Auto Transformer Starter working principle

Auto transformer starter is a type of motor starter that is commonly used in industrial applications. They are designed to reduce the voltage applied to a motor during startup, which helps to limit the amount of current that is drawn from the power supply. In this article, we will discuss the working principle, advantages, and applications of the autotransformer starter.

This can help to prevent damage to the motor and other components in the system, and can also help to reduce energy costs by minimizing the amount of power that is consumed during startup.

There are various starters used to start induction motor like Direct online starter, Soft starters, star delta starter, auto transformer starter, and autotransformer starter is used for large HP motor.

Auto Transformer Starter Diagram


Auto Transformer Starter Working

The working principle of the auto transformer starter is similar to the star-delta starter method. The starting current is limited by (using a three-phase autotransformer) reducing the initial stator applied voltage.

Auto transformer starters work by stepping down the voltage that is supplied to the motor during startup.

The transformer has multiple taps that can be used to adjust the amount of voltage that is applied to the motor, allowing for precise control over the startup process.

Once the motor has reached its operating speed, the transformer is bypassed and the full voltage is applied to the motor.

When the motor is started, the auto transformer is initially set to provide a reduced voltage to the motor.

This reduces the starting current and torque required by the motor, which helps to prevent damage to the motor and other equipment.

As the motor accelerates, the tap on the auto transformer is gradually moved to increase the voltage supplied to the motor until it reaches its full operating voltage.

Auto transformer starters are typically used for motors with high starting currents, such as those used in large industrial applications.

They are also commonly used in situations where there are limitations on the available power supply, such as in remote locations or areas with limited electrical infrastructure.

The auto transformer starter is more expensive, more complicated in operation, and bulkier in construction when compared with the star–delta starter method.

But an auto transformer starter is suitable for both star and delta-connected motors, and the starting current and torque can be adjusted to the desired value by taking the correct
tapping from the autotransformer.

When the star delta method is considered, voltage can be adjusted only by
factor of . 1/√3

An autotransformer starter reduces inrush current by using a transformer in the line just ahead of the motor to step down the voltage applied to the motor terminals.

By reducing the voltage, the current drawn from the line is reduced during start-up. When the set time on the timer has expired, the autotransformer is bypassed.

The auto transformer starter is a closed transition type, meaning that the motor remains connected to the line during the entire acceleration period.

The transformer has three taps which provide 50%, 65% and 80% of full line voltage.

At delivery the transformer is connected to the 65% tap the inrush current will be reduced to 42% of normal; and the starting torque will be reduced to 42%.

The autotransformer starter can be used for any squirrel-cage motor.

Advantages Auto transformer starter

  • The autotransformer starter limits significantly the inrush current.
  • It is used for large motors, which start by direct connection to the network is not possible. For large motors also the star-delta starter cannot be used, especially if they are started with a significant load.
  • The circuit has an advantage over starting with a regular autotransformer, which needs to be at some point completely disconnected during the start inducing high voltage impulses,which can damage the electrical insulation of the stator.
  • The most effective ratio of the autotransformer is between 65-80%.


  • The circuit is quite complex and involves a relatively expensive autotransformer.
  • Due to the physical size of the whole device, it might not be possible to add the Korndorfer starter to an existing machine if space is scarce.


Autotransformer starters are a cost-effective solution for controlling the starting current and voltage of three-phase induction motors. They are easy to operate and can help to prolong the life of the motor by reducing stress on the motor windings.

Autotransformer starters are commonly used in industries such as mining, manufacturing, and transportation, where large motors are frequently used. Overall, the autotransformer starter is an effective and efficient way to start and control large motors.

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