Quote

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

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

@smOkey2, I was sure I replied to this thread a few days ago but the post has gone anyway 100 amps is a very light load especially as the generator would put out less than 5V at 1440 rpm That is why pulsed power applications run the rotors so fast, voltage is propertional to speed and field strength. The 1MA generator had 80 tons of rotors but much smaller generators have been produced which produce similar currents

@SmOkey2

Unless there is relative motion between the source of the field and the inductor, there can be no
'back torque'.

While I don't believe the torque reaction in a homopolar is between the moving conductor and the magnet but rather between the moving conductor and the stationary current return conductor. There is still relative motion between the source of the field and the inductor, otherwise it would violate faradays law or no voltage would be induced. I suspect that you assumed that the rotor is still the source of the induced voltage when the magnets are fixed to the rotor? Draw the complete electric and magnetic circuits including the load and you will see the field lines of the now moving magnet cutting the stationary electric circuit before looping around and entering the back face of the rotor. If the rotor was making the voltage it would be possible to rotate the load and magnets with the rotor and dispense with the brushes. If you still have access to this aparatus remove the brushes and connect a 3V light across the rotor with some hot melt glue to secure it and the wiring to the rotor, run the unit and see if the light glows. This makes sense, if no relative motion of the magnetic to any conductor is required why does the rotor need to spin at all and why is the output voltage proportional to the rotor speed? Something to think about, FWIW this was problem in a first year physics text.

I mentioned ANU's 80 ton machine, it is a very old machine but togther from a surplus betatron magnet and some large steel disks, The university of texas had made much more compact machines which deliver similar current.

dont confuse the 'source' with the field. a rotating disk magnet produces a stationary field.
relative to the field, the disk is spinning.

 I have never heard that one before but lets assume for the sake of argument that it is true. What is the field stationary with respect to, the planet the solar system the galaxy? what happens if you move the source, does the field get left behind? If the field was stationary with respect to the planetary rotation axis just the act of putting magnets near wires would make high voltages For example near the equator he velocity of the earths surface relative to the rotation axis is around 440 m/sec, if a 1.5T magnetic field was put near a conductor it would produce 1.5X440 V/m, I think 660V for every meter of conductor perpendicular to the the earths rotation axis and exposed to a 1.5T field would have attracted some attention by now, the case for the solar system and galaxy voltages are even higher. If you say the the magnetic field is stationary relative to the earths surface, why? There has to be some reason for this field to pick this particular frame of reference over the countless other possibilities. Maybe the field is stationary with respect to the the sources rotation axis, why the field is uniform it has no concept of rotation axis and linear generators also work and these can't have a rotation axis. The most obvious frame of reference for a magnetic field is the source as there is actually some reason for the field to follow the source, it is produced by the source whether it be electron spin or current flowing in conductors.

The homopolar generator derives its voltage from the motion of a conductor in a magnetic field, it doesn't have to be a disk a conductor travelling in a straight line though a field will produce a voltage. FWIW the Voltage is calculated by the equation e=LBV where e is the induced voltage, Length in Metres of the conductor perpendicular to the direction of travel B is the field strength in Teslas and V is the velocity of travel in metres per second. The faraday disk or homopolar generator came out of Faradays work involving motion of a conductor in a magnetic field it obeys exactly the same laws.

i could probably throw together a small unit if i find me a coper disk.
slap it between a pair of magnetron rings, and mount a shaft through it.....
resistor + LEDs...

tie it to multiple points around the outer edge of the disk to balance the load

No need to balance the load the resistance of the disk is negligable, also there is no need to use copper even iron disk machines can put out MA. It doesn't even need to be a disk, a single spoke on a shaft rotated in a field will produce exactly the same voltage, a disk is used becuase it is easy to pick up current off, easy to balance and has a lower resistance than spokes.

As I mentioned a conductor moving in a straight line perpendicular to a magnetic field will also make a voltage, MHD generators and rail guns use this principle. Lets say you stick ten 30mm neos in a line to a bit of wood and stick a length of wire on top, if you lay the stick across the seat of a car and go for a drive, will there be a voltage produced? This is exaclty the same scenario as the rotating magnet Homopolar generator, if the theory of the stationary field is true the voltage will be 24.5 m/s x 0.3m x 1.5T or 11V at 55mph I say no because there is no conductor motion with respect to the magnetic field, if you had 2 sliding contacts behind the car and hooked up meter to them you would see the 11V but this is not very practical. hence the popularity of the disk rather than the linear setup.

You are forcing me to revisit old physics books

Quote from: PulsedPower on February 28, 2008, 08:23:05 AM

Quote

dont confuse the 'source' with the field. a rotating disk magnet produces a stationary field.
relative to the field, the disk is spinning.

 I have never heard that one before but lets assume for the sake of argument that it is true. What is the field stationary with respect to, the planet the solar system the galaxy?

The fact that rotating the magnet with a stationary disk produces no electrical potential leads people to the conclusion that the field itself is stationary.  This has never been proven even though Faraday did numerous experiments in this line.  Tesla thought that the magnet did not contain the magnetism but was merely distorting the field around it, the ether theory.

It is interesting that when the space shuttle broke a long tether that it glowed with potential so that it could be seen for more than 100 miles due to the effect of moving through the magnetic field of the earth.

http://en.wikipedia.org/wiki/Faraday_paradox
"The magnetic field will be independent of any rotation of the magnet. In this configuration, the polarisation is determined by the absolute rotation of the disk, that is, the rotation relative to an inertial frame. The relative rotation of the disk and the magnet plays no role."

Actually, it now looks possible (maybe) to have a true one piece
Homopolar generator that I?ve been discussing here,

http://tech.groups.yahoo.com/group/free_energy/message/30766

wether or not this distortion occurs in a vacuum, or through an aether is not yet known. To know that, we would first have to devise a means of distinguishing an eather, that permeates everything.
how would we even go about doing such? would it matter if we did?

There may be a simpler answer.  The magnetic field may dislodge electrons that are collected or moved by the centrifugal force of the disk conductor. However, this brings up the question of inertial field of reference.  How does the disk know that it is spinning?  Obviously this proves a connection with the rest of the universe, spacetime or whatever.