Shaded Pole Motor

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. 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 counter-voltage 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.

Shaded-pole motors are best suited to low-power household application, because the motors have low starting torque and efficiency ratings. Because of the weak starting torque, shaded-pole motors are built only in small sizes ranging from 1/20 to 1/6 hp. Applications for this type of motor include fans, can openers, blowers, and electric razors.

The universal motor is constructed like a series-type DC motor with a wound series field (on the stator) and a wound armature (on the rotor). As in the DC series motor, its armature and field coils are connected in series. As the name implies, universal motors can be operated with either direct cur rent or single-phase alternating current. The reason for this is that a DC motor will continue to turn in the same direction if the current through the armature and field are reversed at the same time. This is exactly what hap pens when the motor is connected to an AC source. Universal motors are also known as AC series motors or AC commutator motors.

Although universal motors are designed to run on AC or DC, most are used for household appliances and portable hand tools that operate on single-phase AC power.

Unlike other types of single-phase motors, universal motors can easily exceed one revolution per cycle of the main current. This makes them useful for appliances such as blenders, vacuum cleaners, and hair dryers, where high-speed operation is desired. The speed of the universal motor, like that of DC series motor, varies considerably from no load to full load, as can be observed when you apply varying pressure on an universal drill motor.

Both the speed and direction of rotation of a universal motor can be controlled.

Reversing is accomplished just as in a DC series motor by reversing the current flow through the armature with respect to the series field. Varying the voltage that is applied to the motor controls the speed.