Powerflex 525 Overvoltage (air blowers)

I think we have an issue on an air conveying table where the downstream blower is "pulling" the upstream blower slightly faster causing an overvoltage.
Is there a parameter that would help alleviate this?
I was told it was happening while they were running, but I have to wonder if it truly happened during a decel. Just for the heck of it, I increased the decel from 10 sec to 12 sec.

I have never heard of any vfd the you can adjust the buss overvoltage setting it factory set to protect the buss caps in the event of n overvoltage.
2 options I can think of
install a buss loader resister some call it a DB resister on the drive this will dump the excess energy into resistor and keep the buss voltage at a safe level.
or another option is to set both vfd's up as common buss configuration this way the excess energy in the regen unit will be passed back to the motoring unit and reused to power the fan. this will keep both vfd's buss voltage at normal level.

caution some vfd manufactures don't allow you to use the common buss configuration so it would be best to check with them first.

Not sure about the Rockwell VFD's, but you can likely turn the decel / regen / braking current / torque limit down to a minimum, like 0-5%...

phuz said:

I think we have an issue on an air conveying table where the downstream blower is "pulling" the upstream blower slightly faster causing an overvoltage.
Is there a parameter that would help alleviate this?
I was told it was happening while they were running, but I have to wonder if it truly happened during a decel. Just for the heck of it, I increased the decel from 10 sec to 12 sec.

Click to expand...

I'm a bit out of date on the Rockwell drives. Powerflex 700 was the last that I played with.

There was a bus regulation selection. Something like internal Brake, PWM frequency, None ... or something like that.

I never worked that well for me. I ended up changing the speed of the drive that was tripping to 55 Hz instead of 60 Hz, but only when the load current dropped below a threshold. It is still working. My condition was different - a bucket elevator was generating for a brief time only when empty and the buckets were heavier on the dump side. I never got a scope on the bus to prove it was generating, but this fix has been working for about 5 years.

Dynamic brake resistors are also available and will likely work for you. We have a return air fan that we slow down to 'pressurize' a room for an HVAC application. The fan blades work like a cork or a damper. The smallest available brake resistor (2A I think) works fine. It's barely warm to the touch.

Parameter A550 is the DC Bus Regulator function, which makes the drive react to rapid changes in DC bus voltage by dynamically changing the decel rate automatically. The factory default is Enabled [setting = 1]. If set to [setting = 0], someone purposely disabled it. The usual reason is because with it Enabled, the decel time can end up being variable instead of predictable and people sometimes don't like that. That then means you must manually extend out the Decel time to avoid nuisance tripping, something that people overlook if there is some condition, like what you described, that can cause an overhauling load.

Not sure about 525s, haven't had much exposure to them

On Mitsubishi A series drives you can turn on regen avoidance, it works like the function jraef describes, except it is can be valid at all times, not just deceleration.

The reality is if your drives are tripping on over voltage the only thing you can do is increase the running frequency or put the excess bus voltage somewhere - usually as heat through a braking resistor (though could be back onto the line)

I noticed a lot of reference to setting the regen torque limit to a small percentage to control the bs voltage. that will not really help you. You first need to be sure that you a a buss loader resister attached to the drive no control will work without a place to dump the extra energy.
if you are pulling to much air through the fans with the speed set at low speed then a simple solution is to just turn off the fan drive tripping. if it's not running it will not trip.
it looks to me like you really don't need it anyway for that application.