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Stepper Motor Ratings

  1. Jul 18, 2009 #1
    I've got a 12V, 2 Ohm/winding rated motor and I'm trying to figure out how much current it'll take. From Ohm's law 12/2 = 6A - and thats a lot!

    Is this much current needed to drive the motor? Can I use a lower amount of current? If so, how do I limit it?

    I don't have the datasheet for the motor. It does have a model number, but a google search did not reveal anything.

    Its a bipolar motor - only guessing because its got 4 wires coming out of it.
     
  2. jcsd
  3. Jul 18, 2009 #2

    vk6kro

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    That Ohms Law calculation looks OK.
    It will rotate at lower voltages but with less power. But if you don't have data, you will have to test the motor.

    Does the motor have two windings or four? If it is two, are they isolated from each other?

    You can put a voltage across the windings and watch for the shaft to step. It happens very fast, so put a mark on the shaft so you can see if it has happened.
    Once it moves, you have to put the voltage across another winding or reverse the polarity across that one to get more rotation.

    When you can predict which way the motor is going to turn, try holding it with your fingers to get an idea of how much power it has at lower voltages than the rated 12 volts.

    Try to work out how many degrees it is turning each time it steps.
     
  4. Jul 18, 2009 #3
    Hi, I just finished wiring up the 2nd half - the L298 chip. I, again as a test, attached 4 LEDs + some resistors at its 4 outputs and whenever I apply the clock, it changes its output.

    So I think thats good.

    However, when I attached the motor and powered up with the 9V battery, the motor just stood in one place. It did take some force to move the motor, so it was working. But it didn't move whenever I applied the pulse.

    It took less force to move it when I attached the wallwart (5V, 1A).

    Howcome its not rotating on its own? Is it the current? Do I need more? The max. I can supply is 2.5A due to the limitation of L298.
     
  5. Jul 18, 2009 #4

    vk6kro

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    How did you attach the stepper motor without testing it?

    Have a look at the lollowing page which explains the operation of stepper motors quite well.
    http://www.haydonkerk.com/ResourcesandDownloads/StepperMotorTheory/tabid/192/Default.aspx

    In the diagrams follow the movement of the rotating north pole.

    You should need two drivers for a stepper motor and they may have to be made up from power transistors to carry the large current of that motor. See fig 5 in the above reference which shows two H Bridges.

    Do you really need a stepper motor, or would a geared down DC motor be better? You could drive that with just one H bridge. Stepper motors might be a bit difficult for you at this stage.
     
  6. Jul 18, 2009 #5
    Well, from what I've read Unipolar motors have 5 wires coming out of them and mine only has 4. So I'm assuming mine is a bipolar motor with one winding per phase, as there is no common.

    I attached one winding (2 wires) to one set of outputs of the L298. And the other to second set of outputs. The L298 is a Dual H-Bridge driver. I made sure that a pair of wires did, infact, form a winding by measuring the resistance between them.

    I might get another stepper motor, however (they're cheap here). Something which demands a lesser current.

    But I just wanted to know if you do think its the current thats limiting the motor's movement?
     
    Last edited: Jul 18, 2009
  7. Jul 18, 2009 #6

    vk6kro

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    There are stepper motors with 4 wires where one is common and the others are fed in sequence. I have several of those.

    You need 2 H Bridges but the L298 has two in it already. BUT they are not capable of driving that motor apparently. Are you still using 9 V batteries?


    An alternative would be to make up a higher powered dual H Bridge with power transistors if you needed the power of that motor.
     
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