ROTATING MAGNETIC FIELDS
The
principle of rotating magnetic fields is the key to the operation of most ac
motors. Both synchronous and induction types of motors rely on rotating
magnetic fields in their stators to cause their rotors to turn. The idea is
simple. A magnetic field in a stator can be made to rotate electrically, around
and around. Another magnetic field in the rotor can be made to chase it by
being attracted and repelled by the stator field. Because the rotor is free to
turn, it follows the rotating magnetic field in the stator. Let’s see how it is
done.
Rotating
magnetic fields may be set up in two-phase or three-phase machines. To
establish a rotating magnetic field in a motor stator, the number of pole pairs
must be the same as (or a multiple of) the number of phases in the applied
voltage. The poles must then be displaced from each other by an angle equal to
the phase angle between the individual phases of the applied voltage.
TWO-PHASE ROTATING MAGNETIC FIELD
A
rotating magnetic field is probably most easily seen in a two-phase stator. The
stator of a twophase induction motor is made up of two windings (or a multiple
of two). They are placed at right angles to each other around the stator. The
simplified drawing in figure (1). illustrates a two-phase stator.
 |
| figure (1) |
If
the voltages applied to phases 1-1A and 2-2A are 90º out of phase, the currents
that flow in the phases are displaced from each other by 90º . Since the
magnetic fields generated in the coils are in phase with their respective
currents, the magnetic fields are also 90º
out of phase with each other. These two out-of-phase magnetic fields,
whose coil axes are at right angles to each other, add together at every instant
during their cycle. They produce a resultant field that rotates one revolution
for each cycle of ac.
To
analyze the rotating magnetic field in a two-phase stator, refer to figure (2).
The arrow represents the rotor. For each point set up on the voltage chart,
consider that current flows in a direction that will cause the magnetic
polarity indicated at each pole piece. Note that from one point to the next,
the polarities are rotating from one pole to the next in clockwise manner. One complete cycle of input
voltage produces a 360-degree rotation of the pole polarities. Let's see how
this result is obtained.
 |
| figure (2) |
The
waveforms in figure (2) are of the two input phases, displaced 90º because of the way they were generated in a
two-phase alternator. The waveforms are numbered to match their associated
phase. Although not shown in this figure, the windings for the poles 1-1A and
2-2A would be as shown in the previous figure. At position 1, the current flow
and magnetic field in winding 1-1A is at maximum
(because
the phase voltage is maximum). The current flow and magnetic field in winding
2-2A is zero (because the phase voltage is zero). The resultant magnetic field
is therefore in the direction of the 1-1A axis. At the 45-degree point
(position 2), the resultant magnetic field lies midway between windings 1-1A and
2-2A. The coil currents and magnetic fields are equal in strength. At 90º (position 3), the magnetic field in winding
1-1A is zero. The magnetic field in winding 2-2A is at maximum. Now the
resultant magnetic field lies along the axis of the 2-2A winding as shown. The
resultant magnetic field has rotated clockwise through 90º to get from position 1 to position 3. When
the two-phase voltages have completed one full cycle (position 9), the
resultant magnetic field has rotated through 360º . Thus, by placing two windings
at right angles to each other and exciting these windings with voltages 90º out of phase, a
rotating
magnetic field results. Two-phase motors are rarely used except in
special-purpose equipment. They are discussed here to aid in understanding
rotating fields. You will, however, encounter many single-phase and three-phase
motors.
THREE-PHASE ROTATING FIELDS
The
three-phase induction motor also operates on the principle of a rotating
magnetic field. The following discussion shows how the stator windings can be
connected to a three-phase ac input and have a resultant magnetic field that
rotates. Figure (3), views A-C show the individual windings for each phase.
Figure (3), view D, shows how
the
three phases are tied together in a Y-connected stator. The dot in each diagram
indicates the common point of the Y-connection. You can see that the individual
phase windings are equally spaced around the stator. This places the windings
120º apart.
 |
| Figure (3) |
The
three-phase input voltage to the stator of figure (3) is shown in the graph of
figure (4). Use the left-hand rule for determining the electromagnetic polarity
of the poles at any given instant. In applying the rule to the coils in figure (3),
consider that current flows toward the terminal numbers for positive voltages,
and away from the terminal numbers for negative voltages.
 |
| figure (4) |
The
results of this analysis are shown for voltage points 1 through 7 in figure (4).
At point 1, the magnetic field in coils 1-1A is maximum with polarities as
shown. At the same time, negative voltages are being felt in the 2-2A and 3-3A
windings. These create weaker magnetic fields, which tend to aid the 1-1A
field. At point 2, maximum negative voltage is being felt in the 3-3A windings.
This creates a strong
magnetic
field which, in turn, is aided by the weaker fields in 1-1A and 2-2A. As each
point on the voltage graph is analyzed, it can be seen that the resultant
magnetic field is rotating in a clockwise
direction.
When the three-phase voltage completes one full cycle (point 7), the magnetic
field has rotated through 360º .
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