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De-rating Factors On Vfd

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Dear All,


I believe that de-rating factors are vital while engineering a system. I want to know exactly the de-rating factor normally applied on Variable Frequency Drives on its capacity over and above the ambient temperature and altitude.



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It's going to depend upon the manufacturers.


There is no universal standard to which they must adhere, and temperature is an area where many of them tend to become less than conservative when it comes to making their product appear less expensive. In other words, you are right to be concerned about it, but unfortunately the responses to this issue will differ from brand to brand. A good rule I have always followed it to de-rate a VFD by 2.5% for every 1°C over the basic temperature design spec, but only up to the stated maximum. So for instance if the basic temperature rating is 40°C and your environment is 46°C, you must de-rate the VFD by 15%. I have also seen some drives that say to de-rate only 1% for every 100m.


One thing to watch out for is that some drives have a basic temperature rating of 50°C to start with, and you cannot assume that it is OK to over-rate them if your ambient is only 40°C. Most VFDs will have a maximum temperature rating of 50°C anyway, so beyond that the de-rate factor cannot help but again, you need to be careful because sometimes a manufacturer will have a MAX rating of 40°C as well, which means you cannot use de-rating at all. You also need to be careful of footnotes in the specifications. Danfoss for instance states that their VFD current ratings are based upon 45°C, 55°C max. with derating. But if you read the footnote about that 45°C rating it says that "the 24hr average must be below 40°C". What does that mean? If means it can tolerate 45° for a short time, but not continuously, so to be conservative you really need to ignore the 45° rating and work based on 40° like everyone else.


Altitude is a similar issue, although every VFD I have ever seen is rated to 1000m (3300ft.) without derating. I then use a de-rate of 1% for every 100m over the basic rating. So at a 10,000ft altitude (3000m), the de-rate would be 2000/100=20, so a 20% de-rate. Another formula used a lot is 2% for every 1000ft above 3300ft., so that equates to 1% for every 150m, a little less stringent.


But with altitude you also need to be cognizant of voltage derating as well. I also use 1% voltage de-rate for every 100m above 1000m. So at that same 3000m, the voltage rating would also go down by 20%, meaning if you had a 500V class VFD, the new max. voltage rating would be 400V. If you had a 400V rated VFD however, it would go down to 320V, meaning it may be problematic to use it on a 400V system. That's why I tended to recommend 690V rated drives to my clients who were sending 460V equipment to Chilean mines which are often at around 10,000ft elevation, because when you de-rate the voltage, it is still over 480V dielectric.

"He's not dead, he's just pinin' for the fjords!"
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Further to the excellent comments from Jraef,

The semiconductor devices are affected by the internal junction temperatures. Typically, the maximum junction temperatures are in the order of 125 C to 150C depending on the devices. As the ambient temperature increases, the junction temperature increases if the current remains the same. In order to limit the maximum junction temperature, the current must reduce as the junction temperature increases.


There are other components that are more sensitive to temperature. The power Electrolytic capacitors age quicker as the temperature rises. A standard rule of thumb is that the life of a capacitor is halved for every ten degree C temperature rise.

Optocouplers are used to transfer signals to the power output stages, and these can be greatly affected by temperature. Hence there are definite limits placed on the maximum operating temperature that are not directly related to the cooling of the power components. It is common for the maximum temperature to be limited to 55 - 60C


Elevated altitudes reduce the density of the air. This reduces the cooling efficiency of the heatsinks and so the maximum current reduces with altitude.


You need to refer to the manufacturer (or agent) for the figures for your specific product.


Best regards,

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