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.
I must defend the iron powder cores... Gaps reduce permeability and hence magnetic coupling - that's different than motor efficiency. Some of the most efficient motors have air core coils with no iron at all. But they don't have much torque.
My motors may have a little less torque than they did with the laminate cores. (Much depends on the flux gap setting, which is huge (1/2") and easier to change on an axial flux motor than to keep exactly constant. My "Electric Hubcap" motors' very approximate figure is 1.5 foot-pounds per 10 amps, at 36 volts, so 10 foot-pounds at around 70 amps.) But the no-load currents went down at any RPM when I switched to iron powder toroids. Relatively, it's fairly close to having an iron laminate core, but iron losses are low, and it's far from having an air core.
The painted-on coating of ilmenite in sodium silicate on the coils also dropped the motors' idle currents another 25-35%. This is where I get my figure of around 95% efficiency instead of the typical 90-92% for BLDC.
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,
Are you sure? To magnetize supermagnets (AKA NIB magnets, Neo magnets, cobalt samarium magnets, rare earth magnets...) I hear a pulse of a million amps for a microsecond through a coil is used. Don't get your fingers between two supermagnets, even tiny ones. Depending on size and strength, they go from making blood blisters to actually crushing entire fingers! And typical motor size ones like 1" x 2" x .5" have great depth of field.
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.
Amen. There's certainly no way I could match the cost of an induction motor. On the other hand, if you have to buy more lithium batteries in order to accommodate a less efficient motor, the savings may evaporate.
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.
Still, with axial flux, you can thow the magnets on the rotor however you like and it works. Things I've done to make 6 magnet pole rotors:
* 12 evenly spaced magnets (1"x2"x.5"): NNSSNNSSNNSS (my usual arrangement)
* 6 large magnets (2" x 2" x .5"): N S N S N S
* 12 unevenly spaced magnets (1"x2"x.5"): N_NS_SN_NS_SN_NS_S
(The last arrangement was because I had planned to put on 18 magnets on an early motor, NNNSSSNNNSSSNNNSSS, but at the last minute I decided that would be overkill.)
Doubtless one could optimize for smallest torque ripple and other desirable properties. Maybe I have more magnetism than is really useful, or some uneven spacing would be an improvement over the even spacing, or maybe 18 thin magnets would be better - I don't know. That's where the software could indeed come in handy. But the basic arrangement of 2 rotor magnet poles per 3 coils is a given for 3-phase BLDC, and the motors ran great with any of the above arrangements.
Now I'll mention that my first new 24V, 3KW, 0-3000 RPM, 9" O.D. x 4" long, 15 pounds, "Mini Electric Hubcap" motor that I started last month is almost complete... I just have to put 8 supermagnets on the rotor.
This motor is a good fit under the hood of my Honda 7.5 outboard leg, where my larger motor wouldn't quite go in. More later.
Cheers,
Craig
Victoria BC
http://www.saers.com/recorder/craig/TurquoiseEnergyNews/
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