Working Principle and Types of Induction Motors
What is an Induction Motor?
An induction motor is an AC electric motor in which the rotor current needed to produce torque is generated by electromagnetic induction from the magnetic field of the stator winding. An Induction Motor is also known as "Asyncronous Motor" because it always runs at a speed lower than the synchronous speed (rotating magnetic field - RMF in the stator). These motors are widely used in industrial and domestic applications due to their robustness and simplicity.
Types of Induction Motors
Induction motors can be classified in two ways:
- Based on Input Supply:
- Based on Rotor Construction:
- Squirrel Cage Induction Motor
- Slip Ring (Wound Rotor) Induction Motor
Working Principle of an Induction Motor
Unlike a DC motor, where power is supplied to both stator and rotor, in an induction motor, only the stator is connected to the AC supply. Here's how it works:
- AC supply to the stator creates an alternating magnetic field.
- This magnetic field rotates at a speed known as synchronous speed (Ns). Thus, called as Rotating Magnetic Field or RMF.
- The rotating field (RMF) cuts across the rotor conductors, inducing an EMF according to Faraday’s Law.
- Since the rotor is short-circuited, current flows, generating its own magnetic field.
- The direction of induced rotor current, according to Lenz's law, is such that it will tend to oppose the cause of its production.
- This rotor magnetic field interacts with the stator field, producing torque.
- The rotor starts rotating in the same direction as the RMF but never reaches the synchronous speed — this difference is called slip.
Fun Fact: Induction motors act like rotating transformers due to similar electromagnetic induction principles.
Key Terms and Formulas
Synchronous Speed (Ns)
The speed at which the stator’s magnetic field rotates:
Ns = (120 × f) / P
- f = Supply frequency (Hz)
- P = Number of poles
Slip (S)
The difference between the synchronous speed (Ns) and the rotor speed (N):
% slip S = ((Ns - N) / Ns) × 100%
Slip is essential; without it, there would be no torque generation.
Why the Rotor Always Rotates Below Synchronous Speed
The rotor in an induction motor always tries to catch up with the synchronous speed of the stator’s rotating magnetic field (RMF). However, in practice, it never actually reaches that speed.
If the rotor were to match the synchronous speed exactly, there would be no relative motion between the rotating magnetic field and the rotor conductors. As a result, no electromagnetic induction would occur — meaning no induced current, and therefore no torque would be produced to keep the rotor turning.
In such a case, the rotor would begin to slow down due to load or friction. This slowdown reintroduces relative motion, which restores induction and produces torque again. This self-correcting process ensures that the rotor speed always remains slightly less than synchronous speed, a difference known as slip.