Constant vs Variable Torque VFD 1

[MNMaint]

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

 

[jraef]

The issue is a difference in the VT rating, consider the CT rating as "normal" for this purpose.

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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[DickDV]

What you have there, MNMaint, is some of the "snake oil" that runs around in the VFD world. The problem is the terminology. A variable torque drive has nothing to do with a variable torque load. It simply means that at the variable torque continuous current rating (in this case 48amps) it can deliver 10% overload current or 53 amps for one minute before the heatsink gets too hot and the drive faults out.

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.

 

[MNMaint]

Thanks a lot jraef & DickDV. Your posts helped clear that up a lot for me!

 

[majesus]

Just now I have my MCC Model 6 square D book open to check out typical bucket sizes for VFDs and saw the variable torque and constant torque ratings. Curious I checked up on it and boom google takes me right to this thread. Thanks for info guys. I love this forum.

 

[jraef]

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