AC or DC motor control voltage

[cesgibis]

Hi,
I have specified a 110V DC for motor control , our client prefers 230V AC. Could anyone please tell the advantages of DC to AC control.
I've been told about problems with using AC: when local control (stop/start) is located at a large distance (say >400m) from the MCC, the cable capacitance leakage currents could prevent the stop circuit from being interrupted (the motor does not stop when the local stop button is preessed). This probleme is avoided when using Direct Curent.
Could anyone please give me any feedback or advice.
Thanks

 

[jraef]

Capacitance keeping a stop circuit from opening is a problem only on really low low powered solid state circuits and in that case it can happen at any voltage. Proper circuit design can eliminate it anyway. I have never seen it on an AC control circuit that is powering an inductive coil. DC doesn't hold up well over long distances without being seriously high voltage. I don't know of anyone doing 110VDC control circuits outside of emergency switchgear where they need battery backup.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

400 m is quite a distance. And I have seen 230 V AC coils "hang" on shorter cable lengths than that. Especially if RC connected in parallel to coil.

The pull-in current is also rather high in an AC coil and that may be a problem when cable lengths are long.

Even rather "small" installations like gantry cranes have had problems with AC stray currents keeping their opto-isolated input modules activated in spite of open limit switches.

DC, on the other hand, doesn't have that problem. But DC coils (especially 110 V ones) tend to be more complicated and expensive. There is also the concern that electrolytic corrosion shortens life of the rather thin wires used in DC coils. Lower voltage coils will not work over 400 m distance - if cable dimensions are to be kept reasonably small.

So, there are reasons to think this over twice. I would not use AC or DC in distances like that, but some bus system. There are lots of "cable savers" out there. I have used the Siemens ET 200 (I think the number is) in the past. It works quite well and makes lots of channels available for copper saving and later modifications.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[HCBFlash]

What I've seen is that 230VAC control systems are ungrounded, with ground detection. I really like that idea for systems that need to be "fault tolerant". The problem is that most of todays electricians, machinists, plant maintenance folks see that as old and obsolete. Many of them simply can't understand that either a 120, 230, or other voltage, AC or DC can be used with either ungrounded or grounded systems. Hey man, ignorance is at least innocent, but it sure is common.

I'm currently working on a system with 120 VAC (grounded) controls where we have a conveyor with cable-pull e-stops that are presently approx 3000 feet end to end. Yup, a 6000', and growing, circuit length,,,, and it works just fine thank-you. I'd rather see 230 VAC ungounded at least on this particular circuit, but suspect that it'll never be needed and therefore never used. I worked on a project a few years back with e-stop circuits of approx 8000' in length, 230VAC ungrounded that had been in service for about 25 years. It was recently replaced (or is still being worked on) by people that insisted that 120VAC grounded was safer and more reliable,,, but couldn't see how "220" could be grounded.

This is obviously the USA on both these systems, and we're surrounded by trained, licensed folks that run a "neutral" (grounded conductor) as well as a "green ground" to all loads, and if they can't find a white wire or "silver" screw to terminate to, will connect the "neutral" to.... THE APPLIANCE (load) CHASSIS!!!

Why do I mention all this crap? Hey, you MUST consider the user and the people that will be maintaining the equipment. You can do all the "factory representative", "approved service personnel" etc. warnings and contract and conditions of sale language you want, it will still be a present factor.

Please let us all know how this works out for you! and don't forget faq731-376

 

[dpc]

I would say that 400 meters using 230 V ac is problematic for the reason you mention, especially if the cable could get wet. The shunt capacitance of the conductors may prevent the contactor from dropping out. I have experienced this at shorter distances than 400 meters.

You could retain the normal ac contactor but use dc interposing relays for the long control circuit.

I would not use ac control power for this distance.

 

[peebee]

Maybe a resistor (such as an incandescent light bulb) at the end of the control circuit run would be enough to allow the line capacitance to discharge.

 

[Skogsgurra]

It is not about "discharging" anything. The coil has a low resistance already.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[cesgibis]

Thanks for all.
Actually I found one previous thread248-128558 on this forum discussing this problem.
the discussion referred to a Square D article that seemed to be of interest.
The link seems to be not valid anymore. Does anyone know how can this document be accessed ?

 

[HCBFlash]

If you have a copy of that thread you could put the whole thing in a post without commiting any sins as far as eng-tips is concerned, as well as I can tell...

Let me (us) know Square D's publication number if you've got it.

My experiences are pretty much with jraef's, and a low current/high sensitivity device can be helped by putting a higher current device, like your suggested light bulb, in parallel. A 20w resistor won't have lamp life problems though.

 

[cesgibis]

Document is M-379 rev F (square D site)

this link could help:

http://ecatalog.squared.com/techlib/docdetail.cfm?oid=0900892680079349

 

[HCBFlash]

Great document from Square D! Thanks for the link cesgibis!