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brushless true dc motor?

brushless true dc motor?

brushless true dc motor?

(OP)
Has anyone heard of a rotary motor without brushes that can work on true dc?

(unlike the so-called brushless dc motor that cannot be powered straight from dc).

Or would such a motor be outside the realm of existing technology?

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RE: brushless true dc motor?

(OP)
also I am excluding any motor that uses electronics to process the dc input.

Pure stationary dc supply hardwired to copper windings, using iron, permanent magnets, structural materials.... no electronics, no brushes.

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RE: brushless true dc motor?

(OP)
Also no mechanical gadgets driven off the rotor that switch the external power.  External dc power flows continuously unswitched through the winding.

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RE: brushless true dc motor?

You didn't specify continuous rotation, which is the hard part.  Galvanometers and DC torque motors both meet your specification as stated.

I don't know the details, but I think that magnetically commutated alternators exist, and are sold into the heavy truck market.  I have not heard of a magnetically commutated motor.  Such a thing would be of immense commercial value, so if I figure it out, I'll be talking to a patent lawyer before anyone else.



Mike Halloran
Pembroke Pines, FL, USA

RE: brushless true dc motor?

(OP)
yes, continuously rotating.

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Eng-tips forums: The best place on the web for engineering discussions.

RE: brushless true dc motor?

Without breaking the circuit or having an alternating supply commutation is not possible.

TTFN

Eng-Tips Policies FAQ731-376


RE: brushless true dc motor?

Well, sort of. Are you aware of the old DC watthour meters? A couple of designs used a copper disc as both the rotor and the armaturre winding. Line current was passes through the disk from one edge to the other using a mercury bath insead of brushes. The disk was in a magnetic field energised by the line voltage.
Continous rotation, no brushes, no comutator.
If there was a short, I would not want to be near until the mercury had a chance to recondense.
There was a connection to a rotating member by the mercury pool.
This is as close to your criteria as I can come Pete.
respectfully

RE: brushless true dc motor?

(OP)
Thanks for the comments.

No brushes and no devices (like mercury) that act like brushes.

Everyone thinks it's impossible?

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Eng-tips forums: The best place on the web for engineering discussions.

RE: brushless true dc motor?

Barlow's disc is a way to do that: don't know of any practical power application for it.
Also it requires a sliding contact, but no commutation: don't know if this satisfies your criteria, electricpete.
In case this device was known under a different way, here is a brief description.
A conductor disc is immersed in a constant magnetic field perpendicular to its plane. A dc supply is connected between its center and a point on the periphery (here is the sliding contact): when current flows, the disc rotates.

prex
http://www.xcalcs.com : Online tools for structural design
http://www.megamag.it : Magnetic brakes for fun rides
http://www.levitans.com : Air bearing pads

RE: brushless true dc motor?

(OP)
I have seen that device that you called Barlow's disk but it was called a homopolar motor / generator (can operate in either mode) in an electromagnetics textbook.

The open circuit voltage in generator mode is V = w* B * I *(b^2-a^2) where w is radian speed, B is axial flux, I is current in the disk, b is disk od, a is disk id.   From this we see the voltage is very low which tells us the current will be very high (a consequence of the fact that it is essentially a one-turn coil), which limits the usefulness.

It does have brushes on the outer periphery of the rotating copper disk... doesn't meet the criteria.

Are there any brushless true dc motors meeting the requirements?
- can work on true dc
- excluding any motor that uses electronics to process the dc input.
- No brushes and no bush-like devices (like mercury)
- Pure stationary dc supply hardwired to copper windings, using iron, permanent magnets, structural materials.... no electronics, no brushes.
- Also no mechanical gadgets driven off the rotor that switch the external power.  External dc power flows continuously unswitched through the winding.
- continuously rotating.

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RE: brushless true dc motor?

(OP)
Correction regarding the homopolar generator:
 V = w* B * (b^2-a^2)

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RE: brushless true dc motor?

Electricpete,

No, I don't know of any but it would appear that you have either found or invented one.

RE: brushless true dc motor?

