mikeybmdb32
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I'm having difficulty understanding why variable torque VFD's have a higher current rating than constant torque VFD's. Is it assumed that the variable torque VFD will be running below it's maximum speed all of the time?
Constant Vs Variable Torque Ratings for VFD's
- Thread starter mikeybmdb32
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It is usually to do with the short term overload capability. I have seen variable torque drives with a short term overload capacity of 110% (i.e. rated capacity plus 10%) and constant torque at 150%.
Typically it will make one step difference in the rating. For example, the same drive might be rated for 90kW VT and 75kW for CT.
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When connected to a true "variable torque" load such as a centrifugal pump or fan, it is essentially acknowledged that the power requirements of the load is what is called "quadratic", meaning the power requirement will drop at the square of the the speed decrease (look up "affinity laws" for more details). Since the power devices in a VFD are rated based upon heat rejection under load, if the load is decreasing faster because it is a VT load, then your power devices will not heat up as much. So manufacturers of VFDs can essentially say that the VFD is capable of a higher overall current rating IF the load fits that quadratic profile.
They ALSO reduce the "overload" capacity of the VFD as well. Normally, a CT rated VFD is capable of at least 150% overload for 1 minute. But a VT rated VFD will say something like 120% for 30 seconds, or 110% for 60 seconds (it varies by mfr), again a concession based on the fact that it is theoretically less likely that a VT load will overload the VFD for that long unless it needs to trip anyway (i.e. a jammed impeller etc.).
So in essence, the higher current/power rating of a VT rated VFD is akin to a "definite purpose" rating. Any use outside of that narrow spectrum of load profiles will result in the VFD tripping off to avoid burning up.
And for those of you who also frequent the other fora I participate in, yes this is a copy of another post of mine...
They ALSO reduce the "overload" capacity of the VFD as well. Normally, a CT rated VFD is capable of at least 150% overload for 1 minute. But a VT rated VFD will say something like 120% for 30 seconds, or 110% for 60 seconds (it varies by mfr), again a concession based on the fact that it is theoretically less likely that a VT load will overload the VFD for that long unless it needs to trip anyway (i.e. a jammed impeller etc.).
So in essence, the higher current/power rating of a VT rated VFD is akin to a "definite purpose" rating. Any use outside of that narrow spectrum of load profiles will result in the VFD tripping off to avoid burning up.
And for those of you who also frequent the other fora I participate in, yes this is a copy of another post of mine...
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Nothing wrong with doing that.
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