# Danfoss micro VLT FC 51 problem: Auto switches to OFF



## Phanx12 (Dec 20, 2019)

Hello everyone

I have a problem with several single phase Danfoss micro VLT FC 51 drives. All are ran by a PLC through digital contact, reference is supplied by analog input and all parameters are read over Modbus RTU. 

The problem is i sometimes see the drive wen't from Auto to Off and just sitting. They do sometimes, like once a day get an alarm, number 8 (Undervoltage DC bus) and in the drive setting i have set to infinitely recover. But it should still stay in Auto mode, not revert to OFF. 

I've looked many times in all the manuals online that i could find and couldn't find the answer. At the same time, perhaps someone knows if any of the drive's digital inputs can be tied to remotely activate AUTO mode or if it can be done via modbus? It's incredibly annoying to press AUTO each time on the LPC...


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## Cow (Jan 16, 2008)

So is the problem auto to off? Or the fact you keep getting a daily under voltage trip that in turn causes the auto to off condition?

From where I'm sitting, I'd be looking to correct the under voltage trip. At that point, one solution will fix both problems.


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## oliquir (Jan 13, 2011)

Did you try disabling hand mode button via parameters 0-40, 0-41 maybe if disabled vfd has no choice to go in auto mode?


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## gpop (May 14, 2018)

weird. Its sounds almost like the voltage at the motherboard has dropped to a level that it has to shut down so it can not do a self reset. 


WARNING/ALARM 8, DC under voltage
If the intermediate circuit voltage (DC link) drops below
the under voltage limit, the frequency converter checks if a
24 V DC backup supply is connected. If no 24 V DC backup
supply is connected, the frequency converter trips after a
fixed time delay. The time delay varies with unit size.
Troubleshooting
Check that the supply voltage matches the
frequency converter voltage.
Perform input voltage test.
Perform soft charge circuit test.


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## MotoGP1199 (Aug 11, 2014)

That's weird that it goes to OFF. Usually if I trip an alarm its still in AUTO but with the alarm present and motor not running. I've had a few drives do weird stuff on the programming side that I couldn't figure out. Now I'm in the habit of performing an initialization (I think 14-22) on the drive and programming from scratch. I haven't had issues since doing that. On another note I had a drive that kept getting the Under voltage alarm. I found a loose wire(factory connection) inside the drive on one of the terminals. Its been over two years since I tightened it up and the issue has not come back.


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## JRaef (Mar 23, 2009)

How is this powered? Any chance it is powered by a generator? The reason I ask is because I have had this issue with Danfoss (and other) drives; if powered by a generator, and someone stops the generator by just killing the fuel (not uncommon in some circles) without opening the generator breaker first, it allows the generator to "spin down" slowly and results in the system receiving low frequency power. That then causes the control boards to get scrambled because it gets internal data errors and check sum faults, so the MPU (Main Processor Unit) resets itself, losing it's memory of your previous programing and command state. If you were using the default programming for everything, you don't notice anything has changed there, but you do lose the last command state.


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## Phanx12 (Dec 20, 2019)

> So is the problem auto to off? Or the fact you keep getting a daily under voltage trip that in turn causes the auto to off condition?
> 
> From where I'm sitting, I'd be looking to correct the under voltage trip. At that point, one solution will fix both problems.



The problem is AUTO to Off. The drives (though only the single phase, there are 3 phase FC 51 and they are fine) even turn off sometimes when there wasn't an alarm. Parameter 15-30 is empty, so error log is clean. 

Under voltage trip for these might be normal, they are sitting on a 1000 amp power supply from a nearby enclosure, where the power jumps from 150 amps to 780 in 1,5 seconds. 




> Did you try disabling hand mode button via parameters 0-40, 0-41 maybe if disabled vfd has no choice to go in auto mode?


Already did, but those are just buttons. Those parameters disable the physical buttons to generate error 85 if pressed when disabled. I suppose it should keep out unauthorized personnel interacting. 



> weird. Its sounds almost like the voltage at the motherboard has dropped to a level that it has to shut down so it can not do a self reset.
> 
> 
> WARNING/ALARM 8, DC under voltage
> ...


Drive is configured to resume in AUTO if the power drops. It will be in AUTO and running at 50Hz, i'll shut it's power off completely, apply power and it resumes in AUTO back to normal operation. So, it can't be that. 



> That's weird that it goes to OFF. Usually if I trip an alarm its still in AUTO but with the alarm present and motor not running. I've had a few drives do weird stuff on the programming side that I couldn't figure out. Now I'm in the habit of performing an initialization (I think 14-22) on the drive and programming from scratch. I haven't had issues since doing that. On another note I had a drive that kept getting the Under voltage alarm. I found a loose wire(factory connection) inside the drive on one of the terminals. Its been over two years since I tightened it up and the issue has not come back.



Could be, i cleaned them thoroughly by each individual component, board just to be sure. I also made sure i'm not missing anything after searching 30 pages on Google. So i called Danfoss, they confirmed that there are no digital inputs to switch the drive over from Off to Auto. You can't do it via Modbus either, only read the coil status. 




> How is this powered? Any chance it is powered by a generator? The reason I ask is because I have had this issue with Danfoss (and other) drives; if powered by a generator, and someone stops the generator by just killing the fuel (not uncommon in some circles) without opening the generator breaker first, it allows the generator to "spin down" slowly and results in the system receiving low frequency power. That then causes the control boards to get scrambled because it gets internal data errors and check sum faults, so the MPU (Main Processor Unit) resets itself, losing it's memory of your previous programing and command state. If you were using the default programming for everything, you don't notice anything has changed there, but you do lose the last command state.



Powered from a 400V 1000 amps enclosure that supply a nearby offset printing press. Power is clean but as is normal, can jump from some hundred amps to almost a maximum rating. 



So after my call with Danfoss who confirmed my suspicion, that there is absolutely no way to set the drives from Off to Auto (or From Auto to Off or Hand for that matter) remotely. That pissed me off a bit, so i modded the drives a bit. Simple stuff, just added mplex connection wires to the buttons, should the PLC read that the coil for HAND/AUTO is reading 0, it'll toggle a relay for 500ms and put the drives back to Auto. 




After this, it's working fine. So, take that Danfoss.

Can see it here:


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## Phanx12 (Dec 20, 2019)

Tried posting in here earlier but apparently my replies need to be approved.


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## Dennis Alwon (May 9, 2009)

Phanx12 said:


> Tried posting in here earlier but apparently my replies need to be approved.


Often times a long post from a newbie has to be approved. I approved it


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## Phanx12 (Dec 20, 2019)

Dennis Alwon said:


> Often times a long post from a newbie has to be approved. I approved it


Hey thanks! Rules are rules, understood.


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