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Electrical and Magnetic Circuits of a Motor

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Patrick

Sat Jan 25 2014, 06:51PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

(In the following discussion don't confuse the k sub v notation with kV -- kilovolts)

First, im searching for a efficient motor, duct, prop and battery combination. To that end ive got the best, most energy dense battery I can afford, so now its on to the motor.

Background:
the motors im experimenting with are called the BC2836-7, which means 28mm x 36mm x 7 turns. there are other versions, the -11, -9, and -6. all in the same physical package. however the difference in turns varies two important characteristics. The K sub V and the contiguous wattage for flight.

as the turn count goes up, the Kv goes down, and power goes down.
So I presume magnetizing current and DC resistance are adding heat as the turn count becomes increasingly high. Eventually approaching burn up. But as the Turns decrease more and more current is needed for each rpm x torque unit, making it less efficient than a lower turn counterpart.

So given the motors as purchased and used in the bicopter and tricopter applications were the BC2836-7 (1120 kv) iteration.

Known Data:
BC2836-6, 368 W, 1500 kv. A/T = 180 @ 12V
BC2836-7, 336 W, 1120 kv. A/T = 196 @ 12V
BC2836-9, 243 W, 880 kv. A/T = 182 @ 12V
BC2836-11, 206W, 750 kv. A/T = 187 @ 12V

Notice there is no dash 8 or 10 version...
There are no verisons below the -6, or above the -11, which for aerodynamic reasons would become useless do to insufficient or excessive RPM.

Question and possible goal:
if I interpolate between the the -7 and the -9 (where a mythical -8 would live) can I get a sloppy approximation of the -8 capabilities?

if so, then :
1120kv / 7t = 160kv per T, for the -7.
880kv / 9t = 98 per T, for the -9.
And (160-98) / 2 = 31+98 = 129kv per T for 8 turns. (129kv x 8T = 1032 kv)

I suscpect this ugly approximation underestimates the DC resistacne and over estimates the magnetic limitaions, since the -6 works just fine at 180 A/T.

Better still, if I reverse the above math to find Watts, then:
(Assume 190 A/T on the steel), (190 A/T) / 8T = 23.75 A x 12 V = 285W, right in the middle of the -7 and -9 !

IF im right then:
BC2836-6, 368 W, 1500 kv. A/T = 180 @ 12V
BC2836-7, 336 W, 1120 kv. A/T = 196 @ 12V
BC2836-8, 285 W, 1032 kv, A/T 190 @ 12V
BC2836-9, 243 W, 880 kv. A/T = 182 @ 12V
BC2836-11, 206W, 750 kv. A/T = 187 @ 12V

Caveat:
However, Just because its drawing 285 watts, doesn't neccsarily mean its more efficient or putting that power to the air.

Ash Small

Sat Jan 25 2014, 09:52PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

If I understand this correctly, can you not rewind one with 8 turns and measure it?

BigBad

Sun Jan 26 2014, 12:27AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

I've been studying motor efficiency.

Protip: with a few caveats, it's not the number of turns, it's the weight of copper: Copper resistance losses are inversely proportional to weight of copper.

The turns set the voltage/current trade-off for the motor. For a given weight of copper, lots and lots of thin turns = high voltage, low current, whereas small number of thick turns = low voltage, high current.

There are a few caveats, if the turns are too thick, then you hit skin effect. Also, eventually (quickly) you run out of space for copper.

Dr. Slack

Sun Jan 26 2014, 07:48AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Take the physics approach and try to find some invariant ratios, and then you can stop worrying about things that don't matter.

Motors have got steadily more efficient in the last century or two as magnets have delivered higher fields. Copper has not changed at all (though some bigass generators have got superconducting windings, but that's not possible for small motors, silver windings anyone?)

The thing that generates the torque in a motor is the total pole current, that is number of turns * current. The thing that generates the back voltage is the change in total flux linkage per second, that is number of turns * magnetic field * speed. You can do similar sums for losses. The bottom line is that *other things being equal* (like packing fraction, power losses in external circuit, and if you keep the speed and motor geometry constant than things like power losses in air resistance/turbulence, power losses in eddy currents, friction will be equal), the number of turns on a motor makes NO DIFFERENCE to the power rating or the efficiency.

So, why do motors have different numbers of turns?

a) To be matched to different power supply voltages
b) To experiment with their own manufacturing process to see whether a different wire gauge makes for a better packing fraction, or an easier to wind motor
c) And dare I say it, to get experimenters to buy several different types and compare them, generating free publicity

Now motors *are* different, in power/weight, and in efficiency, and I'm not saying otherwise. These differences may even correlate with the numbers of turns they use. But the differences aren't caused by the numbers of turns.

The difficult thing is to separate out what is necessarily making a difference ("ya canna changes the laws of physics Cap'n") and what just happens to (for instance the size of the lam-stack means that 10 turns uses all the space, 9 turns can only go up one wire size, but 8 turns can go up three wire sizes, so 9 turns will be less efficient than either 8 or 10 in this case, things like that).

