronous motor?
Answer:- induction motor
They only required 3 phase AC supply
They run at speed less than synchronous speed which varies with load.
Power factor at full load is maximum. reduced to a very low 0.2 value at a load below 50%.
It is a robust motor applied in all type of duties in engine room.
Most economical.
Least maintenance.
They are self starting.
Synchronous motor:-
They require both DC to Rotor and three phase AC supply to stator.
Run at constant speed at all loads.
Synchronous motor can run at a constant power factor at all loads by varying the DC excitation voltage.
Selectively used to their constant speed is required at all loads.
They are costly.
Require regular maintenance of slip rings and brushes.
They are not self starting.
Question 41:- Difference between acid and alkaline battery?
Answer:- Lead acid battery contain two plate emerged in an electrolyte. The positive terminal is lead peroxide and negative terminal is lead. The electrolyte used is dilute sulphuric acid, during discharge the hydrogen ion from sulphuric acid react with oxygen dissolved from lead peroxide and forms water. The water formed dilutes the electrolyte and its specific gravity decreases.
At negative plate the sulphate ion combines with the lead and form lead sulphate. The lead sulphate covers the active material of the plate the further reaction is stifled.
Fully charged cell is capable of producing two volt and relative density will be 1.285@ 15 degree Celsius
After a period of discharge the voltage on load drops to 1.75 volts and relative density to 1.110.
Flow of current from the supply source reverse the discharge action of the cell. the sulphate ions from lead sulphate goes into solution leaving the plate as pure lead. Water in the electrolyte breaks down returning hydrogen ion into the solution and allowed oxygen to recombine with the lead of positive plate and form lead peroxide.
Differences:- Acid battery the electrolyte is sulfuric acid, during discharge specific gravity decreases, lead acid battery voltage drops as it discharges take more time in charging and discharging.
Alkaline Battery, the electrolyte is potassium hydroxide, no change in specific gravity during charging or discharge, alkaline battery maintains voltage until discharge completely. Alkaline battery has low internal resistance that allow quick charging and discharging.
Question 42:- working of a brushless alternator?
Answer:- in brushless alternator slip rings and brushes are eliminated and excitation is provided by small alternator. The AC exciter has three phase output windings on the rotor and magnetic poles fixed in the casing.
The Poles are supplied with DC from an AVR.
3 phase current generated on the winding on the exciter rotor pass through a rectifier assembly on the shaft & then to the main alternator poles.
No slip rings are needed
There is no direct electrical connection between rotating and stationary winding of a generator. Exciter field is supplied from DC from a static AVR. Rectification of the exciter voltage is achieved by shaft mounted silicon diode and then supply to the main alternator field.
Question 43: - principle of induction motor?
Answer: - when a 3 phase supply is connected to the stator winding the current produce rotating magnetic field which rotates at a speed called synchronous speed. The value of synchronous speed depends on the number of magnetic poles on the stator and the frequency of the supply connected to the stator. The magnetic field produced by the current flowing through the stator winding is cut by the rotor winding and this induces a magnetic flux in them. Since this flux is changing an EMF is induced which cause a current to flow in these windings. The flow of current produced a torque in the rotor winding which causes them to rotate in the direction of RMF at a speed which is less than the synchronous speed.
Question 44: - Application of induction motors?
Answer: - 3 phase squirrel cage induction motor used for main water pump Motors, main jacket cooling water and lube oil pump Motors.
Small single phase synchronous motors for Radar motor, gyro compass motor.
three phase synchronous motor of large size is used for main electric propulsion and bow thruster.
Synchronous motor act as synchronous condenser in shaft generator system.
Double cage induction motor turning gear motor for high starting torque.
Single Phase induction motor exhaust fan motor used in toilet hand drill hand blower mixer grinder.
Question 45:- what are intrinsic safe circuits?
Answer:- In these circuits there is no Spark or thermal effect is produced under normal and faulty condition to cause ignition. The circuit condition is limited to 50 Milliamps and 30 volt.
This protection is used in low power instrumentation alarm and communication system.
Question 46:- reduced voltage starters?
Answer:- auto transformer starter star delta starter soft starter.
Question 47:- direct online starter?
Answer:- high starting current low starting torque.
Question 48:- what is the purpose of seath on cable?
Answer:- The seat of cable protects the insulation from damage or injury.
Material required to be heat, oil, chemical and flame retardant.
Polychloroprene(PCP) or neoprene.
chlorosulphonated polythelene(CSP) or bypalon.
Question 49: - types of fuses?
Answer: - HRC, kit Kat type, cartridge type, semi-closed type.
Fuse material:- silicon, Copper or Aluminium.
Question 50:- Two alternators are in parallel have same kilowatt load rating but they are at different power factor what is the reason for this?
Answer:- KW load sharing is done by droop of the Governor.
KVAr load sharing is done by droop of the AVR.
Reasons:- if there is fault in the droop of AVR.
Excitation of one of the alternator more than the other.
Question 51:- What is the reason for one alternator failing to build voltage?
Answer:- The generator is remaining in stand still for a long time there is a loss of the residual magnetism.
If the shunt field DC generator excitation field resistance is more than the critical resistance.
Wire may be broken
Voltmeter is faulty.
Question 52:- Relay
Answer:- it is an electrically operated switch. Many relays use an electromagnet to operate a switch mechanism.
Relays are used where it is necessary to control a circuit by low power signal. A type of relay that can handle high power is required directly to derive an electric motor is called a contactor.
Types of relay: - Magnetic,
Thermal bimetallic strip,
Electronic
Parts of relay: -
Electromagnet relay have armature that can be attracted by electromagnet, spring and set of electrical contacts.
Question 53: - Thyristors?
Answer: - they are solid state switches which are turned on by the application of lower switch static voltage through the trigger connection known as Gate electrode.
Examples are SCR & triacs.
Are used to control current greater than thousand ampere and voltage in excess of thousand voltage.
Used as latching switches in an alarm circuit. Conversion of AC to DC.
Control of electric motor speed.
Electric propulsion.
Dimmer lights.
Question 54: - alternator generating low voltage there is a problem with the AVR, find out where the problem is?
Answer: - problem with diodes
Question 55: - what is the function of the eco-sounder and where it is placed?
Answer: - it sends sound waves to the sea bed for measuring its depth, it is fitted at the forward bottom portion of the ship.
Question 56: - what was the specification of electrical distribution system of your ship?
Answer: - 3 phase 440 volt AC 60 hz system
Question 57:- why is the speed of the motor 900 not 950?
Answer:- If we use 950 rpm then the effective number of pole we get would be a decimal value.
N=120f/P.
For example f=60, N=900 then P=8
If N=950, then P=7.74
Question 58:- What to do if the voltage produced by a generator is 438 volt instead of 440 volts.
Answer:- adjust the AVR, adjust the trimming voltage of AVR.
Question 59:- is it correct to put a fuse of 5 ampere into a motor which is running at 4ampere current?
Answer: -yes, because the fuse current rating will always be higher than that of the current rating of the motor, also during starting the current is high let’s say 6Amps, but there is a time delay for the blowing of the fuse.
Question 60: - how is the RPM signal from a turbocharger measured?
Answer: - on the compressor shaft the compressor wheel tightening nut has a projection with a magnet mounted on it. This rotates with the shaft and the rotation is picked up by the two magnetic pickup sensors which are just like a pair of tongs. The projection rotates between the pair of the tongs cutting the flux, there by inducing a current which is calibrated to give RPM. Magnetic pickup sensor send signal to the transducer. The output from the transducer is filtered, chopped and the signal is conditioned and finally converted to square waves.
Square waves pass through a phase lock loop multiplier. This output is given by a precise time base generated through pieze crystal and then counted. The count is displayed on the LED as RPM.
Question 61: - what is the difference between MCB and MCCB?
Answer:- MCB or miniature circuit breaker with optimum protection facility for current protection only.
These are manufactured for fault level up to 10 kilo ampere only. The operating current range of 0.5 to 63 ampere. Single double or three pole versions. These are used for small loads electric circuit, domestic wiring extra.
MCCBs are moulded case circuit breaker with the production facility of over current earth fault. It has variable range of 50% to 100% over current. Can be wire for remote as well as local operation. They are manufactured for fault current up to 50Kapms.
They are used for application related to large current flow requirement.
Question 62:- what is proximity effect?
Answer: - In a conductor carrying AC, if current is flowing through one or more nearby conductors. Such as within a closely wound coil of wire, the distribution of current within the first conductor will be constraint to a small regions. Resulting current around is termed as proximity effect. This gives an increase in effective resistance of the circuit which increases with frequency.
Explanation:- a changing magnetic field will influence the distribution of electric current flowing within an electric conductor by electromagnetic induction. When an AC flows through an isolated conductor it creates an associated alternating magnetic field around it. The alternating magnetic field induces Eddy current in the adjacent conductor, alternating the overall distribution of the current flowing through them. The result is that the current is concentrated in the area of conduction furthest away from nearby conductor. Carrying current in the same direction.
Example:- If two wires carrying the same AC lines parallel to one another the magnetic field of one wire will induce longitudinal Eddy current in the adjacent wire, which flow in the longitudinal loops along the wire facing away from other wire, and back in opposite direction on the side of the wire facing the other wire. Thus the Eddy current will reinforce the main current on the side facing away from the first wire and what was the main current in the side facing the first wire. The net effect is the redistribution of current in the cross section of the wire into a strip on the side facing away from the other wire. The current is concentrated in the smaller area of the wire the resistance will increase.
Question 63:- ICCP, how to know it’s working correctly?
Answer:- the correct working of ICCP can be monitored by checking the value of reference electrode voltage. The voltage measured between the hull and reference electrodes of an unprotected ship with sea water as an electrolyte is
Zinc electrode: 450mV negative to hull
Silver electrode: 600mV positive to hull
When satisfactory protected, the protection current will make the hull 200mV more negative, i.e 250mV in case of zinc and 800mV in case of silver.
