Consider a portion of 3-phase induction motor as
shown. The working principle of a three phase induction motor is basically Lenz's law as explained below.
- When 3-phase balanced stator winding is energised from a 3-phase balanced supply, a rotation magnetic field is set up which rotates around the stator core at synchronous speed, Ns=120f/P in rpm.
- The rotation field passes through the air-gap and cuts the rotor conductors, which as yet, are stationary. Due to this relative speed between field and rotor conductors, emfs are induced in rotor conductors. Since rotor is always short circuited, whether it is squirrel cage type or wound type, currents starts flowing in the rotor conductors.
- Since, the current carrying rotor conductors are placed in stator magnetic field, they experience mechanical force. These mechanical forces on all rotor conductors add up to make the rotating torque, which tends to rotate the rotor in the same direction as the rotating magnetic field.
- Question as to why the rotor rotate in the same direction as the rotating magnetic field can be explained by Lenz’s law. According to this law, the direction of induced current would be such as to oppose the cause of its generation. Here, the cause producing the rotor currents is the relative speed between the rotating field and the stationary rotor conductors. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to catch it, but never catches it.
The above principle is also the induction motor principle for any type i.e slip ring induction motor working principle or squirrel cage induction motor working principle.
