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when the rheostat kept in minimum to maximum position in
motors what will happen?
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Answer Posted / bhavani
In dc motor v=Eb-IaRa is the voltage equation.at starting Eb=0 so V=IaRa and the armature resistance is very small nearly 0.01ohm.so Ia increases ,to avoid this problem we kept 3-point starter.in motors field resistance is kept in minimum in order to get high starting torque.if v increases speed also increase and flux in field rheostat and field current are decreases. at the instant of starting to control the speed not to go beyond its rated value.that's why we kept field resistance in minimum.If we kept in maximum,R increases,I decreases,N increases so the motor will exceed its rated speed,then motor will damaged.or to avoid this probiem,we can control the speed of motor ,when armature rheostat is in maximum,voltage applied is minimum,so N decreases.so we will get rated speed by controlling any one of this.either by incresing applied voltage to armature or by decreasing the current in field winding.THANK YOU
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