Variac Transformer


Variable-voltage transformers are efficient means of changing voltage, especially if you want a greater degree of flexibility in changing the ratio between the primary and secondary coils. They are simple to operate, and controls are available for automatic adjustment to maintain “constant” (regulated) voltage output. The amount of voltage change obtained depends upon the type of variable transformer used.

What is Variac/Autotransformer?

The Variac/Autotransformer has the usual magnetic core but has only one winding, which is common to both the primary and secondary circuits.The single winding is “tapped” at various points along its length to provide a percentage of the primary voltage supply across its secondary load. Therefore,in an autotransformer the primary and secondary windings are linked together both electrically and magnetically.

The section of winding designated as the primary part of the winding is connected to the AC power source with the secondary being part of this primary winding. If the primary is the total winding and is connected to a supply, and the secondary circuit is connected across only a portion of the winding, then the secondary voltage is “stepped-down” as shown in fig(1).

An autotransformer is used mainly for the adjustments of line voltages to either change its value or to keep it constant.

Autotransformer Design

  • When the Primary current(IP) is flowing through the single winding in the direction of the arrow as shown, the Secondary current (IS) flows in the opposite direction.
  • Therefore, in the portion of the winding that generates the secondary voltage (VS) the current flowing out of the winding is the difference ofIP and IS.

“The Autotransformer can also be constructed with more than one single tapping point.”

Autotransformer with Multiple Tapping Points


Standard method for marking an auto-transformer winding

The standard method for marking an auto-transformer winding is to label it with capital (upper case) letters. For example, A, B, Z etc. to identify the supply end.

Generally, the common neutral connection is marked as N or n. For the secondary tapping’s, suffix numbers are used for all tapping points along the auto-transformers primary winding. These numbers generally start at number 1 and continue in ascending order for all tapping points as shown in fig(3).

Autotransformer Terminal Markings


If the voltage adjustment is by a small amount, either up or down, then the transformer ratio is small as VP and VS are nearly equal. Currents IPand IS are also nearly equal.Therefore, the portion of the winding which carries the difference between the two currents can be made from a much smaller conductor size, since the currents are much smaller saving on the cost of an equivalent double wound transformer.

Note: – However, the regulation, leakage inductance and physical size (since there is no second winding) of an autotransformer for a given VA or KVA rating are less than for a double wound transformer.

Comparison of Autotransformer with Two winding Transformer

Autotransformers are clearly much cheaper than conventional double wound transformers of the same VA rating. When deciding upon using an autotransformer it is usual to compare its cost with that of an equivalent double wound type.

This is done by comparing the amount of copper saved in the winding. If the ratio “n” is defined as the ratio of the lower voltage to the higher voltage, then it can be shown that the saving in copper is: n.100%.

For example, the saving in copper for the two autotransformers would be:


Main Advantage and Disadvantage

-The main advantage of this type of transformer design is that it can be made a lot cheaper for the same VA rating.

-The biggest disadvantage of an autotransformer is that it does not have the primary/secondary winding isolation of a conventional double wound transformer.


-An autotransformer can also be used to step the supply voltage up or down by reversing the connections.

-Auto-transformers can be used to provide different voltage points along its winding or increase its supply voltage with respect to its supply voltage VP as shown in fig.

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