Question

How do you calculate the slip of an AC induction motor?

Answer 1

The slip of an AC induction motor is a measure of the difference in speed between the motor's rotating magnetic field (synchronous speed) and the actual rotational speed of the motor's rotor. It's an important parameter because it determines the motor's ability to produce torque.

The formula to calculate slip (S) of an AC induction motor is given by:


=


−




×
100
%
S=
N
s
    ​

N
s
    ​

−N
r
    ​

    ​

×100%

Where:


S is the slip (expressed as a percentage).


N
s
    ​

 is the synchronous speed of the motor's rotating magnetic field, which is given by:



=
120
×


N
s
    ​

=
P
120×f
    ​




f is the frequency of the AC power supply in Hertz.


P is the number of poles in the motor.


N
r
    ​

 is the actual rotational speed of the motor's rotor in revolutions per minute (RPM).

In the context of this formula, a positive slip indicates that the motor is operating at a speed slower than its synchronous speed, which is the case during normal motor operation when it's producing mechanical work. A negative slip would imply that the motor is operating above its synchronous speed, which is not physically possible for an induction motor.

Keep in mind that induction motors have varying slip depending on the load they are driving. Under no-load conditions, the slip is very close to zero, but as the motor's load increases, the slip also increases, which is necessary for the motor to maintain sufficient torque to overcome the load's resistance.