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ompensation is one of the main reasons why people would want to change the secondary voltage so quickly. When the incoming line voltage changes, it’s best that the secondary voltage that serves the load remains regulated. This lessens the risk of intense fluctuation or continual voltage hunting conditions. This is why a voltage tolerance limit—from half a volt to a handful of volts—is maintained.

Variable voltage transformers are honestly the best choice for professionals and hobbyists who desire a more versatile option for changing the ratio between primary and secondary coils. They’re widely accessible, easy to operate, and (depending on the model and brand) can be highly intuitive. You can even program variable voltage transformers to automatically adjust to maintain constant or regular voltage output.

What are Variacs?

An “autotransformer” is a transformer that consists of only one coil shared by both the primary and secondary side of the circuit. The term “variable” in a variable autotransformer basically refers to the ratio of the primary windings to the secondary windings—i.e., the ratio of the secondary voltage to the primary voltage.

“Variac” is the generic name for variable autotransformers.

Variacs are arguably the most popular type of variable voltage transformers. They are AC power supplies that are cheaper, smaller, and a lot more portable than dual-winding transformers. They also have a number of useful casual and industrial applications that make them highly sought after.

Variac Structure

Although different types of Variacs will have variations in design, all models follow roughly the same basic structure.

Parts of a Variac/Variable Autotransformer:

• Winding
• Tappings on the winding
• Laminated magnetic core
• Carbon brush (for secondary voltage, rotates)
• Carbon brush (variable tap, moves up and down)

How a Variac Works:

Variable autotransformers have a single partially exposed winding the primary winding wrapped around a laminated magnetic core. There is a carbon brush (also known as the movable wiper) that is positioned in such a way that it can create an electrical connection with the winding. The transformer’s primary connection is made to both ends of the primary winding.

It is secondary connection called the common connection is made to just one end of the winding and is tapped through a movable carbon brush. This carbon brush can rotate or slide along the exposed section of the primary winding. The transformer’s ratio changes as the wiper moves.

Variable voltage transformers are typically designed with several primary windings enough to produce a secondary adjustable voltage that can be tuned from a few volts to fractions of a volt per turn. As long as the carbon brush is always in contact with the primary windings, the secondary voltage can be adjusted.

Common Applications for Variacs

There are a lot of common uses and practical applications for Variacs. As we mentioned earlier, they’re more portable and more cost-effective than regular AC power supplies, which pretty much makes them a top pick for short-term projects and casual hobbies. Variacs can be used to gradually revive previously dormant electronic equipment. They can also be used to regulate servo motors and control the temperature of ovens and heaters.

Voltage adjustment, when used with thermostatic control, provides more uniform heating.

You can also use Variacs to simulate various voltage and line conditions for experiments, or power electrical equipment designed for voltage that isn’t the typical 220V or 440V supplied domestically.