How does a rotating magnetic field work in an AC motor?

How does a rotating magnetic field work in an AC motor?

• A. Three-phase AC applied to stator windings creates rotating magnetic fields that peak in sequence, producing a single rotating field at synchronous speed.
• B. A single-phase current creates a rotating field automatically.
• C. The rotor generates the magnetic field that causes rotation.
• D. The stator is energized by DC to produce rotation.

Answer: (A)

A rotating magnetic field in an AC motor comes from energizing the stator with a balanced three-phase supply. The windings are spaced 120 degrees apart, and each phase’s magnetic field peaks at a different time. The combination of all three fields adds up to a single magnetic field that appears to sweep around the stator, i.e., rotate. The speed of that rotating field is the synchronous speed, set by the supply frequency and the number of stator poles. The rotor sits in this rotating field and has currents induced in it; those rotor currents create their own magnetic field that interacts with the rotating field, pulling the rotor to follow the rotation and producing torque. The other ideas don’t fit because a single-phase supply doesn’t produce a continuous rotating field, the rotor doesn’t create the rotating field by itself, and energizing the stator with DC would yield a static magnetic field, not rotation.

A rotating magnetic field in an AC motor comes from energizing the stator with a balanced three-phase supply. The windings are spaced 120 degrees apart, and each phase’s magnetic field peaks at a different time. The combination of all three fields adds up to a single magnetic field that appears to sweep around the stator, i.e., rotate. The speed of that rotating field is the synchronous speed, set by the supply frequency and the number of stator poles. The rotor sits in this rotating field and has currents induced in it; those rotor currents create their own magnetic field that interacts with the rotating field, pulling the rotor to follow the rotation and producing torque. The other ideas don’t fit because a single-phase supply doesn’t produce a continuous rotating field, the rotor doesn’t create the rotating field by itself, and energizing the stator with DC would yield a static magnetic field, not rotation.