A three-phase asynchronous motor can only rotate if it has a rotating magnetic field, which is generated by the stator windings. The three-phase power supply provides a voltage that is 120 degrees out of phase with each other. Additionally, the stator windings are spatially arranged at 120-degree intervals. When the three-phase current flows through these windings, they create a rotating magnetic field in space. As the current alternates, the magnetic field rotates synchronously, completing one full rotation per cycle. The rotational speed of the magnetic field, known as the synchronous speed, is calculated using the formula: n = 60f/P, where f is the frequency of the power supply and P is the number of pole pairs. This means the motor's speed depends on both the frequency and the number of poles. In contrast, a single-phase AC motor has only one winding and uses a squirrel-cage rotor. When a single-phase sinusoidal current passes through the stator winding, it generates an alternating magnetic field that changes direction and strength over time. However, this magnetic field remains fixed in space, making it a pulsating field. This pulsating field can be broken down into two rotating fields that move in opposite directions at the same speed. When the rotor is stationary, these two fields produce equal but opposite torques, resulting in zero net torque, so the motor cannot start on its own. However, if an external force is applied to rotate the motor in one direction, the interaction between the rotor and the rotating fields changes. The torque in the direction of the applied force becomes stronger, while the opposing torque weakens, allowing the motor to continue rotating in that direction.
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