Electric power for dummys......

Discussion in 'Electric Propulsion' started by MadProfessor8138, Jul 7, 2019.

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  1. Jul 10, 2019 #21

    MadProfessor8138

    MadProfessor8138

    MadProfessor8138

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    Two cannibals are eating a clown when one looks at the other and says "does this taste funny ??? " ...............
    Sorry....couldn't help myself.

    Kevin
     
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  2. Jul 10, 2019 #22

    proppastie

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    I thought one of the great things about electric was the fantastic low end torque.
     
  3. Jul 10, 2019 #23

    RonL

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    Maybe this will be of some help to you or others, it is just a minimal amount of information.
    The motor pictured is a 10kw @10,000 RPM rated and measures about 4.5" X 7" and weighs 12 pounds.
    Bob Boucher (the owner) published a book a few years back titled " The Electric Motor Handbook" I think it is a very good book :)

    https://www.astroflight.com/explanation-of-motor-terminology.html

    https://www.astroflight.com/4535-astro-brushless-motor-details.html
     
    Last edited: Jul 10, 2019
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  4. Jul 10, 2019 #24

    emir_82

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    That is true, until the flight of my prototypes you only have my word.
    Also I will publish bench test and coherent values of power to weight ratios.
    But for now I have only that.
     
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  5. Jul 10, 2019 #25

    RonL

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    I only intended to draw out an answer along the lines of, what grade and quality of materials would go into a motor.
    There are so many variations from "it will work" to "I can't afford that" that many people don't have a clue why some motors are able to perform at many different values when they appear to be the same in physical dimensions. :)
     
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  6. Jul 11, 2019 #26

    emir_82

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    Of course that at the same apparent size the motors will perform kind of similar. Core material and magnet quality can make a difference. But generally they go cheap in copper quantity.
    The problem also is that they put numbers that the best quality motor doesn't have at the same weight and shape.
     
  7. Jul 11, 2019 #27

    pictsidhe

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    Electric motors can briefly produce much higher than rated torque. Torque is directly proportional to current in the windings. Resistive heat loss in the windings is proportional to the square of the current. Many motors are fan cooled, so the cooling increases with speed. You end up with motors that have an increasing continuous torque capacity with speed. What motors can do briefly until things overheat and what they can do continuously are very different. For less than a second, you can run most motors at 10 times continuous power. Hold them there, they will soon let the magic smoke out.
    How you control the peak current that a motor takes will influence its power curve. BE (before electronics), motors were often just switched directly to the supply. This leads to very high current and torque at first, decreasing as the motor speeds up. That's OK, they can take brief overloads. Bog an electric motor and it's torque will increase, but then the magic smoke escapes. We now have electronic control that can limit current. That keeps motors safe across the whole rev range, but means we lose that low rev torque boost.
     
  8. Jul 11, 2019 #28

    Steve C

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    I've never seen a Chinese motor that couldn't be improved by rewinding. Sometimes you can increase copper fill by as much as 30% and that will always make the motor more efficient.

    If you can find a high pole count, large diameter motor, that would be my choice over any reduction drive. The more poles a motor has, the more torque it will generate because the magnets are closer to the coil at the time it gets switched on. Magnetic strength is inversely proportional to the distance squared, so it makes a difference!

    High rpm motors with reduction drives can have high efficiency, but it's more stuff to go wrong and there's maintenance.

    You really need a dyno chart to see where a motor is happy to run. Different motor designs have max efficiency at different points. If you have a low pole count motor wound for low rpm (small wire - lots of turns), it will not like a lot of amps. On the other hand a high pole count motor wound for max rpm (large wire - few turns) will be more efficient drawing very high amps.
     
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  9. Jul 11, 2019 #29

    Hephaestus

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    If you guys are interested in the windings part. The old RC guys have some great info on rcgroups and others - rewinding/building your own was the only way to get the performance back in the early y2k era. Before the off the shelf Chinese motors were an option.

    Google cdrom motor winding and you'll find a treasure trove.

    Explanations of the how to, differences in the patterns, wire choices (material and guage) calculations and all that good stuff are all there in the archives. It's still done fairly frequently because chinesium quality isnt always adequate, but you start seeing motors as the parts to build not the black box of mystery.

    The only difference becomes size.
     
