Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Powering a DC motor

  1. Oct 10, 2011 #1
    Hey guys, I'm working on a project involving a DC motor. I'm in school for mechanical engineering.. Electrical problems haven't really interested me so this is the first I've ever really looked into it apart from studying basic circuits in a physics course last year. Anywho, the project:

    I want to power and control a 2.5 HP, 120 volt GS Electric DC motor (part number 119022 if it makes a difference). The motor was taken from a treadmill along with anything else of electrical importance. However, I don't want to use the old circuit boards because they're probably damaged and they do several other things besides power the motor. The motor says its speed is 5600 rpm and I want an rpm adjustable from the range of 80 to 250 rpm. I don't know how the powerband of electric motors works, but if it's anything like a combustion engine, it won't perform well that far below 5600 rpm. I may have to gear things differently as opposed to using the shaft directly from the motor if that's the case.

    How are DC motors generally powered? Could a lamp dimmer be used in this to navigate to different rpm's?
  2. jcsd
  3. Oct 10, 2011 #2


    User Avatar
    Gold Member

  4. Oct 10, 2011 #3
    You will need to modify the above kit to get more than ~16 volts out of it.
  5. Oct 10, 2011 #4
    I've been looking into it before and after posting.

    What do you have to vary to get a DC motor to spin at a different speed? Would rpm from a 120 volt power supply be roughly double the rpm from a 60 volt power supply, or is it based on current?

    Edit: I have an old laptop charger that outputs 20 volts DC and 3.34 amps.. Couldn't I just use that? I forgot to mention that this motor is overkill for what I'm doing so it doesn't have to be running at max.
    Last edited: Oct 10, 2011
  6. Oct 10, 2011 #5
    This is my understanding of it right now, whether it's right or wrong


    C = wall outlet
    B = laptop charger outputting 20 volts DC
    A = one of the output wires is split and some sort of variable resistor is placed there (preferably with a handy little twist knob lol) to control the speed of the motor
  7. Oct 10, 2011 #6
    The motor speed for most of the treadmills I have used are controlled by adjusting the frequency of the signal applied to the motor. These are typically BLDC motors. Give this a read and see if it helps answer your questions. http://en.wikipedia.org/wiki/Brushless_DC_electric_motor

    What is your project requirement? Do you just need to have a variable speed motor? If so, what are your torque/speed requirements?
  8. Oct 10, 2011 #7
    err....if it's _really_ 2.5HP, that's about 2Kw of power or 16 amps at 120v...which is at the limit speced for the beefed-up version of the suggested "ck1400.htm" board. And, I might add, about 5 times the amps (and 30 times the power) of your usual laptop charger...

    Maybe it's .25 (1/4)HP?

    And if it's BLDC, you are SOL in the do it yourself control department. You can however find BLDC controllers, so some more research might be in order.

    I will go with mdjensen22 on getting the real specs before proceeding. However, I did find this tidbit page that might help you as is:
  9. Oct 10, 2011 #8
    Nice link Schip.
    I was going to recommend a universal motor if they just needed speed control - it's a lot easier to implement control on if you don't want to get to far into the electronics. There are a lot of demo boards available for this...such as ST's STEVAL-IHM029V1

    Also, by adding a diode bridge, you can easily gain some efficiency back, among other advantages...http://educypedia.karadimov.info/library/ACMotorControl.pdf
  10. Oct 11, 2011 #9
    The laptop charger did not work.

    I hooked up the two cables from a cordless drill to the treadmill motor and it seems to work, rpm is dependent on trigger pull, etc.. However, powering a 2.5 horse motor with a small hand drill battery leaves something to be desired. Perhaps a better course of action would be to somehow up the voltage (or power? not sure which would technically be right here) and running it through the drill as if it were the normal battery. Maybe a battery could be added in parallel with the original battery or maybe there's a way to channel power from a charger into where the battery went..
  11. Oct 11, 2011 #10


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Actually it's the other way round for a simple DC motor. The torque is maximum when stalled. As it speeds up, there will be a back emf generated that actually reduces the current flowing. Unlike an IC engine, it doesn't have to admit fuel and air and expell it before it can work. It's more like a steam engine that has loads of torque at zero revs - you just let the steam in and it pushes the pistons down even when stationary.

    The upper speed limit of a DC motor usually would be for a very light load and will drop as load is applied (just grip the chuck of a DC drill and feel the torque as the revs drop). For zero load or friction, the speed of an ideal motor would be infinite if you ignore the effects of the switching time in the commutator and a few other 'mere' details.
  12. Oct 11, 2011 #11
    Your motor is likely a "universal" DC motor with a front end bridge rectifier so it can be used with 120 VAC. It would help if you knew if the rectificer were internal to the motor or on the circuit board.

