Shaded-Pole motor, how does it work?

Question

I need some explanation about Shaded-Pole Motor.

In the book: "Electric Motors and Control Systems" by Frank D. Petruzella (published by McGraw-Hill), in Chapter 5, Part 4, on page 113 under "Shaded-Pole Motor" it is written that:

Unlike other types of single-phase motors, shaded-pole motors have only one main winding and no start winding or switch. As in other induction motors, the rotating part is a squirrel-cage rotor. Starting is by means of a design that uses a continuous copper loop around a small portion of each motor pole, as illustrated in Figure 5-51. Currents in this copper loop delay the phase of magnetic flux in that part of the pole enough to provide a rotating field. This rotating field effect produces a very low starting torque compared to other classes of single-phase motors. Although direction of rotation is not normally reversible, some shaded-pole motors are wound with two main windings that reverse the direction of the field. Slip in the shaded-pole motor is not a problem, as the current in the stator is not controlled by a countervoltage determined by rotor speed, as in other types of single-phase motors. Speed can therefore be controlled merely by varying voltage, or through a multitap winding.

But when looking on the motor I have, it seems that copper wire is circulating through whole main winding, instead of only part of it. So how is it that additional magnetic field can be generated, to turn the rotor into motion?

Answer 1

I suspect that your confusion is because you may be more familiar with the construction of Figure 1.

Figure 1. A simple shaded pole motor. Note that this simplified diagram does not show the closed magnetic loop between the top and bottom of the poles (so it would be brutally inefficient as drawn). Source: Slideshare.

In the case of Figure 1 the unshaded coil will follow the magnetising current of the primary coil. The shaded poles will follow a little later with a resulting clockwise rotation.

Figure 2. A better diagram. Source: HPACmag.

The motor you have seems to have a core of the type shown in Figure 3.

Figure 3. A multi-pole shaded-pole motor core. Source: Made-in-China.

I couldn't find any authoritative information on this online but my guess is:

1. The primary magnetic circuit is in area 1, as usual.
2. This large shading ring shades about 1/4 of that half of the core. The flux in the green and blue areas will be delayed by the shading.
3. The second shading ring further delays the flux in this area. The result is, I suspect, better starting torque as the rotating field carries on for longer than in Figure 2.
4. The core is closed to avoid an assembly step but notice the pinch-off at this point and opposite. This is probably determined to be enough to provide mechanical strength without too much flux leakage.
5. A straight section with a bobbin on it can easily be dropped into this portion.

Answer 2

Your photo is only showing the core, not the "winding". The entire motor looks domething like this, the winding is the red part.

The copper rings shown in your photo are called "shading coils" and are the thing that makes the single phase power source appear to "rotate" so that the motor spins in the same direction every time you give it AC power. Otherwise without it, the motor rotation would either be random, or there would be no rotation at all, it would just shake back and forth as the AC goes from positive to negative over and over.