Exam Question

(a) Describe the possible causes and the effect of running a three phase motor with one phase open circuited (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 resisance, having an inductance of 0.01H? If the frequency rises by 5 per cent and this voltage remains constant, what would be the resulting change of voltage at the heater terminals? (10)