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Electronics Dewalt motor's and pwm controller's

Discussion in 'Modding' started by BagUd, 7 Mar 2007.

  1. BagUd

    BagUd What's a Dremel?

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    Ok i have a dc motor that i want to drive with (2*12V 55A) car batteries in parallel. but when the moter gets too much load it draws to much current.
    is there an easy way to make a current limiter ? ore is there some thing else i can use.

    thank you.

    the motor is actually a 18v DeWalt hammer drill motor, but will be run of 24v.
     
    Last edited: 19 Apr 2007
  2. cpemma

    cpemma Ecky thump

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    How 'easy' depends on what max current you want to set, do you have a figure?
     
  3. BagUd

    BagUd What's a Dremel?

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    the motor is 420watt's i think. 420W/18V = 23,3A...

    is there a way i can measure the amp's from the 18v DeWalt battery to see how much it can deliver.
     
  4. cpemma

    cpemma Ecky thump

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    A couple of 0.47Ω resistors in parallel to give 0.235Ω placed in series with the motor will drop 5.5V at 23.3A bringing the 24V supply down to a safer level.

    To check the actual current draw at 18V you'd need a much lower resistance in series with the motor, say 0.01Ω, measure the mV drop across the resistor. 100mV = 10A
     
  5. Macaba

    Macaba What's a Dremel?

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    cpemma- Your resistor suggestion leaves me slightly confused- what power rating will the resistors need to be?
     
  6. cpemma

    cpemma Ecky thump

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    Power Law, I2 x R, 23.3 x 23.3 x 0.235 = 128W @ 23.3A

    Rather a lot, but it's split over 2 resistors, a pair of 62-8174 look OK. It is a 420W @18V motor after all, and you're trying to waste about 1/4 of the available voltage. Maybe a PWM solution would be better, 75% duty cycle.
     
  7. bender386

    bender386 What's a Dremel?

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    are you trying to run the motor at 24v or 12v because 2 car batteries in parallel will give you 12v if you want 24v then you will need to connect them in series.
     
  8. ConKbot of Doom

    ConKbot of Doom Minimodder

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    Rather then limiting current, which would limit power, and possibly waste power Why not change the gear ratio to something more suitible.
     
  9. BagUd

    BagUd What's a Dremel?

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    yes i meant in series. 24V at 55A.


    the gearing is 1:5 at 2000rpm input and 450rpm output at no load. Going lower will be to slow. it is powerful enough though, but going up small hills it just pulls too much current.


    what im making is a 1:6 scale Abrams M1A2 tank

    [​IMG]
    [​IMG]
    [​IMG]
     
    Last edited: 9 Mar 2007
  10. BagUd

    BagUd What's a Dremel?

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    ok i just got the info of the motor from my work.

    [​IMG]
    it is a 450 watt motor. the 2,6Ah dos that mean 2,6 Amp hours, if so wouldnt it be smarter to limit the current to the motor to something like 4-5 amps or less ?
     
  11. ConKbot of Doom

    ConKbot of Doom Minimodder

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    A permanent magnet DC motors current controls the torque, and the voltage controls the speed.

    As the torque demanded of the motor increases, the impedance of the motor decreases. Drawing more current increasing the magnetic field, outputting the torque.

    If you limit the current, you limit the torque the motor can put out, if it needs more then that, excess voltage is eaten up in the resistor, limiting the speed, and decreasing the demanded torque.

    Also, since you are using a 18v motor on a 24v supply, there is 1.33333 times the voltage, and 1.3333 times the current that will flow compared to normal, which is a 77% increase in power drawn.


    The motor is designed to use/dissipate a certain number of watts, as long as you keep dont push that too much(I.E. not almost 180% power;) ), you can run the motor from any voltage. I dont know if the 450 watts is peak or maximum continuous output. If they are smaller then a large soup can (like a 16oz one), and since its from a drill, most likely it isnt :lol: I would make an educated guess that 450w would be peak.
    Either way, you cant let the motor draw more power then it would if you were running it off the normal voltage, without having to deal with overheating. With a proper datasheet, you could figure out where you need to be.

    You could add a resistor to drop excess voltage and limit the power, or use PWM to limit the average current and the power, or change the gearing so you arent using more power then the motors can put out constant. But no matter how you do it, you are limiting your power, and if you need more then the motor puts out to reach the speed/acceleration you want, its time for bigger motors.

    Plus, treads eat up a ton of torque, especially turning, a high speed drill motor with only a 5:1 reduction just doesn't seem like it would have the torque to move it around well. Plus I don't really trust that datasheet. 56 Nm is a ton of torque. A little over 41 foot pounds. which would be obscene for a motor in a handheld device.

