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IndustryArena Forum > MetalWorking Machines > HURCO > Hurco KM3P y-axis runaway
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  1. #1
    Join Date
    Jan 2011
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    6

    Hurco KM3P y-axis runaway

    The y-axis servo in my KM3P died. I bought a used one off eBay and hooked it up. Immediately upon powering up the machine, the Y-axis goes into runaway in the negative direction. It bangs into the limit stops and bounces there until I turn off the power. Upon closer inspection of the wiring in the servo's junction box, I can see that the heavy red and black wires (pins 1 and 2) are reversed in the replacement servo. It is as simple as reversing the red and black wires to correct the problem? I can't say that I fully understand how servo motors work or even if they are AC or DC. I've seen some chatter in other threads about reversing the Channel A and B (green and blue?) wires when a motor is internally wired backwards from the original but I don't see "green and blue" wires in the junction box nor do I understand what "channel A and B" are. Can someone suggest a solution? The servo is a Hurco/ElectroCraft E26-2 part number 727-902-0767.Any help would be appreciated.

  2. #2
    Join Date
    Dec 2013
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    5720

    Re: Hurco KM3P y-axis runaway

    That is a DC servo motor. Yes, reversing the red and black (armature) wires should solve the problem. The motor is running the opposite direction from what the controller expects.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3
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    Jan 2011
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    6

    Re: Hurco KM3P y-axis runaway

    Thanks Jim. I will give it a try and post what happens. I learn something new everyday.

  4. #4
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    Jan 2011
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    6

    Re: Hurco KM3P y-axis runaway

    Status update: Still not working correctly. I swapped the heavy red and black wires (pin 1 and 2) on the servo motor. Now, when powering up, the Y servo does its normal little jiggle during power up and remains stationary. The runaway seems to be stopped. However, the rest of the machine starts acting weird. The console will become unresponsive randomly after Manual - Power - Start is pressed. Neither Z or X axis will jog manually (+ / - buttons or with wheel). Tried restarting the machine several times and then noticed that the X axis servo is getting very hot even though I am not trying to use it (other than trying to jog it manually occasionally). Powered it off again and let it set for an hour until X servo was cool to the touch. Disconnected the Y servo at the servo (in the knee) and powered back up. Now console is acting correctly and I can jog Z and X normally.

    So, while it appears that reversing the red/black on the Y servo makes it sort of happy from the runaway standpoint, the rest of the machine doesn't seem to like that solution at all. Any comments or other ideas?

  5. #5
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    Dec 2013
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    5720

    Re: Hurco KM3P y-axis runaway

    Wow

    I'm at a bit of a loss here. I can't imagine any condition that would cause the Y axis to have that effect on the other two axes.

    Makes me wonder if there is some other issues with the controller or drive(s). Another possibility would be to reverse the red and black wires again, and then reverse the A & B on the encoder, this should eliminate the runaway condition. But I still can think of any reason why that would affect the other axes. Especially why it would cause the X axis servo to overheat. That makes no sense.

    Assuming that there is an encoder on the motor, the one possibility that comes to mind is that the encoder on the Y axis motor is either shorted out or not wired correctly and that is causing the encoder power for the whole system to be affected.

    How many wires are coming out of the motor and maybe more importantly is it an exact match for the motor you replaced?

    Maybe post some pictures if you are able to.
    Jim Dawson
    Sandy, Oregon, USA

  6. #6
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    Jan 2011
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    6

    Re: Hurco KM3P y-axis runaway

    Jim.
    The system reaction was a total surprise to me too. I cannot explain it either. I have attached a pic of the junction box. It uses a 15 pin connector but I think only 13 are used. There seems to be 4 heavy gauge wires from the motor (red, black, white, and blue). There are 4 light-gauge wires *red, black, white, green) which I believe go to the limit switch. Lastly, there is a gang of 5 wires (seen in the upper center of the photo). I assume these come from the encoder. These are red, black, green, yellow, and white. The only difference in the wiring patterns between the two servos is the red/black in Pin1 and Pin2 (closest to you in photo). The servo has 4 brushes which I have inspected/cleaned. The motor spins fast and noiselessly when in runaway (belt removed so it won't move the bed).

    Which wires at the A and B encoder wires?

    Sorry...picture seems to be rotated 90 degrees from how I took it. :-(

    Attachment 459898

  7. #7
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    Dec 2013
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    Re: Hurco KM3P y-axis runaway

    I'm not exactly sure what I am looking at here.

    The small green, white, red, and black seem to come from an external device (limit switch maybe?)

    The other small cable going into the motor, green, red, white, black?, could be encoder? or maybe tach?. Normally I would expect a tach to have only two wires, and an encoder would have 4 or more. It would be common for an A/B encoder to be wired as follows: Red=5V, Black=0V, Yellow= A channel, and Green= B channel.

