When power is first applied to an electric motor, the lack of motion by the armature in the magnetic field results in the supply seeing only the D.C. resistance of the windings as a load. This results in large currents being drawn until the motor has built up speed to its normal running condition. This high starting current is typically 6 to 8 times the normal operating current and usually lasts for about 10 to 15 seconds depending on the size of the motor. This extended duration for the high starting currents can cause problems in specifying suitable protection devices (fuses and MCCB’s) that will not trip during starting but that will still trip under a genuine fault condition. The high starting currents can also have implications for the size of the conductors in the supply distribution gear.
A means of reducing these staring currents is to use a 'Korndoffer' motor start transformer.
This transformer is an autotransformer that is switched into circuit when starting and switched out of circuit once the motor is up to its running speed. The autotransformer is a multi-tapped output transformer offering a range of output voltages typically from 50% to 85% of the nominal supply voltage. Since the motor presents a load equivalent to the DC coil resistance when starting, the reduction in voltage applied to the motor provides a direct reduction in the starting current required by the tap ratio. Also, the action of the autotransformer will reduce the load current drawn from the supply.
The diagram shows the typical circuit for 'Korndoffer' transformer operation.
Initially the contactors C2 and C3 are closed, C1 is open and the main input isolator is open. C3 being closed forms the star point of the autotransformer. When the main input isolator is closed the supply energises the transformer and a percentage of the supply voltage is applied to the motor. Once the motor has reached its operating speed the contactor C3 is opened. The autotransformer stops acting as a transformer and the main supply voltage is applied to the motor through the overwind part of the coils which act as small inductors. Finally the contactors C1 and C2 are switched simultaneously disconnecting the autotransformer from the circuit and placing the motor directly onto the main supply.
In many cases 'Korndoffer' transformers have been superseded by electronic motor speed controllers that also offer a soft start function that restricts the starting current. However in applications where variation in motor speed is not required the 'Korndoffer' can still offer a competitive solution for simple motor starting applications.
The size of the 'Korndoffer' transformer is determined by the size of the motor and the number of times it is required to be started each hour.

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