Constant vs Variable Torque VFD
Constant vs Variable Torque VFD
(OP)
Hello All,
My question concerns a particular VFD that, as indicated in the manufacturer's literature, is rated for a maximum current of 31 amps when used in a constant torque application or 48 amps when used in a variable torque application. Can someone please explain why the large difference in maximum amps?
Thanks
My question concerns a particular VFD that, as indicated in the manufacturer's literature, is rated for a maximum current of 31 amps when used in a constant torque application or 48 amps when used in a variable torque application. Can someone please explain why the large difference in maximum amps?
Thanks





RE: Constant vs Variable Torque VFD
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 will 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.
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 or burning up.
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RE: Constant vs Variable Torque VFD
On the other hand, if you take the same drive and derate it to 31 amps continuous and call it a constant torque drive, then you can say that it delivers 50% overload current or around 48amps for one minute. As you noticed, its the same drive, just different overload ratings. Why the two ratings are called variable and constant torque is buried in the early history of VFD's and doesn't matter anymore. In fact, some drive manufacturers are now calling them "normal duty" and "heavy duty" drives to avoid the confusing terminology.
In fact, both types of drives can operate both variable and constant torque loads as long as the overload requirements stay within the drive limits.
Hope that clears up what, to me, is an unfortunate set of misleading terminology.
RE: Constant vs Variable Torque VFD
RE: Constant vs Variable Torque VFD
RE: Constant vs Variable Torque VFD
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies