VFD sizing calculator — drive selection
Pick the right variable-frequency drive for a three-phase motor: enter motor kW and the load duty, and read the recommended VFD output current and the standard drive size to order.
VFD drive selection
Service factor
Load type
Formula: I (A) = (kW × 1000) / (√3 × V × PF × η) with PF 0.85 and η 0.90 — the required continuous current is FLA × service factor × load factor, and the drive rating is the next standard size above it with at least 10% margin.
Heavy-duty VFDs deliver 150% overload for 60 s; normal-duty drives provide 110–120% for 60 s. If the application has frequent stops and starts or high start-up torque, pick the next size up and keep this continuous-current margin.
Browse related parts — AO Ctrl stocks the drives, motors and braking hardware for variable-speed control:
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Frequently asked questions
How do I size a VFD for a three-phase motor?
The tool computes motor FLA = (kW × 1000) / (√3 × V × PF × η) with PF 0.85 and η 0.90, then applies the load factor — 1.0 for variable torque (pump/fan), 1.1 for constant torque (conveyor/compressor) and 1.25 for high-inertia shock loads — and any service factor. The recommended drive current is that figure plus 10% headroom, matched to the next standard drive rating.
What size VFD do I need for an 11 kW motor?
At 400 V with the default PF 0.85 and η 0.90, an 11 kW motor has an FLA of about 20.8 A. With a 1.0 service factor and variable-torque load the required continuous current stays at 20.8 A, and the recommended VFD current is about 23 A (10% margin). The tool then matches that to the smallest standard drive whose 400 V output current covers it — 11 kW, whose rated output of about 24.6 A covers 22.8 A. The next size up is recommended for frequent starts or high start-up torque.
Why is a service factor and load type applied to VFD sizing?
A VFD must supply the continuous current the motor can actually demand. The service factor (1.0 or 1.15) raises the required current when the motor is allowed to run above nameplate, and the load-type factor accounts for torque profile — crushers and hoists pull more than pumps. Heavy-duty VFDs deliver 150% overload for 60 s while normal-duty units give 110-120% for 60 s, so the extra margin prevents nuisance tripping.
AO Ctrl provides this tool for reference only.