The hull potential should remain constant in a properly protected/ regulated system.
Question 64: - what are transistors and how do you test them?
Answer: - the transistor is basically a diode that shares one end. The shared end is called the base and other two ends are called emitter and collector. The collector accepts input current from the circuit, but it can send the current through the transistor via an emitter only if the base allows the current to pass. The base acts like a gate. When a small current is applied to the gate it opens and large current can flow from the collected to the emitter. Types are PNP and NPN. Use is switch and Amplifier.
Testing: - insert probes into the multi-meter, black probe into common and red into terminal marked for testing diodes.
Turn the selector knob to the electrode test function replace the probe tips with alligator clamps. Clamp the black Probe with one of the lead of the transistor.
Touch the red probe to each of the other two lead.
If the display shows a high resistance, when each of the leads are touched, you have found the base and it's a good PNP transistor.
If the display shows two different readings for the two leads, clamp the black probe to another lead and repeat the test.
After camping the black probe to each of the three leads if you don't get the same high resistance reading when touching the lead with the red probe you either have a bad transistor or PNP transistor.
Remove the black probe and clamp the red one of the lead.
Touch the black to each of other two leads touch the black probe to each of the other two leads are touched. You have found the base and you have a good PNP transistor
If the display shows two different reading from the other two leads, clamp the red probe to another lead and repeat the test.
After clamping the red probe to each of the three leads if you don't get the same high R reading of resistance then touching the other lead with black probe, you have a bad PNP transistor.
Question 65:- cascade control system for jacket cooling water?
Answer:- in the master controller if there is a deviation between ‘mv’ of the jacket cooling water temperature and ‘sv’ set by the operator, the master controller sends signal, which act as the ‘sv’ of the slave controller. Any deviation between the ‘sv’ and ‘mv’ of the cooling water inlet temperature cause the slave controller to send signal to reposition the 3-way valve to increase or decrease cooling water flow from the cooler.
Any change in the inlet cooling temperature initiates the slave controller to act in a self-contained control loop to adjust the three way valve to bring the inlet cooling water temperature to the correct value. Thus any change in the cooler outlet temperature (engine cooling water temperature) due to sea water pressure or temperature is prevented from react in the engine and causing any temperature change in the engine system which would have taken a long time to sense and correct temperature deviation. If the inlet temperature of the cooling water is low, slave controller output signal passes through the range splitter to the diaphragm control valve for the steam inlet to open steam to the heater.
Question 66: - tank scope?
Answer:- It is a combustible gas indicator it works in any atmosphere used during cow and purging operation. Calibration gas is butane, electrode are not catalytic. Heated to 300 degree Celsius temperature and R decreases
Scale 0-20% HC by volume.
Question 67:- advancement in alkaline batteries.
Answer:- maintenance free and Shield. No evolution of gases cause Cathode is covered with a little part of cadmium Hydroxide, so that if it is partially charged when anode is fully charged. No vent plug. No spillage.
Question 69:- Star delta starter?
Answer:- three sets of star windings have end connections which are brought out to a starter box. Change over contacts in the starter box enable the 6 ends to be star connected for starting and then as the rotor comes upto speed, to be reconnected in delta. Star starting has the effect of reducing the voltage to 57.7% of line voltage. Starting current and torque are only one third of that in DOL starter. The low current start is obtained at the expanse oof torque and star-delta motor can be used only with light starting loads.
Question 70: - barrier potential and depletion layer?
Answer: - the potential build-up in PN junction by the diffusion of negative electron from N to P region and appearance of the holes on the N side is called barrier potential. Very few current carrier remain in the junction area as a result of the diffusion and it is for this reason called depletion layer.
Question 71: - accelerometer
Answer: - accelerometers are increasingly used in vibration studies because they are small and respond to a wide frequency range then other transducers. The accelerometers consist of the piezoelectric material, part of which is held between some masses and the body of the instrument. If the instrument is fastened to some piece of the vibration machinery, the inertia of the mass causes the piezoelectric material to be subjected to a stress and electrical signal generated. As force is equal to the product of the mass and acceleration, the electrical output is proportional to the acceleration of the mass acting on the piezoelectric material. Use to obtain vibration signature of the machines like pumps, turbochargers extra.
Question 72: - what is the cause of Humming sound in a transformer?
Answer: - This occurs in core transformers. The core of the materials length is extended and contracted by a very small amount at the frequency at which the transformer operated, known as magnetic process. Magnetostriction causes the air being flow back and forth at that frequency causes the sound generation which is the humming sound. Thransformers without core donot generate that sound
Other reasons:-
Insulation is deteriorating
Loose Terminals
No rubber footing installed
Ways to reduce Humming:-
Use cusion padding and oil barriers.
Use good design and materials like stiffners while constructing the walls oif transformer tanks.
Proper fixing and clamping of laminations and frames of transformers.
Question 73: - purpose of seathing on cable?
Answer: - To protect insulation of the cable from damage and injury. It is not classed as insulation. Sheath materials are required to be heat, oil, fire and chemical resistant they should be tough and flexible.
Question 74: - purpose of armouring?
Answer: - they gives extra mechanical protection to the cable braided wire of galvanized steel or fined phosphor bronze Airtel also act as a screen to reduce interference caused by magnetic field in adjacent communication and instrumentation cables.
Question 75:- what determines the current carrying capacity of a cable?
Answer:- restriction imposed by the insulation limits the current carrying capacity of the cable. Greater the current flowing in the cable the higher will be the heat radiated. Heat radiated is equal to the square of current X resistance. So greater will be the rise in temperature. Rise in temperature can damage translation and damaged insulation can not serve its purpose of confining current flow in the conductor.
Question 76:- properties of conductor?
Answer:- there is no electric field inside the conductor. Electric field does not penetrate into conductor. Charge exist only at the surface of the conductor.
All points of a conductor are at the same potential.
Resistance increases with increase in temperature.
Ductile, malleable, valence band and conduction band overlap.
Copper is used commonly as a conductor sometime Aluminium is also used which have less current capacity require greater diameter.
Question 77:- continuity check of very long cable?
Answer:- continuity is checked by measuring the resistance of wire in ohms. It should be very small for healthy cable. In case of breakage the resistance will be very high in Mega ohms.
In order to measure/ check continuity of a very long Cable one end of the cable can be connected to the hull of the ship/earthed. At the other end resistance should be checked between the cable terminals and hull.
Question 78:- Cable insulation and its purpose?
Answer:- main purpose of cable insulation is to keep the current in the conductor and to prevent contact with live wire. Insulation is adversely affected by humidity, temperature, moisture, oil, chemical, vibration, stresses, dirt and old age. Traditional insulants – cotton, Silk, paper, mica, glass, fibre
Modern insulants:- PVC, butyl rubber, EPR.
Question 79: - hotspot in the induction motor?
Answer: - centre of the coil winding where the temperature would be higher than at the surface.
Question 80: - battery capacity Test procedure?
Answer: - battery capacity is expressed in ampere hour. It is measured how much ampere hour the battery can deliver before its terminal voltage drop down to end of discharge. Voltage X number of cells.
The discharge current should be maintained constant during the test. The test time can be selected from the table which will also give the corresponding discharge current and also the terminal voltage at the end of discharge. Normally the test is carried out for five or eight hours. As the discharge time is increased, the discharge current will have to be reduced. Test can be conducted in 1 hour also but then the discharge current has to be increased. At the end of discharge the voltage of a lead acid battery cell is 1.75 volt to 1.80 volt. If the battery reaches the end of discharge voltage before the specific time, the capacity has reduced. Before carrying out the test, check and record specific gravity of electrolyte, cell terminal voltage each and battery terminal voltage. Charging should be discontinued and check all the battery connections.
Question 81: - Clamp metre working?
Answer: - clamp metre basically act as a current transformer. The instruments tongs are clipped around a single conductor without any physical contact. The conductor carrying AC current act as primary and the magnetic flux due to the current flowing in the conductor is alternating flux. As this flux is continuously changing there must be change in flux linkage in the secondary of current transformer. As per Faraday's law of electromagnetic induction the rate of change of flux will produce EMF. If the circuit is closed current will flow which is measured and displayed.
Question 82:- purpose of electrical motor on governor.
Answer:- The motor is called synchronising motor or speed setting motor. The motor runs on DC power only. The purpose is to provide a mean of remote speed adjustment it enable the switch board operator to match the frequency of an alternator to that of other alternator. To change the load distribution when operating with other units in droop mode.
Change in Governor Speed setting produce the same effect governor moment as to change in the load on the engine. The motor is coupled to governors speed setting mechanism through friction clutches.
Question 83:- material of fuse wire other than silver? why can't we use mild steel as fuse wire?
Answer:- other materials are tin lead alloy, aluminium, zinc, antimony etc.
Properties of fuse material:
Low resistivity: that is the less opposition to flow of current.
Low melting point: So that when the current is too large, the temperature of the fuse wire will rise and it will melt and break the circuit.
Mild steel has comparatively higher melting point which is approximately 1400 degree Celsius which is quite high. Also its resistivity is higher. Silver has a melting point of 962 degree Celsius. Resistivity of mild steel is 15 yocto ohmmeter where as resistivity of silver is 1.59 yocto ohmmeter.
Question 84: - what is CE written mostly on electrical equipment?
Answer:- CE is the abbreviation for confirmite europieenne.
Manufacturer’s declaration that the product meet the requirements of applicable EU directive.
Question 85: - fuse markings?
Answer: - ampere, voltage, time current characteristics, approvals, manufacturer part number serial number and breaking capacity. T 3.15A H 250V
In the above first letter means FF = very fast acting, F= fast acting, M= medium acting, T= slow acting, TT= very slow acting.
Question 86:- basics of transformer?