So the best motor will be one from a manufactruer who
a) uses the strongest magnet and bought it for a good price and
b) is clever about using all the remaining space for copper and
c) doesn't make any idiot mistakes in making the rest of the motor

There are so many more variables to confuse things. Some users will want good power/weight, some good power/space, some good power to losses. Run copper hot, it's more lossy, but you can get better power/weight. More variables to try to ignore when making valid comparisons.

I guess in your application, you care more about weight and efficiency than size

Shrad

Sun Jan 26 2014, 09:30AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

in the laser industry, there have been scanners which steer the beam in an Y/X fashion at rapid speeds

the scheme has been the same for years, two windings and a magnet on an axle... the magnetics have always been implemented the same way, with normal laminations and normal windings driven the normal way, and this is the amplifier which determined the maximum speed along with the quality range of the mechanics

now, with the same mechanics, if you use thin wire and carefully made laminations, if you wind it really neatly and compactly, you end up with a denser field along the active area and thus way higher efficiency

now pour thermal epoxy and vacuum-pot the coils, and you have better heat transfer and ruggedness

they did this at Pangolin with their new scanner set, and while the standard scanners perform at say 40k points per second at 12 degrees of spread and decrease in performance if you widen the angle, theirs are outperforming anything existing at their widest angle by keeping the 40kpps speed without overheating or screaming like others (even cambridge) would do

maybe you would have a chance at applying the same idea to your motor, and just rewind one by hand with thinner, high quality wire in the most compact way you can, then thermally pot it so you squeeze the maximum of heat outside the motor laminations and coils

I don't know how the overall magnetic field of such a motor behaves, but I'm certain the torque depends greatly on field density, and I guess this is the same principle... a thinner wire will occupy more space, more evenly, and give a denser field than big diameter copper wound with greater inter-turn space and greater bend raduis

Patrick

Sun Jan 26 2014, 06:39PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

these are two of the same motors supposedly, but the wire was different in 3 of 4 examples I bought. note the scorching at 21 Amps for 3 seconds.

I already scorched the above motor seen with the dark windings, so I might as well try it with 8T and compare in a test stand. ill also need to know wheter to use delta or wye connections. I have better magnet wire with a higher heat rating. let me count thier stranding...

Ash Small wrote ...

If I understand this correctly, can you not rewind one with 8 turns and measure it?

yes, but I needed to know if it'd be better to aim for another turn other than 8, the math proves 8 will be best, if any is better than the other.

Dr. Slack wrote ...

So the best motor will be one from a manufactruer who
a) uses the strongest magnet and bought it for a good price and
b) is clever about using all the remaining space for copper and
c) doesn't make any idiot mistakes in making the rest of the motor

I guess in your application, you care more about weight and efficiency than size

first, you are being very charitable to those Chinese crap motor makers. I may end up having to undo some of there mistakes.
Third, Dr. Slack, it seems when the windings become to few, and the A-t become to high, the heat factor builds faster than the RPM factor or power factor.

Second, its efficiency at a certain RPM that I need, for benefit of the propeller diameter x pitch. Im thinking I need more torque at a lower RPM, but I need formal categorized testing to be sure, which will be attempted soon.

EDIT: 28 strands of wire means 4 strands of 0.011" diameter wire. (scorched example) the undamaged better made motor appears to be 0.014 or 0.015" diameter wire. but I had to use calipers so not sure if that's exact. However, I cant be sure if its 2 or 3 strands :( I cant count the turns for sure without destroying it, ad I keep estimating 18 strands, which isn't a 2 or 3 multiple of 7 ! I wondering if theres 21 or 14 loops.

Ash Small

Sun Jan 26 2014, 08:31PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

ok, I'm not sure how practical this is, but if you are going to re-wind one, how about trying copper foil insulated with Kapton tape, or similar? this should mean a greater mass of copper if done correctly. (I'm sure you could even enamel it if you put your mind to it, but I imagine kapton should do the trick for evaluation purposes.)

The tricky bit would be joining wires to the ends, but maybe this could be accomplished around halfway along the length of the armature. It looks like you have 'some' space to play with. If you use multi-stranded wire 'flattened out', it wouldn't take up a lot of space.

Patrick

Sun Jan 26 2014, 10:04PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

that's tempting as i have copper foil for murdering snails and slugs. but im going to use 4 strands (I hope) of larger 0.014" diameter wire. with higher temp insulation.

but, I was pondering adding heat conducting epoxy, as mentioned above. to prevent hot spots from short duration surges.

BigBad

Sun Jan 26 2014, 11:09PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Can you vary the pack/supply voltage?

So far as I know, the motors should give almost exactly the same performance, just at different voltage points.

Patrick

Mon Jan 27 2014, 06:48AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im trying to keep the cell count low. First, the more cells, the more likely one cell seems to go bad. Second, the larger the capacity the less interconnect mass and more energy containing mass.

when i turn over to fuel cells, ill need to prefer the lower voltage, higher current mode of operation.

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