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Author Topic: Limit switch error when connecting VFD shielding to ground???  (Read 2632 times)

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I was having problems with the machine getting false positives saying the limit switch had been triggered in the middle it running, the diagnostic screen showed no switches enabled. After hours of troubleshooting, turning off inputs, etc. I found that two things were causing it:

1) if the ground from my PC to the C10 controller board was not connected to earth ground
2) if the shielding on my VFD cable was connected to the ground pin in the VFD

I do not understand why connecting the VFD shielding to the ground terminal on the VFD would cause these limit switch errors, does anyone have an idea? That shielding is not connected on the other side, there is no connections between the VFD and the rest of the system (it is ran stand alone) I just cant figure it out.

Thanks for any feedback. For now I just disconnected the VFD shielding to keep working.

Offline Davek0974

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Re: Limit switch error when connecting VFD shielding to ground???
« Reply #1 on: April 28, 2016, 02:19:44 AM »
do not gound the pc port side to ground, the control side of the pc is usually isolated, connecting it to ground can cause serious ground fault loops as the supply side of the pc is grounded as well.

not sure about the vfd part, is this supply or motor side cables?

i do not use shielded cable on the supply side but if i did i would only ground it at the supply point not the vfd, the protective ground wire is obviously connected at both ends and again from the output side to the motor, the output cable shielding is only connected at at vfd point.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives
Re: Limit switch error when connecting VFD shielding to ground???
« Reply #2 on: April 28, 2016, 11:56:55 AM »
I have attached a photo of how I have the VFD wired. The 220v from the wall has three wires. I have the two hots going to 'R' and 'S' on the left and the Ground going to the ground point on the far right side. The wire is not shielded from the wall as it is just a modified extension cord. The wire going out to the Spindle from the VFD is shielded, it is not connected to anything on the spindle end and when I connect it to the same ground lug on the far right you see in the picture on the VFD end it throws a "Limit Switch Triggered" error in Mach3 as soon as I start it up. Again, the VFD is completely isolated from anything else on the machine, it is not wired anywhere near the control box, to the PC or anything... If I disconnect the shielding from the ground lug than I do not get the limit switch error unless I disconnect the PC side ground from earth ground as explained below...

Okay, so the other issue causing "Limit Switch Triggered" error is if I don't have the PC side 5v connected to the controller box earth ground. I thought that I was supposed to isolated the PC power from the ground used by the A/C and toroid but if I do I get the limit switch triggered error (with no further details). Here is my setup, perhaps it will be obvious to one of you where my problem lies:

1. I have 5vdc from the Computers USB port going to the C10 control board to power it.
2. I have 12vdc from the Computers Power Supply going to the Opto22 relay in the control box to turn on 120v AC power to the toroid.
3. The limit switch shielding is all bundled in the box side together and attached to the earth ground bar coming from the wall A/C
4. The 5vdc from the Computers USB port has the ground wire also connected to the control box ground bar (if it is not, I get the "Limit Switch Triggered" error as soon as I turn on the VFD)

Also, it probably goes without saying but when I get these "Limit Switch Triggered" errors there is not actually limit switches that are triggered on on diagnostics page...

Hope these details are enough to help. Thanks!
Re: Limit switch error when connecting VFD shielding to ground???
« Reply #3 on: April 28, 2016, 03:51:46 PM »
I have an update on this. I am able to run the system without "Limit Switch Triggered" errors without the PC 5v ground connected to the chassis ground if I remove the 100ohm resister on the 5V COM on the limit switches as shown in this diagram: http://cnc4pc.com/Tech_Docs/3_C10R10.pdf

Offline Davek0974

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Re: Limit switch error when connecting VFD shielding to ground???
« Reply #4 on: April 28, 2016, 03:58:18 PM »
Sounds a bit like the 100ohm is stopping the inputs being pulled far enough down and the logic level is on the border between on and off, removing the resistor pulls it down harder.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives
Re: Limit switch error when connecting VFD shielding to ground???
« Reply #5 on: April 28, 2016, 04:04:19 PM »
So is this an indication of another problem or should I just leave the resistor off and call it good?

Offline Davek0974

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Re: Limit switch error when connecting VFD shielding to ground???
« Reply #6 on: April 28, 2016, 04:12:00 PM »
I'm not familiar with that board so I cant really say yay or nay - not sure why they want a restore there - none of my machines have it.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives
Re: Limit switch error when connecting VFD shielding to ground???
« Reply #7 on: April 28, 2016, 04:31:08 PM »
You have a ground loop path through the PC. It's bad practice to use the PC 5V or 12V power supply to power external BOB's. The VFD cable shield should be connected to a positive earth ground along with VFD ground terminal.  Its very possible your circuit breaker panel neutral and panel ground are not connected together and by connecting the VFD shield it's creating a ground loop. Most building codes in the US mandate neutrals and ground are seperate.
Re: Limit switch error when connecting VFD shielding to ground???
« Reply #8 on: April 28, 2016, 04:40:10 PM »
What is an external BOB?

The panel on my shop does not have the neutral and ground busses connected but they are connected on the house box. I remember because subpanels are not supposed to have the ground and neutral connected.

I do not understand what you mean about a "positive earth ground". The 220v coming into the VFD has two hots and a ground, I confirmed that the 220 ground wire does connect to the sub-panel ground do I need to ground the VFD ground with the Chassis ground in the control box (which is the grounding bus for everything in the box including the 120v ground)?

I think what confuses me the most is how the VFD shielding can effect anything considering it is only connected on one side, there is nothing for it to conduct.

What should I do in my setup to fix this or do you recommend keeping the VFD shielding not connected to anything?

Re: Limit switch error when connecting VFD shielding to ground???
« Reply #9 on: April 28, 2016, 04:56:49 PM »
You don't have two hots and a ground... you have two hots and a neutral. Neutral and ground are two separate animals. Sounds like what you are doing is using the panel neutral as a VFD ground. Run  #10 or 12 wire from a earth ground such as a water pipe or circuit breaker ground bus to the machine. Then connect the VFD and VFD shielded to the new ground wire. Then use a separate power supply for the C10 or BOB (Break Out Board). Do not use the PC power supply for external device powering.