VFD (?) tripping RCD

[Jonathan]

[Edit - sorry this has ended up being a very long post. Wanted to mention everything so that everyone doesn't reply saying to do something I've already done!]
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Refrained from posting this yesterday afternoon when the problem started as I expected to have found what's wrong by now. Sadly that's not the case.

In short RCD has started tripping when I run the router.

It's happened 4 times this weekend, all on the same aluminium part. Today and yesterday the machine ran for roughly half an hour then the main 40A breaker for the workshop went (switching off everything including the lights which is a bit annoying). Yesterday when that happened I switched everything back on, zeroed it ... lasted about 30 seconds and tripped again. Then I tried to start again, but found the parallel port on that computer is now broken :cry: Switched to backup computer, all seemed fine after running for a couple of minutes then bang... electric went again. By that time it was past midnight and I'd spent about 5 hours trying to cut a simple Z-axis plate, so I left it. Tried again today, same again ... worked for about half an hour then tripped ... then worked for 10 mins without tripping which was enough to finish that part. At least the cutter survived every time. Spindle was left at 185Hz throughout ... not getting hot either.

The RCD trips at a seemingly mundane time - not when the VFD is turned on, or when the current changes quickly or anything like that.

Things I've tried, all of which made no difference:

• After the first fault I measured the voltage between machine frame/spindle at various points and earth. Less than a volt AC and DC in all cases. Machine frame and spindle are both earthed.
  
• Measured the voltage above with the spindle running, same result. If I disconnect the earth wire I get 180VAC between frame and earth...so I didn't do that for long!
  
• Measured voltage as above between everything I could reach with the meter with the spindle running and touching the aluminium it was cutting.
  
• Switched of all non essential appliances - i.e. left air compressor, heater and de-humidifier off. So just stepper drivers, spindle, computer+monitor and lights. It has only tripped with the router on, never without so it must be the router or something related.
  
• Connected the earth wires a bit more securely.
  
• Cleared about 5kg of aluminium swarf, made by the router, off the floor. Though it might be conducting or something...yes I was desperate!
  
• Searching on google.
  
• I've not changed anything since last weekend and the weekend before when the machine was fine. Even using the same cutter and same aluminium!
  
• Measured current to earth ... negligible reading.
  
• Measured insulation resistance from the 3 phase wires to earth (spindle body). It's high, megaohms.
  

I've heard and read before that you shouldn't use an RCD with a VFD, but mine's been working fine for almost a year now with one so I didn't worry about it. How could it suddenly be causing a problem. I don't really want to just bypass it without knowing why it's tripping and without good reason.

Maybe the VFD is on it's last legs? Hope not...bit worried about starting again as whatever the fault is appears to have already killed one computer which is quite impressive seeing as it's optically isolated!

Really want to get this sorted soon as I've got parts that need cutting asap.

Left my oscilloscope at uni, so if it's not sorted by next weekend I'll bring it back to aid diagnosis - might pick up something the multimeter wont.

I'll go and make some bits on the lathe now, which I power from the same VFD (got a big switch to swap them over). Hope it works.