SQUIRREL-CAGE INDUCTION MACHINE

Motor and generator principle of operation

This animation illustrates the running of a squirrel-cage induction machine as speed ω_m varies from below synchronous speed ω_s (equal to 1 the per-unit  system) when it acts as a motor, to above ω_s when it operates as a generator.  The rotating magnetizing field  represented by the space vector B_m  (or, equivalently by the magnetizing current I_m)  is created by exciting the 3-phase stator winding with balanced 3-phase currents. For ease of representation, the stator winding is shown as 3 concentrated ABC windings. The space vectors B_m and I_m rotate at synchronous speed ω_s=1 with respect to a stator (or stationary) observer  and at the slip speed ω_sl = 1 - ω_m with respect to a rotor observer. In the motor mode of operation where ω_m< 1, the rotor  effectively  moves backwards (clockwise) with respect to the field, inducing in each bar a voltage having the polarity indicated and a magnitude proportional to slip velocity u and to the field strength acting on the bar (in accordance with the flux-cutting rule v = Blu). Since the magnetic field is sinusoidally distributed in space, so will the induced voltages in the rotor bars. Ignoring the effects of rotor leakage ( i.e. assuming that the rotor is purely resistive), the resulting rotor currents are in phase with the induced voltages and are thus sinusoidally distributed in space varying sinusoidally in time at slip frequency. The rotor current axial flow directions (in and out) are characterized by the red-blue colors and their magnitudes by the size of the circular bars; they may then be represented by the space vector I_r which rotate at the slip speed ω_sl  with respect to the rotor and at synchronous speed ω_s with respect to the stator.  Because B_m cannot change with a fixed stator input voltage (in accordance with Faraday's law), a stator space vector I_R is created in order to compensate for the rotor effects so that the resultant stator current becomes I_s = I_R + I_m. The electromagnetic force exerted on a rotor bar can be derived from the f = Bli rule and it is acting in the positive or anticlockwise direction (same as rotor) in the present case of a motor. The resultant torque developed on the rotor also acts in the same direction.

In the case of a generator where ω_m > 1, the rotor now is running ahead of the magnetizing field so that all polarities and directions are reversed when compared to the motor mode (except for the magnetizing components). The electromagnetic torque is also reversed acting in  opposite direction to the rotor rotation.

A standard method for controlling the speed of induction machine is to operate with a variable-frequency, variable-voltage supply, maintaining the ratio V_s/ω_s or equivalently B_m constant. The result of  this method of speed control can be visualized by following this simple procedure: access the 'Object Properties' of the slider θ_r  and readjust the 'Speed' of the 'Animation'.