View Full Version : OT high freguency power line
01-18-2006, 01:51 AM
One day in the electronics shop I got the bright idea of hooking up a roll of cable tv wire (co-ax) to the flyback transformer in a tv. I was amazed to see that the coax was up to the task. I could get a good long spark off the end of the wire without the coax itself shorting out inside. Knowing that you can conduct higher power with less loss using higher voltages, I had always figured that you could use coax to conduct a few tens of kilowatts as long as the voltage was high enough. With modern techniques in high frequency power supplies, you could also make your step-up and step-down transformers much lighter and smaller. Just something to power up your remote cabin from your hydro generator that just happens to be a good distance away. Roll out the coax, and forget about the #6 wiring. Should I be keeping an eye out for the men in white coats or is there some semblance of sanity in this?
01-18-2006, 01:54 AM
The only time sanity stops is when you quit experimenting.
Dave da Slave If it don't agree; get a bigger hammer, BFH
01-18-2006, 07:25 AM
Typically, losses go up at higher frequencies, although the weight of equipment goes down.
You have a lot of VA into the wire capacitance, which is an issue.
Then there is the problem of having a long enough line that you get to "antenna" size at the frequency you are using. That can be fairly short as far as transmission line distances go.... and long lines (not coax) radiate power into space
Added to that is the rectification at one end, and step-up in frequency and voltage at the origin. Neither is as efficient at higher frequencies.
The coax has a voltage limit.... ask one of the folks here who deals with high power radio transmitters.... and teh dielectric absorbs power also, at some point it overheats.
Going the OTHER end, high voltage DC is quite efficient for transmission, so if you can do teh conversion efficiently, you get a boost for long lines.
We have an audio amplifier of 2.5 kW output. It has an SMPS, not dissimilar to the techniques for boosting voltage at high frequency such as you suggest. It is about 86% efficient. With more advanced techniques, that might reach 93% or maybe even a bit more.
An "iron" transformer for 60 Hz could probably be 99%+ efficient.
But the whole product weighs 23 lb, where the iron transformer for 99% would weigh far more than that, plus structure weight to hold it.
It's all in what's important. For power transmission, normally efficiency rules, although cost of losses is weighed against physical plant cost.
01-18-2006, 09:25 AM
Power transmission is a well developed subject. Different styles of transmission line have evolved for different purposes. First, the flyback of a TV set is designed to deliver both medium voltage AC and high voltage DC, not high frequency AC. The horizontal sweep frequency on the primary of a flyback is about 15735 Hz (15.735 Khz), and I don't call that a high frequency by any means. The secondary delivers something between 15,000 and 40,000 Volts which is rectified into DC to energize the CRT. It has an AC component which is the same 15.735 Khz as the primary and this is filtered by the capacitive characteristic of the CRT itself.
Inside the TV the 15 Khz is carried around by plain wires. And the high voltage is also carried to the CRT with a plain, but heavily insulated, wire.
Coax cable has both a voltage and a power rating. The voltage rating is determined by the dielectric material used and it's resistance to arcing. The power rating is based on the voltage developed when the cable is carrying a high frequency signal and is operating at it's characteristic impedance. Hence, it is based on the basic voltage rating.
The advantages of coax are that it's characteristic impedance can be predicted and controled, thus you see 50 Ohm, 75 Ohm, and coaxs of other impedances. [The characteristic impedance is the impedance in Ohms that, if use as both a source and a load impedance on the line, will produce the maximum transfer of power to the load.] This becomes an important factor as the frequency of the signal is increased. At power line frequencies (under 1000 Hz) it is of little importance. Hence coax is not often used for power distribution, AC or DC. It would be a pure waste of money in most cases.
So coax could be used for power transmission but is not because it offers no real advantages and it's a lot more expensive than plain wire.
Now get me started on speaker wire.
01-18-2006, 12:09 PM
Then you get into the dielectric over time decay.
It happens with generator cables coming out of the power plant to the switch yard.
Don't think I'd try it. Buy you some spark plug wire from JcWhitney. That is what I used for my tesla coil.
01-18-2006, 01:41 PM
Darryl, have another beer the white coats are on the way.
Beleive me you don't want to know about the challenges of sending kilowatts down coax at high frequencies. A subject dear to my heart.
David mentioned the Tesla coil. Here is a useless bit of information, Nikola Tesla was born 150 years ago this year and there are special stations on the air to celebrate this.
Anyone interested have a look here.
Nikola Tesla (http://www.inet.hr/9a6aa/9a150nt.htm)
There is a link at the bottom right of the text to more information about Nikola Tesla.
[This message has been edited by zl1byz (edited 01-18-2006).]
01-18-2006, 04:15 PM
You mean they are'nt a band? Tesla?
Some old dude who liked to throw sparks?
01-18-2006, 04:56 PM
Nic wrote music?
I guess he really was really was a genius http://bbs.homeshopmachinist.net//wink.gif
Today I will gladly share my experience and advice, for there no sweeter words than "I told you so."
01-18-2006, 05:09 PM
I understand the Tesla museum in Colorado Springs has been closed for a while, never did get to see it. What a guy, favorite story: "Tesla's experiment burned out the dynamo at the El Paso Electric Company and the entire city lost power."
You just gotta admire somebody like that!
01-18-2006, 09:49 PM
Well I guess I'll have that beer while I contemplate various alternate uses for other things. http://bbs.homeshopmachinist.net//smile.gif
Tesla was indeed a man ahead of his time. The latest 'tesla' thing I've been intrigued about (again) is fireball lightning. I wonder about the current state of knowledge on this.
A good read is Tesla-man out of time- by Margaret Cheney ISBN 0-7432-1536-2.