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DC Motors

Discussion in 'Electronic Basics' started by Jon Slaughter, May 28, 2007.

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  1. Does anyone know what kinda motors that most drills use? I was thinking it
    was a torque motor but not sure.

    Also, for DC motors can I get away with changing the voltage to a lower
    setting to increase the torque or do I run a huge risk of burning up the
    motor?(i.e., are they usually designed for a very narrow operating voltage).
    I have this old broken 18v drill and I want to turn it into opening a gate.
    I figure that it has enough torque to do it and I can control the speed
    quite easily. I plan on using some gears so I can open and close it with a
    change of polarity. I'm a little worried that it might have to much torque
    or that it might be to fast/slow for the gate(which the gears have a lot to
    do with of course).

    Just wondering how much I can play around with the voltage to get what I
    want if I need too. Also, I assume that P is pretty much constant for
    motors? i.e., if the rated power for a drill is P then P = I*V for a large
    range of I and V? So if I know its a "100w" drill at 20V then I can
    calculate the current at 15 voltages = 100/20 A = 5A and it should be
    approximately what the drill will pull? (obviously the load has a lot to do
    with it but just curious as to some simple approximation)

  2. Chris

    Chris Guest

    Hi, Jon. My Milwaukee rechargeble drill uses a DC permanent magnet

    You can reduce the DC voltage applied to the cordless drill without
    burning up the motor. It will just reduce the speed. It's
    overvoltage that will fry the motor. And current is proportional to
    the voltage applied. That is, if you have an 18V drill, it will
    require only half the current at 9V. The thing is, it will then only
    be using 1/4 the power (P = I^2 * R), which means your output power
    will decline accordingly, too.

    Reducing the power supply voltage will reduce the torque. You'll have
    to look at your project to see what's best, but your work output from
    the motor will decrease a lot if you run it at less than 6V. I'd
    suggest using some additional gearing if you need to run it at lower

    Also, most rechargeables require several amps of current. You might
    want to look at using PWM control if you need to have an adjustable
    power source. Here are some good circuits:

    The "A Practical PWM Circuit" toward the top of the page will work
    well for a cordless motor requiring several amps DC.

    Good luck
  3. You might be able to get a higher torque at a slower speed by using a low
    frequency PWM, which will provide a full power pulse long enough to cause
    the mechanical movement you need, and then waiting a little while before
    the next pulse. This might work well if there is some springiness in the
    mechanical drive, so you can essentially wind up a spring and then wait
    until the load moves, and then apply more pulses as required. It would be
    much like a hammer drill, which works well to overcome initial friction and
    inertia to get the load moving.

    Otherwise, probably starting with full power and then throttling back once
    it is moving might be a good option.

  4. Phil Allison

    Phil Allison Guest


    ** It will need two additional things to do that job:

    1. A diode across the motor, rated for 7 amps or more, preferably a
    Schottky type.

    2. A heatsink for the MOSFET and the diode.

    A higher current MOSFET should be used - one with an on resistance less
    than 20 milliohms.

    ....... Phil
  5. Chris

    Chris Guest

    Hi, Phil -- you're right. Thanks.

  6. Why not just use an scr and AC instead?
  7. Well, a triac of course...
  8. Phil Allison

    Phil Allison Guest

    "Jon Slaughter"

    ** Cos it is a DC motor - you PITA bloody fool.

    ........ Phil
  9. Jasen

    Jasen Guest

    a farrly conventional permanent magnet motor, Jacobs is a popular brand.
    that won't happen.
    don't stall them for too long. or run them at low speed under load.

    if you want slow use a gearbox.
    the biggest problem is heat, which is the difference Energy in - Work out.
  10. OMG.. your a fucking moron... calling me a fool yet you have no idea!

    who the **** says you can't use rectification?

    whats your problem you moron?

    Now go shut the **** up! You fucking piece of shit!

    BTW, if you use just one SCR then it will rectify the wave anyways in a
    sorta half wave rectifier.

    Now I know you obviously don't know shit and so theres no bother replying to
    future posts because you will be ignored as I ignore all idiots who think
    there smart but don't know shit.
  11. Phil Allison

    Phil Allison Guest

    " John Slaughter = ASD Fucked, fucking TROLL IDIOT "

    ** YOU DID - **** BRAIN !!

    YOUR stupid, fucking post above says " AC " !!!!!!!!!

    Two minutes later you posted this to confirm it:

    " Why not just use an scr and AC instead?

    Well, a triac of course... "

    Go EAT SHIT - MUTHERFUCKER !!!!!!!!!!!

    ....... Phil
  12. Rich Grise

    Rich Grise Guest

    If you already have speed control, don't mess with the voltage. What's
    broken about the 18V drill? Right after saying it's broken, you say it has
    torque and speed control.

