Question

A 50-KW, 220V, 50 Hz, squirrel-cage three-phase induction motor is needed to rotate a mine
conveyor belt drive. The motor starts sometimes under load, but most of the time is needed to run
at rated speed and slip of 3.5%.
(a) Propose an appropriate design of this drive motor in order to suit the application. Motivate
and then illustrate your design with suitable drawings and speed (slip)-torque graphs. [12]
(b) If the supply voltage of the motor in (a) above fluctuates with a range of ± 10 %, determine
the developed torque range of the motor at its rated speed. Full load speed of the motor is
1440 rpm [10]
(c) An induction motor is rated 5% slip on full load. The motor is started on reduced voltage to
limit starting current to three times the full load current. Find the ratio of the starting torque
to full load torque [6]

Answer 1

(C) Given data : Full Load slip Scl= 5;%. 0,05 starting current (Ist) to full = 3 times = 3 load current (ISL) Soletion!- full The ratio of starting Lead torque (Tse) torque (Tst) to TS (te) xSFL = (3)2 X0.05 = 9x0.05 = 0.45