Answer:- it works on the principle of mutual inductance of two coils of Faraday's law of electromagnetic induction primary coil is carrying AC current it produces alternating magnetic flux this alternating magnetic flux cuts through the secondary and using and EMF in it.
Primary secondary are wrapped around a common soft iron core made of individual laminations to reduce Eddy current and hysteresis losses.
Role of change of flux linkages depends on the amount of heat flux with the secondary winding. So it's a desired that almost all the flux of primary winding be linked to the secondary winding.
But in open a very tiny portion of the flux of first winding will link with second. In order to overcome this Ellora elektrans part in the form is placed common to both the windings. Maximum number of eggs produced by primary winding is passed through and linked to the secondary through this low reluctance path.
Question 87:- losses in transformer?
Answer:- copper losses depends on the load current.
Core losses or iron losses = the sum of hysteresis and Eddy current losses magnetic properties of material used to construct the core of transformer.
Question 88:- Magnetostriction?
Answer:- flux in a ferromagnetic material such as the core, causes it to physically expand and contract slightly with each cycle of magnetic field magnetostriction buzzing sound in transformer.
Question 89:- methods to reduce Eddy current losses?
Answer:- thin laminated cores prevent most of the Eddy current from flowing at all. area decreases and hence resistance increases eddy current losses are proportional to the square of thickness of lamination.
Thickness = 0.25 mm to 0.5 mm insulated from each other.
Increasing resistivity by using core material as Silicon Steel.
So content up to 3% results in dramatic increase in resistivity three to four times higher the increase in Silicon impaired the Steels Mechanical properties causing difficulty for rolling due to brittleness .
Question 90: - Factors affecting hysteresis loss?
Answer: - maximum flux density, frequency and core material.
Use high grade silicon Steel.
Question 91: - how to reduce flux leakage in transformer?
Answer: - carefully selecting core material having low reluctance.
Careful design of the core, geometry of the core effects leakage flux.
- Making Transformer window long and narrow.
- arranging primary and secondary winding concentrically.
- interleaving/ sandwiching the primary and secondary winding.
- using shell type construction in which both primary and secondary are founded on the centre leg and limbs which has enlarged the cross sectional area.
Question 92: - reluctance?
Answer: - it is an opposition to magnetic flux in a magnetic circuit analogous to resistance in electrical circuit.
Question 93: - leakage flux?
Answer: - amount of flux lines that do not follow the core and are lost to the surrounding air.
Transformer core design is of two type core type and shell type.
Question 94:- single phasing?
Answer:- loss of current through 1 phase in a 3 phase supply is single phasing. This can be due to blown fuses, cable break, loose contacts.
Running 3 phase motor after single phasing will continue to run at slightly reduced RPM. Stopped three phase motor will not start if single phasing has occurred because in the absence of 3 phase rotating magnetic field it will not available.
Question 95: - How to determine positive and negative terminals of battery when there is no marking on battery.
Answer: - battery polarity can be checked by using multimeter when the red probe is connected to positive terminal and black probe to negative terminal multimeter will show positive value of voltage and vice versa
At positive plate there is some sulphate accumulation.
Question 96: - white deposits on battery terminals? Why there is salt deposition on battery terminals.
Answer:- White deposits on battery Negative terminals is the result of sulphation. Sulphate ions combine with pure lead during discharge to form lead sulphate crystal (amorphous form). During use, small sulphate crystals form, but these are normal and are not harmful. During prolong charge deprivation, however, the amorphous lead sulphate converts to a stable crystalline that deposits on negative plate. Sulfation lower charge acceptance .
Question 97: - battery maintenance?
Answer: - a. Batteries should be placed in well ventilated rooms.
b. Ventilation fan should be Spark proof room should be kept dry and clean no smoking sign should be displayed in and out of the room no naked light is allowed only explosive proof light is allowed.
c. Condition of battery should be checked regularly, in Lead acid battery the specific gravity is checked by hydrometer. Fully charge battery has a specific gravity of 1.280, fully discharged battery has a specific gravity of 1.12.
d. In alkaline batteries the condition is checked by voltmeter.
e. Tightness of terminals is to be checked.
f. Terminals are to be applied with petroleum jelly to prevent corrosion.
g. Battery should be secured in a wooden tray.
h. All tools used in a battery room should be insulated.
i. Temperature of the electrolyte should not exceed 45 degree Celsius.
j. Vent hole should be kept clean. Clean the battery top to keep dry and clean with baking powder and water solution.
k. Excessive Rapid charging should be avoided.
l. Always top up with distilled water. Battery plates Should always be submerged in electrolyte.
m. At least once in a month the battery should be completely discharge and recharge to increase performance. Check voltage under load.
n. Check container for any leak or deformation.
Question 98:- MSB safety and maintenance.
Answer:- safeties
1. sectionalization through bus tie.
2. Rubber matting of at least 3 mm thick front and back of the MSB.
3. Earthing of hinged doors.
4. Dead front – All switchgears and equipments are enclosed in sheet steel compartments.
5. No pipelines in the vicinity of bus-bar.
6. Insulated handrails.
7. Cabinet doors cannot be open if circuit breaker is closed.
8. Circuit breaker and Switch Gears are of draw out type.
9. Earth lamps.
10. UV, OV protection
11. UF, OF protection.
12. Short circuit protection.
13. Overcurrent trip.
14. Reverse power trip.
15. Co2 fire extinguisher nearby.
16. 24 volt DC emergency lighting provided.
17. Fuse puller, Lamp holder and circuit breaker operating handles.
18. MSB on raised platform.
19. Drip proof protection.
Maintenance:-
1. Should be clean inside and outside dust removed by vacuum cleaner.
2. Connection fixing should be checked for tightness.
3. Busbar insulation should be checked for deterioration and tracking.
4. Operation of bus tie should be checked.
5. All auxiliary wiring should be checked for soundness.
6. Incoming cable should be checked for soundness and their gland inspected.
7. Earth bonding of panel doors to be checked.
8. Switches and indication lamps checked.
9. Operation of all safety devices should be checked.
Question 99:- Sf6 circuit breaker?
Answer:- low voltage circuit breaker use air, meaning contact separate in air. While High Voltage 6.6 KV and 11KV uses vacuum type circuit breaker or gas field type circuit breaker.
Sf6 circuit breaker is a gas filled circuit breaker, in which Sf6 is under pressure of 5 bar at 20degree Celsius. Contact separate in this pressurized gas chamber. Insulation is quite high. Dielectric constant is high. Quicker arc extinction.
Question 100: - power factor and its significance?
Answer: - residential areas are charged for electric power consumption on the basis of KW-hr, while industries are charged for KVA consumption.
So power factor has got no significance for residential areas.
While in industries same load if supplied with a lower power factor shall require to draw higher KVA and would result higher electric bill.
For example:- KW=2 with P.F=0.9, when compared with a supply of P.F 0.7 found that later will consume 2.85KVA and while former had 2.22KVA consumption. Also the current value is increased to 12.4Amps which was 9.66Amps initially.
So the higher power factor is economical.
Question 101: - methods for improving power factor?
Answer: - 1.Static capacitors when connected in parallel with the inductive loads provide leading current. Static capacitors in parallel with devices working on low power factor. These provide leading current which neutralizes (totally or approx) the lagging inductive component of load current. They have low losses, low maintenance and no moving parts, work in ordinary atmosphere and lightweight.
2. Synchronous condenser: - when a synchronous motor operates at No-Load and over excited, then it provides leading current and work like a capacitor.
When a synchronous condenser is connected across supply voltage (in parallel), then it draws leading current and partially eliminates the reactive component and this way power to improved.
Phase advancer: - it’s a particular type of A.C exciter which is connected to the rotor circuit of an induction motor to improve P.F. It’s mounted on the shaft of main motor.
It supplies exciting ampere turns to the rotor circuit at slip frequency. By providing more ampere-turns than required the I.M can be made to operate on leading P.F like an over-excited synchronous motor.
Current is injected through the slip rings of motor, which is leading with regards to the rotor voltage.
This current relieves the stator circuit of the duty of magnetisation thus improves power factor.
Question 102: - Cogging and crawling of induction motor?
Answer: - when the no. of rotor slots and stator slots have common factor, the rotor in a squirrel cage exhibits a tendency to remain in one place and not rotate at all, when energized from mains. When the rotor teeth force stator teeth, the reluctance of the magnetic path is minimum, that is why rotor tends to remain fixed. (Magnetic locking occurs). This phenomenon is called cogging or Magnetic locking of induction motor.
Crawling: - when an induction motor runs at very low speed (precisely 1/7th of synchronous speed). Even if full voltage is applied to it, then it is called at crawling. It’s caused by 7th harmonics, which may be present in power supply.
Question 103: - losses in induction motor? Core loss constituents? Hysteresis loss in detail? Methods incorporated to reduce both eddy and hysteresis losses.
Answer: - Losses: 1. Stator cu losses + stator core losses.
2. Rotor copper losses + Rotor core losses (depends, in f’=1/s, which is, very small so negligible).
3. Friction losses + winding losses.
Core loss: - Hysteresis loss- because of magnetisation and de-magnetisation
And eddy current loss.
Hysteresis: - it’s the time lag when the magnetic field is first generated and when the field is fully developed. Addition of silicon makes the steel more efficient and faster in forms of building and maintaining magnetic field. Silicon reduces reluctance and hysteresis loss.
Eddy current loss: due to alternating flux linking the core which will induce an EMF in the core due to which a current called eddy current is being circulated in the core. As there is some resistance in the core, with this eddy current circulation concerts into heat called eddy current loss.
Question 104: Condition for maximum efficiency of transformer? How?
