What is an Autotransformer Starter & Its Working

Squirrel cage type induction motor mainly works with complete line voltage using DOL starter. Here this starter draws a very huge starting current so motor winding can be damaged & also creates a current surge over the power system. Therefore an alternate starting technique is used to trigger 3- phase induction motor known as an autotransformer starter. This can be mainly used for induction motors which are connected in star or delta connection. The autotransformer starter is very famous for the voltage starters which are reduced electromechanically. This kind of starter can decrease the inrush current within the distribution system toward the less level of all the various types of electromechanical starters. Additionally, the taps over the autotransformer allow alteration of motor torque & voltage. It is preferably suitable for activating most of the industrial loads. This article discusses an overview of an autotransformer starter and its working.

What is an Autotransformer Starter?

An Autotransformer starter is used to decrease the applied voltage to a motor throughout its beginning. When the big motors are used for a long time in the circuit, then a voltage drop may occur, so they can lead to damage to the electrical equipment which runs on a similar supply line. We know that both the voltage and current are directly proportional to each other like V∞I. If we reduce the voltage supply, then automatically the current at the beginning will also reduce. So, the voltage can be reduced by using different methods. One technique to reduce the input voltage given to the electric motor is using a transformer.


The same decreased voltage can be given to the electric motor until it gets its regular running speed. Once this motor’s running speed reaches its normal running speed, then the decreased voltage can be replaced through the main full voltage line.

However, it must be kept in mind that the transformer which performs the purpose in such kind of starter is not a usual step down transformer which has special primary & secondary windings. This transformer includes simply a single winding which is utilized for input & output.

Such kind of transformers has several taps for providing outputs. In general, autotransformer tapping can be arranged in between 50% to 80% voltage. So these tapings are mainly used to supply several voltage choices to trigger the motor. The operation of these motors is simple, with less weight & less cost as compared to a normal transformer through two windings.

The starter of this transformer doesn’t need difficult cabling because it is allied to the motor through three terminals simply. While triggering the motor, it is connected through the output of the autotransformer. The main purpose of an autotransformer starter is it decreases the initial current of the motor toward the square of the transformer’s voltage ratio.


Autotransformer Starter Principle

The operation of the autotransformer starter will be like this. The circuit diagram of the autotransformer is shown below, where the starter switches are represented with 1, 2, and 3. To activate the motor, the two switches in the circuit like 1 & 2 are closed. Once the switches are closed then less voltage can be supplied to the motor from the autotransformer.

Autotransformer Starter Circuit
Autotransformer Starter Circuit

This voltage restricts the input current toward the stationary motor to accelerate. The motor’s torque can also be decreased. The electric motor enhances its speed till the torque of the motor as well as the load torque balance to each other & a stable speed is attained.

In this stage, the switch2 is opened & temporarily the motor can be supplied through even less voltage due to the windings of the autotransformer perform like inductors which are connected in series through the motor.

This time is small to separate switch 1 & connect switch 3, which provides the complete voltage toward the motor. Further speed increases and the motor achieves its complete rated speed. At this position, the soft start can be finished & this motor works under complete load. The autotransformer is not required for a long time & is deactivated through opening switch 1. This motor can be supplied directly from the 3-phase network. So, to deactivate the motor, switch 3 is released.

Theory of Autotransformer Starter

The auto-transformer starter can be used with any kind of squirrel cage induction motor, which supplies throughout taps of 3-phase autotransformer starter. In this starter, the motor can be connected directly to the secondary winding of the transformer.

The taps of the auto-transformer discontinue the starting voltage as well as supply to motor in steps of 50% 65% otherwise 80% of a small voltage. This transformer is used for starting purpose and the line current is low always as compared to motor nominal current throughout the start, due to the motor connected toward the secondary winding of the transformer starter throughout acceleration.

If the motor is connected to 50% tap of the transformer, then motor current would be decreased up to 50% of small starting current. However, the line current will be simply 25% of the small starting current. The variation among motor current and line current will be because of the autotransformer within the circuit.

Because of the lower line, the existing autotransformer starter is used to decrease the voltage starter. As the motor starting current is higher as compared to the line current through an autotransformer starter, then the starter will generate more torque for each ampere of line current as compared to any other kind of voltage starter


The advantages of an autotransformer starter include the following.

  • The inrush current can be restricted by using this transformer
  • The starting voltage adjustment can be done through the choice of an appropriate tap on the transformer.
  • The supply current is less as compared to the motor current
  • The usage of excitation current is less
  • This motor is applicable for large motors, where the motor start through direct connection toward the network cannot be possible. The star-delta also cannot be used for large motors, particularly if they are activated through a significant load.
  • This technique is appropriate for long starting phases.
  • This transformer ratio ranges from 65% to 80%
  • For each ampere of current supply, the highest torque will be there
  • While designing this transformer, less copper is required.
  • Its efficiency is high as compared to the conventional one.
  • Voltage regulation is much better.


The disadvantages of autotransformer starter include the following.

  • Less power factor
  • The circuit is fairly complex & involves a comparatively expensive autotransformer.
  • Because of the big size of the device, the Korndorfer starter cannot be added to an accessible machine if the gap is limited.


The applications of autotransformer include the following.

  • It is used to reduce voltage starter in induction motor
  • Used like booster at the finish of long transmission line to pay off for line losses
  • Used as starting device for particular kinds of fluorescent light fixtures
  • Autotransformer is used in electrical devices within the testing labs
  • Used to increase the incoming voltages
  • Used in AC feeders like boosters to enhance the required voltage level.
  • This transformer is used in induction & synchronous motors like a part for triggering purposes.
  • Used to activate squirrel cage & slip-ring induction motors.
  • Used to interconnect systems that operate in threshold voltages.

Thus, this is all about an overview of autotransformer starter. These transformer ratings are low as compared to the usual motor starter ratings for the higher kW motors. The size of this transformer is extremely small, so effective material will decrease the price. The copper, as well as iron losses, will effectively reduce the material less so this transformer has high efficiency as compared to normal isolation type transformers. Here is a question for you, what are the different types of transformers available in the market?