DieselShip's Online Content Library

Q1. a) Briefly explain the principle of Operation of Induction Motors (4)

b) What is slip for an induction motor? (4)

c) Draw a simple ladder logic diagram of star delta starting of an induction motor. (8)

Q2. What is Zener diode and how does it regulate the voltage? What happens to the series current, load current and Zener current when the d.c. input voltage of a Zener regulator increases? Draw a neat diagram of Zener regulator and explain. (16)

Q3. Discuss the criteria of the classification of marine high voltage for A-C. and D.C. Systems. Sketch a Ships high voltage distribution system and explain its features. Discuss the various methods of testing the insulation of HV system. Mention the significance of PI Test, why 3 terminals insulation testers are used in HV insulation measurements? (16)

Q4. What are the conditions for producing sustained oscillations? Classify oscillations with respect to frequency range, principle involved, etc. It is possible to produce oscillations with RC networks in phase shift oscillators. Discuss in detail. (16)

Q5. What is soft starting of an Induction motor? Describe with a circuit using thyristors used for soft starting. Discuss its advantages and dis-advantages. (16)

Q6. a) Describe International protection rating and types of insulation. (6)

B. A 72 KVA transformer supplies a heating and lighting load of 12 KW at unity power factor and a motor load of 70 kVA at 0.766 (lagging) power factor: Calculate the minimum rating of the power-factor improvement capacitors which must be connected in the circuit the ensure that the transformer does not become overloaded. (10)

Q7. A. Which of the following three motors has the poorest speed regulation: shunt motor, series Motor or cumulative compound motor? Explain.

B. A 440V shunt motor takes an armature current of 30A at 700 rev/min. The armature resistance is 0.7ohm. If the flux is suddenly reduced by 20 per cent, to what value will the armature current rise momentarily? Assuming unchanged resisting torque to motion, what will be the new steady values of speed and armature current? Sketch graphs showing armature current and speed as functions of time during the transition from initial to final, steady-state conditions.

Q8. A. What is back emf? Derive the relation for the back emf and the supplied voltage in terms of armature resistance.

B. A three- phase induction motor is wound for four poles and is supplied from a 50 Hz system. Calculate.

i. The synchronous speed; (4)

ii. The speed of the rotor when the slip is 4 per cent; (3)

iii. The rotor frequency when the speed of the rotor is 600 r/min. (2)

Q9. A. What are the factors which determine the synchronous speed of a motor? (6)

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. (10)

Q10. A. Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse. (6)

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 100 V, 50 Hz 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? (10)