Voltages, Currents, and Tesla

To clarify on question one, I mean like who you bump up the voltage for power lines to transfer AC current.

I guess I just don't get how raising the voltage lowers the current. It seems to me that the same number of electrons still need to pass through to deliver a given amount of energy. Are they just faster due to more potential energies which means you have less "traffic?" I do see how, in relation to equations, it gives a lower I-squared R losses.

You are missing the energy carried by the electron. Higher voltage gives faster moving electrons that have higher energy.

Take the water analogy. Squirting water at a water wheel. You can either deliver a lot of water at low pressure or deliver less at high pressure to obtain the same movement of the wheel.

Thank you Rich, helped a lot, I guess that should of been obvious. Does anyone know about Tesla's egg of columbus. I believe it was copper plated and had induced currents in it. I want to build one.

----------------------------- Individual electrons in a conductor, at AC, simply wobble (relatively slowly) back and forth- the energy transfer has nothing to with their speed. Energy flow is at near light speed. The best way to deal with this involves EM theory but circuit theory does work very well.

Consider a single electron falling through a potential difference V = then the work it does is determined by the charge on the electron and the voltage V (in fact potential difference in volts is defined in terms of the work per unit charge) Thus 1 electron falling through 1000V does the same work as 1000 electrons falling through 1V. Both voltage and current must be considered. This is what you are not doing. If you look at a hose- consider big hose low pressure vs small hose, high pressure can deliver the same energy in a given time. Beyond that -forget the hose as it can lead you astray.

We can express energy in terms of power* time where power is proportional to the product of voltage and current. A transformer has voltage*current on one side the same as on the other side (ideally) This is a consequence of conservation of energy.

Consider a transformer with a 1:10 turns ratio. Suppose the applied voltage on the primary is 100V and there is no load. There will be no secondary current and the primary current will also be 0 (ideal case). No power transfer. The secondary voltage will be 1000V. Turns ratio a =100/1000 =0.1

Now consider a l00 ohm secondary load. At 1000V the secondary current will be 10A and the secondary power will be 1000*10 =10,000 watts. The secondary current tries to reduce the magnetic flux in the transformer but for "constant" AC voltage, this can't happen so a primary current of

If you are wanting a mechanical analogy of a transformer, think of a gear train which trades speed for torque at the same power.

I don't know about his "egg" but it appears to be what is called a "drag-cup" induction motor. You can make an induction motor using a beer can as a rotor. Even a paper clip can be made to turn. I've done it with 3 coils and a 3 phase supply but it should be no problem to make a single phase version. Another scheme is to attach a magnet to a crank arm so that it can be rotated. Put a copper or aluminum disk on the same axle but free to rotate independently. Turn the crank and the magnet will cause eddy currents in the disk so that it will rotate as well.

With a 3 phase supply, how do you make one? The power at a house is one phase correct? If so how would you create 3 phases from one, if possible. I'm thinking of doing this in an undergrad physics lab, and looking for ideas. This being said, I have a DC supply, an AC supply, and a few other things which may be useful, but I would like to use the AC. Is it possible to make 3 phase with common materials?

The phenomenon being described does not depend upon the microscopic properties of electrons. "Upping voltage and lowering current" does not produce less resistance. The point is to reduce I^2*R losses. Mere lowering the current does that. If you insist on understanding that at the microscopic level, go to a good book on solid state physics.

I can only guess what "Tesla's Egg of Columbus" can be. I think that it is a hollow egg shape made from metal. Placed in a rotating magnetic field, it acts like a squirrel-cage so that when it speeds up it goes on end.

Bill

-- Ferme le Bush

One marvelous thing about induction motors is that a single phase can produce TWO counter rotating magnetic fields. Once you get the egg rotation started, it will react much more strongly with the field rotating in the same direction as the egg. It should then speed up.

Bill

-- Ferme le Bush

Probably the easiest way is to hook the shaft of a single phase motor to the shaft of a 3 phase motor / generator. Hook the single phase motor to the single phase power supply and you get 3 phases out of the other motor. You can also alter frequency this way. Make sure the output is the frequency you want it to be.

Josh

You won't get 3 phase from single phase unless you use a relatively complex rectifier/inverter setup or a single phase motor driving a 3 phase generator as J. Drumel suggested. You can do as Salmon Egg suggests, give it a spin in the right direction. Otherwise use 2 coils at right angles and put a motor starting capacitor in series with one of them. You then have a "single phase" motor . Not a good one as there is no iron in the core so the flux distribution is not great. However, it should suffice. P.S. A small, light compass may, if it can be accelerated quickly enough, will run as a synchronous motor. You'll need a strobe to see it though. (I am thinking of a cheapie about 1/2 inch diameter. )

We ran our facility on some weird transformer conglomeration to synthesize three phase from single phase. MSC and the like of industrial tool suppliers sell single to three phase converters for running your three phase mill off a single phase supply. Phase-A-Matic static phase converters 1-3 HP model about $110. I don't think that's a motor running a generator.

Doug T

You need more than a weird transformer hookup. You would need to have some means of shifting the phase of the input to at least one winding. There are also rotating machines which will do this- even from DC but these are now antiques. However, the static phase converter you mention is what I called a "relatively complex rectifier inverter setup" -which is the way to go for most such conversions. I have also seen the use of a 3 phase motor supplied from single phase - with some means to get it turning in one direction, such as a small single phase motor, or whatever is at hand. Then the 3 phase motor will produce 3 phase voltages which can drive a smaller load. This is physically simple but is pretty cumbersome. For the purpose intended by the originator of this thread, all are relatively expensive. The use of a series capacitor with one of the coils is a way to get the needed phase shift for that purpose.

Would putting a motor driven horseshoe magnet under the table be cheating?

bud--