Key takeaways
- Use the motor’s rated current as the primary sizing check, with kW as a cross-check.
- Match the drive input voltage and phase to the installed power supply, for example 220–380 VAC three-phase systems.
- Identify the load type: variable torque, constant torque, or high peak torque.
- Choose V/F control for simple fans and pumps, or vector control where speed accuracy and low-speed torque matter.
- Plan for cable length, dust, heat, vibration, harmonics, and protective shutdown requirements.
1. Collect the information from the motor nameplate
The motor nameplate is the starting point for every VFD selection. Photograph it before ordering a drive and record the rated power, rated current, voltage, frequency, speed, power factor, service factor, insulation class, and connection method. If the nameplate is missing, ask the machine builder for the original motor specification rather than estimating only from machine capacity.
Rated current matters more than rated power. Two 7.5 kW motors can have different rated currents because of efficiency, poles, voltage, and design. A drive must continuously supply the motor’s full-load current at the site supply voltage while still meeting the application’s overload requirement.
2. Match the power supply and motor voltage
Confirm whether the incoming supply is single-phase or three-phase and whether the motor is intended for 220–380 VAC. Using a drive outside its voltage range can damage the input rectifier, output stage, or motor insulation. Also check supply frequency stability and whether the installation has a dedicated breaker, suitable earth, and correct wire size.
For export equipment, document the site voltage tolerance. A drive selected at nominal voltage may still trip if the plant supply is consistently low or high. Qingtian’s EC590 vector VFD and EC6000 high-performance VFD pages list the voltage and power ranges for the two product families.
3. Identify the load type
| Load type | Typical machines | Selection focus |
|---|---|---|
| Variable torque | Centrifugal fans, centrifugal pumps | Normal overload is usually enough; energy saving can be significant |
| Constant torque | Conveyors, mixers, compressors, extruders | Check continuous current, low-speed cooling, and heavy-duty overload |
| High peak torque | Crushers, punch presses, hoists | Verify starting torque, overload duration, and braking/resistor requirements |
A fan usually needs less torque at low speed, while a conveyor may require full torque from near zero speed. High-inertia or impact loads need more attention to acceleration time and overload capacity. If the machine cycles frequently, size for the worst continuous current and the required acceleration, not only the steady-state current.
4. Check overload and current demand
Most general-purpose drives specify a normal-duty overload such as 120% for a short period and a heavier rating for a shorter period. Constant-torque and impact loads may require a larger frame or a heavy-duty rating. Do not hide a marginal selection behind a long acceleration time if the process needs a defined cycle time.
For retrofit projects, measure the running current with a clamp meter at normal production and at the highest-load step. Compare both values with the drive’s continuous rating. If the measured current is close to the drive limit, use the next drive size.
5. Select the right control performance
Basic V/F control is reliable and easy to commission for fans, pumps, and simple conveyors. Vector control provides better speed accuracy and low-speed torque for extruders, winding systems, hoists, test benches, and machine tools. Sensorless vector control avoids an encoder, while closed-loop vector control uses a motor encoder for more demanding speed or torque control.
If you are comparing control methods in detail, read V/F Control vs Vector Control VFD. If you are still calculating drive size, start with VFD Sizing Guide: Motor kW, Current and Voltage.
6. Review installation and environmental conditions
- Ambient temperature: maintain clearance around the drive and avoid mounting it above hot equipment.
- Dust and fibers: use a clean cabinet, filtered ventilation, or sealed enclosure where appropriate.
- Humidity and condensation: avoid temperature swings that form water inside the panel.
- Altitude: high-altitude installations may require derating.
- Motor cable length: long cables can require output reactors or dv/dt filters.
- Braking: decelerating high-inertia loads may need a braking resistor or extended stop time.
7. Define protection and operator requirements
Decide how the operator will start, stop, jog, reverse, and adjust speed: local keypad, external potentiometer, 0–10 V signal, 4–20 mA signal, fieldbus, or relay control. Also define fault behavior. For example, pumps may use automatic restart after a transient fault, while saws, hoists, and personnel-adjacent machinery should require a controlled reset.
Finally, check required wiring protection, emergency stop logic, motor thermal protection, earth-fault protection, and panel labeling. A drive can only protect the process reliably when its parameters and external safety circuit are configured together.
Practical selection checklist
| Check item | Why it matters |
|---|---|
| Motor rated current | The drive must supply continuous current without sustained derating |
| Supply voltage and phase | Prevents incorrect rectifier and output selection |
| Load type and inertia | Determines torque, acceleration, and overload requirement |
| Speed range | Affects motor cooling and control-mode choice |
| Control signal | Keypad, analog, digital, or communication must match the machine |
| Environment | Heat, dust, humidity, and altitude affect reliability |
| Braking and stopping | High-inertia loads may need resistors or extended ramp times |
If you supply the motor nameplate, photos of the existing control panel, and a short description of the machine cycle, Qingtian can review the selection before quotation. Send the details through the contact page to avoid ordering an undersized or oversized drive.
Need a second opinion on your VFD selection?
Send the motor nameplate, supply voltage, load type, and target speed range. Qingtian will identify a suitable EC590 or EC6000 configuration.