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Author Topic: Just for fun: AC vs DC high-current switching  (Read 3806 times)

Frank Koenig

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Just for fun: AC vs DC high-current switching
« on: December 07, 2017, 10:39:35 pm »

I ran across the video on an electric vehicle forum. Pretty cool, I thought.

--Frank

https://youtu.be/Zez2r1RPpWY
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Mike Sokol

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Re: Just for fun: AC vs DC high-current switching
« Reply #1 on: December 08, 2017, 12:35:03 am »

I ran across the video on an electric vehicle forum. Pretty cool, I thought.

Yes, very cool indeed. Looks like one of my tabletop demonstrations. Of course the reason that the AC side has no visible arc when opening the switch is that AC shuts off 120 times per second in the US, and 100 times per second in Europe. While the DC is continuous and thus will keep the arc going until the gap gets too great.
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Robert Lofgren

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Re: Just for fun: AC vs DC high-current switching
« Reply #2 on: December 08, 2017, 02:38:52 am »

Cool!
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Keith Broughton

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Re: Just for fun: AC vs DC high-current switching
« Reply #3 on: December 08, 2017, 08:32:12 am »

Cool demo!
Another reason to not use Edison's DC power distribution.
That said, the elements heat up much faster with DC.
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TJ (Tom) Cornish

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Re: Just for fun: AC vs DC high-current switching
« Reply #4 on: December 08, 2017, 09:06:44 am »

Cool demo!
Another reason to not use Edison's DC power distribution.
That said, the elements heat up much faster with DC.
That's the initial observation, but assuming the 220v AC is RMS, the heating power of A/C and D/C are actually the same.  If it was 220v peak, then yes, DC has more power at a given voltage.

I presume the reason the elements heated faster with D/C is they were already warm.
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Chris Hindle

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Re: Just for fun: AC vs DC high-current switching
« Reply #5 on: December 08, 2017, 12:16:32 pm »

That's the initial observation, but assuming the 220v AC is RMS, the heating power of A/C and D/C are actually the same.  If it was 220v peak, then yes, DC has more power at a given voltage.

I presume the reason the elements heated faster with D/C is they were already warm.
Don't discount alternating voltage (passing 0 120 times a second) compared to  steady voltage. That's gotta mean something.
Chris.
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TJ (Tom) Cornish

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Re: Just for fun: AC vs DC high-current switching
« Reply #6 on: December 08, 2017, 12:31:24 pm »

Don't discount alternating voltage (passing 0 120 times a second) compared to  steady voltage. That's gotta mean something.
Chris.
Yes - it does mean something - the difference between RMS and peak-to-peak measurements of the same waveform.  RMS is the average, or heating power of an AC waveform - in loose terms, it's DC-equivalent power.  There is much written about this online if you care to study further.
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Stephen Swaffer

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Re: Just for fun: AC vs DC high-current switching
« Reply #7 on: December 08, 2017, 01:24:39 pm »

A couple of comments on this demo:

DC welding is a lot easier to keep and maintain an arc-if the process allows for DC.

Second, this illustrates the importance of paying attention to switch ratings.  Switches rated for AC may not work well at all with DC.

This is also why safety switches, etc have spring loaded mechanisms-to prevent operators from slowly opening a switch.
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Steve Swaffer

TJ (Tom) Cornish

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Re: Just for fun: AC vs DC high-current switching
« Reply #8 on: December 08, 2017, 01:31:28 pm »

A couple of comments on this demo:

DC welding is a lot easier to keep and maintain an arc-if the process allows for DC.

Second, this illustrates the importance of paying attention to switch ratings.  Switches rated for AC may not work well at all with DC.

This is also why safety switches, etc have spring loaded mechanisms-to prevent operators from slowly opening a switch.
Even as a kid ordering parts from Radio Shack I thought it was weird that a switch rated for 20A at 120V AC was only rated at 24v DC and/or at a lower amperage.  This video shows why.
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Frank DeWitt

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Re: Just for fun: AC vs DC high-current switching
« Reply #9 on: December 11, 2017, 06:10:38 pm »

I toured Mt Wilson observatory in CA  They are still powered with 120 v DC  When a light switch needs replacing they need to find a old one that is rated for DC.  The new ones wont work two times.

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