Stators demystified

Discussion in 'Thumpers' started by Luke, Dec 17, 2006.

  1. Luke

    Luke GPoET&P

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    The KTM is 3 phase, like the KLR. The XR that we've been trying to troubleshoot is 2 phase. For the purposes of this analysis, they are the same. The difference is that when you go to rewind the stator, you have three windings to change instead of one. The changes are all relative, so you rewind with X% more or fewer windings than whatever was on there in the first place.


    Regarding a change to a Delta connection on the stator.. I suspect the results may be similar to this Look for the big post by Earthscape. He used a different stator, but got the sort of output I'd expect from reconnecting the stock stator. The end result looked like one fried aftermarket regulator, one flaky stock regulator, and probably one fried CDI. Very bad news.

    ....edit.... as of today, it seems that the CDI didn't fry after all!
    #41
  2. Luke

    Luke GPoET&P

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    Flummo;
    It looks like you have everything right- the only thing I'd question is the OCV at full rpm. It seems very low.

    The spreadsheet has a table for predicting output power based on the parameters... if you don't have excel you can do it yourself.

    Stator parameters:
    OCVA = open circuit voltage at a particular RPM
    RPMA = rpm that the above open circuit voltage was measured
    SCCA = short circuit current

    Operating conditions:
    LV = load voltage
    RPM = engine speed

    output current = (OCVA*RPM/RPMA - LV)/((OCVA*RPM/RPMA)/SCCA)
    which is the same as: SCCA*(1-LV/(OCVA*RPM/RPMA))

    Alternately, see if this works. It's a google spreadsheet. I think it's viewable by anyone. The graphs didn't copy over, unfortunately


    One ohm (cold) sounds pretty high for winding resistance, I can include that in the model, but it does make the math more complicated. I figured that if it affected the output less than 10% that I should not worry about it, but it sounds like it is important after all.

    Even with an inaccurate meter, go ahead and measure the winding resistance anyway and subtract the resistance the meter measures when you touch both probes together directly. With a digital meter this should give a rough sanity check for your calculations.

    Changing the winding resistance by going with a thicker wire should help- with the resistances you have, it should more than make up for the extra losses in the rectifier. If you go with fewer windings, don't forget that you also reduce the length of the wire, which helps even more.

    You'll probably want a battery to provide the start-up current that the HID lights require.
    #42
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  3. Flummo

    Flummo n00b

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    Thanks for the reply. Too bad my computer crashed after I wrote you a long reply...:cry Oh, well. Here we go again.

    I was also suprised by the low OCV. I will make all the measurments seriously as soon as I get a chance.

    The google spreadsheet worked fine, will come in handy once I get all the numbers.

    I have measured the cold resistance with a couple of meters, and it seems to be 0,6-0,7 ohm. Less than I expected, the wire must be shorter than I estimated, but at 1 ohm when hot it still isnt good.

    I will have a battery (or even two) for the HID, it wouldn't work without it. I think I'll use a cheap regulator/rectifier for snowmobiles, that'll probably be the simplest solution.


    Do you know how a stator reacts to dual windings? Instead of a single 400 turn winding (for example) you put two 200-400 turn windings on top of eachother. On low rpm they are connected in series (with relays & simple electronics) to give a single winding with many turns, charging early but only low current. On higher rpm they are connected in parallel, giving low number of turns and low resistance = high current. In theory I think it sounds good, but then again, so did Titanic...

    A spinoff on this: How does a stator react when one part of the winding is disconnected? Example: the generator has 400 turns in total, but with outlets at 300 and 400 turns. On low rpm all is used, but on higher rpm the power is instead drawn from the "300 outlet". How does the 100 unused turns affect the output of the windings that actually are being used?


    Now, if you excuse me, I'll go get the axe and take a chat with my computer about that crash...:bash
    #43
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  4. Luke

    Luke GPoET&P

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    1 ohm is a lot, it might not be that high. I came up with the +50% by taking the tempco of copper (.4%/degreeC) and guessing that an aircooled motor would run up to 140C

    The only direct experience that I have was running my XR while measureing the stator output. It made no difference (that I could tell) to the ignition output what I was doing to the lighting output. (short, open, in between)

    Everything after this based on one guess:
    My guess is that the different poles on the core are mostly independant from one another, but that windings on the same pole would be coupled together like a transformer with multiple secondary windings. If you think about it, the stator core is the same as a transformer core except that the flux is induced by the flywheel magnets, instead of a voltage applied to another set of windings. Since the poles are independant, you want to use all the poles, all the time to get the most power out.

