VFD wiring

[MuzzerParticipant @muzzer]

Posted by Stuart Bridger on 20/06/2017 22:23:29:

Posted by Mike Poole on 20/06/2017 21:45:50:

You could let the scope float by disconnecting the earth, to indicate the earth was disconnected we used to have a long earth and pull it out of the plug top so you could see it, I had a nice shock one day when I had the bare croc clip in my hand and leaned on the chequer plate platform round the machine, the earth cable in the plug top had been left long enough to touch the live pin of the 13amp top in the days before pins were shrouded to protect small fingers so the scope was live. A pretty dodgy setup at the best of times but it works, sooner or later it will bite you though. My boots must have been good insulators as I didn't even get a tingle until I was earthed. Why is an electric shock so hilarious for everyone but the victim? although I was on the receiving end that time I still can't help laughing when it's someone else.

Mike

When I worked in an electronic repair shop, one of my colleagues used this trick. A certain piece of kit had a logic board where for some reason the signal ground was floating at I think about 350V DC. Highly unsafe, but apparently the only way to troubleshoot. Everyone kept well clear when he was working in this way.

Edited By Stuart Bridger on 20/06/2017 22:25:56

When I used to work in SMPS power supply etc development we obviously had to work on live equipment with scopes, power analysers etc. Scopes aren't designed to be "grounded" to live mains, as the user interface (knobs, buttons, display etc) and exposed metalwork are intended to be grounded. We would use isolation transformers to make the AC side somewhat safer in case of touching and generally would use voltage and current isolation probes like the ones Andrew mentioned to investigate high impedence circuits.

In some cases it would be possible to connect the scopes directly to the (largely floating) high voltage circuit, with a 100k resistor between true ground and the scope signal ground. But due to the stray capacitance, measurements made like this tend to be distorted and potentially inaccurate. It would also be done by experienced, well trained engineers. It's certainly not "the only way" and a failure to use safe instrumentation could land someone in serious legal trouble.

Murray

[David JuppParticipant @davidjupp51506]

Posted by Lee Goulding on 24/06/2017 19:08:32:

Hi David

So would I use the existing 4 core wire to wire to the mains section of the VFD ?

Which wiresas there's only a L1, L2 and Earth on the live side of the VFD

Thanks

Edited By Lee Goulding on 24/06/2017 19:09:03

Sorry – I'm not familiar with the switch, the lathe, or your VFD. If you are not clear how to wire it, get some help from an electrician. Personally, I'd probably start again, but it may well be possible and safe to re-use some of the existing wiring if you know what you are doing.

[Lee GouldingParticipant @leegoulding74917]

Hi David

Ok thanks for the advice 👍

[SillyOldDufferModerator @sillyoldduffer]

On the 18th June I posted measurements showing that the VFD in my Warco WM280 was radiating considerable interference. I looked at the lathe and noted three points of concern:

• The lathe is earthed via the mains connection. Not a safety issue, but a glorious opportunity for a long earth wire inside my house to act as an antenna. (My garage is connected to the ground floor ring main and happens to be at the maximum possible distance from the consumer unit and earth.)
• The mains filter inside the lathe is not fully effective, potentially leaking RF to Line and Neutral. (I haven't proved that it actually does leak.)
• The 3-phase output from VFD to motor was apparently unshielded. Actually it is, but the shield isn't earthed. This, I thought, was the most likely source of interference.

Therefore, as a first step I replaced the 3-phase wiring with a shielded cable and fitted brass glands at each end to ensure that the shield was earthed immediately it left the metal control box, and as soon as it entered the metal motor terminal box.

Re-measuring shows that I appear to have made the EMC problem worse!

Next attempt will be to put a hefty filter on the mains input to the lathe. Not sure how long this will take because I have to organise a box to put it in.

Nothing is ever easy…

Dave

[martin perman 1Participant @martinperman1]

Dave,

I was always told that you should only earth one end of the cable sheilding.

Martin P

[SillyOldDufferModerator @sillyoldduffer]

Posted by martin perman on 06/07/2017 13:55:53:

Dave,

I was always told that you should only earth one end of the cable sheilding.

Martin P

Yes, that's what's my sources recommend too. Well worth a try, though not a 5 minute job because I'll have to rip it all out and install another length of shielded cable to try it. (The fittings I used happen to suit grounding at both ends and it's a slight faff to leave one end floating.)

I'm a little suspicious of my shielded cable too. The braid mesh has more gaps than I like.

Dave

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