The so-called compensating key is made up of an autotransformer with different derivations, designed to suit the starting process. This autotransformer is activated to the stator circuit, where the star point of the same becomes accessible and at the moment of starting is short circuited and this connection ends as soon as the motor is connected directly to the network.
First, this type of starting is used in electric motors of greater power, applying loads with high friction index, as well as crushers, machines driven by belts, calenders and the like. The advantage is that at 65% bypass, the starting current in the line can approach the value of the drive current using a switch.
In the star-triangle, the switching of the reduced voltage branch to the supply voltage does not cause the current rise, since the autotransformer behaves in the same way to a reactance that prevents the increase of the same. Browning 2BK80H
In a simple way, in the case of a compensating switch designed from three autotransformers, the leads often found in compensator switch autotransformers are on average 50%, 65% and 80%. The star-triangle keys may be to enumerate some advantages and disadvantages of the compensator key.