    The yellow stripe on the black & red is a little odd, that might indicate that those are not the armature wires?

    And that leaves the question what are the large blue and yellow wires?

    Do you have the electrical documentation for that machine? A very quick google search failed to turn up any wiring information on that motor.

    It may be time to trace back the wires and see exactly what they connect to inside the electrical cabinet.
    Jim Dawson
    Sandy, Oregon, USA

  8. #8
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    Jan 2011
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    6

    Re: Hurco KM3P y-axis runaway

    here is the Hurco drawing that I have for the servo wires. Maybe it will help make sense of things? The tach is the heavier blue and white wires. From the drawing, would pin9 and pin10 be the A and B channels you referred to earlier?

    Attachment 459912

  9. #9
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    Dec 2013
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    Re: Hurco KM3P y-axis runaway

    Ok, that all makes sense.

    Not sure what ''MKR'' is, maybe index pulse?, and I'm guessing ''PSC'' is power supply common.

    But none of this explains what the heck is going on.

    Swapping the White (9) and Green (10), and reconnecting the red and black as they were might have the same effect as reversing the armature wires, and would maybe keep the tach polarity happy.

    I don't have a definitive answer for you. But you would not hurt anything by making that wiring change.
    Jim Dawson
    Sandy, Oregon, USA

  10. #10
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    Jan 2011
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    6

    Re: Hurco KM3P y-axis runaway

    Jim,
    It looks like maybe I (we) been chasing my own tail. I have been trying a lot of different things and have learned several (interesting) things. First, based on some stuff I read elsewhere, the polarity of the tach is important. To prove this to myself, when I wire Pin1 and Pin2 according to the schematic but reverse pins 3 and 4 (tach), the servo goes into runaway. Putting pin3 and 4 back to their correct locations, stops the runaway. Like you, I was pretty much out of ideas so I put all the servo wiring (pins) back to where the schematic said they should be and sat down to consider my options. I decided it was time for "an act of desperation": I normally don't like to do this because, if there really is a problem in the servo (a short?) I'm afraid I will damage something else. The Y axis amp has been showing a red fault light ever since I first tried the replacement servo. I was "assuming" this was the result of something bad in the servo since the amp did not have the red light previously. But, as my "act of desperation", I decided to move the the X-axis wiring from its amp to the Y axis amp with the fault light. It is simple to do with the connectors that are used on the amps. I know the X axis servo is good and it works properly when connected to it own amp. Surprisingly, the Y amp still had the red fault light when attached to the X servo and X half of the dual axis PCB. Now I'm thinking maybe there isn't anything wrong with the replacement servo after all. To finalize my act of desperation, I attached the Y servo to the X amp and the *#%$#% thing WORKED! The replacement Y servo works just fine when attached to the X amp. This seems to indicate that the servo is ok and that the dual axis PCB is also ok but, my Y amp has failed. I am wondering if the initial test where the red/black (pin1 and 2) were backwards somehow damaged the amp and I (we) have been chasing a ghost since.

    In any case, I appreciate all your help trying to figure this out. It was a good learning experience although I now have more gray hair than before. I could use less stress in my life. I guess I will need to see about getting the Y amp repaired. Thanks again for the help.

  11. #11
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    Dec 2013
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    5720

    Re: Hurco KM3P y-axis runaway

    My pleasure to help out. We all learn from trying to solve these problems. Sounds like you are making good progress.

    I wonder if it's time for a complete retrofit, new motors, drives, and software? I have a pretty low tolerance for older systems that are failing. As soon as I have a failing system, I just retrofit with modern hardware/software. In the case of my CNC lathe, I did the retrofit just because I didn't like the way the 30 year old Fanuc software worked.
    Jim Dawson
    Sandy, Oregon, USA

  12. #12
    Join Date
    Aug 2014
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    17

    Re: Hurco KM3P y-axis runaway

    You should be able to find boards on eBay. I will be selling the controls out of a KM3P in a while also. I also bought a pair of KM3's and the shipping company destroyed the panel on one and a retrofit was the only practical thing. The other one is now in process.

    Your diagnostic method is about what I would have done also but then I am not a machine tool mechanic and have never played one on TV...

  13. #13
    Join Date
    Feb 2021
    Posts
    1

    Re: Hurco KM3P y-axis runaway

    Hi all, I have a km3p mill. Y axis runaway issue, swapped amp between X,Y,and Z and fault moves with the amplifier board. Got a second hand one on Ebay but it was also faulty, same thing happens. I was going to get another from Ebay but my boards are REV R but I only see same boards with different revisions, do you know if these are compatible. Maybe it's time to retrofit of scrap it.
    Thanks in advance.

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