3-phase IM principle; speed less than sync

Operating principle

A three-phase induction motor works on the principle of a rotating magnetic field and electromagnetic induction.

   Stator (3-phase winding)         Rotor (short-circuited bars)
        R Y B                             ______
   +--------------+                    __/ |  | \__
   |  rotating    |    air gap        |    bars   |   -> torque
   |  field Φ at  |  ------------->   |___________|
   |  speed Ns    |
   +--------------+

1. A balanced three-phase supply is given to the stator winding, whose coils are displaced by 120° in space. The three currents, 120° apart in time, produce a rotating magnetic field of constant magnitude (1.5 Φ_m) rotating at the synchronous speed N_s = 120 f / P rpm.
2. This field sweeps across the stationary rotor conductors. By Faraday's law, emf is induced in the rotor conductors.
3. The rotor circuit is closed (short-circuited bars or rings), so rotor currents flow.
4. The rotor currents lie in the stator field, so each conductor experiences a force (F = B I l, Fleming's left-hand rule). Together these forces give a torque.
5. By Lenz's law, the rotor tries to oppose the cause of the induced currents, i.e. the relative motion between the field and the rotor, so the rotor starts turning in the direction of the rotating field.

Why the rotor speed is always less than the synchronous speed

• The induced emf and current in the rotor exist only because of the relative speed between the rotating field and the rotor (slip speed N_s − N).
• If the rotor were to reach N_s, the relative speed would be zero; no flux would be cut, so the rotor emf, rotor current and torque would all become zero.
• With no torque the rotor would slow down due to friction and windage; the relative motion would reappear, and torque would develop again.
• Therefore the rotor runs at a speed N slightly less than N_s, just enough to induce the current needed for the load torque. The difference is expressed as slip:

s = (N_s − N) / N_s, typically 2 to 5 % at full load.

For this reason the motor is also called an asynchronous motor.