Question about power capacity of electrical motors.

About at this point, even the best superconductors, at LHe temperature, can't sustain operation. These two cases, and many at lower intensities (including thousands of MRI machines in the 1-5T range), are all superconducting.

Superconductors are a big win, because the field isn't so high that it's impossible, and because most of these require tons of total energy (it's intense /over a large volume of space/, too).

It's impractical to try and charge one of these things every time you need to use it, let alone run steady-state, while burning a tremendous amount of power in a resistive material. You'd have problems with mechanical expansion, thermal gradients, circulating fluids (I mean, you need LHe and LN2 otherwise, but not at the pressure and flow rate that you need for the cooling fluid to run a copper magnet this big) and so on.

...These ones, however, may not be superconducting! Or at least not wholly, AFAIK.

My familiarity with critical field is probably lacking as well; I just remember it's somewhere in that range.

You'd have to ask the designers of that particular NMR. It might be feasible: after all, magnets can be stacked in a particular way so that their vectors add up (Halbach array and such). They might be using a similar trick to optimize the use of superconductors.

The strongest are either fully resistive, or hybrid. The ref shows how this one is built:

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A metric shitton of current, and water, is supplied to a perforated copper sheet, wound into a helix. Around that, superconductors supply as much field as they can (evidently, around 20T), which superimposes with the resistive magnet's field. It has to be done this way, and not the other way around (superconductor inside), because superconduction is not linear at this level.

And note that boosting a 20.0T superconducting magnet, up to 21T as a hybrid magnet, takes exactly as much additional effort as making a 1T air core magnet does (not easy!). So, to achieve another 20-some T extra from that resistive magnet is no small feat!

Tim

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Seven Transistor Labs, LLC 
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Reply to
Tim Williams
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Magamps are certainly radiation resistant and EMP hardened, but 99% efficiency? Doubtful. It takes some energy to saturate a magnetize core even of the best materials. Core losses are significant. Resistive losses are significant (you have to keep core saturated in spite of output currents). Sure switchers have lots of currents flowing that can be used but 99% seems rather high to me for a magamp control unless you are going super conducting or something.

Reply to
benj

99% is right, look at transistor switchers, they run 70 to 83% or so efficient, and they have loss due to Vbe, they cannot get to the rails.

magamps do not have that limitation, (they do have some distortion for audio) I have not designed with them that much.

Reply to
Serg io

Actually they do get to "rails" In switching mode they go from full "off" to low impedance "on" where losses are very small. All the loss takes place during the switching transition. The switching time is not zero plus there is a delay factor due to sweeping carriers out of junction. The actual percentage dissipation depends upon the structure of the device which is not only the type of device but the kind of structures used in fabrication. Of course above I'm especially talking about junction structures, though principles still apply to other structures. Yeah, I've measured this crap.

Yes, but you are still driving a core to saturation and current in coils of wire. I mean transformers (and motors) do pretty well, but 99% seems really pushing the edge to me.

Reply to
benj

Purely resistive, with 38T continuous field strength, has been achieved: (that's at my alma mater!) ). And they are now also building a hybrid for 45T, the same value as in your reference, but one would expect that if 38T is possible resistively, a hybrid could ultimately reach close to 60T. (Pulsed magnets can of course get even higher.)

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Jos
Reply to
Jos Bergervoet

Thanks for the informative response. So at least one of these high magnetic field labs does it without superconductors. If they do this by being additive of the magnetic fields that raises the possibility it would also work with electric motors.

BTW, what is the relation between the magnetic field strength, the current, and the power output? That is to say, even if the magnetic field is limited, can we increase the power output by increasing the current nevertheless.

Bob Clark

---------------------------------------------------------------------------------------------------------------------------------- Carbon nanotubes can revolutionize 21st-century technology IF they can be made arbitrarily long while maintaining their strength.

Some proposals to accomplish that here:

From Nanoscale to Macroscale: Applications of Nanotechnology to Production of Bulk Ultra-Strong Materials. American Journal of Nanomaterials. Vol. 4, No. 2, 2016, pp 39-43. doi: 10.12691/ajn-4-2-2 | Research Article.

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Reply to
Robert Clark

Thanks for that link. It has this interesting passage:

"Large scale and precise

38 tesla magnetWith this new design, HFML also challenged a number of Dutch suppliers and manufacturers. For example, the coils had to be made of another material. Wijnen: 'Many of the HFML magnet coils are made of copper, which is not strong enough for this new magnet. That is why we switched to a compound of copper and silver. Brandsma Metaalveredeling (Brandsma metal processing) managed to silver-plate this complex material and MCi punched the cooling holes. "

Interestingly the copper-nanotube composites I discussed in the first post of this thread have twice the strength of regular copper with comparable conductivity. So they could improve the magnetic field strength.

