- 4 Point Starter Definition: A 4 point starter is a device used to protect the armature of DC shunt or compound wound motors from high starting currents.
- Diagram Components: The 4 point starter includes four main points: L (line terminal), A (armature terminal), F (field terminal), and N (no voltage coil).
- Working Principle: The 4 point starter operates by independently connecting the no voltage coil across the supply, maintaining consistent performance.
- No Voltage Coil: The NVC ensures the handle stays in the RUN position, using a fixed resistance to control the current.
- Operational Difference: The key difference between a 4 point and 3 point starter is the independent connection of the NVC, ensuring stable operation regardless of field circuit changes.
Working Principle of 4 Point Starter
A 4 point starter protects the armature of a DC shunt motor or compound wound DC motor from high starting currents. Its hold coil is on a fourth supply point, not in the field circuit.
It is built much like a 3 point starter, with one extra terminal and coil. Starting resistance is still cut out of the armature as speed rises. In a 4 point starter the hold coil is taken out of the shunt field and put across the line through a current-limiting resistance in series.
The construction diagram for the operation of 4 point starter shows that extra N terminal.
Construction and Operation of Four Point Starter
A 4 point starter has four terminals:
- ‘L’ Line terminal (Connected to positive of supply.)
- ‘A’ Armature terminal (Connected to the armature winding.)
- ‘F’ Field terminal. (Connected to the field winding.)
- 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 No Voltage Coil (NVC) is fed on its own from terminal N, beside L, F and A. Field current can then be reduced for speed control without starving the NVC. A fixed resistance in that N path sets NVC current so the magnet can still hold the handle on RUN.
The rest of the hardware matches a 3 point starter. A variable resistance is split into sections whose studs are marked OFF, 1, 2, 3, 4, 5, RUN. Moving the handle across those studs cuts starting resistance as the motor speeds up.
When supply is on and the handle is on stud 1, current splits three ways as in the diagram.
- 1 part flows through the starting resistance (R1+ R2 + R3…..) and then to the armature.
- A 2nd part flowing through the field winding F.
- And a 3rd part flowing through the no voltage coil in series with the protective resistance R.
A change in the shunt field therefore does not change NVC current, because the two circuits are separate.
NVC pull on the handle’s soft-iron bar must still beat the spring on RUN, whatever setting the field rheostat has. Supply failure still drops NVC current so the spring can return the handle to OFF.
That separate hold-coil feed is the working difference between a 4 point starter and a 3 point starter. Both still limit starting current in a shunt wound DC motor or compound wound DC motor.
