Alternating Current Motors (AC Motors)

AC motors are considered as a significant portion of the world's electric load. Nowadays, industrial and domestic usages of AC motors are very popular. In such usage, induction motors are very common.

Motors are devices which convert electrical energy into mechanical energy. According to supply system, motors are of two types AC and DC motors. The two main parts of a motor are a field winding and an armature. The part which is stationary is called stator while the rotating part is called rotor. In AC motors, the field winding rotates and armature is stationary. In DC motors, the arrangement is just the reverse.

Preference Of A.C. Motors Over D.C. Motors

AC motors are preferred over DC motors due to the following reasons:

  1. Due to the wide spread use of AC power transmission systems, AC motors are used very popularity in the world.
  2. AC motors do not require converting machinery or devices as required in case of DC motors.
  3. As compared to DC motors it requires less repairing and maintenance.
  4. Due to the availability of three-phase AC supply system, high power motors can be used easily.
  5. AC motors can be used to improve the power factor.

However, DC motors (though are used very rarely), have their own importance. For example, some vehicles and trains contain DC supply and thus require DC motors for their usage. Another application of DC motor is a situation in which a wide variation in speed is needed. Nowadays due to the availability of solid-state devices, DC motors can be used easily for these purposes.

General Working Principle Of A.C Motors

The general working of an AC motors is based on the principle that when a current carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming's left-hand rule and whose magnitude is given by:



B = flux density in wb/m2
L = length of conductor in meters
F = Force in Newton
I = Current in amperes

This process is also called interaction. It is discussed briefly as follows:

In Fig.1 is shown a conductor carrying current I amperes outward, is placed in a magnetic field of flux density B Wb/m2. Let the active length of the conductor is L meters. The conductor is at right angled to the field. A magnetic field is setup round the current carrying conductor:

AC Motors

Due to interaction of these two fields, a mechanical force is developed. Whose magnitude is equal to, F = BIL Newton and direction is out of the paper (by Fleming's left-hand rule). If the numbers of conductors (i.e. coils) are increased, the action of different forces can be converted into a rotating torque.


  1. According to principle of operation:

    1. Synchronous Motor

    2. Induction motor

      1. Slip ring

      2. Squirrel cage

        1. Single cage

        2. Double cage

  2. According to type of current:

    1. Single phase

    2. Three phase

  3. According to their speed

    1. Constant speed

    2. Variable speed

    3. Adjustable speed