Four point starter is connected to a long-shunt compound motor. If we compare it to the generally used 3 point starter, we can easily mark one remarkable difference than its counterpart. In four point starter, the most critical ‘Hold ON coil’ (electromagnet) is placed out of the shunt field circuit. Rather to its place in three point starter, this Hold On coil is directly connected across the line. A resistance is connected in series with the hold On coil for protection.
As soon as the starting handle comes in contact to stud 1, the line current is divided into three sub sections.
First section flows through external resistance Rs, armature of motor and the field in series
Second section flows through the shunt field
Third section flows through the Hold ON coil and current resistance.
With such type of configuration, the current passing through the electromagnet (Hold ON coil) is completely unaffected by any change of current in the shunt field circuit. This happens due to the independency of both the circuits from each other. This means that the functioning of electromagnet will not be affected by the regulator (or field rheostat) adjustments. In any case, the Hold ON coil will be able to exert sufficient electromagnetic pull to the starting handle and hence the starting handle will always remain in ON position except when the circuit gets open.
Disadvantage of four point starter:
The four point starter does not have a lot of disadvantages. However, as i have experienced, The four point starter has one major drawback i.e. it does not offer high speed protection to the motor. If the field opens while the motor is in running stage, the field current will decrease to zero. It should be noted that there is always a little amount of residual flux available in the field winding. We also know that the speed of the motor is directly proportional to flux available. Hence, the motor will attempt to run with extremely high speed .