AC Motors
AC MOTOR THEORY
Torque Production
When alternating current is applied
Figure 3 Induction Motor
to the stator windings of an AC
induction
motor,
a
rotating
magnetic field is developed. The
rotating magnetic field cuts the
bars of the rotor and induces a
current in them due to generator
action.
The direction of this
current flow can be found using
the left-hand rule for generators.
This induced current will produce
a magnetic field, opposite in
polarity of the stator field, around
the conductors of the rotor, which
will try to line up with the
magnetic field of the stator. Since
the
stator
field
is
rotating
continuously, the rotor cannot line
up with, or lock onto, the stator
field and, therefore, must follow
behind it (Figure 3).
Slip
It is virtually impossible for the rotor of an AC induction motor to turn at the same speed as that
of the rotating magnetic field. If the speed of the rotor were the same as that of the stator, no
relative motion between them would exist, and there would be no induced EMF in the rotor.
(Recall from earlier modules that relative motion between a conductor and a magnetic field is
needed to induce a current.) Without this induced EMF, there would be no interaction of fields
to produce motion. The rotor must, therefore, rotate at some speed less than that of the stator
if relative motion is to exist between the two.
The percentage difference between the speed of the rotor and the speed of the rotating magnetic
field is called slip. The smaller the percentage, the closer the rotor speed is to the rotating
magnetic field speed. Percent slip can be found by using Equation (12-1).
(12-1)
SLIP
NS
NR
NS
x 100%
Rev. 0
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