Introduction 3 Phase Auto Transformers

3-phase auto transformers

A transformer is used to increase/decrease or increase voltages in an electric circuit using the principle of electromagnetic induction. This comes useful when the supply voltage or line voltage available to you does not match the input voltage requirements of the device you want to operate.

Regular transformers have two coils – primary and secondary.

The three-phase auto-transformer has a single coil that acts both as the primary and secondary coil. They are used for starting induction motors, in audio systems, in power transmission and distribution, and even in railways. Read further to find out what features you should look for while buying a 3-phase autotransformer.

Which are the options you should consider:-

1. Power Rating

The power rating of a transformer indicates the maximum load it can handle. It is expressed in terms of kVA and can be determined by the following formula:

kVA = (1.73*Volts*Amperes)/1000

The volts and Amps in the formula are the voltage and currently experienced under maximum load conditions. Each device has an operating voltage and current that you can find from its specifications. It’s recommended to choose a transformer with a power rating more than your maximum capacity.

2. Primary Voltage

You should select a transformer whose primary voltage matches the supply voltage or the line voltage available.

3. Secondary Voltage

The secondary voltage should match the input voltage requirements of the load that is connected to the transformer. The main purpose of the transformer is to convert the available supply voltage to this value.

4. Frequency

The transformer can only step up or step down the voltage, and it cannot alter the frequency of the supply voltage. You should check the frequency of the operation of the autotransformer. In Canada, the frequency of the AC supply is usually 60Hz.

5. Winding

Although copper windings are expensive when compared to aluminum windings, they are also more durable and resistant to corrosion. Opting for an auto-transformer with copper windings will ensure that it lasts for a long period with minimum maintenance.

3 Phase Auto Transformer Pros and Cons

Every type of transformer has its own merits and demerits. Sometimes the disadvantages of using a particular type of transformer will make you rule it out. It is important that you are aware of these so that you can make an informed decision.


  • They are smaller and cheaper than traditional transformers.
  • The leakage flux in these transformers is less, and this allows for improved regulation.
  • Copper loss is also less for these transformers, thereby making them more efficient.


  • Since there is no isolation between the primary and secondary windings, it is not safe to be used for stepping down very high voltages to those suitable for a smaller load.
  • Three-phase auto-transformers are not very effective in suppressing harmonic currents and mostly act as a source of ground fault currents.
  • They are not the economical option if the ratio of voltages is greater than 3:1.


An autotransformer is crucial to the operation of different heavy types of machinery. Knowing the maximum power that it is expected to handle and its input and output voltages are important while choosing the transformer you want to purchase.

Elect Power Inc. is among the top 3-phase auto transformer manufacturers in Canada. You can choose from among transformers with different power ratings and voltage requirements from their website.