INDUCTION MOTOR:

INDUCTION MOTOR:WOUND ROTOR MOTOR

A wound rotor induction motor has a stator like the squirrel cage induction motor, but a rotor with insulated windings brought out via slip rings and brushes. However, no power is applied to the slip rings. Their sole purpose is to allow resistance to be placed in series with the rotor windings while starting. This resistance is shorted out once the motor is started to make the rotor look electrically like the squirrel cage counterpart.

Why put resistance in series with the rotor?Squirrel cage induction motors draw 500% to over 1000% of full load current during starting.This is not a severe problem for small motors, but it is for large motors.

Placing resistance in series with the rotor windings not only decreases start current, locked rotor current but also increases the starting torque, locked rotor torque.Wound rotors are used when variable speed is required.Carbon brushes connect the slip rings to an external controller such as a variable resistor that allows changing the motors slip rate.Wound rotors are used in applications requiring high starting torque such as:CrancesElevatorsHoistsEtcVIDEOPHASE WOUND INDUCTION MOTORSQUIRREL CAGE INDUCTION MOTORConstruction is complicated due to presence of slip ring and brushesConstruction is very simpleThe rotor winding is similar to the stator windingThe rotor consists of rotor bars which are permanently shorted with the help of end ringsRotorresistancecan be added by using slip ring and brushesSince the rotor bars are permanently shorted, its not possible to add external resistanceDue to presence of externalresistancehigh starting torque can be obtainedStaring torque is low and cannot be improvedSlip ring and brushes are presentNo Slip ring and brushesFrequent maintenance is required due to presence of brushesLess maintenance is requiredMore expensiveCheaperRarely usedWidely usedRotor copper losses are high and hence less efficiencyLess rotor copper losses and hence high efficiencySpeed control by rotor resistance method is possibleSpeed control by rotor resistance method is not possibleUsed where high starting torque is required i.e in hoists, cranes, elevator etcUsed in lathes, drilling machine, fan, blower printing machines etc