During operation, the stator of a polyphase
alternating current motor has a rotating magnetic
field. This is developed by the direction and
amount of current flow in the stator coils.
Figure 1 represents a two-pole, three-phase
stator, having one slot per pole per phase in which
there are six stator coils. These coils are connected
to a three phase power supply having phase
rotation A, B, C.
At the instant shown, the current in phase A in
maximum positive enters the motor at A+ and
leaves at neutral where the three conductors meet.
The “A” arrows in Figure 1 represent the magnetic
flux (lines of force) developed by this current. At
this instant the current in phases B and C is negative,
each equal to one-half the current in A. The
magnetic flux developed by B is shown by the “B”
arrows, each equal to half the “A” arrows. The flux
developed by C is represented by the “C” arrow,
each equal to half of A. B and C also have horizontal
components but, as shown, they cancel out.
The result is a vertically polarized stator, with
a North pole at the top and a South pole at the
bottom. The arrows show the flux flowing
downward through the rotor.
One-twelfth revolution later (Figure 2), the
positive current in A has dropped in value and is
equal to the negative value in C, which has
increased in value. The current in B is zero. Equal
field strengths are developed by A and C and,
canceling out the opposing components, we find
the total flux in Figure 2a to be represented by the
two “A” and two “C” arrows, 30° from the former
position.
At the 60° position, shown in Figure 3, the
current in phase B and the corresponding stator
coils are positive and equal to phase A. The
current in phase C is maximum negative. After
canceling out the opposing components, we find a
resultant field, as shown in Figure 3a composed
principally of current in phase C, supplemented
by smaller equal amounts in phases A and B.