Frequency inverters are electronic devices that let you control the speed of an AC motor. Background: If electric motors or AC motors are operated directly from an AC voltage supply system, they can only avail of a fixed speed based on the number of poles and the supply frequency of the power supply system on location. If, however, an application or production process requires an adjustable AC voltage (that is, a controllable speed), frequency inverters are used. These frequency inverters can generate an AC voltage that is variable in amplitude (the output voltage level) and frequency from a constant AC voltage.