How to Choose a VFD for Your Motor & Load Type

Choosing a VFD starts with the load, not the catalogue: identify whether your application is variable-torque (pumps, fans), constant-torque (conveyors, mixers) or constant-power (spindles, mills), then match the motor ratings, pick a control mode and confirm the protection class. A 7.5 kW pump drive and a 7.5 kW conveyor drive are usually different selections even though the power rating is the same. This guide walks through the five selection steps with a load-type table.

Step 1 – Identify Your Load Type

Step 2 – Match Motor Ratings

The drive must match the motor's rated voltage, full-load current and frame characteristics. Key checks: the rated current of the drive at the operating voltage must cover the motor's full-load current (plus a sensible margin, typically 10–20% for constant-torque duty); single-phase input limits output power; and the maximum motor cable length needs output reactors on long runs. For the full current-based method see our VFD sizing guide.

Step 3 – Choose the Control Mode

• V/f (scalar) control: simplest; fine for most pumps and fans and for multi-motor parallel operation
• Sensorless vector control: better low-speed torque and speed accuracy for conveyors, mixers and compressors without an encoder
• Closed-loop vector control: highest accuracy (0.01% of rated speed typical) for hoists, winders and precision machines with an encoder

Match the control mode to what the process needs – a fan rarely needs closed-loop accuracy, while a hoist absolutely does.

Step 4 – Environment and Derating

Check the installation environment before finalising the model: enclosure IP rating for dust and moisture; ambient temperature derating (most drives derate above 40 °C, and many are limited to 50 °C); altitude derating above 1000 m (typically 1% per 100 m above); and input voltage quality including harmonic limits – see our energy efficiency article for why harmonics matter to the whole installation.

Step 5 – Verify Against the Application

• Confirm overload duty: constant-torque drives need higher overload capability (typically 150% for 60 s) than light-duty drives
• Confirm speed range: field-weakening range and minimum speed must cover the process
• Confirm communication needs: Modbus, fieldbus or I/O must match the control system
• Plan spares: common sizes and models reduce downtime – see our component maintenance article

Frequently Asked Questions

Can one VFD model cover both pumps and conveyors? Many drives have light- and heavy-duty ratings in one frame, so one model family covers both – but the continuous and overload ratings you select differ. Specify the duty class at order time.

How important is the control mode for a pump? V/f is usually sufficient for pumps and fans; vector control helps when you need torque at low speed or better speed holding. Our VFD FAQs cover control questions in more detail.

Not sure which drive matches your motor and load? Our selection team can work from your motor nameplate and process data. Start with our VFD selection guide, and see the high-voltage VFD page for larger ratings.