Answer: - for the transformer the efficiency is maximum when "Iron loss= copper loss"
Proof:
$\displaystyle \small \mathrm{Efficiency, \eta =\frac{Output}{Input}=\frac{Input-losses}{Input}=1-\frac{losses}{Input} }$
Considering primary side,
$\displaystyle \small \mathrm{Input = V_1I_1cos\theta _1}$
If $\displaystyle \small \mathrm{R_{01}}$ is the total resistance referred to primary, the cu-loss, $\displaystyle \small \mathrm{W_c =I_1^2R_{01}}$
Say, Iron loss= $\displaystyle \small \mathrm{W_i}$
Then $\displaystyle \small \mathrm{\eta = 1- \frac{W_c+W_i}{V_1I_1cos\theta _1}}$
or
$\displaystyle \small \mathrm{\eta = 1- \frac{I_1^2R_{01}}{V_1I_1cos\theta _1}-\frac{W_i}{V_1I_1cos\theta _1}}$
differentiating wrt load current $\displaystyle \small \mathrm{I_1}$ , and equating to zero.
$\displaystyle \small \mathrm{\frac{d\eta }{di_1}=0-\frac{R_01}{V_1cos\theta _1}+\frac{W_1}{V_1I_1^2cos\theta _1}=0}$
or
$\displaystyle \small \mathrm{W_i =I_1^2R_{01}}$
Thus, "Iron loss= copper loss"
Question 105: properties of transformer oil? Why it is not used on board?
Answer: - Properties
1. Thermal stability.
2. Excellent insulating properties.
3. High flash point higher than 140 degree Celsius.
4. Low pour point less than -6 degree Celsius.
5. Viscosity should remain low at temperature.
The cooling oil used in a transformer is flammable at high temperatures and presents a considerable fire hazard due to the large quantity used.
A defective or aged insulation of winding or a high electrical surge may cause overheating in the transformer and leads to the ignition of oil. The situation is compounded by damage to the transformer tank or oil-carrying lines resulting in a spray fire.
Question 106: - you are given a Y- starter and a motor. How will you say that the given starter is compatible with the motor?
Answer: - For a motor to be compatible with a starter all six terminals of the 3-phase winding shall be brought out and available at the terminal box. So that the connection can be made.
Answer: 1. Checking contacts of the circuit breaker for damage and dirt.
2. Clean by electro-cleaner for contacts.
3. Dirt and dust are to be removed vacuum cleaner.
4. Circuit breaker contact is never to be filed and scrapped.
5. Defective or burnt contact to be replaced.
Question 2:- Safeties of MSB?
Answer: -1. Minimum 3 mm thick rubber mat on front and back.
2. Insulated handgrip and rail ebonite handle.
3. Dead front is required for the switchboard for AC.
4. No pipeline to pass in the vicinity.
5. Earth fault indicator.
6. If the breaker is on the top cover cannot be opened.
7. Ventilation grid.
8. Manual tripping device.
9. under voltage relay, 0.8 x normal voltage (3-5 seconds).
10. Reverse power trip (5 - 15 % below normal power).
11. over voltage trip- 1.3 X normal voltage.
12. Preferential trips- 110% full load current.
Question 3: How sound is transmitted through the telephone from the engine room to Bridge?
Answer: There is a diaphragm from where we speak due to the sound wave diaphragm vibrating. A coil is there, due to the vibration of the diaphragm the coil rotates in a magnetic field, which means the sound wave is converted into an electrical wave.
The diaphragm is vibrated by the sound wave and moves the driving rod and armature rack out forth through the stationary magnetic field existing between the magnet and the armature.
The moving armature Alters the magnetic field making it move and cutting the conductor in the coil thereby inducing a small AC voltage that produces a fluctuating magnetic field about the armature. The armature is repelled and attracted by the field of the magnet and moves back and forth. The diagram of the receiver vibrated in and out by the driving rod, thus creating sound waves, which correspond with the sound waves striking the transmitter diaphragm.
Question 4: As a second engineer how will you order a battery for emergency lightning?
Answer: voltage, ampere hour, make and model number, dimension and type of battery.
Question 5: What is an ampere-hour?
Answer: The power rating for a battery is given in Amp-hr, which means how many amps can a battery deliver to be completely discharged over an hour.
Example of required amperage
Emergency load (navigational equipment + lighting) if the load 100 Amps need a minimum of 8 hours so the required capacity is a 24-volt 800-ampere hour.
Question 6: if your ship is designed for 60 hertz at 440 volts. shore supply volt 380 Volts is operating at 50 Hertz reasons?
Answer: supply voltage should be 380 volts. Has the frequency been reduced by 16.7% proportionally the voltage should be reduced by 16.7% ratio of voltage and frequency can be maintained a speed of the motor is directly proportional to the frequency and torque current is directly proportional to the voltage so the increase in torque current is controlled by the proportional reduction in voltage?
Question 7: difference between a switch and a circuit breaker?
Answer: a circuit breaker is an automatically operated electrical switch designed to protect a circuit from damage caused by circuit faults.
Its basic function is to detect a fault condition while maintaining continuity and immediately disconnect the circuit upon the fault detection.
A Switch is manually operated by using a contactor to open and close the circuit.
Question 8: current transformer secondary winding is earth why?
Answer: the reason is to reduce the insulation costing. When you earth one secondary terminal you can insulate the current transformer to phase voltage else you have to insulate for line voltage, which might increase the cost as well as size of the current transformer. By earthing one end of the current transformer, the circuit is anchored to zero voltage and any fault can be detected by Earth fault meter.
Question 9: checks when taking shore supply?
Answer: connection box at entrance of accommodation or emergency generator room.
The connection box must have suitable terminals to accept the Shore supply cable including an earthing terminal to earth the ship’s hull to shore earth. The connection box must have circuit breaker or an insulator switch and fuses to protect cable linking to connection box to main switch board.
A voltmeter is fitted to indicate polarity of DC shore connection.
For an AC shore supply, phase sequence indicator is used to indicate correct supply phase sequence.
Shore supply switch on MSB is interlocked with generator circuit breaker so that it cannot be closed generator is still connected
Reverse phase sequence will produce reverse shaft rotation because the rotating magnetic field will be reversed with disastrous result.
Phase sequence can be corrected by interchanging any two conductor of shore supply.
Higher shore supply frequency will cause motor to run faster, overloaded and overheated. A higher voltage will generally cause equipments to take excessive current cause overheating. Lower voltage is generally not so serious but may cause motor to run slower and overheat may cause motor to stall.
Voltage, frequency and phase sequence should be same. Metre reading should be taken. Earthing, insulation of shore supply cable and shore connection box to be tested.
Question 10: what is slip?
Answer: the difference between synchronous speed (Ns) and the actual speed (N) of the motor. Expressed as percentage of the synchronous speed.
Question 11: undervoltage trip?
Answer: It is intended to open the brakes at loss of excitation for any reason, has a time delay. The solenoid is wired across the main leads on the generator side of circuit breaker and when not energized leave the plunger in trip position and breaker cannot be closed. Normal voltage output with the machine running energizes the coil and the plunger is pulled down against loading spring to release the trip. This allow closure of breaker and means the device is set.
Question 12: Measurement of air gap in alternator?
Answer: It is measured at 4 location 90 degree apart on both ends using feeler gauge rotate rotor by 90 degree and repeat the readings shall not be beyond 10% on average at each end. Values of air gap 1.5 mm maximum.
Question 13: What is SCR?
Answer: A silicon controlled rectifier is a four layer pnpn semiconductor device that does not allow the current to flow until it is triggered. The flow of current is in one direction. It has three terminals gate - input terminal. Anode - output terminal. Cathode which is common to both the gate and anode application SCR is used in alarm circuit.
Question 14: Circuit breaker types?
Answer: Air Circuit breaker, Oil circuit breaker, Gas circuit breaker with sf6 (Sulphur hexafluoride), vacuum circuit breaker - The high dielectric strength of a vacuum allow a very short contact separation and a rapid restrike free of arc intruption is achieved.
Question 15: what is single phasing and method of protection against it?
Answer: single phasing is usually caused when one of the three backup fuse blows if one of the contactor contact is open circuited the effect of single phasing is to increase the current in the remaining two lines and cause the motor for become noisy due to uneven torque produced in the Rotor.
The loss of current through 1 phase in 3 phase supply is single phasing prevents a motor from starting but running motor may continue to operate with fault.
Method of protection:
Bi metallic single phase protection. Three bimetals cold or three bimetals hot is balanced. 2 bimetals hot and one cold is differential.
Electromagnetic overload relay.
Short circuit protection fuses.
Question 16: Factors on which stresses losses depends on?
Answer: Applied voltage, material of the core and frequency.
Eddy current losses is proportional to the square of the applied voltage and is independent of frequency.
Transformer losses = iron losses + copper losses.
Iron losses is equal to hysteresis losses and Eddy current losses.
Question 17: How to reduce flux leakage in the Transformer?
Answer: To reduce flux leakage primary and secondary are wound on the same Core. page 496 BL theraja.
Question 18: What is zener diode? Uses?
Answer: It is a diode across which the voltage drop is control over the range of current.
It is used as a means of providing a stabilized voltage in same circuit. It acts as a electrical relief valve.
Uses: Voltage stabilizer and battery charging circuit.
Another definition of zener diode: A properly doped crystal diode which has a sharp breakdown voltage is known as zener diode.
Question 19: Function of Zener barrier and use and diagram?
Answer: Zener barrier prevents the transfer of unacceptably high energy from safe area to the hazardous area.
The Zener diodes in the zener barrier are connected in the reverse Biased direction. The breakdown voltage of the diode is not exceeded in normal condition.
If this voltage is exceeded due to fault in safe area, the diode start to conduct use to blow.
Application of Zener barrier:
Alarms, proximity sensors, fire and smoke detectors.
Question 20: Cable is wet how to bring back insulation resistance?
Answer: Cable drawing out by injecting heating current from: -
Current injection set
Welding transformer
The cable should be disconnected at both ends from equipment. The injection cable must have good connection at each end.
Current flow and cable temperature should be carefully monitored.
Criteria during heating:
The injected heating current must never exceed the rated current for the cable (connect an ameter).
