Q1. 1.(b) A 12 A, 7.5 kW, 440 V, 3-phase, star-connected, 50 Hz, 4-pole squirrel cage induction motor develops full load torque at a slip of 3% when operated at rated voltage and frequency. The leakage reactances of stator and rotor windings are five times the respective stator and rotor resistances. The ratio of stator to rotor winding is 3:5. Determine the percentage increase in stator reactance to limit starting current to 2.5 times the full load current. Assume R1 and R2 are equal and of requisite amount; and motor has negligible magnetising reactance and core losses.
(a) 1.(a) Give the circuit diagram of a negative peak clamper circuit using op-amp, and
(b) 1.(b) A A, 7.5 kW, 440 V, 3-phase, star-connected, 50 Hz, 4-pole squirrel cage induction motor develops full load torque at a slip of 3% when operated at rated voltage and frequency. The leakage reactances of stator and rotor windings are five times the respective stator and rotor resistances. The ratio of stator to rotor winding is 3:5. Determine the percentage increase in stator reactance to limit starting current to 2.5 times the full load current. Assume R1 and R2 are equal and of requisite amount; and motor has negligible magnetising reactance and core losses.
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(c) In a short-circuit test on a 3-pole, 110 kV circuit breaker, power factor of the fault was 0.4, the recovery voltage was 0.95 times full line value. The breaking current was symmetrical. The frequency of oscillation of restriking voltage was 15,000 cycles/sec. Estimate the average rate of rise of restriking voltage. The neutral is grounded and fault involves earth.
(d) 1.(d) Induction motor rotor standstill impedances are given as Zic = (0.01 + j0.5) Ω and Zoc = (0.05 + j0.1) Ω respectively. Assuming the stator impedance to be negligible, determine the approximate ratio of the torques produced by the outer cage Toc to the torque produced by the inner cage Tic at a slip of s = 0.05? Also determine the net torque developed as a function of Toc and comment on performance as compared to a single cage motor.
(e) 1.(e) A MVA, 13.8 kV turbo-generator having Xd'' = X2 = 15% and X0 = 5% is about to be connected to power system. The generator has current limiting reactor of 0.7 Ω in the neutral. Before the generator is connected to the system, its voltage is adjusted to 13.2 kV. When a double line to ground fault develops at terminal 'b' and 'c', find the initial symmetrical r.m.s. currents in the ground and in line b.
