Introduction
The difference between open and closed transition auto transformer starting is the cost of one timer the benefit is that the motor remains connected to the power supply throughout the starting period. That places less strain on the motor and the run contactor and consequently increases their life. It is for that reason the open transition version of auto transformer starting is not considered in this article.
Power Circuit
Control Circuit
Circuit Operation
When the start button is pressed its contacts a3-a4 close, the main contactor coil MC is energised which causes it’s main contacts on terminals 1-2, 3-4 and 5-6 to close, it’s normally open auxiliary contact on terminals 13-14 also closes. The auxiliary contact on terminals 13-14 maintains the supply to the main contactor coil when the start button is released. Simultaneously the star contactor coil SC is energised which causes the main contacts on terminals 1-2, 3-4 and 5-6 to close. Power is connected from the power supply to the motor terminals U, V and W at a reduced voltage, the motor accelerates to a speed determined by the load. At the same time the star contactor timer SCT energises which causes it to start its timing cycle. After a preset time the star contactor timer times out the change-over contacts 15-16 open which de-energises the star contactor SC, the motor is still connected to the power supply but with the auto transformer acting as an in line current limiting choke. The star contactor timer SCT contacts 15-18 close when that timer times out, this energises the run contactor timer RCT after a preset time its contacts 17-18 close which energises the run contactor RC which causes the main contacts on terminals 1-2, 3-4 and 5-6 to close. The motor is now connected to the full line voltage and accelerates 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 of the star SC and run RC contactors are placed into each others coil circuit so that electrically the two contactors cannot be simultaneously energised which would cause up to only half the winding to be connected to the full line voltage which will cause an overload within the auto transformer, this can be seen by examination of the above circuit. It is also normal to mechanically interlock the star and run contactors in addition to the electrical interlock.
There is a variation of the power circuit where the run contactor is connected to the power supply and is in line with the auto transformer, this lowers the current in that contactor to 80% of the line current thus savings could be made in the cost of the contactor, however due the short time withstand and type 2 coordination requirements, which are discussed later in this article, the savings are marginal.