Configure Variable Torque VFD to run a Constant Torque Motor

The VFD should be capable of constant torque just fine. It should be able to easily produce 100% torque from somewhere around 5-10% speed to 100% speed.

The constant and variable torque ratings are kind of stupid these days, the reality is that any new constant torque VFD is just an oversized variable torque VFD so it can provide a higher current above FLA longer. So, more overload torque.

So, you think manually setting V/F will work?

By the default, the Motor Control type was on VC Quad. And the VFD would fault out after running for a few hours.

What was the fault?

Motor overload fault (OLF)

twoPlusPlus Yes, you're looking at the VFD which is commendable. Because I'm a rotating machines guy, I would also advise that you be sure the MOTOR is also rated for constant torque across the speeds you need. Some are, some are not. And a bit of "are not" goes a long way to letting that magic smoke out.

Converting energy to motion for more than half a century

Motor overload is likely caused by setting the overload up for a variable torque load. It can be set to constant torque use, but that assumes the motor doesn't rely on spinning it's own cooling fan. It has nothing to do with manually setting the V/Hz ratio.

is it an ATV630D75M3?

You can usually set up a VFD that was sized for Variable Torque to run a SMALLER motor at Constant Torque. The issue is ONLY about the overload capability of the VFD (not the motor). A VFD sized as Variable Torque (Normal Duty) can typically deliver 110% current for 60 seconds, maybe 150% current for 2 seconds or so. A VFD sized for Constant Torque (Heavy Duty) can typically deliver 150% current for 60 seconds, 180-200% current for 3 seconds. The issue is that a Variable Torque load with a properly sized motor is difficult to overload because of the nature of what a Variable Torque load does, so the VFD is not EXPECTED to have to deal with that. So the under the VT application use, the transistors inside of the VFD are smaller for a given HP rating. This can lead to a VFD tripping on "VFD Overload" because your load has demanded more current than the VFD is capable of delivering, but would not affect the MOTOR overload conditions.

In doing the changes for a VT load, the VFD will often ALSO have a different V/Hz output "curve" (as opposed to a fixed V/Hz ratio) because the load does not NEED as much torque as it slows down, so the VFD can reduce some losses in the motor by reducing the excitation current. That is done by enabling a "V/Hz²" output curve. If your VFD was configured like that, then applied to a Constant Torque load, you can indeed cause the drive to trip on MOTOR overload because it is EXPECTING much less from the motor. Whether or not your VFD can be re-programmed for a constant V/Hz pattern is going to be specific to your VFD, I don't like the Schneider drives (the manual sucks in my opinion), so I can't help you there, but MOST of the major player VFDs will have the ability to change that. What might happen though is that the internal programming for the VFD may then lower the maximum amp setting you can enter for the motor. Some do, some don't.

This would potentially solve the MOTOR overload issue, but will NOT change the VFD's capacity for running a CT load. That however is all going to be dependent on the ACTUAL LOAD on the motor (and thus the VFD) in your machine. So for example if you have a 75HP motor that is only running at 82A at full speed, and the VT rated drive is rated for 96A, then the OL capacity of the VFD is 110% of 96A, so 105.6A, which is going to give you 129% current capability for that load.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

can I boost the torque in this range to be 60% of max torque at 40Hz and 90% of max torque at 55Hz?

Click to expand...

This was the question about the torque load on the motor and a VT rated VFD which should be capable of this torque.

I finally got a chance to work on this yesterday. Changing the Motor Control Type to Standard V/F (from its default Quad) seems to have helped somehow. Maybe because the motor itself seems to be oversized for what the application needs so it never needs 100% torque.

@LionelHutz If I set the VFD to Constant Torque settings (DRT - Heavy Duty), then it de-rates the Motor HP to 60HP (Motor I have is 75hp).

@jraek I cant say that I completely understand what you said, but it looks like VFD behaved exactly the way you described (V/Hz^2 output curve). I also made sure that the VFD does not operate below 40Hz because the motor nameplate says 2:1 CT. Which I understand is its way of saying "at lower Hz, the motor needs 50% no matter what". Is this correct?

The “2:1 CT” means that below 50% speed on a CT load, the motor cannot properly cool itself.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden