Technical Article

Motor Starters Part 10: Autotransformers

January 30, 2023 by Simon Mugo

Motor starters like autotransformers can solve induction motor challenges. Here’s all you need to know about the workings, connections, and applications of autotransformers.

This is the final article in this series. To catch up on the other articles in Simon Mugo’s series, please follow these links:


Some induction motors, such as the squirrel cage, use the Direct On-Line motor starter, which is prone to drawing too much starting current and can destroy the motor windings or create high voltage and current surges. To solve these problems, engineers have used other motor-starting techniques to start three-phase induction motors


An autotransformer. Image used courtesy of Adobe Stock


Such techniques involve using the autotransformer starter, which is suitable for induction motors connected in delta or star connection. The starter has become popular for voltage starters that can be reduced electromechanically due to its ability to reduce the inrush current that can occur in power distribution systems to safe levels.

The autotransformer taps are good for altering the motor voltage and torque. In most industrial load activation, autotransformers are the most suitable type of starters.


What Is an Autotransformer?

In the motor starting connection, an autotransformer is deployed in the connection to reduce the voltage applied to the motor during starting. Remember, the current and voltage supplied to the motor are directly proportional, and hence reduction in the voltage means a reduction in the current. Therefore, many techniques can be used to reduce the voltage input to the electric induction motor. Deploying a transformer is one such method. 

The reduced voltage is supplied to the motor until it achieves the maximum expected running speed. Once the maximum speed is achieved, the reduced voltage is reconnected to the motor using the full voltage line from the main. This function is not achieved using the normal step-down transformer composed of special secondary and primary copper windings. The transformer in use has a single copper winding which serves as both the input and the output.

An autotransformer has a significant number of tapings that serve as outputs. Generally, these output tapings are connected between a minimum of 50% to a maximum of 80% voltage. The tapings offer different levels of voltages used for motor triggering. Its operation is simple and economical, with low weight density compared to other types of transformers. An autotransformer starter does require difficult cabling because it has three terminals used to connect to the motor.

The main purpose of the autotransformer motor starter is to reduce the initial starting current of the electric motor to the voltage ratio of the transformer’s square.


The Starting Principle of the Autotransformer


Figure 1. Circuit diagram of an autotransformer starter. Image used courtesy of Simon Mugo


Above is the autotransformer starter circuit diagram. The circuit has switches 1, 2, and 3. For the motor to be activated, witches 1 and 2 should be in closed mode. On closing switches 1 and 2, the motor is supplied with less voltage through the autotransformer. The voltage serves as a limit of the input current that gets to the motor at a stationary position. The action also decreases the torque of the motor. The electric motor gains its speed gradually until it gets to a point where the motor torque and the load torque are at equilibrium and the motor has achieved a stable rotating speed.

At such a point, switch 2 goes open, and the electric motor is now temporarily with less voltage because the autotransformer windings behave like an inductor connected in series with the electric motor. Here switch 1 and 3 are connected to provide full voltage to the electric motor. The speed of the electric motor further increases to the maximum rated speed. At the position, soft starting is completed, and the motor attains full load. Now the purpose of the autotransformer is not needed for a long time, and hence it is disconnected through switch 1. The motor is now supplied with voltage directly through the connected 3-phase voltage. To stop the motor, you have to release the designed switch 3.


Advantages and Disadvantages of Autotransformer Starters


  • The advantages of using the autotransformer starter are as follows.
  • The transformer helps in limiting the inrush currents
  • There is the possibility of adjusting the starting voltage because the autotransformer provides different and appropriate tapings to meet your design requirements.
  • The autotransformers utilize fewer excitation currents.
  • These types of starters have fewer supply currents as compared to the current of the motor.
  • The technique has solved the problem of long starting phases. It has been deemed the most suitable for such purposes.
  • For every supplied current, there will be the attainment of the highest torque.
  • Such types of transformers will be cheaper to make because less copper is utilized in their construction.
  • The autotransformer starters have higher efficiency when compared to conventional starters.
  • They offer a much better voltage regulation function.



  • Below are the limitations of the autotransformer starters.
  • The autotransformer offers a very low power factor
  • The autotransformer circuit is somehow complex and fairly expensive because of the involvement of the autotransformer.
  • The starter is somehow bigger and, therefore, cannot be used in machines with limited access.


Applications of Autotransformers

  • A good device for reducing voltages in the induction motors starters
  • Nice gadget to be used as a booster at the long transmission lines finish enabling compensation of high losses experienced.
  • Applicable in specific types of fluorescent lamp fixtures.
  • Some electrical devices, especially those in testing labs, utilize autotransformers to perform their functions well.
  • They are used in boosting the circuit incoming voltages
  • They are also used in the area of AC feeders as boosters to enable the system to achieve the required level of voltage
  • Slip ring and squirrel cage induction elect motors activation
  • Employed in the interconnect systems to operate in the required threshold voltages.


Key Takeaways of Autotransformer Motor Starters

  • Direct-On-Line motor starters drain too many currents and voltages and hence are not best-suited for starting squirrel gauge motors, which has led to increased use of autotransformer starters in starting induction electric motors.
  • An autotransformer is deployed to reduce the voltage of the starting motor to safe amounts. A voltage reduction means a reduction of the current since they are directly proportional.
  • An autotransformer has several numbers of voltage tapings with voltage ranging between 50% to 80%, which gives a different amount of output voltage to be supplied to the motor.
  • The starter circuit has three switches labeled 1, 2, and 3, which control when the motor should operate under limited or full voltages.
  • The advantages of the autotransformer starter range from limiting inrush currents, adjusting starting voltage, fewer excitation currents, much better voltage regulation, and many more.
  • The disadvantages include low power factor, higher cost, and space limitations due to its larger size.
  • The autotransformer finds much use, such as slip-ring and squirrel cage induction motor starting, lighting some fluorescent lights, AC feeders, circuit voltage boosters, transmission line boosters, and many more.