Limit Switches Tripping when using VFD and Spindle

[prestondocks]

Having purchased a Chinese VFD and Spindle from Ebay I found that as soon as I turned it on my limit switches started to trip. Assuming that this was due to EMF I Replaced the power cable to the Spindle with a Shielded one, but found that the tripping of limit switches got worse. I just finished rewiring all my limit switches with Shielded wire. This has greatly improved the situation but is still not perfect. If the spindle power cable comes within about 500mm of one of my limit cables it trips. This is a real problem for my Z axis home switch.

As I am not a electrician, I don't know what else to try. Can any one suggest what I should do next?

Thanks
Simon

[Al_The_Man]

Do your limits switches go directly into the parallel port or through a BOB?
It is most likely a earth grounding issue.
Do you have Ground bonding wires run back from the spindle and gantry to the VFD and to a central earth ground point?
Are your motor wires twisted tightly and preferably run in flexible metallic conduit
All metallic parts of the machine should have a ground bonding conductor coming back to the central earth point.
Al.

[prestondocks]

Hi Al,

Your really helping me out this week. Thanks very much.

when you say do I have a ground wire running back from the spindle to the VFD. Can you elaborate. There are just three wires going to the spindle, all are power. Are you saying I should attach a ground wire to the spindle casing? If so, do I attach the other end to the Mains Earth in the VFD?

Thanks
Simon

[Al_The_Man]

I have noticed before that some of the ROC water cooled 3ph spindles only have 3 wire plugs on them, they should have 4 pin, one for ground, you should have one from motor frame that can go back to the VFD ground, ideally this ground should also go to a central Earth ground point on the machine, proballey yours goes back to the 220v power socket?
Also, as the spindle is usually on a powered slide, the gantry itself should also have a ground bonding conductor back to earth ground.
I have a couple of forum links on my other PC that I can post when I get back to it.
Al.

[UUU]

I had a simlar problem - but I'm using my limit switches also as home switches. This is in Mach 3. So I can reference all my axes using while the spindle isn't on - then use soft limits to control the extent of table movement. Not perfect, but it gets round the problem until I can sort out what's causing the crosstalk between he circuits.

[Al_The_Man]

Here are a couple of past links to digest.
http://www.cnczone.com/forums/phase_...lp_please.html
http://www.cnczone.com/forums/phase_...se_issues.html
http://www.cnczone.com/forums/genera...questions.html
Grounding.pdf - DivShare
http://www.automation.siemens.com/do.../emv_r.pdf?p=1
Al.

[prestondocks]

Thanks Al,

I have take a quick look already at some of the links you posted and got some pointers to things I have not done yet, like grounding the shielding.

I also had a thought, that although my motor is 3 phase and has no specific ground connection, by connecting the shielding to the power connector (as I should have already done), I can connect the other end to earth in the VFD.

Thanks very much for your help, I will let you know how I get on.

Simon

[prestondocks]

Just want to say Thanks Al. The problem did seem to be because of a lack of grounding. I have not tried any cutting yet, but I have run the spindle at full speed and jogged around the table with out any of the home switches being fired.

The key problem seemed to be the lack of grounding in the Parallel port card. This only had a two wire 12v power supply. I have wired one of the ground ports to the earth in the VFD.

If I remove this ground cable the problem returns. So I am sure I will not have problems from now on. If I get time I will try to post a short video on youtube and put a link in here to help others who might be having the same problem.

Thanks again
Simon

[Al_The_Man]

Thats good.
What I do on all PC based systems that have an external device powered by the PC. P.S. is to make sure the P.S. common goes to a central Earth Ground point.
Many overlook the fact that the PC. P.S. has a common connection to the spindle through the grounded neutral.
Al.