DieselShip's Online Content Library

Q1. a) Explain the construction and the periodic maintenance required of a Vacuum Circuit Breaker (VCB). (8)

b) Compare the construction, operation, and usage of Vacuum Circuit Breakers (VCB) with Air Circuit Breakers (ACB). (8)

Q2. a) Explain the construction and working principle of Star-Delta Starter with the help of circuit diagram. What are the advantages and limitations of using a Star-Delta Starter for starting an induction motor. (10)

b) Describe the maintenance procedures for a motor starter. What are the common faults, and how would you troubleshoot them. (6)

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.

Q4. What is the meant by "excitation" in an alternator? With the help of a neat diagram of brushless alternator labelling all the important parts. explain how the excitation is achieved in a brushless alternator.

Q5. a) Explain the construction, working principle, and characteristics of a Zener Diode. Discuss its applications in electronic circuits. (8)

b) What is a Zener Barrier? With the help of a diagram, explain how a Zener Barrier works in an intrinsic Safe Circuit and discuss its importance in hazardous environments. (8)

Q6. A. What are the characteristics of PN junction diode? Point out its specifications. Also point out the significance of dynamic and static resistances. (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)

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.

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?