    So on your 5-pole stator you could put on two windings, 40 turns per pole each. Connect them in series at low rpm to get the equivilent of a 400 turn winding. At higher rpm you could use only one of the 200 turn windings and have a higher current output. Putting the second winding in parallel shouldn't change the resistance in the stator, but it will halve the wiring resistance.
    The windings don't have to have the same number of turns, but it makes thinking about it easier.
    If the windings are dependent (coupled) then the voltage on one winding will always be a fixed multiple of the voltage on the other windings. Eg, if you are powering a 15V load with a 300 turn winding, the other 100 turn winding will have 5 volts across it even if it is unloaded.

    I looked at trying this and in my case the potential gain wasn't worth the added complexity. One thing to know is that if you disconnect a coil with a relay it will arc the relay unless you have some sort of snubber built in.


    As for your computer, show it this:
    [​IMG]
    #44
  5. PackMule

    PackMule love what you do

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    Thanks for the detailed write-up. I'm going to have to revisit it a few times to get things squared in my head, methinks. :deal
    #45
  6. hoggjaww

    hoggjaww DOG ASS

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    Does a Stator or slash R/R hold (excited electrons) until they are needed? I hope so. Have you read the group buy thread in Thumpers, and was moved to Venders? I am hoping that when there is not a load on the stator that there will not be any (killing) heat generated to the the R/R.

    Harry
    #46
  7. Luke

    Luke GPoET&P

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    Sorry to say, no. If your new stator puts out more power then the old one, you are going to have to deal with extra heat in the R/R. The good news is that if you don't use all the power the stator wants to generate the difference does not all end up in your regulator. The regulator actually short-circuits the stator coil for a brief period of time. Most of the heat generated by the short-circuit is dissipated in the stator, but it's cooled by the engine oil and can presumably handle it. There is extra power dissipated in the regulator, but better designed regulator will dissipate less power than others.

    You want to know if your regulator will survive with a powerful stator and not much load? Obviously I can't say- but a low load won't help. If it is designed for this sort of stator it shouldn't be a problem.
    #47
  8. Flummo

    Flummo n00b

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    Okay... I havent measured anything new yet, dont have any suitable resistors, but I have been doing some counting with the numbers I had. Right now I think 10% less turns will be a good compromise, and 1,0mm wire instead of 0,7 (meaning exactly half the resistance for the same length of wire). Should make a nice improvement and will hopefully be more than enough to power the HID light.

    Thankyou again, info like yours is priceless when you need it!:clap
    #48
  9. kellyk7

    kellyk7 Who knows

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    I still have not gotten a chance to test this, but one member PMed me with the advice that my stator may be too beefy for my needs and thus the reg/ shuts it down full time. This would also be supported by the heated/ electric burning smell I get when running the DC reg and not just AC. His thoughts were that the stator I rewond may be trying to dump over 250 watts into a system wanting 50 watts and the reg is rated for 150 watts and because of the extra 50 watts it is shutting down to save itself. Makes sense to me, then couple that with what you are saying from above, the stator is disipating the heat from the extra 200 watts and may be burning the epoxy.

    I guess now I am going to pop the side cover off and get a look at the stator to see if it may show signs of overheat.

    This really has been a good thread, thanks
    #49
  10. Luke

    Luke GPoET&P

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    Let the magic smoke out and all bets are off....

    If you are feeling daring, fire the bike up as-is and check to see if anything feels like it's overheating. Start with the regulator, of course, but also check the wires going between it and the stator.

    I don't know what to look for to see if the stator has overheated (besides the obvious)


    Good luck, and let us know what you find out.
    #50
  11. kellyk7

    kellyk7 Who knows

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    I hope to get time this weekend to put it back on DC, right now it is on the stock AC reg only and works fine, and the Reg does not overheat nor do I get that heated smell. so far the smoke has stayed where it should, and the only smoke I have let out was the plastic headlight lens. who would have figured a 55/65 watt cool blue halogen bulb would get so hot,
    #51
  12. arrcrussell

    arrcrussell Gimme Dirt

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    Luke, great article – thanks for the education.
    I’ve an XR650R and have unwound the stock 4 poles and gone with a 10 pole set up and a pure AC system. A friend recently purchased an XR650R and wants to do the same – however, I’m second guessing my approach and considering suggesting simply leaving the stock 4 poles alone and winding the 6 empty poles to give 2 circuits but combine the circuits externally to get a single AC circuit. Query – will the output of the combined circuits (4 poles and 6 poles) be the same as a rewound single circuit (10 poles). The reason I ‘m considering the 2 circuit approach is that if a failure occurs, one can simply disconnect the failed circuit and limp home with the remaining circuit – at reduced power, I know, but better reduced than none at all. I’d appreciate hearing your thoughts on this approach. Thanks.
    #52
  13. Luke

    Luke GPoET&P

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    Yes, that should work. I think it would be best if the windings both had the same number of turns. Since the stock windings are 300 turns, put 50 turns on each of the other six poles. Connect the two windings in parallel. The polarity of the windings will matter. If it doesn't work when you first hook it up, reverse the connection to one of the windings.

