# Coincidence or screw up!



## just the cowboy (Sep 4, 2013)

Background:
Yesterday we ripped an unwind belt unit of off a machine and repaired some bearing issues, strictly a mechanical job and we did not disconnect anything electrical at all. After reinstalling the unit we turned the unit by hand to align the belts to the motor, no problems all spins free and tracks correctly. After unlocking and turning power back on “DC drive did not power back up". Checked drive and power is good but it is dead. 
Question! 
Could it of blown the drive by turning the motor by hand? 
My thoughts:
No,  
First: It is a field motor not a permanent magnet motor so turning it by hand should not generate any voltage. 
Second: It is a 4 quad drive made to go in either direction, so turning forward or reverse would not hurt it even if the motor generated some voltage. 
Third: If the machine was running and we lost plant power this same situation would occur where the load would keep turning the motor for a short time just like we did when checking by hand. We never lost a drive like this after an outage. 
So again Coincidence or screw up! 
What are your thoughts?
 Thanks
Cowboy


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## wildleg (Apr 12, 2009)

I have no idea as to the answer to your question, and I know that this is hindsight, but one thing that I try to be cautious about is allowing coincidences to happen. What I mean by this is that , say you had cycled the machine down, then cycled back up prior to performing any work - you would know that the machine is in proper order. Then when you took down to do the work, if it didn't come back on line you would know that it was due to the work and the question as to whether or not it was a coincidence would not exist. Similarly, some times, with complicated machines, it is also better (if at all possible, for time and money reasons), to use a single point failure design to the work, so that failure can be keyed on one item (ie, replacing mulitple controls and a motor and no knowing what has failed on restart)


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## just the cowboy (Sep 4, 2013)

wildleg said:


> I have no idea as to the answer to your question, and I know that this is hindsight, but one thing that I try to be cautious about is allowing coincidences to happen. What I mean by this is that , say you had cycled the machine down, then cycled back up prior to performing any work - you would know that the machine is in proper order. Then when you took down to do the work, if it didn't come back on line you would know that it was due to the work and the question as to whether or not it was a coincidence would not exist. Similarly, some times, with complicated machines, it is also better (if at all possible, for time and money reasons), to use a single point failure design to the work, so that failure can be keyed on one item (ie, replacing mulitple controls and a motor and no knowing what has failed on restart)


We power up/down the machines all the time to work on them for lockout reasons. Never saw a need to power a machine back up every time we work on one. 

And it was a one point repair not multiple repairs.


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## John Valdes (May 17, 2007)

Coincidence.
Check all E-Stops too.


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## mitch65 (Mar 26, 2015)

I cant imagine turning that motor be hand would be the cause. More likely an internal fuse on the drive gave up the ghost


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## JRaef (Mar 23, 2009)

So technically, you CAN regenerate energy from the motor by spinning it by hand, because there is always a small amount of residual magnetism. But could it damage the drive? If that were possible, that's one weak ass drive... All of your other scenarios would hold true as well, i.e. power losses, etc. causing the same thing. Bottom line, that's not what damaged the DC drive.

Was it a rubber belt? I have, on two occasions, seen a visible static discharge spark from removing a rubber conveyor belt that was running on isolated rollers. So as soon as the belt brushed against a metal frame, it sparked. On one of those, a little PLC failed to power up again, which we attributed to that static discharge spike on the ground plane.


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## John Valdes (May 17, 2007)

JRaef said:


> So technically, you CAN regenerate energy from the motor by spinning it by hand, because there is always a small amount of residual magnetism. But could it damage the drive? If that were possible, that's one weak ass drive... All of your other scenarios would hold true as well, i.e. power losses, etc. causing the same thing. Bottom line, that's not what damaged the DC drive.
> 
> Was it a rubber belt? I have, on two occasions, seen a visible static discharge spark from removing a rubber conveyor belt that was running on isolated rollers. So as soon as the belt brushed against a metal frame, it sparked. On one of those, a little PLC failed to power up again, which we attributed to that static discharge spike on the ground plane.


I hate when **** happens!
I was troubleshooting a line of automated machines and had the alligator clip on one of my meter leads.
I saw a tiny little spark and I found out I took out a PLC.

Lucky for me, the people understand mistakes happen and we worked it out.


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## JRaef (Mar 23, 2009)

You admitted it???!!! 

I did something similar once, luckily it didn't damage anything, but it did prevent it from working. I had left a jumper on a circuit after testing it, so although it worked fine in "Hand" for my testing, when they put it in "Auto" after I left, it wouldn't work (the system detected the conflict). They called my back out, gave me a PO and everything. When I saw my jumper, I took it off, luckily nobody saw it. I knew that was the problem, but I went ahead and reloaded the program, turned it on and everything worked. I was the "hero", but of course all I really did was correct my own mistake. I told them they didn't have to pay me for that 2nd call because I should have tested it in Auto before leaving (which was true, but I had finished after the operators had gone home). I never admitted my other error though. 

