Slightly OT- Electrical

I'll agree with you there. I'm envisioning lighting fixtures scattered across a factory floor.

Kevin Gallimore

Reply to
axolotl
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Since it is under discussion, here is a link to measurements of 10 ft. of zip cord.

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Kevin Gallimore

Reply to
axolotl

Interesting link, but they don't have the propogation velocity listed there.

Should I put a few feet of zip cord on our TDR and see what it is?

Jim

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Reply to
jim rozen

Well OK, for those who care about this, I wired a short (75 inch) piece of cordage up and measured it. This was about number 16 three-conductor jacketed cord, which is what our sunnex lamps use on the primary of the transformer.

The answer is that a pulse will propogate at a rate of

1.8 nanoseconds per foot in that cordage. So the lamps would have to be really far apart to have the spikes dephase. Probably hundreds of feet, given that they'd have microsecond- like riseitmes.

Jim

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Reply to
jim rozen

I see from the other post that you measured 1.8 nS/FT., which is pretty close to the 1.9 nS delay of the PVC ribbon cable I measured 20 years ago. I don't see, however, why you have such an emotional investment in this. The OP said nothing about zip cord- that is your assumption. I wouldn't guess the the premises are wired with zip cord, but it is possible. I saw no distances mentioned. If you can accurately predict what is going to happen in a system with the information offered you are a better man than I.

Kevin Gallimore

Reply to
axolotl

Emotion has nothing to do with it - it was an interesting question and I was running the TDR at work today anyway. Pretty much any signal in a transmission line like that will propogate at about that speed. As you say, good coax will be less.

The OP said nothing about zip cord- that is your assumption. I

LOL. If you want I'll do some 12-2 romex monday morning, we'll be measuring stuff all day with that instrument.

Jim

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Reply to
jim rozen

Actually, he does 56% of the velocity of light in free space. Also v is proportional to 1/Sqrt(LC). I don't recall the constant off hand but it's easy to look up and he gave the L and C for the cable in question.

BTW, are you careful to use the same length of cable between the amp and each speaker? ;-)

Ted

Reply to
Ted Edwards

That's calculated for a cable of arbitrary construction. I wanted to get closer for the transformer issue.... :^)

Jim

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Reply to
jim rozen

Transformer? Why would you be concerned with propagation properties of a power supply transformer? Surely you don't still build amplifiers with output transformers in them!

Ted

Reply to
Ted Edwards

Have you been following the thread? The OP asked what kind of transients would be created when switching off many lighting transformers, simultaneously. The transformers are all wired in parallel.

I said it would be a problem, the transients from each unit would sum up on the line. Another theory is that because each transformer (*power* transformer) is at a different location on the power buss, the transients will not sum - but would rather de-phase from their physical separation.

Because of the propogation velocity compared to the risetime of the transients, I think they will add up.

Jim

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Reply to
jim rozen

I would agree that they would add up but why should the transient be any larger than that which you would get from a single transformer rated for same total power? Why would this be any different than any other low voltage lighting scheme?

Ted b

Reply to
Ted Edwards

It won't. At least that's what I would contend. There was another approach to the matter that maintained that the spikes would all cancel out and go away for many small transformers.

Jim

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Reply to
jim rozen

Here's my take :

- the 'spark' is the dissipation of the energy stored in a magnetic field

- each transformer has some energy stored in it when energized

- if you turn off 50 transformers, then the total energy dissipated will be

50 times the energy stored in a single transformer

Now how this affects the transient that one sees at the contacts is another story, and not one I feel totally comfortable with. Having said that, I don't think that the transient voltage would rise, but it might. Perhaps the lenght of time the transient lasts will go up, but I can't really see that happening.

Dave (who's not sure if he contributed or not in this case)

Reply to
Dave Keith

I am equally not sure of what would happen. With 50 transformers the amount of energy in the magnetic fields would be 50 times greater. But I am not sure that the voltage would be any higher than with just one transformer. I am assuming that there is no arcing across whatever switch is used to open the circuit. With no arcing the energy in the magnetic field goes into the voltage in the transformer winding capacitance. With fifty transformers you have more energy in the magnetic field, but also more distributed capacitance. So the same voltage. If the voltage causes arcing across the switch, then the voltage is nearly the same as the voltage across an arc does not vary much with current.

My guess is that one transformer that has fifty times the power capacity of a small transformer would have less distributed capacitance than 50 small transformers. So my guess is that fifty small transformers would act somewhat differently than one bigger transformer.

I agree with Jim Rozen that the delay thru the cabling is not significant.

Dan

Reply to
Dan Caster

If the OP wanted to suppress any transients, an optimum approach would be to reverse the phase of every other transformer, shouldn't affect a lighting circuit and the transients would cancel each other out, or very nearly so...

Bob

Reply to
Bob Robinson

Won't work, Bob. Because all the loads are isolated resistive loads, and the magnetic fields of the separate transformers aren't coupled, there is no phasing to be done with the transformers. The primaries all look like independent low Q chokes in parallel across the line.

Gary

Reply to
Gary Coffman

A standard air-break contactor cannot chop the supply current to the transformers. The parting contacts will arc until a natural current zero occurs. Then the arc will extinguish and will not restrike. Thus the circuit is interrupted without a significant spike. This action explains why there is no problem with large office building fluorescent lighting systems controlled by lighting contactors. Thousands of buildings are switched twice a day with no ill effects and no spike suppressors are needed.

Randy

Reply to
Randal O'Brian

Very true. My point was that the spark, or arc, will be 50 times more 'powerful' when disconnecting 50 transformers compared to disconnecting a single transformer. If you switch off a large inductive load you can get quite large sparks/arcs, which in time can pit the heck out of switch or relay contacts, a little unwanted EDM as it were ...

Reply to
Dave Keith

You're absolutely right, I was having a brain fart and thinking that by reversing the leads on the primary that the current would be flowing in the opposite direction...I had to pick up a coil spring and look in either end to see the error of my ways. Thanks for keeping me honest...

Bob

Reply to
Bob Robinson

Yep, this is something I have to keep reminding myself - if you saw off teh head of a bolt and turn it around, that doesn't make it left-hand thread!

I think it was Gary who had the great story about one of the early telecommunication satelites where they put a right hand circularly polarized transmitting antenna on the bird.

They were suprised at the small signal on the ground, until they realized that they had left-hand polarized antennas there.

Somebody thought that if the radiation was right hand polarized on the way out, it would be left-hand on the ground. Like the bolt, even if you look at it from the other side, it's still right-handed.

Jim

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Reply to
jim rozen

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