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3-Phase-Induction-Motor

By Ashik





The AC induction motor is a rotating electric machine designed to operate from a three-phase source of alternating voltage. The stator is a classic three-phase stator with the winding displaced by 120°.
  • 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
  • Inverter shoot-through' possible, requires dead-time' circuits and compensation










The three-phase AC induction motor has a squirrel cage rotor in which aluminum conductors or bars are shorted together at both ends of the rotor by cast aluminum end rings. When three currents flow through the three symmetrically placed windings, a sinusoidally distributed air gap flux generating the rotor current is produced. The interaction of the sinusoidally distributed air gap flux and induced rotor currents produces a torque on the rotor. The mechanical angular velocity of the rotor is lower then the angular velocity of the flux wave by so called slip velocity.

In adjustable speed applications, AC motors are powered by inverters. The inverter converts DC power to AC power at the required frequency and amplitude. The inverter consists of three half-bridge units where the upper and lower switches are controlled complimentarily. As the power device's turn-off time is longer than its turn-on time, some dead-time must be inserted between the turn-off of one transistor of the half-bridge and turn-on of its complementary device. The output voltage is mostly created by a pulse width modulation (PWM) technique.The three-phase voltage waves are shifted 120° to one another and thus a three-phase motor can be supplied.


The stator windings of an AC induction motor are distributed around the stator to produce a roughly sinusoidal distribution. When three-phase AC voltages are applied to the stator windings, a rotating magnetic field is produced.

The rotor of an induction motor also consists of windings or more often a copper squirrel cage embedded within iron laminates. Only the iron laminates are shown. An electric current is induced in the rotor bars which also produce a magnetic field.

The rotating magnetic field of the stator drags the rotor around. The rotor does not quite keep up with the rotating magnetic field of the stator. It falls behind or slips as the field rotates.

In this animation, for every time the magnetic field rotates, the rotor only makes three-fourths of a turn. If you follow one of the bright green or red rotor teeth with the mouse, you will notice it change color as it falls behind the rotating field. The slip has been greatly exaggerated to enable visualization of this concept. A real induction motor only slips a few percent.







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