MET-AUG-2019

Q1. 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.

 

Q2. 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.

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. 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.

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. What is meant by “Resonance” in RLC circuits? Compare the series and parallel resonance circuits.

B. Find the frequency at which the following circuit resonates.

 

Q7. 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.

 

Q8. Explain What is meant by, and the significance of, four of the following terms.
I. Voltage Stabilization,
II. Filter choke;
III. Impedance,
IV. Rectification,
V. Grid bias voltage.

Q9. 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.
 

Q10. A. Write shorts notes on various types of DC Motors.

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.