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Guide

How to choose a variable frequency drive (VFD)

Step-by-step VFD selection: match motor current and voltage, load type, overload rating, braking, filters and communication so you order the right drive first time.

  • 7 min read
  • Updated

The selection checklist

  1. Read the motor nameplate: rated voltage, full-load current (FLA), power, frequency and speed.
  2. Match the drive to the supply: voltage class (for example 230 V, 400 V or 690 V) and single- or three-phase input.
  3. Choose a drive whose continuous output current is at or above the motor current. Current is the real limit.
  4. Check overload capacity against the load. Pumps and fans (variable torque) need less overload than conveyors, crushers or hoists (constant torque).
  5. Decide on control: V/f, sensorless vector or closed-loop vector with an encoder, depending on the speed accuracy and torque required.
  6. Add the communication option your controller uses, for example PROFINET, EtherNet/IP, Modbus or CANopen.
  7. Add accessories: braking resistor for fast stops, line reactor or EMC filter, output reactor for long motor cables.

Derating

Drive ratings assume a standard ambient temperature and altitude. Above the rated temperature, above roughly 1,000 m altitude, or at a higher switching frequency, the usable current drops. Check the derating curve in the datasheet and size up if needed.

Constant torque or variable torque?

  • Variable torque (fans, centrifugal pumps): torque falls with speed, light overload is enough, and large energy savings are possible.
  • Constant torque (conveyors, mixers, positive-displacement pumps): the same torque at all speeds, so the drive needs a heavy-duty overload rating.

Safety and protection

  • Safe Torque Off (STO) is built into many drives and reduces the need for an extra contactor.
  • Install correct short-circuit protection and follow the manufacturer's cable and grounding guidance.
  • Long motor cables may need an output filter to protect the motor insulation.

Replacing an existing drive

Match the old drive's rating and mounting first, then check the control terminals, firmware and communication. Many manufacturers publish replacement guides from older to current drive families.

Frequently asked questions

Can I use a VFD with a motor that is rated higher or lower?

Size the drive to the motor current. A drive can run a smaller motor, but a motor larger than the drive's rating risks overload trips and is not protected correctly.

Do I need a braking resistor?

Only when the load must stop quickly or an overhauling load regenerates energy. Many fan and pump applications do not need one.

What does derating mean for a drive?

It means reducing the allowed output current at high ambient temperature, high altitude or high switching frequency. The datasheet gives the curve.

This guide is general engineering guidance. Always confirm part-specific details, ratings and compatibility against the manufacturer’s datasheet. We confirm price, stock and specifications for the exact part in a written quotation.