AC Motors
AC MOTOR THEORY
AC MOTOR THEORY
AC motors are widely used to drive machinery for a wide variety of applications.
To understand how these motors operate, a knowledge of the basic theory of
operation of AC motors is necessary.
EO 1.1
DESCRIBE how a rotating magnetic field is produced
in an AC motor.
EO 1.2
DESCRIBE how torque is produced in an AC motor.
EO 1.3
Given field speed and rotor speed, CALCULATE
percent slip in an AC motor.
EO 1.4
EXPLAIN the relationship between slip and torque in
an AC induction motor.
Principles of Operation
The principle of operation for all AC motors relies on the interaction of a revolving magnetic
field created in the stator by AC current, with an opposing magnetic field either induced on the
rotor or provided by a separate DC current source. The resulting interaction produces usable
torque, which can be coupled to desired loads throughout the facility in a convenient manner.
Prior to the discussion of specific types of AC motors, some common terms and principles must
be introduced.
Rotating Field
Before discussing how a rotating magnetic field will cause a motor rotor to turn, we must first
find out how a rotating magnetic field is produced. Figure 1 illustrates a three-phase stator to
which a three-phase AC current is supplied.
The windings are connected in wye. The two windings in each phase are wound in the same
direction. At any instant in time, the magnetic field generated by one particular phase will
depend on the current through that phase. If the current through that phase is zero, the resulting
magnetic field is zero. If the current is at a maximum value, the resulting field is at a maximum
value. Since the currents in the three windings are 120° out of phase, the magnetic fields
produced will also be 120° out of phase. The three magnetic fields will combine to produce one
field, which will act upon the rotor. In an AC induction motor, a magnetic field is induced in
the rotor opposite in polarity of the magnetic field in the stator. Therefore, as the magnetic field
rotates in the stator, the rotor also rotates to maintain its alignment with the stator’s magnetic
field. The remainder of this chapter’s discussion deals with AC induction motors.
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