A variable frequency drive (VFD, Variable Frequency Drive or AFD Adjustable Frequency Drive) is a system for controlling the rotational speed of an alternating current (AC) motor by controlling the power frequency supplied to the motor. A variable frequency drive is a special case of a variable speed drive. Variable frequency drives are also known as adjustable frequency drivers (AFD), AC drivers or microdrivers. Since the voltage (or voltage) is varied at the same time as the frequency, they are sometimes called VVVF drivers (variant voltage converter).

Variable frequency drive devices operate under the principle that the synchronous speed of an alternating current (AC) motor is determined by the supplied AC frequency and the number of poles in the stator, according to the relationship:.

Where RPM = Revolutions per minute, f = AC supply frequency (Hertz), p = Number of poles.

In asynchronous motors, the revolutions per minute are slightly lower due to the asynchronous motor that its name indicates. In these, there is a minimum gap between the rotation speed (RPM) of the rotor ("real" or "output" speed) compared to the amount of RPM of the magnetic field (which should comply with the above-mentioned equation in both synchronous motors and asynchronous motors) because it is only attracted by the external magnetic field, which always surpasses it in speed (otherwise the motor would stop having torque at the moments in which it reaches the magnetic field).

The way to vary the frequency basically consists of changing the duty cycle (ON time and OFF time in a period) of a periodic square wave, in such a way that the mean value of the voltage (the average) over time varies between V maximum and V minimum (see figure to the right). The speed with which we vary the work cycle, that is, its average value, will be the frequency of variation of the average value.

This is physically achieved through switching "electronic keys" that are IGBTs (insulated gate bipolar transistors) that act as switches that, when closed and opened by means of specific software, form the square wave, which allows obtaining the sinusoidal signal.