MET-MAR-2020

Q1. With respect to the High Voltage power systems installation, explain the different types of circuit breaker that are used, comparing them on merits and de-merits. Describe the theory of arc phenomenon and the mechanism fitted to mitigate the arc.

Q2. Explain the methods used to control the speed of a 3 Phase induction motors. Draw and Explain a Variable Frequency Drive used for optimization of energy efficiency of auxiliary machineries on board vessels.

 

Q3. What is the meant by "excitation" in an alternator? With the help of a neat diagram of brushless alternator labeling all the important parts. explain how the excitation is achieved in a brushless alternator.

Q4. Diesel electric propulsion is now being chosen as the power plant for an increasingly wide variety of vessels.
(a) Sketch a simple layout of such an installation.
(b) Explain the advantages of selecting such a plant.

Q5. With reference to an emergency source of electrical power in cargo ships: -
A. Describe a typical power source.
B. Give a typical list of essential services, which must be supplied simultaneously;
C. Explain how the emergency installation can be periodically tested.

Q6. A. Describe how protection against short circuit is provided in a 3 phase induction motor circuit.

B. Explain how rotating magnetic field is produced in three phase winding with three phase supply.

A 4-pole, 3-phase induction motor operates from a supply whose frequency is 50 Hz. Calculate
(i) Speed at which the magnetic field of the stator is rotating,
(ii) speed of the rotor when the slip is 0.04,
(iii) the frequency of the rotor current when the slip is 0.03.

Q7. A. Explain the potential hazards of liquid-cooled transformers are used onboard ships.

B. What are the losses in transformers? Mention the various factors which affect these losses.

In a 25 KVA, 3300/233 V, single phase transformer, the iron and full-load Cu. Losses are respectively 350 and 400 watts.
Calculate the efficiency at half-full load, 0.8 power factor.

 

Q8. A. Why is it important to maintain high efficiency of operation? And low values of voltages regulation for power transformers?

B. A 100 KVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary and secondary resistances are 0.3 Ω and 0.01 Ω respectively, and the corresponding leakage reactance’s are 1.1 Ω and 0.035 Ω respectively. The supply voltage is 2200 V. Calculate:
(i) The equivalent impedance referred to the primary circuit;
(ii) The voltage regulation and secondary terminal voltage for full load having a power factor of (i) 0.8 lagging and (ii) 0.8 leading.

 

Q9. A. List the factors that determine the starting torque of the three-phase induction motor. How does this torque generally compare with the value of the rated torque?

B. The low-voltage release of an a.c. motor-starter consists of a solenoid into which an iron plunger is drawn against a spring. The resistance of the solenoid is 35 ohm. When connected to a 220 V, 50 Hz, a.c. 2supply the current taken is at first 2A, and when the plunger is drawn into the “full-in” position the current falls to 0.7 A.
Calculate the inductance of the solenoid for both positions of the plunger, and the maximum value of flux- linkages in weber-turns for the “full-in” position of the plunger.
 

Q10. A. With the aid of delta and star connection diagrams, state the basic equation from which delta – star – delta conversion equation can be derived.

B. Three batteries A, B and C have their negative terminals connected together, between the positive terminals of A and B there is a resistor of 0.5 ohm and between B and C three is a resistor of 0.3 ohm,
i. Battery A 105 V, Internal resistance 0.25 ohm
ii. Battery B 100 V, Internal resistance 0.2 ohm
iii. Battery C 95 V, Internal resistance 0.25 ohm
C. Determine the current values in the two resistors and the power dissipated by them.