AC Drive Sizing: Matching PowerFlex to Your Motor

An AC drive that's too small trips on overload; one that's too big wastes money. Follow these six steps to size a variable-frequency drive (VFD) to your motor — and pick the right PowerFlex family.

1. Read the motor nameplate

Write down the motor's voltage, full-load amps (FLA), horsepower (or kW) and enclosure. The drive must match voltage and provide at least the motor's FLA — sizing by amps is more reliable than sizing by horsepower alone.

2. Classify the load

• Variable torque (centrifugal pumps, fans): the drive can usually be sized at 100% of motor FLA.
• Constant torque (conveyors, extruders, mixers, hoists): size the drive for the motor's full torque across the speed range.

3. Apply derating

At high altitude (above ~1000 m), high ambient temperature, or when the drive is mounted in a sealed enclosure, a drive's output drops. Standard practice is to upsize by one frame or apply the manufacturer's derating table.

4. Pick the right PowerFlex family

5. Check control and safety features

Confirm the drive supports your network (EtherNet/IP, DeviceNet, Modbus, PROFINET) and whether you need Safe Torque-Off (STO) for functional safety. The PowerFlex 525 and 755 both include embedded EtherNet/IP and STO.

6. Don't forget the basics

• Line and load reactors for harmonic reduction on larger drives.
• Braking resistors for applications that must decelerate quickly.
• Proper enclosure (IP20 vs NEMA 4X) for the installation site.