An AC Variable Frequency Drive (VFD) is commonly referred to as an “Inverter”. This is because of the way a VFD works. The following details the inner workings of a VFD:

Alternating Current (AC) power is applied to the input of the VFD and feeds a bridge rectifier.
The rectifier converts the Alternating Current (AC) voltage into Direct Current (DC) voltage.
The Direct Current (DC) voltage then feeds the Direct Current (DC) buss capacitors on the VFD where it is stored for use by a transistor or Insulated-Gate Bipolar Transistor (IGBT).
Direct Current (DC) from the capacitors feed the input of the transistor(s).
The transistor(s) then continuously turns on and off at the appropriate frequency to build a new sine wave for use by the motor connected to the output of the VFD.

The process above is often referred to as inversion because it changes from one form to another then back again.

The voltage frequency, as distributed in the USA, is 60 cycles per second and the unit of measurement is Hertz (Hz). The output frequency and voltage of an AC Variable Frequency Drive (VFD) is variable and controlled by the speed at which the output transistor is continuously turned on and off.

The variable speed is controlled digitally in modern VFDs and changed by the operator through programming, an operator interface, or by changing an analog input to the VFD that is programmed as “speed reference input”.