The voltage should be in the region of 30 - 55 volts depending upon the current setting.
The temperature should not be allowed to exceed 30 degree Celsius.
Temperature and insulation resistance should be measured and recorded every hour.
When Insulation resistance become study the heating should be carried out for a further 4 hours before switching off.
When satisfactory insulation resistance value have been restored a final check should be made with the cable at normal ambient temperature.
Insulation resistance final values reading:-
Between Core and earth is approximately 20 mega ohms.
Between Core and core is approximately 100 Mega ohms.
Question 21: Cycloconverter, thyristor and transistor?
Answer: The cycloconverter method of controlling speed also relies on the ability of the converter to accept current from the switch board at constant frequency and voltage but to pass this current to the AC motor at a reduced frequency and with voltage adjusted.
The cycloconverter is different in that it operates without an intermediate stage of conversion.
Question 22: - Fuses
Answer: - tinned copper in re-wirable fuse melts at 1080 degree Celsius.
Silver in Cartridge fuses at 90 degree Celsius.
Current rating gives the normal current that may be carried.
Minimum fusing current is the smallest current that will cause melting.
HRC fuses have silver wire enclosed in quartz powder filled ceramic tubes with metal end caps.
They have accurate time/ current characteristic and reliability for discrimination.
Question 23:- semi enclosed fuses
Answer: - Re-wirable fuse has an insulated carrier for safe handling and containment of the wire in an asbestos lined tube.
Fault: - is that of premature failure of wire oxidation on contact with air or being stretched when fitted in.
Current characteristics are found by testing few of the same type of fuses at different current and plotting the result. The bottom current testing is not more than 1.05 % minimum fusing current. Top current is the one that will melt wire is not for more than 0.5 of a second. The other test currents are equally spaced between these.
Question 24: - how you restore residual magnetism in an alternator?
Answer: - to restore residual magnetism connect a 12 volt battery to the exciter field generator is at rest. Remove exciter field lead F+ & F- from the AVR. Measure the exciter field resistance from the F+ and F- lead. You should be able to read some resistance as you are measuring the continuous winding. An infinite resistance reading would indicate and open in the exciter field. Also check to be sure there is no path to ground.
connect F+ to positive pole of the battery.
Hold the F- led by the insulated portion on the lead wire, touch F- in the negative pole of the battery for 5 to 10 seconds, then remove.
Reconnect F+ and F- to the AVR regulator. Repeat the procedure if the generator fails to build voltage.
Question 25:- principle of a tachometer.
Answer:- types of tachometers are AC and DC.
It works on the principle of generation of EMF when a Rotary conductor cuts the magnetic flux. When magnetic flux is constant then EMF produced is directly proportional to the speed of armature that is coupled to the shaft of motor.
Question 26: - parts of MSB
Answer: - 1. Synchronoscope.
2. Air circuit breaker
3. Frequency meter.
4. Synchronising panel
5. Earth fault indicator.
6. Feeders
Question 27:- what is a diode and how it is tested?
Answer: - it is a semiconductor device that is formed by joining p-type and n-type materials and allow the current to flow only in one direction.
- Act as a non-return valve for the current.
- Use to convert AC to DC.
Testing: -
- Check the multi-meter for proper functioning. Short the probes and check resistance and/or continuity. Switch the instrument to diode test mode.
- Switch off the power of circuit. Discharge the capacitors and remove the fuses.
- Connect the two probes of multi-meter across the diode.
- In forward biased measurement, a healthy should be showing 0.5-0.8volts
- If display is over range in both the directions, the diode has open circuit fault.
- If displays less than 1volts in both the directions, the diode may be short circuit fault.
Question 28: - Alternator maintenance.
Answer: - before maintenance, safety should be taken.
1. Shut down and lock off generator prime mover.
2. Generator circuit breaker is locked off.
3. Electric heaters are switched off and isolated.
4. Put the tag. “MAN AT WORK”.
Checks: -
All wiring of the generator should be inspected for damage.
Check oil and water contamination of cable insulation within terminal box.
Check cooling air filter and exhaust opening are not blocked and free from dirt and dust.
Remove dust and dirt from winding by vacuum cleaner.
Oil deposited on insulation will reduce the insulation resistance the oil deposition can remove by washing with special detergent liquid.
Winding insulation can be improved by suitable air drying and varnishing.
Measure the insulation of stator and motor winding to earth and between stator phase minimum insulation resistance should be 1 Mega Ohm.
Check pedestal bearing lubrication.
The area between rotor and stator( 1.8- 3 mm).
Check the space heater.
Question 29:- alternator safeties
Answer:- under voltage protection, reverse power protection, preferential tripping of non essential load, over-current protection, overload trip, short circuit trip, Prime mover over speed trip and emergency stop.
Question 30:- an induction motor has been flooded with seawater insulation resistance is down to zero ohm. What is the procedure for putting back into the service?
Answer: - isolated electrically discharge all Static Current (by earthing). Dismantel rotor and stator. clean all parts through fresh water, allow stator and rotor to dry completely with low power electrical heater or lamp. Provide sufficient ventilation to allow the moisture to escape. Once dried check insulation resistance. if it is zero continue heating till it get good value of insulation resistance is achieved.
If it is not improving then send motor ashore for rewinding. if insulation resistance improves then varnish can be applied. Varnish has to impregnate through the winding. Commutator separated to be cleaned thoroughly. Commutator required to be cleaned by appropriate contact electro cleaner and soft brush. Resurface the commentator by sanding to remove pitting arcing marks and embedded dirt. Check brush contact by pressing. If the pressing is not ok change the spring. Change both the bearings. Assemble and check insulation resistance again. Run on no load if everything is ok. After 2 hours is temperature rise and noise are normal then connect the motor to the load.
Question 31:- what kind of stator winding in generator? Why so is required? Hard reduces cable size? Why you wish to reduce cable size?
Answer:- in case of star connection phase and line current are same. For same number of turns in star connection voltage generation is more but insulation of winding is less. it provide neutral for earthing. Size, cost and maintenance are reduced. In case of delta connection line current is equal to the √3 x phase current. For same voltage we need larger cross sectional area of cable to carry more current.
We want to reduce cable size for lowering the cost of installation as well as maintenance
Question 32:- AVR
Answer:- it is a automatic voltage regulator which senses the generator output voltage and alters the field current to maintain the voltage at its set value.
Question 33: - ensure the fair current sharing when generators continues to run.
Answer: - ensure the overvoltage and undervoltage alarm & trips are operational.
Question 34:- how capacity of battery is decided?
Answer- by passing fixed amount of current till the voltage drops to set value. The time for discharge is multiplied to current and amp-hr is found out.
Question 35:- what are motor protections?
Answer:- overload trip, fuse, thermistors and bimetallic strips
Question 36:- difference between welding Transformer and normal/lighting transformer?
Answer:- welding transformer is a step down transformer that reduces the voltage from the source voltage, that is suitable for welding usually between 15 to 45 volts. Secondary current is quite high 200 to 600 amps. The secondary may have several steps to adjust the secondary voltage to control the welding current. For welding with DC a rectifier is connected to the secondary of the transformer.
Question 37:- purpose of motor varnish
Answer:- it refers to the secondary layer of insulation that is applied to the coils on the rotor and stator winding in a motor. It immobilizes windings so that coils do not move. If the coils are allowed to move coating of inner primary insulation would quickly wear out as the rotating magnetic field cause the winding to vibrate. Motor varnish also increases the dielectric strength of the winding which is more important for transformer winding then motor winding. Dielectric strength is the maximum voltage that can be applied to a material without causing it to break down.
Question 38:- purpose of interpole in DC machine?
Answer:- interpoles or auxiliary poles are placed in between main poles. Their function is to assist commutation by producing auxiliary flux which thereafter helps in eliminating brush sparking. Interpole winding are connected in series with the armature windings.
Question 39: - MSB trips and method of testing?
Answer:- mSB trips
Overcurrent undervoltage reverse power
Testing
Reverse power can be tested by lowering the power of DG.
Question 40:- difference between induction and synchronous motor?
Answer:- induction motor
They only required 3 phase AC supply
They run at speed less than synchronous speed which varies with load.
Power factor at full load is maximum. reduced to a very low 0.2 value at a load below 50%.
It is a robust motor applied in all type of duties in engine room.
Most economical.
Least maintenance.
They are self starting.
Synchronous motor:-
They require both DC to Rotor and three phase AC supply to stator.
Run at constant speed at all loads.
Synchronous motor can run at a constant power factor at all loads by varying the DC excitation voltage.
Selectively used to their constant speed is required at all loads.
They are costly.
Require regular maintenance of slip rings and brushes.
They are not self starting.
Question 41:- Difference between acid and alkaline battery?
Answer:- Lead acid battery contain two plate emerged in an electrolyte. The positive terminal is lead peroxide and negative terminal is lead. The electrolyte used is dilute sulphuric acid, during discharge the hydrogen ion from sulphuric acid react with oxygen dissolved from lead peroxide and forms water. The water formed dilutes the electrolyte and its specific gravity decreases.
At negative plate the sulphate ion combines with the lead and form lead sulphate. The lead sulphate covers the active material of the plate the further reaction is stifled.
Fully charged cell is capable of producing two volt and relative density will be 1.285@ 15 degree Celsius
After a period of discharge the voltage on load drops to 1.75 volts and relative density to 1.110.
Flow of current from the supply source reverse the discharge action of the cell. the sulphate ions from lead sulphate goes into solution leaving the plate as pure lead. Water in the electrolyte breaks down returning hydrogen ion into the solution and allowed oxygen to recombine with the lead of positive plate and form lead peroxide.
Differences:- Acid battery the electrolyte is sulfuric acid, during discharge specific gravity decreases, lead acid battery voltage drops as it discharges take more time in charging and discharging.
