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Thread: DC Motor

  1. #1
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    Default DC Motor

    Dear Members,
    Not sure if this question belongs in this forum. I construct automata posting WIP etc in the "Automata" section on this site.
    Previously, all the works have been hand cranked, producing movement by cams attached to a crankshaft. The current project is a 750mm high figure which will be controlled by 13 cams around 80mm dia. The power source in this case will be a DC motor, revolving around 3 rpm, attached to the camshaft. My knowledge of DC motors is next to non existent, and so asking for any assistance from knowledgable members, such as, what size 12 V battery would be needed to drive the shaft; how is the torque required to operate the shaft calculated; how to decide what power the motor should be to drive what is essentially a robot with moving parts, like eyes, arms/elbows, hear turn etc.
    Thanks in advance for any assistance,
    Regards,
    Gus

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  3. #2
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    Look for a geared output DC motor. Add a cheap electronic speed control and you can dial in the correct speed for your automaton.
    Something like this: DC12V Geared Motor 25mm 5-1000RPM 25GA-370 Low Electronic DIY High Quality | eBay
    Speed controller: DC Motor Speed Switch Regulator Controller PWM Variable Adjustable 6/12/24/28V | eBay
    Those were the droids I was looking for.
    https://autoblastgates.com.au

  4. #3
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    NCArcher’s advice is spot on. However, if you need a driver for higher current, this crowd provides top notch gear. They also supply DC motors.

    DC Motor Driver

    I use their PWM drivers to drive automotive electronic throttle bodies, electronic EGRs and Smaller DC motors that would be suitable for your application

  5. #4
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    Default

    Thank you NCA and Lappa !
    May I ask a couple more ? I want to use a battery, not plug into a socket. Does that raise other issues with the motors, such as battery Wattage etc ? Does using a speed controller reduce motor power ? Which specifications of a motor relate to its power output ?

    Regards,
    Gus

  6. #5
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    The motor I linked to is 3.5W, draws 0.06 amps (according to the specs but that seems too low). The speed controller, basically, reduces the voltage to the motor to make it go slower but the power doesn't matter that much for your usage. The geared output has loads of torque, relatively speaking, and should have no trouble turning what I imagine is the fairly light load of your camshaft. You can run it off a 12V battery no problem. The size of the battery depends on how long you want to run it for without recharging or replacing batteries.If we believe the specs and say that it runs at 60mA, add a generous 40mA for the speed controller, a AA battery pack (8 batteries to give 12V) assuming around 1800mAH for alkaline batteries would run for over 10 hrs before the voltage got too low to turn the motor. Probably longer as the speed controller accepts a pretty wide voltage range. If you had the room, a small sealed lead acid battery would run it for a very long time.
    Those were the droids I was looking for.
    https://autoblastgates.com.au

  7. #6
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    Thank you NCA !
    I always over engineer everything. In this case, 12 cams pull cables attached to elements which have return springs. Imagine 12 cams, each pulling cables against resistance, some at the same time as others, some not. When some other works, with less cams, being hand cranked, there is a noticeable resistance as cams kick in. Hence my concern about power. Am I incorrect in assuming that reducing power by speed controllers reduces available power ? Also my assumption that low voltage equates to low power, even if high motor rpm, is reduced by gearing ?

    Regards,
    Gus

  8. #7
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    how big a crank do you need to operate this by hand?

    Reducing speed by lowering voltage reduces available power, yes.

    Low power does not mean low torque though. You would choose to start with an appropriately geared motor so your desired rotation comes somewhere near the motor's rated RPM at it's nominal voltage. No point choosing a 20rpm motor if you only need 5rpm, you'd be operating in the lowest part of the voltage envelope, and that would likely lead to more power loss and harder life on the components.

  9. #8
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    Thank you Poundy !
    I haven't cut any cams yet. The usual method involves hand cranking a blank cam, while it is attached to the element I want to move. The scribed crank is cut, then the next, and so on. Some cranks are large, some quite small, it all depends on how much resistance there is when everything is connected. By contrast, the last project used a worm gear input to the crankshaft, there is almost no load at all on the small crank, despite a heavy load.
    Maybe I need to rethink the whole motor concept, use worm gear input, vary the crankshaft speed with further gears
    if it needs it, use a higher speed motor.
    I have read some user reviews on small, geared motors, there is a lot of varying views on reliability. In itself, that doesn't mean much.
    Regards,
    Gus

  10. #9
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    My take on any geared motor is you're unlikely to actually cause them a problem if you buy one close to the required RPMs. Design your cams toward something that's available and you'll be fine.where in Sydney are you ? I probably have a bunch of random cheap geared motors that I have acquired over the years that I could loan you to test if you really wanted.

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