VFD disconnect aux contacts

proof · Feb 1, 2018

DickDV said:
On the most recent designs of VFD's, ABB no longer requires or recommends pre-action switches on disconnects in VFD output circuits.

I think most disconnects are not rated for opening and closing under power. They are for keeping power from flowing, like when equipment needs to be locked out, but they are not for interrupting current. For that you need a contactor, breaker, VFD, or etc. So ABB may not require it, but the disconnect manufacturer probably does.

jraef · Feb 1, 2018

DickDV said:
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.

Meh, not really that uncommon because many of the IPM manufacturers changed to adding better fly-back diodes in the package so the transistors don't see the same di/dt. That took place maybe 10 years ago. The problem is that not ALL VFDs are going to have that, so you can't GENERICALLY say its OK to not have the aux contacts, plus most drive mfrs stop using the IPMs at around 50HP, reverting back to discrete power transistors again. So whether or not the beefier fly-back diodes are there becomes a function of the quality of the design and as we know, not all drives are created equal. So it's likely that ABB has made a decision to always put the beefier diodes in, and more power to them, but they are not unique in that.

But like you, I'd worry about this on larger drives in particular anyway. A few hits from disconnecting under load and those diodes short, then the next one takes out the transistors. So the beefier diodes basically just buy you some added fudge factor, but it's still not something I'd bet the cost of a 100HP drive on, especially when the REAL cure is to simply add some $10 aux contacts.

Bit_Bucket_07 · Feb 1, 2018

jraef said:
Meh, not really that uncommon because many of the IPM manufacturers changed to adding better fly-back diodes in the package so the transistors don't see the same di/dt. That took place maybe 10 years ago. The problem is that not ALL VFDs are going to have that, so you can't GENERICALLY say its OK to not have the aux contacts, plus most drive mfrs stop using the IPMs at around 50HP, reverting back to discrete power transistors again. So whether or not the beefier fly-back diodes are there becomes a function of the quality of the design and as we know, not all drives are created equal. So it's likely that ABB has made a decision to always put the beefier diodes in, and more power to them, but they are not unique in that.

But like you, I'd worry about this on larger drives in particular anyway. A few hits from disconnecting under load and those diodes short, then the next one takes out the transistors. So the beefier diodes basically just buy you some added fudge factor, but it's still not something I'd bet the cost of a 100HP drive on, especially when the REAL cure is to simply add some $10 aux contacts.

I've always insisted upon early break auxiliary contacts on a local disconnect even for an across the line starter. As has already been mentioned, the disconnect stabs are designed to carry a load -- not to make or break a load.

Rob... · Feb 2, 2018

Bit_Bucket_07 said:
I've always insisted upon early break auxiliary contacts on a local disconnect even for an across the line starter. As has already been mentioned, the disconnect stabs are designed to carry a load -- not to make or break a load.

In the UK it's within out IEC standards, disconnects with a red/yellow handle must be able to disconnect or make contact with the full locked rotor current of a motor.

Grey and black handles denote off load switching.

paulB · Feb 2, 2018

Rob,

Do you mind to site specific paragraph, which is stating that disconnect with red / yellow handle must be able to make or brake full locked current of a motor.

PaulB

Sydney · Feb 2, 2018

jraef said:
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.

I remember an incident at the plant that could have been caused by a disconnect being used as a emergency stop. We had additional down time because of a VFD failure, but didn't thing to contribute it to the disconnect.

Excellent info...Thanks to all.

janner_10 · Feb 3, 2018

paulB said:
Rob,

Do you mind to site specific paragraph, which is stating that disconnect with red / yellow handle must be able to make or brake full locked current of a motor.

PaulB

We have to pay to download the standards so people maybe reluctant to share, but quite a good bit of info here:

http://www.moeller.net/binary/ver_techpapers/ver950en.pdf

VFD disconnect aux contacts

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