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Variable frequency drive, a three-phase AC output motor controller. Questions about vacuum fluorescent displays should not receive this tag.

In general, a variable-frequency drive includes an inverter stage, converting DC to three-phase AC power to drive an AC motor. This inverter is usually six IGBTs in three phase legs; older designs may use SCRs or other switchable semiconductors. The inverter is controlled in such a fashion as to create a desired output waveform, controlling the speed and torque of the driven motor.

The DC power to feed the inverter may come from various sources. Most VFDs include a rectifier section, to convert AC grid power to DC. These designs must also include a precharge circuit, to prevent inrush currents into the DC bus capacitance from damaging the rectifier. Many VFDs also include DC bus terminals, allowing external DC power feeds from rectifiers, other VFDs, or storage devices.

Rarely, VFDs may use a direct matrix topology, converting AC line power directly into the frequency desired to operate the motor, with no interstitial DC bus.

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