# Danfoss FC-103 VLT Drive Issues



## JRaef (Mar 23, 2009)

To me this sounds like more of an issue with whatever is controlling the bypass and/or cycling the loads. Cycling loads on the output of a VFD is a recipe for disaster in my opinion. People do it all the time and "get lucky", but luck is not a valid design strategy and this is not something that any VFD manufacturer says is OK. When contactors open and close on the output of a VFD while it is running, the switching will cause transients on the output side of the drive that can damage the transistors; opening the contacts under load causes a voltage transient, closing when the drive is already running subjects the drive to the motor inrush current transient. The transistors are not designed to deal with either scenario very well.


But, that's not what you are experiencing here because (so far) your drives are not failing. Does that drive have a fault history queue that you can look back on? I'd be curious if the VFD was tripping on OC from the transients and is transferring over to bypass, then resetting itself. But in the process of transferring over to bypass, the motors going from a low speed (from the VFD) to full speed in bypass while they are still spinning when that takes place, is occasionally causing a current spike that in some cases is high enough to blow fuses. This can happen because the motor's were at a different frequency when the bypass switched over and their magnetic fields may not have collapsed yet when the bypass closes so you are essentially connecting two power sources out of sync. If there is a fault history in the drive, you could look at what happened BEFORE the bypass kicked in.


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## gpop (May 14, 2018)

JRaef said:


> People do it all the time and "get lucky", but luck is not a valid design strategy and this is not something that any VFD manufacturer says is OK.


AB blessed the design where i worked as they could not build a drive that could handle the over hauling cam on one machine. By using more than one machine the regenerated voltage could be dumped into the second motor. (odd problem sometimes takes a odd solution, They did oversize the drive add line reactors and a brake in there design) 

when it said it was running yet showed no amps or hertz was the "running" flashing. If so it thinks that the stop button was pressed on the drive display panel. If it was not flashing then it sounds like the control card is have issues. 

Check the cooling fan on the drive as this can cause problems if the fan is locked up. 


what are the motor sizes, what size and type of fuse are you using (time delay, fast acting), what size is the drive, what is the ramp time, what is the max amps setting on the drive. (wondering if all 6 motors are being called for at the same time)


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## frenchelectrican (Mar 15, 2007)

gpop said:


> what are the motor sizes, what size and type of fuse are you using (time delay, fast acting), what size is the drive, what is the ramp time, what is the max amps setting on the drive. (wondering if all 6 motors are being called for at the same time)


I been wondering too with 6 motors on single drive IMO that is not the best combation to deal with it. 

I am thinking either inrush current or phase sequence is not right on the par and went to bypass mode quick.

I would run each motor on it own VSD and use the interface card to daisy chain the unit in parallel mode type. so that way if one of the motor deciced to drop out of the line the other stay on and throw a code for single VSD instead of whole thing.


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## SpaceMonkey (Sep 19, 2018)

Thanks for the help so far guys, really appreciate it.

In the fault history, there are no faults at all in the past. Neither in the fuse blowing scenario or the intermittent stopping did any sort of error code generate. 

When it was shut down, I tried to press "stop" and then "run". drive would not do anything. Take in mind it was still in "run". Only when I cycled power did it immediately start and ramp up. 

I swapped the control card with another working drive, and it shut down again within a couple hours. I restored the drive to factory defaults, copied program from one of the other drives(same drive, controlled the same), and it did it again.

During normal operation, the contactors on the output of the drive are being cycled frequently, as in usually the controller will bring on a set or drop a set of fans every 5 minutes or so.

The following specs for the motors are:

6 motors
1.5HP each
460v 
5 motors have an FLA of 3.5 
1 motor has an FLA of 2.9
Total FLA: 20.4

Only thing that is throwing me off about the motor data, is the drive asks for what the KW is, not the HP. (Which I'm assuming is going the be the sum of all 6 motors). 

I took 1.5(6) x 746 = 6.7KW 

I selected the closest value to that in the drive (slightly higher), and it seems I get a "torque limit" warning when in full output(60hz), and the current limit is set to 110%. For some reason the drive shows a higher KW being consumed. Current draw though is under 20.4amps. 

Each motor with full voltage and at 60 hertz draws its FLA.

The drive seems to always put out a balanced voltage and current.

Otherwise I've made sure the motor data is correct.

I will have to update you guys with the other parameters in the next day or so, as I do not remember off hand what the ramp time what set to.


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## SpaceMonkey (Sep 19, 2018)

Fuses are Bussman Type T JJS-40, as called for in the manual.


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## gpop (May 14, 2018)

3.5 amps for a 1.5hp motor sounds odd.

The drive isnt protecting the motors so normally the drive would be set at what ever its max rating is. If you calculate the amps of all 6 motors to kw your calculation looks well off.


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## gpop (May 14, 2018)

SpaceMonkey said:


> Fuses are Bussman Type T JJS-40, as called for in the manual.


so fast acting and current limiting 40 amp fuses.


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## SpaceMonkey (Sep 19, 2018)

gpop said:


> 3.5 amps for a 1.5hp motor sounds odd.
> 
> The drive isnt protecting the motors so normally the drive would be set at what ever its max rating is. If you calculate the amps of all 6 motors to kw your calculation looks well off.


Please elaborate? can you correct my calculations and show me where I went wrong? I will double check those FLA values I posted tomorrow, but I am near certain that's what the nameplate states. I thought I've read before that sometimes the HP rating of a motor can be a bit skewed across different manufactures. Could that be part of why my calculations are off?


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## scotch (Oct 17, 2013)

Years ago we had a similar problem after a VSD retrofit on a 6 fan cond. unit ; when some fans were taken offline they started to spin in reverse with airflow being drawn thru' by the others and when re-energised would cause all sorts of issues. we had to fit backflo dampers in the end.


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## SpaceMonkey (Sep 19, 2018)

Swapped the problem drive with another rack's drive. Now I've went 2 weeks with no faults on either drive. Seriously confused now....before the drive wouldn't run more than a day.


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## Taz069 (Oct 8, 2018)

Space this brings up a couple of interesting issues. Did you reinstall the problem drive into another slot. To me it sounds like a programing issue with this particular drive be it paramaters to sync start, ramping speed etc. Please understand I haven't fooled with these systems in close to 15 years. With the beta raven system I last installed programing all the variances into the system was the most problematic issues, seeing all the different variables that weight into the equation. It would be interesting if you put that problem drive into another system and same issues appeared.


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## SpaceMonkey (Sep 19, 2018)

I'm bumping this as I had an idea. In an application like this with multiple motors and motors being started and stopped on the output, could having the drive not set to a "flying start" possible cause some of my issues?


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## JRaef (Mar 23, 2009)

Probably not, but you are sailing in uncharted waters as far as what a VFD is DESIGNED to do. Flying Start works by the drive injecting a small DC current into one winding, using it to measure a SINGLE spinning motor's back EMF to determine its speed, so it matches its frequency to that before fully energizing the output. When you have multiple motors and some are already connected, there is no way for the VFD to even notice the disconnected motor being added in, other than the sudden and immediate change in current.


It's more likely that the snubber circuits that are protecting the output transistors are being stressed by the act of turning motors on and off on the output, and eventually those snubbers are failing, which then affects the transistors. Again, VFDs are not designed for that type of application and IF you get away with it, it will only be for a while. You installed another drive in which the snubbers have not yet failed.


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