Key takeaways
- The drive’s continuous output current must be greater than or equal to the motor’s rated current.
- Use kW to compare product families, but use current for the final size check.
- Match input voltage, input phase, and motor voltage before comparing power ratings.
- Constant-torque and high-inertia loads may require a larger drive than a fan or pump of the same power.
- Measure real running current during the heaviest normal production step when replacing an existing drive.
Why current is the first sizing value
Motor power is useful for communication, but current is what the drive must supply. Motor efficiency, power factor, pole count, voltage, and design standard can produce different rated currents for motors with the same nominal kW. A drive selected only by kW can therefore be undersized even when the power label appears correct.
Use this rule:
Drive continuous output current ≥ motor rated current at the application voltage.
Then check the overload rating against the starting or acceleration requirement. If either condition fails, choose the next larger drive.
Collect the motor data
| Nameplate data | How to use it |
|---|---|
| Rated power / kW or HP | Initial product-family comparison |
| Rated current | Primary continuous-current check |
| Rated voltage | Match to drive output and supply system |
| Rated frequency and speed | Base speed and V/F pattern setup |
| Power factor and efficiency | Power and input-current estimates |
| Service factor | Consider motor continuous-margin expectations |
| Duty cycle | Continuous, intermittent, or repeated high-load cycles |
For existing equipment, use a clamp meter to measure current at normal production, maximum load, and acceleration. Record the duration of each high-current condition. A 60-second overload and a 5-second overload are very different sizing problems.
Confirm voltage and phase
List the site supply and the motor connection separately. A common industrial installation may use a three-phase 220–380 VAC supply, but export projects can encounter different local systems. Confirm whether the drive input is single-phase or three-phase, and whether the motor is wired in star or delta where applicable.
Do not assume a drive can be pushed above its voltage class. Also check panel voltage drop, breaker rating, cable sizing, and the maximum acceptable motor cable length. Long cables may require output filtering to protect motor insulation and reduce electrical noise.
Classify the load
Variable-torque loads
Centrifugal fans and pumps generally require less torque as speed falls. They often allow a normal-duty drive rating and provide strong energy savings when flow is controlled by speed instead of a damper or valve. Verify minimum speed, pipe or duct conditions, and possible water-hammer or surge behavior.
Constant-torque loads
Conveyors, extruders, mixers, positive-displacement pumps, and compressors can demand useful torque across the speed range. Check low-speed motor cooling, acceleration time, and continuous current. A forced-ventilation motor fan may be needed if the process runs for long periods at low speed.
High-inertia or impact loads
Crushers, centrifuges, flywheel machines, and punch equipment may need high starting torque, controlled acceleration, or braking. Calculate the inertia and target acceleration time, then verify that the drive overload duration and braking option match the duty cycle.
A simple sizing workflow
- Record the motor nameplate data and machine load type.
- Confirm site voltage, phase, breaker capacity, and cable route.
- Select the drive voltage class and product family.
- Compare drive continuous current with motor rated current.
- Check overload percentage and duration against starting, acceleration, and impact load.
- Check minimum and maximum speed, motor cooling, and control mode.
- Review braking, stopping method, cable length, harmonics, and enclosure conditions.
- Document the selected drive, motor, parameters, and protection settings.
Common sizing mistakes
| Mistake | Better practice |
|---|---|
| Choosing only by kW | Verify rated current and overload first |
| Ignoring duty cycle | Measure current for the highest normal production condition |
| Ignoring acceleration | Check ramp time against load inertia and process cycle |
| Undersized braking | Calculate regenerated energy during deceleration |
| Poor enclosure planning | Respect clearance, cooling, dust, and humidity requirements |
| Mixing voltage classes | Match drive, supply, and motor voltage before comparing price |
You can compare the two Qingtian product families on the product overview, then inspect the detailed ranges for the EC590 vector VFD and EC6000 high-performance VFD.
Send your sizing data for review
Provide the motor nameplate, measured current, supply voltage, load type, and duty cycle. Qingtian will recommend an appropriate drive rating.