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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Salvador wrote ...
Ok, Dr.Slack, just to confirm , a single disc , yes it has some load atached to it via brushes which also completes the circuit because wthout that there is no path for the current to flow , so brushes , disc and load say a light bulb , and that's it no permanent magnets no field coils no nothing just a start up current kick say from a battery after the disc starts to spin to create the first current and the b field , and this will work?
With the return circuit, you already have all the topology you need to create a suitable field. However, you need to get the geometry right to intereact with the disc. Follow Radiotech's suggestion and study the arrangement of the Barlow's Wheel on wikipedia. The return circuit has been bent into a U shape to enhance the field in the correct direction to cut the disc. If you loop the U shape several times, you can enhance it further.
Now whether you call the magnetic field generating coils 'field coils' or 'an appropriately shaped return circuit' is just a question of semantics, the generator itself doesn't care. Presumably one meets your criteria, the other doesn't. Are you asking 'can I make a self-excited generator when I try hard to NOT provide an approparite field generating winding?'
You might want to do the sums to see what the maximum lamp reisistance is for self-starting for any speifici residual magnetism and speed. Note that where some descirptions of self excited generagtors assume parallel field connection, this series connection requires a short circuited load for easy starting. Note that some iron in the field path not only provides some remnance for self starting, but also some field regulation as it gets into saturatnion.
Registered Member #54402
Joined: Mon Feb 02 2015, 11:09PM
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Posts: 86
Well a changing magnetic field can create a force which is not transferred with electricity , also a permanent magnet exerts a force on a spinning copper disc which is perpendicular to the field lines and that is a homopolar disc or faraday disc. I have to say I didn't quite understood the underlying message of your post radiotech , could you please elaborate some more ?
If the generator is to generate electrical power, there has to be some torque on the disk to supply the energy. Without an external field, the disk won't feel any torque. AFAIK, the motors in the Tesla electric vehicles don't use neodymiums but use a rotating magnetic field to drive a conductive rotor. These motors can recuperate mechanical energy once they are "kick started", but they do need a stator.
Registered Member #54402
Joined: Mon Feb 02 2015, 11:09PM
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Posts: 86
Uspring , but field is not something that has to have a point at which it is tied like a ship to an anchor or like a string or something does it?
Remember that in the faraday paradox it made no difference either the copper disc rotates and the magnet is stationary or if the magnet rotates together with the disc in a single assembly, current was produced in the disc in both situation because all the disc needed was a field cutting its path at 90 degrees ,
as can be seen in this video demonstration
when the magnet rotates at the same speed as the disc it can't put any torque as you speak because they are both at the same frame of reference at that situation yet current is induced. My situation is similar only except I don't want to use a physical magnet attached to the disc I want to use the current running through the disc as the magnet, okay what do you think folks ? what is right and what is wrong here ?
Thanks, Slack for your input I understand what you mean , that the part that would complete the circuit from brush to brush goes close enough to the disc as to interfere with the field and serve as the " excitement coil" but what if the wire that complete the circuit from the brushes go away from the disc as far as possible and are connectet together some place far away as the field from them could be said negligible to effect the disc , would then the disc still produce current from its own magnetic field which is then a result from the current that it produces running from the rim to the ceter or vice versa?
This is kinda chicken and egg which came first quesion I guess.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Salvador wrote ...
Thanks, Slack for your input I understand what you mean , that the part that would complete the circuit from brush to brush goes close enough to the disc as to interfere with the field and serve as the " excitement coil" but what if the wire that complete the circuit from the brushes go away from the disc as far as possible and are connectet together some place far away as the field from them could be said negligible to effect the disc , would then the disc still produce current from its own magnetic field which is then a result from the current that it produces running from the rim to the ceter or vice versa?
This is kinda chicken and egg which came first quesion I guess.
No, you couldn't get the other wire far enough away for its field to be negligible.
I feel that the current in the disc itself does not contribute, in fact is the only part of the current loop that does not contrubute, to the field that generates the emf in the moving disc. However, I am having a hard time coming up with an explanation that convinces me. I kinda feel that if the disc could react with its own field, pull itself up by its own bootstraps so to speak, that could create force with no reaction, breaking conservation laws. And we're very hot on conservation laws on this site.
Let me try to explain why via a motor, as a generator is just a motor driven backwards, but for some reason I find it easier to reason about a motor. But it is closer to a homopolar motor or a railgun than it is to a 'conventional' rotary motor.
Consider a battery, driving current through a wire, which of topological necessity loops from one terminal to the other, enclosing an area. Drive a current through the wire, and it generates a magnetic field, which tries to expand the area enclosed. Let some of the conductor move in the direction of this force, the armature in a railgun, or one of the side conductors in a homopolar motor (look at Barlow's Wheel carefully and identify the moving and stationary parallel conductors) and you have a motor. Drive the conductors in to reduce the enclosed area, and you have a generator, which is what you get when you explosively compress a cylinder carrying a current, for some one-shot high flux experiements.
