Squirrel Cage Induction Motor Animation

Stator view

Three-phase sinusoidal balanced excitation in 3 stator phases produces a sinusoidally distributed field rotating at the excitation frequency ω_s. This field can be viewed as being created by a single equivalent sinusoidal winding which is excited with dc current and which is also rotating. The flux density distribution is represented here by the state vector B_m (or, equivalently, by the magnetizing current I_m). It is portrayed in the air gap with shaded colors for which the more intense the color the higher the field intensity and where blues point to the rotor and reds to the stator.

This distribution rotates at the stator frequency ω_s as seen from the stator stationary frame and at the slip frequency ω_slip=ω_s– ω_m with respect to the rotor turning at ω_m, thereby generating sinusoidal voltages of slip frequency in the rotor bars and corresponding sinusoidal slip-frequency currents with a delay due to the presence of rotor leakage. This rotor current distribution rotates at ω_slip with respect to the rotor and atω_s with respect to the stationary stator observer. It can thus be represented by the rotor space vector I_r. Since B_m  (or I_m) cannot change, the stator current I_s must include a component I_R = – I_r .

Rotor view

In the rotor reference frame, the rotor speed ω_m is effectively zero. The flux density wave represented by the space vector B_m (or, equivalently, by the magnetizing current I_m) now moves at slip speed with respect to the rotor inducing in the rotor bar slip-frequency currents which are sinusoidally distributed. The result is seen as a sinusoidally distributed rotor winding whose axis is represented by the space vector I_r; both winding and space vector I_r rotate at the slip speed ω_sl . Similarly, the stator space vector I_R which compensates for the rotor effects and the resultant stator current I_s = I_R + I_mall move at slip speed.

Synchronous view

In the synchronous reference frame, all variables appear as dc quantities in steady state. All space vectors are  frozen in space in the shown fixed position based on the assumption that the original stationary and the synchronously rotating reference frames coincide at time t = 0.  These vectors may then be directly associated with the (per-phase) phasors of the induction motor, combined in a phasor diagram, and interpreted by means of an equivalent cicuit made up of a magnetizing inductance in parallel with the rotor leakage inductance acting in series with a resistor.