Homopolar Generators (N-Machine) by Bruce de Palma

[PulsedPower]

If you attach the magnets to the shaft so they spin WITH the disk,
there IS NO BACK EMF.

theres nothing to push against, the magnetic source is spinning with the disk.

but the field, being circular, remains stationary

In my previous post I mentioned that there is no torque reaction on the magnets anyway in fact quite a few homopolar machines put the rotor inside a large air core solenoid. Because the magnetic field produced by rotor current flow is orthogonal to the magnet field there can be no torque reaction on the magnet. This is the case with a uniform field, if the field is not uniform in the direction of motion then there is a some induction in the rotor producing a field and consequently some torque reaction on the magnet assembly, If the magnets travel with the rotor then there will be no eddy currents induced in the rotor  as even a nonuniform field will be unchanging with respect to the rotor. But as the magnets are not moving with respect to the return current path eddy currents can be induced in it instead. Non uniformity of the field in the radial direction produces no eddy currents. It is difficult to describe without drawing it. AFAIK as the previous poster mentioned having the magnets travel with the field will still produce voltage when measured from a stationary point most likely because the magnetic field is now moving with respect to the return conductor.

I am guessing that this lack of torque reaction with the field magnet is what you mean by nothing to push against, but the return current path provides plenty to push against the current in it sets up a magnetic field opposite to the current flowing though the rotor. That 27000 rpm 140 lb rotor mentioned in my other post didn't come to a stop by itself. I understand that there might be some difficulties measuring the power input to a homopolar generator if it is measured by running it on cradles like a brake dynanometer as some torque will appear outside the machine if care is not taken in locating the output terminals maybe this is why some people asociate them with over unity power output. Also the strong stray magnetic fields associated with them may also play havoc with the instruments used to measure the power input and output. In a pulsed power application the input power is well defined as the polar moment of inertia and the rpm of the rotor are easily quantified and the output power is measured with instruments which are designed to deal with the strong sources of interference associated with this environment

[argona369]

If you attach the magnets to the shaft so they spin WITH the disk,
there IS NO BACK EMF.

theres nothing to push against, the magnetic source is spinning with the disk.

but the field, being circular, remains stationary

In my previous post I mentioned that there is no torque reaction on the magnets anyway in fact quite a few homopolar machines put the rotor inside a large air core solenoid. Because the magnetic field produced by rotor current flow is orthogonal to the magnet field there can be no torque reaction on the magnet. This is the case with a uniform field, if the field is not uniform in the direction of motion then there is a some induction in the rotor producing a field and consequently some torque reaction on the magnet assembly, If the magnets travel with the rotor then there will be no eddy currents induced in the rotor  as even a nonuniform field will be unchanging with respect to the rotor. But as the magnets are not moving with respect to the return current path eddy currents can be induced in it instead. Non uniformity of the field in the radial direction produces no eddy currents. It is difficult to describe without drawing it. AFAIK as the previous poster mentioned having the magnets travel with the field will still produce voltage when measured from a stationary point most likely because the magnetic field is now moving with respect to the return conductor.

I am guessing that this lack of torque reaction with the field magnet is what you mean by nothing to push against, but the return current path provides plenty to push against the current in it sets up a magnetic field opposite to the current flowing though the rotor. That 27000 rpm 140 lb rotor mentioned in my other post didn't come to a stop by itself. I understand that there might be some difficulties measuring the power input to a homopolar generator if it is measured by running it on cradles like a brake dynanometer as some torque will appear outside the machine if care is not taken in locating the output terminals maybe this is why some people asociate them with over unity power output. Also the strong stray magnetic fields associated with them may also play havoc with the instruments used to measure the power input and output. In a pulsed power application the input power is well defined as the polar moment of inertia and the rpm of the rotor are easily quantified and the output power is measured with instruments which are designed to deal with the strong sources of interference associated with this environment

Hi Pulse,
It sounds like you know your homopolar generators.
I never thought of the lack of torque back into the magnetic source.
But just the closing path producing torque relative to
The rotor. that is interesting.

I?ve always wondered if there was a way to do away
with any slip rings. A ?one piece? device that would rotate as a whole
(Closing path included) driving a load.
But I think it would be fundamentally impossible.
But maybe there would be a way?

Btw , the spiral disk I believe was not for a higher voltage
But to counteract torque and reinforce the magnet?

