T-Pain Is On A Boat: Re: [Electric Boats] Magnetic Circuits & The Prototype Motors

Oops, I think we jumped off into deep waters without much explanation. My apologies.

The short answer is that laminations or cores need to be magnetic. And aluminum (or copper) is not. In fact, the best cores would be magnetic but NOT electrically conductive. Made of 100% Pure Unobtanium.

Magnetic circuits are just like electric circuits in that both need to be a closed loop to operate. Lets look at both:

Taking a flashlight for the electric example, its loop is from the + terminal of the battery, then through a wire and switch, on to the light bulb, and then back to the - terminal of the battery. Push the switch to "on", and current flows around the loop doing work wherever it meets an appropriate (not too much nor too little) resistance.......like in the light bulb. The main requirement is that all parts of the loop conduct electricity. Copper and aluminum are good for conducting electricity.

For a magnetic example I can't think of a real world example that we all know. Part of the problem is that magnetic circuits are less obvious. But magnetic circuits are pretty much the same as electric circuits. There is one big difference and it gets back to your question. To conduct a magnetic field we need something that is attracted to a magnet. The most common materials are iron or steel and various other iron (ferrous) compounds. So we make laminations of iron or steel. You can't make laminations out of aluminum or copper because although they will conduct electricity, they won't conduct a magnetic field.

So think of it this way: we have a magnet which works like a battery, and in order to get it to do anything you have the requirement to hook the N terminal of the magnet to it's S terminal in order to get magnetic current - which is called "flux" - to flow around the loop. Instead of copper wires, we use solid iron, or steel laminations. Then, just like the resistance of the lamp interrupted the electrical circuit, we interrupt the magnetic circuit with an air gap. We need that air gap because we can cause the flux to do work in the gap if the gap is small enough so that the magnetic flux flows across it. Often we put a copper coil in that gap and let the magnetic field do work on the coil...or we can go vice versa. One way we have a motor and the other way we have a generator.

What we have been talking about on this forum is the problem with steel as a lamination material. The problem springs from the fact that steel not only conducts magnetic flux, but it also conducts electricity. Because of that, as steel conducts the magnetic flux it tends to form within itself little tiny electrical whirlpools called "eddy currents". I think of them as being little whirling universes within the lamination....that's probably as weird as it is handy.... Anyway, these eddy current universes soak up lots of energy and generate quite a bit of heat. We don't want either of those to happen. Eddy currents only grow when there is continued electrical conductivity. So the problem becomes a need to retain the magnetic conductivity while limiting the electrical conductivity. One way to do that is by making the core as thin laminations of steel separated form each other by varnish insulation. That way the eddy current universes can only grow in two dimensions instead of three. Or maybe we use some sort of iron powder and restrict the size of eddy current universes to the size of an iron powder particle.

It's a game like any other. There are lots of ways to solve the problem, but each has a downside. Balancing the pros and cons is where the fun is. It's also where the money and prizes are........!!

The big difference today is that with computer modeling we can do all this thinking using a computer generated 3-D model on a screen. Back when I started working in magnetics the computer models didn't exist. After the pencil and paper work, we had to actually build the device and measure it in order to prove a new concept. Now computer models are accurate enough that we accept their output as proof of a concept.

And that's huge.....

Enjoy!! Roger L.

----- Original Message -----

From: Aaron

To: electricboats@yahoogroups.com

Sent: Thursday, January 03, 2013 10:20 PM

Subject: Re: [Electric Boats] Magnetic Circuits & The Prototype Motors

Is there a reason one dose not use aluminum?

Aaron

From: Craig Carmichael <craig@saers.com>
To: electricboats@yahoogroups.com
Sent: Thursday, January 3, 2013 8:05 PM
Subject: Re: [Electric Boats] Magnetic Circuits & The Prototype Motors

>...you need thin laminations of electrical
>steel, either stamped or (for small quantities) cut by NC water-jet
>machines.

...

>Sadly, dust-iron or ferrite materials can't take the high field strength
>of NiFeB magnets (or don't have a high enough permeability), so although
>an interesting idea, I'm not sure any type of printed magnetic core
>materials are likely to be suitable for these type of motors (not in the
>foreseeable future, though I could be wrong...)

There's a big difference between ferrite and iron... I quit using
laminations when I found commercially made iron powder cores. Inside
are actual iron particles, so it is an actual iron core, not ferrite,
and magnets clamp on strongly.

For my axial flux "Electric Hubcap" motors (36V, 4.6KW 0-2000 RPM...)
I use a separate 2" O.D. x 1" thick iron powder toroidal core for
each coil. (Unlike typical usage, my coil is wound as a doughnut
around the outside, the wires epoxied with heat conductive epoxy.)
These donut coils are individually attached in a ring to non-metallic
(molded PP-epoxy) body parts with no electromagnetic drag. At 2000
RPM the frequency is just 100Hz, so high frequency losses don't come
into play. It's almost lossless. Iron losses at 1200 RPM (60Hz)
calculate as 1W per coil, total 9 watts. No eddy currents because
every powder particle is insulated from all the others.
http://www.micrometals.com/ makes them.

To further improve motor efficiency I paint the coils with ilmenite
in sodium silicate, a para- ferro-magnetic skin that bends magnetic
lines of force that are headed for open air around and into the
cores. It's also something of an internal completion of the magnetic
circuit. AFAIK no one else has ever done that. Motor idle currents at
various speeds on two motors dropped 25-35% when painted. Peak
efficiency of the motors is probably about 95%, but has never been
properly measured.

I would have inclined to solid cores, but toroids with no center is
what there is, and it actually is better - 40% less iron to have
losses in, and the field is more concentrated at the edges where it's
more useful. After all, the coil is turned off whenever a magnet pole
is over its center.

I've done it all without computer modelling or even a proper study of
magnetic circuits and materials. More just recognizing and coming to
understand problems with the early ones and trying out promising
solutions whenever I come across them, even to the point of pretty
radical construction changes. I have no doubt that simulations could
help find the optimum magnet spacing and flux gap for smoothest
torque ripple or other desired characteristics. I will however say
that the motors run great and take a fraction of the power of my
earlier ones to spin them.

(A mild steel plate rotor backs 12 supermagnets to complete the
magnetic circuits of the rotor.)

BTW: My previous implementation of "laminates" for the coil cores
were strips of nail gun finishing nails broken to length and spray
painted - rather thick but the metal was good.

BTW 2: I'm working on a 24V, 3KW, 0-3000 RPM version with 6 coils
instead of 9, the "Mini Electric Hubcap" motor.

>A switched reluctance motor having no permanent magnets is desirable, as
>a PM motor always has a strong magnetic field in the gap between the
>stator and rotor, even when not running.

With an axial flux motor, the flux gap is around 1/2 an inch. I put a
PP-epoxy plastic wall between the rotor compartment and the stator so
the coils are entirely out of harm's way in any event.

>The canal silt has a lot of
>ferrous material in it which will quickly get trapped in this field on
>the sort of rim-drive motor I envisage (outer stator, ring rotor, with a
>4-blade prop fitting inside the ring). An S-R motor has no residual
>field, but has the problem that the gap between the salient poles of the
>rotor and stator has to be narrower than with a PM type to ensure a high
>efficiency. Hence my interest in the simulation software.
>
>Anyone else here have any working knowledge of rim-drive ring thrusters?

Not me. Would a prop with a ring around the outside, driven from the
center axle, accomplish about the same thing? I'd rather keep my
motors out of the water.

Craig
http://www.TurquoiseEnergy.com/
http://www.saers.com/recorder/craig/TurquoiseEnergyNews/

=====

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