Video: Everything you wanted to know about DC motors

Continuing on with our series of Hackaday original videos, this week we are presenting a video all about DC motors. DC motors are relatively simple electromechanical devices that turn electrical energy into rotational movement. In this video, [Jack] takes apart a small DC motor and shows off all of the parts inside and describes how it all works. He also talks about how to modify DC motors to increase their speed or torque as well as how to change their directional preference. In this video, he rewinds a motor and shows how this changes the performance characteristics of the motor.

Is mechanical stuff not your thing? Stay tuned for next week when we launch into a series of videos where we show how to program for the Atmel ATmega328 processor using C. In this series, we’ll show you the real nuts and bolts of programming for this processor by working with its I/O pins,timers, A/D, interrupts, and more.

Video after the break.

32 thoughts on “Video: Everything you wanted to know about DC motors

  1. Great introduction.

    Slot car racers have taken small DC motor technology to extremes. Do a search for Koford, Camen or Alpha slot car racing motors.

    They have cobalt magnets in single, quad and radial configurations; ball bearings, .001″ tolerances and are dynamically balanced. Most of the motors are designed to specs for a certain class; they all try to optimize RPM an torque. I’ve seen 150K-300k RPM numbers thrown around for some of these motors.

    I have a bunch of these and I’m always looking for another use for them.

  2. nice info of DC motors
    i have one question the motor from a 12volt dc pc case fan have a stationary rotor an a magneet ring to spin the motor.i have one time take 12 9volt bloks to get over 100volt He usually ran max 15000rpm so fast that I had to look for me fingers.
    because the fan was about to take off.
    the engine does not burn out by the air the fan moved keep the engine cool
    can i make it even faster bij rewinde it

  3. Cool video although im still confused about the configuration of the wire in relation to the commutators poles. I used to think that each coil was its own circuit but in reality that would not work. Maybe you could do another video of an AC motor rewind. Knowing how to rewind an AC motor can come in handy when working with motorcycles charging systems

  4. neat! I thought that I had already known all that I have ever wanted or needed to know about DC motors, but I was quite wrong indeed. I am glad that I have watched the entire video.

    ok, so thicker wire consumes more current, and provides a higher RPM with lower torque. How does a motor rewound with thicker wire affect its ability to generate electricity when turned by an external motion source as opposed to an unmodified version of itself?

    1. The voltage produced is proportional to number of turns in the magnetic field so

      2x the turns -> 2x the voltage

      unfortunately…

      smaller gauge wire and/or more turns means a higher coil resistance which in turn means less current

      so more turns means a higher voltage but less current.

    2. Pretty much the same in reverse. More turns equals more voltage, but since the wire is thinner it also leads to a higher resistance that limits the current.

      The amount of power you can put in or out of a motor depends on whether you can keep the rotor cool enough, and not exceeding the voltage where the insulation breaks down.

    3. The explaination is misleading about the wire thickness though.

      Thicker wire does not consume more current on its own. When the motor is running without a load, the back-EMF generated in the coils (the motor acting as a generator) almost exactly counters the voltage acting on it, so the current in the armature is actually I = (V – V_emf) / R.

      R is defined by the lenght and thickness of the wire, and V_emf is directly proportional to the speed of the rotor and the number of turns in the armature.

      This actually explains why the motor turns faster. As it speeds up, the current it draws approaches zero, so the power it puts out approaches zero. At the nominal top speed, it is drawing just enough current to generate just enough torque to overcome the bearing friction, and no more, because if it was to turn any faster, its power output would decrease while the power requirements to turn would increase.

      When you slow down the motor from the top speed, it starts to draw current proportional to the torque you’re taking out, and here the smaller number of turns – not only the thickness of the wire – defines the IMPENDANCE of the coil, which defines how much current it will draw. If you can fit more turns of the same thick wire in, it will draw less current to generate the same torque.

      Impendance, usually marked Z is sqrt(X^2 + R^2) where X is the reactance, or the sum of the inductive and capacitive properties of the coil, and R is naturally the ohmic resistance.

      X here mostly depends on frequency (speed) and the inductance of the coil – that is, the number of turns (and the rotor material, and the airgap, and the geometry…)

      You’ll notice how in the earlier equation I used R to make things conceptually simple. I really should have used Z as in V=ZI which is the Ohm’s law for alternating current.

      So, the current that a motor will draw is:
      I = (V – V_emf) / Z

      where V_emf and Z are proportional to the speed the motor is spinning at, and the number of turns in the coil. The thickness of the wire affects the ohmic resistance part, but that’s a lesser effect.

      1. Also notice, that when the motor is at a standstill, Z = R which explains the high current draw when you start an electric motor, since R is typically very low in order to maintain efficiency.

        Once the motor starts spinning, the reactance of the coil picks up because here X = 2*pi*f*L and it starts to limit the current more than the actual resistance of the wire.

        Anyhow, this is the reason why it’s bogus that electric cars don’t need to change gears. The slower you run the motor, the less efficient it is because the proportion of X to R is smaller. Full torque at standstill only means that you’re wasting 100% of your power.

    4. Hopefully I do not confuse things:

      With less windings (thicker wire) the coil will build up less of an couter-magnetic field.

      I.e. if we draw a fixed power from the terminals it still should be more easy to turn.

      With more windings (thin wire) it should be a stronger field that makes easier hard to turn the shaft when the same power is drawn from it.

      So at the same rotation speed, more windings equals more power, but you have turn harder.
      If it was an ideal mechanical system with no friction and such, the energy you put in by rotating with a higher force with many windings should be the same as with less windings where you rotate faster but with less resistance.

      1. Nope. With that statement you’re building an overunity device.

        Drawing P amount of power out means you will always drag the rotor back by P + any losses inherent in the system.

        The only leeway you get is by making that power either through torque, or through speed, because power equals torque times speed.

  5. nice hack…
    can someone say more about the relations amoung speed, torque, turns, voltage etc?
    also how to select the wire!

    regards..

    1. Hey @marcus,

      I just googled your four terms, and found this: http://zone.ni.com/devzone/cda/ph/p/id/46

      It’s a tutorial on DC motor calculations that would answer your questions about the math very completely.

      I thought the simple concepts were well presented in the video: thicker wire draws more current, the thicker the wire the fewer turns will fit on the same armature, the fewer turns the faster the motor. If you don’t want to do the math, do the experiments!

      To figure it out you’d have to know the area you have available to fill, and that’s determined by the geometry of the armature and how carefully you can wind them. Look up “circle packing in a square” for examples of the math.

      The video talked a bit about buying the wire. It’s simply called “magnet wire” or “enameled wire”, and is sold by standard wire gauge. Wikipedia has a very good article on the relationship between wire diameter and AWG sizes. http://en.wikipedia.org/wiki/American_wire_gauge

      The wire diameter affects its resistance, which in turn affects power draw. A thicker wire has lower resistance, which means it will draw more power. (It will also generate more heat.)

    2. Most of it was said (or implied) in the video
      more turns
      less speed,
      more torque,
      lower current higher (more resistance)
      voltage is nearly static
      less turns
      more speed,
      less torque,
      higher current (lower resistance),
      voltage is nearly constant

      As for the exact details of the complex interplay between static magnets and the dynamic magnetic fields that just hurts my head even trying to thinking about it even before hysteresis is factored in. And if the heating in the armature is factored in, then the maths just gets scary.

      I enjoyed the video, it is a good introduction.

  6. I had no idea that much was going on with the choice of magnet wire. DC never was my thing, but that was very informative.

    But please, turn off the autofocus if you can, and get some brighter lights. I couldn’t make out the separations on the commutator. And the camera choosing when to focus on your fingers vs when to focus on the motor was kinda distracting. If you can’t get rid of autofocus, shoot from further back with a telephoto and brighter light, that might buy you a little more depth of field.

  7. I had to stop it after a minute or so. Not to be elitist, but if you’re going to present something, get your terminology correct for the video! “End bell” (You didn’t call it that) “Bronze bearing” (It’s brass) “Some metal coated in another metal”. (Look it up before you tell everyone about it.) It makes you look like you don’t know what you’re talking about and doesn’t inspire confidence in the rest of what you’re saying.

    1. you know, i didnt want to come out and say that but…agreed. it’s nice this guy spent 18 minutes there but i would have watched the whole thing if he spent the hour, laid it all out and made something decent.

      instead of “this thing here (waves pencil around whole area) does… stuff and uhh this and uhh that”

  8. To add, I don’t know a lot about electric motors and I was interested to learn more. Instead, within the first minute or so I’m presented by an apparently ill-informed host who seems to know less than I do! While I’m sure this isn’t actually the case, your lack of presentation skill and basic mistakes make me apathetic to watch more. I expect more from a Hack-a-day presenter.

  9. ok guy’s quit roasting his nuts he is trying to teach….albeit he does need to study up on names of the objects he is dealing with but at least he is trying if you can do better then do it. it is easy to be a back seat driver in a tornado but try being the driver he was probably nervous

    1. No, we’re offering (constructive) criticism. We’re trying to teach him back. I detest whiners like you and your catch-all “You do better” argument.

  10. Thanks for the video, I’m looking forward to the next one. Some bits were a bit difficult to see and there’s a few assumptions that people know about the current and magnetism relationship (if its focused on NOOBS?).

    I think its great to see an original video also, rather than a rehash of textbook dc motor theory.

  11. One thing I didn’t see in the video that I may have missed but is really important is the direction in which you wind the wire. That determines which way your north pole is facing when the coil is energized. If you wind one of them backwards, you will probably get an oscillating effect.

  12. Can you replace the power stems in the plastic with wire? If so can you tell me how or point me in the direction of a tutorial please?

  13. Wow, give the guy a break. The video was pretty useful and a good introduction.

    The bearing in that motor may indeed be bronze, not brass.
    Many small hobby motors use a phosphor bronze bearing. In fact, I don’t think any of the ones I’ve ever used had a brass one. Sometimes, if they are *really* cheap, there is no bearing at all and the shaft just rides in the slippery plastic. But if there’s a bearing, it’s very often bronze.

    As for the “metal coated with some other metal”, it’s not so easy for him to really know what that coating metal might be. Some are precious metals like silver or gold; some are not. I give him points for knowing that the motor he is showing is a little better because it has the graphite blocks to increase current-handling capacity (which they do by dissipating heat from the brushes).

    Sure, he made some mistakes (like saying the motor could have “5, 6, 10 or more poles”, when in fact, the number of poles would surely be an odd number). And I really wish he had explained how he knew which way to go from coil to coil and where to solder.

    But it’s still a darn handy presentation.

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