How and When to Derate Variable Frequency Drives
Sizing a variable frequency drive correctly can be complicated, but there are a few issues that can make it even more difficult. Many customers have been told in the past that they need to “derate” a drive, but don’t know exactly what that means or how to do it. For general help, see our “How to Specify a VFD” article. To learn specifically about derating, let’s dive into the details.
When you derate a VFD, you are reducing the ratings by a set amount, effectively leading to upsizing the drive. It’s not always as simple as increasing one size. Sometimes the VFD may end up significantly larger, and other times you lose some spare capability but don’t actually have to change the product you buy.
There are three main reasons to derate: heat, elevation, and phase. We also are often asked about derating based on overload capability, but this one is not actually a derate and can lead to some problems. We’ll cover all these topics in this article. With everything we discuss, remember that these are the general rules of thumb, but you should always verify for each manufacturer, series, and model how and when to appropriately derate.
Derating for Temperature
Every VFD has a temperature range that it’s rated to operate in listed in the manual. Be careful when looking this up as there are often two temperature ranges listed: one for storage and one for use.
Common ranges have a low temperature point of -10°C to 0°C (14°F to 32°F) and a high temperature of 40°C (104°F). This means that the ambient temperature around the VFD needs to stay in that range. Be aware that operating electrical equipment can cause the temperature in a room or panel to rise, and that would need to be accounted for or cooled to stay in the range. Also keep in mind that direct sunlight can make the equipment hotter than the air around it.
The manual will also list the maximum temperature the drive can be derated to and how to derate it. Let’s look at a Galt G500 VFD as an example. The manual says:
“Check whether the ambient temperature of the inverter during actual application exceeds 40°C. If yes, derate 1% for every additional 1°C. In addition, do not use the inverter when the ambient temperature exceeds 50°C. Note: For cabinet-type inverter installation, ambient temperature is the air temperature inside the cabinet.”
If your environment will be 55°C then your drive won’t function properly no matter how you derate it. Instead, let’s run an example where you’re operating at 48°C. Based on the Galt manual, you need to derate 8%.
If your motor is 480V and 172A while running a fan, you are likely using a Galt G540-01800UL-03 because of its 180A rating. We now need to reduce that rating by 8%, or about 15A. That means the drive is now effectively rated at 165A, below the motor’s full load rating.
In this case we would look at the next size G500, the G540-02150UL-03. This is rated at 215A when normal, and with an 8% derate it’s down to about 197A. This is sized appropriately for the motor and application after the derate.
For the lower temperature limit, there’s typically not a derate. For this line of Galt VFDs, the manual asks you to find a way to increase the temperature to meet the minimum temperature.
“Check whether the ambient temperature of the inverter during application is below -10°C. If yes, install space heaters.”
Derating for Elevation
Elevation derates are for VFD installations at high altitude, common in mountainous areas. Within North America, this means often through the Rocky Mountains, and occasionally throughout the continent in other areas. These derates are very similar to temperature, and in many ways are a branch on the temperature derate tree. The air at higher elevations is thinner and less effective at cooling, which means that you derate the VFD to make sure it cools properly.
The process is similar, including looking to the manual for the information. The standard ratings on VFDs vary more here just like it does with temperature, but one that is often seen is 1,000m (3,280ft). Let’s look at the same situation for a Galt G500, assuming the same 480V 172A motor is running a fan, but this time under 40°C but all the way at 2,500m (8,200ft). The manual says:
“Check whether the altitude of the application site exceeds 1000m. If yes, derate 1% for every additional 100m.”
We’ll take the 180A rating and reduce it by 15%, meaning it’s now rated at 153A. The larger drive is reduced from 215A to about 182A, which is still appropriately sized. We would select the larger drive in this application.
Note that there’s no upper limit on this section. That doesn’t mean that it doesn’t exist or that other lines don’t have an upper limit. If your application goes above a 10% derate for any specific factor, it’s worth verifying that you’re not exceeding the capabilities of the VFD. We’ve had many requests for VFDs at mines around 4000m because of mountainous terrain. In this case you’re derating by 30%, which is significant in terms of increase.
Derating for Phase
We cover derating for phase conversion in depth in our article “Can a VFD Convert Single Phase to Three Phase.” Go to that article for more details on exactly how to derate.
As a summary, when a standard 3-phase drive is being fed an input of single phase, the motor will draw the same current, but the input of the VFD has all of that amperage focused on two incoming terminals and their associated diodes and components, so they need to be upsized in order to handle that load. The safe rule here is to double the amp rating of the motor. This means that you would half the capability of the VFD (or implement a 50% derate).
