Stator has three main parts: Outer Frame – It is the outer body of theof the motor.It protects the inner part of the machine. Stator Core – Built up of high gradesilicon steel.Carries the alternating magnetic field. Stator winding – Has a three phasewinding.
There are two types of rotors which areemployed in 3 – phase induction motor. Squirrel Cage Rotor. Phase Wound/ Slip Ring Rotor.
It consists of a laminated cylindrical corehaving semi closed circular slots at the outerperiphery. Copper or aluminum bar conductors are placedin these slots and short circuited at each end bycopper or aluminum rings called shortcircuiting rings. The rotor winding is permanently shortcircuited and it is not possible to add anyexternal resistance.
The rotor slots are not parallel to the shaft butskewed to – Reduce humming .Provide smoother torque for different positions ofrotor.Reduce magnetic locking of stator and rotor.
It is also called SLIP RING ROTOR Consists of a laminated core having semi closed slotsat the outer periphery and carries a 3-phase insulatedwinding. The rotor is wound for the same number of poles asthat of stator. The three finish terminals are connected togetherforming a star point and the three star terminals areconnected to three slip rings fixed on the shaft.
Principle of OperationA rotating field is set up in the stator when a 3- Phasesupply is given.The stationary rotor cut the revolving field and due toelectromagnetic induction an e.m.f. is induced in the rotorconductor.As the rotor conductor is short circuited current flowsthrough them.It becomes a current carrying conductor in magnetic fieldand start rotating.
SlipInduction motor rotor always rotate at a speed less thansynchronous speed.The difference between the flux (Ns) and the rotor speed (N)is called slip.% Slip = (Ns – N) 100NSlip speed = Ns – N