Match the drive’s rated output current to the motor nameplate full-load current (FLA), confirm the supply voltage class, then choose an overload class that fits the load. The kW rating alone is never enough — two 11 kW motors can draw noticeably different currents depending on efficiency, poles and voltage.
What information do you need before sizing?
Collect these points before you request a quote — a complete picture avoids the most common sizing mistakes:
- Motor nameplate: rated power (kW or HP), full-load current, rated voltage, frequency (50/60 Hz) and rated speed.
- Supply: voltage class and phases available at the cabinet — for example, the Qingtian EC590 accepts 220–380 VAC input classes.
- Load type: constant torque (conveyors, compressors, mixers) or variable torque (centrifugal fans and pumps).
- Duty cycle: how often the machine starts, and whether it runs long periods at low speed.
- Environment: ambient temperature, dust, altitude and motor cable length.
- Control interfaces: required speed range and accuracy, plus 0–10 V, 4–20 mA or RS-485/Modbus signals.
Why current matters more than kilowatts
A VFD protects and limits the motor through current, so the drive’s rated output current — not its kW label — decides whether it will run your motor cleanly. Two motors with the same power rating can carry different full-load currents because of efficiency class, pole count and rated voltage. If the motor’s FLA sits above the drive’s rated output current, the drive will nuisance-trip on overload even though the “kW numbers” look equal. The practical rule: drive rated output current ≥ motor FLA, with margin for the duty cycle.
Which overload class does your load need?
Classify the load first, because it sets how much starting and running overload the drive must deliver:
| Load type | Typical examples | Sizing approach |
|---|---|---|
| Variable torque | Centrifugal fans, centrifugal pumps | Size to nameplate FLA; normal duty is usually sufficient |
| Constant torque | Conveyors, mixers, compressors, positive-displacement pumps | Keep heavy-duty margin — typically a 150% overload capability |
| Shock / low-speed duty | Hoists, crushers, extruders | Upsize one frame and review braking requirements |
The overload percentages above describe typical industry classes, not a substitute for the datasheet — check the overload rating of the specific frame before ordering.
When should you upsize the drive?
- Ambient temperature or altitude above the datasheet’s continuous rating.
- Motor FLA within roughly 10% of the drive’s rated output current.
- Frequent starts, jogging or shock loading.
- Long motor cable runs that raise leakage current and reflected voltage.
- Dusty workshops where cooling channels will gradually restrict airflow.
A five-step sizing checklist
- Read the motor nameplate: kW/HP, FLA, voltage, frequency and rated speed.
- Classify the load (variable torque vs constant torque) and the duty cycle.
- Select a drive whose rated output current meets or exceeds the motor FLA with duty margin.
- Confirm the supply voltage class and phases match the drive input rating.
- List the options you need — braking, filters, communication, keypad — and confirm the exact model with your supplier before ordering.
How the Qingtian range maps to your motor
The Qingtian EC590 covers 0.75–315 kW on 220–380 VAC input classes, with V/F and sensorless vector control for induction and PM synchronous motors — the default choice for general machinery. When your line needs torque control, closed-loop speed stability or more I/O, step up to the EC6000 high-performance drive. If you are weighing the two platforms, see our EC590 vs EC6000 comparison.
You can also download the EC590 parameter guide to review the control modes before ordering. Not sure which frame fits? Send us the motor nameplate, voltage and application — our engineers reply within 24 hours on business days.
FAQ
How much bigger should the VFD be than the motor?
In most cases the same current class is right: choose the drive whose rated output current covers the motor’s FLA. Move up one frame for heavy-duty loads, hot environments, long motor cables or very frequent starting.
Can one VFD run two motors at the same time?
Yes, in V/F mode a single drive can supply several motors if its rated output current covers the sum of all motor FLAs and each motor has its own thermal overload protection. Vector control is designed for one motor per drive, so keep separate drives wherever speed accuracy matters.
Does an oversized VFD waste energy?
No — a drive draws power in proportion to the load. Oversizing mainly costs budget and panel space rather than efficiency, but avoid going far beyond what current matching requires, because electronic protection settings become less precise.