VFD Bus Overvoltage on startup

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philly

Senior Member
We have a 125hp 480V fan motor which is started with a VFD. We have recently had problems with the drive tripping on overvoltage during startup of the fan. When the drive operated fine it had a long ramp up time of about 3 minutes or so. Somehow the program got dumped and the ramp time defaulted back to a ramp time of 10s. This is when the problems started occuring.

My question is in theory how a smaller ramp time can cause DC bus overvoltage as oppposed to a longer one? Can a fan overhaul itself during startup?

Is it possible that other fans in the same ductwork are pulling a draft which is spinning this fan faster than motor frequency? Wouldn't this get worse for a longer ramp time however since the fan would spend more time at slower speeds while the draft was pulling it?

This drive has a DC brake which I'm not sure is not dumping this excess voltage. Should the chopper and brake be active during startup? I'm double checking to make sure that this brake is enabled.
 

TxEngr

Senior Member
Location
North Florida
I'm don't really have an answer but do have a few thoughts.

If you're tripping off on DC bus overvoltage, I suspect that your braking resistor circuitry is not working and probably not set up properly anymore. Otherwise the excess bus voltage would be dumping to the braking circuit. What I don't understand is why your load would be 'dumping' suddenly while starting up causing the bus overvoltage. The long ramp in the initial settings indicates that this fan is a high inertial load and must be started fairly slowly to keep from tripping on overcurrent. Is it possible that there's an overcurrent trip immeidately followed by the bus overvoltage trip? The bus overvoltage would be the last trip in the queue. Does your drive have the ability to see the last several trips or faults and not just the last one? I'll be interested in hearing the final conclusion on this one.

TxEngr
 

Jraef

Moderator, OTD
Staff member
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Occupation
Electrical Engineer
Most likely there was something else in that programming that you lost which was taking care of it. If you accelerate some types of fans too fast, they can indeed get ahead of themselves and overhaul a bit. If you had current limit set up too low, AND the dynamic braking is no longer enabled (with would likely be the factory default), then it's entirely possible to get an OV trip on acceleration, regardless of the status of other fans. In other words it's not likely a simple change in the accel ramp time that caused the problem, the problem probably exists and there was something done in the initial commissioning that took care of that, but you lost it when the VFD was reset to the factory defaults.
 

philly

Senior Member
If you had current limit set up too low, AND the dynamic braking is no longer enabled (with would likely be the factory default), then it's entirely possible to get an OV trip on acceleration, regardless of the status of other fans. In other words it's not likely a simple change in the accel ramp time that caused the problem, the problem probably exists and there was something done in the initial commissioning that took care of that, but you lost it when the VFD was reset to the factory defaults.

Can you explain how the current limit setting could play a role if the braking was not enabled? How can the current limiting lead to OV trips in this case?

So you dont think by simply chaning the ramp time from 3min down to 10sec would cause the OV problem?

I was somewhat involved during the commisioning of this drive and dont recall any other particular settings other than the Dynamic braking and the long acceleration time which was set from the begining prior to ever running the drive.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Can you explain how the current limit setting could play a role if the braking was not enabled? How can the current limiting lead to OV trips in this case?

So you dont think by simply chaning the ramp time from 3min down to 10sec would cause the OV problem?

I was somewhat involved during the commisioning of this drive and dont recall any other particular settings other than the Dynamic braking and the long acceleration time which was set from the begining prior to ever running the drive.
Current limit in a VFD is accomplished by monitoring current and if it attempts to climb above the setting, the output frequency is reduced regardless of the speed command from the speed control settings; current limit is an over ride condition. If there is enough inertia in the moving air or even in the fan itself, the fan shaft speed basically stays the same or reduces more slowly than the VFD output frequency is dropping, which creates an overhaul condition and regeneration. If the DB was turned on, the regen energy would be dumped off into the resistors and you may never know it. If the DB was reset to Disabled, the regen energy has nowhere to go.

Changing the ramp time alone may have done it too, but that depends on how the VFD handles current limit; if it reduces the commanded speed at the same ramp slope, increasing the ramp slope time might have the effect of allowing the regen to be dissipated by the drive itself without the DB being enabled. It also may have not allowed the VFD to go into current limit in the first place because with a long ramp time, the current never exceeded the limit setting.
 

sgunsel

Senior Member
I'm not sure either, but 10 seconds is a pretty demanding start for a 125 HP fan motor. I have had smaller fans trip similarly with short acceleration times. I suspect the VFD tries to supply enough amps by increasing the voltage to accelerate the fan but can't do it in the time available. You should reset the acceleration time to 3 minutes and it will probably be OK again. If it must start faster, reduce the start time until you get VFD tripping again. Just don't start too often - my 20 HP fan motor is recommended to start no more than 2 times per hour, which probably assumes across the line starting.
 
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