Why do most of the motors of household appliances use shaded pole motors, and what are the advantages?

Shaded pole motor is a simple self-starting AC single-phase induction motor, which is a small squirrel cage motor, one of which is surrounded by a copper ring, which is also called a shaded pole ring or a shaded pole ring. The copper ring is used as the secondary winding of the motor. The notable features of the shaded-pole motor are that the structure is very simple, there is no centrifugal switch, the power loss of the shaded-pole motor is large, the motor power factor is low, and the starting torque is also very low. They are designed to remain small and have low power ratings. The speed of the motors is as precise as the frequency of the power applied to the motors, which are often used to drive clocks. Shaded-pole motors only rotate in one specific direction, the motor cannot rotate in the opposite direction, the loss generated by the shaded-pole coils, the motor efficiency is low, and its structure is simple, these motors are widely used in household fans and other small-capacity appliances.

How Shaded Pole Motor Works

Shaded-pole motor is an AC single-phase induction motor. The auxiliary winding is composed of copper rings, called shaded-pole coil. The current in the coil delays the phase of the magnetic flux at the magnetic pole part in order to provide a rotating magnetic field. The direction of rotation is from the non-shaded pole. to the shaded pole ring.

Shaded pole coils (rings) are designed so that the axis of the magnetic pole is offset from the axis of the main pole pole, and the magnetic field coil and additional shaded pole coils are used to generate a weak rotating magnetic field. When the stator is energized, the magnetic flux of the pole bodies creates a voltage in the shaded pole coils, which act as the secondary winding of the transformer. The current in the secondary winding of the transformer is not synchronized with the current in the primary winding, and the magnetic flux of the shaded pole is not synchronized with the magnetic flux of the main pole.

In a shaded-pole motor, the rotor is placed in a simple c-core, and half of each pole is covered by a shaded-pole coil that generates pulsating flux when an alternating current is passed through the supply coil. When the magnetic flux through the shading coil changes, voltage and current are induced in the shaded pole coil, corresponding to the change in the magnetic flux from the power coil. Therefore, the magnetic flux under the shaded pole coil lags the magnetic flux in the rest of the coil. A small rotation is generated in the magnetic flux by the rotor, so that the rotor rotates. The following figure shows the magnetic flux lines obtained by the finite element analysis.

Shaded Pole Motor Structure

The rotor and its associated reduction gear train are encased in an aluminum, copper or plastic housing. The enclosed rotor is magnetically driven through the housing. Such gear motors typically have a final output shaft or gear that rotates from 600 rpm to 1 per hour. /168 revolutions (1 revolution per week). Since there is usually no clear starting mechanism, the rotor of a motor powered by a constant frequency supply must be very light to be able to reach operating speed within one cycle of the supply frequency, the rotor can be equipped with a squirrel cage, so that the motor starts like an induction motor, once the rotor is Pulled to synchronise with its magnet, there is no induced current in the squirrel cage and therefore no longer plays a role in operation, the use of variable frequency control enables the shaded pole motor to start slowly and deliver more torque.

Shaded pole motor speed

Shaded pole motor speed depends on the design of the motor, the synchronous speed (the speed at which the stator magnetic field rotates) is determined by the frequency of the input AC power and the number of poles in the stator. The more poles of the coil, the slower the synchronous speed, the higher the applied voltage frequency, the higher the synchronous speed, the frequency and the number of poles are not variables, the common synchronous speed of 60HZ motor is 3600, 1800, 1200 and 900 rpm. Depends on the number of poles in the original design.

in conclusion

Since the starting torque is low and cannot generate enough torque to turn large equipment, shaded pole motors can only be manufactured in smaller sizes, below 50 watts, low cost and simple for small fans, air circulation and other low torque applications. Motor speed can be reduced by series reactance to limit current and torque, or by switching the number of motor coil turns.