relay arcing

[Mr Dave]

What it sounds to me is that you are trying to open the contact under a high current load.

A common solution is to redistrubte the current load across multiple contacts in the same relay.

 

[elevmike]

Bitmore,

I dont know what size, or how old your crane motors were but I cant imaginge the arching involved. I have to ask though what happened to the idea of using the ward-lenard dc generator field control? The system was origionally invented for use on cranes some 80+ years ago. Until AC drives came along all the large crans I saw had generator field control.

 

[panic mode]

If the picture above was unclear, maybe this one is better.
The point is to use inverting contact and not separate NO and NC
contacts from the same relay (see "construction detail" on the right)
Reason for suggestion is to overcome potentially wrong wiring
that could create intermittent short circuit with currents
exceeding normal levels by far (see BAD example). The "bad" example
can work but it's more delicate to control. Even if the coils
are interlocked using NC of other relay, you still can have short
circuit because of response time of the relay.
Contacts in series are used for high voltage. Contacts in parallel
for high currents. I never worked on cranes (like to stay on ground)
but I'm pretty sure that those DC motors aren't 12V.
I fail to see why clock signal needs to be as powerful as crane.
To get an accurate answer we need better question. All this is just
unnececary elaboration on vague question of someone who probably just
stopped by once, didn't bother to register and is unlikely to come again.

zzzzz

 

[BobB]

Getting back to the original problem of DC arcing across a relay. I mentioned that I had used contactors in the past and seriesed the contacts to quench the arc. The simple fact is that DC does not go through a "zero" point, as does AC. Therefore, the arc is harder to put out as there is no zero point.

Effectively, wiring a contactor with the contacts in series effectively increases the air gap and allows the arc to be quenched. If the contacts in series does not increase the air gap enough for the DC voltage supply arc to be quenched, the contactor will burn up the same way as a relay.

The last time I used this technique was in a power station. Switching of the HV circuit breakers was at 110vDC. A calculation was performed by my contactor supplier to establish the air gap required to extinguish the arc developed by breaking 110VDC. Current also has to be taken into account, of course.

When the contactors were installed correctly, the "burning up" of relay contacts was no longer an issue. The system has been working satisfactorily for some 6 years without a problem.

I would like to relate a story an ex boss of mine told me years ago. He was, for some years, the test engineer for a large electrical manufacturing company in UK. They designed some DC switch gear to be installed in Canada. Apparently, they were for a high voltage transmission line. The transmission was in AC and then converted to DC where the line went under a lake. When the juice came out the other side, it was converted back to AC for further transmission. I am unsure of the reason for the conversion but I am sure there was a good one, and possibly one of the engineers here would know.

They set up to test the switches one night on the weekend. They were in an industrial area and chose this time to test when there was minimal load. Apparently they required a lot of power to carry out the tests. Full load must have been substantial.

They started to test the switches and very quickly discovered that an error had been made in the design of the arc shutes. The arc was not quenched. The arc then started to fire all over the place and, of course, went to the nearest ground point. This happened to be a steel walkway and hand rail installation so that the engineers could walk around the test rig. Apparently, the walkway MELTED AND BURNED . There were people running everywhere trying to escape the arc.

Just goes to show the problems that can be created when using high volrtage, high power DC, and how careful one needs to be when working with it. SCARY

He had a lot of other stories to tell about testing with high voltage AC and DC, but another time when a suitable topic is started.

 

[panic mode]

Here are some links I've seen in another forum:

Above a certain break-even distance (about 50 km for submarine cables, and perhaps 600-800 km for overhead cables [3]), the lower cost of the HVDC cable outweighs the cost of the electronics.

http://encyclopedia.thefreedictionary.com/HVDC
http://www.worldbank.org/html/fpd/em/transmission/technology_abb.pdf

 

[tzbul]

Well,the load is not too big,the voltage is not too high,just use a capacitor in paralel and that's it.YOU don't even need contacts connected in series.

 

[Bitmore]

TZbul said use a capacitor, well that and a series resister and you have a "snubber" and that will quench the arc, but you have to size the resister and capacitor right or all you will have is a firecracker!
Series contacts are a common approach to quenching a D.C. arc. Some people said they have never heard of such a thing, but given the fact it is a popular method (proven by a simple search on Gooogle) shows that allot of other people have run into the same problem.

bitmore

 

[parimalam]

You may try by connecting low value non polarized capacitor across the contacts to reduce the contacts arcking.

regards
K.Parimalam