Actually, I once had an idea for a brushless tachometer that I think would work.  The trick was to thread the coil windings through the back iron so that only one side of the coil was exposed to the magnetic field.

RE: brushless true dc motor?

(OP)
Yup, that is very much along the same lines of what I am thinking about.

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RE: brushless true dc motor?

I was shown a sketch for a proposed DC generator that was meant to work without brushes or a comutator.
A magnet was rotated on the north-south axis and the lines of force were cut by the windings on one side of an adjacent coil.
When you drew a sketch of the coil, the magnet and the lines of force, it looked good. I pointed out that that the lines of force must be cut in such a way that the conductor was moveing either from a weak field into a stronger field, or vice-versa.
Although the sketch showed the lines representing the magnetic flux appeared to be cut, the conductors remained in an area of the same flux density. No EMF would be induced.
Remember, you must cut lines of force in such a way as to go from a weaker field to a stronger field or the opposite to generate or motor.
respectfully

RE: brushless true dc motor?

(OP)
Yes, the idea of the sketch sounds similar to what I am thinking.

You might very well be right  (I am wondering what I am missing, if this is something that has not been done before), but I don't understand your objection.

[QOUTE]Although the sketch showed the lines representing the magnetic flux appeared to be cut, the conductors remained in an area of the same flux density. No EMF would be induced.
Remember, you must cut lines of force in such a way as to go from a weaker field to a stronger field or the opposite to generate or motor.[/QOUTE]

Take for example the "homopolar" motor generator (Barlow's disk) discussed above.  The disk rotates in a constant uniform field.... but EMF and force are still developed.    

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RE: brushless true dc motor?

I have the same question about "Weak Field-Strong Field." The force in a current carring conductor is

Force = Field Strength x Wire Length x currrent

RE: brushless true dc motor?

I decided that my Brushless Tach would not work, the error being the belief that a conductor burried in iron would not be cut by a moving magnetic flux.

For example, if the rotor bars in an induction motor rotor are completely burried in the rotor iron (no slots), I believe that a strong current is still induced in the rotor bars.

RE: brushless true dc motor?

(OP)
Going back to the homopolar motor.  If you search youtube for homopolar motor you will see many varieties.

One is like this:
http://www.youtube.com/watch?v=u9_YojwhlmQ

It's a battery.  Then a neodymium magnet hanging below the battery by a screw (the magnet keeps it all together, the screw acts like a bearing).   Then connect the opposite terminal of the battery to the circumference of the magnet.  You have radial current flow in the magnet interacting with tangential flux from the magnet to produce tangential force (torque).

It is the same as the homopolar motor described in my textbook  (Barlow's disk)except the magnet plays a dual role of providing the axial flux and providing the disk conductor.

This has brushes and it's not the same thing I have in mind, but it seems to illustrate that  a varying flux cutting a conductor is not a prerequisite for motor action.

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RE: brushless true dc motor?

(OP)
Correction: "You have radial current flow in the magnet interacting with axial flux from the magnet to produce tangential force (torque)."


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RE: brushless true dc motor?

A corona wind spinner.  (You didn't say no ions!)

RE: brushless true dc motor?

I could not resist (after reading Bobbys contribution): Convert DC to heat and run a heat engine with it...

RE: brushless true dc motor?

(OP)
Here is the brushless true dc motor that I had in mind:

1 - PM magnets on the shaft in a homopolar orientation (i.e. all North poles facing outward).

2 - Take a piece of steel pipe as a stator core.  Cut several notches at each end.    Wind the stator coil continuously around the stator core so that there are conductors on the ID and OD of the stator core  (current on the ID flows in opposite direction from current on the OD).   

3 - Assemble a steel endplate onto the stator core to complete the flux path.  It sits on the "teeth" between the notches and it has a hole in the center for the shaft with only a very small clearance.

The flux path is from the magnets, radially outwadr through the ID conductors, axially in the pipe to the endplate, then radially inward on the endplate to the shaft.

Something like this:

http://home.houston.rr.com/electricpete/eng-tips/brushlesstruedc.ppt

We expect to develop tangential force on the ID conductors due to the interaction of radial flux and axial current (F = N I L x B).

