Definition
A variable frequency drive (VFD) regulates motor speed by rectifying incoming AC to DC and then inverting it to an AC output whose frequency and voltage it controls. Motor speed and torque follow that output directly, which lets a VFD replace fixed-speed running with ramped starts, load-matched speed, and measurable energy savings on fans, pumps, and conveyors.
Key facts
- Internally rectifies AC to DC across a DC link, then inverts it back to a controllable variable-frequency AC output.
- Motor speed follows output frequency; torque and current limits are typically parameterized per application.
- "Frequency converter" and "VFD" name the same device family in this context; Siemens documents SINAMICS G120 as both.
- Distinct from a soft starter, which limits inrush current at fixed frequency rather than varying speed continuously.
How does Variable Frequency Drive work or appear?
A VFD converts fixed-frequency mains AC to DC across a DC link capacitor bank, then uses power switching devices to synthesize an AC output at a chosen frequency and voltage. Because motor synchronous speed is proportional to supply frequency, the drive controls speed by controlling that output frequency, typically holding a fixed voltage-to-frequency ratio or using vector control for tighter torque response.
What should technicians inspect for Variable Frequency Drive?
An inverter is the specific stage inside a VFD that produces the AC output; the VFD is the complete packaged drive including rectifier, DC link, control electronics, and protection. A servo drive adds closed-loop position feedback for precise motion, which a standard VFD does not provide. A soft starter changes voltage at line frequency to limit starting current, then bypasses to full voltage: it does not vary running speed the way a VFD does.
FAQ
Frequently asked questions
What task does a VFD perform in motor speed control?
It converts incoming AC to a controllable variable-frequency, variable-voltage output so the connected motor runs at the speed and torque the process requires, instead of a fixed line-frequency speed.
Is a frequency converter the same thing as a VFD?
In this equipment context, yes: "frequency converter" and "variable frequency drive" refer to the same device family; Siemens SINAMICS G120 documentation uses both terms for the same product.
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