    You will lose a little power due to the fact that the stock wire is a little bit on the thin side but that shouldn't be a big deal.
    #53
  14. dentedvw

    dentedvw Where did I put that

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    Maybe it was posted, but can someone tell me why my new (used) stator has three wires whereas the old one had only two? It's a DR350, and was previously a dirt only bike, no battery, etc. I am working on adding bits as I can, and I just got a used stator from an S model, that had battery, etc.
    I wasn't really planning on using a battery, but maybe I can squeeze one on somewhere, but it's not looking good. Maybe a cap would work though.

    Well, i haven't been able to turn up a stock regulator/rectifier mess, but I did buy a rectifier at Radio Shack. They were all out of regulators though.
    Would I be better off getting one from another bike at the boneyard?

    A while ago I took off the stator that's on the bike and was sorely dissapointed to find only a few posts were wired at all. I guess that's what you get with a dirt bike though. That's why I got the other style stator, hoping it would help with some added load. I want to put some different and additional lights on, such as turn signals, and a more suitable headlight.
    #54
  15. mouthfulloflake

    mouthfulloflake Not afraid

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    Dented VW,

    that sounds like a 3 phase stator versus the typical dirt DR single phase.
    You can use it, just use a regulator/rectifier from a street bike, such as this. ( notice the 3 yellow wires)

    http://forum.svrider.com/showthread.php?t=91050&highlight=rectifier

    sho12

    Could you post a pic of the stator? and or count the terminals that are wound?

    IM wondering if I can wind my dirt 350 stator, to be 3 phase, since it will be assemytrical due to the CDI charge winding in there.

    street bikes normally have 3 phase stators, right?

    dirt bikes, do they normally?

    Luke, any input on this?
    #55
  16. dentedvw

    dentedvw Where did I put that

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    Fantastic, thank you very much!

    I have the dirt model off the bike, and the SE model on there already, since last fall. I doubt the picture of the dirt would help you, as that is what you have already.

    I had quite a bit of trouble with electrics in November, the last time I got to ride the bike at all, and haven't touched it since. My plans to fix it up over the winter have fallen apart, as I purchased a more suitable bike. I may still finish it up though, it's a fun bike.
    #56
  17. Luke

    Luke GPoET&P

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    I don't think there's any rule to whether a type of bike has a 2 or 3 phase stator. I've never heard of a three phase stator on a bike without e-start, but that's as close to a rule as I could get. Otherwise, pretty much anything goes.

    Three wires doesn't necessarily mean three phases. It could just be two two-phase windings, like on a KTM RFS stator.

    If you want to convert from two to three phase, you'll need a different stator core or flywheel. The number of phases is determined by the position of the magnets relative to the poles on the core.
    #57
  18. YamaGeek

    YamaGeek Skeletor sparklemuffin.

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    I have an old C102 honda that was originally 6 volt. It's a electric start for a 50cc, which seems like overkill, but it works.

    I've also got a bunch of unused 7 cell 1.25 nominal volt per cell Nimh battery flight packs from my electric model airplane days that are unused. I've converted the bike's electrical system over to this roughly 8~9 voltage to not have to buy a new 6 volt 11 amp hour battery. The batteries I'm using are 2 amp hour cells and I'm using 2 in parallel.

    Now this battery package spins the motor over with a lot of authority, and with new 12 volt bulbs it runs the bulbs at a fair brightness. I'm sort of concerned about the lack of regulation in this system and that there's no regulator that works on a 9 volt system. So I'm balancing the load of consumers to the battery to keep the voltage so I don't bake the NiMH cells.

    Since the system is going to be run as a street bike with the lights on all the time, I'm wondering if I might have some over head that can allow for additional small voltage consumers like some small cell phone or camera rechargers.

    Unfortunately I've not done a current voltage load test of this system, so I'm going on the nominal use of the original 6 volt system which was a 25 watt headlight, with a standard 6 volt 1155 tail lite. No turn signals in the system yet.

    Not to get long winded, and I hope your eyes haven't glazed over yet, but am I getting started at this and trying keep it simple enough to not have to do a complete rewind of the stator.

    Thanks for your stator tutoring, BTW
    #58
  19. XRider

    XRider Almost Lifelike

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    The stator is part of the alternator. An alternator is simply an AC generator and the stator is the static or stationary windings.
    #59
  20. rivercreep

    rivercreep Banned

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    But I'm still too dense for all this to sink in! :huh

    Sure did appreciate all the hard work though!
    YOU ROCK!:freaky
    #60