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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[electricpete]

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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[electricpete]

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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[MikeHalloran]

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

 

[electricpete]

yes, continuously rotating.

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[IRstuff]

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

TTFN

Eng-Tips Policies FAQ731-376

 

[waross]

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

 

[electricpete]

Thanks for the comments.

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

Everyone thinks it's impossible?

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[prex]

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

 

[electricpete]

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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[electricpete]

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

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[sreid]

Electricpete,

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

 

[sreid]

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.

 

[electricpete]

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

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[waross]

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

 

[electricpete]

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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[sreid]

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

 

[sreid]

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.

 

[electricpete]

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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[electricpete]

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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