AC MOTOR THEORY
AC Motors
Summary
The important information covered in this chapter is summarized below.
AC Motor Theory Summary
A magnetic field is produced in an AC motor through the action of the three-
phase voltage that is applied. Each of the three phases is 120° from the other
phases. From one instant to the next, the magnetic fields combine to produce
a magnetic field whose position shifts through a certain angle. At the end of
one cycle of alternating current, the magnetic field will have shifted through
360°, or one revolution.
Torque in an AC motor is developed through interactions with the rotor and
the rotating magnetic field. The rotating magnetic field cuts the bars of the
rotor and induces a current in them due to generator action. This induced
current will produce a magnetic field around the conductors of the rotor,
which will try to line up with the magnetic field of the stator.
Slip is the percentage difference between the speed of the rotor and the speed
of the rotating magnetic field.
In an AC induction motor, as slip increases from zero to ~10%, the torque
increases linearly. As the load and slip are increased beyond full-load torque,
the torque will reach a maximum value at about 25% slip. If load is
increased beyond this point, the motor will stall and come to a rapid stop.
The typical induction motor breakdown torque varies from 200 to 300% of
full-load torque. Starting torque is the value of torque at 100% slip and is
normally 150 to 200% of full-load torque.
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