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Old January 29th, 2018, 11:02 AM   #1
JeffKiper
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VFD disconnect aux contacts

Is there an electrical code requirement that calls out the a need for an auxiliary contact in a field mounted disconnect?
I have a customer that doesn't want to put auxiliary contact in the field disconnect. I though there was a call out for it but I can't find it.
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Old January 29th, 2018, 11:07 AM   #2
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Just tell him you will be glad to sell him a new drive after he blows up the one he has by skipping the aux. contact.

I am pretty sure you can find a VFD manual that states this as a requirement for any downstream disconnecting device.
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Old January 29th, 2018, 11:09 AM   #3
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I don't know if there is something specific about that, but without it you will never know to stop the drive (or starter for non-vfd applications) or not start the drive. That opens the distinct possibility that some one will throw the disconnect on and have the motor suddenly start.

From a code point of view, you may want to look into unexpected motion or the fact that this scenario would equate to the disconnect acting as a starter.
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Old January 29th, 2018, 12:54 PM   #4
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It's not a "code" issue, it's a drive protection issue. Opening the contacts of a disconnect switch while the drive is RUNNING can cause severe damage and failure of the output transistors of the VFD. So it's possible that just ONE mistake can cost them the entire cost of a replacement VFD + down time revenue loss. The cost of adding and wiring the aux contacts is insignificant in comparison. You can buy VFD cable that already has the two extra wires needed for that included in the bundle and already separately shielded for this exact purpose.

What the aux contact does is that you tie it back to the VFD control / command system to IMMEDIATELY turn off the output transistors. Because aux contacts on disconnect switches are (almost*) ALWAYS designed to open just BEFORE the main contacts, by the time the main contacts open, the transistors are already off-line and no damage occurs. Anyone who tells them / you that this is not an issue is clueless.

* MOST of the time that is going to be true, but it's actually incumbent on the specifier to ENSURE that the aux contacts open ahead of the main contacts.
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Old January 29th, 2018, 01:23 PM   #5
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Make you case in writing why you suggest to have aux contact. Then move on.

He who pays the piper, calls the tune.

We have had plenty of call backs to the retro fit things we initially advised on some months / weeks later.
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Old January 29th, 2018, 02:31 PM   #6
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Quote:
Originally Posted by janner_10 View Post
Make you case in writing why you suggest to have aux contact. Then move on.

He who pays the piper, calls the tune.

We have had plenty of call backs to the retro fit things we initially advised on some months / weeks later.
Amen to that. "Told-you-so" billing is always sweet, but I never actually rub it in to the customer unless they complain first.

I recently got called out to a large facility what I did 4 years ago; smart MCCs, Ethernet to everything etc. etc. Everything works great, but I gave them a $25k recommended spares list when we were done and they ignored it completely. Fast forward 4 years and a 500HP VFD blew up (because they changed to a 600HP motor and "assumed" it was OK). No spare parts on hand, so they were down for 5 days as everyone scrambled to find a way to get them on line again. $200,000 per day in lost production; that "savings" of $25k by not buying spares turned into $1 million in losses and emergency repair costs of another $100K... NOW they are buying the spare parts...
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Old January 29th, 2018, 05:01 PM   #7
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I agree that it's bad practice not to use the aux contacts on the disconnects to stop the drive, but I've personally seen operators open and close disconnects on running drives enough times to know that it's not necessarily a sudden death situation. These were all <25HP drives, however...

Understand that I'm not advocating for doing that!

Actually, I've even seen major companies standardize on VFD control panel designs that perform an emergency stop that only stops the drive by first opening up a contactor on the load side of the drive. An aux contact on the load contactor then breaks the "Run Enable" signal to the drive. So every e-stop results in exactly the same damage as opening a disconnect on a running drive. Crazy!

Last edited by rupej; January 29th, 2018 at 05:08 PM.
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Old January 29th, 2018, 06:13 PM   #8
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Originally Posted by rupej View Post
Actually, I've even seen major companies standardize on VFD control panel designs that perform an emergency stop that only stops the drive by first opening up a contactor on the load side of the drive. An aux contact on the load contactor then breaks the "Run Enable" signal to the drive. So every e-stop results in exactly the same damage as opening a disconnect on a running drive. Crazy!
Wow I hope that wasn't recent, with STO being pretty standard these days.
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Old January 29th, 2018, 07:19 PM   #9
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Luck is not a valid strategy for reliable operations...
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Old January 29th, 2018, 09:42 PM   #10
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“What the aux contact does is that you tie it back to the VFD control / command system to IMMEDIATELY turn off the output transistors. Because aux contacts on disconnect switches are (almost*) ALWAYS designed to open just BEFORE the main contacts, by the time the main contacts open, the transistors are already off-line and no damage occurs. Anyone who tells them / you that this is not an issue is clueless.”

That’s good to know. I figured the aux and main opened/closed at same time. Thank you for sharing.
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Last edited by seth350; January 29th, 2018 at 09:46 PM.
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Old January 29th, 2018, 10:54 PM   #11
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Quote:
Originally Posted by rupej View Post
...
Actually, I've even seen major companies standardize on VFD control panel designs that perform an emergency stop that only stops the drive by first opening up a contactor on the load side of the drive. An aux contact on the load contactor then breaks the "Run Enable" signal to the drive. So every e-stop results in exactly the same damage as opening a disconnect on a running drive. Crazy!
If you use an early break contact to disable the VFD, this strategy can work. It is much easier on the VFD than killing the input power via e-stop.
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Old January 29th, 2018, 11:11 PM   #12
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Not code, and others have mentioned why to use the aux disconnects, however at my hold job we can a 60A CH disconnected, mounted on a board, mounted on the wall with big black letters that said "This is not a light switch!" I regularly saw production people stand in front of and throw a 200A disconnect on a meat grinder while the grinder was heavily loaded for the days production like it was no big deal, not to mention the countless other smaller disconnects around there, so it is a pretty basic assumption that it will happen.
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Old January 30th, 2018, 02:54 AM   #13
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Had a similar scenario with a 400hp motor connected to a fan for LEV.

Refused the additional minor cost for aux contacts on the local disconnect.

Operator starts the fan one morning, it doesn't move but the VFD is giving an output. The PID at this point is ramping the speed up slowly.

Operator realises the motors switched off at the local disconnect. An turns it on. A big flash and bang as the disconnect blows itself to pieces, an operator with brown underwear later and I get a call to price an emergency repair.

Thankfully nobody was hurt. An the VFD took it like a champ.

I was nicely covered too explaining the consequences of what potentially could happen in a previous email chain with the client. 400hp motors take some serious grunt starting them DOL!!
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Old January 31st, 2018, 08:50 PM   #14
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On the most recent designs of VFD's, ABB no longer requires or recommends pre-action switches on disconnects in VFD output circuits.

Not sure I'm comfortable with that especially over 100hp, but that is the official line.
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Old February 1st, 2018, 09:09 AM   #15
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Quote:
Originally Posted by DickDV View Post
On the most recent designs of VFD's, ABB no longer requires or recommends pre-action switches on disconnects in VFD output circuits.

Not sure I'm comfortable with that especially over 100hp, but that is the official line.
One has to remember ABB is also in the business of selling drives.
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