Working principle and classification of single-phase motors

Single-phase motors generally refer to low-power single-phase asynchronous motors powered by single-phase AC power (AC220V). This type of motor usually has two-phase windings on the stator, and the rotor is a common squirrel cage type (usually made of aluminum). The two-phase windings are distributed at 90° on the stator.

1. Working principle:

Single-phase motor winding diagram

When the starting winding is not energized, a single-phase sinusoidal current passes through the stator working winding, and the motor will generate an alternating magnetic field. The strength and direction of this magnetic field change sinusoidally over time, but it is fixed in spatial orientation, so this magnetic field is called an alternating pulsating magnetic field. This alternating pulsating magnetic field can be regarded as two rotating magnetic fields with the same speed but opposite rotation directions. When the rotor is stationary, these two rotating magnetic fields generate two torques of equal magnitude and opposite directions in the rotor, making the synthetic torque zero, so the motor cannot rotate. When we use external force to make the motor rotate in a certain direction (such as clockwise), the movement of the cutting magnetic lines of force between the rotor and the rotating magnetic field in the clockwise direction becomes smaller; the movement of the cutting magnetic lines of force between the rotor and the rotating magnetic field in the counterclockwise direction becomes larger. In this way, the balance is broken, and the total electromagnetic torque generated by the rotor will no longer be zero, and the rotor will rotate in the direction of the push.

We have experienced this when repairing electric fans. When the starting capacitor of the fan motor is completely out of capacity or the starting winding is short-circuited, the fan will not rotate when powered on. At this time, we will find that the motor can be easily turned without power, but when powered on, the motor is like being attracted by magnets, vibrating and the resistance increases. At this time, if we apply force to make the motor rotate clockwise, it will rotate clockwise, and if we apply force to make the motor rotate counterclockwise, it will rotate counterclockwise. In other words, its rotation direction is uncertain and only depends on the direction of our force.

Let's think about the household refrigerators we often repair. Some refrigerator compressors have no running capacitor, only a starting resistor. The starting resistor becomes an open circuit state after completing its mission, that is, only the working winding is energized when the compressor is working.

In order to make the single-phase motor rotate automatically and the direction of rotation is fixed, a starting winding is required. The starting winding is 90° different from the main winding in space. A suitable capacitor is connected in series with the starting winding so that the current of the main winding is approximately 90° different in phase, which is the so-called split-phase principle. In this way, two currents with a time difference of 90° pass through two windings with a spatial difference of 90°, which will generate a continuous fixed rotating magnetic field in space. Under the action of this rotating magnetic field, the rotor can start automatically, and the direction is fixed, and there will be no phenomenon of uncertain direction. To change the direction of this motor, just swap the terminal of the auxiliary winding.

In single-phase motors, there is another method to generate a rotating magnetic field - the shaded pole method, and the motor is called a single-phase shaded pole motor. The stator of this motor is made into a salient pole type, and there are two types: two poles and four poles. Each magnetic pole has a small groove at 1/3 to 1/4 of the full pole surface, which divides the magnetic pole into two parts. A short-circuit copper ring is set on the smaller part, as if covering this part of the magnetic pole, so it is called a shaded pole motor. When the stator winding is energized, the main magnetic flux is generated in the magnetic pole, and the main magnetic flux part passing through the short-circuit copper ring generates an induced current in the copper ring that lags 90° in phase. The magnetic flux generated by this current also lags behind the main magnetic flux in phase. Its function is equivalent to the starting winding of a capacitor motor, thereby generating a rotating magnetic field to make the motor rotate.

2. Classification of Split-Phase Single-Phase Motors

In summary, single-phase AC asynchronous motors are divided into two categories: split-phase type and shaded-pole type. The split-phase type with starting winding and starting capacitor is commonly used. The split-phase type also has the following working conditions:

1. Startup method

The first type is the starting winding normally working type. As shown in the figure below, the starting winding assists in starting and is not disconnected after starting. Its starting torque is not large. It is mainly used in electric fans, air conditioner fan motors, washing machines and other motors.

Regular working type

The second type is the centrifugal switch type. The centrifugal switch is turned on when the motor is stationary. After power is turned on, the starting capacitor participates in the starting work. When the rotor speed reaches 70% to 80% of the rated value, the centrifugal switch will automatically trip, the starting winding will not participate in the operation, and the motor will continue to run with the running winding coil, as shown in the figure below.

Centrifugal

The third type is the centrifugal switch plus running capacitor type. The centrifugal switch is also turned on when the motor is stationary. After power is turned on, the starting capacitor is connected in parallel with the running capacitor to participate in the starting work together. When the rotor speed reaches 70% to 80% of the rated value, the centrifugal switch will automatically jump off, the starting capacitor completes the task and is disconnected. The running capacitor is still connected in series with the starting winding to participate in the operation. This connection method is generally used in places with large and unstable loads such as air compressors, cutting machines, and woodworking machine tools. When the starting capacitor is connected, the starting torque is increased to facilitate the rapid start of the motor, as shown in the figure below.

Centrifugal parallel type

2. Forward and reverse control:

The following figure is a wiring diagram with a forward and reverse switch. Usually, the starting winding and running winding parameters of this motor are exactly the same. This motor is generally used in the washing motor of a double-bar washing machine. This forward and reverse control method is simple and does not require a complicated conversion switch.

The parameters of the starting winding are the same as those of the running winding.

For motors with different parameters for the starting winding and the running winding, that is, the starting winding and the running winding must not be mistaken, the forward and reverse control can only be done by changing the connection direction of the starting winding or the running winding. For motors with common terminal wiring, if the starting winding and the running winding are separated, its rotation direction is designed. For example, the general double-bar washing machine that we often repair has a dehydration motor that is divided into forward and reverse, and the rotation direction must not be changed as shown in the figure above.