When an older variable frequency drive (VFD) fails, the fastest fix is usually a drive that fits the same place, the same wiring and the same motor. Whether you look for an identical used spare or a current-generation model, the checks are the same. Matching only the horsepower or kilowatt rating is not enough: this guide lists what to compare before you order.
1. Input voltage and number of phases
Start with the supply. Check the nameplate input voltage range (for example a 380-480 V class or a 200-240 V class), the frequency, and whether the drive is rated for single-phase or three-phase input. Some three-phase drives accept a single-phase supply only with a significant derating, and some do not accept it at all; the manual tells you which. Also note the type of network: on ungrounded or high-resistance grounded systems, many drives require the internal EMC filter to be disconnected.
2. Output current, not only HP or kW
The power rating on a drive is a convenient label, but the drive actually protects itself on output current. Two motors with the same power can have different full-load currents depending on their speed, efficiency and voltage. Compare:
- The motor's full-load current on its nameplate.
- The drive's continuous output current at the duty you need.
- The overload current the drive allows and for how long.
The replacement drive's continuous output current should be at least equal to the motor's full-load current. Our articles on IEC motor nominal currents give typical values, but the motor nameplate always wins.
3. Normal duty or heavy duty
Many drives carry two ratings. Normal duty (sometimes called light or variable-torque duty) suits fans and centrifugal pumps, where the load needs little overload. Heavy duty (constant-torque duty) suits conveyors, mixers, hoists and positive-displacement pumps, which need a higher overload for starting and peaks. The same drive frame often has a higher current rating in normal duty than in heavy duty. The overload percentages and durations differ by manufacturer, so read them in the data sheet and choose the rating that matches the old drive's application.
4. Braking: chopper, resistor or regeneration
If the old drive decelerated a high-inertia load quickly or held back an overhauling load, it probably used dynamic braking. Check:
- Whether the braking chopper (transistor) was built into the drive or was a separate module.
- The ohmic value and power rating of the braking resistor. Every drive specifies a minimum resistance; a resistor below that value can damage the chopper.
- Whether the application used a regenerative unit or a common DC bus between several drives.
Inside a variable frequency drive.
5. Control mode and motor data
Older drives were often set up in simple V/Hz mode; others used sensorless vector or closed-loop control with an encoder. If the application needs torque at low speed or accurate speed regulation, the replacement must support the same mode and, when used, the same encoder type and voltage. Enter the motor nameplate data and run the auto-tune routine when the drive offers one.
6. I/O, safety and communication
List every wire landed on the old drive's control terminals and what it does:
- Digital inputs: start, stop, direction, preset speeds, fault reset. Note whether they are wired as sinking or sourcing (see NPN vs PNP: sinking and sourcing) and whether they use the drive's internal 24 V supply.
- Analog inputs and outputs: 0-10 V, ±10 V or 4-20 mA, and their scaling.
- Relay outputs: fault, running, at-speed signals used by the control system.
- Safety function: if the old drive had a safe torque off (STO) input, the replacement needs an equivalent function and the safety circuit must be revalidated. If it had none, keep the existing safety design (often a contactor) unless a qualified person redesigns it.
- Communication: Modbus RTU, PROFIBUS DP, DeviceNet, EtherNet/IP, PROFINET or another network. The option card, the node address, the data mapping and sometimes a device description file (EDS or GSD) must match what the PLC expects.
7. Parameter backup and firmware
Before removing a working drive, back up its parameters: with the keypad copy function, with the manufacturer's PC software, or at least by writing down every changed parameter. A like-for-like spare makes this easy, because the parameter file can often be downloaded directly, but only if the firmware is close enough. Check the firmware version of the old and the replacement drive; some software tools refuse a parameter file made on a different firmware level, and a few parameters may change meaning between versions.
8. Motor cable length and dV/dt
The fast voltage edges of a VFD output can create voltage peaks at the motor terminals, especially with long motor cables. They stress the motor insulation and the bearings. Check the maximum cable length the drive allows, whether the old installation used an output reactor, a dV/dt filter or a sine filter, and whether the motor is designed for inverter use. Keep any filter that was there unless the replacement's documentation says it is not needed. Shielded motor cable and correct grounding at both ends also matter.
9. Enclosure, cooling and mounting
- Enclosure rating (for example IP20, IP55, NEMA 1 or NEMA 12) suited to the location.
- Dimensions, mounting holes and clearances above and below for airflow.
- Heat loss in watts, which the panel cooling must remove.
- Derating for high ambient temperature, high altitude or a higher switching frequency.
- Line reactor or harmonic filter on the input, if one was installed. See the effect of harmonics.
Like-for-like used spare or current-generation model?
| Point | Identical used spare | Current-generation model |
|---|---|---|
| Mounting and wiring | Usually same footprint and terminals | Often different; may need a panel change |
| Parameters | Can often be copied directly | Must be translated and re-entered |
| Communication | Same network and mapping | May need a different option card or PLC changes |
| Downtime | Short swap | Longer engineering and commissioning |
| Long-term support | Depends on the legacy product's status | Current manufacturer support |
For guidance only. A used identical drive is often the quickest way to restart the machine; a planned upgrade can follow later.
One more point for any drive that has been in storage: after extended storage, many manufacturers call for reforming the DC bus capacitors before full voltage is applied. Follow the procedure and storage limits in the drive manual.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions. Drive DC bus capacitors can stay charged after power is removed: respect the waiting time marked on the drive.
Key takeaways
- Match input voltage and phases, then output current at the right duty, not just HP or kW.
- Check braking, control mode, I/O wiring, safety inputs and communication options.
- Back up parameters and compare firmware versions before the swap.
- Respect cable-length limits and keep existing output filters and reactors.
- Check enclosure, cooling and derating for the panel the drive goes into.
Related parts at 3JIndustry
Browse used legacy drives in our Drives collection and motors in our Motors collection. All parts are used, with a 30-day warranty and worldwide shipping.
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