Three-phase A.C commutator Motors.
Written by V R Saturday, 26 June 2010 16:04
Such motors have shunt speed characteristics which means change in their speed is only moderate ,as compared to the change in the load.They are ideally suited for drives,requiring a uniform accelerating torque and continuously variable speed characteristics over a wide range.Hence,they find wide use in high-speeds lifts,fans and pumps and in the drives for cement kilns,printing presses,pulverised fuel plants stokers and many textile machines.Being more complicated,they are also more expensive than single speed-motors.Their efficiency is high over the whole speed range and their power factor varies from low value at synchronous speed to unity at maximum speed.
The speed control is obtained by injecting a variable voltage at correct frequency in to the secondary winding of the motor through its commutator.If injected voltage assists the voltage induced in the secondary winding,the speed is increased but if it is in the opposing direction,then motor speed is reduced.The commutator acts as a frequency changer because it converts the supply frequency of the regulating voltage to the slip frequency corresponding to the speed required.
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