WORKED EXAMPLES

1. A 12 pole, 3 phase alternator is coupled to an engine running at 500 rpm. It supplies an Induction Motor which ahs a full load speed of 1440 rpm. Find the percentage slop and the number of poles of the motor.

Solution: NA = synchronous speed of the alternator

PNA 12 X 500F =------- = --------------- = 50 Hz (from alternator data) 120 120

When the supply frequency is 50 Hz, the synchronous speed can be 750 rpm, 1500 rpm, 3000rpm etc., since the actual speed is 1440 rpm and the slip is always less than 5% the synchronous speed of the Induction motor is 1500 rpm.

NS – N 1500 - 1440s = --------- = ----------------- = 0.04 OR 4% NS 1500

120f 120 x 50 NS = ------------ = -------------- = 1500 P P P = 4

2. A 6 pole induction motor is supplied by a 10 pole alternator, which is driven at 600 rpm. If the induction motor is running at 970 rpm, determine its percentage slip.

P NA 10 X 600From alternator date: f =------- = --------------- = 50 Hz 120 120Synchronous speed of the induction motor

From I.M. data:

N S=120 fP

=120×506

=1000 rpm

% slip=NS−NNS

×100=1000−9701000

=3%