OK to Ground AC Cord to MC-60 Chassis?

Discussion in 'General Electronics Chat' started by Zero Potential, Mar 25, 2015.

  1. Zero Potential

    Thread Starter Member

    Mar 25, 2015
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    Hi. I'm new here. Thanks for the opportunity to participate.

    I took two electronics courses in college, and I build guitar amps, but I am not exactly an EE. Right now I am adapting a treadmill motor and MC-60 controller to run a 1" belt sander.

    I am getting ready to make the AC cord. The board only has two AC connections, and neither is a ground. I am thinking I would like to ground the cord to the chassis. I have no other place for the ground wire to go, and I figure I might as well use it.

    There are five transistors with heat sinks attached to the chassis. Am I correct in assuming they will not cause any problems if I use the chassis as ground?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The TM controller is not isolated from earth ground, so the board and circuitry in general should not be grounded, But a ground conductor should go to any metallic enclosure and continue on to the motor frame.
    Max.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    It's hard to say in your specific case. Grounding the chassis used to be standard practice, if anything broke loose and touched the case then whoever was holding it would be safe.

    However, without seeing your controller no one can tell if the transistors are isolated from the case. I do such testing everyday here, but I have equipment that can automatically apply whatever voltage I want and trip if the current goes over a certain limit.

    The units presently passing thru our area get tested for 1,500 VAC (input side) and 500V DC (output side) with a limit of under 2mA.

    I feel safe grounding those cases (and we do in other tests).

    Do you want to guess with yours?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The MC-60 uses insulated tabs on the power semi's, in any case they are all mounted to the same HS so they would need insulation in any event.
    The input is direct from the AC in so any other part of the circuit, including the control pot conductors could be live WRT Gnd.
    Just important to make sure that the circuitry itself is kept isolated.
    Max.
     
  5. Zero Potential

    Thread Starter Member

    Mar 25, 2015
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    Thanks for the help.

    It sounds like Max is familiar with this thing. If I understand what he's saying, it appears that grounding the chassis will not cause unwanted results such as explosions or death.

    I may as well admit that I don't know what "HS" stands for.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    HS = Heat Sink, just take a resistance reading from power input to any metallic points you intend to connect to earth ground, they should read infinity.
    Max.
     
  7. Zero Potential

    Thread Starter Member

    Mar 25, 2015
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    Wonderful. Thank you.
     
  8. Zero Potential

    Thread Starter Member

    Mar 25, 2015
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    Well, I found a thread on another forum, and it says grounding the chassis will destroy the controller. I may have done that. I did not make a ground attachment, but it is possible that the braided conductor in the cord touched the chassis after I fired it up.

    I don't get any motor motion. The controller sucks a lot of juice and tries to blow the circuit breaker. Testing at the DC output, I get no voltage when I move the 5K control pot.

    I have read that there is supposed to be a common ground for the DC and AC, somewhere on the board. Also, I have seen diagrams where a choke is included between the DC output and the motor.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I have a reversed engineered schematic for it I can post?
    The power devices are very common Littlefuse devices.
    Max.
     
  10. Zero Potential

    Thread Starter Member

    Mar 25, 2015
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    There are lots of schematics, but apparently none are considered reliable. Here is one people refer to.

    http://i.stack.imgur.com/rQXn4.jpg

    Maybe there is a short in there somewhere. It shouldn't be sucking current with no motor connected.
     
  11. Zero Potential

    Thread Starter Member

    Mar 25, 2015
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    When I check the resistance between the DC outputs with no AC connected and no motor connected, I get less than one ohm, so there is a short. That can't be right.
     
  12. Zero Potential

    Thread Starter Member

    Mar 25, 2015
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    I thought the big components on the chassis were transistors, but they are actually rectifiers. I am wondering if I blew one or two out. Can anyone tell me how to check if they are shorted, and which lead does what? I think the long middle leads are grounds, but I'm not sure.
     
  13. Zero Potential

    Thread Starter Member

    Mar 25, 2015
    64
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    They are thyristors. Something I knew nothing about until a few minutes ago. I am trying to see if I fried them.

    I am measuring resistance between the cathodes and anodes, and out of the five thyristors, only one has any real resistance.
     
  14. Zero Potential

    Thread Starter Member

    Mar 25, 2015
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    I am researching this on my own, and I have seen things indicating that a cathode-gate short is a symptom of a toasted thyristor. The result is runaway current, which happens to be what I have.

    Right now it's looking like grounding the cord to the chassis on a DC motor controller is a really bad idea.
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The control is done by a bridge, two rectifiers and two SCR's.
    Max.
     
  16. Zero Potential

    Thread Starter Member

    Mar 25, 2015
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    I located the D4020L and S4020L rectifiers and replaced them. Now the motor runs, but adjusting the 5K pot does not alter the speed. It's either on or off. If the motor isn't running when I plug the MC-60 in, and then I turn the pot knob, it will start, but after that, the pot doesn't change anything.

    Any ideas?
     
  17. Zero Potential

    Thread Starter Member

    Mar 25, 2015
    64
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    The pot goes to three terminals, obviously. The top one is supposed to be +12V, and the bottom one is supposed to be 0. When the controller is plugged in and the pot is removed, the middle AND top terminals measure 12V, which seems a little funny. I checked the resistance between them, and there is no short. The resistance is somewhere around 130K.
     
  18. Zero Potential

    Thread Starter Member

    Mar 25, 2015
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    HERE IS THE ANSWER: if you're using an MC-60 connected directly to a DC motor on one end and a power cord on the other, CLIP THE RPS-1 RESISTOR!

    This will prevent it from running full speed all the time. Look for a zip file called "MC-60modcolletion.zip." The misspelling is in the actual file name. It doesn't really explain it, because the writing is incomprehensible, but it hints at it, and it contains some useful stuff.

    Do NOT ground the power cord to the chassis. The chassis is a HEAT SINK only. Do NOT let the potentiometer lugs touch the chassis. Do NOT touch the potentiometer lugs; the voltage is only 12, but it's a floating system, so the 12 doesn't mean much. You can get a nasty shock. Ask me how I know.
     
  19. MCU88

    Member

    Mar 12, 2015
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    The idea behind earthing the chassis of an metal box is that, if an high voltage active wire comes loose and touches the box, then the fuse blows. Thus the box is not live.
     
  20. Zero Potential

    Thread Starter Member

    Mar 25, 2015
    64
    2
    If you ground an MC-60 heat sink, you will destroy the controller instantly, and you will have to replace components.
     
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