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Working Principle of Four-Point Starter
The four-point starter is used to overcome the drawbacks of the 3-point starter.
Schematic Diagram of Four 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.
Working of the Four-Point Starter
The working of the four-point starter can be described as follows −
- Initially, the DC supply is switched on with handle in the OFF position.
- The handle is now moved to the first stud. When it comes in contact with the first stud, the whole starting resistance is inserted in series with the armature winding and the shunt field winding is directly connected across the DC supply
- As the handle is gradually moved over to the final stud, the starting resistance is cut out from the armature circuit in steps. After reaching to the final stud, the handle is held magnetically by the NVC which is directly energized by the DC supply.
- If the supply voltage is interrupted, the NVC is de-energized and the handle goes back to the OFF position under the pull of the control spring. If the NVC were not used, then in case of failure of supply, the handle would remain in contact with the final stud. When the supply is restored, the motor will be directly connected across the full supply voltage, resulting in an excessive armature current and may damage the motor.
- If the motor is overloaded or if a short circuit is occurred, it will draw a large current from the supply. This excessive current will increase the MMF of the OLC and pull the plunger P, which short-circuits the NVC. Hence, the NVC is de-energised and the handle is pulled to the OFF position by the control spring. Therefore, the motor is automatically disconnected from the supply.
Drawbacks of Four-Point Starter
The only drawback of the four-point starter is that it does not provide protection against the open in the field circuit.
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