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Q4. (a) Why is a synchronous motor not self-starting? What are the various ways in which it can be started?

(b) Differentiate Ge and Si diode characteristics. The reverse bias saturation current for a P-N junction diode is 1.0 µA at 300 K. Determine its AC resistance at 150 mV Forward bias. (10)

Q5. (a) Describe the no-load saturation characteristic of a d.c. generator. (6)

(b) A 400V shunt motor has an armature resistance of 0.3 ohm and a shunt-field circuit resistance of 300 ohm. Calculate the speed attained by the motor when the current taken from the supply is (i) 30A and (ii) 50A. The open-circuit characteristic of the machine, when driven as a generator at 1500 rev/min, is given by the following figures:

Neglect the effects of armature reaction (10)

Q6. a) What is back emf? Derive the relation for the back emf and the supplied voltage in terms of armature reactance. (6)

b) Two earth lamps at 120 V, 15 W are used on a 110 V distribution circuit. Due to damage to a motor, the resistance of the positive line is reduced to 10 ohm and that of the negative line to 5 ohm. Find the voltage across each lamp and the extra load on the generator due to the fault. (10)