Comment by leggomylibro | Hacker News Reader

Comment by leggomylibro | Hacker News Reader

I started reading some of Tesla's speeches recently, and one thing that struck me was how he happily talks about enormous flaws in the implementations of his designs, but still went on to prove that they were POSSIBLE even if the practical design might have to wait.

His idea for ultra-efficient high-voltage, high-frequency AC power lines is a good example. It would probably work, but his proposed system required liquid insulation on the wires; it would be important to keep gases out of the medium, and the slightest break in a solid insulation like rubber or silicone (or gutta-percha) could basically turn the wire into the business end of a tesla coil.

So...I don't understand enough about the subject to say for sure, but we probably landed on 50-60Hz for a reason. I wonder why this one didn't wind up working out, but I'll bet it's described somewhere in the application which I've only skimmed.

Also, for anyone else trying to dive into it, 'condenser' = 'capacitor' in olde-speak.

The choice of 50/60Hz is an engineering trade off. Engines, Turbines and alternators are happiest spinning in 100s to 1000s of revolutions per minute range. At that RPM range, alternators with 2, 4, 6 etc magnetic poles translate to AC line frequencies in the 10's of Hz range. AC power on aircraft is usually at 400Hz because it makes equipment lighter.

It is similar with the choice of 3 phases for electrical power distribution, you can add more phases but it just adds to the cost with little benefit.

Huh, cool; you learn something new every day!

Jeez, when it comes to electricity, it is just like peeling an onion with layers of ignorance...

Another relevant trade off is transformer size vs. efficiency.

AC is used because it's very simple to step up to very high voltage for efficient long distance transmission and then back down to safer low voltages for use, while doing the same with DC requires significantly more complex electronics.

However, at very low frequencies you need to use very large transformers which cost a lot and look ugly, and at higher frequencies you lose efficiency due to skin effect [1].

50 Hz is a good compromise - transformers are of a practical size and skin depth is about 9 mm, which is larger than most conductors and therefore doesn't add much loss.

If you increase the frequency to just 10 kHz (which is still considered very low) the skin depth decreases to only 0.6 mm, and you would have significant loss on even moderate sized conductors.

[1] https://en.m.wikipedia.org/wiki/Skin_effect

You want low frequency for high efficiency in transmission lines. That's why HVDC is becoming more popular [1]. It wasn't really economical to use DC current for a long time because it's more difficult to step up/down the voltage than with AC (you just need a transformer). But power electronics and in particular power semiconductors have advanced so much that HVDC becomes more and more prevalent. As for AC: The higher the frequency the higher the losses, but you can use smaller transformers (without using any fancy power electronics).

[1] https://en.wikipedia.org/wiki/High-voltage_direct_current

We already use high voltage for power transmission. Step up transformers at the power plant bring the voltage up to 230kV for transmission and step down transformers within the city bring the voltage down to normal.

My guess for the frequency, is that it is a harder conversion to perform with acceptable loss. Compatibility reasons doesn't allow for just bumping it up. Also let's not forget that AC frequency actually comes from the revolutions per second the power generator does, so there may be some other physical limiting factors here.

Last, some underground high power transmission lines are actually emerged into oil, to keep them cool.

Higher frequency on power lines would be less efficient, not more. High voltage DC is the most efficient, but is most expensive because you can no longer use transformer for step-up and step-down.

> we probably landed on 50-60Hz for a reason

I'm pretty sure that's the minimum to prevent perceptible light flicker.

Incandescent lights were more forgiving in that regard due to thermal inertia: https://physics.stackexchange.com/questions/13400/what-invis...

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