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HDD Stepper Motor Questions

  1. Jun 22, 2010 #1
    I recently pulled apart two old hard drives, and extracted the stepper motors. I already know they would have to be fairly accurate to be used in hard drives.

    So here are my questions:

    1. These were 7200 RPM hard drives. I'm guessing the motors are probably capable of going much faster than that. How fast can I safely run these motors? I can't find any specs for them.

    2. I neglected to measure the voltage the hard drive used to run the motors, and as of yet I haven't found anything. About voltage should be going to the coils?

    3. Am I correct in saying that they have three coils because the motors have 4 leads?

    4. How much torque can I get from one of these things? Would it be enough to run a small remote vehicle? Or would it be more suited to something like a camera mount?
  2. jcsd
  3. Jul 4, 2010 #2

    Hello there,

    There are 4 lead wires, so your stepper motor have two coils which is two phase hybrid stepper motor most of the time. The driver's speed correspond to its resolution. Full, half, quarter, tenth, 16th, etc. If you want to run a stepper motor too fast, you will lose torque. I don't exactly know it can run your remote vehicle or not because I don't know your motor's holding torque at all. You can try it out.

  4. Jul 4, 2010 #3


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    I thought only the head actuator in a disk drive was a stepper, the main spindle drive is just a servo controlled DC motor ?
  5. Jul 5, 2010 #4
    Sounds simple enough. It should be a simple matter to code a PIC to run it. I'm curious though, aren't there some motors that have a common ground for all the coils, and then have individual inputs?
  6. Jul 5, 2010 #5
    It's likely a brushless DC motor. How do the leads ohm out?
  7. Jul 5, 2010 #6
    There are 4 leads. 1-4 left to right:
    Motor 1:
    1-2 = 1.8[tex]\Omega[/tex]
    1-3 = 1.9[tex]\Omega[/tex]
    1-4 = 1.7[tex]\Omega[/tex]

    2-3 = 3.6[tex]\Omega[/tex]
    2-4 = 2.4[tex]\Omega[/tex]

    3-4 = 3.3[tex]\Omega[/tex]
    This seems like a bad thing...everything connects to everything else. Motor 2 is the same.
  8. Jul 9, 2010 #7
    That would be a three phase brushless dc motor wound in star with a neutral wire. The simplest way to drive it would be a speed controller for rc planes and cars, you dont need the neutral with these, so just leave it disconnected. Oh BTW, wire 1 would be your neutral.
  9. Jul 10, 2010 #8
    Sounds good. I'm planning to get a PIC and program that to drive it. I'm planning to build an RC vehicle so I might as well start there.
  10. Jul 10, 2010 #9
    Now, if you could use the head actuator for steering...
  11. Jul 10, 2010 #10
    I have another motor better suited for that. It's also a stepper, but it was fairly simple to figure out. One common power and 4 grounds for each of the coils. I'm not sure how strong it is because I've never fired it up, but if it can handle the load it should be very accurate. It has detents every degree, and I should be able to make it do half steps.

    Just so I can get the basic control interface going, I stripped down a lego truck I had and motorized the steering and drive wheels with some lego motors...nice and simple. It's a pity Lego went away from making that stuff. It was incredibly fun. Anyhow I guess I'll start by making it run, and after I get a reliable stepper controller working it should go.

    Somehow I don't think the HDD steppers would be able to move a vehicle, but I'll see once I get a controller going.
  12. Jul 10, 2010 #11
    Think low torque, and include a gear box.
  13. Jul 10, 2010 #12
    The only gears I have are lego...the parts probably wouldn't hold up too well. I guess I could make some sturdier gears. I wouldn't need a mix of ratios, so that simplifies the design.
  14. Jul 10, 2010 #13
    famousken: How would one use an RC motor controller to run a BLDC motor? Doesn't this motor need a 3-phase sequenced drive, similar to a stepper minus one phase?

    Lancelot59: save yourself some trouble and go directly to a modified hobby-servo (gear) motor. e.g.: the motors used in this: http://www.junun.org/MarkIII/Store.jsp with instructions for modification here: http://www.junun.org/MarkIII/Manual/kevin/servohack.html and even further mods on my site: http://www.etantdonnes.com/ROBOCAR/encoder/
    Last edited by a moderator: Apr 25, 2017
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