Electrical Machines EE2005
Assignment
1. A 25-kVA, 60-Hz, 2000/220-V distribution transformer has a leakage impedance of
0.42 + j 0.52 Ω in the high-voltage (HV) winding and 0.004 + j 0.05 Ω in the low-voltage
(LV) winding. When seen from the LV side, the shunt branch admittance Y0 is (0.002 – j
0.015) Ω-1(at rated voltage and frequency) is employed to step down the voltage at the loadend of a feeder having an impedance of 0.25 + j 1.4 Ω. If the sending-end voltage of the
feeder is 2 kV.
(a)Draw the equivalent circuit referred to (i) HV side and (ii) LV side, indicating all
impedances on the circuit.
(b)Find the voltage at the load-end of the transformer, when the load is drawing rated
transformer current at unity pf, 0.85 pf and 0.7 pf lagging as well as lagging.
(c)The voltage drops due to exciting current may be ignored. Plot voltage regulation curve
against various load power factor.
2. A 440 V, 6-pole, 3-phase, 50 Hz star-connected induction motor running light at rated
voltage takes 7.5 A with a power input of 700 W. With the rotor locked and 150 V applied to
the stator, the input current is 38 A and power input is 3500 W; the stator resistance/phase
being 0.55 Ω under these conditions. The standstill reactances of the stator and rotor as seen
on the stator side are estimated to be in the ratio of 1: 0.5. When the motor is operating at a
slip of 4%, calculate the stator current, its pf, net mechanical output and torque. What is
motor efficiency under these conditions of operation? Also calculate the motor performance
as above by using the parameters of the approximate circuit model. Compare the results.
3. 6.6 MV A, 13.8 kV, 6 pole, 50 Hz synchronous generator having saturated synchronous
Reactance of 0.6 pu is synchronized to 13.2 kV bus. Calculate its synchronizing power and
torque coefficient per degree mechanical shift of rotor angle at (a) no load, and (b) at full load
0.85 lagging pf.