Until now...:innocent:


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## Big John (May 23, 2010)

Ain't no way you spinning a motor by hand damaged that drive. What did you get it to, all of 1% of the nameplate speed? You may have been regenerating all of 5 volts.

There's a maxim when working on old equipment that is rarely powered off: Just let it run. 
Because it's very common to see that either during the cooldown period or power surge on restart that something will suddenly fail even after years of running fine. Has nothing to do with the poor bastard pushing the button.


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## micromind (Aug 11, 2007)

JRaef said:


> You admitted it???!!!
> 
> I did something similar once, luckily it didn't damage anything, but it did prevent it from working. I had left a jumper on a circuit after testing it, so although it worked fine in "Hand" for my testing, when they put it in "Auto" after I left, it wouldn't work (the system detected the conflict). They called my back out, gave me a PO and everything. When I saw my jumper, I took it off, luckily nobody saw it. I knew that was the problem, but I went ahead and reloaded the program, turned it on and everything worked. I was the "hero", but of course all I really did was correct my own mistake. I told them they didn't have to pay me for that 2nd call because I should have tested it in Auto before leaving (which was true, but I had finished after the operators had gone home). I never admitted my other error though.
> 
> Until now...:innocent:


My basic philosophy is to admit to any and all small errors, even ones that aren't my fault. This way, when I screw up something big and say 'it's not my fault', they'll believe me.......lol.


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## frenchelectrican (Mar 15, 2007)

I know John V did mention e stops that is one of few spots ya have to check it out.,,

JRaef did bring up couple good points on rubber belts and ya it can wreck hovac on drive control system .,,

espcally with static sparks .,,

even thru I do undestand if this is a DC motor genrally with old school control system do not affect too much with static electricity but with modern one it kinda a toss up depending on which type of drive system it being used on it.,,

but it will be wise before you do pull the belts off from the machine with either AC or DC drive system try to islolated or ground it. before you pull the belts to prevent any damage to some of modern controls which they may get too senstive with static electricty.,,

otherwise try to ground all the leads after you cut the power supply and lock it out properly it may really reduce the amount of damage on it.,

I know it may take little more time but I did try that couple time with that will reduce the amount of damage to drive system.


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## Galt (Sep 11, 2013)

Repair did not involve any welding?


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## John Valdes (May 17, 2007)

JRaef said:


> You admitted it???!!!


Hard to hide when the customer saw the little spark too and he was the one that found out what got fried.
No hiding from that. I would have told them anyway. No way to treat a very good customer. Besides, its lying.


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## just the cowboy (Sep 4, 2013)

Just got the drive back from repair. They replaced bad and aged componants in the power supply section.

Thanks for input, plant manager still thinks we did it.


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## just the cowboy (Sep 4, 2013)

*oops*



micromind said:


> My basic philosophy is to admit to any and all small errors, even ones that aren't my fault. This way, when I screw up something big and say 'it's not my fault', they'll believe me.......lol.


I've always said sooner or later you are going to let the smoke out, when you do you are going to say ah sh1t, then you are going to look to see if anyone saw it. Then fix it and move on.


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## macmikeman (Jan 23, 2007)

Surely there must be some way to line filter small static ''spark'' incidents from completely taking out plc's. I'd invent one if I was working on them and it happened to me more than once.


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## sparky970 (Mar 19, 2008)

micromind said:


> My basic philosophy is to admit to any and all small errors, even ones that aren't my fault. This way, when I screw up something big and say 'it's not my fault', they'll believe me.......lol.


I had a JW tell me this 20yrs ago. Never had to say it's not my fault


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## knomore (Mar 21, 2010)

It's your fault in the customers mind no matter the cause.


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## Darr (Aug 19, 2016)

For what it's worth, I have seen motors/generators with field windings that did NOT necessarily keep any "residual magnetism" if you had them torn apart for more than a few hours... meaning when you put it all back together you'd need to use, say, a 9 volt battery to 'flash' the field with the correct polarity else the motor simply would not start (or the generator would not produce any output).


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## MechanicalDVR (Dec 29, 2007)

Darr said:


> For what it's worth, I have seen motors/generators with field windings that did NOT necessarily keep any "residual magnetism" if you had them torn apart for more than a few hours... meaning when you put it all back together you'd need to use, say, a 9 volt battery to 'flash' the field with the correct polarity else the motor simply would not start (or the generator would not produce any output).


Be kind of strange if you found any electromagnet that has residual magnetism.


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## Bird dog (Oct 27, 2015)

MechanicalDVR said:


> Be kind of strange if you found any electromagnet that has residual magnetism.


Or strange for a motor to have a "field flash" ckt like a genset. Do motors get weak permanent magnets?


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