The simplest and least expensive type of single phase motor is the shaded pole motor. Due to low starting torque, its use is limited to applications that require less than 3/4 horsepower, usually ranging from 1/20 to 1/6 horsepower.
Shaded pole motors use no starting switch. The stator poles are equipped with an additional winding in each corner called a shade winding. These windings have no electrical connection for starting but use induced current to make a rotating magnetic field.
The pole structure of the shaded pole motor enables the development of a rotating magnetic field by delaying the buildup of magnetic flux. A copper conductor isolates the shaded portion of the pole forming a complete turn around it. In the shaded portion, magnetic flux increases but is delayed by the current induced in the copper shield. Magnetic flux in the unshaded portion increases with the winding current forming a rotating field.
Rotor torque initiates as the magnetic field sweeps across the face of the pole between the unshaded and shaded portions. The rotor is highly resistant in order to maximize the torque.
Shaded pole motors function best with low torque applications and usually rate less than 1/10 horsepower. They should never be used to replace single phase motors.
Shaded pole motors are best suited to low power household application because the motors have low starting torque and efficiency ratings. Some compatible applications include hair dryers, humidifiers and timing devices.
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