voltage-changer

from The Century Dictionary.

• noun In electricity, a device by which the voltage or electrical pressure in a circuit may be altered either up or down. More usually from a lower to a higher voltage.
• noun A motor-driven dynamo or generator whose windings are so related in each machine that the current which drives the motor shall generate one of a different voltage in the dynamo: used in regulating-systems. Sometimes called a booster, when the voltage is to be raised.