Rotation by Lorentz or by Faraday? 3

[Intermesher]

The following assumes the application of a constant DC current to rotors that have no rotational inertia to start with.

Some illustrations show the flux from the pole of a permanent magnet causing rotation by 'pulling' the laminate core of a coil wound electromagnet into alignment with the permanent magnet; before stopping.

http://www.unicopter.com/1904_7.gif

However, other illustrations show the flux from the pole of a permanent magnet causing a portion of a coil of wires to transition across the full width of the flux; before stopping.

http://www.unicopter.com/1904_8.gif

Are both of the above statements correct ~or~ where is my limited intelligence coming off the tracks.


Thanks,
Dave

 

[Intermesher]

Peter, in the book

http://ocw.mit.edu/NR/rdonlyres/resources/RES-6-002Spring-2008/Textbookcontents/chp06_text.pdf,

which you previously mentioned, Fig 6-22 states "It is impossible to design a commutatorless dc machine.". This sketch and the paragraph above it appear to have come from a 1946 publication.

I don't think that transistors (and particularly power transistors, MOSFETs and IGBTs, etc.) existed back then.

Today's brushed DC motors are being replaces by BLDC motors. In addition, it is looking like Sensorless BLDC motors may start replacing the Hall Effect device, and, encoders in some applications.

Might it be possible to produce an operational 'unhomopolar' motor by pulsing the power and electronically reading the electrical activity?????

There have be 96 patent since 1977 that use the phrase "homopolar motor". I have not looked at any yet, but these inventors must be thinking of something.


Dave

 

[waross]

"Brushless DC motors are accepted by many in the trade as AC motors fallen prey to marketing jargon.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

"It is impossible to design a commutatorless dc machine.". This sketch and the paragraph above it appear to have come from a 1946 publication.

I don't think that transistors (and particularly power transistors, MOSFETs and IGBTs, etc.) existed back then.

I think the statement made in 1946 remains true, if we use the word "brushless" instead of commutatorless. If we want to power the device with true dc where the dc lives in a stationary reference frame, then we get the required voltage induction only when conductors are rotating with respect to the stationary ref frame where the dc power supply lives, as shown by your wikipedia article. So I think we can say the true brushless dc motor cannot exist.

Today's brushed DC motors are being replaces by BLDC motors. In addition, it is looking like Sensorless BLDC motors may start replacing the Hall Effect device, and, encoders in some applications.

As Bill suggested the term BLDC seems a little bit of a marketing misnomer. My understanding is typical BLDC resembles polyphase PM rotor synchronous motor, except instead of supplying it with sinusoidal ac, we supply it with switched and there can be a lot of intelligence built into the switching controls which perhaps resemble similar control flexibility in dc motors. There are others on the forum that know a lot more about BLDC than me.

Might it be possible to produce an operational 'unhomopolar' motor by pulsing the power and electronically reading the electrical activity?????

Again, I think it would be a challenge to try to apply your existing device as BLDC since you have a single winding rather than polyphase winding. And if you made it a polyphase winding, I think it would make sense to swap your magnets to alternating polarities rather than same polarity.

I have to step back and ask the big picture question now....what is your main objective from this thread:[ul]
[li]to modify the existing motor?[/li]
[li]to learn about motors in general?[/li]
[li]to build a motor from scratch?[/li]
[li]to choose an available motor?[/li][/ul]
On the last two it usually helps to describe your application requirements and ask forum members for suggestions. Maybe even start a new thread with your clarified objective.

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[electricpete]

Clarification:
"we supply it with switched"
should have been
"we supply it with switched dc"

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[electricpete]

And by switched dc, I mean alternating polarity so there is an ac component.

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[Intermesher]

I think the statement made in 1946 remains true, if we use the word "brushless" instead of commutatorless.

Perhaps true if the word 'commutator' can be applied to IGBTs.


I'm confused.
This diagram

http://www.unicopter.com/1904_8.gif

is probably correct when it relates to a generator, where the Force is creating a Current.
However, could it be incorrect when it relates to a motor, where the Current is creating a Force. Consider that the sketch appears to represent a Homopolar motor. And, this will be particularly so if the PMs are considerably lengthened in the vertical direction. But, we are told that a Homopolar motor will does not work.

_________________________

My objective is the satisfaction that is derived from conceptualizing, researching and developing improvements in the field of vertical takeoff and landing (VTOL) aircraft. Home page

http://www.unicopter.com

This thread is part of an attempt to see if it is possible to find a previously unconsidered or unobtainable means of providing a lightweight, powerful, slow turning, reliable and simple electric drive for rotorcraft. Asking a lot for sure.

Patenting or financial gain is not a personal objective. This note appears at the bottom of all pages that contain ideas, which do not conflict with the desires of participants in the specific idea.
"The above utility invention is openly and publicly disclosed on the Internet to negate an entity from patenting it, to the exclusion of all others whom may wish to use it. ~ Reference patent law 35 U.S.C. 102 A person shall be entitled to a patent unless - (a) the invention was known ... by others in this country, ..., before the invention thereof by the applicant for patent."


