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What advantages does ac electrical power have over dc electrical power

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What advantages does ac electrical power have over dc electrical power

Old 5th Jul 2013, 21:17
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VK2TVK,

Thanks for the compliment. I do have to say, though, that I disagree on altitude. Air is a pretty good insulator most of the time. Ionized air is a pretty good conductor, though, and air gets ionized when electricity flows through it.

The typical problem with a switch is that when the contacts open, there's always an instant when they're still very close together. An arc will jump that gap, starting ionization, then grow as the contacts spread further apart and the ionization process continues.

Simple switches are spring loaded, so the contacts will separate faster than ionized air can fill the gap and maintain an arc. Some high-voltage switches actually keep the contacts in a vacuum to avoid ionized gas (though the contacts themselves can also create enough ionized particles to sustain an arc in some circumstances).

Other high-voltage switches use a blast of air to physically blow out the arc. These might not work well at altitude, but I don't think they're used for typical aircraft voltages anyway.

Other than that, I'm not sure why arcing would be a bigger problem in unpressurized areas. But there's probably one I haven't thought of.

*** Should have googled before posting. Apparently electrons that bump into too many gas molecules get slowed down and aren't effective at ionizing. So up to a point -- somewhere around 150,000 feet -- less air means more ionization and more arcing.

Last edited by Chu Chu; 5th Jul 2013 at 21:42.
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Old 7th Jul 2013, 23:52
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Keep in mind when avionics started, we Had to have AC to make the avionics work. Synchros and resolvers need AC in most applications. These are the gyzmos that drove the compass cards pointers and such in the steam gauges. And the tube type radios were able to me made much lighter with an AC power supply that could be easily stepped up and down in voltage.


Some aircraft like the newer King Airs with the proline 21 avionics are now DC only, getting rid of the weight associated with the inverters.
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Old 9th Jul 2013, 08:54
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To refer back to the earlier posts on poly-phase systems: a major advantage of these is that for a balanced system, there is no neutral return current. For example: 3 single phase AC circuits would need 6 conductors; the same result can be achieved with 3 conductors if connected 3 phase. In practice, the 3 phases would not balance perfectly so there would be a residual neutral current but it can be of reduced section (it is also possible to use a 3 wire system with no neutral). But wait there's more...because there is no neutral current, the total volt drop across the circuit is halved for each phase, allowing further economies in conductor size.

This great idea was invented by Tesla in the late 1880s and works so well that it has not changed in principle since. And I think these systems still will be around in another 100 or 200 years time.
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Old 9th Jul 2013, 09:37
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And we haven't even started with 3rd harmonic currents, high frequency iron core saturation, power factor and switchmode psu waveform distortion yet!
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Old 9th Jul 2013, 16:05
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Chu Chu:

Other than that, I'm not sure why arcing would be a bigger problem in unpressurized areas. But there's probably one I haven't thought of.
The effect you refer to is described by Paschen's Law. Here is a curve describing breakdown voltage per inch vs air pressure:

Breakdown Voltage | Paschen Curve | Altitude and Pressure | Corona | High Voltage Spacing | Insulation | from High Voltage Connection

Short of [no pun intended] an complete arc, this breakdown voltage also affects something called corona discharge around charged conductors. That's the crackling noise you hear in the vicinity of EHV transmission lines. For long lines (utility) this actually represents a small amount of power loss and inefficiency. But even on relatively short lines in an aircraft, this effect can produce a considerable amount of RF interference as well as ozone, which can affect some materials adversely.
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