Alkaline Battery, the electrolyte is potassium hydroxide, no change in specific gravity during charging or discharge, alkaline battery maintains voltage until discharge completely. Alkaline battery has low internal resistance that allow quick charging and discharging.
Question 42:- working of a brushless alternator?
Answer:- in brushless alternator slip rings and brushes are eliminated and excitation is provided by small alternator. The AC exciter has three phase output windings on the rotor and magnetic poles fixed in the casing.
The Poles are supplied with DC from an AVR.
3 phase current generated on the winding on the exciter rotor pass through a rectifier assembly on the shaft & then to the main alternator poles.
No slip rings are needed
There is no direct electrical connection between rotating and stationary winding of a generator. Exciter field is supplied from DC from a static AVR. Rectification of the exciter voltage is achieved by shaft mounted silicon diode and then supply to the main alternator field.
Question 43: - principle of induction motor?
Answer: - when a 3 phase supply is connected to the stator winding the current produce rotating magnetic field which rotates at a speed called synchronous speed. The value of synchronous speed depends on the number of magnetic poles on the stator and the frequency of the supply connected to the stator. The magnetic field produced by the current flowing through the stator winding is cut by the rotor winding and this induces a magnetic flux in them. Since this flux is changing an EMF is induced which cause a current to flow in these windings. The flow of current produced a torque in the rotor winding which causes them to rotate in the direction of RMF at a speed which is less than the synchronous speed.
Question 44: - Application of induction motors?
Answer: - 3 phase squirrel cage induction motor used for main water pump Motors, main jacket cooling water and lube oil pump Motors.
Small single phase synchronous motors for Radar motor, gyro compass motor.
three phase synchronous motor of large size is used for main electric propulsion and bow thruster.
Synchronous motor act as synchronous condenser in shaft generator system.
Double cage induction motor turning gear motor for high starting torque.
Single Phase induction motor exhaust fan motor used in toilet hand drill hand blower mixer grinder.
Question 45:- what are intrinsic safe circuits?
Answer:- In these circuits there is no Spark or thermal effect is produced under normal and faulty condition to cause ignition. The circuit condition is limited to 50 Milliamps and 30 volt.
This protection is used in low power instrumentation alarm and communication system.
Question 46:- reduced voltage starters?
Answer:- auto transformer starter star delta starter soft starter.
Question 47:- direct online starter?
Answer:- high starting current low starting torque.
Question 48:- what is the purpose of seath on cable?
Answer:- The seat of cable protects the insulation from damage or injury.
Material required to be heat, oil, chemical and flame retardant.
Polychloroprene(PCP) or neoprene.
chlorosulphonated polythelene(CSP) or bypalon.
Question 49: - types of fuses?
Answer: - HRC, kit Kat type, cartridge type, semi-closed type.
Fuse material:- silicon, Copper or Aluminium.
Question 50:- Two alternators are in parallel have same kilowatt load rating but they are at different power factor what is the reason for this?
Answer:- KW load sharing is done by droop of the Governor.
KVAr load sharing is done by droop of the AVR.
Reasons:- if there is fault in the droop of AVR.
Excitation of one of the alternator more than the other.
Question 51:- What is the reason for one alternator failing to build voltage?
Answer:- The generator is remaining in stand still for a long time there is a loss of the residual magnetism.
If the shunt field DC generator excitation field resistance is more than the critical resistance.
Wire may be broken
Voltmeter is faulty.
Question 52:- Relay
Answer:- it is an electrically operated switch. Many relays use an electromagnet to operate a switch mechanism.
Relays are used where it is necessary to control a circuit by low power signal. A type of relay that can handle high power is required directly to derive an electric motor is called a contactor.
Types of relay: - Magnetic,
Thermal bimetallic strip,
Electronic
Parts of relay: -
Electromagnet relay have armature that can be attracted by electromagnet, spring and set of electrical contacts.
Question 53: - Thyristors?
Answer: - they are solid state switches which are turned on by the application of lower switch static voltage through the trigger connection known as Gate electrode.
Examples are SCR & triacs.
Are used to control current greater than thousand ampere and voltage in excess of thousand voltage.
Used as latching switches in an alarm circuit. Conversion of AC to DC.
Control of electric motor speed.
Electric propulsion.
Dimmer lights.
Question 54: - alternator generating low voltage there is a problem with the AVR, find out where the problem is?
Answer: - problem with diodes
Question 55: - what is the function of the eco-sounder and where it is placed?
Answer: - it sends sound waves to the sea bed for measuring its depth, it is fitted at the forward bottom portion of the ship.
Question 56: - what was the specification of electrical distribution system of your ship?
Answer: - 3 phase 440 volt AC 60 hz system
Question 57:- why is the speed of the motor 900 not 950?
Answer:- If we use 950 rpm then the effective number of pole we get would be a decimal value.
N=120f/P.
For example f=60, N=900 then P=8
If N=950, then P=7.74
Question 58:- What to do if the voltage produced by a generator is 438 volt instead of 440 volts.
Answer:- adjust the AVR, adjust the trimming voltage of AVR.
Question 59:- is it correct to put a fuse of 5 ampere into a motor which is running at 4ampere current?
Answer: -yes, because the fuse current rating will always be higher than that of the current rating of the motor, also during starting the current is high let’s say 6Amps, but there is a time delay for the blowing of the fuse.
Question 60: - how is the RPM signal from a turbocharger measured?
Answer: - on the compressor shaft the compressor wheel tightening nut has a projection with a magnet mounted on it. This rotates with the shaft and the rotation is picked up by the two magnetic pickup sensors which are just like a pair of tongs. The projection rotates between the pair of the tongs cutting the flux, there by inducing a current which is calibrated to give RPM. Magnetic pickup sensor send signal to the transducer. The output from the transducer is filtered, chopped and the signal is conditioned and finally converted to square waves.
Square waves pass through a phase lock loop multiplier. This output is given by a precise time base generated through pieze crystal and then counted. The count is displayed on the LED as RPM.
Question 61: - what is the difference between MCB and MCCB?
Answer:- MCB or miniature circuit breaker with optimum protection facility for current protection only.
These are manufactured for fault level up to 10 kilo ampere only. The operating current range of 0.5 to 63 ampere. Single double or three pole versions. These are used for small loads electric circuit, domestic wiring extra.
MCCBs are moulded case circuit breaker with the production facility of over current earth fault. It has variable range of 50% to 100% over current. Can be wire for remote as well as local operation. They are manufactured for fault current up to 50Kapms.
They are used for application related to large current flow requirement.
Question 62:- what is proximity effect?
Answer: - In a conductor carrying AC, if current is flowing through one or more nearby conductors. Such as within a closely wound coil of wire, the distribution of current within the first conductor will be constraint to a small regions. Resulting current around is termed as proximity effect. This gives an increase in effective resistance of the circuit which increases with frequency.
Explanation:- a changing magnetic field will influence the distribution of electric current flowing within an electric conductor by electromagnetic induction. When an AC flows through an isolated conductor it creates an associated alternating magnetic field around it. The alternating magnetic field induces Eddy current in the adjacent conductor, alternating the overall distribution of the current flowing through them. The result is that the current is concentrated in the area of conduction furthest away from nearby conductor. Carrying current in the same direction.
Example:- If two wires carrying the same AC lines parallel to one another the magnetic field of one wire will induce longitudinal Eddy current in the adjacent wire, which flow in the longitudinal loops along the wire facing away from other wire, and back in opposite direction on the side of the wire facing the other wire. Thus the Eddy current will reinforce the main current on the side facing away from the first wire and what was the main current in the side facing the first wire. The net effect is the redistribution of current in the cross section of the wire into a strip on the side facing away from the other wire. The current is concentrated in the smaller area of the wire the resistance will increase.
Question 63:- ICCP, how to know it’s working correctly?
Answer:- the correct working of ICCP can be monitored by checking the value of reference electrode voltage. The voltage measured between the hull and reference electrodes of an unprotected ship with sea water as an electrolyte is
Zinc electrode: 450mV negative to hull
Silver electrode: 600mV positive to hull
When satisfactory protected, the protection current will make the hull 200mV more negative, i.e 250mV in case of zinc and 800mV in case of silver.
The hull potential should remain constant in a properly protected/ regulated system.
Question 64: - what are transistors and how do you test them?
Answer: - the transistor is basically a diode that shares one end. The shared end is called the base and other two ends are called emitter and collector. The collector accepts input current from the circuit, but it can send the current through the transistor via an emitter only if the base allows the current to pass. The base acts like a gate. When a small current is applied to the gate it opens and large current can flow from the collected to the emitter. Types are PNP and NPN. Use is switch and Amplifier.
Testing: - insert probes into the multi-meter, black probe into common and red into terminal marked for testing diodes.
Turn the selector knob to the electrode test function replace the probe tips with alligator clamps. Clamp the black Probe with one of the lead of the transistor.
Touch the red probe to each of the other two lead.
If the display shows a high resistance, when each of the leads are touched, you have found the base and it's a good PNP transistor.
If the display shows two different readings for the two leads, clamp the black probe to another lead and repeat the test.
After camping the black probe to each of the three leads if you don't get the same high resistance reading when touching the lead with the red probe you either have a bad transistor or PNP transistor.
Remove the black probe and clamp the red one of the lead.
Touch the black to each of other two leads touch the black probe to each of the other two leads are touched. You have found the base and you have a good PNP transistor
If the display shows two different reading from the other two leads, clamp the red probe to another lead and repeat the test.
After clamping the red probe to each of the three leads if you don't get the same high R reading of resistance then touching the other lead with black probe, you have a bad PNP transistor.
Question 65:- cascade control system for jacket cooling water?
Answer:- in the master controller if there is a deviation between ‘mv’ of the jacket cooling water temperature and ‘sv’ set by the operator, the master controller sends signal, which act as the ‘sv’ of the slave controller. Any deviation between the ‘sv’ and ‘mv’ of the cooling water inlet temperature cause the slave controller to send signal to reposition the 3-way valve to increase or decrease cooling water flow from the cooler.
