WHAT IS A VARIABLE AUTOTRANSFORMER?

A variable autotransformer is a type of autotransformer that allows continuous adjustment of output voltage via a sliding brush (wiper contact) on the winding. It provides a simple method of continuous adjustment of voltage by the turn of a knob. Its versatility lies in its ability to tap output voltage in almost stepless way from zero to about 117% of input voltage, in steps of one turn.

History dates back to 1933 when General Radio Company offered smooth, distortion-free voltage adjustment for testing and power control, using a single winding with a sliding carbon brush on a toroidal core to vary turns ratio. They introduced it under a trademark “Variac” a short for variable AC. It soon became mainstay in labs and electronics for controlling light, heat, motor speed and much more. The term variac became a generic term over time, though General Radio held the trade mark till 2002.  Automatic Electric, India, introduced Dimmerstat in mid-1940s. Other brand names include Dimmer Dot, Powerstat, Green Dot, etc.

Construction

• Laminated magnetic (CRGO) steel core in toroidal (doughnut) shape, forms the magnetic path.
• Single-layer coil varnished / lacquered wire wound on core, serves both primary and secondary roles. Winding carries extra turns extending beyond supply terminals. This enables the variable point to get about 17% higher voltage than the supply.
• Commutator track is precisely machined along the periphery of wound core.
• Conductive carbon brush makes contact with this track, allowing it bro move and tap winding at any point.
• Input/ Output terminals: AC input is connected to designated AC input side of winding, while wiper in connected to third point for output.
• The assembly is housed in a protective case.

Sometimes the housing case also carries power switch, fuse and voltmeter.

Variacs are usually single phase with input of 230 V, while output may vary from zero to 270V AC. In countries with 110 V supply, they may be rated for 110 V, with maximum output of 135V. It is possible to get variac for 425/440 V rating. Commonly two variacs in series are used for this voltage, where input is connected across two phases, while output is taken across the two wipers.  Variacs may as well mounted on a common frame, with one common knob for tandem operation.

For three-phase applications, it is customary to operate three single variacs in tandem by a single common knob, by mounting them on common operating shaft. This is very useful for three phase loads like motors, heaters, lighting loads etc.

Motorized autotransformers

A single-phase motorized autotransformer, often called a motorized variac, provides continuously variable voltage output from 0 to 270V AC for an input of around 230V in single-phase systems. The motorized version uses an electric motor to automatically adjust the brush or wiper contact along the winding coil, enabling remote or programmed control unlike manual versions. This design suits applications needing precise voltage regulation, such as testing labs, industrial automation, or battery charging setups relevant to electrical engineering.

These devices feature a single winding with a sliding contact for smooth output adjustment, low no-load current (typically under 2% of rated capacity), and rugged construction for continuous heavy-duty use. Motorized models include microswitches, insulated shafts, and high-torque motors for reliability, with current ratings from 2A up to 35A or more. Variacs ratings may be as low as 2 A 230 V single phase to high rated three-phase construction large as few hundred KVA. Two or three phase variacs are made by mounting multiple variacs on a common frame and operated by a common knob.

Drive Mechanism

A servo or DC motor rotates a contact arm gliding across the winding, responding to control circuits for smooth transitions. In three-phase setups, star-connected windings ensure balanced operation, with overload protection halting motion if faults occur. Closed-loop feedback via encoders allows precise positioning for variable voltage profiles. ​

Advantages of motorized operation

• Automated sequencing reduces wear on mechanical contacts and operator errors.
• Programmable logic supports acceleration for high-inertia loads like crushers or pumps.
• Remote control via PLC integrates into SCADA systems for industrial automation.

Common Applications of variac

Variacs excel in lab voltage regulation, motor speed control, dimming, and equipment testing like capacitor or battery charging setups, offering better regulation than switched transformers They are commonly used in manufacturing for smooth power delivery to equipment of enough level, from a fluctuating supply.  In power distribution networks, they help in adjusting voltage on the face of varying loads, thereby improving efficiency.

Used for voltage stabilization in equipment sensitive to fluctuations, such as electronics testing or power supply simulation. In labs, they enable controlled charging/discharging or resonance testing.  Used in automated production lines for stabilizing supply to motors, compressors, CNC machines, or lathes needing step-up/down from line voltage (e.g., 240V to 200V). Remote motor drive supports PLC integration for consistent process control.

Single-phase motorized autotransformers integrate into servo control systems primarily as the variable transformer (variac) component in servo voltage stabilizers (SVS). They are used in closely-controlled motor starters above 50 kW to limit voltage dips, dynamic voltage regulators in substations, and test rigs requiring stepped supply. They excel where reliability demands hands-free operation in harsh environment.