What is a shaded pole AC motor?

The shaded pole AC motor is a single-phase AC motor. It has characteristics such as simple structure and low cost, and is widely used in some small electrical appliances.

Structural principle

Stator structure

The stator core is generally salient pole type and has multiple magnetic poles. On a part of each magnetic pole, there will be a short-circuit ring. This short-circuit ring is like "covering" a part of the magnetic pole, so it is called a shaded pole.

For example, in a two-pole shaded pole motor, there are two salient poles, and a part of each salient pole is surrounded by a short-circuit ring. The short-circuit ring is usually made of copper and cooperates with the main magnetic field part of the magnetic pole.

Working principle

When an AC power source is connected to the stator winding, an alternating magnetic field is generated in the magnetic poles. Due to the presence of the short-circuit ring, the magnetic flux passing through the short-circuit ring lags behind the main magnetic flux in time.

This is because according to Faraday's law of electromagnetic induction, the alternating main magnetic flux will induce an electromotive force in the short-circuit ring, and then generate an induced current. This induced current will in turn generate a magnetic field. According to Lenz's law, this magnetic field will hinder the change of the main magnetic flux, making the magnetic flux passing through the short-circuit ring lag.

For example, when the main magnetic flux reaches its maximum value, the magnetic flux in the short-circuit ring is still in the rising process. This phase difference of the magnetic flux will produce an effect similar to a rotating magnetic field on the surface of the magnetic pole, so that the rotor of the motor is subjected to a torque and rotates.

Performance characteristics

Starting characteristics

The shaded pole motor has the ability to start by itself. Due to the magnetic field lag generated by the short-circuit ring, the motor can automatically start rotating after the power is turned on.

However, its starting torque is relatively small. This is because the magnetic field distribution and the generation method of the rotating magnetic field of the shaded pole motor determine that its starting torque is limited, and it is generally suitable for occasions with small starting loads.

For example, in a small fan, the starting resistance of the fan blades is small, and the shaded pole motor can easily start and drive the fan to rotate.

Operating characteristics

During operation, the speed of the motor is basically stable. Its speed will change according to the power supply frequency and the number of magnetic pole pairs. Generally, the speed is relatively low.

For example, under a 50Hz power supply, the synchronous speed of a two-pole shaded pole motor is 3000 revolutions per minute, but the actual operating speed will be slightly lower than the synchronous speed, and the speed fluctuation is small, which can provide a relatively stable power output.

Efficiency and power factor

The efficiency of the shaded pole motor is low. This is because its magnetic field generation method and structural characteristics lead to a certain amount of energy loss in the energy conversion process, including copper loss, iron loss, etc.

At the same time, the power factor is also relatively low. Since it is a single-phase motor, and the generation and distribution method of the magnetic field is relatively complex, the ratio of active power to apparent power is small during the operation of the motor.

Application scenarios

Due to its simple structure, low cost, and ability to start by itself, shaded pole motors are mainly used in occasions where the motor performance requirements are not high and the load is small.

Common ones are small electric fans, hair dryers, electric models, etc. In these devices, the shaded pole motor can meet the basic power requirements, and its low cost also meets the economic requirements of the product.