Slip:
It is virtually impossible for the rotor of an AC induction motor to turn at the similar speed at which of the rotating magnetic field. No relative motion among them would exist if the speed of the rotor were the similar as that of the stator, and there would be no induced EMF in the rotor. (Recall from previous modules which associative motion among a conductor and a magnetic field is required to induce a current.) Without this induced EMF, there would be no interaction of fields to generate motion. The rotor must, thus, rotate at a few speeds less than that of the stator if associative motion is to exist among the two.
The percentage difference among the speed of the rotor and the speed of the rotating magnetic field is known as slip. As the percentage smaller, the closer the rotor speed is to the rotating magnetic field speed. Percent slip could be found through using Equation (12-1).
SLIP = ( NS - NR/ NS )x 100% (12-1)
where
NS = synchronous speed (rpm)
NR = rotor speed (rpm)
A speed of the rotating magnetic field or synchronous speed of a motor could be found via given Equation (12-2).
NS = 120 f/P (12-2)
where
Ns = speed of rotating field (rpm)
f = frequency of rotor current (Hz)
P = total number of poles
Example: A two pole, 60 Hz AC induction motor has a full load speed of 3554 rpm and what is the percent slip at full load?
Solution:
Synchronous speed:
NS = 120 f/ P
NS = 120 (60 Hz)/2
NS = 3600 rpm
Slip:
SLIP = ( NS -NR/NS)x 100%
SLIP = (3600 - 3554 rpm/3600 rpm)x 100% = 1.3%