Single phase motor and three-phase motor are defined by motor power supply! If we enter a factory, most of the motors involved are three-phase motors. If we enter a family, especially a modern urban family, single-phase AC motor almost covers all our electric appliances: washing machine, lampblack machine, electric vegetable cutter, cooking machine, meat grinder and so on, all of which need single-phase motor.
01 Winding of single phase asynchronous motor
One of the most important characteristics of single-phase asynchronous motor is that it is supplied by single-phase power supply. The stator of the motor consists of two windings, one working winding and one starting winding. Generally, the starting winding works in the starting process of the motor, and only the working winding works after the starting. This is a characteristic of single-phase motor stator winding different from three-phase motor winding.
02 Starting of single phase asynchronous motor
The stator magnetic potential of single-phase asynchronous motor is a pulsating magnetic potential, which cannot produce starting torque, so the motor cannot start automatically. In order to realize the starting effect of the motor, it is necessary to create a rotating magnetic field.
(1) Split phase starting
we mentioned earlier in this paper that the winding of single-phase asynchronous motor includes a working winding and a starting winding. The difference between the starting winding and the working winding in spatial distribution is 90 degrees. In addition, capacitors are connected in series in the starting winding to generate a circular rotating magnetic field in the air gap of the motor, which can produce a large starting torque and make the motor start smoothly.
Generally, the starting winding is designed for short-term operation. In order to avoid winding damage due to overheating, when the motor speed is close to the synchronous speed (generally 70 ~ 80% of the synchronous speed), the starting winding is automatically cut off by necessary control. This kind of motor is called capacitor starting motor.
If the design considers that the starting winding can not only be used for starting, but also can be connected to the power grid for a long time. From the internal structure of the motor, it is a two-phase asynchronous motor, which can improve the overload capacity and power factor. This kind of motor is called capacitor motor. Since the required capacitance of the motor is smaller than that of starting, the excess capacitance will be cut off after the motor is started.
The starting winding can also be connected in series without series capacitor, which can produce a certain starting torque, but the value is small, and it is only used in the occasion where it is easier to start. When starting by resistance method, the starting winding should be designed to have a larger resistance and no additional series resistance is needed.
(2) Shaded pole starting
The stator core of shaded pole starter motor is usually made of silicon steel sheet with salient pole. Each pole is equipped with working winding and connected to single-phase power grid. The starting torque of shaded pole starting motor is very small, which is only suitable for the occasion of low power.