    Run it at 18 V and either figure out a trigger setting, or hack the
    controller to do with what you want.

    Good Luck!
  13. Its broken at the shaft. The motor seems to run fine but overall the drill
    is broken(you know, there are other parts to a drill). The speed control is
    through the trigger and I need to automate it... The "control" is
    encapsulated so not much I can do with it. I didn't say it had speed
    control though... I said I can control the speed quite easily(by suppling a
    lower voltage).

    Since it runs on dc either I can use some dc method of power control such as
    pwm or AC method such as triacs and rectification(both of which probably
    result in essentially the same result). But seems to me using dc and just
    lowering the voltage is the easiest thing? I really don't understand what
    the point of pwm is as it would ultimate seem to still average out things to
    an equivilent voltage supply? (i.e., why not just just lower the voltage in
    the first place if I can do that(remember, I don't need to automate the
    speed of the motor but just easily be able to find the right speed which
    makes the gate open fast enough).

    i.e. the issue is not to automate the speed of the drill but to know how the
    torque and speed are related to the current and voltage so that when I
    guesstimate the right settings needed for the drill to run properly. On
    wiki it seems to say that the torque is inversely related to the speed and
    that the torque is proportional to current while the speed is proportional
    to the voltage. This is very confusing to me and I can't explain why it
    happens that way(except that maybe the motor sets up a reverse MMF that
    counters the current or something like that) but if its the case then it
    seems that running the drill motor slower will give me more torque.... but
    might be to slow.

    I'm just a little confused about that aspect ultimately I can hard wire the
    circuit for the speed as it won't need to be changed(and the motor will not
    run much so it doesn't have to be efficient).

  14. Circa Mon, 28 May 2007 18:08:57 -0500 recorded as
    <YhJ6i.4124$> looks like "Jon Slaughter"

    I had similar questions. The first question I had of the original post was
    what was meant by the term "torque motor." Seems a redundancy.
    If the shaft is bent, then I would not count on being able to use the motor
    for any other purpose. For the motor shaft to be broken yet...
    ....then I must assume that there is a clean break somewhere, and that you
    intend to mechanically splice a new shaft on what's left of the old?
    So, figure out what the trigger does to regulate the speed, and mimic it
    via automation. The same goes for the polarity control. Reverse-engineer
    I will assume that the input to the "control" is a constant 18V DC. In
    which case all you need do is measure the output voltage and polarity as
    you fiddle with the trigger speed control and the switched direction
    control. Gather the empirical data, and if you need more help, post again.
  15. no. The shaft uses several gears and at some point the gears are not turning
    the shaft... the motor itself works fine as far as I can tell. I didn't
    investigate it much but it could easily be fixed but tha is not the goal.
    I'm not trying to repair the drill but use the motor.
    Thats not the issue... you guys keep thinking I'm trying to control the
    speed of the motor.. I'm not... i'm trying to find the effects that changing
    the voltage has on the motor's properties... I guess I should have asked
    this question in a physics or mechanics NG instead...

    look, I don't know how else to say it. The drill runs on 18V and it has the
    ability to control the speed using the trigger which my guess is simply a
    pot. It also has a switch to run it in two speeds which is also probably
    just a switched in resistor.

    The issue is not the electronics that control the motor for varous speeds
    but how the motor works w.r.t to those speeds. I will not be using need to
    automate the speed of the drill(which I never said but you all have assumed
    that). When you are opening a gate you do not need to have it at more than
    one speed... theres no need to open it fast in some cases and slow in

    The issue is, I need to find the right voltage to run the motor at so that
    it opens the gate at the speed I want. Surely I can accomplish this to some
    degree with gears but I want to be able to adjust the speed manually instead
    of buying a new gear just cause its to fast or slow. Say at 18V it turns
    the gate to fast... if I decrease the voltage to 10 voltages it will slow it
    down.. but now maybe the torque is not enough to turn the gate... or is it?
    How do motors work? At 15V will it draw more current than 18 and produce
    more torque at the lower speed(this is what I got from wiki but doesn't make
    sense to me). I do not need to automate the speed but only find out how the
    motor works w.r.t to different voltages. I can only do so much with the
    gears. If I need 25V to get the speed and torque then it could burn up the
    motor... but maybe it would be ok if it only runs for a few seconds at a
    time(With enough delay to cool off).

    The way I see it is that when I run the motor at a higher voltage it will
    increase not only the speed but the torque... not that they are necessary
    directly related but that more voltage means more current by ohms law. I do
    know that it is not necessarily true but not sure. I can't make sense of
    wiki as to me it implies that increase teh voltage decreases the current...

