Chapter 41 discussed the operation of consequent pole motors that change speed by changing the number of stator poles per phase. Although this is one method of controlling the speed of a motor, it is not the only method. Many small single-phase motors change speed by varying the amount of voltage applied to the motor. This method does not change the speed of the rotating magnetic field of the motor, but it does cause the field to become weaker. As a result, the rotor slip becomes greater, causing a decrease of motor speed.
Variable voltage control is used with small fractional horsepower motors that operate light loads such as fans and blowers. Motors that are intended to operate with variable voltage are designed with high impedance stators. The high impedance of the stator pre- vents the current flow from becoming excessive as the rotor slows down. The disadvantage of motors that contain high impedance stator windings is that they are very limited in the amount of torque they can produce.
When load is added to a motor of this type, its speed will decrease rapidly.
Single-phase motors that use a centrifugal switch to disconnect the start windings cannot be used with variable voltage control. This limits the type of induction motors to capacitor start capacitor run and shaded pole motors. Capacitor start capacitor run motors are employed in applications where it is desirable to re- verse the direction of rotation of the motor, such as ceiling fans.
Another type of alternating current motor that can use variable voltage for speed control is the universal or AC series motor. These motors are commonly used in devices such as power drills, skill saws, vacuum cleaners, household mixers, and many other appliances. They can generally be recognized by the fact that they contain a commutator and brushes similar to a direct current motor. Universal motors are so named because they can operate on AC or DC voltage. These motors
are used with solid-state speed control devices to operate electric drills, routers, reciprocating saws, and other variable speed tools.