Like all single phase induction motors, if you simply energize the coils it
will induce currents in the rotor. But currents will lag behind the induced
voltage by nearly 90 degrees. So the current in the rotor winding will be
such that half the rotor bars have a force on them to turn the shaft
clockwise while the other half will have a force to turn the shaft
counter-clockwise and there is very little net torque produced to start the
With a split phase motor, you have two sets of windings. The windings are
such that they have different inductance values. So the AC current through
one set of windings is not in phase with the AC current through the other
winding. If you think how the magnetic field of each 'peaks' at a different
time, you can think of the magnetic field 'shifting' from one to the other.
The currents in the rotor bars induced by the first set of windings to peak
(and generate no torque in relation to the first set of winding's magnetic
field). But when the magnetic field of the stator 'shifts' to the second
position, now the rotor bars carrying current are not exactly balanced on
each side of the 'new' magnetic field position. The imbalance of current
carrying rotor bars on one side versus the other results in a net torque and
the shaft accelerates.
With a shaded pole motor you only have one set of windings. The shorted
turn around part of the pole face has a current induced into it (like a
transformer). This current creates a magnetic field that opposes the one
that induced it. So the magnetic field in that portion of the pole face is
initially less than the 'unshaded' portion. When the current in the main
winding drops and the magnetic field starts to collapse, a current is again
induced into the shorted turn, but this time in the opposite direction and
the result is the magnetic field in the shaded portion is maintained
slightly longer than the unshaded portion. So what you effectively get is a
'shift' in the magnetic field. When increasing, it is stronger in the
'unshaded' portion, then momentarily equal in both portions and then when it
is decreasing it is stronger in the 'shaded' portion. This 'shifting' of
the magnetic field has a similar effect as with the split phase motor. The
current induced in the rotor bars creates a balanced torque for the magnetic
fields 'original' position, but when the magnetic field 'shifts', the
currents in the two halves of the rotor bars are no longer balanced and a
net torque is produced.
Another way to think of both of these is that by creating a short 'shift' in
magnetic field, you get action similar to the constantly rototating magnetic
field of a polyphase machine. Just not a complete rotation, just a sort of
short 'step' in rotation.
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