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Why not use integrated contactor Aux contacts for making a reversing contactor set?

Why not use integrated contactor Aux contacts for making a reversing contactor set?

Why not use integrated contactor Aux contacts for making a reversing contactor set?

(OP)
I have looked at a few videos that use ABB IEC-frame contactors to assemble a reversing contactor set and the preferred method of interlocking is using a hybrid, modular, electromechanical interlock like the VE5-1.
A perfect example of such a video can be seen on you tube if one does a search for "ABB reversing"
My question is this. If I am using 3 pole contactors that have a 4th de-rated N.C. aux pole then why isn't that pole used to form an electrical interlock between two contactors?
Is it the proximity to the 3 phase hi-power that is a concern? Does the external ABB interlock module offer some required spacing?
I would still use the cheaper mechanical only interlock between the two contactors but it just seems a waste that perfectly good aux contacts are often sitting there unused.
Am I missing something here?
Also in this scenario I am using 24vdc coils and driving 480vac 3 phase power.

Thanks!

RE: Why not use integrated contactor Aux contacts for making a reversing contactor set?

It's done that way frequently. It works fine. The issue is that since it's not mechanically interlocked then it means someone can do it by hand accidentally. Say the motor is toast and is actually being commanded ON electrically via the electrically interlocked method you mention, because there is nothing moving a repair guy could accidentally stick a screwdriver on the reverse contactor and if he smartly activates the contactor they can close the second one before the first has had time to disconnect. Can't be done with a mechanically interlocked set of contactors. (Well except I've seen some pretty lame ones that you could actually do that with.)

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why not use integrated contactor Aux contacts for making a reversing contactor set?

(OP)
@itsmoked:

I agree with what you are saying about the possibility of a tech manually pushing the contactors in and causing a dangerous short.....I will admit that I have participated in such stupid acts :)
But here is why i brought this up to begin with.
ABB sells two snap-on interlock units. One is purely mechanical @ $8.00 (VM5-1) and the other is both mechanical and electrical unit @ $20.00. (VE5-1).
Every video I have seen uses the more expensive $20.00 interlock.
I sit scratching my head wondering why they never used the Aux contact set that is already in the contactor in combination with the cheaper $8.00 mechanical interlock. Seems like a wast of copper.
It was at that point that I started wondering if it was an electrical code thing and the proximity of low voltage and high voltage wiring.

RE: Why not use integrated contactor Aux contacts for making a reversing contactor set?

Nothing to do with proximity. There are no issues with that as long as the insulation ratings of the HV and LV wiring are both rated for the HV present.

I do not know for sure but it is likely for two reasons. 1) They actually need or want available the AUX contacts for other functions or 2) The AUX contacts tend to be in close timing with the main contacts and the ones in the expensive interlock may be timed much earlier. 3) bonus reason :) no one stocks the cheap one?

Bit wait! There's more!

Oh yeah! Just because there is a mechanical interlock preventing both contactors from pulling in at the same time, without the electrical interlock you could try to pull in the second contactor which mechanically can't make it. This would burn out it's coil in about 3 seconds or drag down the machine's control voltage into machine insanity-range or blow the control transformer fusing. Hence they always want the electrical interlock to protect the contactors and control circuit. By using all that on the same interlock device you reduce troubleshooting confusion.

Keith Cress
kcress - http://www.flaminsystems.com

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