What is Auto Transformer Starter?

auto transformer starter

Auto Transformer Starter (ATS) are starting devices, for large induction motors, using reduced voltage initially, where availability of current is limited and minimum starting torque is required.

The reduced voltage applied results in lower starting current and higher torque.

Advantages of Auto Transformer starters

  • More Efficient
  • Smaller Size
  • Better Voltage Regulation
  • Lower Cost
  • Low Excitation Current
  • Optimum Design and Construction

Auto-transformer Starter – How it Works

The reduced voltage is applied to the star contactor while starting the motor. The motor accelerates for a preset time of 8 to 12 seconds, limiting input current the star contactor is opened and even lower current is applied momentarily by the auto transformer using the winding as inductors connected in series with motor. The time is short just enough to disconnect star contractor and engage main contactor to supply full voltage in order to achieve full speed simultaneously opening the run contactor and disengage the auto transformer.


  • Pumps Submersible pumps
  • Mixers
  • HVAC
  • Blowers/Fans
  • Extruders & grinders
  • Crushers
  • Conveyors

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