AC MOTOR THEORY
AC Motors
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 three-phase AC to three
windings symmetrically spaced around a stator, a rotating magnetic field is generated.
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