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Help with making a motor.

  1. Oct 4, 2004 #1
    was wondering if you could reverse the engineering of a standard motor to the effect that the shaft would stay stationary and the outer portion would spin

    to do this i thought that it would be better to have the electromagnet on the outside and the permanent magnet on the inner portion.

    now i realize that i would be losing torque by doing this (small wheel moving a large wheel) but that i would gain speed by the same principle ...

    so i am pretty sure that all that can be done ... i guess my real question is .. would it be practical ...

    for my intended application/fantasy the permanent mag would be a total of a 13 inch diameter circle and the copper wire would be in a 15 inch diameter circle. a total of 3 inches wide...

    the outside of the spinning surface would be 27" making a total of 84" movement per revolution. what kind of power supply would be needed to make such a motor be able to push 500 to 650lb's at 1260 RPM
    (approx 100mph)

    if you need more information i will give hat i can or just have some general formulas that i can use to try to figure out what i need to use or do to make it work ...
    thanks for any ones time.

    Edit: found a few online calculators that basically told me (mind you this calc was for a normal shaft spinning motor) that i would need approx 3000 amps@ 48V to produce 100 HP (more then enough) now i was going to have a small gas emgine working a generator to supply the need power ... is that even possible to configuresomething that cuold supply that amount of peak power ???
    Last edited: Oct 4, 2004
  2. jcsd
  3. Oct 5, 2004 #2


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    someone done beat ya to it bud, they're called "Hub Motors"
  4. Oct 5, 2004 #3
    lol funny that you say that ... just found a site with that on there..... doh !!!!
  5. Oct 5, 2004 #4


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    Wrong. Power is power. Power is torque and the RPM its applied at, doesn't matter what part is stationary or moving.

    3000A is a massive amount of current. Using the largest typical wire gauge 0000 you'd lose about 6HP just to heating the wire getting to the motor.

    If your generator is 80% efficient and your electric motor is 80% efficient and you're wire is 94% efficient you'd need 166% of the desired end result at the gas motor. I guess you could borrow a motor from a 1300cc Susuki Hayabusa but why have all this extra junk in between the engine and wheel.

    By the way, I get around 200HP for 3000A at 48V so now you need more than 300HP at the gas motor. Got room for late model LS1 Corvette motor with its additional 500lbs? :smile:

    I'm just playing with your numbers a bit but understand that economies of scale do not always work out.

  6. Oct 6, 2004 #5
    i have been starting to see that from the information that i have been able to pick up ... but there is a company (E-cycle) that has made a hybrid that can hit 80MPH
    but they dont use pure they use the Gas and Electric engine inline with each other and the bike can only go from 0-60 in 6 sec. but they rate theres @ 80HP. weight of the bike is listed at 275lbs. i also found this ....

    Pout (W).............28000 found a page that says 746W = 1 HP (would be a 37.5 HP)
    Voltage (V)..........120
    Speed (RPM)........4000 with a 27" wheel diameter this would equal approx 321MPH
    Torque (Nm)........65
    Efficiency (%)... >85%

    now seeing that i am not nearly as smart as most of you tell me if i got this right ...
    65 Nm = 65 units of force needed to accelerate 1Kg @ 1 meter per sec
    so it can acceletrate 65Kg @ 1m per sec or 143 LBs at the same rate.

    so if i had 400lbs does that mean that it would only be able to get to a lesser RPM seeing that it weighs more then it can push or that it will take longer to do so ?

    found another Calc online that said this would be pulling 233 Amps (assuming much more mangeable)

    But i am not sure if this has enough power to get me where i want to be ???
    still looking for a online calc that would tell me ... but if you know let me know
  7. Oct 6, 2004 #6


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    Ok, those look like motor specs - the bike works as a system and not a 1-dimensional problem.

    You need to factor in the gearing after the motor and the rolling resistance of the bike plus the air resistance of the bike to figure out top speed or acceleration after you factor in the mass.

    Simplified, if you are looking to create a hybrid bicycle that works really really well, you need expensive rare earth magnets, a very nice motor controller, and a compact motor-generator combo.

    If you want to make one on your own, guys have used ~750W (1 HP) wheel chair motors to make electric bicycles with a couple small 12V motorcycle batteries and a golf cart motor controller. Figure maybe $300 for the motor, $100 for batts, $200 for a controller (or much cheaper if you can find salvage). Now look for a generator, tougher but you're farther than many if you get this far and could likely salvage a moped motor and automotive alternator for cheap.

    There is a good reason hybrids aren't 80% of the market - the challenge is to make them practical and affordable (or competitively priced to be more exact).

