# Drive tripping on over current



## ponyboy

Has anyone looked at the current? Has the mechanical load on the motor changed or is it somehow changing when the trips occur? I'd put a power logger on it and tell the jackoffs in charge to STOP throwing parts at it for the love of god.


----------



## Big John

If it's tripping on overucrrent, what are the current values per phase during a trip? What's the normal running current? Is it belt-driven or direct? Is the plenum shared by any other blowers?


----------



## union347sparky

Replace the fan. Then the bearings and belts. If it still does it replace the whole unit. If that don't do it move to a new building.


----------



## Wirenuting

Check the belt sizes, make sure they are a "matched" set. If they aren't the load will spike as the belts flex. 
Insure the bearings are not over greased, insure the sheaves are the correct size and not one of them darn adjustable ones. 
Make sure they are correctly installed by using the alignment equipment. 
Check for closed fire dampers and check to insure the outside, face/bypass dampers operate correctly. Check down stream VFV boxes and return ducts. Check total volume of air being moved for correct amount. 
Check the filter rack for clean filters. 
Check heating/cooling coils to insure they are clean. 
Lastly if "everything" is fine, throttle VFD back slightly and see if it still kicks off, but insure your logging it. 

If you have a building automation system, log it and check all code and subroutines for changes.


----------



## Wirenuting

Oh ya, one more thing, if your in a cold weather area insure that no one replaced your freeze stat, low temp cut off, with one that auto resets. If your using a temp sensor to protect from coil freeze up, look hard at that line of code & install a real freeze stat. 
If someone pulled the freeze stat out because it trips sometimes, check your heating coil. If its a newer coil it might be a tube in tube design for steam. Use a heat spy on the dead end side of the coil to find the rotted inner tube. It will be the tube end that has the lowest temps 
You'll understand what I mean when you see it, if you have this PITA style. Steam and condensate are piped on the same side of this efficient coil..


----------



## gnuuser

I dont highly recommend using a vfd to drive a fan, or blower
due to the fact that loss of power will result in feedback from the magnetic field collapse will create a massive and catastrophic spike back through the vfd and frying it.
these setups require a strong clamping circuit.
even shutting them down improperly can damage them

you are far better off using a rheostat type controller


----------



## erics37

Saw this same fault on a drive last week at the fish plant. It's running a submersible grinder pump for waste water.

It tripped out on overcurrent fault, the maintenance guy reset it, it worked, then tripped again, he reset it, it tripped again even quicker, etc etc  That's when they got around to calling me.

I disconnected the T leads from the drive and megged it, barely even registered on my megger. Went down under the building to the pump well, unplugged the motor cable and megged it right there - still barely any resistance. Megged the supply cable, it was in great shape.

Thought it was kind of weird that it got this toasted without ever tripping the VFD's ground fault alarm, dug through the parameters and found that someone had turned the ground fault alarm off :laughing:

Anyway that's sumpin' to check. If I recall, it was an ACS 550 and the ground fault alarm indicator is Parameter 3017 or thereabouts (pretty sure it's in Parameter group 30 at least).


----------



## Wirenuting

gnuuser said:


> I dont highly recommend using a vfd to drive a fan, or blower due to the fact that loss of power will result in feedback from the magnetic field collapse will create a massive and catastrophic spike back through the vfd and frying it. these setups require a strong clamping circuit. even shutting them down improperly can damage them you are far better off using a rheostat type controller


We have hundreds of drives on hour air handlers with great results. Need to keep them clean as heat kills. 
But the improper shut down can kill a drive when it spikes the line.


----------



## Big John

gnuuser said:


> I dont highly recommend using a vfd to drive a fan, or blower
> due to the fact that loss of power will result in feedback from the magnetic field collapse will create a massive and catastrophic spike back through the vfd and frying it...


 I don't follow. How does turning off a blower differ from turning off any other motor load?

You never disconnect the load side of a running VFD regardless of what it's connected to.


----------



## ponyboy

Wirenuting said:


> We have hundreds of drives on hour air handlers with great results.


