# Thread: Amount of copper wire in a generator

1. ## Amount of copper wire in a generator

I need help coming up with a realistic amount of copper on hand for a story. I have a fictional company that makes propane powered generators that range from the size of lawnmowers to the size of cargo vans. I have no idea how much power these could generate, but that doesn't matter.

To make them, I assume they will have a lot of copper wire on hand. I found a place that sells 5000 foot spools of 22 guage copper wire for 40 dollars, which is just about what it's worth to recycle (3.177 cents per pound this morning).

The point I'm trying to make is that there is enough copper wire sitting around to justify the need for a compound style layout to the place. To do that, I need some idea of how many of these copper wire spools they might go through in a week. To do that, I need some idea of how much wire is in a small generator. That's where the whole thing falls to the ground. I can't seem to find anything that tells me in a way I understand. Yahoo answers* had a question about copper in a 230KW generator and the answer was 1000 to 3000 pounds. Mine are much smaller than that, but I don't know how to describe them accurately as far as output.

*As I once saw on Twitter: "Yahoo Answers - For those times you need to know what some random guy more or less reckons."

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To know how many spools they go through in a week you would need to know how many generators they made in a week, as well as how much copper is in each.

Try your local scrap yard/salvage yard and ask how much copper they take out of wrecked generators.

3. It's in the range 1 to 2 kg per kW for small generators. The design to get rid of heat makes larger ones more efficient but factors of safety also have to increase.

4. Besides how much room they need, you might just go bigger or smaller and claim that the market for commercial space was limited.

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I have torn down many small motors for the copper. My rule of thumb is a pound in the 1/4 HP - 1/2 HP size. Above that I go about a pound per horsepower. Biggest one I have stripped out was 50 hp and yielded only 33 pounds.
Last edited by billslugg; 2014-Jun-01 at 05:33 AM.

6. Thanks for the replies. I Think I've got enough for what I need, but digging into it I found questions I didn't know I'd have.

I found a place that makes the same type of generators I was picturing. They have large ones, powered by propane, that use car motors to turn the (whatever the turny-bit is called). Why?

Why does the motor need to be a full V-8 or V-10 just to generate electricity? Why can t a smaller engine be geared up like a bicycle to just spin the turny-bit faster?

7. Originally Posted by Tog
Thanks for the replies. I Think I've got enough for what I need, but digging into it I found questions I didn't know I'd have.

I found a place that makes the same type of generators I was picturing. They have large ones, powered by propane, that use car motors to turn the (whatever the turny-bit is called). Why?

Why does the motor need to be a full V-8 or V-10 just to generate electricity? Why can t a smaller engine be geared up like a bicycle to just spin the turny-bit faster?
It's the second law again, you cannot get out more power than you put in. And when the power is in the form of current in copper wires at normal temperatures there is resistance and therefore heat losses in the copper. The practical consequence is that you need more copper to handle more power, it's just the same in transformers for example, a bigger transformer handles more power. In practice there are also iron losses. Iron is used to concentrate the magnetic fields but it is also lossy as heat and in fact saturates too, so if you push too much power through, it melts.

The bicycle gears are to match the load to the way humans put out power, there is an optimum pedal rate for max power so you gear to suit the hill.

8. Originally Posted by Tog
Why does the motor need to be a full V-8 or V-10 just to generate electricity? Why can t a smaller engine be geared up like a bicycle to just spin the turny-bit faster?
Would you do the same to run a pump faster? The generator just converts mechanical power (rotation rate*torque) into electrical power (current*voltage). To get more power out, you need to put more power in...using gears to increase rotation rate at the expense of torque won't do the job.

9. Originally Posted by cjameshuff
Would you do the same to run a pump faster?
I probably would. My path to a physics education stopped at the second year of algebra 1.

I do understand why a pump that was pushing water would need* to have a larger motor. It's the same reason the "go fast downhill" gear on a bike is a poor choice for going back up.

What isn't clear to me is that the implication is that electricity generation creates a physical load. That idea combines with the above gave me this idea:

1. A copper thing rotating in a magnetic field generates an electric current.
2. As this current is generated, it creates a force that acts to slow the rotation of the rotating thing, which produces heat.
3. To dissipate this heat, more copper needs to be added to the rotating thing.
4. The additional copper increases the mass of the thing to the point that a larger motor is needed to drive it fast enough to generate current at a constant level.
5. This is why a mouse on a treadmill can't power Chicago.

Do I have the basics correct? Electricity is basically witchcraft to me.

* Totally unrelated but the dislexia kicked in there and I wrote "been" instead of "need," which is almost mirror image. That's a new one for me.

10. Not quite. It's been many years since I studied this stuff but as I recall it the electric load creates a "back-emf" which applies a torque on the rotor. It's nothing to do with cooling or the mass of the rotor. Cooling, if required, will be by blowing air over it.

As an example, years ago I had a 1974 1200cc Honda Civic. Not much power. Turning on the headlights would reduce the idle speed by a couple of hundred RPM. Even operating a turn signal would cause the engine speed to vary in time with the signal. All because of increased torque required to turn the generator against the increased electrical load.

11. As Trebuchet said, it has nothing to do with cooling or mass, it's simply balance of forces. The magnetic field and the electrons in the coils on the armature exert forces on each other causing the electrons to circulate through the circuit and the armature to resist motion. The greater the load, the more work it takes to turn the generator.

That's why it takes a larger engine to produce more power. The need for a larger generator is due in part to the need for thicker wire, but also because the iron cores can only handle so strong of a magnetic field before they saturate. This limits the maximum amount of power a generator (or motor) can handle.

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