An AC induction motor essentially consists of two parts namely a stationary part called the "stator" and a rotating part called "rotor". The rotor is placed inside the stator and is supported on both sides. Energy is supplied to the windings placed in the stator slots. Energy is transferred to the rotor windings through electromagnetic induction and hence such motors are called "induction motors". Three Phase Induction Motor Construction: The stator consists of three-phase winding which are placed in the slots of a laminated stator core. The rotor core is a laminated steel cylinder, having slots in which conductors are cast or wound. The rotor bars are shortened at the both ends by rotor end-rings.

Principle:When a three-phase supply is connected across the stator windings, a rotating magnetic field, constant in magnitude but rotating at synchronous speed, Ns, is produced. The speed of the rotating field so produced depends upon the supply frequency and the number of poles for which the winding is made. The direction of the rotating magnetic field produced by the stator depends upon the supply phase sequence. This field induces an electromotive force (emf) in the rotor conductors which in turn produces the current flow. Thus magnetizing the rotor. Due to the tendency of the rotor magnetic field to be aligned with the stator field, the rotor develops the torque in the same direction and it starts rotating. The speed of the rotor however is less than synchronous speed Ns (the speed of rotating magnetic field developed by the stator). If the rotor runs exactly at the synchronous speed induced emf in the rotor will be zero. Hence there will be no rotor current and rotor torque.

The synchronous speed is a function of the no of poles of the motor and supply frequency. This is given by:Ns = 120 * frequency (f) / number of poles (P) Hence the speed of an AC motor is a function of frequency and the number of motor poles. The speed of the rotor relative to that of the stator-rotating field is called as "SLIP". This slip is the difference between the synchronous speed, Ns and actual speed N and is denoted by S. This is generally expressed as a fraction of the synchronous speed. Thus slip is S = (Ns-N) / Ns where N- is actual rotor speed, Ns - Synchronous speed The primary function of the motor is to provide torque, which makes the shaft / loads to rotate at the required speed.

 

	The "torque" of an induction motor depends upon the flux in the air gap.
	Further, flux is directly proportional to V / f.... where V is supply voltage and f is the supply frequency. It can therefore be said that, the torque T is directly proportional to flux & flux is directly proportional to V / f.
	Thus the torque producing capability of the motor at the rated / required speeds can be retained constant, by maintaining the voltage v/s frequency ratio constant.
	Conclusively one can say that to vary the speed of an induction motor the frequency of the supply going to the motor should be varied. In order to maintain the torque producing capability the voltage applied to the motor needs to changed in the same proportion as that of frequency.