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Author Topic: How many mA in an Ampere?  (Read 22947 times)

Jonathan Johnson

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Re: How many mA in an Ampere?
« Reply #70 on: April 08, 2016, 01:53:03 PM »

Interesting, they transmit DC the length of California?

But with 1.21 gigawatts you can go back in time.   ;D

The length of California... plus Oregon. All the way from the Columbia River at The Dalles.

The Sylmar converter station is, in an oversimplified sense, a massive audio amplifier. It's powered by DC, and takes a low-amplitude 60Hz signal as an input, outputting a high-amplitude 60Hz signal. I don't know what its bandwidth is.

Seems I've heard that the power flow can be reversed, using power generated on the southern California power grid to feed the Northwest power grid.
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Jonathan Johnson

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Re: How many mA in an Ampere?
« Reply #71 on: April 08, 2016, 01:59:39 PM »

Something I've noticed in my recent China travels and wondered about.  Almost all of the transmission lines I see there have 4 conductors in parallel.  Where in the states you typically see singles and I occasionally see doubled conductors.  That is on each side of the tower you see the three phases hanging from insulators draped from tower to tower and each run is typically a single wire.  e.g. the Hetch Hetchy system that supplies power to SF and surrounding areas has 6 wires, 3 on each side draped across the state.  But in China they have these plates holding 4 separate wires about 8" apart (guessing there) with 3 sets on each side of the tower.

Besides the other suggestions posted here, one of the primary reasons for using multiple, parallel conductors on the same phase/circuit is simply to be able to use smaller conductors that are easier to handle. If you are transmitting high current, you might need to specify a 10 cm diameter conductor. That's a huge, heavy wire that's going to be tough to handle, especially when you're stringing it on towers a thousand feet apart. Much easier to handle a 5 cm wire, and gang 4 of them together to get the same cross-sectional area. As a bonus, you double the surface area, which besides the skin effect, improves heat dissipation.

The joining plates are probably a LOT larger than you're guessing -- those conductors could actually be 2 feet apart. It's really hard to judge sizes of that stuff because there is no nearby reference you can use as a comparison.

EDIT: Fixed my math.
« Last Edit: April 08, 2016, 07:28:33 PM by Jonathan Johnson »
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Mark Cadwallader

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Re: How many mA in an Ampere?
« Reply #72 on: April 08, 2016, 04:08:05 PM »

Besides the other suggestions posted here, one of the primary reasons for using multiple, parallel conductors on the same phase/circuit is simply to be able to use smaller conductors that are easier to handle. If you are transmitting high current, you might need to specify a 10 cm diameter conductor. That's a huge, heavy wire that's going to be tough to handle, especially when you're stringing it on towers a thousand feet apart. Much easier to handle a 2.5 cm wire, and gang 4 of them together to get the same cross-sectional area. As a bonus, you double the surface area, which besides the skin effect, improves heat dissipation.

The joining plates are probably a LOT larger than you're guessing -- those conductors could actually be 2 feet apart. It's really hard to judge sizes of that stuff because there is no nearby reference you can use as a comparison.

I understand your point, but I think you need four 5cm cables to equal the cross section of a 10 cm cable.
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John Roberts {JR}

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Re: How many mA in an Ampere?
« Reply #73 on: April 08, 2016, 06:00:24 PM »

I understand your point, but I think you need four 5cm cables to equal the cross section of a 10 cm cable.
4x5cm cables have similar cross section area but 2x the circumference (62,8) of 1x10cm cable (31,4). depending on how much current crowding (skin effect) is going on, 2x the skin could easily be lower resistance for same amount of copper (actually power cables often wrap aluminum conductors around a steel core). 

JR
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Jonathan Johnson

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Re: How many mA in an Ampere?
« Reply #74 on: April 08, 2016, 07:26:50 PM »

I understand your point, but I think you need four 5cm cables to equal the cross section of a 10 cm cable.

You're right, I messed up that math! Fixed the post.
« Last Edit: April 08, 2016, 07:28:57 PM by Jonathan Johnson »
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Bob Leonard

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Re: How many mA in an Ampere?
« Reply #75 on: April 09, 2016, 11:51:27 AM »

Back in 1970 when I got drafted I was working at MIT Instrumentation Labs working on a navy project (DSRV rescue vehicle seen in "Hunt for red october" movie). Yup, I did mil-spec soldering back then.

