Four point starter:
In four point starters, the hold on coil is connected directly across the supply line through a protective resistance R. when the armature touches stud no 1.the line current divides into three parts i) armature starting resistance and overload release. ii) a variable resistance and shunt field winding. Iii) Holding coil and current limiting resistance.
The basic difference between three point and four point starters is the manner in which the hold on coil is connected. The unnecessary tripping of starter can be stopped by connecting separately or parallel both magnetizing and field coil. They are connected in such a way that both should carry their individual current. Thus voltage drop in one coil will not affect the voltage in other coil.
Disadvantages of four coil starter:
The only limitation of the four point starter is that it does not provide high speed protection to the motor. If under running condition field gets opened; the field current reduces to zero. As there is some residual flux present and speed (N) is directly proportional to flux (ø) the motor will tries to run with dangerously high speed .this is called high speed action of motor. In three point starter as no volt coil is directly connected to across the supply; its current is maintained irrespective of the current through the field winding .hence it always maintain the handle in run position as long as supply is there .and thus it doesn’t protect the motor from field failure conditions which returns into high speeding of the motor.
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