Why do single-phase motors require capacitors, while three-phase motors do not?
The reason why single-phase motors require capacitors
Generating a rotating magnetic field: A single-phase power supply provides a single-phase sine current, which can only generate an alternating pulsating magnetic field when passing through the main coil. If there is no other assistance, the motor may stop after turning 90 degrees. Connecting capacitors in series into the circuit can change the phase of the secondary coil current, creating a phase difference with the main coil current, causing the magnetic field force to move forward and backward, creating a rotating magnetic field effect that drives the motor to rotate.

Provide starting torque: In a single-phase capacitor starter motor, the capacitor is connected in series with the starting winding to provide a large starting torque during startup, helping the motor overcome the inertia of the initial stationary state and start smoothly.
The reason why three-phase motors do not require capacitors
Power characteristics: The three-phase power supply can provide three-phase AC power, with a phase difference of 120 degrees between each phase. After connecting the stator winding of a three-phase motor, a stable rotating magnetic field can be naturally formed.

Winding structure: The stator winding of a three-phase motor is a three-phase winding, with a difference of 120 degrees in structure. When a three-phase power supply is turned on, three-phase currents flow in such windings, generating a rotating magnetic field that cuts the copper bars of the rotor, causing the rotor to produce induced electromotive force and induced current. The interaction between the induced current and the magnetic field produces electromagnetic force and torque, driving the motor to operate.

1、 The reason why three-phase motors do not require capacitors

The natural advantages of three-phase power supply
The three-phase motor is connected to three-phase alternating current (three single-phase currents with a phase difference of 120 °), and the current flowing through the three-phase winding generates a rotating magnetic field. This rotating magnetic field directly drives the rotor to rotate, and the starting torque is large enough without the need for additional auxiliary devices.

Principle: The phase difference of three-phase current causes the magnetic field generated by the winding to rotate clockwise or counterclockwise, and the rotor rotates with the rotating magnetic field under electromagnetic induction.

Symmetrical structures come with phase separation function
The three-phase windings of a three-phase motor are symmetrically distributed in space (with a phase difference of 120 °), and with the phase difference of the three-phase power supply, a rotating magnetic field is naturally formed without the need for external components (such as capacitors) to separate phases.

2、 The reason why single-phase motors rely on capacitors

Limitations of single-phase power supply
The single-phase motor is connected to single-phase AC power (with only one phase), and the single-phase winding generates a pulsating magnetic field (the magnetic field strength varies periodically with the current, but the direction is fixed), rather than a rotating magnetic field. Pulsating magnetic fields cannot directly generate starting torque, and the rotor will vibrate in place without rotating.

The "phase separation" effect of capacitors
In order to generate a rotating magnetic field in a single-phase motor, it is necessary to artificially create a phase difference, and capacitors play a key role in this:

Classification and Function of Capacitors

Starting capacitor (short-term operation): Only connected during startup, providing a large phase difference and starting torque, and disconnected by a centrifugal switch after startup (such as a "capacitor starting" motor).

Operating capacitors (continuous operation): Balancing start-up and operation, providing a small phase difference to maintain torque (such as "capacitor operated" motors).

Dual capacitor motor: using both starting and running capacitors to balance starting torque and operating efficiency.