How does an AC electric motor rotate?

How does an AC electric motor rotate?

• A. Direct current in rotor
• B. A rotating field from three-phase power
• C. A static magnetic field
• D. Single-phase power

Answer: (B)

The rotation comes from a rotating magnetic field created in the stator by the three-phase supply. When the stator windings are fed with currents that are 120 degrees apart in phase, their magnetic fields add to form a single field that moves around the stator. This rotating field interacts with the rotor: in an induction motor, it induces currents in the rotor, and those rotor currents produce their own magnetic field that torques the rotor to turn. The rotor can’t reach the speed of the rotating field; it runs at a slip below the field speed, with slip set by load and supply frequency. A static magnetic field wouldn’t rotate on its own, and single-phase power can start rotation only with extra means to create a rotating field; it doesn’t provide the steady, continuous rotating field that a three-phase system does.

The rotation comes from a rotating magnetic field created in the stator by the three-phase supply. When the stator windings are fed with currents that are 120 degrees apart in phase, their magnetic fields add to form a single field that moves around the stator. This rotating field interacts with the rotor: in an induction motor, it induces currents in the rotor, and those rotor currents produce their own magnetic field that torques the rotor to turn. The rotor can’t reach the speed of the rotating field; it runs at a slip below the field speed, with slip set by load and supply frequency. A static magnetic field wouldn’t rotate on its own, and single-phase power can start rotation only with extra means to create a rotating field; it doesn’t provide the steady, continuous rotating field that a three-phase system does.