SQUIRREL-CAGE INDUCTION MOTOR
Three-phase sinusoidal balanced excitation in 3 stator phases produces a sinusoidally distributed field rotating at the excitation frequency ωs. 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 Bm (or, equivalently, by the magnetizing current Im). 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 ωsas seen from the stator stationary frame and at the slip frequency ωslip=ωs- ωm with respect to the rotor turning at ωm, 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 ωslip with respect to the rotor and at ωs with respect to the stationary stator observer. It can thus be represented by the rotor space vector Ir. Since Bm (or Im) cannot change, the stator current Is must include a component IR = - Ir .