Published February 2000

MOTORS: A LOST ART

by Bob Perrin

Start • A Few Words on Words • The DC Motor • Polyphase AC Motors • Single-Phase AC Motors • Winding Down • Sources and PDF

SINGLE-PHASE AC MOTORS

Single-phase AC motors are ubiquitous. Just about any electric shop tool or household appliance has a 110 AC electric motor in it.

As it turns out, single-phase motors are somewhat more difficult to construct and understand than three-phase motors. The difficulty arises in trying to create a rotating magnetic field in the stator with only a single phase.

Photo 2 shows the stator from a single-phase induction motor. The stator was removed from a ceiling fan. The windings look like they were hand-wound. This is an interesting contrast to the machine-wound stator shown in Photo 1.

At first glance, the pole configuration appears similar to that shown in Figure 5. In the single-phase stator in Photo 2, the magnetic polarity of all the poles is changing from north to south at the same time.

If a compass needle were lined up between a pole pair, on the next half cycle, the fields would reverse and the compass would rotate 60^o and line up with an adjacent pole pair. If it were not for a mechanism in the stator shown in Photo 2 called a shaded-pole, the compass would be equally likely to rotate clockwise or counterclockwise.

The stator in Photo 2 has had two sets of windings removed to expose the shaded-pole mechanism. Notice how the poles have a slot cut into them. The exposed copper band is "shading" the small portion of the pole from the magnetic field that is developed in the large pole. This causes a slight delay in the growth and collapse of the magnetic field in the smaller shaded-pole with respect to the larger pole.

Considering this shaded-pole mechanism, let’s revisit the compass through experiment. If the compass starts aligned between two poles. As the single-phase AC begins to change polarity, the large portion of the poles will begin to change magnetic polarity. The shaded-poles will be slightly delayed from the larger pole pieces. Thus the compass will be forced to rotate in only one direction. This is how the single-phase AC shaded-pole motor solves the problem of ambiguous rotor start direction.

Also in Photo 2 you will notice that most of the stator appears black. This color is not the color of the stator lamentations, but rather the color of the resin or epoxy the stator was dipped in. I had to carefully remove the epoxy on the pole where the copper band was exposed. All the poles have a copper band shading the shaded-pole.

In Photo 2, the outline of the copper band can be seen on the pole piece where the winding was removed, but the epoxy was left intact. In this type of stator, often the copper shading bands are hidden by the windings.

In transformers and motors, one prominent failure mechanism is the shorting of windings. This is caused by mechanical vibration wearing the lacquer insulation off of the winding’s wires. This is why windings and stators are often coated in epoxy. It keeps mechanical vibration down.

To keep eddy currents to a minimum and thus iron losses to a minimum, the stator and rotor are constructed from thin laminations. Each steel lamination is insulated from the next by a thin coat of lacquer. Like the windings, the stator can be potted to reduce the chance of mechanical vibration developing and wearing the lacquer.

In Photo 2, you can see a yellow dot near the black and white AC wires. This is a bimetal switch that protects against overheating. The switch actually looks a lot like a neon lamp with a yellow end. Only the yellow end is visible in Photo 2.

Other techniques exist to resolve the field rotation ambiguityin single-phase AC motors.Richardson’s text describes a reluctance start motor. This technique eliminates the shaded-pole in favor of a modified pole geometry that alters the shape of the magnetic field of the pole. Figure 6 shows a simple reluctance start stator.

Figure 6—A reluctance start motor uses the stator geometry to shape the magnetic fields so the rotor will always turn in the proper direction.

Another common type of single-phase AC motor is the series connected commutated motor, more commonly called a universal motor. If you own an old AC-powered hand-drill, most likely you own a universal motor.

The series connected commutated motor is called a universal motor because it will run off of single-phase AC or DC. Although, when run from DC the field windings are likely to overheat and meltdown unless the motor is designed specifically to operate from DC. A universal motor has the brushes and field windings placed in series. [1]

Capacitor-run motors are another type of single-phase AC motors. Photo 1 shows a stator from a capacitor-run motor. This type of motor has two or more sets of windings. A capacitor is used to couple AC power to one winding, while the other winding is energized directly from the incoming AC power. The capacitor introduces a phase shift between the windings. The two separate windings with different phases can set up a rotating magnetic field similar to the one discussed in the section on polyphase AC induction motors.

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