# VFD control transformer problem



## Charger77 (Jan 29, 2011)

I may be stupid, but I can't figure out how to edit my post. Here is the transformer in question:

http://datasheet.octopart.com/9070TF100D1-Telemecanique-datasheet-83412.pdf


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## erics37 (May 7, 2009)

So you grounded the neutral on the transformer secondary?

Was there a ground fault and maybe the terminations weren't secure? Or it had some crud in it affecting the return path, so it didn't trip the OCPD?


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## Charger77 (Jan 29, 2011)

> So you grounded the neutral on the transformer secondary?


Correct.



> Was there a ground fault and maybe the terminations weren't secure? Or it had some crud in it affecting the return path, so it didn't trip the OCPD?


I'm thinking that I had a ground fault somewhere, but I can't seem to locate it. I'm not sure whether the terminal was secure because another non-electrician took it apart before I had a chance to investigate. I follow your logic and I agree. I'm mostly looking for an unbiased opinion to confirm my suspicions. 

Thanks for the input!


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## xlink (Mar 12, 2012)

Maybe the original problem is gone. Like someone welding on the equipment, etc.


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## John Valdes (May 17, 2007)

Most/All VFD's have on board power supply. 24 volt dc.
Are you using a expansion card to accept a different/separate voltage for control and or cooling?


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## Charger77 (Jan 29, 2011)

The 120v powers a few relays on the bottom. It does have a separate 24 volt power supply for everything else.


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## JRaef (Mar 23, 2009)

If the VFD powered motor is not properly frame grounded, common mode noise that it creates could have found a path back through your control system neutral and used that ground connection, which was likely only sized for the transformer capacity so it was too much for it. Your VFD might be attached to a rubber isolated skid with the motor of the cooling tower, and if you grounded the motor to the frame assuming the frame was grounded, or you lost the frame grounding connection due to corrosion, it builds up like a capacitor until it finds a path. This is a gross simplification of course, but I'm giving you a course to investigate. I have seen common mode noise from VFD powered motors actually spark across gaps, and it takes a lot of energy to do that.


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## Charger77 (Jan 29, 2011)

I appreciate the suggestion. I will check it out next week.


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## erics37 (May 7, 2009)

JRaef said:


> If the VFD powered motor is not properly frame grounded, common mode noise that it creates could have found a path back through your control system neutral and used that ground connection, which was likely only sized for the transformer capacity so it was too much for it. Your VFD might be attached to a rubber isolated skid with the motor of the cooling tower, and if you grounded the motor to the frame assuming the frame was grounded, or you lost the frame grounding connection due to corrosion, it builds up like a capacitor until it finds a path. This is a gross simplification of course, but I'm giving you a course to investigate. I have seen common mode noise from VFD powered motors actually spark across gaps, and it takes a lot of energy to do that.


This is a great post :thumbup: I've been passingly familiar with this phenomenon but only through cursory reading at materials. I've only dealt with VFDs mostly in new applications or extensive retrofits where everything was ripped out and proper drive cable was installed, etc. Haven't had any issues with those things yet. But after reading your post, I decided to google it and the first link I clicked on was this one:

http://www.powerqualityanddrives.com/emi_rfi/

Seems to be a wealth of information (though a bit dated). From what I understand, if one was to go to an older facility with traditional electromagnetic starters, with the motor T leads consisting of plain ol' THHN in plain ol' metal conduit, and replace the starter with a VFD and reuse the existing wiring, that could potentially lead to significant issues. That could lead to RF stuff being conducted along conduit and other grounded metal bodies to make its way back to the drive the long way. Or, perhaps like in the OP's case, through the control system ground.

Is this why the use of proper shielded drive cable is so imperative? Or at least a significant factor?


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## Trigger_442A (Sep 15, 2012)

Must have been an overloaded or undersized tx or loose terminal connections, that is my guess


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## Charger77 (Jan 29, 2011)

To offer an update: I found out today that they are replacing the drive due it going bad (news to me....) and they already replaced the transformer without doing any sort of troubleshooting. Unfortunately I work for a company that prefers putting a bunch of band aids on the issues instead of fixing it right the first time. I'll still poke around and see if I can come up with the root cause, but I'm not sure what to expect with someone else tinkering around in it.

I appreciate everyone that has offered some input in this thread.


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## John Valdes (May 17, 2007)

Charger77 said:


> To offer an update: I found out today that they are replacing the drive due it going bad (news to me....) and they already replaced the transformer without doing any sort of troubleshooting. Unfortunately I work for a company that prefers putting a bunch of band aids on the issues instead of fixing it right the first time. I'll still poke around and see if I can come up with the root cause, but I'm not sure what to expect with someone else tinkering around in it.
> 
> I appreciate everyone that has offered some input in this thread.


I once had a customer. A very large international corporation.
I sold and serviced drives, motors and gearing for them.

Your update reminded me of them.
If they lost a motor, drive or any other equipment, regardless of how important it was, they had to have the identical product to replace it with. Identical.
If they lost a Baldor 1/2 HP, 230/460, 1800, 56C everyday motor. I could not replace it with anything but a Baldor. Even though I had several other brands of motor on my shelf that were exactly the same. Direct replacements.

Seems the thinking is similar to the people running your show. Tell them what you think and if they want to jump through their asses backwards. Let em.


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## Charger77 (Jan 29, 2011)

John Valdes said:


> I once had a customer. A very large international corporation.
> I sold and serviced drives, motors and gearing for them.
> 
> Your update reminded me of them.
> ...


God forbid I actually suggest a product that is a higher quality product. They replaced the transformer with a different brand/style just because my boss is buddies with the salesman. I do what I can and I document everything via e-mail just in case. 

They replaced with xfmr without replacing any of the wiring (yes there are still burn marks on them) and I found that they stacked the fork terminals in such a way that the top one was bent and extremely loose under the screw. Full disclosure I'm a maintenance man fresh from electrical construction and I'm the only licensed electrician at this place. The guy that installed the new transformer is a former cellphone repair guy and used to be a groundskeeper for a golf course.


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## John Valdes (May 17, 2007)

Charger77 said:


> God forbid I actually suggest a product that is a higher quality product. They replaced the transformer with a different brand/style just because my boss is buddies with the salesman. I do what I can and I document everything via e-mail just in case.
> 
> They replaced with xfmr without replacing any of the wiring (yes there are still burn marks on them) and I found that they stacked the fork terminals in such a way that the top one was bent and extremely loose under the screw. Full disclosure I'm a maintenance man fresh from electrical construction and I'm the only licensed electrician at this place. The guy that installed the new transformer is a former cellphone repair guy and used to be a groundskeeper for a golf course.


A good salesman helps you decide whats best. That is how I treated my customers. Of course I tried to sell MY products whenever possible. That was my job.

Sounds like you are in for a time in this facility.
Hopefully things work out, your boss sees who you are and gives you input.
Nothing worse than a front office making all decisions with no feedback from the actual people installing and using the equipment.
Good luck.


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