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  • Electric motor modification

    In the electric bike I have built I am using an old starter motor from a 15 hp Briggs lawn tractor engine. It has worked out surprisingly well after various mods to improve the cooling. I drilled holes in one end of the case and made a new end plate plus stub shaft for the brush end of things.

    Yesterday I was fiddling with the motor to change the mounting slightly and had to take off the end plate to machine it. I was thinking about how much more efficient it would be with a couple of NIB super magnet arcs instead of the standard ceramic magnets. Then I noticed several items. There was a lot of extra space between the ends of the ceramic magnets and the brush assembly. There was quite a bit of room on the commutator for the brushes to run closer to the windings, about 1/4" if the commutator was remachined to remove a slight ridge. Also, since I had removed the closed bearing on the commutator end and replaced it with a ball bearing and stub shaft I could easily move the armature 1/4" toward that end.

    Ding. A small light came on. I always keep some NIB magnets on hand for experiments and had some just the right size for what I wanted to try.

    This is what I did. I epoxied 4 pairs of magnets aligned with the poles of the ceramic magnets and at a 90 degree spacing to agree with the 4 pole winding of the motor. Each pair of magnets is oriented with the pair having the same polarity out. Each adjacent pair is opposite polarity to agree with the polarity of the ceramic arcs. I diamond turned the commutator to allow the armature to be shifted as much as possible to that end.



    I have been testing the bike and keeping track of current draw vs speed and hill climbing ability. The current is displayed by a regular digital meter for now and is measured as the voltage drop on a set length of cable. In this case I see about a 20 millivolt drop on 1 foot of cable at 30 amps as calibrated by a lab ampmeter.

    When I reinstalled the armature it was immediately obvious that the new magnets had a significant effect as the armature was pulled strongly to the new position.

    In testing after this mod the motor seems to be producing at least 20 to 30% more torque and heating much less since I don't need to run it as much. That also translates to longer battery life.

    Now I have it in mind to buy a handful of NIB magnets from one of the advertisers here that will be the right size to completely replace the current ceramic magnets. By using some 2" x 1/2" x 1/4 inch NIBs I can approximate the ceramic magnet size and pole configuration pretty closely with about $20 worth of magnets.



    I am curious, has anybody here tried this?
    Free software for calculating bolt circles and similar: Click Here

  • #2
    If you're going to that extent, wouldn't it be a candidate for brushless operation?
    Yeah, complicates the driver, but.......?
    Just got my head together
    now my body's falling apart

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    • #3
      Brushless motors in the kilowatt range are expensive, certainly more than $20.
      Free software for calculating bolt circles and similar: Click Here

      Comment


      • #4
        Originally posted by Evan
        Brushless motors in the kilowatt range are expensive, certainly more than $20.
        Ebay

        Seriously, you can find brushless motors pretty cheap on Ebay, but like Lin says, the controllers are a PITA. If you see a mated pair of controller + brushless motor, it goes for a pretty substantial premium.

        I just paid $250 for the 1.5 KWatt AC Brushless motor that I've installed on my Bridgeport head, but that's an industrial-grade servo with the encoder and cables, and the encoder cables (double shielded with Milspec DIN connectors) are expensive as Hell.

        You can probably pick up a 1 - 2 KW brushless motor (not servo) with the controller for around $100.
        "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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        • #5
          I don't do ebay.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            Err, I meant, given the mods you're already up to, couldn't you homebrew a brushless?
            Just got my head together
            now my body's falling apart

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            • #7
              Sounds like a win win Ev, But iv got to admit im with the other guys about finding something that would perfectly suit your needs and be ultra efficient --- im also wondering what effects it might have if you actually moved your NIB magnets in closer to the stator -- like the original motor magnets, it should make even more of a diff. as the fields will be amplified no?

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              • #8
                Just out of curiosity, how far do you get from a single charge?

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                • #9
                  Hmm, thoughts here....
                  If you end to end that motor with a small universal motor and strap both commutators.......
                  remove the brushes.... from both

                  Or have I mis-understood the concept?
                  I know the driver circuitry is not trivial.
                  Just got my head together
                  now my body's falling apart

                  Comment


                  • #10
                    Originally posted by Swarf&Sparks
                    Err, I meant, given the mods you're already up to, couldn't you homebrew a brushless?
                    That reminds me Lin: the model airplane guys use brushless motors a lot because they have, by far, the highest power density, and you can wind the motor so that it has immense torque at low RPM.

                    There was an article in Nuts & Volts about these guys who make home-brew brushless motors, from 20W up to 2 KW:



                    http://www.aerodesign.de/peter/2001/...ilder_eng.html

                    They have a Yahoo group -- neat bunch of guys:

                    http://groups.yahoo.com/group/lrk-to...yguid=92583912
                    Last edited by lazlo; 07-04-2008, 10:31 AM.
                    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                    • #11
                      I have no idea what the endurance is yet. I'm still sorting out the mechanical bugs (pretty much sorted now) and testing hill climbing ability. I decided to modify the drive system by making it a semi automatic 2 speed transmission. I am really pleased with that and it shift gears by the press of a button. I'll be posting a complete update when I have completed some more road trials.

                      Err, I meant, given the mods you're already up to, couldn't you homebrew a brushless?
                      Yep.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        Yup, that's the sorta deal I meant Rob.
                        Knew I'd seen it somewhere.
                        Just got my head together
                        now my body's falling apart

                        Comment


                        • #13
                          You using your "2 hub" idea Evan, or some sorta solenoid derailleur?
                          Just got my head together
                          now my body's falling apart

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                          • #14
                            That looks interesting, thanks Robert.

                            Lin, I'll give complete details in the next update. It really needs pictures to explain how it works.
                            Free software for calculating bolt circles and similar: Click Here

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                            • #15
                              My little RC plane (peter-out) never would have gotten off the ground without its brushless, Now that you guys are talking about them can I ask if I have the right concept on how they work, it has three wires and its DC, does it use one of the wires as a feedback sensor? it has to have a motor controller so does this motor controller control the signal timing instead of how the brushes automatically do? and does it do it with this feedback wire? Thanks.

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