Three Phase Ac Induction Motor

AC induction machines are popular due to their simplicity, reliability and direct operation from an AC line voltage. ACIMs are asynchronous machines and always have a lower mechanical rotor speed than the power line frequency.

Historically, variations to speed requests was a limitation for AC machines, but the development of frequency converters has simplified motor speed changes, and are now widely used.

ACIM fabrication begins with a three-phase winding placed in the winding slot of the stator. There are two basic types of rotor winding concepts. The first type places the rotor winding placed in the winding slot with a slip ring on the shaft, which historically was used for speed regulation and startup. The more common type consists of a rotor in a squirrel cage form.

The principle of operation for an ACIM is based on the voltage induction from the stator to the rotor. When the stator winding is fed by a three-phase supply voltage, the current flows in the winding and the stator rotating magnetic field is generated. Induced voltage in the rotor windings will create the rotor current and the rotor magnetic field. The interaction between two magnetic fields creates the mechanical torque needed to turn the rotor.

The operational rotor speed is lower than the stator magnetic field speed in order to achieve rotational torque.

• Medium construction complexity, multiple fields on stator, cage on rotor
• High reliability (no brush wear), even at very high achievable speeds
• Medium efficiency at low speed, high efficiency at high speed
• Driven by multi-phase inverter controllers
• Motor EMI good, but terrible EMI from inverter
• Sensorless speed control possible
• Low cost per horsepower, though higher than for 1-phase AC induction motor
• Higher start torque than for 1-phase, easy to reverse motor