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Why start a single-phase synchronous motor with capacitor?

The three-phase synchronous motor is composed of the stator and rotor, in which the stator windings are energized to generate a rotating magnetic field, and because the phase difference between any two phases of the three-phase power supply is 120, three-phase current is generated in the windings when the stator core is connected to the three-phase power supply, thus generating a rotating magnetic field.

Then the rotating magnetic field and cutting rotor copper bar, will generate the induced electromotive force, and therefore generate the induced current, at this time the induced current and the magnetic field interact to produce the electromagnetic force, and then generate the electromagnetic torque.Therefore, when the electromagnetic torque is greater than the load carried by the motor, the rotation.

Why do single-phase synchronous motors need capacitor?The main reason is that there is no rotating magnetic field, so to start the motor it is necessary to start the winding in series.The purpose of using capacitor is to make the two winding current phase difference of 90 degrees, thus generating a rotating magnetic field for the motor to start and work.

Why can’t a single-phase synchronous motor generate a rotating magnetic field by itself?

Since the single-phase synchronous motor winding is switched on, if there is no series capacitor in the starting winding, a magnetic field (pulsating magnetic field) will be generated at its axis. And this pulsating magnetic field is two magnetic fields of equal magnitude and opposite velocities.Once the synchronous motor motor is stopped, the two magnetic fields generate torque in opposite directions of equal magnitude, and the two torque cancel each other out.Therefore, to start the single-phase motor again, the capacitor is needed to provide assistance. As long as the current phase difference between the two windings of the motor is 90 degrees, a rotating magnetic field can be generated and the motor can be started again.

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