  8. Oct 6, 2004 #7
    this is a hub motor. so there is no gearing ... and i was more looking to make a street leagal motorcycle then a "bike" ... my goal would be a prototype under $10K (motor for about 2k rolling chasis from junk yard 500-1k and then various parts and testing to fill the rest)

    and the reasoning that i want to go with an electric is that i was watching a show on discovery where a team out in cali had built an all electric car that could do 0-60 in 2.6 sec (Seeing that an electric motor gives all tourqe from 0 as apposed to a combution engine that does so on a curve).... my thought was that a 2 wheel version should be able to do the same in as much or little time. the car had be PACKED wi battieres in every free space that it could be. and you would not have that luxuary on a 2 wheel so that is why i thought of haveing a 125 CC motor providing power.

    from what i can see that motor that i speced above is not enough to do what i want it to ... but if im only 10-20 % away from the power that i need then mabey i can get there.

    i just really need to find out what HP and Torque is need to move 400-500 lbs to 60MPH in 3 sec or less
  9. Oct 6, 2004 #8


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    Thats pretty nice really. I've a DRZ 400S (motorcycle), single cylinder engine, makes about 40hp, weighs about 240 lbs, does 0-60 in around 4.5 seconds.

    I also weigh about 240 lbs, so me and the bike, including full tank, back pack with a 2 liter and spare clothes, weigh in right around 500lbs.

    However, I can change the gearing on the bike, either increasing the front sprocket a tooth (output shaft) or decrease the rear sprocket by a few teeth, and decrease my acceleration, but increase my top speed. I could do the opposite as well, lowering the tooth count on the front or raising it on the rear, and increase my acceleration, but decrease my top speed.

    I'm not sure if my bike could be made to go 0-60 in 3 seconds flat, but I'm sure I could get it down into that range. But problem is it would top out at 62.

    I'd say 40-50 hp could get you the results you desire, so long as things are geared properly.
  10. Oct 6, 2004 #9
    well 2 things there is no gearing.... this would be a hub motor so that the wheel is turnning it self 1 rev of the motor = 1 rev of the wheel.

    and my plan was to top out at just about 100MPH ... i have ridden alot over the years ... and yes i have gone over 100mph ... mabey 4 times .... you just dont do it that often ... but if i got 0-60 in the 2.6 i was looking for that i would be doing all the time !
  11. Oct 6, 2004 #10


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    duh, forgot about the hub motor.

    give me a stupid point!

    uhm, I don't know a whole lot about the hub motors, but I would assume the RPM and circumference would tell you an approxiamate speed.

    ya, I'm not math wiz so someone will probably have to correct me. But say you have an 18" wheel (size of my rear tire) the circumference is 56.52 (3.14*18) or 4.71 ft.

    For simplicity, and cause I hate math, lets call it 5'.

    so if the wheel is spinning at 1000 RPM, 1000*5=5000' per minute. This is just under 1 mile in a minute, so an aproxiamate speed of 55 mph.

    I have no clue about the accuracy of this math, as I mentioned, I hate it. But as best as I can tell this circumference*rpm should give you a rough estimate of your speed.

    So, to do 0-60 in 2.6 seconds, you would need to work out how to accelerate that wheel to 1000 RPM.
  12. Oct 6, 2004 #11
    just to let you know the 18" tire that you are reffering to most likely is the size of the inner circumfrence (ie the size of the rim) up above somewhere i figured the outter dia. of the tire @ 27" giving me an approx of 84" cir. @ the rated 4000 RPM of the motor that would be just about 318MPH (much higher then speed i want to top at) but my question is how long will it take it to get to that RPM pulling a load of 400LBs...

    also 60MPH with 84" cir would be @ 750 rpm
  13. Oct 6, 2004 #12


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    cool, so I'm not as dumb as I thought. I was just estimating on tire size, I didn't actually think about the size of the tire as well, which I believe is about 24 Inch.

    well, I suppose the trick would be figuring out how much current it needs to reach 1000 RPM. Also, you got to make sure you don't feed it to much, or it will break traction and spin. That's pretty much as far a I know really.
  14. Oct 6, 2004 #13


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    I'd assume you're talking about the T-Zero car that has been featured on Discovery channel a few times. Remember also their estimated selling point was like $150,000 for that car and the 0-60 was like 3.6 seconds. It runs the 1/4 mile in 13 seconds which is very nice but a regular Corvette beats it for 1/3 the money and a used Camaro or Mustang could easily be setup to be faster for much cheaper.

    Ok, a little talk about gearing and power. Power is what makes things accelerate quickly. Gearing allows you to maximize the power applied from a motor. Remember that power is torque and RPM for a motor just like power is current and voltage in electricity. Full torque at zero RPM means zero power until it starts rotating. Even then, this is no advantage.

    You need to find the average power applied to a vehicle during the acceleration.

    Look at this performance graph for an electric motor:

    Notice that for an electric motor you have these parameters graphed as speed, power, current, and efficiency. I address each curve seperately.

    Speed is simple, starts a little over 2000RPM and goes to 0RPM in a straight line. Seems backward the way the chart is setup, but you just need to understand to read the RPMs as it is graphed.

    Current is simple as well, goes from near zero at 2000RPM to off the chart before you get to 0RPM. Delivering huge amounts of current is difficult and will be a contraint on your generator. If you setup a generator for huge current, it will not be able to produce much voltage, or if setup for high voltage then current capacity will suffer. Well unless you can oversize but this is engineering and not throwing stuff at a wall. So at zero RPM (called stall) the motor wants massive current and you can't deliver it, so now you have much less torque than you imagined and you're heating up the motor which is lowering the amount of power it can make too.

