Commonly used in motors where capacitors are used are the start-up of single-phase motors. This capacitor is also called start-up capacitor. From the perspective of circuit principle, it plays a role of phase shift, because single-phase motors have only one set of running windings. The rotor is squirrel-cage type, single-phase electricity cannot generate a rotating magnetic field, and another set of starting windings are needed to match. Because there is only single-phase 220 volt AC, a capacitor is connected in series with the starting winding. The characteristic of the capacitor is that the voltage cannot be abrupt. , So the current of the starting coil will be advanced by 90° to the running coil, so that two rotating magnetic fields can be formed in space, so that the motor starts normally.

There are two sets of single-phase motor stators, which are 90 degrees apart in space. They are the starting winding and the running winding, which can obtain a current of 90 degrees. Under the action of the phase shift of the starting capacitor, the single-phase electricity will make the starting winding and running The phase difference of the winding current is 90 degrees, which generates a rotating magnetic field in the stator space, and 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 are also connected with a capacitor in parallel at both ends of the armature. This capacitor is mainly used for filtering and anti-interference. Because DC motors have carbon brushes, a peak voltage is generated when the carbon brush and the commutator are disconnected. The capacitor is added to release this voltage. Of course, some capacitors connected between the carbon brush and ground can also play a similar role.

One use of the capacitor to the motor is reactive power compensation. The motor is an inductive load. After use, the power factor is relatively low. Power supply rooms need to use capacitors for reactive power compensation. The long-distance reactive power compensation loss is relatively large, and the effect is not very good, so sometimes it is necessary to directly start the reactive power compensation in the vicinity of the motor. For example, the star-delta startup circuit below, add a capacitor to complete this function.

On-site reactive power compensation directly reduces the power consumption and significantly improves the power factor, which fully enhances the potential of power grid transmission.