Dave

 

[electricpete]

I don't have much more to contribute, but I'll comment where something sounds possibly a little off... not being picky, just repeating back for understanding.

Perhaps true if the word 'commutator' can be applied to IGBTs.

Imo the BLDC motor is not a dc motor. As shown in wikipedia, for a true dc motor, the dc supply needs to be in a different reference frame than the conductors in order for the counter emf to be generated. That is not the case for the BLDC motor where we the supply is connected to the stator. imo the BLDC uses electronic switches to create ac... it does not substitute switches for the commutator function on a dc motor. Maybe that's the same thing you were saying.

This diagram

http://www.unicopter.com/1904_8.gif

is probably correct when it relates to a generator, where the Force is creating a Current.
However, could it be incorrect when it relates to a motor, where the Current is creating a Force

Both a motor and a generator need to have motor and generator action. i.e. they both need to create voltage and torque. It just may be a counter voltage (for motor) or a counter torque (for generator).

But, we are told that a Homopolar motor will does not work

Some homopolar motors work. i.e. Zahn page 420 or Bill's link. In all cases brushes are required since these are dc devices.

But the homopolar motor I tried (and I think you tried) without brushes won't work because there is no mechanism for generator action.

Now there is one point of confusion maybe you have that I cannot address. Our homopolar designs put a conductor in a field. So we know there is a force on the conductor. We also presume at least some of the counter-force will occur across the arigap on the PM of the rotor (and I believe I proved it with F.E. for my design). So we think our devices have the capability to produce tangential force and torque in the static condition. Yet we know they can't function as a motor because they can't fulfill the required generator action (required to satisfy conservation of energy). How do we reconcile the device apparently produces a torque in static condition but cannot act as a motor? Seems like only two possibilities:
1 - We were wrong to conclude there is a torque in the static condition (?)
2 - The torque is there in static condition but somehow disappear when the rotor begins to rotate even with current held constant (?)
Beats me.

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[electricpete]

If I look back at my F.E., I only analysed a 2-magnet, 2-partial-arc coil. For that particular geoemetry, I only proved it could generate torque with the rotor in one particular position. But looking at that analysed 2-magnet 2-partial arc coil, it's easy to imagine that in other positions it might stop producing torque or generate opposite torque. And if I added magnets uniformly around the periphery circumference of the rotor, it might act differently. I'm guessing item 1 above might be the more likely logical error (these devices don't actually produce torque in static condition).

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[waross]

I find the sketch in

http://www.unicopter.com/1904_8.gif

to be less useful than it may be. The sketch neglects the location of the return conductor of the current carrying wire. When the wire moves, does it enclose more or less lines of flux? Many homo-polar designs will both generate and motor.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Continuing my earlier line of thought... I am now sure that the brushless dc design that I posted will not produce torque even in the static condition. Assume that the rotor has a lot of tiny magnets... so many that they can be considered axially uniform.

Now we calculate torque as Te = d/dtheta (Wm)
where
Te = electrical torque
theta is angle of rotor with respect to stator
Wm is stored magnetic energy
we choose to hold current constant (alternative choice is flux linkage constant... results in sign reversal).

From angular symmetry of the rotor, we can see that moving the rotor a small angle dtheta will not change the stored energy at all. At first we may think that we still have force on conductor but equal/opposite force on stator core. But another experiment where we move only stator conductor (not stator core) results in same Te = 0 so there is in fact no force even on the stator conductors.

It seems a paradox F = L I x B does not apply. But I believe it is correct. Sticking with the energy approach, if move a current carrying conductor strip within an airgap, there is a change in stored energy which occurs at the leading edge of the conductor strip and at the trailing edge of the conductor strip, but no change in energy at the middle of the conductor strip (see the Long Version page 32 for rough picture). It is the shifting of the flux rising/trailing edge associated with flux from conductor that creates that change in energy. If we have stator conductors all the way around the circumefernce of the airgap, there is no rising/trailing edge (*) and so no change in energy. If we have stator conductors only part way around, there is an equal/opposite rising/trailing edge. It can create torque when it interacts with poles, but we have nothing resembling a pole on these machines. (* it raises yet another question, what if we apply the same logic to the conductor disk of a textbook homopolar motor which we know does produce torque... a question for another day... I believe homopolar motor has some real complexities).

I have kind of hogged the thread talking through a subject that is of interest to me. I apologize for that. I will be glad to continue to talk throught these things or whatever you want in this thread...

However some people may have tuned out on this thread (my fault) and you may miss some good input on your real question of finding a good motor for your application. May I suggest that you post a new thread with title something like "slow lightweight motor?". I'm sure you will get some new suggestions and ideas.

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[electricpete]

Correction:

so many that they can be considered axially uniform.

should have been

so many that the rotor can be considered to have angular symmetry.

