A frequency converter is a power electronic device, which transforms DC voltage to AC voltage of desired frequency and magnitude.

Frequency converters are used to control the speed and the torque of the AC motors. The most typical frequency converter topology is the three-phase two-level voltage source inverter. The phase voltages are controlled using the power semiconductor switches and pulse width modulation (PWM). Semiconductor switching devices and anti-parallel connected freewheeling diodes form a bridge, which can connect each motor phase to the positive or negative dc-link potential. The PWM changes the connections of the phases between the positive and the negative dc-link potentials so that the fundamental wave voltage has the desired frequency and magnitude. The motor reacts primarily to the fundamental voltage and filters out the effects of the harmonic voltages. Nowadays, the insulated gate bipolar transistor (IGBT) is the most widely used semiconductor switching device. The low-voltage (< 1000 V) frequency converters are manufactured in powers ranging from couple of hundreds of watts to several megawatts.

Frequency converters are typically used to control the speed of pumps and fans. In many applications significant energy savings are achieved. The most demanding application areas are found on the industrial processing lines, where the control accuracy requirements can be very high.