A variator of speed, or variator of frequency (abbreviations VFD, of English: Variable Frequency Drive or AFD Adjustable Frequency Drive) is a system for the control of the rotational speed of an engine of alternating current (AC) by means of the control of the frequency of power supplied to the motor. A frequency inverter is a special case of a speed variator. Frequency inverters are also known as adjustable frequency drivers (AFD), CA drivers or microdrivers. Since the voltage is varied at the same time as the frequency, they are sometimes called VVVF drivers (variator voltage variator frequency).

The frequency variation device driver (the drive itself) is formed by solid state electronic conversion devices. The usual design first converts the AC input power into DC using a bridge rectifier. The intermediate DC power is converted into a quasi-sinusoidal AC signal using a switched inverter circuit.

The rectifier is usually a three-phase diode bridge, but controlled rectifiers are also used. Because the energy is converted to continuous, many units accept both single-phase and three-phase inputs (acting as a phase converter, a variable speed drive).

As soon as the semiconductor switches appeared, they were introduced in the Frequency Variators, being applied in inverters of all the available voltages. Currently, insulated gate bipolar transistors (IGBTs) are used in most inverter circuits.

The characteristics of the AC motor require a proportional variation of the voltage each time the frequency is varied. For example, if an engine is designed to work at 400 volts at 50 Hz, the applied voltage should be reduced to 200 volts when the frequency is reduced to 25 Hz. Thus the ratio volts / hertz should be regulated at a constant value (400 / 50 = 8.00 V / Hz in this case). For optimal operation, other voltage adjustments are necessary, but nominally the constant is V / Hz is the general rule. The most novel and extended method in new applications is the voltage control by Pulse Width Modulation (PWM).

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