Stacking Derates
Can you derate for multiple factors at once, such as heat and temperature? The answer is yes but be careful to do your math correctly. You have to calculate the derate for each of the factors and add them together, then calculate based on that. Let’s use our same example above, combining all factors. As a reminder, we’re looking at:
- 480V system
- 172A motor FLA
- Fan application
- 48°C ambient temperature
- 2,500m elevation
We need to factor in a 23% derate, the combination of the 8% temperature and 15% elevation derates. With this situation, neither of our other drives are appropriately sized, with effective ratings of 138A and 165A, both lower than the motor’s FLA.
If we go up one more size, we’re looking at the G540-02600UL-03, normally rated at 260A. When we take off the 23% derate, it’s now rated at 200A, which works for this application.
Note that some manufacturers may give different directions on combining derates. They may ask you to multiply, so in this case it would be (1 – 0.08) x (1 – 0.15), which leads to a similar result in this case: 0.782, or about a 22% derate. For other situations it may vary more.
Not Derating for Overload
Many heavy-duty applications require that you size a drive appropriately to give the right amount of overload. This isn’t a derate, however, as derates reduce the continuous rating on an inverter, while overload is for the temporary capability for load variances.
For this example, let’s look at the Mitsubishi F800 series, a drive built for pump and fan applications. This means that the VFD is often set up for 110% overload. What if you have a heavier duty application like a positive displacement pump, or something else that requires 150% overload? Your first instinct may be to derate the drive and upsize to give the components room to handle that overload, but you can cause the VFD to lose the protection it gives your motor.
Let’s look at the same system, 172A motor. You’d originally want to install an FR-F840-01800-E360, rated at 180A. The parameters will allow you to set an overload rating at 110% of the drives rating for 60 seconds, meaning 198A. Your motor may require up to 258A, a 150% overload of its 172A rating.
The temptation here is to do the math and upsize a VFD, maybe to the FR-F840-02160-E3U6 which would allow you to set an overload rating up to 237A (based on 110% of its 216A standard rating). This isn’t exactly 150% but gives you some extra room.
The issue is the range between the motor’s full load rating and the VFD’s standard rating. The VFD will see anything less than 216A as appropriate, but the motor sees anything above 172A as an overload. Your motor could consistently operate at 200A, burning itself up without the VFD ever seeing a problem.
If you’re limited by the ability of a VFD to derate, avoid running your own math on a derate. Find a line that can be programmed for the higher overload that you need or find an expert to help you figure out the math.
Some lines have this ability built in. The 480V models in the G500 line have ratings for both 120% and 150% overloads, meaning that it has been designed for and works with a wider variety of applications.
Get Help with Derating
Adding the math needed to derate on top of all the other sizing concerns can make getting the right VFD a tricky process. The consequences of getting it wrong can be severe, with the potential for equipment failures and downtime.
The experts at VFDs.com handle VFD sizing and derates every day. If you have questions or a tricky situation, they’re ready to make sure the VFD runs right and can handle whatever you throw at it. Reach out today to start sizing your next VFD.
Frequently Asked Questions
What is VFD derating?
Derating is reducing the continuous rating of a VFD.
When should a VFD be derated?
Common reasons to derate a VFD include ambient temperature, elevation, and phase conversion.
How much should a VFD be derated?
Derates vary by manufacturer and series and should always be verified in the manual of your specific VFD.
Does derating a VFD affect the motor?
Derating a VFD doesn’t directly affect a motor, although improving the health of a VFD can improve its ability to monitor and protect the motor.
Is derating and oversizing a VFD the same thing?
While they’re related, derating is not the same as oversizing a VFD. Derating adjusts the usable rating of the VFD while oversizing means selecting a larger model. Derating often results in upsizing a VFD, but it’s not always required.
Can I avoid derating by adding extra cooling?
Because the temperature derate is based on the ambient temperature, if you can be sure that you have lowered the ambient temperature then you can adjust that derate. Extra cooling will not offset elevation derates.
I live in the Rocky Mountains, what derates should I be worried about?
No matter the location, you need to evaluate the exact environment of the installation. It can vary wildly even within the same city, and you may be looking at conditions like indoor installations or direct sunlight that affect conditions. In the Rocky Mountains, elevation derates are usually involved (but may be minor), and temperature derates are also common.
My derate is in between two numbers, what do I do?
If you’re in between two numbers, you should default to the more conservative number. If you’re at 250m above the maximum elevation but it’s rule that gives 1% for every 100m, assume 300m. If your math leads to a new rating of 158.7 amps, round down to 158.