What is Auto transformer Starter – Working Principle, Diagram

Different types of starting techniques and starter circuits are used for starting the induction motors. The method used is selected based on the motor rating. Such as DOLs are used to start the low-rating motors. Its main benefit is that it cannot limit current at the starting condition. If it is used for larger-rated induction motors, it draws a high current, so as a result, the motor winding gets damaged. So the commonly used starting method for induction motors is the autotransformer starter.
This starter circuit can limit or decrease the high current at the starting time of the motor. As the transformer reduces the voltage or steps down, So it controls the voltage provided to the motor. Let’s get started with What is the Autotransformer Starter? Working Principle, Diagram

What is an auto-transform starter?

The Autotransformer starter is best to use for both star and delta connection circuit motors. Here we made a circuit to limit the starting current with the use of three three-phase autotransformers to decrease the voltage applied at the stator.

There are many toppings on the circuit. The starter is connected with one certain tapping to get the best value of the starting voltage for the motor. The double-through switch makes the connection of the autotransformer to perform starting processes.

If the switch is in starting condition, the primary side of the transformer makes a connection with the power supply, and the motor is connected to the autotransformer. When the motor gets to a speed of about 80% of its rated speed, the switch handle moves to run the state. So the autotransformer is disconnected from a circuit, and the motor makes a connection with the line and gets its rated voltage. The handle of the switch stays connected to the run state by a voltage relay.

If the supply voltage is less than a certain point, the handle is released and moved to the off state. Thermal overload relay as protection if needed.

What is an auto-transform starter



Autotransformer starter diagram

The main components of the autotransformer starter are shown here.


An autotransformer is the main component of a starter that is used to decrease voltage at the starting time of the motor. It comes with nine terminals, and out of them, three are main inputs that are connected to the incoming power source through the transformer contactor. Three input terminals are connected with shooting contactors. The remaining 3 are from tapping of windings and connected with the induction motor.

Main Switch

This switch is used to turn on and off the power supply connected to the starter. It can be MCCB or any other type of breaker, and in some cases, fuses are used. The main power supply is connected to the main switch as an input. The output is connected to the input terminal of the starter. If this main switch is off, no power will start, and the connected circuit

Shorting Contactor

It is connected to one side input pin of the autotransformer. Its main work is to make connections between autotransformer windings. This short circuit is called a shorting contactor. If this contactor is off, the coils of the autotransformer will be disconnected, and when the contactor is on transformer windings again, make connections.

Transformer Contactor

It is connected between the main input terminals of the transformer and the power supply of the main switch. If this contactor is closed, the autotransformer is connected to the power supply. If this contactor is open or off, the autotransformer will be disconnected from the input supply. So it is known as a transformer contactor.

Running Contactor

It is configured between the incoming power supply of the main switch and the motor points. It makes the connection between the motor and the input supply. If the motor is running, it is in closed condition, and the motor will operate at full input voltage, known as a running contactor.

Autotransformer Starter Diagram and Parts

Autotransformer Starter Working

Here is the step explained.

  • First of all, the shorting contactor is in a closed state, and the transformer contactor gets on. When this contactor is on, the motor will get a power supply and start operating. This supply voltage based on tapping is configured. Normally, the initial voltage used is 50%, 60%, or 70% of the full voltage value. This shorting contactor makes connection transformer windings in a star configuration.
  • After a set time, the shorting contactor becomes open, and a coil of the transformer works as a series inductor with a motor.
  • When the motor has a stable speed, the contactor gets closed. It is set by the timer. In this condition, the transformer works as a shunt for the motor.
  • The transformer contact is open, and the autotransformer is insulated from the circuit and motor operating at full load voltage. In a running condition, the running contactor will be in the closed state, and other contactors will be in the open state.

Advantages of an autotransformer starter

  • It is a low-cost circuit.
  • Its high power ratio is between 65 and 80 percent.
  • It helps set the starting voltage with the use of a tap on the transformer.
  • It has maximum torque per ampere of supply current.
  • The motor has a higher current than the supply current.
  • It operates for a long start-up duration.
  • The inrush current is decreased with this starter.
  • For motors of high rating, it is used that cannot be started directly.
  • It increases regulation.


  • It is complicated and uses a costly autotransformer.
  • It needed a larger control panel for the transformer, making it costly.
  • Its physical structure is larger, so it is not easy to connect it in a limited space.


  • It is best to use it for reducing voltage at the start of an induction motor.
  • It is used as a booster for the long transmission line.
  • It is used in fluorescent lamp fixtures.
  • It boosts the circuit input voltage.
  • It is used for slip rings and squirrel cage induction motors.

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