The Holy Grail though remains using carbon nanotubes themselves for the purpose. They have 1,000 times greater current capacity and 1,000 times greater strength than copper, so should result in a marked increase in the magnetic field strength. Perhaps their usefulness for the purpose could be demonstrated at the small lengths available now to create small scale, i.e., in volume, but intense magnetic fields.

Bob Clark

---------------------------------------------------------------------------------------------------------------------------------- Carbon nanotubes can revolutionize 21st-century technology IF they can be made arbitrarily long while maintaining their strength.

Some proposals to accomplish that here:

From Nanoscale to Macroscale: Applications of Nanotechnology to Production of Bulk Ultra-Strong Materials. American Journal of Nanomaterials. Vol. 4, No. 2, 2016, pp 39-43. doi: 10.12691/ajn-4-2-2 | Research Article.

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Reply to
Robert Clark

And there is no "Vbe" restricting the voltage drop, first of all because you can use a MOSFET if you want, but even for a BJT the driving circuit can lift the base voltage above the rail.

Now you exaggerate to the other extreme! There certainly is loss during conduction. Not Vbe, but still a finite Vce due to the internal collector resistance.

That *is* mainly the switching time in many cases! The other part is (re)charging the parasitic capacitances on the different circuit nodes.

And Sergeio is of course completely wrong to claim that it is "70 to 83% or so" for transistors. They also can reach 99%, if the switching is done at relatively low frequency (or even up to a few MHz with resonant drive). A value of 70% would be typical only if you use GHz frequencies, and nobody in his right mind would do that purely for power conversion purposes.

Actually measuring the precise losses if you are around 99% efficiency is not so simple! For instance you'd need the small difference between large AC input power and DC output power, by necessity measured in not exactly the same way.. Probably calorimetric measurement (how much heat does the converter produce) will be the best way. Is that how you did it?

They are said to use materials with small hysteresis losses. Of course if the loop area in those BH curves becomes almost zero then those losses will vanish. But the magamp will need to drive the field into complete saturation (as opposed to ordinary transformers, where hysteresis losses are a problem already, so it does sound difficult! I cannot disagree with you this time..)

--
Jos
Reply to
Jos Bergervoet

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If you look at production video on AC motors for Tesla as well as others. The coils have to be flexible enough for the insertion process. Carbon nan otubes are too rigid to survive the insertion process. It might be possibl e, but they would have to figure out different ways to make motors.

Reply to
edward.ming.lee

Yeah, but going over 2T is obviously an economic and thermal challenge.

Room temp superconductors, even if they have to be cooled a bit below room temp to be useful, would open up a lot of interesting possibilities, though. You could dump the iron, and shrink things quite a bit, even if the Bmax isn't any higher.

Incomplete: voltage and current, or velocity and torque, is power.

Some motors overlap responsibilities, for example the series wound machine has load current in the field winding, so B depends on current, which is torque. But torque depends on B, which is current, so torque actually goes quadratically with current. Similarly, EMF and RPM vary with current, so the RPM is highly variable, too. (Which is the observation: series wound and universal motors have very high torque at low RPM, and very high RPM at low torque.)

But, broadly speaking, B goes as I, and P goes as B^2. The force applied to the motor is due to a difference in flux densities; flux density gives a pressure, the Maxwell strain, p = B^2 / (2*mu).

(Pressure is also energy density, so this gives the energy density of an inductor, for instance.)

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

No exaggeration. But obviously "it depends" because while Vce is finite, and losses therefore low, it's all a matter how long you are "on". If you are switching on and off at fast rates I assure you majority of loss is during transition. No exaggeration.

I already noted that switching rate matters.

No. Actually was done with high speed high accuracy multipliers. Looked at the product of current through the device and voltage across it. You get a large (and very interesting) power spike as the device switches.

Cannot Disagree? Jos? Jos, is that really you or some sock puppet pretending to be you?

Reply to
benj

That is true but the "majority of loss" should not be replaced by "all the loss" (as you did). That is an exaggeration.

Yes it is! It is!

...

OK, the multipliers might be high-accuracy but the current sensors and the voltage probes should then be completely phase error free, and with perfectly matching delay too.

But it's size is very sensitive to probe errors..

Your current post is already a lot better!

--
Jos
Reply to
Jos Bergervoet

No it's not! It's not!

Recess over yet?

Well duh.

Well duh.

I know. I didn't even mention Jefimenko once!

