Shaded-Pole Motors

Shaded-pole motors are commonly found in applications that require light-duty fans such as small window air conditioners and exhaust fans used in rest rooms. If you are a maintenance electrician or technician, you may be requested to service all of the electrical equipment in the plant, including the office areas. If this is the case, you will run into shaded-pole motors. Shaded-pole motors are also used in the paddlewheel-level sensor.

Fig. 1 shows a diagram of the shaded-pole motor. Note that the motor has only one winding. It doesn't have a start winding and a run winding like other single-phase motors. Instead, it has a shading pole that provides the magnetic field phase shift that is required to start the motor. The shaded-pole motor has a copper bar that is inserted around the front of the run winding. The bar is connected at the ends to make a complete circuit called a pole.

Above: Fig. 1: Diagram of a shaded-pole motor. Notice the shading pool located near each winding.

When voltage is applied to the motor to start it, current will flow through the run winding and build up a magnetic field. A current will be induced in the single winding of the shading pole, and it will cause a phase shift to occur that is large enough to make the rotor start to spin. Once the rotor starts to spin, it will begin to build its own magnetic field and come up to full rpm.

The shading pole also helps the motor when its load changes at full rpm. If the motor shaft begins to slow down, the phase shift in the shading coil becomes stronger and provides enough torque to bring the rotor back up to full speed. Another unique feature of the shaded-pole motor is that it can withstand LRA for an extended period. Since the motor does not have a start winding, the run winding is large enough to carry locked-rotor current if the rotor becomes stuck. This is important since it provides burnout protection without any additional devices or equipment being added to the motor.

Note that the shaded-pole motor is used as a level indicator. A paddle is attached to the shaft of the shaded-pole motor and it's turned very slowly. The paddle is mounted in a bin where granular material is stored. When the height of the material increases to the point where it covers the paddle, the paddle will stop turning and stall the motor. Since the motor is a shaded-pole motor, its current will increase, but the extra current won't damage the motor. A sensor is used to detect the change in current, which indicates the level of material is covering the paddle.

Since the shaded-pole motor has these characteristics, it's commonly used for small fan applications. If the fan becomes immovable for any reason, such as lack of lubrication or dirt, the motor will become warm, but it won't overheat and destroy itself like the split-phase or capacitor start motors.