    Power is the arc at the bottom. Notice that an electric motor has zero power at full RPM and zero RPM, and makes maximum power at about 1/2 max RPM. Many TV shows on the Discovery and Science channel have said that eletric motors have the same power regardless of RPM - a bit of a mistake from the oversimplification to make things easy to understand to lay people.

    Efficiency is the tough one, its arc is different and notice how the efficiency peaks just below full RPM (a point of low current and low power) and really goes down quickly as the RPM falls.

    CONCLUSION: At low RPMs an electric motor needs massive current, makes little power, and has extremely poor efficiency. To balance power and efficiency it should be operated from around 1/2 to 7/8 its maximum RPM.

    Then read this website on building a competition electric car. You need to fully grasp the way each change in variable can affect another.

    The T-Zero team was part of the original team that built the GM eletric car that became the EV1. They are obviously very talented and have no rivals that have even come close in terms of electric car performance. They have done very well in their goals and have obviously inspired you to learn more about it. That's awesome.

    But yet the T-Zero was optimized in every way by engineers to get its performance level and yet any motivated teenager who doesn't even need a licence could build a faster car with hand tools, swap meet parts, and for under $5k. Plus have side windows that go up and down, air conditioning, storage room, a roof over their heads, etc. Even cheaper and potentially faster with zero mods would be a sport motorcycle.

    Hope you understand that I'm saying there's a lot more to the story then I could type here and that reading about what is being examined by some of the engineering challenge programs (like that UK one I linked to) is only the begining. Most of the stuff explored is simplified with linear equations and few of the dynamics sorted. A bunch of calculus and hopefully a computer is next for that complex a model that includes the dynamics....

    And for awesome acceleration, you'd be as well off exploring how to add a hydraulic hybrid assist motor to the driveline of a regular gasoline motor vehicle (this is being explored by Detroit engineers for light truck applications) as you would by adding an electric hybrid assist motor.

    You may also want to note that Ford and others are purchasing software from Toyota for their hybrids - they have plenty of engineers on staff to figure out the software but obviously it must be fairly complex and involved if they decided to lease the software instead of create it.

  15. Oct 6, 2004 #14
    Cliff your right ... and when your right your right ... lol and i just did find the formula that tells me the needed Ft-Lb or torque that you need to get to 60 MPH in 3 sec ... its 406Ft-lb no where close to the items that i need ... but alas i have an extremely boring job and i get to sit for 8 hours a night and then every 40 min or so load paper into the printers ... leaves alot of time for thought ... but i have this thing that when i get an idea in my head that i think would be awesome if you could do it ... cant stop thinking about it till i find out if you can or not, me being a stubborn a$$ (my own mothers description), wont just take "nope" for an answer ... and i must admit ... you have been really good at NOT just saying nope cant do it ... and rather saying well if you do "that" then "this" and so forth ... and i really do have to thank you for the effort that you have put into your responses. but alas ... i am pretty sure with current tech. this one can not be reasonably done ...(friend of mine suggested nuclear powered) but i thought that the parts might be hard to come by for that : )

    Thank you again for all your time !
    Last edited: Oct 6, 2004
  16. Oct 6, 2004 #15


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    No prob. Used to have a job like that.

    I hope you learned at least a little bit from this little adventure. Its cool stuff though, isn't it?

  17. Oct 6, 2004 #16
    that it is,as long as your talking about the motors and not the job, i also have another little scheme to build something else too, an electric beer cooler. think of a golf cart 1' skinnier. but you may say what use would there be for that !?!?! well i live in indianapolis, and every year we go to the indianapolis 500 the night before the race ... and if you havent ever been there, its kinda like marty gras on the street infront of the track. every one walks around with a cooler. so we got the biggest cooler that we could find and attached 4 caster wheels to the bottom of it (we were able to fit 6 cases of beer in there, dont worry theres 10 of us to drink it! ) well allthough that did make our lives easier .... my goal for next year is to get an electric powered one ...will see how this goes ... seeing that my wife has put a very strict budget on this one ! just hopin to find a junked golf cart that i can use parts from. but it should be fun !
  18. Oct 7, 2004 #17


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    maybe you get try to find a pretty large RC car for cheap, and build your cooler onto it. This way, not only will it be electric, but have a wireless remote!

    or perhaps what would work good would be one of those kid's trucks. Lil things about the size of a push mower.
  19. Oct 7, 2004 #18
    Well seeing that the lil RC cars are made to hual about 40-75lbs and that 6 cases of bear and the needed ice to keep it cool run about 250lbs i dont that that that is going to be enough power to pull it off ... golf cart size motor and battery life is ideal for the use so ... i have just been call junk yards to see if any have junked ones ... next stop golf corses for any that they are getting rid of ... that i might be able to get off them cheap ... once i have that depending on how many of the parts are working ... i can do the rest by hand and use a few other materials not totaling more then 100 bucks ! my wife knows a couple that run the budwieser top fuel drag team ... i jokingly asked them if they thought that budwieser would sponser it .... yeah dont think that that was going to happen but hey worth a shot .....

    i figure that there are ppl out there that race lawnmowers mabey i can get a cooler racing team yeah thats the ticket !!! LOL
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