    The more I think about it the more I'm coming to the conculsion that your motors are probably undersized for your application. Try increasing the ratio, and see if its as slow as you think it would be. If it is, get bigger motors, or add 2 more drill motors in parallel :naughty:
     
  12. BagUd

    BagUd What's a Dremel?

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    so would it be better to use two scooter batteries (12v 5A) ?
     
  13. Wolfe

    Wolfe What's a Dremel?

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    Using resistors to drop that much energy is a BAD idea.

    The best thing would probably to either control the current using PWM, which should be easily doable provided you're using a solid state motor controller, or using batteries able to supply the required current.

    A resistor producing 128w, aside from being enormous, would require several square feet of heatsinking surface area, and active cooling would almost definitly be necessary.

    Seriously, high power resistor should be a last resort.
     
  14. BlackMan

    BlackMan Minimodder

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    its suprising. we use resistors at work to lower the starting current on our motors. but they are used in wound rotor motors. the rotors are put through resistors between phases, then shorted when they get up to speed. the 3.3kv is put to the stator. they are a fair size, these motors do a fair bit of work, lifting water from the dam up to the storages. i think they draw around 180-200A
     
  15. cpemma

    cpemma Ecky thump

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    You're missing the point. The motor is overheating because you're feeding it 24V, not the 18V it's built for. A 5Ah lead-acid battery can supply just as much current as a 55Ah battery, just not for as long between charges.

    Best solution is 3 x 6V batteries in series, failing that a 12V or 24V motor, they're common enough on golf carts and invalid scooters. You'll see the effect of limiting current if you try running the motor from a single 12V battery. Limit the current too much and it won't even get the tank moving from a standstill.
     
    Last edited: 12 Mar 2007
  16. Kipman725

    Kipman725 When did I get a custom title!?!

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    Controll it's speed (and power consumption over a time period) using pulse width modulation. By adjusting the percentage of the time that the moter is on per time period the power disapated by it can be reduced to safe levels and it's speed can be controlled This solution also has minimal power disapation (unlike the silly idea of huge resistors which would make the battery go flat VERY fast). Unless you have truly crazy power draw you can make a simple pulse width modulator with A 555 astable and a darlington driver circuit/mossfett. I think there are transistors out there capable of driving fork lift truck moters so if you can't find anyhting suitible just keep looking.

    555 info:
    http://www.uoguelph.ca/~antoon/gadgets/555/555.html

    As the 555 has a Vmax of aprox 15v you need to lower it's supply voltage to this level using a zenner diode and resistor (as the 555 dosn't have a constant power consumption so a resistor on it's own might not work)


    good luck with your tank it looks groovy :D
     
  17. cpemma

    cpemma Ecky thump

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    You haven't told us, how do you control the speed of this tank? Any decent radio-control system will already have a PWM system so (a) you don't want two PWM systems in series to interfere with each other, (b) limiting the maximum power of the one will keep heat down. If that doesn't work, it just shows the motor isn't powerful enough for the job.

    "resistors ... which would make the battery go flat VERY fast"... anything which reduces the motor current will make the battery charge last longer. Resistors are an inefficient way of doing it, but they certainly won't shorten the time between charges.
     
    Last edited: 14 Mar 2007
  18. Munterofamodder

    Munterofamodder What's a Dremel?

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    does this tank carry the batteries? 2 55Ah car batteries will weigh *lots*, the scooter batteries would I imagine be several times lighter. As both will give more than enough current (car batteries can give short bursts of several hundred amps - think about car starter motors) the only difference is the amount of running time. If the motor dissipates 480W you will be drawing roughly 20 amps @ 24V, so you'd have about 15 minutes run time of the 5Ah batteries. If it carries the batteries, I'd have thought the weight saving from lighter batteries alone would vastly improve the hill climbing performance.
     
  19. BagUd

    BagUd What's a Dremel?

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    been looking in to pwm and i like it... but i can't find one in europe that can handle the amps. and im gonna need a h-bridge i think ? i got a 6 ch. reciver from graupner.
     
  20. Macaba

    Macaba What's a Dremel?

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    You talk alot of drawing too much current, but you havn't said how its detrimental to your purpose. If you are powering this thing off car batteries, car batteries should be ok for huge amounts of current draw.

    Hmm, whilst we are on the case, I'm looking for a motor similar to the one you describe for an electric transportation project, where are you getting your motor from?
     

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