Electrical Engineering ⇒ Topic : Three-point starter

Three-point starter. Refer Fig. (a) The starter has three terminals L,'F and A. The line terminal L must be connected to either side, positive or negative of the D.C. source on the main switch; the field terminal F is connected to one field terminal on the motor; the armature terminal A must be connected to either one of the motor armature terminals. The final connection must then be made from the second line terminal on the main switch to a junction of the remaining two armature and field terminals of the motor. If it is desired that the speed of the motor is controlled, a field rheostat is added as shown in Fig. (b).

   When the motor is at rest, the starter arm [represented by an arrow in.Fig.(b)] is held in the OFF position by a strong spiral spring.

                       Fig. (a) Three-point starter connected to a shunt motor

Drawback of a three-point starter. The use of a three-point starter presents a problem.

The speed of the motor is controlled by means of the field rheostat. To increase the speed of motor necessitates the setting of the field rheostat to a higher resistance value. The current through the shunt field is reduced, and so is the current through the coil of the holding electromagnet.The reduced current through the coil weakens the strength of the magnet and makes susceptible to line-voltage variations. In the weakened condition as Light reduction in line voltage would further weaken the holding magnet, releasing the arm of the starter and thus disconnecting the motor from the line. Unscheduled stoppages of the motor make the three-point starter quite unpopular