# Problem at the trade show.



## JayH (Nov 13, 2009)

The combined inrush of the XFRMR and the drive sounds like the most likely culprit.

Try letting the transformer start up and then turn on the controller.


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## electromanj (Dec 18, 2007)

I have the xformer conected straight to the drive. I agree It would help to let the trans energize first, then the drive. This morning I powered up and everything came on line OK. The reason I suspect the capacitors in the drive is because after the breaker trips the first time it never trips again. Like the caps charge on the first power up and after they are charged no more problems. Nice point though, maybey the LC of the transformer and capacitors together are the problem. I would like to try without the transformer and see what happens.


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## micromind (Aug 11, 2007)

If the secondary of the transformer is connected directly to the input of the drive, and the primary is switched, eventually the MOVs in the drive will blow up. 

The reason is because of the spike generated in the transformer due to the collapsing field. A lot like the ignition coil on a gas engine. The MOVs can take this only so many times, then they short out, then on the next powerup....Blam!

It's also possible that the MOVs are clamping the voltage on power-up, and the breaker is seeing a short. Or more likely a ground fault.

Rob


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## electromanj (Dec 18, 2007)

micromind said:


> If the secondary of the transformer is connected directly to the input of the drive, and the primary is switched, eventually the MOVs in the drive will blow up.
> 
> The reason is because of the spike generated in the transformer due to the collapsing field. A lot like the ignition coil on a gas engine. The MOVs can take this only so many times, then they short out, then on the next powerup....Blam!
> 
> ...


Hey, Thanks! I hadn't thought of that back EMF on powerdown. Will definitly be adding a disconnect between drive and xformer! We do these shows about every three years and the KABOOM never happened yet but I'm sure that would happen. Thanks for saving me the embaressment of a mushroom cloud in the middle of the expo hall!:laughing: 
So, do you think the tripped breaker on power up is more likely a GFCI issue or an inrush issue? Or a comination of both?


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## JayH (Nov 13, 2009)

Get some secondary protection on that XFRMR. Cheap insurance.


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## micromind (Aug 11, 2007)

I'd say the power-up breaker tripping is more of an inrush issue than GFI. 

A 125 HP VFD has some pretty good-sized capacitors in the rectifier section, and combined with the transformer inrush, the breaker might see a short circuit. 

If you brought the transformer up first, then the VFD, the breaker has a much better chance of holding.

Rob


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## John Valdes (May 17, 2007)

Measure first start current and subsequent start currents with no load. There is a small amount of lag time for the electronics to fire up. There is a distinct relay click on most every drive I have worked on during start up. Nothing can work until this relay contact closes. It very well may be a current sensing relay. For the life of me I cannot remember what this relay is called, and it may not be present on all drives. You might never hear this click in the normal application, but you should hear it in your display booth.

Do you have line reactors. I know its a stretch here at the show, but worth looking into.

Why a 125 HP drive in a display? Even without load you are still pulling some decent current, especially if its a torque control.

Please let us know what you find out. I personally want to know.......Thanks


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## micromind (Aug 11, 2007)

Some drives have phase monitors in them. They simply look for all 3 phases to be hot and reasonably balanced. 

If a drive is single phased, it'll still work, but the rectifier section is in danger of burning up. This is because only 4 of the 6 diodes/SCRs are conducting. If the drive is operating at more than about 1/2 of its capacity, the diodes/SCRs are overloaded. 

A line reactor will limit inrush current during start-up, especially when the drive is connected to a low-impedance system. Meaning one that can produce a huge amount of current. 

This isn't as common as it used to be, but every now and then I'll get called in because a brand-new drive simply blew up on its first power-up. Almost always, it's a fairly small drive connected very close to a big system. The diodes/SCRs can't handle the inrush to charge the capacitors. A line reactor will solve this. 

This is why I always recommend that drives be fed with the smallest possible wire. Anything that will cut back on inrush current will extend the life of the drive.

Rob

P.S. A line reactor is nothing more than a simple current limiting reactor (CLR). A CLR will pass current with minimal voltage drop until it reaches a certain point. Then it'll cause as much voltage drop as needed to keep the current from going over the limit. It works on the principal of inductive reactance.


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