  10. Jul 11, 2019 #30

    pictsidhe

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    The torque of a motor is the product of ampere-turns in the stator and the field strength, which depends mainly on the strength of the magnets. The stator field will demagnetise the rotor magnets so strong magnets are needed to resist that for high torque. Number of poles does not alter it. Ampere turns is limited by the amount of copper you can stuff in the stator slots. There is a limit to the current density that you can use.
    Motor design is a juggling act. Improve it by enlarging one part, other parts becomes sub-optimal.

    I've just not come across poorly wound Chinese motors yet. Not filling the slots 100% is a heinous crime!
     
  11. Jul 11, 2019 #31

    Hephaestus

    Hephaestus

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    I've got a few kicking around with shorted windings right out of the box... QA/QC is pretty pathetic at times. But they make good cores.

    Put your ohmmeter across the windings sometime, even "good" motors can have >15% variation between windings.
     
  12. Jul 11, 2019 #32

    RonL

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    Before anyone gets turned off because it seems the discussion is about model airplane motors, consider that 3 or 4 of the astro motors I linked to above, could be connected in series to a jackshaft (timing belts) and have an output of 45 to 60 horsepower.
    I think this power range would be a good start point.
     
  13. Jul 11, 2019 #33

    Steve C

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    Ok, the experts say the pole count effect on torque is negligible when conparing motors with the same kv. I guess where I was going is that you can use a higher pole count motor in place of a gear reduction and I'm not considering the same kv.

    In practice, high pole motors turn bigger props.
     
  14. Jul 11, 2019 #34

    pictsidhe

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    Bigger motors need more torque. Bigger motors turn bigger props. Increasing the poles won't help a small motor turn a larger prop, as the effect on torque is 'neglible'. If you keep the current density the same, changing the kV or poles won't affect torque.
     
  15. Jul 11, 2019 #35

    Hephaestus

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    Not quite correct lower kv tend to have higher torque lower rpm.

    There was one of the windings configurations that you could effectively rewire in operation. Wye and delta(?) Memory fails don't want to look up.

    One wind was great for high torque high amp draw, ones more of a low torque high rpm wind.

    Pair of contactors is all that's needed to switch the wiring configuration.

    I remember it because it was a stupid expensive vfd upgrade solution to a piece of equipment we were continually stuck repairing. But the repair made so much sense, new motor, add a super spendy VFD, pair of contactors, bit of wire, then - never had to do it again.
     
  16. Jul 12, 2019 at 3:43 PM #36

    Steve C

    Steve C

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    Go get a 14 pole model motor and a similar size 2 pole. You will fry that 2 pole trying to make it turn the prop the 14 pole will turn. It won't do it. If you wound that 2 pole with enough turns to get the kv down where it's in the ballpark, it would have such tiny wire that it couldn't handle enough amps to be useful.

    I've been playing with this stuff for a long time. I'm not an electric motor expert (I know who they are though) but I do know about matching the type to the job. Matching the right number of poles and the winding scheme to your operating range results in max efficiency.

    Btw, I've taken Chinese motors apart that didn't even have the same number of turns per phase. If they start running out of room to quickly wind a tooth, it seems to be perfectly acceptable to then to leave a turn off one. You don't have to have the same number per tooth, but per phase yes please!
     
  17. Jul 12, 2019 at 4:02 PM #37

    delta

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  18. Jul 12, 2019 at 5:26 PM #38

    J.L. Frusha

    J.L. Frusha

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    Look up the LRK-TorqueMax on both RC Groups and Yahoo Groups. There are plans for a variety of outrunners.

    If I can, I'd like to build one that uses bobbins for the coils, so I can try a particular type of winding.
     
  19. Jul 12, 2019 at 7:03 PM #39

    pictsidhe

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    I have rebuilt motors, rewound motors and also built a few from scratch. I use physics to design them.
    My best motor book is by Philip Kemp, i forget the title, but can tell you when I get home. Anyone wanting to rewind a motor should understand the physics. Books ae highly recommended.
     
  20. Jul 12, 2019 at 7:23 PM #40

    pictsidhe

    pictsidhe

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    F=BIL. F is force, B is field strength. I is current, L is length. You can trade I and L against each other. One turn and 100 amps has the same current density and resistive heating as 100 turns ans 1 amp. It also produces the same force. Changing the number of poles does not alter the flux density, which is generally limited by the flux density that the laminations can take without excessive losses.
     

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