I can see a legitimate question about this device:  
1 - As was pointed out above, what is the mechanism for developing a voltage in the conductors which is required for conservation of energy if this device is to do work?  I'm not sure I know the mechanism of how voltage would be induced in the stator conductor.  But then again, I can’t explain how the voltage is developed in the homopolar disk motor, and we know that it works just fine.

But if you tell me it won’t work, I have another question:
2 - Why wouldn't it do work if a force is generated F = N I L x B?  If there is current flowing and flux flowing in a perpendicular direction, the only way it can NOT work is if there is some other equal/opposite force generated when this thing begins to rotate.  I'm not sure I know what the mechanism would be for this equal opposite force, either.

So #1 it doesn't work or #2 it does work.  Either way I can't fully explain it.  Likewise I can't answer question #1 above for the disk motor but it still works.     I could build one and try it I guess.   Would it be a waste of time or would it make me a millionaire?   

As you may guess since I’m posting my “secret” design rather than spending time to build a prototype, I am suspicious that it won’t work... if it did someone would have done it before.

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RE: brushless true dc motor?

There would need to be some asymmetry to create a rotational force.  Sure, you may get a tangential force, but you will also get an equal and opposite tangential force at the same time.  Apply AC to your stator and you will have a rather inefficient synchronous motor

RE: brushless true dc motor?

(OP)
"There would need to be some asymmetry to create a rotational force."

Why do you say that?  Once again, the homopolar disk machine does not have any angular assumetry, but it works and according to one textbook is used as a motor in some very low speed applications (ship propulsion) and as a generator in some very high current applications (electroplating or something like that).

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RE: brushless true dc motor?

I'm not familiar with the homopolar disk machine, so I can't say how it works.

In a conventional DC motor you have two tangential forces on the rotor created by opposite magnetic poles.

In a three-phase AC motor, synchronous or induction, you have a rotating magnetic field that can drag the rotor around with it.

In a single-phase AC motor, you have an additional winding with capacitors to change the pulsating field (that won't cause a rotor to turn) into a rotating field.

All motors I'm aware of use asymmetric forces to create rotation.  A review of basic Statics and Dynamics shows the need for a net force to create motion.

RE: brushless true dc motor?

(OP)
Can you explain how you conluded there is no net force?

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Eng-tips forums: The best place on the web for engineering discussions.

RE: brushless true dc motor?

The description you provided does not provide a mechanism that I could see that would provide a net force.  The rotor would be pulled in both direction simultaneously.

RE: brushless true dc motor?

electricpete,
can't understand your question #1. The homopolar motor works because there is a current flow perpendicular to the magnetic field and to the allowed direction of motion. The countervoltage is induced in the conductor because if you join the conductor to the generator, this forms a loop that concatenates part of the magnetic field.
On the other hand I think that your machine works.
In fact one could say that it doesn't work, because the force exerted on a conductor on the OD is balanced by an equal opposite force exerted on the nearby conductor on the ID, as current flows in the opposite direction.
However the conductor on the OD is not traversed by any magnetic field, because the magnetic field is captured by the stator iron and diverted in the axial direction (you didn't specify this feature in your description, but I'm sure you are aware of it evil).
Another arrangement similar to yours could be as follows:
-one polarity cable goes to one stator end, here it is divided into a multiplicity of wires; the wires are distributed on the periphery of the stator, go longitudinally on the inside face to the other stator end, then rejoin into the other polarity cable.
Still another arrangement:
-the inside face of your stator cylinder is covered with a copper foil, separated from the stator iron (as iron is quite electrically conductive) by an insulating sheet.
However your arrangement is possibly more flexible, because it allows for using many thin wires: note that to facilitate the winding the stator may be divided into two halves (or more parts), as the cylindrical part of it has longitudinal magnetic flux only.

My compliments for your idea: you should definitely make a prototype.

prex
http://www.xcalcs.com : Online tools for structural design
http://www.megamag.it : Magnetic brakes for fun rides
http://www.levitans.com : Air bearing pads

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