In computing the total field passing through the current carrying area, we use Biot-Savart and do a line integral along the length of the conductor. You can therefore see that the entire loop of current, including the loop through the battery, and the path through the disc, is generating a field. How far the loop is away is just a multiplier in the expression for the field.
What I'd like to do is produce a similar simple thought experiement for the work done when a wire moves, and integrate the forces and displacements to get energy. I suspect that would show that the self field of the armature does not appear in the total expression for work done. But I'm not going to think about that myself at the moment.
I suspect that if you look up railgun, you might find the sort of analytic treatment I'm after, it's only a slight difference in geometry. Railguns tend to express the efficiency of their geometry, rail spacing and the like, as a delta inductance per distance moved by the armature. I'm sure the same measure could usefully be used to summarise the geometry of a homopolar machine.
There is a mistake in the logic of the video. Spinning the disc creates voltage/current in the circuit so it seems like spinning just the magnet must do the same thing as it's equivalent except for the frame of reference. Not so.
The equivalent action to the spinning disc is the disc stationary and the magnet rotating along with the *whole of the rest of the instrument*. In both situations you need a commutator and one part of the electrical path cuts flux and the other avoids a counter EMF by not cutting flux.
Registered Member #54402
Joined: Mon Feb 02 2015, 11:09PM
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Posts: 86
Your right Wastrel, just by spinning the magnet nothing happens but if we would alos spin the brushes and connecting wires then it would induce current just as if the disc is spinning , but at the end of the video that is actually shown. they keep the disc and the magnet stationary and spin just the brushes and current is induced , so for any practial reasons there is no need to spin the magnet because the field lines or I should say the field strength doesnt change for a symmetrical magnet so we can just spin the disc or the brushes.
Ok some further thinking here , also as to what you Dr.Slack said , well youtube is full of the famous heart shaped copper wire around a AAA finger battery , in this case where does the b field comes from or in other words as Uspring earlier on here said there is nothing " against" which the wire could push itself? well there is a small round magnet at the bottom usually but that magnet is rotating at the same speed as the wire so from relativity their both at the same reference point.So from another observer looking at the device he couldn't tell the difference between the field coming from the wire due to current going through it from battery terminals and the magnet , yet still this field that is corotating with the wire provide a torque against which the wire can push itself to rotate. So as you said earlier Dr.Slack about the bootsrapping this seems just like the case , there is no torque exerted on the magnet because the motor would work both if the magnet would be kep stationary and if the magnet moves together woth the wire as it is in most cases.
Maybe this beggs a broader question like for example, does it matter where the field comes from whether from some external magnet or the disc itself , as long as its in the right direction and angle it should work right? I;m wondering about this now because there are no distinction between the fields of the rotating magnet and the wire their both practically the same.
Oh and if you will , another one , what would happen if i used a changing b field in this scenario with the single disc , just say I attached a capacitor as a load to the disc via the brushes , the current would oscillate back and forth would a chaging b field act differently from a static dc field as normally found in a faraday disc? I assume yes because a changing field would be able to induce some current and voltage even without the physical rotation of the disc but once it rotates , well excuse me I really can't back this up right now but I have a strong feeling this should work, I just want to ask before I spend time and effort cutting wires and making a test rotor to know it it's worth it at all. Hope to hear your opinions.
Maybe this beggs a broader question like for example, does it matter where the field comes from whether from some external magnet or the disc itself
Yes it does. Think e.g. of instead of conducting a current from the center of the disk to a brush at the rim, you use a stationary wire from the center of the disk to where the brush is and conduct a current through it. This will cause a braking effect on the disk like an eddy current brake. You'll have a force on the wire and an equal and opposite one on the disk, braking it. When you instead conduct the current through the disk, as in your original proposal, you'll have similar forces, but from the disk to the disk. So the forces remain inside the disk and no torque will appear.
Registered Member #54402
Joined: Mon Feb 02 2015, 11:09PM
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Posts: 86
Are you basically telling that since every current generates a magnetic field which indeed generates an equal and oppposite counter EMF using a single disc for both excitation and current production would fail because the current going in one way would be met by a current trying to go the other way in the same conductor which menas they would cancel out , is this what you thought?
I guess the only way this would work after all is if there would be relative motion between , like two discs and with some clever gears or something once you spin the axis both discs go say in separate direction or one stays at rest while the other one spins , or for a very simple yet quite inneficient probably setup as Dr.Slack said just twist the wire coming from the brush so that it makes a few turns close to the spinning disc.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600
Dr. Slack wrote ...
I feel that the current in the disc itself does not contribute, in fact is the only part of the current loop that does not contrubute, to the field that generates the emf in the moving disc.
It will certainly contribute, any neutral charge current inevitably creates a magnetic field; but the amount of contribution will be lower, because the current is diffuse, and so it will tend to self cancel to a degree.
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self excited homopolar generator (no PMM!!!)
