What is an Autotransformer

Autotransformers are a form of transformer where only one winding is used and having taps on the winding, it is able to provide voltage transformation.

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Most transformers have two or more windings, but it is also possible to create a transformer that uses a single winding - an autotransformer.

With an autotransformer, various taps are used on the winding to enable the required step up or step down ratio to be used.

Some autotransformers even have a variable tap on the winding to enable a variable output voltage to be obtained. These variable autotransformers are often known as Variacs, although the actual word 'Variac' is a trade name and the term variable autotransformer is probably more appropriate for general use.

Autotransformer basics

As already mentioned an autotransformer is a form of transformer that has only one winding, which acts as both the primary and secondary and yet it can still transform voltages up and down.

The basic concept of an autotransformer can be seen in the circuit symbol or diagram above.

An autotransformer must have at least three connections - one common one connected to ground or neutral,, one of the input and one of the output. If the input and output were connected to the same tap, then there would be no transformation of the incoming voltage and no point in using the autotransformer.

In the circuit symbol shown, it can be seen that the input is connected to a fixed tap and the output can be connected to any one of a number of taps.

Any taps lower down the coil than the point where the input is connected and any taps used that are higher up the coil will give a higher voltage than the input.

The autotransformer follows the same rules that govern normal transformers in terms of the voltage transformation.

\frac{E_{s}}{E_{p}} = \frac{N_{s}}{N_{p}}

Where:
Ep is the primary side EMF
Es is the secondary side EMF
Np is the number of turns for the input tap
Ns is the number of turns for the output tap

Similarly if the voltage at the output is higher than the input voltage, then it provides less current than that taken at the input.

The input and output power ratings are the same, if any losses in the transformer are ignored. This is generally a fairly safe assumption as losses are relatively low and exact values are seldom needed - although for loss measurements, etc, this will be important.

This the input and output current levels can be calculated:

\frac{I_{s}}{I_{p}} = \frac{N_{p}}{N_{s}}

Where:
Ip is the primary side current
Es is the secondary side current
Np is the number of turns for the input tap
Ns is the number of turns for the output tap

Autotransformer advantages and disadvantages

The autotransformer is an interesting development of the basic transformer format and like any electronic component, it has its advantages and disadvantages.

Autotransformer advantages:

Auto transformers have many advantages and for this reason they are sued in a variety of different applications from power supplies to RF circuits and many more.

Weight: As an autotransformer uses a single winding, and the core can be smaller as it does not have to accommodate two or more windings, an autotransformer can be much lighter in weight than a comparable multi-winding transformer.
Cost: As the autotransformer only has a single winding, and a smaller size, the cost of an autotransformer will be less than that of an equivalent multi-winding transformer.
Flux leakage : As only a single winding is used for the autotransformer the flux linkage is better than that of a multi-winding transformer.
Increased VA rating for given size: Many autotransformers are used for supplying power to various electrical items. Here the VA rating is of great importance. Autotransformers provide a much greater VA rating for a given size of cost when compared to a multi-winding transformer and this can be a significant advantage in many circumstances.

Autotransformer advantages:

Isolation: In view of the fact that a single winding is used there is no isolation between the input and output.
Safety: One of the issues with an autotransformer that is used with high voltages is that if the winding breaks or if the neutral or ground connection becomes disconnected, then the full input voltage can be applied to the output - the transformer then acting as a series inductor.

Like all other electronic components, autotransformers have their advantages and disadvantages. These need to be weighed up and decision made about their use for the particular electrical or electronic design or particular application.

Autotransformer applications

There are many areas where autotransformers can be used both for higher power electrical uses as well as for lower power ones within electronic equipment.

For electrical uses, autotransformers are widely used for transforming voltage levels, often to enable 240V equipment to be used from a 110V power line of vice versa. In these applications the isolation of a full transformer is not needed because the equipment being sued will provide this. Using an autotransformer will save on cost size and it will also be smaller than an equivalent full transformer.

Autotransformers can also be sued within RF circuits for impedance matching. Using a single inductor with the relevant taps will provide a more effective and efficient component than one using two windings as losses can be higher.

Although the applications of autotransformers within RF design may not always be thought of as being an autotransformer, the principle is exactly the same as that used for high power low frequency voltage transformation.

What is a Variac

A variable autotransformer, also known as a Variac is a form or autotransformer where the output can be varied using a manual control knob.

The name Variac is actually a trade name, but the word has come into everyday use to describe a variable autotransformer.

The trade name Variac name was owned by General Radio from 1934, but was taken over by Instrument Service Equipment in 2002.

The variable autotransformer is often wound on a circular or toroidal former and typically has a winding where the wire insulation has been removed on an area to enable a slider (normally with a circular action) to slide along the winding and 'tap-off' the required voltage level.

The output is taken from this conducting sliding brush or slider to the output.

The full input voltage is applied across the input terminals and the output is taken from the tap point.

Often the input voltage is applied across the full winding, but this only allows for output voltage levels less than the input to be generated. If the input is applied to a tap, typically near the top of the coil, then it is possible to provide a step up capability.

There are many uses for variable autotransformers. In one application, they can be used to change the input power line or mains voltage to various forms of electrical or mains / power line powered equipment. They can also be used as light dimmers that do not produce the large quantities of harmonics and electromagnetic interference that are produced by SCR or Thyristor based dimmers.

Variable autotransformers can also be automated to automatically regulate the output voltage and this can be particularly useful for situations where the input voltage may vary.

Autotransformers, both static and variable are ideal for many applications. They find very many uses and they are particularly known fr their use in electrical applications, although the same principle can be sued within many RF designs as well.