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 1.1DESCRIBE how a
rotating magnetic field is produced in an AC motor.
EO 1.2DESCRIBE how
torque is produced in an AC motor.
EO 1.3Given field
speed and rotor speed, CALCULATE percent slip in an AC motor.
EO 1.4EXPLAIN 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 threephase stator to which a threephase 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.
Figure 1 ThreePhase
Stator
From
one instant to the next, the magnetic fields of each phase 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 (Figure 2). Since the rotor has an opposing magnetic
field induced upon it, it will also rotate through one revolution.
For
purpose of explanation, rotation of the magnetic field is developed in Figure 2
by "stopping" the field at six selected positions, or instances.
These instances are marked off at 60° intervals on the sine waves representing
the current flowing in the three phases, A, B, and C. For the following
discussion, when the current flow in a phase is positive, the magnetic field
will develop a north pole at the poles labeled A, B, and C. When the current flow
in a phase is negative, the magnetic field will develop a north pole at the
poles labeled A', B', and C'.
Figure
2 Rotating Magnetic Field
At point T1, the current in phase C is at its maximum
positive value. At the same instance, the currents in phases A and B are at
half of the maximum negative value. The resulting magnetic field is established
vertically downward, with the maximum field strength developed across the C
phase, between pole C (north) and pole C' (south). This magnetic field is aided
by the weaker fields developed across phases A and B, with poles A' and B'
being north poles and poles A and B being south poles.
At Point T2, the current sine waves have rotated through
60 electrical degrees. At this point, the current in phase A has increased to
its maximum negative value. The current in phase B has reversed direction and
is at half of the maximum positive value. Likewise, the current in phase C has
decreased to half of the maximum positive value. The resulting magnetic field
is established downward to the left, with the maximum field strength developed
across the A phase, between poles A' (north) and A (south). This magnetic field
is aided by the weaker fields developed across phases B and C, with poles B and
C being north poles and poles B' and C' being south poles. Thus, it can be seen
that the magnetic field within the stator of the motor has physically rotated
60°.
At Point T3, the current sine waves have again rotated 60
electrical degrees from the previous point for a total rotation of 120
electrical degrees. At this point, the current in phase B has increased to its
maximum positive value. The current in phase A has decreased to half of its
maximum negative value, while the current in phase C has reversed direction and
is at half of its maximum negative value also. The resulting magnetic field is
established upward to the left, with the maximum field strength developed
across phase B, between poles B (north) and B' (south). This magnetic field is
aided by the weaker fields developed across phases A and C, with poles A' and
C' being north poles and poles A and C being south poles. Thus, it can be seen
that the magnetic field on the stator has rotated another 60° for a total
rotation of 120°.
At Point T4, the current sine waves have rotated 180
electrical degrees from Point T1 so that the relationship of the phase currents
is identical to Point T1 except that the polarity has reversed. Since phase C
is again at a maximum value, the resulting magnetic field developed across
phase C will be of maximum field strength. However, with current flow reversed
in phase C the magnetic field is established vertically upward between poles C'
(north) and C (south). As can be seen, the magnetic field has now physically
rotated a total of 180° from the start.
At Point T5, phase A is at its maximum positive value,
which establishes a magnetic field upward to the right. Again, the magnetic
field has physically rotated 60° from the previous point for a total rotation
of 240°. At Point T6, phase B is at its maximum negative value, which will
establish a magnetic field downward to the right. The magnetic field has again
rotated 60° from Point T5 for a total rotation of 300°.
Finally, at Point T7, the current is returned to the same
polarity and values as that of Point T1. Therefore, the magnetic field
established at this instance will be identical to that established at Point T1.
From this discussion it can be seen that for one complete revolution of the
electrical sine wave (360°), the magnetic field developed in the stator of a
motor has also rotated one complete revolution (360°). Thus, you can see that
by applying threephase AC to three windings symmetrically spaced around a
stator, a rotating magnetic field is generated.
