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what is the difference between four point starter and three point starter?

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Answer :

The only difference is in the NO VOLT COIL connection in the starter, in three point starter NVC is connected to the field whereas in four point starter it is connected separately. The advantage of using 4 point starter is that it can work well in field controlled motor as well.

If we try to perform field control of DC shunt motor using a 3 point starter, the field current might get very low(for above rated speed) and lead to DEENERGISE the NVC leading to turning off of the motor.
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Answer :

The shunt connection in four point starter is provided separately form the line where as in three point starter it is connected with line which is the drawback in three point starter.
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