Slip Speed in an Induction Motor

Slip Speed of the Induction Motor is defined as the difference between the synchronous speed and the actual rotor speed. An induction motor cannot run at synchronous speed. Let us consider that the rotor of the induction motor is running at the synchronous speed. At this condition, the rotor conductors do not cut the flux and as a result, there is no generation of voltage, current and hence, no torque.

The speed of the rotor is slightly less than the synchronous speed. Thus, the slip speed expresses the speed of the rotor relative to the field.

  • If Ns is the synchronous speed in revolution per minute
  • Nr is the actual rotor speed in revolution per minute.

The slip speed of the induction motor is given as

SLIP-IN-INDUCTION-MOTOR-EQ-1

The slip speed expressed as a fraction of the synchronous speed is called the Per Unit Slip or Fractional Slip. The per unit slip is generally called the Slip. It is denoted by s.

SLIP-IN-INDUCTION-MOTOR-EQ-2

Therefore, the rotor speed is given by the equation shown below.

SLIP-IN-INDUCTION-MOTOR-EQ-3

Alternatively, if

  • ns is the synchronous speed in revolution per second
  • nr is the actual rotor speed in revolution per second.

Then,

SLIP-IN-INDUCTION-MOTOR-EQ-4

The percentage slip in revolution per second is given as shown below.

SLIP-IN-INDUCTION-MOTOR-EQ-5

The slip of the induction motor varies from 5 percent for small motors to 2 percent for large motors.

Importance of Slip

Slip plays an important role in Induction motor. As we know that the difference between the synchronous speed and the rotor speed is known as Slip. Hence, the magnitude of the induced emf is represented as

IN-INDUCTION-MOTOR-EQ-6

The rotor current is directly proportional to the induced emf.

SLIP-IN-INDUCTION-MOTOR-EQ-7

The torque is directly proportional to the rotor current.

SLIP-IN-INDUCTION-MOTOR-EQ-8

Therefore,

SLIP-IN-INDUCTION-MOTOR-EQ-9Hence, torque is directly proportional to slip.

SLIP-IN-INDUCTION-MOTOR-EQ-10

Thus, greater the value of the slip greater will be the induced EMF and the rotor current. As a result, the rotor will develop a large amount of the torque.

At no-load induction motor requires small torque to overcome mechanical, iron and other losses, therefore slip is small. When the motor is loaded, greater torque is required to drive the load. The slip of the induction motor increases and the rotor speed decreases. Thus, the slip in an induction motor adjusts itself to such a value so as to meet the required driving torque under normal operation.

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