Any change in the inlet cooling temperature initiates the slave controller to act in a self-contained control loop to adjust the three way valve to bring the inlet cooling water temperature to the correct value. Thus any change in the cooler outlet temperature (engine cooling water temperature) due to sea water pressure or temperature is prevented from react in the engine and causing any temperature change in the engine system which would have taken a long time to sense and correct temperature deviation. If the inlet temperature of the cooling water is low, slave controller output signal passes through the range splitter to the diaphragm control valve for the steam inlet to open steam to the heater.
Question 66: - tank scope?
Answer:- It is a combustible gas indicator it works in any atmosphere used during cow and purging operation. Calibration gas is butane, electrode are not catalytic. Heated to 300 degree Celsius temperature and R decreases
Scale 0-20% HC by volume.
Question 67:- advancement in alkaline batteries.
Answer:- maintenance free and Shield. No evolution of gases cause Cathode is covered with a little part of cadmium Hydroxide, so that if it is partially charged when anode is fully charged. No vent plug. No spillage.
Question 69:- Star delta starter?
Answer:- three sets of star windings have end connections which are brought out to a starter box. Change over contacts in the starter box enable the 6 ends to be star connected for starting and then as the rotor comes upto speed, to be reconnected in delta. Star starting has the effect of reducing the voltage to 57.7% of line voltage. Starting current and torque are only one third of that in DOL starter. The low current start is obtained at the expanse oof torque and star-delta motor can be used only with light starting loads.
Question 70: - barrier potential and depletion layer?
Answer: - the potential build-up in PN junction by the diffusion of negative electron from N to P region and appearance of the holes on the N side is called barrier potential. Very few current carrier remain in the junction area as a result of the diffusion and it is for this reason called depletion layer.
Question 71: - accelerometer
Answer: - accelerometers are increasingly used in vibration studies because they are small and respond to a wide frequency range then other transducers. The accelerometers consist of the piezoelectric material, part of which is held between some masses and the body of the instrument. If the instrument is fastened to some piece of the vibration machinery, the inertia of the mass causes the piezoelectric material to be subjected to a stress and electrical signal generated. As force is equal to the product of the mass and acceleration, the electrical output is proportional to the acceleration of the mass acting on the piezoelectric material. Use to obtain vibration signature of the machines like pumps, turbochargers extra.
Question 72: - what is the cause of Humming sound in a transformer?
Answer: - This occurs in core transformers. The core of the materials length is extended and contracted by a very small amount at the frequency at which the transformer operated, known as magnetic process. Magnetostriction causes the air being flow back and forth at that frequency causes the sound generation which is the humming sound. Thransformers without core donot generate that sound
Other reasons:-
Insulation is deteriorating
Loose Terminals
No rubber footing installed
Ways to reduce Humming:-
Use cusion padding and oil barriers.
Use good design and materials like stiffners while constructing the walls oif transformer tanks.
Proper fixing and clamping of laminations and frames of transformers.
Question 73: - purpose of seathing on cable?
Answer: - To protect insulation of the cable from damage and injury. It is not classed as insulation. Sheath materials are required to be heat, oil, fire and chemical resistant they should be tough and flexible.
Question 74: - purpose of armouring?
Answer: - they gives extra mechanical protection to the cable braided wire of galvanized steel or fined phosphor bronze Airtel also act as a screen to reduce interference caused by magnetic field in adjacent communication and instrumentation cables.
Question 75:- what determines the current carrying capacity of a cable?
Answer:- restriction imposed by the insulation limits the current carrying capacity of the cable. Greater the current flowing in the cable the higher will be the heat radiated. Heat radiated is equal to the square of current X resistance. So greater will be the rise in temperature. Rise in temperature can damage translation and damaged insulation can not serve its purpose of confining current flow in the conductor.
Question 76:- properties of conductor?
Answer:- there is no electric field inside the conductor. Electric field does not penetrate into conductor. Charge exist only at the surface of the conductor.
All points of a conductor are at the same potential.
Resistance increases with increase in temperature.
Ductile, malleable, valence band and conduction band overlap.
Copper is used commonly as a conductor sometime Aluminium is also used which have less current capacity require greater diameter.
Question 77:- continuity check of very long cable?
Answer:- continuity is checked by measuring the resistance of wire in ohms. It should be very small for healthy cable. In case of breakage the resistance will be very high in Mega ohms.
In order to measure/ check continuity of a very long Cable one end of the cable can be connected to the hull of the ship/earthed. At the other end resistance should be checked between the cable terminals and hull.
Question 78:- Cable insulation and its purpose?
Answer:- main purpose of cable insulation is to keep the current in the conductor and to prevent contact with live wire. Insulation is adversely affected by humidity, temperature, moisture, oil, chemical, vibration, stresses, dirt and old age. Traditional insulants – cotton, Silk, paper, mica, glass, fibre
Modern insulants:- PVC, butyl rubber, EPR.
Question 79: - hotspot in the induction motor?
Answer: - centre of the coil winding where the temperature would be higher than at the surface.
Question 80: - battery capacity Test procedure?
Answer: - battery capacity is expressed in ampere hour. It is measured how much ampere hour the battery can deliver before its terminal voltage drop down to end of discharge. Voltage X number of cells.
The discharge current should be maintained constant during the test. The test time can be selected from the table which will also give the corresponding discharge current and also the terminal voltage at the end of discharge. Normally the test is carried out for five or eight hours. As the discharge time is increased, the discharge current will have to be reduced. Test can be conducted in 1 hour also but then the discharge current has to be increased. At the end of discharge the voltage of a lead acid battery cell is 1.75 volt to 1.80 volt. If the battery reaches the end of discharge voltage before the specific time, the capacity has reduced. Before carrying out the test, check and record specific gravity of electrolyte, cell terminal voltage each and battery terminal voltage. Charging should be discontinued and check all the battery connections.
Question 81: - Clamp metre working?
Answer: - clamp metre basically act as a current transformer. The instruments tongs are clipped around a single conductor without any physical contact. The conductor carrying AC current act as primary and the magnetic flux due to the current flowing in the conductor is alternating flux. As this flux is continuously changing there must be change in flux linkage in the secondary of current transformer. As per Faraday's law of electromagnetic induction the rate of change of flux will produce EMF. If the circuit is closed current will flow which is measured and displayed.
Question 82:- purpose of electrical motor on governor.
Answer:- The motor is called synchronising motor or speed setting motor. The motor runs on DC power only. The purpose is to provide a mean of remote speed adjustment it enable the switch board operator to match the frequency of an alternator to that of other alternator. To change the load distribution when operating with other units in droop mode.
Change in Governor Speed setting produce the same effect governor moment as to change in the load on the engine. The motor is coupled to governors speed setting mechanism through friction clutches.
Question 83:- material of fuse wire other than silver? why can't we use mild steel as fuse wire?
Answer:- other materials are tin lead alloy, aluminium, zinc, antimony etc.
Properties of fuse material:
Low resistivity: that is the less opposition to flow of current.
Low melting point: So that when the current is too large, the temperature of the fuse wire will rise and it will melt and break the circuit.
Mild steel has comparatively higher melting point which is approximately 1400 degree Celsius which is quite high. Also its resistivity is higher. Silver has a melting point of 962 degree Celsius. Resistivity of mild steel is 15 yocto ohmmeter where as resistivity of silver is 1.59 yocto ohmmeter.
Question 84: - what is CE written mostly on electrical equipment?
Answer:- CE is the abbreviation for confirmite europieenne.
Manufacturer’s declaration that the product meet the requirements of applicable EU directive.
Question 85: - fuse markings?
Answer: - ampere, voltage, time current characteristics, approvals, manufacturer part number serial number and breaking capacity. T 3.15A H 250V
In the above first letter means FF = very fast acting, F= fast acting, M= medium acting, T= slow acting, TT= very slow acting.
Question 86:- basics of transformer?
Answer:- it works on the principle of mutual inductance of two coils of Faraday's law of electromagnetic induction primary coil is carrying AC current it produces alternating magnetic flux this alternating magnetic flux cuts through the secondary and using and EMF in it.
Primary secondary are wrapped around a common soft iron core made of individual laminations to reduce Eddy current and hysteresis losses.
Role of change of flux linkages depends on the amount of heat flux with the secondary winding. So it's a desired that almost all the flux of primary winding be linked to the secondary winding.
But in open a very tiny portion of the flux of first winding will link with second. In order to overcome this Ellora elektrans part in the form is placed common to both the windings. Maximum number of eggs produced by primary winding is passed through and linked to the secondary through this low reluctance path.
Question 87:- losses in transformer?
Answer:- copper losses depends on the load current.
Core losses or iron losses = the sum of hysteresis and Eddy current losses magnetic properties of material used to construct the core of transformer.
Question 88:- Magnetostriction?
Answer:- flux in a ferromagnetic material such as the core, causes it to physically expand and contract slightly with each cycle of magnetic field magnetostriction buzzing sound in transformer.
Question 89:- methods to reduce Eddy current losses?
Answer:- thin laminated cores prevent most of the Eddy current from flowing at all. area decreases and hence resistance increases eddy current losses are proportional to the square of thickness of lamination.
Thickness = 0.25 mm to 0.5 mm insulated from each other.
Increasing resistivity by using core material as Silicon Steel.
So content up to 3% results in dramatic increase in resistivity three to four times higher the increase in Silicon impaired the Steels Mechanical properties causing difficulty for rolling due to brittleness .
Question 90: - Factors affecting hysteresis loss?
Answer: - maximum flux density, frequency and core material.
Use high grade silicon Steel.
Question 91: - how to reduce flux leakage in transformer?
Answer: - carefully selecting core material having low reluctance.
Careful design of the core, geometry of the core effects leakage flux.
- Making Transformer window long and narrow.
- arranging primary and secondary winding concentrically.
