There are three things that rotate in an induction motor. The first one is the magnetic field of the stator which rotates at synchronous speed (Ns). The second part is the magnetic field of the stator which also rotates at synchronous speed with respect to the stator. The third rotating part is the rotor which rotates at motor/ rotor speed (N).
The speed of the rotor magnetic field of the rotor with respect to the rotor is known as the slip in induction motors. In other words slip is the difference the speed of revolving field which is known as the synchronous speed (Ns) and the rotor speed (N). It helps to determine the rate by which the flux is cut by rotor conductors and the magnitude of induced emf.
Slip in an Induction Motor
Slip or slip speed is the speed difference synchronous speed (Ns) and the rotor speed (N).
Slip Speed = Synchronous Speed – Rotor Speed
Slip Speed = Ns – N
Note: The slip of induction motor can’t be a negative value since the speed of rotor is always less than synchronous speed.
Synchronous Speed is determined by the frequency (f) and the number of magnetic poles (p).
NS = 120f/p
Fractional Slip in an Induction Motor
When the slips of an induction motor is divided by the synchronous speed (Ns), it’s known as the fractional slip and is represented by letter S.
Motor Speed/ Rotor Speed/ Actual Speed of the Motor
N = NS(1-S)
Percentage Slip in an Induction Motor
The slip of a induction motor is commonly expressed as a percentage fractional slip is represented by % S.
Relationship Between Slip and the Induced EMF
T = K(NS – N)
T = KNS × S
T = K1 S
Power, Torque and Angular Speed
Power = Tω
Angular Speed ω = 2πN/60
Frequency of Rotor Currents/ EMF Induced
Frequency of the rotor currents depend upon the slip speed which is the relative speed between the magnetic field of the rotor and the speed of the rotor.
So the frequency of rotor currents or the frequency of EMF Induced can be calculated using the equation fr = Sf.
S is the fractional slip of the motor
f is the frequency of the supply voltage