Answer / udit

3pt starter : No volt coil and field coil are in series .
4 pt starter : No volt coil and field coil are in parallel.

To get speed above rated speed we use field weakening control as speed increases if we reduce flux. So to do it we reduce field current . In 3pt nvc and field are in series so nvc current also reduces hence the electromagnetic force reduces and magnet releases the handle and motor gets turned off. Thus we cannot use 3 pt starter for field weakening control hence we use 4 pt starter .

If there is field failure (field coil open circuited)then speed becomes dangerously high and motor can damage. To avoid this if field C oil is in series with nvc then whenever there is field failure the current in nvc also becomes zero and motor is turned off .hence no damage to motor. This is advantage of 3pt over 4pt.

Nvc is no volt coil

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