An inverter when used in the context of motor speed control can also be known as a variable frequency drive (VFD). It essentially generates a varying frequency three phase AC voltage to effect a change in the speed of a motor. It achieves this by converting the incoming power supply into a DC voltage and then generating a three phase AC voltage from this DC supply. The development of electronics since the manufacture of the first semiconductors has seen the speed and processing power increase enormously which has made it possible to, not only digitally synthesise the required AC frequency for any given speed of the motor but to also analyse the motor current and rotor position.
Why is it called an inverter?
The term inverter only relates to the final part of the VFD's electronic architecture, the part that converts DC voltage to AC. There is no clear technical reason for the use of the term 'inverter' as it is generally believed to refer to the inversion of the early mechanical process of converting AC voltage to DC, sometimes referred to as an 'inverting converter'.