What is a Variable Transformer?
A variable transformer provide a simple, rugged method of controlling electrical voltage, current and power. It takes in utility line voltage and provide continuously adjustable output voltage. When the value of the voltage on the AC line is incorrect, just a tum of the variable transformer dial operates a sliding brush that makes it right. The same dial can act like a rheostat to control the speed of motors; cutting input voltage reduces the motor speed. A turn of the dial converts power line output into the desired voltage.
Versatility of the variable transformer lies in its ability to tap in anywhere from 0 to 100% of the line voltage, plus another 17% usually available because of a few extra windings in the secondary. Depending on the frame size, the output current ranges from 1 to 50 A at 120 V or half this amperage at 240V. Added versatility is possible by larger size units. Some single coil variable transformers are available with output rates low as 0.13 kV A and "ganged" 3-phase variable transformer units can reach 405 kVA.
Most common use, especially with the smaller 500 to 750VA sizes, is to obtain AC voltages different from the normal 120 or 240 V, single or three-phase supply. The other major use is delivering either standard or test voltages from lines where voltages fluctuate above or below the normal standard.