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Author Topic: Just for fun: AC vs DC high-current switching  (Read 3214 times)

Frank DeWitt

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Re: Just for fun: AC vs DC high-current switching
« Reply #10 on: December 12, 2017, 10:01:13 am »

A motor contactor at Mt Wilson

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Jonathan Johnson

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Re: Just for fun: AC vs DC high-current switching
« Reply #11 on: December 12, 2017, 05:36:38 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.

Typical AC-only light switches now have a cam that separates the contacts. By this design, the switch opens and closes slowly.

AC/DC rated switches have a spring loaded snap action. When you move the switch handle, you push (or pull) on a spring which holds the contacts in position until you cross a point of unstable equilibrium. When you cross the point of equilibrium, the spring causes the switch to quickly snap to the other position, eliminating the slow opening and closing of the cam design. That's why DC-rated switches have a distinctive snap sound.

The cam design is much simpler and cheaper to produce.
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Mark Cadwallader

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Re: Just for fun: AC vs DC high-current switching
« Reply #12 on: December 12, 2017, 05:51:59 pm »

A motor contactor at Mt Wilson



It looks like 3 of the 4 coil springs were hand-wound. (Or were machine wound by a human operator.)
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Jonathan Johnson

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Re: Just for fun: AC vs DC high-current switching
« Reply #13 on: December 13, 2017, 12:31:21 am »

A motor contactor at Mt Wilson



Curious that it has hex nuts instead of square nuts. I'm used to seeing square heads on old electrical equipment.

A little research shows that mass production of hex nuts & bolts was introduced in the mid 1800s, but didn't gain ubiquity until the 1940s, when the advantages of hex nuts became important for arms production for World War II. At least that's what I've been able to gather.
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Frank DeWitt

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Re: Just for fun: AC vs DC high-current switching
« Reply #14 on: December 13, 2017, 09:13:03 am »

Curious that it has hex nuts instead of square nuts. I'm used to seeing square heads on old electrical equipment.

A little research shows that mass production of hex nuts & bolts was introduced in the mid 1800s, but didn't gain ubiquity until the 1940s, when the advantages of hex nuts became important for arms production for World War II. At least that's what I've been able to gather.

These panels have been in steady use for over 100 years, so it is possible that the nuts were replaced at some time. BTW the reason for DC is that there was no power lines up there when the telescopes were built and they didn't want the vibration or the exhaust at night, so a big DC generator was installed along with batteries.  The batteries were charged during the day and the power used at night.   The generator is still there and has been restored by volunteers.     
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Mike Sokol

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Re: Just for fun: AC vs DC high-current switching
« Reply #15 on: December 14, 2017, 04:28:38 pm »



It's a work of art. I love looking at vintage electrical gear. I have a drafting board what was probably made in the early 1900's and the castings are simply beautiful. Shame that we can't spend that much time and care on modern gear, but it's just not cost effective. But there are still boutique shops for those willing to pay for detail work.
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Jonathan Johnson

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Re: Just for fun: AC vs DC high-current switching
« Reply #16 on: December 15, 2017, 07:42:54 pm »

It's a work of art. I love looking at vintage electrical gear. I have a drafting board what was probably made in the early 1900's and the castings are simply beautiful. Shame that we can't spend that much time and care on modern gear, but it's just not cost effective. But there are still boutique shops for those willing to pay for detail work.

There's a local panel shop (Prairie Electric in Vancouver, Washington) that does excellent work. Granted, the parts themselves aren't things of beauty, being run-of-the-mill, off-the-shelf parts, but the workmanship in the finished panel is top notch.

A nearby industrial facility had two large control panels built by Prairie Electric when the facility was built. A few years later they expanded, and had the new panels built by another panel shop. The panels are mostly "identical" in design.

If you were to first look at the panels built by the second shop, you would probably say they were done neatly with decent workmanship. But when you open the cabinets for the panels built by Prairie Electric, the difference is noticeable: Prairie's panels are works of art in comparison.

There still is pride of workmanship out there.
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Stephen Swaffer

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Re: Just for fun: AC vs DC high-current switching
« Reply #17 on: December 16, 2017, 10:45:58 pm »

Unfortunately (apparently), the pride in workmanship was not rewarded with the subsequent contract.  Likely, that change was made becasue it was slightly cheaper to do "decent" work vs "top notch".  I think it is the realities of business that is killing the pride in workmanship.
« Last Edit: December 17, 2017, 02:25:02 pm by Stephen Swaffer »
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Tim McCulloch

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Re: Just for fun: AC vs DC high-current switching
« Reply #18 on: December 17, 2017, 12:10:45 am »

Unfortunately (apparently), the pride in workmanship was not rewarded with the susequent contract.  Likely, that change was made becasue it was slightly cheaper to do "decent" work vs "top notch".  I think it is the realities of business that is killing the pride in workmanship.

"Doing a good job is like wetting your pants in a dark suit - you get a nice warm feeling but nobody notices.". Found on a small poster at my old factory job.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Brian Bolly

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Re: Just for fun: AC vs DC high-current switching
« Reply #19 on: December 17, 2017, 12:10:33 pm »

There's a local panel shop (Prairie Electric in Vancouver, Washington) that does excellent work.

Just flipping through their Instagram account, I'd have no problem hiring them based on workmanship alone.  You're not kidding - their crew clearly takes pride in their work, even down to their van organization.

(For those that are curious: Prairie Electric)
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