What is the role of capacitors in motors?

Generally, it is common to use capacitors in motors. It is the start of single-phase motors. This capacitor is also called starting capacitor. From the perspective of circuit principle, it plays the role of phase shifting, because single-phase motors have only one set of running windings. The rotor is a squirrel-cage type, and single-phase electricity cannot generate a rotating magnetic field, requiring another set of starting windings to cooperate.Because there is only single-phase 220 volts ac, a capacitor is connected in series with the starting winding. The characteristic of the capacitor is that the voltage cannot change suddenly, so the current of the starting coil will be 90° ahead of the running coil, so that two rotating coils can be formed in space. The magnetic field allows the motor to start normally.

The stator of the single-phase motor has two groups, which are 90 degrees apart in space, which are the starting winding and the running winding respectively, and can obtain currents with a difference of 90 degrees.Under the phase shifting effect of the starting capacitor, the single-phase power will make the current of the starting winding and the running winding have a phase difference of 90 degrees, and a rotating magnetic field will be generated in the stator space.The rotor will cut the magnetic field lines to generate current, and then there will be a rotating magnetic field. The stator and rotor magnetic fields interact, and the rotor can rotate continuously.

Some dc motors also connect a capacitor in parallel at both ends of the armature. This capacitor is mainly used for filtering and anti-interference. Because the dc motor has carbon brushes, when the carbon brushes and the commutator are disconnected, a peak voltage will be generated. The capacitor is added to release this voltage. Of course, some indirect capacitors between the carbon brushes and the ground can also play a similar role.

One use of capacitors for motors is reactive power compensation. Motors are inductive loads, and the power factor is relatively low after use. Power supply rooms need to use capacitors for reactive power compensation. However, the long-distance reactive power compensation loss is relatively large, and the effect is not very good, so sometimes it is necessary to directly compensate the starting reactive power near the motor, such as the star-delta starting circuit below, and add a capacitor to complete this function.

On-site reactive power compensation directly reduces power consumption, improves power factor, and taps the potential of power grid transmission.