Select a VFD by motor current and the complete operating duty, then check voltage, motor type, braking, cooling and interfaces. Matching the motor kW is only an initial screening step.
Start with the motor and the process
Collect the motor nameplate, including voltage, current, frequency, power, rated speed and motor type. Record whether the process needs speed control, torque control or positioning. For a permanent magnet or high-speed motor, obtain the electrical data and permitted operating envelope from the motor supplier.
Describe the machine at its hardest operating point: starting under load, reversing, accelerating a large inertia, sustaining low speed or repeatedly braking. A continuous light load and a short heavy overload require different checks even when motor power is identical.
Seven questions that determine VFD selection
| Question | Why it matters |
|---|---|
| What current must the drive supply? | Check continuous current, peak current and overload duration at the specified voltage. |
| How fast must the motor operate? | Check the required output frequency, motor speed limits and low-speed cooling. |
| Does the load return energy? | Evaluate resistor braking, a common bus or AFE regeneration. |
| What motor feedback is needed? | Match the encoder signal, feedback card and required control behaviour. |
| Where will the drive operate? | Consider temperature, altitude, enclosure, contamination and derating. |
| How will it communicate? | Specify the PLC protocol, required data and option cards. |
| What must be included in the cabinet? | Define protection, precharge, reactors, filters, cooling and auxiliary circuits. |
Match the product family to the duty
INOMAX MAX500 is the general-purpose VFD range; MAX510 addresses high-performance requirements; MAX600 is the Special HVAC VFD range. For demanding torque control, regeneration, modular systems or liquid cooling, evaluate the ACS industrial drive portfolio. These are starting points for selection, not substitutes for checking the exact model.
For instance, a ventilation fan with a variable-speed requirement is different from a mine hoist with repeated braking and a controlled brake sequence. A high-speed test motor also requires frequency and current matching; a general-purpose frequency rating cannot be assumed suitable.
Prepare a useful quotation request
- Attach motor nameplates and the supply specification.
- Describe the machine, load cycle and required speed range.
- State acceleration, deceleration, overload and braking requirements.
- Provide installation conditions and motor cable length.
- List the PLC, encoder, I/O and safety requirements.
- For a replacement, include the existing model, drawings and parameter records.
Related INOMAX resources
See ACS880 technical specifications, AFE versus resistor braking and common DC bus systems. The INOMAX product range provides the next step for model-specific enquiries.
Frequently asked questions
Can I choose a VFD with the same kW as the motor?
Only after confirming current rating, overload duty, voltage and derating. The same kW label does not ensure the required output current.
Does a larger VFD always solve overload trips?
No. Check the load, motor data, acceleration profile, wiring and actual fault first. Oversizing cannot correct every mechanical or configuration problem.