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Unipolar stepper motor question

Discussion in 'Sensors and Actuators' started by theforce, Dec 23, 2012.

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  1. theforce


    Dec 23, 2012
    Hi, I recently purchased a couple of stepper motors listed as 1000rpm, 3A, 12-48V,, 6 wire steppers. I'm new to steppers, but have been reading a lot. I was hoping to build a driver with minimum components and drive with an Arduino. As I understand it, I should be able to drive the stepper at it's rated current and voltage without needing a chopper circuit, but understand that I won't get the maximum possible torque.

    When the motors arrived I found that they measure 1.7 ohms across each of the two windings and 1.1 ohms from the center tap to each end. My first question is, is this really a normal unipolar motor? Why is the restistance across the ful coil not exactly twice the half coil?

    Second question, since the resistance is only 1.1 ohms, if I drive the coil at 12v then won't the current be about 12A and not 3A? There are no markings on the motor. Were the specs in the listing incorrect, or is my thinking?

    I stupidly tied the 12v without thinking a melted a wire. Then I set the voltage to 3.3v and do see about 2.7A now. The circuit works ok this way, but I can only get 100rpms without dropping steps even with no load.
  2. theforce


    Dec 23, 2012
    Ahhh, I think I was able to answer my own first question. I'm not used to dealing with such small resistance. I think just the DMM leads are making a difference. I disconnected the leads and held the motor wires directly into the DMM and was able to read 0.7 ohms for each coil.

    I was also able to find what I think is the right motor in a spec sheet. My motors look to be PK266-03B found on page C-238 of . I note that the voltage listed is 2.3 volts as opposed to the 12-48v speced from the seller. Based on what I have read, it is typical to drive steppers at a higher voltage but limit to the rated current in order to increase speed and torque. I'm driving them now at 3.3 volts with an ATX power supply rated at 22A, but I noticed that the actual voltage at the coil has dropped quite a bit. So, I'm guessing that I'm doing everything correctly, but will need to up the voltage and use a current limiter if I want more than the 100rpms that seems to be the best I can do with my current set up.

    I'd really apprecaite a sanity check on my thinking here. Any help?
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