"Because the current is flowing in a large circle at the rim of the disk, the magnetic field created
by the current not only does not work against the field magnet above the circular plate, as in
conventional generators, but it actually reinforces the magnet"

>Tesla's Fuelless Generator,
>http://home.comcast.net/~onichelson/Fuelless.pdf
>from this page which has some good articles,
>http://tesla.nichelson.googlepages.com/home

[sm0ky2]

our set-up in the lab was a 15-inch copper dsk

mounted between two disk-magnets.

S/N facing together on either side of the disk. Steady power from a power-supply to run a DC motor, which turned the device. The power produced was nothing near overunity, since our motor was grossly overpowered for the mass of our homopolar generator.

What i remember distinctly, was the fact that there was no difference in torque when current was being drawn off the disk - between a carbon-brush on the exterior rim and a slip-ring on the shaft. 

I dont know what the set-up was on that massive disk that "came to a stop", but when we replicated the generator, using a rotating magnetic field,  it essentially flywheeled to a stop, wether or not we extracted electricity from it.

The experiment was to show the difference between the disk rotating above the magnet, and the disk rotating WITH the magnet. Voltage/Current are constant, but with a stationary magnet, there is negative forces when electricity is drawn.
as far as back  EMF from a pair of leads... that value is so rediculously small that it appears minute even to the drag effects of friction from the surrounding air.

Moving the magnets with the disk in this manner does not alter the induction effect whatsoever.
the magnetic field remains stationary (provided that your shaft turns straight).


@ Argona - if i understood Tesla correctly, the spiral cut sections of his disk were not to raise the voltage,. but to raise the CURRENT. as it forces the current to flow around the disk. instead of finding "shorter" paths across the radius of the disk as it spins around. - granted the overall current flow, does make a spiral around the disk, not all of it travels in that path and there is a great deal of curent loss with a non-spiral-cut disk. - at least thats how it works according to Tesla.

[PulsedPower]

Hi argona, I am a bit rusty on these machines but I did the math on a 120MJ one just for the fun of it years ago, it is nice to dream, 120 MJ in a package weighing less than 3 tonne would have been an interesting but expensive toy to build.

Btw , the spiral disk I believe was not for a higher voltage
But to counteract torque and reinforce the magnet?

It should reinforce the magnet, though it is far easier to put turns in the stationary return current path  as is done with self exciting homopolar generators, they only need a small current to start them. Navel Research Laboratories built a 10MJ unit in the mid 70's. Only a car battery to start it but the current in the magnet rapidly reached 170kA before the current was diverted into the load. Alas there was plenty of torque with the pair of 170 lb rotors @ 18000 rpm initial speed almost stopping in 1 sec. In pulsed power there are good reasons not to spiral the rotor conductor:
1 it increases windage
2 it reduces the burst speed and thereby the energy which can be stored
3 it increases the rotor resistance
4 it increases the rotor inductance

Putting the spiral (coil usually) in the current return path only increases the system inductance and this can be switched out on pulse delivery without reducing the magnetic field (the field will drop slowly as the current decays in the now shorted coil).

SmOkey2, that sounds like a decent sized homopolar machine what sort of current was it putting out? How was the torque measured? measuring the shaft torque or cradling the motor are the safest, cradling the homopolar machine can introduce experimental errors easily. Did you know that an iron disk works fine for long rise time applications, with the advantage of greatly reducing the reluctance of the magnetic circuit (more voltage less magnet)

The setup on the high speed machine (27k rpm) from memory was a single aluminium slab rotor around 14" diameter with lots of brushes which could be brought into contact quicky with the rim and shaft, special attention was made to bring the current return conductor close to the rotor (to reduce system inductance and therby decrease the current rise time) The magnet was an air cored solenoid fed from another homopolar generator and energized seconds before the power was required (keeping a 4T magnet energised any longer than necessary gets expensive both in power and cooling) All the homopolar machines I have read about have been pulsed power ones and they stop very fast once current (40kA to 1MA) is taken from them. Maybe the current you were taking from it was small, being a low voltage machine it takes many amps to get appreciable power from it. 

Over unity or not they are interesting machines for playing with high current

[sm0ky2]

the torque was measured as a function of RPM on the shaft, vs current draw on the DC motor, which was metered at the source.
this rating was within some % error of , but close to the manufacturers specs for those motors.

as far as the actual  measurements for voltage/current, i know we wrote them down, throughout several experiments in a table, for the lab project, but i couldn' tell you what the actual values were..  it could not have been over 100amps, with the equipment were using, We didn't have anything rated much higher than that. - though, to fathom a megaAmpere', i kind of wish we had!!

drawing current off of the disk did not make it stop spinning or even slow down when the magnets were attached to the rotating shaft