Shaded-Pole motor, how does it work?

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?

• I only see the copper wire and not the whole main winding. Commented Mar 16, 2020 at 9:14
• I think you mean that "copper wire is looped around the small pole but also has parallel branches around the main core". Commented Mar 16, 2020 at 9:28
• You've posted a picture of the pole-shading turns. The explanation you've posted is correct. Currents in this copper loop delay the phase of magnetic flux in that part of the pole enough to provide a rotating field. I'm at a loss to get what you don't understand. The excitation generates current in the shading turns. The field from the shading turn current is in quadrature to the excitation. This effectively rotates the field in time from the middle of the pole to the shaded edge of the pole, producing a weak rotating field to pull the rotor round. Commented Mar 16, 2020 at 9:29
• It is circling the top part of the winding (in the picture) but not the bottom part. Commented Mar 16, 2020 at 11:03
• Could you indicate what part in your picture you're referring to as the "main winding"? Commented Mar 16, 2020 at 15:46

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.