Principle of operation of a three phase induction motor

The three phase induction motor was invented by Nicola Tesla in and around the late 1880's as well as others. there are two parts: stator and rotor seperated by an air gap. Now, when 3 phase supply is given (3 phase means 3 voltage waveforms of same magnitude but 3 different phases) in the stator conductors(coil). The stator coils are connected to the three phase supply which is create the rotating magnetic field. this rotating magnetic field is in clock wise direction It is also called synchronous field. the rotating field act on rotor conductors but rotor conductor rotates in anticlockwise direction in stationary field. By faraday's electromagnetic induction , A voltage will be induce in the rotor conductor. Since rotor circuit is complete, either through the end ring or an external resistance the induced voltage causes a current to flow in the rotor conductors. By right hand rule we can determine the direction induced current in the conductor. Direction of current is outwards. The current in rotor conductor produces its own magnetic field.

We known that when a conductor carrying current is put in a magnetic field a force is produce on it. thus , a force is produced on the rotor conductor. the direction of this force can be found by left-hand rule. it is seen that the force acting on the conductor is in the same direction as the direction of the rotating magnetic field.Since the rotor conductor is in a slot on the circumference of the rotor, this force acts in a tangential direction to the rotor and develops a torque on the rotor . similar torque are produced on all the rotor conductors. Since the rotor is free to move , it starts rotating in the same direction as the rotating magnetic field .thus, A three phase induction motor is self starting.since the operation of this motor depends upon the induced voltage in the rotor conductors, It is called an induction motor.     

3 phase IM ( Induction motor) basically works on the principle of mutual induction ( as in case of transformer). The major difference between them is that transformer is static in nature ( it doesn't have any moving parts) whereas IM is a dynamic one.

Point wise principle 

• 3 phase AC supply given to IM produces a constant amplitude Rotating Magnetic Field-RMF (To know how this occurs, google "Production of rotating magnetic field using 3 phase supply") which rotates at synchronous speed with respect to stator.
• This field cuts the rotor conductors (which are stationary initially) and induces an emf (as per Faraday's law of electromagnetic induction).
• Emf drives current in rotor conductors as they are short circuited.
• So now we see that current carrying conductors of rotor are placed in a magnetic field, hence force acts on each of these conductors (refer Lorentz's law ).
• The resultant of forces on all the conductors produces a rotating effect on the rotor ( torque).
• Now the million dollar question is " In which direction does the rotor rotate?"
• This can be easily answered if you understand Lenz's law correctly. It says " effect opposes the cause".
• In IM, cause is 'rotating magnetic field cutting rotor conductors or the flux cutting action' & end effect is the 'torque'.
• So, for effect (torque ) to oppose the cause (flux cutting action), torque produced makes the rotor rotate in the same direction as that of the RMF.

NOTE:

Rotor rotates close to synchronous speed but not at synchronous speed.