Construction and Working of Three Point starter

Construction and Working of Three point starter Points : Construction and Working of Three point starter Construction The three point starter has a tapped resistor enclosed in a box. Contact buttons located on a panel mounted on the front of the box are connected to the tapped resistor. A movable arm with a spring reset can be moved over the contact buttons to cut out sections of the tapped resistor. Working To operate a three terminal starter, first close the line switch. Then, we move the starting arm from the OFF position to contact 1. Continue to move the arm slowly toward contact 6, pausing on each intermediate contact for a period of 1 to 2 seconds.

Do not hold the arm on any one contact between 1 and 6 for too long a period of time. The starting resistors are designed to carry the staring current for a short period of time only. When the arm of rheostat is moved to the first contact 1, the armature is connected across the source. The shunt field, in series with the holding coil, is also connected across the source. Because the total resistance is connected in series, of the armature, therefore the shunt field current is at a maximum value, and provides a good starting torque. As the arm is moved to the right toward contact 6, the starting resistance is reduced and the motor run to its rated speed. At contact 6, the armature is connected directly across the source voltage.

At contact 6, the holding coil caught the moving arm. The starting resistance is in series with the shunt field when the arm is in the run position at contact 6. This additional resistance has practically no effect on the speed, because the starting resistance is small compared to shunt field resistance. If it is necessary to control the speed of the motor, do not use a three point starter. Because when we change the value of variable resister the current in the shunt field and holding coil may be reduced to a value insufficient to hold the arm against the action of the spring, the spring will return the arm to the OFF position. Thus, the motor will become disconnected from the supply.