brushless true dc motor?

[electricpete]

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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[BobbyNewmark]

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

 

[ijl]

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

 

[itsmoked]

ijl; ahhh that is great!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricpete]

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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[davidbeach]

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

 

[electricpete]

"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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[davidbeach]

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.

 

[electricpete]

Can you explain how you conluded there is no net force?

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[davidbeach]

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.

 

[prex]

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 ).
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

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