A failed variable frequency drive can stop a production cell quickly, but the fastest available unit is not automatically the right replacement. To calculate VFD replacement sizing correctly, start with the motor nameplate and the actual application, then confirm the incoming supply and installation conditions. Matching only the old drive’s kW rating can leave a plant with nuisance trips, poor torque or a drive that is damaged shortly after start-up.
For a like-for-like replacement, the existing VFD part number remains the best starting point. It identifies the manufacturer series, voltage class, enclosure, communications options and likely mounting arrangement. However, the replacement decision should be checked against the motor and duty, especially where equipment has been modified, a motor has been rewound, or the original drive has been repeatedly tripping.
Calculate VFD replacement sizing from the motor nameplate
The motor nameplate provides the figures that determine whether a VFD can supply the load safely. Record the rated output current first. This is usually more useful than motor kW or horsepower because VFD output ratings are stated in amps and vary by voltage class, overload capability and manufacturer.
Also record the motor’s rated voltage, frequency, power, full-load speed, service factor where fitted, and connection arrangement. A dual-voltage motor may be wired differently for 230 V and 400 V operation. Do not assume that a 400 V drive can run a motor configured for 230 V, or that a 230 V input drive can provide 400 V output.
A basic sizing check is straightforward:
The VFD continuous output current must be equal to or greater than the motor nameplate full-load current, after allowing for the application duty and any required derating.
For example, a 400 V, 7.5 kW motor might have a nameplate current of approximately 15 A. A replacement drive rated for at least 15 A continuous output at 400 V may be suitable for light-duty fan or pump service. If the motor drives a loaded conveyor, mixer or crusher, the required drive rating may be higher because the application needs greater overload current.
Motor kW remains a useful cross-check. A 7.5 kW drive is not always interchangeable with every 7.5 kW motor. Two drives with the same power label can have different output-current ratings, particularly when one is rated for normal duty and the other for heavy duty.
Use the motor current, not the failed drive label
The failed VFD may have been oversized, undersized or selected for a different motor before the machinery changed. Its rating label only tells you what was installed. The motor nameplate and the actual mechanical load tell you what is required now.
If the replacement drive will operate more than one motor, add the full-load currents of all motors and account for how they start and stop. In most cases, each motor requires its own overload protection. A single VFD feeding multiple motors also limits individual speed control and can complicate fault finding.
Match the drive to the load duty
VFDs are commonly offered in normal-duty and heavy-duty ratings. Normal-duty drives are typically intended for variable-torque loads such as centrifugal fans and pumps. These loads require less torque as speed falls, which allows the drive to run with a lower overload allowance.
Heavy-duty applications need more capacity. Conveyors, positive-displacement pumps, compressors, extruders, hoists, mixers and high-inertia machinery can require strong starting torque or sustained overload. Select a drive whose heavy-duty current rating covers the motor full-load current and the equipment’s expected overload demand.
A common error is to replace a heavy-duty drive with a normal-duty model of the same kW. It may run unloaded during commissioning, then trip on overcurrent when the line is full or the process starts under load. Where duty is uncertain, review the machine manual, previous fault history and measured running current before ordering.
Starting frequency also matters. A fan that starts a few times per day places a different demand on a drive than a reversing conveyor cycling every minute. Frequent acceleration, deceleration and plugging generate heat in the drive. A larger frame size, braking resistor arrangement or regenerative solution may be required depending on the stopping duty.
Confirm supply voltage, phase and installation limits
A correctly sized output rating is only one part of the selection. The replacement VFD must suit the available supply. Confirm whether the site has single-phase or three-phase input, the nominal voltage and frequency, and the expected voltage tolerance.
Many industrial drives are designed for 380-480 V three-phase input, while smaller units may accept 200-240 V single-phase or three-phase supplies. A drive with three-phase input requirements cannot simply be connected to a single-phase supply. Some drive ranges permit single-phase input with a stated derating, but this must be confirmed in the manufacturer documentation rather than assumed.
Check the input protection arrangement at the same time. The upstream fuse, circuit breaker, isolator, cable size and earth connection must match the selected drive’s installation requirements. Size these from the VFD manufacturer’s data, not solely from motor current. Input current, harmonic content and protective-device recommendations can differ significantly between models.
Ambient conditions can change the final rating. High cabinet temperature, altitude, restricted airflow, dust, vibration and side-by-side mounting may require derating. A drive that is adequate on an open test bench may overheat in a packed enclosure next to contactors and power supplies. Leave the required clearance above, below and beside the unit, and verify whether the enclosure needs forced ventilation or air conditioning.
Output cable length deserves the same attention. Long motor leads can create voltage spikes that stress motor insulation and drive output stages. Depending on the cable length, motor age and drive manufacturer, a dV/dt filter, sine-wave filter or output reactor may be needed. This is particularly relevant when replacing a legacy drive with a newer high-switching-frequency model.
Check control, feedback and mechanical compatibility
A replacement VFD must fit the control system as well as the power circuit. Identify how the failed drive receives its run command and speed reference. This could be local keypad control, digital inputs, analogue 0-10 V or 4-20 mA signals, Ethernet/IP, PROFINET, Modbus, PROFIBUS or another fieldbus.
A drive can be electrically large enough but still delay a repair if it cannot communicate with the PLC or accept the existing signals without additional hardware. Check required I/O count, relay outputs, safe torque off connections, encoder feedback and any process PID function before selecting a part number.
Physical fit is equally practical. Measure panel space, mounting orientation and cable-entry clearance. A newer replacement may have a different footprint even when its electrical rating matches. If the original is door-mounted or uses a remote keypad, confirm keypad compatibility and cable length.
For critical assets, capture the existing parameter set before removing a working but unstable drive. Motor rated current, acceleration time, minimum and maximum frequency, ramp shape, torque boost, switching frequency, control mode and fault-reset behaviour all affect operation. If the drive has already failed, recover the settings from a backup, machine documentation or a matching drive where possible.
A procurement checklist before ordering
Before requesting a replacement, have the motor nameplate photo, failed VFD part number, supply details and application description ready. State whether the machine is a fan, pump, conveyor, mixer, compressor or another load, and mention any repeated faults or braking requirement. These details make it easier to identify an equivalent or an appropriately uprated alternative.
Also specify the condition requirement. A new and sealed unit may be preferred where standardisation, warranty policy or a long planned service life is the priority. A refurbished unit can be a practical route for an obsolete series, a legacy panel layout or an urgent repair where an exact part number reduces engineering time. For discontinued automation equipment, independent multi-brand stock can provide options when the original channel has no immediate availability.
At Automation Planet UK, part-number-led sourcing helps maintenance and procurement teams compare available condition options without losing sight of the technical match. The final selection should still be verified by a competent controls engineer or electrician against the motor, supply and manufacturer instructions.
Commission the replacement before returning to production
After installation, check motor rotation at low speed before coupling the load where the machinery allows it. Confirm that motor parameters match the nameplate, current limits are set correctly and the control signals behave as expected. Run the equipment through its normal speed range while monitoring output current, drive temperature and any PLC alarms.
Do not increase current limits simply to clear overload trips. A trip may indicate an undersized drive, excessive mechanical load, incorrect motor data, too-short acceleration time or a wiring issue. Finding that cause before full production is usually faster than replacing another drive after the next stoppage.
The right replacement is the one that matches the motor current, load duty, supply, controls and environment - not merely the one with the closest kW figure. A clear nameplate photo and a few accurate site details can turn an urgent VFD failure into a controlled, compatible repair.