After I finished my army conscription, I was supposed to get my old job back, but instead they assigned me to a different engineering group within MITIL working on inertial navigation guidance systems for missiles.  :'(

I was not a happy camper for multiple reasons and left a few months later to take my first job working on audio electronics (pitch shifter to restore pitch from speeded up talking book tape recordings). 

JR

Cambridge or Lexington?
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John Roberts {JR}

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Re: How many mA in an Ampere?
« Reply #76 on: April 09, 2016, 12:17:15 PM »

Cambridge or Lexington?
Both gigs before and after were in Cambridge. One apartment I lived in back then was in Cambridge but most of my apartments were in back-bay Boston.

During my brief military interlude, they renamed the place Draper labs, to help MIT distance itself from all the military work the Lab was doing. Back in the '60s-'70s the military was not very popular, and Cambridge was very liberal (like an east coast version of Haight-Ashbury).

The MITIL/Draper labs also did the computer navigation software for Apollo mission, and other high profile non-military projects. 

  JR

PS: I ended up applying for work at MITIL because my brother attended MIT and was working on his doctoral thesis there. The attached picture was of me standing in front of an array of manometers (pressure-vacuum sensors) used for one of my brother's experiments. I helped him run some of the late night trials. Back in those days data capture was performed with Polaroid camera (also from Cambridge). His super-sonic steam nozzle made so much noise we wore hearing protection, and ran the experiments at night. 
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Keith Broughton

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Re: How many mA in an Ampere?
« Reply #77 on: April 09, 2016, 12:22:22 PM »

The great thing about this forum is I learn something pretty much every day. :)
I had no idea that there was a DC power distribution system.
While DC has obvious advanteges, I alway thought everything was AC.

So, for you high voltage  guys...
I sometimes see what looks like some kind of adjustable clamp on either side of the connection point of the HV line to the insulator on high tension towers.
They can be seen in this pic.
What are they?
« Last Edit: April 09, 2016, 12:26:21 PM by Keith Broughton »
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Bill Harvey

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Re: How many mA in an Ampere?
« Reply #78 on: April 09, 2016, 12:47:29 PM »

The great thing about this forum is I learn something pretty much every day. :)
I had no idea that there was a DC power distribution system.
While DC has obvious advanteges, I alway thought everything was AC.

So, for you high voltage  guys...
I sometimes see what looks like some kind of adjustable clamp on either side of the connection point of the HV line to the insulator on high tension towers.
They can be seen in this pic.
What are they?

You're talking about the dumbell-looking weights on the lines outboard of the insulators?

If so, those are vibration dampeners. They're weights on flex couplings designed to eliminate (or at least reduce) line vibrations caused by wind from adversely affecting the insulator's outer attachment points.   https://en.wikipedia.org/wiki/Stockbridge_damper

EDIT: Added wiki link.
« Last Edit: April 09, 2016, 12:52:58 PM by Bill Harvey »
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Frank Koenig

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Re: How many mA in an Ampere?
« Reply #79 on: April 09, 2016, 12:54:46 PM »

The great thing about this forum is I learn something pretty much every day. :)
I had no idea that there was a DC power distribution system.
While DC has obvious advanteges, I alway thought everything was AC.

So, for you high voltage  guys...
I sometimes see what looks like some kind of adjustable clamp on either side of the connection point of the HV line to the insulator on high tension towers.
They can be seen in this pic.
What are they?

Those things that look like a pair of length-wise skewered hot dogs are so called vibration dampers there to prevent the line from turning into a giant aeolian harp and tearing itself apart in a strong wind.

I say "so called" as adding mass, while having the desired effect, is not strictly speaking damping, which would require energy dissipation. Someone more familiar with aeroelasticity or stringed instrument acoustics might be able to explain why they interfere with the unwanted mode.

--Frank
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Re: How many mA in an Ampere?
« Reply #79 on: April 09, 2016, 12:54:46 PM »


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