PF755 Braking Resistors Enabled is Hit or Miss

I installed braking resistors on 15 identical drives over the last few weekends. After enabling them on one line using CCW, they worked fine. On two other lines, they do not seem to use the resistors at all. There is no discernable difference in stopping from standard at about 7 seconds. We are targeting 3 seconds.

I feel like I'm missing something obvious that I did on one line but it's just a handful of parameter changes! I even copied from one drive that was working (identical FW on 14.002) and it still didn't work. Any ideas are appreciated.

Parameters Changed:

372 Bus Reg Mode A Both DB 1st
373 Bus Reg Mode B Both DB 1st
382 DB Resistor Type- External
383 DB Ext Ohms - 20 Ohms
384 DB Ext Watts- 1000 Wats
385 DB Pulse Watts 57200 (calculated with rockwells worksheet)

Stop mode is in Ramp for quickest stop time.

When checking the HIM on each drive the parameters are correct.. but stopping performance is unchanged.

The simple answer would be to swap drives from a line that brakes correctly with a line that doesn't.

Same behaviour, the problem is external to the VFD.

Correct behaviour, the problem is with the VFD.

If the lines are identical, and the drives are the same model, you could try copying the whole of the working drive configuration to a non-working one. There may just be some setting you have overlooked.


Also check firmware revision levels in the drives ....

Are you getting dc bus faults on the ones that won’t stop in time? If not then the braking resistor is doing its job, you may have a parameter issue else where.

The drives may be identical, but maybe the load that has to be stopped is different ?
As Dock suggests, you may get DC bus faults. If the cause is that the inertia is too great to stop quickly, then probably cannot fix it with a parameter change. You may extend the stopping time which may avoid the DC bus faults, but then you will have essentially the same result in the end.

If you are running in FV (which I usually recommend). Make sure P426 is cranked to -800%. That way your other braking and tuning parameter limits come into play before the regen limit does.

While the linked thread references the PowerFlex 70 drive, it is worth reading for some information and opinions on brake resistor setup.

PowerFlex 70 Braking with External Resistor

What size of PF755 drives are we talking about here? Because on Frames 6 and 7 (75 to 350HP) the Braking Chopper does NOT come standard with the drive, you have to order it that way from the factory. If your drives were not ordered with the Brake Chopper installed, you might still have the terminals, but there is nothing connected to them. In the part number of the drive, the second letter after the size number is indicative of whether the brake chopper is there. if it's an "A" it is there, if it is an "N" it is not. So for example;
20G1AND248AA... has a brake chopper,
20G1AND248AN... does not.

If that's the case, they cannot be added in the field, you have to buy a new drive with the chopper from the factory.