What protection does the four point starter offer ? The use of three point starter presents a problem .The speed of the motor is controlled by means of the field rheostat .To increase the speed of the motor it is necessary to set the field resistance to a higher value .The current through the shunt field is reduced and so is the current through the coil of holding magnet . The reduced current through the coil reduces the strength of the magnet. Thus it makes motor susceptible to line voltage variations .In the weakened condition a slight reduction in line voltage would further weaken the holding magnet , releasing the arm of the starter and thus disconnecting the motor from the connection line. As a result unscheduled stoppages of the motor makes the three point starter quite unpopular. The four point starter eliminates the disadvantage of three point starter .In additi...
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