MET-Dec-2020

Q1. A sketch a circuit diagram for an automatic voltage regulator illustrating how the AVR utilizes a silicon-controlled rectifier to control the excitation system for an alternator.

Q2. With reference to the condition monitoring of electrical machinery:
A. State TWO important parameters that may be recorded;
B. Explain how the parameters are measured and what defects may be revealed.

Q3. Compare methods of obtaining speed regulation of three-phase induction motor generally used in tankers by means of:
A. Rotor resistance; B. Cascade system; C. Pole-changing.
Give examples where each system may be employed with advantage.

Q4. Explain why it is necessary to have reverse power protection for alternators intended for Operation. 

A. Sketch a reverse power trip; 

B. Explain briefly the principle on which the operation of this power trip is based and How tripping is activated.

Q5. With reference to alkaline batteries used on board ship.
A. Describe the operation of a battery cell and state the material used;
B. Describe how the cells are mounted to form a battery;
C. State the advantages and disadvantages compared with lead-acid batteries.

Q6. A. Explain the significance of the root mean square value of an alternating current or voltage wave form; Define the form factor of such a wave form.

B. A total load of 8000 kW at 0.8 power factor is supplied by two alternators in parallel. One alternator supplies 6000 kW at 0.9 power factor. Find the KVA rating of the other alternator and the power factor.

 

Q7. A. Discuss different methods of speed control of a d.c. series motor by adjusting field ampere turns. 

B. A 230 V, d.c. shunt motor runs at 1000 r.p.m and takes 5 amperes. The armature resistance of the motor is 0.025Ω and shunt field resistance is 230 Ω Calculate the drop in speed when the motor is loaded and takes the line current of 41 amperes. Neglect armature reaction. 

 

Q8. A. What are the factors which determine the synchronous speed of a motor?

B. Three conductors fitted side by side in the stator of a salient-pole alternator. Each generates maximum voltage of 200V (sinusoidal). The angle subtended at the centre of the stator between adjacent conductors is 20 electrical degrees. If the three conductors are connected in series, find
(i) the r.m.s. value of the effective voltage and
(ii) the ‘breadth factor’. Using the theory that is the basis of this problem, give one reason why three-phase current has been introduced.

 

Q9. A. Briefly describe the maintenance routines carried out for emergency batteries onboard.

B. A power of 36 W is to be dissipated in a register connected across the terminals of a battery, having emf of 20V and an internal resistance of 1Ω.

Find:

(i) What value of resistance will satisfy this condition.
(ii) The terminal voltage of the battery for each of the resistances and

(iii) The total power expenditure in each case.

 

Q10. A. Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse.

B. A coil having a resistance of 10 Ohm, and an inductance of 0.15 H is connected in series with a capacitor across a 100V, 50Hz supply. If the current and the voltage are in phase what will be the value of the current in the circuit and the voltage drop across the coil?