    There are several types of DC motors. Some have perm. magnets and some have field windings. For those with field winding, these can be in parallel or seried with the armature windings.

    The parallel wound DC motors act like those with perm. magnets. Either of these are the most common.

    For perm and parallel wound DC motors, velocity is nearly proportional to voltage applied for small loads. This proportion is called the K factor for a given motor.

    Current is proportional to back torque.

    The common perm. DC motor is very simple to model. It looks like two resistors in series. One resistor is the load, the other models winding resistance. The load resistance is variable depending on load. The ineria of the load looks like a capacitance across the load resistor.
    Last edited: Oct 11, 2011
  13. Oct 11, 2011 #12


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    If a commutator motor doesn't have a permanent magnet field then it doesn't need a rectifier; the direction of the force is the same on both polarities of the AC cycle. The ever familiar AC/DC motors never had a rectifier and were / are series wound. I seem to remember being told that shunt wound motors were used for heavy traction - as in trams and railway locomotives - more torque at very low revs I think.
    I don't know what the present practice is but do they actually make mains operated motors with permanent magnets?
  14. Oct 11, 2011 #13

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    the treadmills i disassembled had a permnent magnet motor of about 2hp(intermittent rating surely it wasn't physically big enough) something like 5000 rpm at 90 volts
    they control armature voltage with that circuit board

    i used a full wave bridge and household lamp dimmer
    it overspeeds it a little unloaded but it held together.
    120v vs 90 - i figure i was in 7000 rpm range
    so dont use a filter capacitor

    be aware hp = 2 * pi * torque * rpm / 33000
    so torque = hp*33000/2*pi*rpm

    so it's a low torque high rpm motor
    and with permanent magnet field it'll have a natural torque limit.

    if you want high torque get a series field motor or planetary gears for the one you have
    maybe an old washing machine transmission

    for speed range he's interested in a windshield wiper motor wouldn't be bad. Or electric window...
  15. Oct 12, 2011 #14
    Facinating. I didn't know that, but it makes sense perfectly good sense once having it explained. I was taking the word of an engineering friend. It looks like me and my friend are going to have a future discussion about universal motors.
  16. Oct 12, 2011 #15
    The motor is a perm. magnet motor. When the treadmill was assembled, the mains went through at least a circuit board before getting to the motor.

    I'm still looking for a power supply.. I watched this http://www.youtube.com/watch?v=cyhzpFqXwdA&feature=related which helped a bit. If the results from a 9 volt drill battery are slightly less than what I want, guesstimating says I should bump it up to 15 or 20. I figure I can just feed whatever 15~20 volt DC source that I end up using through the battery terminals in the drill so I can use the drill's trigger as the speed control. I doubt putting 5 or 10 more volts through the drill will totally screw it up, but I could be wrong.
  17. Oct 12, 2011 #16


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Can you clear up the actual power that you are involved with? 0.25 or 2.5kW?

    If a 9V drill battery nearly runs it then I think you could get a hefty variable bench dc power supply off the shelf that would do the job.
  18. Oct 12, 2011 #17

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    the drill trigger control will probably work only with AC
    because its job is to 'throttle' AC to a drill motor which is the universal type

    but it's a good idea

    since your motor is most likely a permananet magnet type
    which you could determine by spinning it with an electric drill and measure its voltage.
    if it's a permanent magnet motor then voltage should be in proportion to speed,
    that is at 10% rated speed ( 500 rpm?) it should make 10% or so voltage (12 V ?)
    if it's a wound field motor it'll make way less...

    you might try one of these to make AC into DC

    and throttle AC to the bridge with the drill parts.

    connect your motor to the two DC terminals of bridge they're marked + & -
    and AC to the two with the marked with letters AC or ~ symbol...
    ...cut one of the AC lines and splice in speed control

    the bridge will serve as "flywheel" diodes so feel it after a brief run, if it's getting warm it'll need a heatsink.
    and be aware a drill trigger is way undersized for a 2.5 hp motor so it may not last long.

    needless to say dont leave any electrical parts or bare connections exposed where tiny fingers might find them.
  19. Oct 13, 2011 #18
    Try a light dimmer on the AC side of the circuit jim hardy gave you. You might need a bypass capacitor across the motor leads.
  20. Oct 13, 2011 #19


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I still don't understand how the motor you describe in the OP (2.5hp 120V) is going to work with a 20V supply. Granted, you may get it to 'go round' but how is it expected to produce any mechanical power?
    It's a good idea, in any engineering project, to get a few numbers sorted out, from the start or you can waste a lot of time, money and, on a forum, lots of posts.
    What exactly is your requirement?
  21. Oct 13, 2011 #20

    Seems to be a common theme...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook