Introduction
The drawing below shows a two stage rotor resistance starter for clarity and simplicity, additional stages can be added as necessary to suit the characteristics of the motor and it’s driven plant and the limitations of the power supply.
Power Circuit
Control Circuit
Circuit Operation
The contact on terminals a3-a4 of each start push button energises the required direction contactor, either the forward main contactor FMC or the reverse main contactor RMC, the relevant main contacts 1-2, 3-4 and 5-6 close which puts power; with either clockwise or anticlockwise phase rotation; to the motor terminals, normally open auxiliary contacts of the relevant direction contactor on terminals 13-14 close to maintain the power to that direction contactor when the button is released.
When either the forward or reverse start button is pressed it’s contacts a3-a4 close, the relevant main contactor coil FMC or RMC is energised which causes the main contacts on terminals 1-2, 3-4 and 5-6, its auxiliary contacts on terminals 13-14 and 43-44 to close and auxiliary contact 21-22 to open. Power is connected from the power supply to the motor terminals U, V and W and the motor starts to accelerate in the selected direction. The auxiliary contact on terminals 13-14 maintains the supply to the main contactor coil. At the same time the main contactors second normally open contact on terminals 43-44 closes and the resistance stage 1 timer R1T energises which starts its timing cycle. After a preset time the resistance stage 1 timer R1T times out the normally open contact 17-18 closes this energises resistance stage 1 contactor R1C causing its main contacts 1-2, 3-4 and 5-6 to close which short out resistance stage 1. This reduces the rotor resistance and the motor continues to accelerate. At the same as R1C energises resistance stage 2 timer R2T energises and starts its timing cycle. After a preset time the resistance stage 2 timer R2T times out the normally open contact 17-18 closes this energises resistance stage 2 contactor R2C causing its main contacts 1-2, 3-4 and 5-6 to close which short out resistance stage 2. This shorts out the external rotor resistance and the motor continues to accelerate to full speed.
To stop the motor the stop button is pressed its contacts a1-a2 open which de-energises the energised contactors and timers thus stopping the motor by removing its power supply and resetting the control circuit.
The auxiliary normally closed contacts on terminals 21-22 of the forward FMC and reverse RMC contactors are placed into each others coil circuit so that electrically the two contactors cannot be simultaneously energised which would cause a short circuit, this can be seen by examination of the above circuit. It is normal to mechanically interlock the forward and reverse contactors in addition to the electrical interlock. It can also be seen from the above circuit that it is not possible to change the direction without first pressing the stop button to de-energise the energised contactor.