MET-MAR-2019
Q1. Sketch and describe a main engine shaft driven generator arrangement with an electric system for frequency correction;
Q2. With reference to A.C generators:
A. Give a brief outline of care maintenance that should be given to stator and rotor of an AC generator;
B. Explain what is likely to occur if the driving power of one A.C generator suddenly fails when two generators are running in parallel. What safety devices are usually provided for such events?
Q3. A sketch a circuit diagram for an automatic voltage regulator illustrating how the AVR utilizes a silicon-controlled rectifier to control the excitation system for an alternator.
Q4. A. Explain how the efficiency and regulation of a transformer can be assessed by open circuit and short circuit test?
Q5. The direct on line start of squirrel cage motor is used for most electrical drives on a.c powered ships. Describe with sketch as necessary one method of overcoming each of the following problems:
a. high starting current.
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;
i. The synchronous speed;
ii. The speed of the rotor when the slip is 4 per cent;
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.
(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.
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?
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