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  1. #21
    Join Date
    Feb 2009
    Posts
    12
    Alright, well now that we have our terms clear, I will say I am connecting the negative terminal on the plasma cutter to common on the motor. But regardless of terminology, that should theoretically work, correct?

    And yes, my table is actually grounded (earth ground) all the time. Its connected at multiple points along the aluminum frame to be certain.

    Oh and Jim.... I called Hypertherm about ordering a voltage divider and they said you don't actually sell them separately. Is that true? They did offer me one that comes as a replacement part for your PHC system, but it costs $195. That sounds a bit excessive for just a couple resistors and filtering elements so I think I will continue to try to make mine work.

    Now that we have established that my table is grounded to earth ground and the negative terminal on the plasma cutter is only connected to common on the motor, is there any other suggestions you guys might have as to why the plasma cutter won't fire with it connected to common on the motor? Once or twice, it has actually produced an arc that way, but it sputters out within 3-5 seconds. Usually though, just a few sparks are emitted from the torch when you request it to fire while J16 is connected to common. Again, I appreciate everyone's help. Sorry if my terminology is not correct, but I don't feel I am a complete novice when it comes to electronics. The other 100+ I/O on this machine plus a CNC laser cutter, and CNC milling machine was wired solely by myself and works perfectly. This is the first circuit that seems to be kicking my butt and I can't figure out why. Is there any chance that a defect with the plasma cutter could be causing this or is it definitely something specific to the way I have this wired to the motor?
    Attached Thumbnails Attached Thumbnails IO1.jpg   IO2.jpg   IO3.jpg   IO4.jpg  


  2. #22
    Join Date
    Jan 2008
    Posts
    2247
    Without looking at a complete schematic of what you are trying to do....I cannot help. Normally the positive and negative are interfaced to a voltage divider/isolator....which divides this voltage to a smaller (50:1 as one example) analog voltage signal. This voltage signal then feeds a comparator circuit that determines if the arc voltage is higher or lower than a preset voltage...then an adjustment is made through a motor drive circuit to the z axis motor. I cannot for the life of me understand why you want to connect the plasma negative voltage.....which is as much as 300 volts apart from the (earth gounded) cutting machine frame and the work ground (positive side of the plasma power supply output)....to the motor common terminal...without a full schematic I cannot help.

    As far as the voltage divider for $195....as I said in my earlier posts...it is more than a voltage divider. It is actually called the plasma interface.....it is comprised of a circuit board mounted inside and enclosure that is designed to be physically mounted to the Powermax case. All of the plasma interface signals are junctioned in this enclosure (Plasma start, machine motion, raw dc arc voltage)....the dc voltage is divided and filtered. The enclosure is CSA and CE approved for use with our plasma cutting systems. It is designed to safely interface the plasma to the THC....which then interfaces to the cnc. If you want to look at more detail of one of our THC systems, The Sensor PHC, here is a link to the instruction manual .pdf file
    https://www.hypertherm.com/library/f...)/806150r0.pdf

    In this manual you can see pictures and funtionality of the plasma interface box...there also is a parts breakdown of the box....you could order just the board if you desire, however our techs are not authorized to help you install it in any non-standard way.

    Best regards, Jim Colt

  3. #23
    Join Date
    Jun 2009
    Posts
    14

    Re: Builiding a Voltage Divider

    Quote Originally Posted by jimcolt View Post
    As far as the voltage divider for $195....as I said in my earlier posts...it is more than a voltage divider. It is actually called the plasma interface.....it is comprised of a circuit board mounted inside and enclosure that is designed to be physically mounted to the Powermax case.

    Best regards, Jim Colt
    Jim,
    I know this is an old thread, but I have a Miller Sprectrum 1000, and I need a raw arc reducer to 50:1 for my Dynatorch CNC. Would this voltage divider work for my system possibly?

    Thanks

  4. #24
    Join Date
    Feb 2020
    Posts
    4

    re: Building a Voltage Divider

    Quote Originally Posted by jimcolt View Post
    You can easily do this experiment....connect a DVM (digital volt meter) to the terminals J15 and J16 in the Hypertherm Powermax system. Set the meter on a DC scale with a range of up to 300 volts. Fire the plasma torch and cut at a fixed speed....raise the torch....read the voltage on the meter, then lower the torch, read the voltage on the meter. You will notice the voltage rises as the arc length gets longer and lowers as the arc length gets shorter.

    In a torch height control....the circuitry monitors this voltage and adjusts the torch height by driving the z axis motor up or down to maintain the correct voltage, consequently you can use this to control torch height. The voltage divider is used to isolate and filter this signal from the plasma, bringing it down to a useable level that can be used by a comparator circuit that compares the actual arc voltage to a known voltage......and then adjusts the torch height to compensate for plate that is not perfectly flat. Conventional plasmas need to maintain the correct torch to work distance within plus or minus .020"....or within about 5 arc volts. With High definition class plasmas, torch heigh must be maintained to plus or minus .004" of the ideal height, or within 1 arc volt.

    I don't understand why you want to ground anything from the plasma directly to the drive motor.....

    Jim

    the KeyWord here is "comparator" thanks Jim you solved my problem 11 years after you posted this. the DVM is not grounded to your CNC electrical system. but when you just connect a divider directly to the ground of your micro controller you will nuke it or yourself.

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