Automobile starter motor to power bicycle?

Discussion in 'General Electronics Chat' started by DerStrom8, Jul 9, 2011.

  1. DerStrom8

    Thread Starter Well-Known Member

    Feb 20, 2011
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    Good day, everyone! :)

    I am in need of a somewhat large DC motor, as I plan to motorize my bicycle. I am having some trouble, though, finding a motor that would work. I was wondering if you guys think an automobile starter motor would work for this? I know they are intended to run no more than 15 seconds at a time, but I don't imagine running it for 2-3 minutes at a time would cause too much damage. Then again, I could be wrong--the motor could heat up and burn out after 30 seconds or less. For this reason, I wanted to ask for some opinions. Do you guys think that a starter motor can put up with this kind of strain? If not, can you suggest any other motors that would work? This is a low-budget project, so if I can find motors in used equipment, it would be preferable.

    Thank you very much for the help!
    Regards,
    Der Strom
     
  2. #12

    Expert

    Nov 30, 2010
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    Once upon a time, I saw a guy crank a car engine at about 50% duty cycle for 25 minutes by using a HUGE battery charger on the battery. The starter was smoking when I told him to stop, but it did not fail. If your bicycle is easier to turn than an 8 cylinder car engine, you can probably make this work.

    The part that I do not know is whether the starter motor will draw 300 amps to push a marshmallow or is the current load dependent?
     
  3. DerStrom8

    Thread Starter Well-Known Member

    Feb 20, 2011
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    Hi, #12. Thanks for the prompt reply! :)

    This is exactly the type of question that I was hoping to have answered. I would assume the current draw is dependent of the load, but I am not very experienced when it comes to automotive parts.

    Der Strom
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    I know a neighbor who built a gocart with a ford starter(circa 1975) and never had any over heating problems. It was a small vehicle for his kids to use, so the load was less than 100 pounds. Having to lug around a large lead acid battery will do more to increase your load than just your weight alone
     
  5. #12

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    Ummm...I was assuming you knew a bicycle is easier to turn than a car engine.

    I know that a difficult start will use over 400 amps from a battery that usually starts the car with 300 amps, so there is some load dependence, but a bicycle is so efficient that it should run on only a few hundred watts. 8, 16, 24 amps. I don't know if a car starter can go that low. Better pick something cheap and small that was designed to start a 4 cylinder engine.
     
  6. DerStrom8

    Thread Starter Well-Known Member

    Feb 20, 2011
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    I can see how a lead-acid battery could cause a problem here, but I figured that proper placement would distribute the weight evenly and reduce strain on the motor. Then again, I am still looking for a lightweight (i.e. Li-Po battery back) with a large capacity.

    Hahaha, yeah, I figured it would be a bit easier to turn a bicycle wheel :p

    I was planning on using one from a '72 VW super beetle (small, 4 cyl I think). Someone also just mentioned a generator, but they are rather large (physically).
     
  7. MrChips

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    Oct 2, 2009
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    If this is any help, I own and ride a couple of e-bikes.
    Typically they use direct drive stepping motors rated from 250W with 36V 10AH batteries
    to 500W with 48V 15AH batteries. Typical range is about 50Km.
    You can work out some numbers from this.
     
  8. #12

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    Glad to know my guess about wattage was almost correct. Tell us more. Do the e-bike motors run at full power when cruising on a flat street? Well, at some speed they do. There must be a highest speed at full power. Tell us more.
     
  9. DerStrom8

    Thread Starter Well-Known Member

    Feb 20, 2011
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    Thanks, guys :)

    I figured out that I will need a motor with about .4 horsepower to drive my bicycle. That means about 288 watts, so there's another piece of criteria to go from :D

    12 volt, 288 watt DC electric motor. Hmm...
    I had also thought of using a treadmill motor, but I don't know anything about them. Are they DC? What voltage do they run on? What is their HP rating? There are several types of motors to choose from, so thanks for continuing to helping me choose the best one. ;)
     
  10. #12

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    The treadmill motors I know about are steppers so the very accurate variable speed thing can happen. Anyone else is welcome to tell if they know about treadmill motors that are not stepper motors.

    edit: I have to go to work now. Will check back later.
     
