Working Principle of AC Motor and Types AC Motors

Working Principle of AC Motor and Types AC Motors

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Working principle of ac motor is little bit different from DC motor. Why is working principle of ac motor different from DC motor? In single phase induction motor, when a single phase supply is given to the stator winding, a pulsating magnetic field is produced and in a three phase induction motor, when three phase supply is given to three phase stator winding, a rotating magnetic field is produced.

The rotor of an induction motor is either wound type or squirrel cadge type. Whatever may be the type of rotor, the conductors on it are shorted at end to form closed loop. Due to rotating magnetic field, the flux passes through the air gap between rotor and stator, sweeps past the rotor surface and so cuts the rotor conductor. Hence according to Faraday’s law of electromagnetic induction, there would be a induced current circulating in the closed rotor conductors.

Working Principle of AC Motor and Types AC Motors

The amount of induced current is proportional to the rate of change of flux linkage with respect to time. Again this rate of change of flux linkage is proportional to the relative speed between rotor and rotating magnetic field.

As per Lenz law the rotor will try to reduce the every cause of producing current in it. Hence the rotor rotates and tries to achieve the speed of rotating magnetic field to reduce the relative speed between rotor and rotating magnetic field.

Basic working principle of ac motor, AC motors have a stator which is carefully wired so that it creates magnetic poles in a rotary pattern around the perimeter of the stator when it is connected to power.

Then a rotor is suspended inside the stator on bearings so it is free to rotate. A variety of methods are used to then create magnetic poles on the rotor which will be attracted to the rotating magnetic field of the stator as it rotates, causing the rotor to rotate and drive whatever load may be mechanically connected to its shaft.

Types AC Motors

Three main types of AC motors are distinguished by the methods they use to create magnetic poles on the rotor.

  1. Easiest to explain is the permanent magnet rotor, where permanent magnets are mounted on the rotor to create permanent North and South poles on the rotor which will follow the stator’s rotating magnetic field.
  2. Next simplest is the synchronous motor, where two slip rings are used to bring a DC current onto the rotor to create North and South magnetic poles in wire-wound iron pole pieces on the rotor.
  3. Most difficult to comprehend is the induction motor, where a round solid piece of magnetic steel rotor has aluminium or copper bars embedded lengthwise into its face. The bars are connected at each end of the rotor with continuous heavy shorting bars passing around the two sides of the rotor. As the rotating magnetic field of the stator rotates around the rotor it induces (hence induction type) a heavy electric current into these bars which then create magnetic poles in the steel rotor which are then attracted to follow the magnetic field of the stator.

AC Motors Works

Also note that most synchronous motors don’t generate very much torque on the rotor when the rotor is not closely synchronized in speed with the rotating field in the stator, so they need additional help to get started and up to synchronous speed.

This starting torque is normally provided by induction windings embedded into the faces of the pole pieces which only operate until the rotor achieves synchronous speed.

Still, whereas normal induction motors can generate 400% to 600% of rated torque at startup due to the higher slip rate of the rotor bars through the stator’s rotating magnetic field, the best most synchronous motors can do is “in the range of” 250% rated torque for startup, a consideration which must be addressed in application engineering.

The above assumes rotating magnetic field in the stator, a condition which is most easily set up with three phase supply power. With single phase supply power no rotating field is automatically provided so the stator is wired with a second phase winding set at 90 degrees to the primary supply, and is supplied through a capacitor from the main supply usually at startup only.

The capacitor provides a second phase of power at 90 degrees phase angle from the main power, providing definite directional rotation of the stator field for startup.

Working Principle of AC Motor and Types AC Motors Conclusion

After going through the above portion of ac motor we can now establish working principle of ac motor and types of ac motors. I hope you enjoy when reading this article, thank you.