Electrical Machines - Three-Point Starter

The three-point starter is mainly used for starting shunt and compound motors.

Schematic Diagram of Three Point Starter

The circuit diagram of the three-point starter is shown in the figure. It is called three-point starter because it has three terminals viz. L, Z and A. 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.

The three terminals L, Z and A of the starter are connected to the positive terminal, the shunt field terminal and the armature terminal respectively. 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 in shunt field circuit, which provides protection against the open circuit in the field winding. 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.

Working Principle of Three Point Starter

The working of the three-point starter can be stated 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 energized by the shunt field current.
• If the supply voltage is interrupted or if an open-circuit is occurred in the field circuit, 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 isolated from the supply.

Drawbacks of Three-Point Starter

The three-point starter suffers from a serious drawback for motors with large variation of speed by the adjustment of the field rheostat. As in the 3-point starter, the NVC is connected in series with the shunt field circuit, thus it carries the shunt field current.

While exercising the speed control through the field rheostat, the shunt field current may reduce to such an extent that the NVC may not be able to hold the handle in the ON position during the normal operation of the motor. This may disconnect the motor from the line, which is not desirable.