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IndustryArena Forum > MetalWorking Machines > Benchtop Machines > PM25-MV Electronic Spindle Speed Control from GRBL
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  1. #1
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    PM25-MV Electronic Spindle Speed Control from GRBL

    Here is my first information on PM25-MV spindle drive control:

    The machine comes with 750W brushless hall-based spindle motor which is controlled by the electronic controller. I am posting the pictures below. As a first finding, the controller has a 2200pF capacitor from power lines to ground (EMC filter). When I connected this to GFCI protected power outlet, the current through the capacitor will occasionally trip the GFCI protection, which is inconvenient. Do not connect the machine to GFCI outlets.

    A quick disassembly shows the modular BLDC controller which is molded in epoxy in place after assembly, so it is not repairable. The molding protects from moisture which is good. The hall sensors and the direction/speed controls are powered from on-board low voltage regulator which is NOT galvanically isolated from the power lines. This makes it dangerous to wire speed control to PC or microcontroller! Direct connection may lead for complete failure of electronics and it cannot be repaired.

    The speed is controlled by the DC voltage which may be from 0V (stop) to 5V (full speed). The controller has a label which apparently specified the maximum speed of this version of the controller (see Motor_Controller_2). Mine is 0-2500rpm, but there are options to set it to 0-3500rpm. However, since it is molded, apparently the maximum speed cannot be changed.

    Since there is a need of isolation to control it externally, one needs to install an optocoupler as shown on schematics. Two options are available: using a transistor optocoupler (PC817, H11Ax, H11Bx, 4N23, etc) (opto_1) and using a logic optocoupler (H11Lx, NTE309x, 6N137) (opto_3). The second option will be more accurate. Transistor option will provide zero (or close to zero) voltage when the LED is off; and 5V when the LED is on. The logic version will output zero when LED is on and ~+5V when the LED is off.
    The connection of the LED to controller may vary - it may be connected between power and PWM output of the controller, or between PWM output and ground. Depending on the way it is connected, the PWM will be effectively inverted.

    Once the optocoupler is built, it makes sense to check what is the idle state of the controller and set the inversion appropriately.

    In some cases, it makes sense to retain the manual control of the spindle speed - that can be easily done with two diodes as shown in the third ("enhanced") schematics (opto_3).

    One disadvantage of the logic optocoupler is the fact that it sets the full speed when the CNC controller is powered down. It is possible to fix this by adding one logic inverter gate between the output of the optocoupler and the RC filter.

    Disclaimer: use the information herein at your own risk, no warranties of any kind, and you need to understand what are you doing. When testing, make sure that you do not touch energized parts on the motor controller and the high side of the optocoupler to avoid electric shock!
    Attached Thumbnails Attached Thumbnails Motor_Controller_1.jpg   Motor_Controller_2.jpg   Motor_Controller_3.jpg   Opto_1.jpg  

    Opto_3.jpg  
    Last edited by dmderev; 01-12-2020 at 12:25 AM.

  2. #2
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    Additional pictures of the BLDC controller... I could not find schematics or any other information so far. Apparently it is microcontroller driven and there is a programming (?) connector in the middle. The control input impedance is apparently 100kOhm.

    Disclaimer: Use information herein at your own risk, no warranties of any kind.
    Attached Thumbnails Attached Thumbnails Opto_2.jpg   Motor_Controller_4.jpg  

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