Auto Transformer: 7 Important Answers for Beginners

Auto transformers are one of the electrical devices that I rarely met during work. And I realized that I may be not the only to notice this. While I was searching about auto transformer, I found that there are some basic questions about this transformer type. In this article I will answer these questions.

Why is an Auto Transformer Called as Such?

Due to the getting different taps from main and only winding of transformer according to the desired output voltage, it’s called auto transformer, here’s the word “auto” just refers to the single-coil functioning sole, whereas one winding can serve functions at primary and second sides and one end serves as the common; Auto is not referring to any kind of automatic mechanism.

A transformer called auto transformer when they have following characteristics:

  • only one winding that works as secondary and primary winding both.
  • we provide one voltage source on secondary side and can got different voltage levels on output by getting taps from secondary side.
  • it’s some part of winding common with lower and higher voltage side and remain part of winding called series winding.
  • Its primary and secondary winding connected electrically and magnetically with each other.

Difference Between Conventional Transformer and Auto-Transformer

Transformers are devices that transfer electrical energy from one circuit to different circuits by increasing (Stepping up) or decreasing (Stepping down) in voltages as per requirement. The secondary voltage to primary voltage ratio is called the transformation ratio. It can also be described as the ratio between the number of turns on the secondary and primary winding.

  • An auto transformer can be described as a transformer with only one winding wound on a laminated core. The term ‘auto’ comes from a Greek word, meaning that a single coil works alone.
  • The principle of operation of an auto transformer is similar to that of a two winding transformer the only difference is that the single winding part of this transformer operates on both sides of the same winding as the primary and secondary windings.
  • A typical transformer contains two separate windings that are not associated with each other.
  • Auto transformers are lighter, smaller, and cheaper than other transformers, but they do not provide electrical insulation between the two windings.
  • We know that a typical transformer contains two windings that are magnetically connected but electrically isolated, i.e., primary and secondary. However, in an auto transformer, a single winding is used that acts as primary and secondary both and connected with each other electrically and magnetically.
  • The configuration of the auto transformers uses a continuous winding, with taps are drawn at different points that are convenient to load connected at the secondary side.
  • Auto transformer is more efficient; it requires less excitation current when compared to conventional transformers. As compared to a typical transformer, the auto-transformer efficiency is high because the losses in the ohms and the core are small.

What are the Similarities of Conventional Transformer and Auto Transformer?

Both auto and conventional transformers are still transformers, and have some similarities. Let’s find out some of these similarities.

  • Both types of transformers transforms the voltage and current in the form of step-up and step down.
  • Both transformers work on the principle of electromagnetic induction.
  • Copper conductor used for windings of both types.
  • Primary and secondary windings of both transformers are magnetically connected with each other.
  • Both transformers can change voltage and current levels as required by changing number of turns and thickness of wire in secondary line.

Can we Use an Auto Transformer Instead of a Power Transformer?

Yes, we can use an auto transformer instead of the conventional transformer if rated correctly, but it depends on the requirement of the load voltages and power.

If the load requires fixed voltage, then we use conventional transformers, but if the load requires different voltage levels, then auto transformers are the best to convert a fixed AC voltage into a different AC voltage level Because the Auto transformer can be built with multiple tapping.

Another reason to use an auto transformer is that they are cheaper than two winding transformers with the same rating. Low copper and iron usage reduce the cost. The reduced price is still significant, provided that the transformation ratio is maintained.

While using an auto transformer, two following points must be kept in your mind:

  • Loss of isolation is the immediate difference. Grounding will be the same for secondary and primary windings. This is important to remember if your load is sensitive to grounding-related issues.
  • Because the auto-transformer internal works, any winding defect or breaking could potentially cause damage to your load.

Many nuances, including cost, can be added to the equation. However, it depends on the transformer’s function, voltage, power level, risk, and economic analysis. 

Isolation Transformer VS Auto Transformer

Isolating a load from the power source is some times required, may be for safety purposes. In this case, an isolating transformer is used. This transformer is a two winding transformer and provides a galvanic isolation with no no conductive path between load and power source.

But, what is the difference between isolation and auto transformers? I will provide a complete comparison below.

Isolation TransformerAuto-transformer
Transformers having primary (input) and secondary (output) windings isolated from each other are known as isolation transformers.The transformer in which the primary and secondary sides share a common winding is called an auto transformer.
Dielectric insulation isolates the input and output power.The input power and the output power are not electrically isolated. It is connected due to a single winding.
This can be a step-up or step-down transformer, to match the requirements in the electrical system.Voltage can be adjusted according to the load requirement by taking a tap at the winding’s common end.
Its primary and second windings are separated to deter equipment from receiving harmonics or spikes from supply voltages.There is no separation between the primary and secondary windings. Therefore, the protection of the equipment is dependent on the supply devices.
The neutral conductor of the transformer secondary is connected to the safety ground of the electrical system. This eliminates noise and neutral-to-ground voltage. This solves reliability issues for microprocessor-based electronics.The primary and secondary share a common end of winding in which the primary side of neutral must be grounded to get voltage on the secondary side.
Electrically safe to work at the output side.  Failure of the winding insulation in the auto-transformer can result in the application of full input voltage to the output.
Heavy weight and bigger physical size require more windings and a more significant core.It is lighter in weight and smaller in physical size as it requires fewer windings and a smaller core.
Isolation transformer working on the principle of “Induction only”.Auto transformer based on the principle “Conduction and induction.”
Isolation Transformers are less efficient as compare to auto transformer and used where need only fix voltage levels.Auto transformers are more efficient as compare to Isolation Transformer and used where need variable voltage levels.
Auto Transformer vs Isolation Transformer

Why Auto Transformer Used in Substations?

Auto transformers are widely used in grid substations for interconnecting grids at different voltage levels. The auto transformer is used in substation transmission line as a booster that supply the additional voltage drop.

For example, power is generated at a particular station, and it is transmitted through long transmission line, so there is going to be voltage drop associated with that. To compensate this voltage drop, we use auto transformer.

Auto transformers are not use as conventional transformer to do huge step-up and step-down in voltage, it is use for minor variations in voltage levels.

Auto transformers used in substations for adjusting voltage drops that occur in power system  in case of a long transmission line.


Can Auto Transformer Convert AC to DC?

Auto transformer is used to step up or step down the voltage as per requirement; it cannot be converted from Alternate current to Direct current (AC to DC).

In fact all transformers types in any power system are used to transmit and convert electrical energy from primary to secondary in the form of AC voltage. There is no such thing as a transformer converts AC to DC or vise versa. Auto transformer is not an exception, It’s a single winding transformer that takes AC input and provides only AC output, not to convert AC to DC.

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