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Thread: Need help running hobby motor

  1. 1st November 2009 05:13 AM #1
    redmonkey
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    Default Need help running hobby motor

    Hello there,

    I'm new this stuff and don't know much.

    I have a question here. I have a Twin Motor Gear Box (Twin Motor Gearbox) that uses two FA-130 motors (
    Mabuchi FA-130 Motor) and 2 D Cell Batteries (D Cell Batteries I use are about 1.5V-8000mAh). I need to run the motors as fast and as strong as possible with the 2 D Cell batteries, it doesn't matter that the batteries last 10 minutes. I was thinking of connecting the batteries in series which will give me 3V-8000mAh. Then have a circuit that will change 3V-8000mAh into about 6V-2500mAh. I need help finding the circuitry that will do that. I know the circuit won't have 100% efficiency, I'm only looking for about 50-70% efficiency. Battery run time does not matter, I only need about 10 minutes of run time. Currently the batteries will run the motors for 1 day, batteries in series, motors in parallel.

    I don't really know what to search on google, so any help is appreciated.
    If anything I said up there is wrong, please correct me.

    If any of this doesn't make sense, please ask. I really need help.
    All help appreciated.
    Thanks in advance.

    Thanks.
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  2. 1st November 2009 11:15 AM #2
    KMoffett
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    Have you tried hooking the motors to 4 D-cells in series to see if the 1.5v-3v motor will last 10 minutes at 6v?

    Ken
    "To invent, you need a good imagination and a pile of junk."
    Thomas A. Edison (1847 - 1931)
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  3. 1st November 2009 11:32 AM #3
    Boncuk
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    Quote Originally Posted by KMoffett View Post
    Have you tried hooking the motors to 4 D-cells in series to see if the 1.5v-3v motor will last 10 minutes at 6v?

    Ken
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  4. 1st November 2009 11:51 AM #4
    KMoffett
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    Yeah......but, better than spending the time and money building a DC-DC inverter...only to come out with the same result.

    Ken
    "To invent, you need a good imagination and a pile of junk."
    Thomas A. Edison (1847 - 1931)
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  5. 1st November 2009 11:56 AM #5
    Sceadwian
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    I would definitely try that first =) If you stall those motors at twice the voltage the coils will probably melt into little puddles =\ Instead of 2 D's How about using 3 C cells?
    "Because I be what I be. I would tell you what you want to know if I
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  6. 1st November 2009 04:14 PM #6
    redmonkey
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    The motors can run at 6V from 4 D Cells easily. They don't burn out and run perfectly fine for much more then 10 minutes. Is there a way to get the same speed and power from only 2 D Cells, less run time does not really matter, I only need about 10 minutes, before I change the batteries.

    The person who asked about 3 C Cells instead of 2 D Cells, I can only use D Cell Batteries.
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  7. 1st November 2009 04:24 PM #7
    KMoffett
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    Is this for a competition or school project...thus the limitation?

    Ken
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  8. 1st November 2009 04:25 PM #8
    Sceadwian
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    Well you'd need an oscillator circuit powering a transistor to pulse a step up transformer the re-rectify it to power the motor, problem is the voltage is so low you'd be lucky to get 25% efficiency out of it, the other 75% would be wasted as heat. You really need to add more cells in series to get a higher voltage with any degree of usability. The batteries simple can't provide enough current after conversion losses.
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    could, mum, but I be a cat, and no cat anywhere ever gave anyone a
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  9. 1st November 2009 04:34 PM #9
    redmonkey
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    @KMoffett: This is for a robotics race, that's why there's these restrictions.

    @Sceadwian: I didn't get what you said in the first part, haha, if you say efficiency if about 25%, then 3V-8000mAh would equal to about 6V-1000mAh. And what I'm getting that your saying is, I won't be able to run the motor any better then if I connected them to the motor directly. Right? So it's not possible?
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  10. 4th November 2009 02:10 AM #10
    mneary
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    Impossible? Not really, but not exactly practical (with limited knowledge). You're looking for a 2A boost converter that accepts an input of 2.4 volts.

    One thing that might help is an external low current source of 6-12V to bootstrap it. If you don't already have 6V, then you could build a LM2698 circuit with about 9 components for up to $6. This won't be beefy enough to drive the motor.

    Then you could use a fairly common boost controller to drive the gate of a couple of MOSFETs. I say a couple, because you'll need multiple switches to keep the ripple in the battery down.
    de KI6RWX
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  11. 5th November 2009 01:58 AM #11
    Sceadwian
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    mneary, think about the efficiency for a few minutes, then get come back to this post =) We're not talking advanced semi conductor electronics and a high degree of circuit prep. The OP has much simpler goals.
    Last edited by Sceadwian; 5th November 2009 at 01:59 AM.
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  12. 5th November 2009 06:52 AM #12
    mneary
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    Quote Originally Posted by Sceadwian View Post
    The OP has much simpler goals.
    Considering his starting point, true. If he can't handle the LM2698 for starters, his project is dead. Kind of an entrance exam.

    Maybe I'll try to build one and see where the problems lie.
    de KI6RWX
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