In one line
The rotating stator field cuts the rotor conductors, induces currents in them, and the resulting force drags the rotor along in the same direction, always slightly slower than synchronous speed; the fractional lag is the slip.
Explained simply
Move a magnet quickly over an aluminium plate on a pivot and the plate follows the magnet, without touching it. That is induction: the moving field induces currents in the plate, and those currents feel a force that makes the plate chase the field. But if the plate ever caught up, there would be no relative motion, no induced current and no force, so it always lags a little.
Technically, the rotor of an induction motor receives no electrical connection; its currents are induced by transformer-like action, which is why it is also called a rotating transformer.
How it works
- Three-phase supply to the stator produces a rotating magnetic field at \( N_s \).
- At start the rotor is stationary, so the field cuts the rotor conductors at full speed and induces a large EMF (Faraday's law).
- The rotor circuit is closed, so rotor currents flow.
- These currents in the magnetic field experience force; by Lenz's law the rotor turns in the direction of the field to reduce relative speed.
- The rotor reaches speed \( N \) slightly less than \( N_s \). The difference \( N_s - N \) is the slip speed.
- When load increases, the rotor slows a little, slip increases, more EMF and current are induced, and more torque is produced.
Key formula
\( s = \dfrac{N_s - N}{N_s}, \quad N = N_s(1 - s), \quad f_r = sf \)
\( s \) = slip (fraction or %), \( N_s \) = synchronous speed (rpm), \( N \) = rotor speed (rpm), \( f_r \) = frequency of rotor currents (Hz), \( f \) = supply frequency (Hz). Slip is 1 at standstill and a few percent at full load.
Quick example
A 4-pole, 50 Hz motor has \( N_s = 1500 \) rpm. If it runs at 1440 rpm, \( s = 60/1500 = 0.04 \) (4%), and rotor current frequency is \( 0.04\times50 = 2 \) Hz. This is why a nameplate reads 1440 rpm, not 1500.
Common mistakes
- Saying the rotor can run at synchronous speed; at \( N_s \) no EMF is induced and torque is zero.
- Writing slip as \( (N - N_s)/N_s \) (sign reversed).
- Saying the rotor is fed through brushes in a squirrel-cage motor.
How MU asks it
Numericals are not expected in this module (as per the MU syllabus), so questions are theory. Typical: "Explain the working principle of a three-phase induction motor" (5 marks), "Why can an induction motor never run at synchronous speed?" (3 marks), or "Define slip". For full marks explain RMF, induced rotor EMF, current and torque in sequence, use Faraday's and Lenz's laws by name, define slip with formula, and state why \( N < N_s \).