Glossary Updates12 new terms added to the glossaries · October 2, 2026, 22:44 CEST
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Glossary · Motion control and drives

Variable-frequency drive (VFD)

Also known as: Frequency inverter, Frequency converter, AC drive

German: Frequenzumrichter

In industrial automation, a variable-frequency drive (VFD) is a power electronic device that converts the fixed-frequency supply into an output of variable frequency and voltage to control the speed and torque of an AC motor.

  • Motion control

In one sentence

A variable-frequency drive controls an AC motor's speed and torque by converting the supply to variable frequency and voltage.

Example

Fans in a drying line are driven by VFDs that adjust speed to the required air flow instead of using dampers.

How it applies

  • Engineering: A typical VFD has a rectifier, a DC link with capacitors and an inverter. Control modes range from V/f to vector control. Line filters, motor cables and EMC measures are part of the design.
  • Commissioning: Motor data, ramps, limits and control source are set; an identification run improves vector control.
  • Operation: Faults such as overcurrent, overvoltage or overtemperature are shown as codes.
  • Documentation: Warn that the DC link holds dangerous voltage after switch-off (Stored energy) and give the waiting time from the manufacturer. Explain that switching off the drive's enable is not isolation; for work on the motor, use Energy isolation and Lockout/tagout (LOTO).

VFD vs. soft starter

A soft starter only limits current during starting and stopping; the motor then runs at grid speed. A VFD controls speed continuously.

By knowledge.aitechdoc.world · Published September 26, 2026 · Last reviewed

Source: IEC 61800-2, Adjustable speed electrical power drive systems — Part 2: General requirements — Rating specifications for low voltage adjustable speed AC power drive systems

Definitions follow the cited standards and specifications. Where a source is a copyrighted publication, such as an ISO, IEC or EN standard, the definition is a close paraphrase, not a verbatim quotation, so as not to infringe copyright. We recommend reading the original publication. The sections “How it applies” are editorial commentary by AI TechDoc Blog and are not part of any standard.

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