SQUIRREL-CAGE INDUCTION MOTOR
Stator view
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 ωs as 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 .
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