MET-JUNE-2019

Q1. What are semiconductor devices? What are its advantages over thermionic devices? With respect to semi-conductor devices describe working principle and application of the following
A. Zener Diode B. Transistor, C. Photocell, D. Thyristor.

Q2. Tank liquid level sensors are an integral part of ships. Describe with aid of suitable sketches the working principle of,
(a) capacitive type level sensor;
(b) Ultrasonic level sensor;
(c) Float.

Q3. 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.

Q4. 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.

Q5. Explain the effect of reduced voltage on standard cage motors with respect to EACH of the following:
(i) Burn out; (ii) Starting current; (iii) Starting torque; (iv) Speed.

Q6. 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.

 

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 tenets 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;
ii. The speed of the rotor when the slip is 4 per cent;
iii. The motor frequency when the speed of the rotor is 600 r.p.m

 

Q9. 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?