How to Derate a VFD for Carrier Frequency and Altitude
A drive's rated output current assumes reference conditions, and when the real installation runs a higher carrier frequency, sits in a hotter room, or lives at altitude, the drive cannot deliver that full current without overheating. Derating is how you account for those conditions so the drive lasts. This page is for the engineer sizing or commissioning a drive in a demanding environment. It explains why each factor forces a derate, how they stack, and how to apply the manufacturer's derating so the drive is not asked to do more than the conditions allow.
VFD Derating in one line: To derate a VFD, reduce its usable output current below the nameplate rating for each condition that increases its heat or reduces its cooling: a higher carrier frequency raises switching losses, a higher ambient temperature leaves less margin to shed heat, and a higher installation altitude thins the air so it cools less. Apply each derating factor from the manufacturer's tables and combine them, then confirm the derated current still covers the motor's full-load amps. The derating figures are drive-specific, so always use the manufacturer's documentation rather than a generic rule.
Why Each Condition Forces a Derate
Every derating comes down to heat: either the drive makes more of it or the environment removes less. The drive's output devices generate loss as they switch and conduct, and that loss becomes heat the drive must shed to stay within its temperature limits. When the heat produced rises or the cooling falls, the only way to keep the drive within limits is to reduce the current it carries, which is exactly what a derating does. Understanding it as a heat balance makes the individual factors make sense rather than seeming like arbitrary penalties.
Carrier frequency is the factor you often control. A higher carrier frequency, described in the page on VFD carrier frequency, makes the output devices switch more times per second, and each switching event costs energy, so switching losses and drive heat rise with the carrier. That is why a drive rated for a certain current at a default carrier must carry less at a higher one. If the application does not need a high carrier, lowering it recovers current capability instead of derating it away.
Ambient temperature and altitude are set by the site. A higher ambient temperature narrows the gap between the drive's operating temperature and its limit, so it can dissipate less heat and must carry less current, the same enclosure-heat thinking as in the page on cabinet heat dissipation and IO derating. Altitude thins the air, and thinner air both cools less effectively and, above a threshold, reduces the voltage the drive's insulation can stand, so high-altitude installations derate for cooling and sometimes for voltage too.
Apply and Combine the Derating Factors
Work from the manufacturer's derating tables, because the numbers are specific to the drive. Each drive family has published curves or factors for carrier frequency, ambient temperature, and altitude, expressed as a fraction of rated current the drive can carry under that condition. Read the factor for each condition your installation presents, and treat these documented figures as authoritative rather than reaching for a remembered percentage, since the deratings differ between drive sizes and generations.
Combine the factors, because they stack. A drive running a high carrier in a hot room at altitude suffers all three effects at once, so the usable current is the rated current reduced by each applicable factor together, not just the worst one. The manufacturer's documentation states how to combine them, and the honest result can be substantially below the nameplate current. Applying only the most obvious derate and ignoring the others leaves the drive overloaded in the very conditions the derating exists to protect against.
Confirm the derated current still covers the load. After applying every derating, the drive's usable output current must still exceed the motor's full-load amps with margin, the value covered in the page on the motor full-load amps nameplate value. If the derated capacity falls below the motor's needs, the drive is undersized for the conditions and you must either choose a larger drive, lower the carrier frequency, or improve the environment. Sizing a drive on its nameplate current while ignoring the deratings is how a drive ends up thermally overloaded in service.
Choose Between Derating and Improving Conditions
Derating is not the only response; often you can remove the condition instead. If a high carrier frequency is forcing a derate but the application does not need the quieter motor or the smoother waveform a high carrier gives, lowering the carrier recovers current capability directly. Weigh what the high carrier is buying, such as reduced audible noise, against the current it costs, and lower it where the benefit is not worth the derate.
Attack the environment where the site allows. A hotter room can be cooled or the drive relocated, an enclosure can be ventilated or air-conditioned, and a drive can sometimes be mounted where ambient is lower. Improving the cooling recovers current the ambient derate would otherwise take, and on a marginal installation that can be the difference between the drive you have being adequate and needing a larger one. The choice between a bigger drive and a better environment is an economic one, made with the real numbers.
Document the derating basis so it survives changes. Record the carrier frequency, ambient, and altitude assumed, and the resulting usable current, so a later change, raising the carrier for a noise complaint, or a new heat source in the room, is recognized as eroding the margin the sizing depended on. Because the drive's own temperature and load are values a monitoring system can trend, a drive creeping toward its thermal limit as conditions drift shows up in the data, giving warning before a heat-driven fault, and confirming whether the derating assumed at commissioning still holds.
Frequently Asked Questions
Why does a higher carrier frequency force me to derate the VFD?
A higher carrier frequency makes the drive's output devices switch more times per second, and each switching event dissipates energy, so switching losses and drive heat rise with the carrier. To stay within its temperature limits the drive must then carry less current, which is the derate. If the application does not need the quieter motor or smoother waveform a high carrier provides, lowering the carrier frequency recovers the current capability instead of giving it up to a derate.
How do multiple deratings combine?
They stack. A drive running a high carrier in a hot room at altitude suffers all three effects together, so the usable current is the rated current reduced by each applicable factor combined, not just the single worst one. Use the manufacturer's documentation for how to apply them together, because the honest combined result can be well below the nameplate current. Applying only the most obvious derate and ignoring the rest leaves the drive overloaded in exactly the conditions the deratings exist to protect against.
Can I avoid derating by improving the installation?
Often, yes. Lowering the carrier frequency recovers current lost to a carrier derate when the application does not need the high carrier. Cooling or ventilating the room, relocating the drive to a cooler spot, or air-conditioning the enclosure recovers current lost to an ambient derate. The choice between improving conditions and choosing a larger drive is an economic one, but reducing the condition that caused the derate is a legitimate alternative to simply accepting the reduced current.
Automation services
Need help turning this into a working system?
Merobix integrates SCADA, programs Allen-Bradley and Siemens PLCs, and designs and fabricates industrial control panels.
Meeting requests are reviewed before confirmation.