Q4. a) Describe the possible causes and the effect of running a three-phase motor with one phase open circuit. (6)
b) A heater unit of inductance has a resistance of 6.5 ohms and is intended for use with 100V mains. For 50Hz what voltage would it be suitable when placed in series with an external apparatus, of negligible resistance, having an inductance of 0.01H? If the frequency rises by 5 percent and this voltage remains constant, what would be the resulting change of voltage at the heater terminals? (10)
Q5. a) Explain the applications of PN junction diode (6)
b) In a full wave rectifier without filter, the load resistance is 4 k Ω. Each diode has a forward resistance of 0.8 k Ω. If the peak voltage applied to each diode is 120 V at 50Hz, calculate:
(i) The average value of current.
(ii) Efficiency and
(iii) Regulation at a load current of 10mA (10)
Q6.a) Derive an expression for the emf induced in an a.c. generator. (6)
b) A 3000 KVA, 6-pole alternator runs at 1000 r.p.m. in parallel with other machines on 3300V bus-bars. The synchronous reactance is 25%. Calculate the synchronizing power for one mechanical degree of displacement and the corresponding synchronizing torque. (10)
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