A rotating magnetic field is produced when three phase supply is fed to stator of an induction motor. The rotating rotating magnetic field has constant magnitude and has a circular rotation.The speed at which rotating magnetic field rotates is known as a synchronous speed of the motor. The synchronous speed depends on the number of poles and frequency of the supply source of the motor and it is denoted by Ns. The synchronous speed (Ns) of the motor is expressed by the following mathematical expression.

**Definition of Slip**

The lagging of the actual speed(N) of the motor with synchronous speed(Ns) is called the slip.

S = Ns – N

The actual speed of the motor is always less than the synchronous speed of the motor.

**Illustrative Examples:**

A 4 poles, 50 Hz induction motor rotor speed is 1480 RPM. What is the slip speed of the motor? The motor draws 500 kW from the supply source. What is the copper loss of the motor.

The synchronous speed of the 4 Poles,50 Hz induction motor is

Ns= 120 x 50/4 = 1500 RPM.

Actual speed (N) = 1480 RPM

Slip(S) = (Ns-N)/Ns x 100

S = (1500-1480)/1500 x 100

S = 20/1500 x 100

**S = 1.33 %**

Input Power = 500 kW

Copper loss = Slip x Input Power

Copper loss = 0.0133 x 500**Copper loss = 6.66 kW****Read More:****Slip- Torque characteristics of Induction Motor**