Same here. All of our rtus are on drives, we wouldn't have them any other way. Rheostats are extremely impractical for production facility use. 

Last batch I hooked up simply require an end switch to turn unit on and off. No problems at all with the vfds


----------



## Wirenuting

ponyboy said:


> Same here. All of our rtus are on drives, we wouldn't have them any other way. Rheostats are extremely impractical for production facility use. Last batch I hooked up simply require an end switch to turn unit on and off. No problems at all with the vfds


For me getting the bosses here to install aux's switches on the local disconnects is like pulling a 5 year olds Tooth. They don't see or understand the reason. 
I would say that most of our early drive failures are due to people opening the local disconnects when servicing them.


----------



## denny3992

gnuuser said:


> I dont highly recommend using a vfd to drive a fan, or blower
> due to the fact that loss of power will result in feedback from the magnetic field collapse will create a massive and catastrophic spike back through the vfd and frying it.
> these setups require a strong clamping circuit.
> even shutting them down improperly can damage them
> 
> you are far better off using a rheostat type controller


Did 2-700hp id fans on kilns... Installed load side of vfd contactor to disconnect output of vfd cuz on power loss a natural gas engine sarted and turned fan to keep fire going! 

Talk about interlocks...

Had a speed sensor that didnt allow vfd to start unless speed was under 300rpm 

It was a
Relay nightmare!
All existing stuff we had to interface with!!


----------



## Jabberwoky

gnuuser said:


> I dont highly recommend using a vfd to drive a fan, or blower
> due to the fact that loss of power will result in feedback from the magnetic field collapse will create a massive and catastrophic spike back through the vfd and frying it.
> these setups require a strong clamping circuit.
> even shutting them down improperly can damage them
> 
> you are far better off using a rheostat type controller


Gnuuser, most drives these days are design to take large CEMF spikes. There are diodes integrated into the transistor modules just for that purpose. I have heard stories of installers undersizing drives with blowers that then have CEMF problems. A few temperature control systems I have encountered actually have a line contactor turn off the drive instead of using its start/stop controls.


----------



## gnuuser

That I understand 
But you see all kinds of crap happen when you work with 100 HP. And above.
I've actually seen a 250 HP motor blow an inverter apart.
Quite a mess


----------



## JRaef

althecontrolguy said:


> A 15Hp ,460 volt, 17.7 amp ABB drive, is tripping out on over current. The motor is running a fan in an AHU. I didn't start the diagnoses, but I got pulled in, because right now nobody can find the problem. Originally, the motor megged greater 2.2GΩs. So they replaced the drive, same problem. The motor was then replaced, same problem. Replaced the wire between drive and the motor, same problem. Replaced the drive again, same problem. The drive does not trip right away, but seems like there is a 7-10 day interval, between trips. Could dirty power do this? All lugs at the drive, and in the peckerhead are tight.Any other ideas as to what to look for?





ponyboy said:


> Has anyone looked at the current? Has the mechanical load on the motor changed or is it somehow changing when the trips occur? I'd put a power logger on it and tell the jackoffs in charge to STOP throwing parts at it for the love of god.





Big John said:


> If it's tripping on overucrrent, what are the current values per phase during a trip? What's the normal running current? Is it belt-driven or direct? Is the plenum shared by any other blowers?


Looks like our OP went AWOL on us, too bad. These were the right courses of action, maybe he took a look and discovered, wonder of wonders, that the drive was tripping on over current because.... drum roll.... the motor was drawing too much current too quickly!

If you are still there Al, there is a slight difference in in a VFD between an Over Current trip and an Over Load trip. If the motor was running along and the flow from the fan was too much, the motor draws too much current over a long time and the motor thermal over LOAD protection kicks in, just like it would on any motor controller. You can get too much air flow when there is a problem with the plenum, the air duct, not providing sufficient pack pressure, ie "open channel flow". When that happens, the power required by the motor increases at the cube of the flow, so the motor overloads. This can happen in an HVAC system when someone remodels, cuts into a plenum to get air into a room, and pays no attention to the effect that will have on the fan load.

