Question

working principle of 3 phase induction motor

Answer 1

Anelectrical motoris such an electromechanical device which converts electrical energy into a mechanical energy. In case of three phase AC operation, most widely used motor isThree phase induction motoras this type of motor does not require any starting device or we can say they are self-starting induction motors. For better understanding, theprinciple of three phase induction motor, the essential constructional feature of this motor must be known to us.
This Motor consists of two major parts:
Stator of MotorStator of three phase induction motoris made up of numbers of slots to construct a 3 phase winding circuit which is connected to 3 phase AC source. Thethree-phase winding is arranged in such a manner in the slots that they produce a rotating magnetic field when three-phase AC supply is given to them.Rotor of MotorRotor of three phase induction motorconsists of cylindrical laminated core with parallel slots that cancarry conductors. Theconductorsare heavy copper or aluminium bars which fit in each slot, and they are short-circuited by the end rings. The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed because this arrangement reduces magnetic humming noise and can avoid stalling of the motor.Working of Three Phase Induction MotorProduction of Rotating Magnetic FieldThe stator of the motor consists of overlapping winding offset by an electrical angle of 120o. When the primary winding or the stator is connected to a 3 phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed.Secrets Behind the Rotation:According toFaraday’s lawan emf induced in any circuit is due to the rate ofchange ofmagnetic fluxlinkage through the circuit. As the rotor winding in aninduction motorare either closed through an externalresistanceor directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar and due to this emf acurrentflows through the rotor conductor.Here the relative speed between the rotating flux andstatic rotor conductor is the cause of current generation; hence as perLenz's law, the rotor will rotate in the same direction to reduce the cause, i.e. the relative velocity.Thus from theworking principle of three phase induction motor, it may be observed that the rotor speed should not reach the synchronous speed produced by the stator. If the speeds become equal, there would be no such relative speed, so no emf induced in the rotor, and no current would be flowing,and therefore no torque would be generated. Consequently, the rotor cannot reach the synchronous speed. The difference between the stator (synchronous speed) and rotor speeds is called the slip. The rotation of the magnetic field in an induction motor has the advantage that no electrical connections need to be made to the rotor.
Thus thethree phase induction motoris:
*.Self-starting.
*.Lessarmature reactionand brush sparking because of the absence of commutators and brushes that may cause sparks.
*.Robust in construction.
*.Economical.
*.Easier to maintain.