Rotor Construction Of an Induction Motor Squirrel Cage Rotor

• Silicon steel is used for stator as well as rotor construction and in rotor as well as stator it is laminated to reduce eddy current losses.
• The stator frequency is generally higher then rotor frequency hence to reduce eddy current loss we make use of thinner lamination in stator and thicker lamination in rotor
• The rotor winding consists of uninsulated conductors in the form of coppers and aluminum bars embedded in the slots of the rotor.
• The ends of the conductor are short circuited by end rings of the same material.
• End rings also provide good mechanical support to the rotor conductor which during rotation experience centrifugal force.
• In squirrel cage rotor there is no definite number of poles but poles are induced in the same number as there are poles on the stator.
• There is no definite number of phases present on the rotor but we generally treat the rotor as three phase.
• Due to smooth surface of the rotor the air gap between stator and rotor is less as compared to wound rotor.
• Due to less air gap less magnetizing current is required.
• Due to less magnetizing current its no load power factor is better as compare to wound rotor induction motor.
• It has less rotor resistance so starting torque is less as compare to wound rotor.
• The rotor conducts are skewed in order to eliminate the harmonics.
• SCIM as poor starting characteristics but excellent running characteristic.

Drawback of SCIM

• Less starting torque
• High starting current
• Less rotor power factor due to less resistance.

Wound Rotor Induction Motor(WRIM)

• In slip ring Induction motor the rotor also contains winding similar to the stator that is the rotor winding can also be short pitched and distributed just like stator winding.
• The rotor must be wound for same number of poles as the stator winding.
• There can be any number of phases on the rotor but in general we consider 3-phases on the rotor.
• The stator of an induction motor can be stator as well as delta connected but we assume that stator to be delta connected but in a wound rotor induction motor (WRIM) the rotor must always be star connected where the terminals of rotor winding are connected to slip rings on the shaft of the motor which are further connected to the brushes which are then connected to variable resistances so that we can vary the resistance in series with the rotor winding.
• Slip rings are made up of phosphorous bronze whereas the brushes are made of carbon.

What are the function of external resistance in a slip ring induction motor?

• By connecting an external resistance we can limit the starting current in the motor.
• By increasing the rotor resistances by means of external we can increase the starting torque.
• By connecting an external resistance we can improve the power factor.
• If external resistance remains connected even after starting the motor then it can be used for speed control.
• Due to more air gap between stator and rotor it has more leakage flux and hence less torque is produced as compare to squirrel cage Induction motor.

Comparison Between SCIM and SRIM