The key point is that the magnetic field is continuously changing inside the stator.
In an AC induction motor:
- AC current changes direction every half cycle.
- Therefore, the magnetic field also changes direction.
- In a 3-phase induction motor, the three phase currents are 120° apart, so their combined magnetic field produces a continuously rotating magnetic field.
- The rotor experiences this continuously changing magnetic field, so current is induced in the rotor.
- That induced rotor current produces torque, keeping the rotor rotating.
So even though the AC current becomes zero momentarily, the motor doesn't stop. The rotating magnetic field has inertia/momentum in the rotor, and the field immediately changes direction and continues rotating.
But in a 3-phase motor, the magnetic field is changing continuously.
That's why a 3-phase induction motor runs continuously rather than starting and stopping 50 times per second.
A single-phase induction motor is different—it has a pulsating magnetic field and is not self-starting. A 3-phase induction motor has a rotating magnetic field and is self-starting.
3-phase induction motor
The stator has three separate windings, one for each phase:
- Phase A → winding A
- Phase B → winding B
- Phase C → winding C
The three windings are physically arranged around the stator, typically 120° electrical apart.
For a simple 3-wire, star-connected motor, you can have:
3 supply wires → 3 phase windings
Each supply wire carries one phase current.
What about single-phase?
A single-phase induction motor has one main winding connected to the single-phase supply. Many practical motors also have an auxiliary (starting) winding that provides the initial starting torque.