When is an auto-transformer starter used?

When more flexibility is required for starting a squirrel-cage motor than is provided by the star-delta method, which is limited as far as starting torque is concerned. Auto-transformer starting permits the stator to be wound for running in star. The starting torque can be adjusted to suit the load by changing the voltage tapping on the auto-transformer. Both starting torque and current are reduced in the same proportion.

It is used for motors of medium and large size on light starting loads (e.g. centrifugal pumps, fans, compressors and mills). Up to about 75kW the simple auto-transformer starter is employed; above this, the Korndorfer connection is recommended.