  16. Circa Mon, 28 May 2007 19:38:04 -0500 recorded as
    <vBK6i.5918$> looks like "Jon Slaughter"
    Well, you're simply not making sense. In your original post you said:

    "I figure that it has enough torque to do it and I can control the speed
    quite easily."

    "I plan on using some gears so I can open and close it with a
    change of polarity."

    "I'm a little worried that it might have to much torque
    or that it might be to fast/slow for the gate...."

    And now you say you are not trying to control the speed. Well, which is
    it? Have you not found that changing the voltage changes the speed? Can
    you not see that experimenting with the existing controls will help you
    understand how those controls affect the motor operation? Do you
    understand that the motor's properties are inherent, and changing the
    voltage does not change the motor's properties but instead changes its
    operational performance?

    Have you done any research on the subject? Do you understand the
    relationship between rotational speed and torque? Are you making an effort
    to? Are you converting the motor to a practical project (a motorized
    gate), or are you just dinking around? Make up your bloody mind what you
    want to do, present it in an intelligible fashion, and quit blaming your
    lack of ability to understand the help you are getting on the people that
    are helping you. Crikey!! Jung n guvpx phag.

  17. Ok... your on my ignore list... you want to bitch at me about this shit yet
    your the one adding your own assumptions.. I said "Easily controlt he
    speed"... i.e., fucking change the input voltage... I NEVER SAID I NEEDED TO

    Use your fucking brain and maybe it would make more sense instead of trying
    to make it blame it all on me. How fucking hard is it to conceptualize the
    problem? open a fucking gate with a motor... what the **** would I need
    variable speed control for that? who's the fucking moron.

    I never blamed anyone. I said I couldn't fucking understand it... theres a
    difference... your the one who started the blame game and I'll be damned if
    I'm going to take that shit from you. I asked fucking simple questions and
    maybe I didn't do a good job of explaining but you have no fucking right to
    try and put your ego trip on me.

    Anyways, I'll figure it out by myself as usual cause fuckers like you and
    phil who don't know shit who end up being wrong or misunderstand me(doesn't
    matter if its my fault cause you have no right to try and make it worse) and
    then have to put your ego in when I tell you that i not what I needed.

    I guess you get to join the ignore list with Phil...

    Note for all the other assholes who are going jump in and bitch at me for
    bitching at him... just to fucking bad... save your breath.
  18. Circa Mon, 28 May 2007 21:05:32 -0500 recorded as
    <vTL6i.8305$> looks like "Jon Slaughter"
    Thank god for small favors.
  19. Rich Grise

    Rich Grise Guest

    [crossposted to sci.engr.mech and rec.crafts.metalworking,
    but followups-to set to sci.electronics.basics - if you
    want to chime in and think your group might benefit, please
    add a followup-to of your home group.]

    When you decrease the DC voltage, the motor slows down, yes, but the
    torque drops dramatically. With any decent load, as soon as you start
    to decrease the voltage, the speed (under load) decreases twice as
    fast as you thought it would - it's not proportional to the voltage,
    but the current. And, because the motor has DC resistance, when you
    decrease the voltage, you decrease the current, which lowers _both_
    the torque and the speed.

    It might be instructive to go ahead and hack into the gears until
    you get to the useful one, and put some kind of load on it, and
    do your speed control experiments.

    A good load might be a V-pulley, with a cut V-belt dangling over
    it with a weight on either end. When the pulley turns, it wants
    to drag the belt with it, like a capstan. Well, you put a weight
    on the end that it wants to lift, and adjust the weight until it
    just holds position. Or, maybe just a fish scale (upside down,
    of course, fastened to the floor), so it will reach equilibrium. At that
    point, the weight times the diameter of the pulley will give you the
    torque at that speed and voltage. This is a "Prony brake" or, according
    to Wiki, a "de Prony brake":

    But their diagram shows a slipping clamp on a shaft - what I
    described has the pulley slipping inside a loop, something like:

    Let us know what you find out! :)

    Good Luck!
  20. Tim Wescott

    Tim Wescott Guest

    It's most likely a permanent magnet DC motor. A PM DC motor, by itself,
    isn't a 'torque' motor or other type of motor. You can _use_ a
    permanent magnet DC motor to give a controlled torque if you drive it
    with a set current instead of a set voltage, but you don't change the
    motor construction to do that.

    Lowering the voltage to the motor isn't going to make it deliver more
    torque. It will lower the stall torque (because the stall torque
    depends on the stall current, which depends on the terminal voltage).
    If you need to drive the motor at the most torque you can get without
    burning it up, then you need a controlled-current supply, and you need
    to be ready for some interesting accelerations when whatever you're
    driving breaks loose.

    Mostly, if you want more torque you should either get a bigger motor or
    gear it down.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at
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