A single-phase AC motor is a rotating electromagnetic machine used to convert electrical energy into mechanical energy.
Working principle of single-phase AC motor
Using a single-phase capacitor motor as an example: A single-phase motor has two windings: a starting winding and a running winding. These two windings are spatially 90 degrees apart. A large capacitor is connected in series with the starting winding. When single-phase alternating current flows through both the starting and running windings, the capacitor causes the current in the starting winding to lead the current in the running winding by 90 degrees in time, reaching its maximum value first. This creates two identical pulsed magnetic fields in both time and space, generating a rotating magnetic field in the air gap between the stator and rotor. Under the influence of this rotating magnetic field, an induced current is generated in the rotor. The interaction between this current and the rotating magnetic field produces an electromagnetic torque, causing the motor to rotate.
Working principle diagram of a single-phase AC motor
The starting methods for 220V AC single-phase motors can be roughly divided into the following types:
1. The split-phase starter type, as shown in Figure 1, uses an auxiliary starting winding for starting, and its starting torque is not large. The operating speed remains approximately constant. It is mainly used in motors for electric fans, air conditioner fan motors, washing machines, etc.
Figure 1. Capacitor transfer type wiring circuit
2. When the motor is stationary, the centrifugal switch is closed. After power is supplied, 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 capacitor has completed its task and is disconnected. The starting winding does not participate in the running work, and the motor continues to operate with the running winding coil, as shown in Figure 2.
Figure 2. Wiring circuit for capacitor-start type
3. When the motor is stationary, the centrifugal switch is on. After power is supplied, 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 capacitor has completed its task and is disconnected. The running capacitor is connected in series with the starting winding to participate in the running work. This connection method is generally used in air compressors, cutting machines, woodworking machine tools and other places with large and unstable loads. See Figure 3.
Figure 3. Wiring circuit for capacitor-start operation (dual DC capacitors)
For motors equipped with centrifugal switches, if the motor cannot start successfully within a short time, the winding coils will burn out quickly. Capacitor value: Dual-value capacitor motors have a large starting capacitor and a small running capacitor; the withstand voltage is generally greater than 400V.
Forward and reverse control
Figure 4 shows the wiring diagram with a forward/reverse switch. Typically, the starting winding and running winding of this type of motor have the same resistance value; that is, the wire diameter and number of coils in the starting and running windings are exactly the same. This type of motor is commonly used in washing machines. This forward/reverse control method is simple and does not require complex changeover switches.
Figure 4 Wiring diagram for switch control of forward and reverse rotation