- interleaving/ sandwiching the primary and secondary winding.
- using shell type construction in which both primary and secondary are founded on the centre leg and limbs which has enlarged the cross sectional area.
Question 92: - reluctance?
Answer: - it is an opposition to magnetic flux in a magnetic circuit analogous to resistance in electrical circuit.
Question 93: - leakage flux?
Answer: - amount of flux lines that do not follow the core and are lost to the surrounding air.
Transformer core design is of two type core type and shell type.
Question 94:- single phasing?
Answer:- loss of current through 1 phase in a 3 phase supply is single phasing. This can be due to blown fuses, cable break, loose contacts.
Running 3 phase motor after single phasing will continue to run at slightly reduced RPM. Stopped three phase motor will not start if single phasing has occurred because in the absence of 3 phase rotating magnetic field it will not available.
Question 95: - How to determine positive and negative terminals of battery when there is no marking on battery.
Answer: - battery polarity can be checked by using multimeter when the red probe is connected to positive terminal and black probe to negative terminal multimeter will show positive value of voltage and vice versa
At positive plate there is some sulphate accumulation.
Question 96: - white deposits on battery terminals? Why there is salt deposition on battery terminals.
Answer:- White deposits on battery Negative terminals is the result of sulphation. Sulphate ions combine with pure lead during discharge to form lead sulphate crystal (amorphous form). During use, small sulphate crystals form, but these are normal and are not harmful. During prolong charge deprivation, however, the amorphous lead sulphate converts to a stable crystalline that deposits on negative plate. Sulfation lower charge acceptance .
Question 97: - battery maintenance?
Answer: - a. Batteries should be placed in well ventilated rooms.
b. Ventilation fan should be Spark proof room should be kept dry and clean no smoking sign should be displayed in and out of the room no naked light is allowed only explosive proof light is allowed.
c. Condition of battery should be checked regularly, in Lead acid battery the specific gravity is checked by hydrometer. Fully charge battery has a specific gravity of 1.280, fully discharged battery has a specific gravity of 1.12.
d. In alkaline batteries the condition is checked by voltmeter.
e. Tightness of terminals is to be checked.
f. Terminals are to be applied with petroleum jelly to prevent corrosion.
g. Battery should be secured in a wooden tray.
h. All tools used in a battery room should be insulated.
i. Temperature of the electrolyte should not exceed 45 degree Celsius.
j. Vent hole should be kept clean. Clean the battery top to keep dry and clean with baking powder and water solution.
k. Excessive Rapid charging should be avoided.
l. Always top up with distilled water. Battery plates Should always be submerged in electrolyte.
m. At least once in a month the battery should be completely discharge and recharge to increase performance. Check voltage under load.
n. Check container for any leak or deformation.
Question 98:- MSB safety and maintenance.
Answer:- safeties
1. sectionalization through bus tie.
2. Rubber matting of at least 3 mm thick front and back of the MSB.
3. Earthing of hinged doors.
4. Dead front – All switchgears and equipments are enclosed in sheet steel compartments.
5. No pipelines in the vicinity of bus-bar.
6. Insulated handrails.
7. Cabinet doors cannot be open if circuit breaker is closed.
8. Circuit breaker and Switch Gears are of draw out type.
9. Earth lamps.
10. UV, OV protection
11. UF, OF protection.
12. Short circuit protection.
13. Overcurrent trip.
14. Reverse power trip.
15. Co2 fire extinguisher nearby.
16. 24 volt DC emergency lighting provided.
17. Fuse puller, Lamp holder and circuit breaker operating handles.
18. MSB on raised platform.
19. Drip proof protection.
Maintenance:-
1. Should be clean inside and outside dust removed by vacuum cleaner.
2. Connection fixing should be checked for tightness.
3. Busbar insulation should be checked for deterioration and tracking.
4. Operation of bus tie should be checked.
5. All auxiliary wiring should be checked for soundness.
6. Incoming cable should be checked for soundness and their gland inspected.
7. Earth bonding of panel doors to be checked.
8. Switches and indication lamps checked.
9. Operation of all safety devices should be checked.
Question 99:- Sf6 circuit breaker?
Answer:- low voltage circuit breaker use air, meaning contact separate in air. While High Voltage 6.6 KV and 11KV uses vacuum type circuit breaker or gas field type circuit breaker.
Sf6 circuit breaker is a gas filled circuit breaker, in which Sf6 is under pressure of 5 bar at 20degree Celsius. Contact separate in this pressurized gas chamber. Insulation is quite high. Dielectric constant is high. Quicker arc extinction.
Question 100: - power factor and its significance?
Answer: - residential areas are charged for electric power consumption on the basis of KW-hr, while industries are charged for KVA consumption.
So power factor has got no significance for residential areas.
While in industries same load if supplied with a lower power factor shall require to draw higher KVA and would result higher electric bill.
For example:- KW=2 with P.F=0.9, when compared with a supply of P.F 0.7 found that later will consume 2.85KVA and while former had 2.22KVA consumption. Also the current value is increased to 12.4Amps which was 9.66Amps initially.
So the higher power factor is economical.
Question 101: - methods for improving power factor?
Answer: - 1.Static capacitors when connected in parallel with the inductive loads provide leading current. Static capacitors in parallel with devices working on low power factor. These provide leading current which neutralizes (totally or approx) the lagging inductive component of load current. They have low losses, low maintenance and no moving parts, work in ordinary atmosphere and lightweight.
2. Synchronous condenser: - when a synchronous motor operates at No-Load and over excited, then it provides leading current and work like a capacitor.
When a synchronous condenser is connected across supply voltage (in parallel), then it draws leading current and partially eliminates the reactive component and this way power to improved.
Phase advancer: - it’s a particular type of A.C exciter which is connected to the rotor circuit of an induction motor to improve P.F. It’s mounted on the shaft of main motor.
It supplies exciting ampere turns to the rotor circuit at slip frequency. By providing more ampere-turns than required the I.M can be made to operate on leading P.F like an over-excited synchronous motor.
Current is injected through the slip rings of motor, which is leading with regards to the rotor voltage.
This current relieves the stator circuit of the duty of magnetisation thus improves power factor.
Question 102: - Cogging and crawling of induction motor?
Answer: - when the no. of rotor slots and stator slots have common factor, the rotor in a squirrel cage exhibits a tendency to remain in one place and not rotate at all, when energized from mains. When the rotor teeth force stator teeth, the reluctance of the magnetic path is minimum, that is why rotor tends to remain fixed. (Magnetic locking occurs). This phenomenon is called cogging or Magnetic locking of induction motor.
Crawling: - when an induction motor runs at very low speed (precisely 1/7th of synchronous speed). Even if full voltage is applied to it, then it is called at crawling. It’s caused by 7th harmonics, which may be present in power supply.
Question 103: - losses in induction motor? Core loss constituents? Hysteresis loss in detail? Methods incorporated to reduce both eddy and hysteresis losses.
Answer: - Losses: 1. Stator cu losses + stator core losses.
2. Rotor copper losses + Rotor core losses (depends, in f’=1/s, which is, very small so negligible).
3. Friction losses + winding losses.
Core loss: - Hysteresis loss- because of magnetisation and de-magnetisation
And eddy current loss.
Hysteresis: - it’s the time lag when the magnetic field is first generated and when the field is fully developed. Addition of silicon makes the steel more efficient and faster in forms of building and maintaining magnetic field. Silicon reduces reluctance and hysteresis loss.
Eddy current loss: due to alternating flux linking the core which will induce an EMF in the core due to which a current called eddy current is being circulated in the core. As there is some resistance in the core, with this eddy current circulation concerts into heat called eddy current loss.
Question 104: Condition for maximum efficiency of transformer? How?
Answer: - for the transformer the efficiency is maximum when "Iron loss= copper loss"
Proof:
$\displaystyle \small \mathrm{Efficiency, \eta =\frac{Output}{Input}=\frac{Input-losses}{Input}=1-\frac{losses}{Input} }$
Considering primary side,
$\displaystyle \small \mathrm{Input = V_1I_1cos\theta _1}$
If $\displaystyle \small \mathrm{R_{01}}$ is the total resistance referred to primary, the cu-loss, $\displaystyle \small \mathrm{W_c =I_1^2R_{01}}$
Say, Iron loss= $\displaystyle \small \mathrm{W_i}$
Then $\displaystyle \small \mathrm{\eta = 1- \frac{W_c+W_i}{V_1I_1cos\theta _1}}$
or
$\displaystyle \small \mathrm{\eta = 1- \frac{I_1^2R_{01}}{V_1I_1cos\theta _1}-\frac{W_i}{V_1I_1cos\theta _1}}$
differentiating wrt load current $\displaystyle \small \mathrm{I_1}$ , and equating to zero.
$\displaystyle \small \mathrm{\frac{d\eta }{di_1}=0-\frac{R_01}{V_1cos\theta _1}+\frac{W_1}{V_1I_1^2cos\theta _1}=0}$
or
$\displaystyle \small \mathrm{W_i =I_1^2R_{01}}$
Thus, "Iron loss= copper loss"
Question 105: properties of transformer oil? Why it is not used on board?
Answer: - Properties
1. Thermal stability.
2. Excellent insulating properties.
3. High flash point higher than 140 degree Celsius.
4. Low pour point less than -6 degree Celsius.
5. Viscosity should remain low at temperature.
The cooling oil used in a transformer is flammable at high temperatures and presents a considerable fire hazard due to the large quantity used.
A defective or aged insulation of winding or a high electrical surge may cause overheating in the transformer and leads to the ignition of oil. The situation is compounded by damage to the transformer tank or oil-carrying lines resulting in a spray fire.
Question 106: - you are given a Y- starter and a motor. How will you say that the given starter is compatible with the motor?
Answer: - For a motor to be compatible with a starter all six terminals of the 3-phase winding shall be brought out and available at the terminal box. So that the connection can be made.
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