    Last edited: Jul 9, 2011
  11. MrChips

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    Ebike steppers will regulate depending on speed required. Hence the current draw will depend on acceleration required and amount of load. Because of ebike laws in USA and Canada, speed will max out at 32km/h (20mph), current draw drops when cruising on a flat.

    My guess is under full load, a 250W motor will draw about 7A @ 36V while on average it is running at about 150W, 4A @ 36V.
     
    Last edited: Jul 9, 2011
    #12 likes this.
  12. strantor

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    Oct 3, 2010
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    I had a friend in high school who had an old SAAB which had been modified in the ignition circuit. the key had been bypassed and a start button installed that would crank the starter no matter if the steering was locked or if it was in gear. one time he ran out of gas and drove the thing home with the starter about 1/4 mile and up into his driveway. starter didn't burn up if it'll drive a car, it'll drive a bike and probably a lot faster too. My vote is it would work great; but I wouldn't hook it up direct drive and then connect a battery to it; it would probably shoot right out from under you.

    Another thing; there is a company who sells electric superchargers for cars which are made out of 3 starter motors. They may or may not have been rewound or otherwise modified to run continuously though.
     
  13. #12

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    I was also thinking about the Bendix on a standard starter motor because it disengages the drive when the starter motor is not energized. That seems like an advantage to me.

    Good luck on the "shoot right out from under you". This is a possible danger point and makes me think of centrifugal clutches. You might be able to get (or modify) a centrifugal clutch weak enough to soften the starting torque.

    Just tossing ideas around.
     
  14. strantor

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    how were you thinking to control it? maybe a PWM controller? starters draw a lot of amps. maybe you could go simple and use some kind of soft start; put a resistor in series with the motor to take off, and then go direct to batter when you get up to speed. or maybe X different resistors for X different torque settings? I don't know if that would work or not.
     
  15. #12

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    Strantor has pointed out the really difficult part (besides the mechanical design). From this, I get the idea that the generator might be an option because altering the (lower) field winding current alters the torque.

    Still just tossing ideas around.

    ps, work got delayed by an hour or so, so I have a bit of time to fool around with this.
     
  16. strantor

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    how about PWM via a mercury relay. Look into that; I don't know much about mercury relays, like response time an such; maybe too slow for PWM but I know they can switch tons of current and their life cycle in the gajillions of switches range
     
    Last edited: Jul 9, 2011
  17. strantor

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    So far all I found on mercury relays is response time 50mS, 100A for >200$. And I think they have to be mounted upright, maybe not good for a scooter application. Imagine you lay it over and it makes contact, goes balls to the wall, what you going to do? so maybe not the answer. I don't know, any other ideas? SSR maybe; going to be expensive at that kind of amperage also
     
  18. DerStrom8

    Thread Starter Well-Known Member

    Feb 20, 2011
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    Thanks a lot for all the ideas and suggestions, everyone! :)

    I plan on using PWM to control the motor speed, yes. As for starting it up, to avoid high-load starts, I could always pedal and get up to speed, then flip a switch on the handlebars and have the motor take over. Just another idea to ponder :D
     
  19. shortbus

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    The treadmill motors I know about are brushed DC motors, never seen a stepper in one. They run on 100VDC or 180VDC.
     
  20. strantor

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    Yes, I like. Maybe have your controller sense the voltage output from the starter while pedaling, and not output until a certain point; you would need to find the RPM (or, voltage output from the motor at that rpm) at which it point it is safe to start powering without risk of over-amping your circuit. Also you (or, your circuit) would need to account for the amp increase of going uphills.

    As for the motor, and physically connecting it; what's your plan? In other powered bike kits I have seen from the 80's, the motor shaft has a small wheel to rest on the back tire; pretty straight forward. keep in mind the gearing of the specific bike you have when setting it up; most have the freewheel mechanism as part of the rear wheel, so if you connected the motor to the drive chain somehow, you run the risk of turning the pedals into a heel & calf puree machine.
     
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