Working principle of single-phase motor and selection of capacitor

Release Time：

2025-04-28

The three-phase motor flows three-phase currents with a phase difference of 120 degrees, which can generate a rotating magnetic field. The single-phase current flowing through a single-phase motor cannot generate a rotating magnetic field, and a certain method needs to be adopted to make it generate a rotating magnetic field. One of the methods is to use capacitors, which is also a common method.

Working principle of single-phase motor and selection of capacitor
The three-phase motor flows three-phase currents with a phase difference of 120 degrees, which can generate a rotating magnetic field. The single-phase current flowing through a single-phase motor cannot generate a rotating magnetic field, and a certain method needs to be adopted to make it generate a rotating magnetic field. One of the methods is to use capacitors, which is also a common method.

Capacitors are used for phase separation to create a phase difference of nearly 90 degrees between the currents in two windings, in order to generate a rotating magnetic field. In three-phase electricity, there is a phase difference in the current between each two phases, so there is no need to separate them. The capacitive induction motor has two windings, namely the starting winding and the running winding, which are 90 degrees apart in space.

A capacitor with a larger capacity is connected in series on the starting winding. When a single AC current is passed between the running winding and the starting winding, the current in the starting winding is advanced by 90 degrees in time due to the action of the capacitor, reaching the maximum value first.

Two identical pulse magnetic fields are formed in time and space, creating a rotating magnetic field in the air gap between the stator and rotor. Under the action of the rotating magnetic field, induced current is generated in the motor rotor, and the interaction between the current and the rotating magnetic field generates electromagnetic torque, causing the motor to rotate.

1. Voltage selection of single-phase motor capacitor:

The withstand voltage must be greater than the peak value of the AC input voltage: 220V × 1.414 ≈ 311V, and a withstand voltage of 400V or higher can be taken.

It can also be calculated simply according to the withstand voltage formula: UC ≥ 1.5U (V).

2. Selection of capacitance for single-phase motor operation:

2.1 Single phase motor with only one capacitor

Using only one capacitor, it is both a starting capacitor and an operating capacitor, such as small capacity motors like electric fans and washing machines.

The selection of capacitors for electric motors has strict voltage requirements, which must be equal to or greater than 1.5 times the rated voltage of the motor. For power supplies with a rated voltage of 220V, the rated voltage of capacitors should usually not be lower than 400V.

The capacitance value has a certain degree of flexibility, and there is not much relationship between the large and small values in a short period of time, especially for the starting capacitance, which can be selected at 2-5 times the working capacitance. But it is not advisable to randomly choose capacitors with larger capacity and replace them with capacitors in order to improve the starting torque of the motor, mistakenly believing that larger capacitor capacity is better. In fact, although this approach can increase the starting torque, the starting current of the motor will also increase at a greater rate, which is unfavorable for the motor.