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

Kevin, I'm enjoying your explanations and examples of electrical driver circuitry. Please keep it up, and maybe I'll finally get smart.

I'll do some pay back by helping out with the magnetic circuit side of things. That's what I know best.

On powder iron cores: Yes, they can be purchased, but I don't think that you can argue that simply because they are able to be purchased that means much in terms of the internal flux path or flux gap. Power iron makes a very inefficient core for a motor because it is designed to reduce eddy currents; not to enhance magnetic circuit efficiency. Even tiny gaps add up.

On using permanent magnets, I agree that electro-magnets can be made quite a bit stronger than permanent magnets. After all, we use electro-magnets to make permanent magnets....not the other way round. Designing with permanent magnets are only worth the cost when switching speeds have to be very fast, when the input power is limited, or the efficiency has to be very high. As for motor manufacturers moving away from permanent magnets, my experience is that there are always some motor manufacturers moving towards or away from permanent magnets. It's market driven.

But I don't think we want to misleading folks into thinking that permanent magnet motors are somehow not desirable...? All motors have advantages; the chief disadvantage of permanent magnet motors is that they are more expensive. Their advantage is efficiency.

For just everyday household electric motors where you can throw away amperage I agree that permanent magnets have few advantages. I can see how for your controller circuit R&D that simply winding electromagnetic coils makes things more convenient.

Permanent magnets would complicate things by needing a designed flux path. Just trying to measure flux is a hassle; it's not as easy as measuring electricity. Electricity is trapped inside a wire and can be measured between two points along that wire whereas flux has to be summed in three dimensions. That nasty field summation is why the simple computer-drive magnetic FEA that I keep flogging is so valuable.

It seems that we are using different definitions of sintering. The sintering I referred to was in the metallurgical sense where the laser welds metal to metal instead of hardening the resin. Yes I remember that other style where a light beam hardened the resin - old time process. Today the results of a sintering process look more like a porous casting because more and more metal is directly welded together.

But I still wouldn't go to all the trouble of using such a core. I only mentioned it because I like to keep an eye on what is happening in the R&D world but my preference is to stay with what works easily. The problem with sintering is still the same as powdered iron: The tiny gaps and vector changes are additive. Right now all the commercial iron powder cores I know of are oriented towards reducing eddy currents and lower residual magnetism. Those things are nice at high frequency, but they are only useful to the motor designer if the flux path isn't compromised.

As I said, I don't know controllers. But there is a lot of motor torque and efficiency to be gained by using solid or laminated cores shaped with an FEA program like FEMM. Since that is low hanging fruit. Why not pick it?

Roger L.

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

From: Kevin Pemberton

To: electricboats@yahoogroups.com

Sent: Thursday, January 10, 2013 10:36 PM

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

Hi Roger,
Because powder iron cores can be purchased I would guess that the gap
using powder would not be an issue. The major problem I see is perm mags
are only so good. Electro mags can out preform perms anyday. Rare earth
mags are quite good, but as I understand it some motor manufactures are
moving away from them. You can't use sintered cores because you loose
the insulation you are trying to obtain between the iron particles. In
the 80's they had 3D laser proto-typing equipment, and I could see such
equipment producing a core if the iron content were high enough in the
plastic used. I am assuming the 3D printer being discussed here is much
the same type machine. The machine I am thinking of has a table that
drops in a vat of resin while a laser runs over the resin to harden it.

The flux looses it's power as I remember square of the distance in air.
I don't know how much it looses power in iron but the loss I would think
to be to much if using a weak field to begin with as is the case of a
perm mag. The advantage with the wheel motor is an inch of torque on a
24 inch disk is a foot pound of torque on the shaft. Fluid is forgiving,
one may not get what they want, but may get enough to satisfy.

I can't help but believe that designing a wheel prop with the shorted
windings of a squirl cage motor in the ring and 3phs windings in the
nozzle may answer many needs. What intrigues me most about this topic,
is the fact that people are thinking about designing a unit rather than
looking for what they might put together off the shelf. Just think, a
Kort nozzle built with coils embedded in a plastic iron core composite
where no gap is needed to install coils. Think of those coils not as
coils but wires spaced around the circumference with only a little core
material allowed between them. Remembering that with induction motors
power required to energize the armature is measured in slip. The
armature need not be solid so could be the water mover(the prop). The
down side of this approach is surface piercing props would be out of the
question. The only thing I can find wrong with piercing props is safety
issues.

Now back on track. After the parts are formed a vacuum process could
impregnate the parts and the coil would be protected from any water damage.

Don't forget that the motor control has as much if not more losses that
the motor you choose to improve. Picking your battles is a better
strategy than fighting on all fronts. It may be that developing a smart
controller that takes advantage of all energy saving practices may be a
better first battle than improving an already efficient motor. If that
concept is to much, design a useful surface piercing system. Estimates
for such a prop could throw any concern about efficiency in all other
systems out the window.

Think about it for a minute a 24inch prop in 12 inches of water using
about half the energy of a standard prop. I would say a tunnel hull
would do the trick.

Happy boating
Kevin Pemberton

On 01/07/2013 09:44 PM, Roger L wrote:
> But to get back to the original question, I think we were discussing
> whether a the working gap in a magnetic circuit would benefit more
> from doing more work on the geometry and material of the core.....or
> whether the flux density in the gap (the gap being where the coil is
> inserted) would be improved more if we replaced a portion of the core
> with additional magnets.
> Stated that way, my best first guess was that we are better off
> working on the core itself, as one of the weird things about magnets
> is that they don't conduct additional flux very well. But I hedged
> by saying that this is exactly the kind of nasty and debatable design
> problem that is easily resolved today with computer modeling. No
> technical skill needed, just a couple of hours with a free magnetic
> FEA program like FEMM will give a anyone a real answer to the kind of
> problem that used to require several day's work by someone with a
> serious magnetic education.
> And then we kinda got off onto other things.
>      enjoy,   Roger L.

--
Committing murder in exchange for lifestyle
makes you a "thug" not a "Rights Activist"

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