Reply to
benj

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Commercial jet aircraft rely on turbo-fan engines, which use shrouded fans to convert the rotational energy geenrated by the gas-turbine into thrust. These are simply shrouded propellors. Conventional propellors offer variabl e pitch, and are even more effective if youy aren't flying close to the spe ed of sound.

Propellors may look old-fashioned, but for any aircraft which isn't designe d to fly as fast as economically possible they are the preferred means of p ropulsion

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Using wire made of lower resistance material does reduce resistive losses. If it has a higher thermal conductivity as well, it can get rid of more hea t too.

For many motors, the temperature dependence of the magnetic properties of t he pole pieces also come into it, and limit the temperature that you can le t your windings warm up to.

It's sort of complicated.

--
Bill Sloman, Sydney
Reply to
bill.sloman

The question in regards to magnetic field strength in electric motors is whether increasing the power by increasing the current is limited by magnetic saturation of the core material. This magnetic saturation is at ca.

2.2 T ( tesla) for ferrite cores. But do large electric motors commonly approach this value anyway? This is higher for example than the highest magnetic field for permanent commercial magnets made of neodymium, at ca. 1.3 T:

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But these magnetic fields of neodymium permanent magnets are so strong they could crush your hand if you placed it between two neodymium magnets you could fit in your palm. So I'm inclined to think the magnetic field strengths generated by electric motors are not that strong.

Anyone know what are the strengths of the magnetic fields commonly generated by large electric motors.

Bob Clark

---------------------------------------------------------------------------------------------------------------------------------- Carbon nanotubes can revolutionize 21st-century technology IF they can be made arbitrarily long while maintaining their strength.

Some proposals to accomplish that here:

From Nanoscale to Macroscale: Applications of Nanotechnology to Production of Bulk Ultra-Strong Materials. American Journal of Nanomaterials. Vol. 4, No. 2, 2016, pp 39-43. doi: 10.12691/ajn-4-2-2 | Research Article.

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Reply to
Robert Clark

Ah, I get it -- to a certain extent, no. But for practical purposes, yes, I think.

The reason is the same as why transformers can deliver far more current than the magnetizing current required to saturate their cores. This applies to induction motors, which are little more than transformers where the secondary is a shorted turn, allowed to spin. Flux density in the rotor is proportional to slip.

Um, it probably applies to series wound / universal motors as well, but I can't quite intuit whether that should be the case or not. It also depends on load, because RPM/EMF and current are interdependent.

A PM motor is limited by the coercive force of the magnets, and whether that [magnetomotive] force can be transmitted by the laminated core. (For laminated iron and NdFeB, it might not be possible, dunno.)

Transmitted force (mechanical and MMF kinds) is limited by air gap, so you have that to fight.

Whether that means you can destroy an NdFeB based motor, or demagnetize it, by thrashing the winding current and/or rotor velocity, I'm not sure.

Under pulse conditions, even if you saturate the core around the winding, you can always deliver more and more flux density, and eventually the motor looks more like an air core one (these exist, for low inertia applications, like old timey computer tape drives that had to reverse on a dime; and probably similar with some aerospace synchro or LVDT applications, too), and yeah, the force will continue to rise, but not as quickly as it did before saturation, because the effective air gap is now vastly higher.

So, no, the force isn't limited, but yeah, it's only going to go so far. Whereas an ideal diode looks like a limited voltage (with current rising exponentially), a real diode has series resistance; same with saturation, where it gets much harder to put in that extra flux, but you're fighting an increasing amount of air gap in the process.

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

apperently the conductive lubricant is the hot technical item, the critical item

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short pdf on it

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Reply to
Serg io

Had no idea. Thanks. Learn something interesting every day. (If one can do that and make a girl laugh as well, the day is a success.)

--
Goat
Reply to
Lofty Goat

Of course you still have no idea. Rail guns can't work! Just look at the picture in the link above. The magnetic field ends at the projectile so there is no or very little force to accelerate it. The whole design is stupid! Obviously you want current supplied from BOTH ends of the rails so you have a magnetic field over the entire length of the sabot so it feels the maximum force! Obviously all these so-called rail guns are just military hoaxes to try to extract taxpayer research money, None of them can work!

Reply to
benj

you can find you tube vids that show you how to build your own at home using car battories. So you can slam a hole in your Moms New Car. (holyshit!!)

Lubricant: must provide low friction must provide high conductivity must not cause dammage to rails after multipl shots

Mercury ? w ALu ??

any Ideas ?

carbon seems too high in resistance

I think they are using something that generates a plazma

Reply to
Serg io

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