The four-point starter is used to overcome the drawbacks of the 3-point starter.
The circuit diagram of the four-point starter is shown in the figure. It consists of a graded starting resistance to limit the starting current and is connected in series with the armature of the motor. The tapping points of the starting resistance are taken out to a number of studs. It is called 4-point starter because it has 4 terminals viz. L, N, F and A.
The one end of the armature coil is connected to the terminal A and of the shunt field winding to the terminal F. The other ends of the armature and the shunt field windings are directly connected to the negative terminal of the supply.
The no-volt trip coil (NVC) is connected directly across the line with a current limiting resistor R in series. The NVC is also known as under-voltage protection of the motor. One end of the handle is connected to the terminal L through the overload trip coil (OLC) and the other end of the handle moves against the force of control spring and makes contact with each stud during the starting period of operation. The starting resistance is cutting out gradually as the handle passes over each stud in clockwise direction.
Therefore, with this arrangement, a change in the field current for the variation of the speed of the motor does not affect the current through the NVC, as the two circuits are independent of each other.
The working of the four-point starter can be described as follows −
The only drawback of the four-point starter is that it does not provide protection against the open in the field circuit.