Slip in Induction Motor – Slip Explanation and Equation

Slip in an induction motor refers to the difference between the synchronous speed of the rotating magnetic field and the actual rotor speed. It is a fundamental property of induction motors and plays a crucial role in their operation.

The voltage induced in rotor bars of an induction motor depends on the speed of rotor with respect to the magnetic fields. The term slip in Induction motors is used for representing the relative speed of rotor with respect to magnetic fields.

Induction motors work based on the principle of electromagnetic induction. Like all rotating machines, the basic construction of an induction motor comprises of two parts: the stator and the rotor. The stator windings in the motor produce a rotating magnetic field, which induces currents in the rotor windings, resulting in the generation of torque and the rotation of the rotor.

The synchronous speed of an induction motor is determined by the frequency of the power supply and the number of poles in the motor. It represents the speed at which the rotating magnetic field would ideally rotate if there were no slip. The synchronous speed is given by the formula:

Synchronous Speed (Ns) = (120 * Frequency) / Number of Poles

Also see: Induction Motor Formulas with Calculations

However, due to the presence of rotor windings and the load on the motor, the actual rotor speed is slightly less than the synchronous speed. This speed difference is known as slip and is expressed as a percentage or a fraction.

Mathematically formula to calculate Slip:

Slip (s) = [ ηsync – ηm ] / ηsync

where in above equation:

  • ηsync = Speed of magnetic fields
  • ηm = Speed of motor shaft

The factor on the right side is often multiplied with 100 to represent slip in terms of slip percentage.

Slip %age = {[ ηsync – ηm ] / ηsync  } * 100

Important points to remember

  1. The rotor of an induction motor always turns slower than the synchronous speed, so the slip and slip percentage is smaller than 1 and 100 respectively
  2. Slip is actually the difference between synchronous speed and actual speed
  3. In practical terms, slip is necessary for an induction motor to operate and produce torque. If there were no slip (i.e., rotor speed equals synchronous speed), there would be no relative motion between the magnetic field and the rotor, resulting in no torque generation. Therefore, slip is a vital characteristic that enables an induction motor to function as a reliable and efficient motor in various industrial and commercial applications.

Formula Summary:


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