Most AC motors are of the induction type. They are, in general, simpler and cheaper to build than equivalent DC machines. They have no commutator, slip rings, or brushes, and there is no electrical connection to the rotors. Only the stator winding is connected to the AC source, and, then, as their name implies, induction produces the currents in the rotor. A common and particularly simple form of rotor for this type of motor is the squirrel-cage rotor (see Fig. 7-1). It is so named because of its resemblance to a treadmill-type squirrel cage. The induction motor is based on a rotating magnetic field. This is achieved by using multiple stator field windings (poles), each pair of which is excited by an AC voltage of the same amplitude and frequency as, but phase-displaced from, the voltage supplying the neighboring pair. Figure 7-2 shows how the magnetic field rotates in a four-pole induction motor, where the voltages to the two pairs of poles are 90° out of phase with each other. When the rotor is placed in the stator’s rotating field, the induced currents set up their own fields, which react with the stator’s field and push the rotor around.
Note that some rotors are skewed. The skew of a rotor refers to the amount of angle between the conductor slots and the end face of the rotor laminations. Normally, the conductors are in a nearly straight line, but for high torque applications the rotor is skewed, which increases the angle of the conductors. The term full skew refers to the maximum practical amount (see Fig. 7-3). Figure 7-4 shows how the rotor is located in reference to the stator and the end bells that hold it in place.
A number of types of AC motors are available. The types presented here are those most often encountered when working with heating, airconditioning, and refrigeration equipment.
The shaded-pole induction motor is a single-phase motor. It uses a unique method to start the rotor turning. The effect of a moving magnetic field is produced by constructing the stator in a special way (see Fig. 7-5)
Portions of the pole piece surfaces are surrounded by a copper strap called a shading coil. The strap causes the field to move back and forth across the face of the pole piece. In Fig. 7-6, a numbered sequence and points on the magnetization curve are shown. As the alternating stator field starts increasing from zero Fig. 7-6(1), the lines of force expand across the face of the pole piece and cut through the strap. A voltage is induced in the strap. The current that results generates a field that opposes the cutting action (and decreases the strength) of the main field. This action causes certain actions. As the field increases from zero to a maximum of 90°, a large portion of the magnetic lines of force is concentrated in the unshaded portion of the pole Fig. 7-6(1). At 90° the field reaches its maximum value. Since the lines of force have stopped expanding, no electromagnetic field (EMF) is induced in the strap, and no opposite magnetic field is generated. As a result, the main field is uniformly distributed across the poles as shown in Fig. 7-6(2).
From 90° to 180°, the main field starts decreasing or collapsing inward. The field generated in the strap opposes the collapsing field. The effect is to concentrate the lines of force in the shaded portion of the poles, as shown in Fig. 7-6(3).
Note that from 0° to 180° the main field has shifted across the pole face from the unshaded to the shaded portion. From 180° to 360°, the main field goes through the same change as it did from 0° to 180°. However, it is now in the opposite direction Fig. 7-6(4). The direction of the field does not affect the way the shaded pole works. The motion of the field is the same during the second half-hertz as it was during the first half-hertz.
The motion of the field back and forth between shaded and unshaded portions produces a weak torque. This torque is used to start the motor. Because of the weak starting torque, shaded-pole motors are built in only small sizes. They drive such devices as fans, timers, and blowers.