The Operating Principles Of The Shaded Pole Motor

A shaded pole motor is one that operates along standard single phase induction principles. It has poles which are salient, i.e. projected. As with most types of induction motors, the shaded pole motor typically has either two or four poles. Each pole features an inductive copper ring or band. These bands divide each pole into unequal parts; the part upon which the band is mounted is known as the pole's shaded portion.

How It Works

The shaded pole induction motor is started by applying single phase current to its stator, producing an alternating flux. EMF is generated in the pole through the change in the flux. Because the shaded section is effectively a short circuit, the current it produces flows in the opposite direction of the primary flux. This means that there's a significant lag in the flux in the shaded area when compared to the flux in the unshaded area. The net result of this phase difference is a rotating flux.

Thanks to the power supply, we can say that the winding current of the stator is going to be alternating. The same holds true for its current-induced flux. To get a better handle on how the shaded pole motor works, let's look at three key points in the current cycle:

1) When the stator's flux goes from zero to nearly its maximum value.
2) When the stator's flux holds steady at or near its maximum value.
3) When the stator's flux drops from its maximum value to zero.

Point 1

In this part of the cycle, flux is building up very rapidly, producing a very high current. As per Faraday's law, any change in flux levels causes EMF. This induced EMF will set current flowing through the short-circuited copper brand wrapped around the pole's shaded region. The band's current produces a flux of its own. Lenz's law tells us that this type of induced current is going to oppose the direction of the force which caused it - in this case the initially induced current. With two fluxes in opposition, the flux on the stator's non-shaded section will grow while the flux on the shaded section decreases. Because flux distribution in non-uniform, the magnetic axis of the pole is set into motion.

Point 2

Once the flux becomes steady, most of the forces acting on the stator become constant. The amount of induced EMF being generated in the pole's shaded section is minimal, and the counter-flux it produces is not significant enough to affect the primary flux derived from the application of current. As long as flux distribution is uniform, the pole's magnetic axis lies stable in the pole's center.

Point Three

When the flux starts to drop, current begins to rise again. Faraday's law is still in effect, so a significant EMF is reintroduced. The short-circuited band in the pole's shaded area once again starts producing a flux of its thanks to the current flowing through it. Lenz's law also takes effect, dictating that the bands current will be in direct opposition to the decreasing current which caused it. This time around, the flux is the pole's shaded area starts to grow, and the flux in the non-shaded area is the one that becomes weak. The pole's magnetic axis is again compelled to move but in the opposite direction of point 1.

The shift in the pole's magnetic axis continues over and over again through both the negative and positive phases of winding current application. This creates a rotating magnetic field oriented from the pole's non-shaded section towards its shaded section. This rotating field is the key which provides startup torque to the motor and enables conventional induction operation.