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[electricpete]

If we have stator conductors only part way around, there is an equal/opposite rising/trailing edge

That was wrong. The relevant thing is that in the motor I have drawn all conductors within the airgap have the same polarity. (there is no alternation of polarity). That is what prevents the rising/trailing edge in radial flux profile that we're looking for to create a torque.

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[Intermesher]

Peter said;
I will be glad to continue to talk throught these things...

This is what I would like if you, Bill, and perhaps others, are willing.

There are attractive larger diameter, slow-turning, outrunner and inrunner, motors available. One example is this

http://www.alliedmotion.com/Products/Series.aspx?p=10&s=1

This thread was specifically started to see if a phaseless motor was possible and to try and understand the correlation between the magnetic attraction shown here

http://www.unicopter.com/1904_7.gif

and the electromechanical activity shown here.

http://www.unicopter.com/1904_8.gif

Bill said;
Many homo-polar designs will both generate and motor.

Will you direct us to one or two of these motors? This may help in understanding the concern.

__________________

On the subject of the phaseless motor (homopolar motor). I wonder if magnetic reluctance in conjunction with Pulse Width Modulation or Pulse Amplitude Modulation might drive a drive a homopolar PM rotor?

Figure 6-1 on

http://ocw.mit.edu/NR/rdonlyres/resources/RES-6-002Spring-2008/Textbookcontents/chp06_text.pdf

shows the 'On' and 'Off' action of a switch. Perhaps, for my requirement at least, there is some way to direct the 'On' pulses to the CW rotor and direct the 'Off' pulses to the CCW rotor.

Dave

 

[electricpete]

" Many homo-polar designs will both generate and motor."

Will you direct us to one or two of these motors? This may help in understanding the concern.

The MIT / Zahn textbook link above page 421 gives description of homopolar generator. If you read through, it says it can also operate as a motor.

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[electricpete]

On the subject of the phaseless motor (homopolar motor). I wonder if magnetic reluctance in conjunction with Pulse Width Modulation or Pulse Amplitude Modulation might drive a drive a homopolar PM rotor?

If you are referring to the textbook homopolar motor such as shown in Zahn (uses brushes and works as-is), you can use electronics to change the voltage.

If you are referring to my device and your device (no brushes and doesn’t work), I don’t think there is much that can be done with front-end electronics. It would be better to organize the rotor into poles (alternating magnet polarity) and the stator into separate poles and run it is a PM sync motor or BLDC motor.

It seems a paradox F = L I x B does not apply. But I believe it is correct. Sticking with the energy approach, if move a current carrying conductor strip within an airgap, there is a change in stored energy which occurs at the leading edge of the conductor strip and at the trailing edge of the conductor strip, but no change in energy at the middle of the conductor strip (see the Long Version page 32 for rough picture). It is the shifting of the flux rising/trailing edge associated with flux from conductor that creates that change in energy. If we have stator conductors all the way around the circumefernce of the airgap, there is no rising/trailing edge (*) and so no change in energy. If we have stator conductors only part way around, there is an equal/opposite rising/trailing edge. It can create torque when it interacts with poles, but we have nothing resembling a pole on these machines.

That was wrong. The relevant thing is that in the motor I have drawn all conductors within the airgap have the same polarity. (there is no alternation of polarity). That is what prevents the rising/trailing edge in radial flux profile that we're looking for to create a torque.

I think the conclusion was right... there are (paradoxially) no net tangential force on conductors in my device even though those current carrying conductors are located in a radial flux field and right hand rule would predict such a force. But the explanations above fall short. One more try on the explanation. Again Te = d/dtheta(Wm). We proceed as in 12.2 of the long version (add together flux contributions of stator and rotor so we can differentiate to find torque)) There is one contribution of radial flux from the rotor PM’s... easy to figure out – it is constant flux (does not change with theta). The contribution to radial airgap flux from the stator conductors?... Zero! We can show it with Ampere’s law remembering that both conductor currents are in the same direction. A simpler intuitive thought is to compare flux pattern for two conductors carrying current in same direction and opposite direction. If currents flow opposite direction (as those shown in 12.2), then the flux lines encircle each conductor independently and we have flux lines flowing in the area between the conductors... these are the same flux lines that would cross the airgap when we put the conductors 180 apart in the airgap. If currents flow in same direction, the flux lines tend to encircle the pair of conductors (no flux lines cutting the plane between conductors and no corresponding flux crossing the airgap). We also know if we put an array of conductors all flowing the same direction to form a ring of current, the flux inside that ring (where the airgap is) is zero. So if airgap flux from the stator conductors is zero, the energy approach predicts torque on those conductors will be zero. It is a satisfying explanation for something that has been bugging me a long time.

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[Intermesher]

Peter, I read through the first part of your Long paper and skimmed through the latter equations. It is impressive.

This thread has been an interesting and informative one, but there is a need to return to the more practical. Thank you Peter and Bill for your time and contributions.


Dave