Whether an asynchronous motor can start on its own depends on its specific type. Three-phase asynchronous motors can start on their own, while single-phase asynchronous motors usually cannot. The core difference stems from the different characteristics of the stator magnetic field.
For three-phase asynchronous motors, their self-starting capability directly depends on the rotating magnetic field generated by the three-phase alternating current. When symmetrical three-phase alternating current is applied to the stator windings, a magnetic field rotating in a fixed direction and at synchronous speed is immediately synthesized. This rotating magnetic field actively cuts the stationary rotor conductors, inducing a current in the rotor circuit. The rotor conductors carrying the induced current are subjected to electromagnetic force in the rotating magnetic field, forming an electromagnetic torque that drives the rotor to rotate. In this process, no additional starting device is needed; the generation of the rotating magnetic field, the formation of the induced current, and the driving of the electromagnetic torque are a continuous cause-and-effect process. Therefore, three-phase asynchronous motors have self-starting capability; simply connecting a three-phase power supply is sufficient to complete the starting process from standstill to operation. However, when the power supply capacity is insufficient or to ensure the safety of the motor and equipment, necessary auxiliary starting measures are required.
Single-phase asynchronous motors, connected to single-phase alternating current, generate a pulsating magnetic field in their stator windings. This pulsating magnetic field varies sinusoidally with time but remains fixed in space. This pulsating magnetic field cannot provide a continuous starting torque to a stationary rotor: it can be decomposed into two equal-sized, opposite-direction rotating magnetic fields. The electromagnetic torques they produce on the rotor cancel each other out, keeping the rotor in a constant state of equilibrium and preventing it from rotating on its own. Therefore, to start a single-phase asynchronous motor, an additional starting device (such as a capacitor-driven split-phase starting winding or a shaded-pole short-circuit ring) must be installed. These devices convert the stator magnetic field into a rotating magnetic field at startup, generating initial torque to drive the rotor. Once the motor speed reaches a certain value, part of the starting device automatically disconnects. The torque generated by the positive rotational component of the pulsating magnetic field established by the main winding current, aligned with the rotor’s direction of rotation, drives the motor to continue operating. The torque generated by the negative rotational component, aligned with the rotor’s direction of rotation, prevents rotation. Since the electromagnetic torque aligned with the rotor’s direction of rotation is greater than the negative component, the motor continues to operate.
Post time: Dec-16-2025