Homopolar Generators (N-Machine) by Bruce de Palma

[PulsedPower]

@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

[sm0ky2]

i dont have any way of moving 80-tons of mass as such high RPM,

but im sure i understood the principal that was being taught, as i have used this in other applications since, and it holds true in every case i have encountered it.
Unless there is relative motion between the source of the field and the inductor, there can be no
'back torque'.

Induction still occurs because the field is stationary.

[PulsedPower]

@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.

[sm0ky2]

@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.
any moving field-lines relative to the return circuit would be a result of the EMF caused on the inductive surface of the disk, NOT from the magetic field - and this can be avoided by routing your wires at a different angle.
assuming the magnetic disk is polarized evenly, and not in sections, there is a stationary, uniform field sitting in space, as the magnet spins.


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.
(since the brushes cannot slide when the entire circuit is moving)

[PulsedPower]

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