An over CURRENT trip on a VFD however indicates that for some reason, the motor (or circuit) is very suddenly is demanding more current than the VFD is capable of delivering. In the case of a typical HVAC fan drive, that could be as little as 110% of the VFD current rating for 30 seconds, or 150% for 3 seconds. In this case, the drive is tripping off to protect ITSELF. This can happen in HVAC systems if there is something like icing taking place, a bearing is seizing up, a motor winding is going bad etc. etc. You have eliminated the motor winding possibility and a motor bearing possibility by changing the motor, you eliminated the possibility of it being the drive or cables by changing them. What's left that hasn't changed? The belts and bearings, icing or the possibility that the load design is the problem.

What can happen there is that under some circumstances, the fan is being called to run when other fans sharing that plenum are already running, and they have no automatic dampers to prevent this fan from spinning due to the air draft pulling through it, called "windmilling". If the fan is spinning forward, the cure may be as simple as enabling a feature called "Flying Restart" in the VFD that allows the VFD to detect the motor speed, match itself to it, and then turn on, avoiding the current spike that can take place. But if the fan is BACK spinning, that is a more difficult problem. The automatic backflow dampers are supposed to prevent this, but if they are not there, that might be the best cure. If for some reason they can't do that, you may have to find a way to BRAKE the motor to a stop before trying to run it. Some VFDs can do this, it's called an "Anti-Windmilling Brake" feature. If your drive doesn't have that, but it does have Dynamic Braking as an option, you may need to rig up a work around using that. If that's the case, post back and I can guide you on how to do it, I've done it many times.

Or you may be off on another project because this problem went away, and all this was a complete waste of time, other than to get it off my mind so I can sleep at night... :whistling2:


----------



## gnuuser

denny3992 said:


> Did 2-700hp id fans on kilns... Installed load side of vfd contactor to disconnect output of vfd cuz on power loss a natural gas engine sarted and turned fan to keep fire going!
> 
> Talk about interlocks...
> 
> Had a speed sensor that didnt allow vfd to start unless speed was under 300rpm
> 
> It was a
> Relay nightmare!
> All existing stuff we had to interface with!!


i know what you mean! our tank cooling fans for the molten glass have a blower head of cast aluminum 
they weigh about 1/2 a ton each and act as a big flywheel when power is removed.
we have them sequenced to ramp down when initiating the shut down program but it takes a while to come to a complete stop
our biggest enemy is not power failures but impatient management .
rather than use the shutdown program they will just throw the panel main disconnect


----------



## denny3992

gnuuser said:


> i know what you mean! our tank cooling fans for the molten glass have a blower head of cast aluminum
> they weigh about 1/2 a ton each and act as a big flywheel when power is removed.
> we have them sequenced to ramp down when initiating the shut down program but it takes a while to come to a complete stop
> our biggest enemy is not power failures but impatient management .
> rather than use the shutdown program they will just throw the panel main disconnect


Hurry hurry hurry! Time is money!

Ask them why did they waste the money for the programming and hardware to do it the right way then?


----------



## MDShunk

Sort of a dirty method, but it works 99% of the time when a drive trips out randomly on overcurrent (assuming the wire, motor, and local disconnect meg fine).

1) add time to accel
2) add time to decel
3) enable flying restart
4) add an interlock contact to the local disconnect, tied to the enable terminal on the drive for knuckleheads that insist on using the local disco all the time (this should be standard wiring practice by now)
5) add a properly sized load reactor
6) do your drive's equivalent of "HIM Copycat" and copy the whole setup from a similar working drive into your suspect drive. Sometimes a setting you never thought of got changed by ghosts or whoever.


Being that drives are reasonably self-diagnostic, I tend to believe what they say in the fault que, and seldom expect the drive to have failed when overcurrent faults are shown.


----------



## wdestar

Maybe I missed it. Did anyone suggest cleaning the fan blades?


----------

