The variable voltage transformer thrives on trouble. When the value of the voltage on the AC (alternating current) line is incorrect, just a turn of the dial on the variable voltage transformer operates a sliding brush that makes it right. The same dial can act like a regulator to control the speed of motors; cutting input voltage reduces the motor speed.

Versatility of the variable voltage transformer lies in its ability to tap in anywhere from 0 to 100% of the line voltage. Depending on the frame size, the output current ranges from 1 to 50 amps at 120 volts or half this amperage at 240 volts. Even more versatility can be found in larger units. Some single coil variable transformers are available with output ratings as low as 0.13 kVA and three- phase “ganged” units can reach 405 kVA.

The most common use for variable transformers is to obtain AC voltages that are different from the normal 120 or 240 volts, single or three- phase supply. An additional use of this transformer is to deliver either standard or test voltages from lines where voltages fluctuate above or below the normal standard.

Solid state voltage controls “chop” the input voltage using a semiconductor switch called a triac. The triac connects the input to the load for a variable portion of each AC half-life. The devices are smaller per controlled kVA but produce significant current harmonics because of the lack of sinusoidal voltage applied to the load. By nature of the time chopping control method, solid state controls cannot serve constant current loads as a variable transformer can. In addition, triacs do not have the same overload capability as variable transformers of equal load ratings and may have to be significantly derated for loads with short term overloads or inrush currents.

When a rheostat reduces the line voltage that is supplied to an electrical device, the voltage drop in the rheostat must be dissipated in heat. If the line voltage or the load varies, the rheostat must be continually readjusted. With a variable transformer, heat loss is insignificant and output voltage is not affected noticeably by the load.

A reactor is an impedance of small resistance and large inductance. Sometimes, the reactor is inserted in series with a load to vary current and voltage. In contrast with a variable transformer, it has some heat loss and a low power factor. The inductance must be adjusted to take care of input and output variations.

Other types of transformers with tapped primary and secondary windings usually require opening the circuit while changing from one tap to another. Tap changing mechanisms are relatively expensive and voltage changes are not likely to be in small increments.