Four Point Starter: Diagram and Working Principle

Working Principle of 4 Point Starter

A 4 point starter protects the armature of DC shunt motor or compound wound DC motor from the high starting currents that occur when the motor starts.

The 4 point starter has a lot of constructional and functional similarity to a 3 point starter, but this special device has an additional point and coil in its construction (as the name suggests). This brings about some difference in its functionality, though the basic operational characteristic remains the same. The basic difference in the circuit of a 4 point starter as compared to 3 point starter is that the holding coil is removed from the shunt field current and is connected directly across the line with current limiting resistance in series.

Now to go into the details of the operation of 4 point starter, let’s have a look at its construction diagram. This will help demonstrate the difference between a 4 vs 3 point starter.

Construction and Operation of Four Point Starter

A 4 point starter as the name suggests has 4 main operational points, namely

1. ‘L’ Line terminal (Connected to positive of supply.)
2. ‘A’ Armature terminal (Connected to the armature winding.)
3. ‘F’ Field terminal. (Connected to the field winding.)
4. Like in the case of the 3 point starter, and in addition to it there is,
   A 4th point N (Connected to the No Voltage Coil NVC)

The key difference in a 4 point starter is that the No Voltage Coil (NVC) is connected independently across the supply through the fourth terminal ‘N’, besides the ‘L’, ‘F’, and ‘A’. This means that changes in the field supply current don’t affect the NVC’s performance. The NVC must always produce enough force to keep the handle in the RUN position, against the spring’s force. This current is adjusted using a fixed resistance connected in series with the NVC through the fourth point ‘N’.

Apart from this, the 4 point and 3 point starters are similar in other ways. Both have a variable resistance divided into sections. The contact points of these sections are called studs and are labeled OFF, 1, 2, 3, 4, 5, and RUN. The handle can be manually moved over these studs to regulate the starting current as the motor speeds up.

To understand how it operates, let’s look at the diagram. When the supply is connected and the handle is moved to stud No. 1, the circuit is complete and current flows through the starter. The current splits into three paths.

1. 1 part flows through the starting resistance (R₁+ R₂ + R₃…..) and then to the armature.
2. A 2^nd part flowing through the field winding F.
3. And a 3^rd part flowing through the no voltage coil in series with the protective resistance R.

So the point to be noted here is that with this particular arrangement any change in the shunt field circuit does not bring about any change in the no voltage coil as the two circuits are independent of each other.

This essentially means that the electromagnet pull subjected upon the soft iron bar of the handle by the no voltage coil at all points of time should be high enough to keep the handle at its RUN position, or rather prevent the spring force from restoring the handle at its original OFF position, irrespective of how the field rheostat is adjusted.

This marks the operational difference between a 4 point starter and a 3 point starter. As otherwise both are almost similar and are used for limiting the starting current to a shunt wound DC motor or compound wound DC motor, and thus act as a protective device.