Variac Transformer

Variable voltage transformers can control motor speed and help with the testing out the voltage of old equipment, among other uses. Whatever you need it for, TEMCo has the guide to help you select a variable voltage transformer.

How does a transformer work?

Transformers alter voltage as it travels through the coils of the transformer, going from one circuit to another.

A standard transformer contains two coils: the separate primary and secondary. A varying magnetic field is created in the core of the transformer by the current in the primary coil. The field then creates voltage when as it is carried over to the secondary coil.

Transformers are often used for giving applications the correct voltage. They can also be used to sustain consistent voltage in applications where the voltage consistently spikes and/or dips.

Transformers regularly use taps, or set connection points along the coil. Taps only allow for a few specific amount turns to be selected, which severely limits of amount of output voltage that a transformer can produce.

Types of Transformers


Unlike the majority of transformers, autotransformers have only one coil, which acts as both the primary and secondary. Autotransformers are smaller, lighter and cheaper than a standard transformers, because they have fewer windings and a smaller core.

Isolation transformers

An isolation transformer has one coil wrapped around another. It buffers the powered device from the power source while energy is being transferred. The buffering provides safety against electric shock hazards and also blocks interference that may arise from ground loops.

Isolation transformers have insulation between the windings that allow it to withstand higher voltages due to its wire being insulated.

What is a variac transformer?

A variac transformer, also known as a variable transformer, is a transformer that has a range of output voltages. They will most often be autotransformers, though they can also be isolation transformers.

Variable transformers have a tap, in the form of a sliding brush, that can go anywhere along the coil, greatly expanding the number of available output voltage numbers.

Autotransformer coils have insulated magnet wire. They will have their insulation removed on the top, so that the sliding brush can be in contact with the exposed wire.

Isolated variacs will only have a brush on the secondary coil, but not on the primary.

Why choose a variac transformer?

A variac transformer can be used to provide greater speed control in an application, since rotations per minute (RPM) are determined by the amount of voltage running through it.

A large variety of applications can use a variac transformer, such as light dimmers, resistive loads, heating elements, lab gear, switchgear. They can also be used to test the voltage capabilities of appliances.

When using old equipment that has not been turned on or been in use for a long time, especially items such as radios and amplifier tubes, it is probably a good idea to use a variacs, as they can be used to slowly raise the voltage. In the case of old equipment, sending too much voltage, too quickly, may prove to be too much for an old piece of equipment. It may result in the failure of old parts that are not easily replaced.

Variac Selection Guide

Here are the specifications you need to know to size a variac for your application.

Constant Current Rating

In most cases, the current and voltage are inverted, so as voltage goes down, amperage goes up. In the case of a variac transformer, when the voltage changes, instead of the amperage changing, the wattage will change instead.

The output wattage will change by the same percentage as the voltage.

The amount of current running through the transformer will depend on the load of the device connected to it. If the variac is used with load that is fixed, maximum current can be drawn. If the transformer is going to be used for continuous 24 hour duty, it is very important that you do not exceed the maximum output amperage.


A variac transformer will be either single phase or three phase.

A single phase motor starter will probably be found in residential areas. Three phase power is used in industrial environments and other places that run heavy equipment. A three phase variac transformer is simply multiple single phase variacs tied together in either a wye or a delta connection.

Input and output voltage

The input voltage comes from your source power, output voltage goes to your equipment.


A variac transformer has a fixed frequency design, so frequency will not change. Common frequencies include 50,60, 50/60, or 400 Hz.

A 50 Hz transformer will be able to work on 60 Hz power. A 60 Hz transformer, however, will not be able to run on 50 Hz power.

Constant impedance rating

Electrical impedance is how much opposition current experiences when voltage is applied.

The ratio of output impedance to input impedance is the same as the ratio of output voltage to input voltage squared. So, if the impedance ratio of a variac transformer is changed, then the voltage ratio will also change by a small amount.

The constant impedance rating is the current that can be carried when the current is proportional to the voltage output.

Ambient Temperature

The variac transformer will have an ambient temperature rating, which means the surrounding area must not exceed a certain temperature.


There are two types of variacs: bench and open frame.

Bench model transformers are protected from contaminants by a shield, with a knob on top of it. They have an an on/off switch, a fuse, one or more AC outlets, a line cord and sometimes a current or voltage meter.

Open frame models are open, and do not need the added protection of the bench transformer since they are meant to be mounted or incorporated into another piece of equipment.

Many variacs will come with a voltmeter, which is used to measure the potential in the electrical difference between points in a circuit. An ammeter measures the current in a circuit.

Bench Model Variac Transformer

External Devices

Variacs can come with additional instruments, including an on/off switch or an input plug, which would be found on a portable variac.

TEMCo's variacs have an input plug, an on/off switch, a voltmeter, and a 5 amp fuse.

Variac Transformer
Variac Models and Pricing

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