coulomb
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Re: PG&E Shutting off power.

Fri Oct 18, 2019 3:33 pm

GRA wrote:
Fri Oct 18, 2019 3:11 pm
Most streetcar systems ran at around 500VDC.

Here in Australia, they ran/run at around 580VDC, because that's what you get when you mercury rectify 415 VAC three phase (240 V phase to neutral, we skupped the whole 110/115/117/120 V nonsense). The only reason it was DC was that traction electric motors were DC at the time. And of course two wires (one near-ground potential rail and one overhead cable) is much easier for transportation than three live wires.
But AC won because it could cover a much larger area, so was far cheaper in terms of wire,

Yes, because of low frequency transformers.

The other huge advantage of AC is that you can turn it off with a cheap switch. The arc is self quenching because the current crosses zero every 8.3 or 10 milliseconds. Also fuses are cheaper for the same reason. That's a major nuisance for DC over about 24 V.

Edit: yet one more advantage of AC, at least outside of North America where three phase seems to be uncommon, is that induction motors run nicely at utility frequency, at a convenient 1800 or 1500 RPM, less a little. They happily run continuously for years, and don't have brushes that wear out.
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GRA
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Re: PG&E Shutting off power.

Fri Oct 18, 2019 3:48 pm

coulomb wrote:
Fri Oct 18, 2019 3:33 pm
GRA wrote:
Fri Oct 18, 2019 3:11 pm
Most streetcar systems ran at around 500VDC.

Here in Australia, they ran/run at around 580VDC, because that's what you get when you mercury rectify 415 VAC three phase (240 V phase to neutral, we skupped the whole 110/115/117/120 V nonsense). The only reason it was DC was that traction electric motors were DC at the time. And of course two wires (one near-ground potential rail and one overhead cable) is much easier for transportation than three live wires.
But AC won because it could cover a much larger area, so was far cheaper in terms of wire,

Yes, because of low frequency transformers.

The other huge advantage of AC is that you can turn it off with a cheap switch. The arc is self quenching because the current crosses zero every 8.3 or 10 milliseconds. Also fuses are cheaper for the same reason. That's a major nuisance for DC over about 24 V.

Yeah, when I used to design off-grid systems, the cost of DC rated switches was a lot higher than AC. The other problem with DC, lacking any cheap, efficient means of changing the voltage, was that everyone had to string wires for different voltages for every different load - lights, alarms, machinery etc., darkening the skies with all the wires stacked on poles. The book I mentioned upthread, "The Grid", points out that there were something like 7 different DC voltages in common use at one time in the U.S, all of which were often strung on the same poles: 100, 110, 220, 500, 600, 1,200, 2,000. Not that AC immediately brought about standardization, because IIRR there were also nine different frequencies in use. From memory, they included 25, 30, 33.3, 40, 50, 60, 66.6, 83.3, and 125 Hz.
Last edited by GRA on Sat Oct 19, 2019 11:32 am, edited 2 times in total.
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goldbrick
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Re: PG&E Shutting off power.

Fri Oct 18, 2019 8:18 pm

Lots of great points in your post but could you explain this a bit? I don't see how AC vs DC would affect the complexity or cost of a fuse....
coulomb wrote:
Fri Oct 18, 2019 3:33 pm
Also fuses are cheaper for the same reason.

coulomb
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Re: PG&E Shutting off power.

Fri Oct 18, 2019 10:13 pm

goldbrick wrote:
Fri Oct 18, 2019 8:18 pm
I don't see how AC vs DC would affect the complexity or cost of a fuse....
A fuse is like a switch that operates by melting a conductor. When DC current is reduced, any inductance in the circuit will tend to keep the current flowing. So the fuse will tend to arc, that is, the very air at the gap ionizes and conducts. An arc has a resistance of very roughly four ohms, so if say 10 amps is being interrupted, or the current manages to fall to 10 amps, it's only dropping some 40 volts, or 10% of a typical Electric Vehicle battery. Meanwhile, that 40 volts and ten amps is 400 watts, quite a lot of heat in a very small volume. This heat is enough to keep the air ionized, and this can keep the current flowing until something burns up.

A DC rated fuse will have the conductor surrounded by glass beads or ceramic to keep the gap cool, so the air can't ionize. A 100 amp DC cartridge style fuse is almost an inch diameter and two inches long (pardon the caveman units, but this is a US hosted forum so I try to speak the vernacular). It's heavy and costs some US$10.

Once an arc starts, its self sustaining nature means that the gap has to be increased to ridiculous lengths to extinguish it, like 1 mm per volt, or well over a foot for 400 V. It's like starting a fire, its just best not to let one start at all.

Switches and contactors rated for say 500 V DC have to contain magnetic blowouts or other clever arrangements.
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nlspace
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Re: PG&E Shutting off power.

Sat Oct 19, 2019 8:04 am

Great explanation, thanks for sharing this information.

LeftieBiker
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Re: PG&E Shutting off power.

Sat Oct 19, 2019 12:44 pm

I think that this is how DC arc welders work. ;)
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Oilpan4
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Re: PG&E Shutting off power.

Sat Oct 19, 2019 1:27 pm

goldbrick wrote:
Fri Oct 18, 2019 8:18 pm
Lots of great points in your post but could you explain this a bit? I don't see how AC vs DC would affect the complexity or cost of a fuse....
coulomb wrote:
Fri Oct 18, 2019 3:33 pm
Also fuses are cheaper for the same reason.
Just search YouTube for differences between AC and DC arc.

All the big fuse I have cut open are packed with glass beads.

I weld with AC and DC for different applications, big differences in the arcs.
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Oilpan4
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Re: PG&E Shutting off power.

Sat Oct 19, 2019 1:30 pm

bearlight.jpg
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Nubo
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Re: PG&E Shutting off power.

Sat Oct 19, 2019 4:04 pm

Oilpan4 wrote:
Sat Oct 19, 2019 1:30 pm
bearlight.jpg
LOL. I'd love to see that flying in front of PG&E's corporate offices.
I noticed you're still working with polymers.

goldbrick
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Re: PG&E Shutting off power.

Sat Oct 19, 2019 9:13 pm

Thanks for the explanation. I knew about the difference in switches but never connected the dots on a fuse, which is just a one-time use switch in a way. Thanks again, great info.

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