3 phase IM ( Induction motor) basically works on the pr […]
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 (i.e. it doesn't have any moving parts) whereas IM is a dynamic one.
An induction motor consist of two main parts stator and rotor. The stationary or outer part is called stator while rotating part from which we get the mechanical output is called rotor. Like any other machine, induction motor is also an electromechanical device.
As we call it an 'induction' motor, the working principle is simply based on electromagnetic induction. The stator of the machine houses coils on it while rotor is cylindrical and carries conductor bars. When AC supply is given, it excites the stator coils. Now, AC supply is time varying and we know a time varying current through a coil produces time varying magnetic flux which can induce emf in a nearby conductor.
Consider a stator and rotor. On the stator side there are three phase balanced winding which are excited by three phase balanced current which produces a rotating magnetic field. On the rotor there are also three phase balanced winding but they are shorted onto itself. At the time of starting rotor is stationary. Due to which there is a relative motion between the rotating magnetic field and stationary rotor, due to which an emf is induced in the rotor conductors as per the faraday law of electro magnetic induction. As rotor winding is shorted onto itself, due to this induced emf a current start flowing in it. The direction of this induced current will be in such a way that it will oppose the very cause of its production as per the lenz law. The cause of production here is relative motion between the stationary rotor and rotating magnetic field. So in order to oppose the relative motion rotor also starts rotating in the direction of magnetic field but with a lesser speed Nr. The difference of speed between stator magnetic field and roto speed is called slip.
Between stator and rotor remains a very small air gap. Thus stator produces a time varying magnetic field in this air gap. For a 3 phase stator coil it can be shown that the generated magnetic field is a rotating one. This rotating magnetic field flux is cut by the rotor conductor bars which induces emf through them. As we know from Lenz's law, the induced emf is so as to oppose the very cause of its own creation. Thus the rotor induced emf tries to reduce the effect of stator air gap rotating magnetic field flux by trying to rotate at the same speed as that of the magnetic field. In other way, the rotor tries to achieve a speed where relative velocity between rotor and the rotating air gap flux becomes zero (although never happens). This is the working principle of an Induction Motor although loosely explained.