How to size a variable frequency drive for a motor

Sizing a variable frequency drive looks like it should be a matter of matching kilowatts. The motor says fifteen kilowatts, so order a fifteen kilowatt drive. That shortcut works often enough that people keep using it, and it fails in exactly the applications where failure is most expensive. The drive does not care about the motor’s rated power. It cares about current, and current depends on what the load is doing.

Why current matters and power does not

A drive is a current source feeding an inductive load. Its semiconductors have a thermal limit expressed as a continuous current rating and a short term overload rating. The kilowatt figure printed on the enclosure is a convenience, derived by assuming a standard four pole motor at a standard voltage with a typical power factor and efficiency.

Change any of those assumptions and the relationship breaks. A high pole count motor running at low speed draws more current per kilowatt than a standard one. A motor rated for a different voltage draws different current for the same power. The correct starting point is always the full load current stamped on the motor nameplate, not the kilowatt rating.

The load type decides the overload requirement

Variable torque loads

Centrifugal fans and pumps have torque that rises roughly with the square of speed and power that rises roughly with the cube. These loads never ask for more than rated torque once running, and they ask for very little at low speed. Drives can be selected close to the motor current, and manufacturers publish a variable torque or normal duty rating for exactly this case.

Constant torque loads

Conveyors, extruders, mixers, positive displacement pumps, hoists and compressors demand full torque across the speed range and sometimes considerably more than full torque to break away from standstill. These need the drive’s constant torque or heavy duty rating, which is a lower current figure than the normal duty rating on the same hardware.

This is the single most common sizing error in the field. The same drive may be listed as suitable for twenty two kilowatts in variable torque service and eighteen and a half kilowatts in constant torque service. A technician reading the first figure and installing it on a conveyor has undersized the drive without doing anything that looks wrong.

High inertia and frequent starting

Loads with large rotating mass need sustained overload to accelerate, and the duration matters as much as the magnitude. A drive that can deliver a short overload for a few seconds may not sustain it for the thirty seconds a large centrifuge needs. Applications that start and stop many times an hour accumulate thermal stress, and the drive should be selected with margin rather than at its limit.

Altitude, ambient temperature and derating

Published ratings assume a reference ambient temperature and altitude. Above either reference, continuous current capability falls, because cooling becomes less effective. Installations in hot plant rooms, in enclosures without forced ventilation, or at elevation need the derating curves from the manufacturer’s manual applied before selection, not after commissioning.

Switching frequency has a similar effect. Raising the carrier frequency reduces audible motor noise and improves current waveform quality, and it increases switching losses in the drive, which reduces available output current. A drive set to a high switching frequency to quieten a motor in an occupied area may need to be a size larger.

Cable length between drive and motor

Long motor cables behave as a distributed capacitance, producing charging currents the drive must supply and voltage reflections that stress motor insulation. Beyond the manufacturer’s stated limit, output filters or reactors become necessary, and the filter itself consumes voltage headroom.

This matters when the drive sits in a central panel and the motors are distributed across a plant. It is worth establishing cable runs at the design stage, because the remedy is cheaper to install than to retrofit.

A practical sequence

Read the motor nameplate and record full load current, voltage, frequency, pole count and service factor. Classify the load as variable torque or constant torque and establish the required breakaway torque. Select the drive from its rating for that duty class, using current rather than kilowatts. Apply derating for ambient temperature, altitude and chosen switching frequency. Check motor cable length against the manufacturer’s limit. Confirm the short term overload capability covers the acceleration requirement for the actual inertia. Then, and only then, compare the result with the motor’s kilowatt rating as a sanity check.

If that final comparison shows the drive is a size larger than the motor, that is usually correct rather than wasteful. An oversized drive runs cooler, lasts longer and costs less than an unplanned shutdown. General background on how these converters work is summarised in the overview of variable frequency drives.

Frequently asked questions

Can I size a drive from the motor kilowatt rating alone?

Only for straightforward variable torque applications at standard voltage. For anything else, use the full load current from the nameplate and the drive rating for the correct duty class.

What is the difference between normal duty and heavy duty ratings?

Normal duty assumes a variable torque load with limited overload demand. Heavy duty assumes constant torque with substantial overload. The same drive carries a lower current rating in heavy duty service.

Does a higher switching frequency require a larger drive?

Often yes. Higher switching frequency increases losses in the drive and reduces its continuous current capability, so the derating curve should be checked before selection.

How long can the cable between drive and motor be?

It depends on the drive and the cable type, and the manufacturer states a limit. Beyond it, output reactors or filters are needed to control charging current and voltage reflections.

Is oversizing a drive a problem?

Generally no. An oversized drive runs cooler and tolerates overload better. The main consideration is that current sensing resolution at very low loads can be poorer, which matters only in precision applications.