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IndustryArena Forum > CNC Electronics > Gecko Drives > Gecko G540 Current Set Resister
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  1. #1
    Join Date
    Nov 2015
    Posts
    2

    Question Gecko G540 Current Set Resister

    FIRST POST!

    I am building a custom controller box for my eBay 3040 CNC frame using a G540 controller. I got four VEXTA PV267-02BA-C1 stepper motors. They are 6-wire unipolar , 2-phase, 2A/phase motors:



    According to the Gecko Motor Wiring guide (Figure 10), this is how to connect a unipolar motor as a bipolar motor:


    It says "just remember to set the drive current to exactly half the motor’s rated unipolar current rating".

    So I just want to confirm with the experts so I dont blowup my G540. I need to set the current limit resisters for these motors to 1 Amp (1/2 the unipolar current (2 Amps)), correct?

    The G540 document states "The resistor is calculated by taking the current in amps and multiplying by 1000. If your motor is rated at 2.8A per phase you will have to use a 2.8K 5% 1/4W resistor." So, in my case, for a full-winding configuration, I want to use a 1K Ohm resister to set the output current to 1 Amp ((2A/phase) /2 * 1000 = 1000 Ohm)?

    IF I wired the motors up in half-winding mode, I would use the 2A/phase rating and a 2K Ohm set resister?


    Also:
    Full-Winding Mode = Slower with more torque?
    Half-Winding Mode = Faster with less torque?


    Thanks for helping me solidify my understanding here!

  2. #2
    Join Date
    Dec 2015
    Posts
    15

    Re: Gecko G540 Current Set Resister

    I have the exact same question. Did you figure it out?

  3. #3
    Join Date
    Nov 2015
    Posts
    2

    Re: Gecko G540 Current Set Resister

    Quote Originally Posted by atarijeff View Post
    I have the exact same question. Did you figure it out?
    Yes, I believe I did.

    I decided to go with a half-winding configuration. After doing more reading, I came to the realization that this was the best configuration for my setup. Since my motors use a single coil with a center tap (per phase), the inductance the driver sees quadruples if you use the full coil! This can bee seen in the data sheet specs in my original post. The half winding (Unipolar) spec is 3.54mH while the full-winding (Bipolar) spec is 14.2mH (or 4x3.54mH). This is still within the spec for the G540 (max is 30mH) but higher inductance means the motor is slower to react. Also, with full winding, you have to run at half current due to increased resistance (using the full coil vs half the coil). This gives you less torque as torque is a function of magnetic field and magnetic field is a function of current! With half-winding you can use the full phase rated current (2A in my case).

    The 6 wire design allows for less transistors in the driver. It is important to understand that a Unipolar motor only uses half if its coil anyway. Current is applied at the center tap and a transistor drives one side of the coil or the other creating differently polarized magnetic fields (for forward or reverse) per phase. The Bipolar drivers (like the ones in the G540) use an H-Bridge to drive current in both directions within the same coil (or half coil in my case). So, as I understand it, a half-winding configuration on a Unipolar motor operates as a bipolar motor at the rated motor specs.

    So, running at half-winding means spec speed and torque. The drawback is that higher current will cause more power consumption and hotter motors and drivers. My rig has an abundant power supply and active cooling on the G540, so neither of those drawbacks affected me.

    Hope that helps!

  4. #4
    Join Date
    Dec 2015
    Posts
    15

    Re: Gecko G540 Current Set Resister

    Thanks for the info. Of course now I have more questions than answers to think about. I'm in the middle of a build that someone else abandoned. My motors are the one spec'd here with a knockoff TB6560 that I'm already regretting. I think a G540 is in my future.

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