VFD cable article

[Kitwn]

On another tack I did a bit of searching whilst nibbling at my spicy turkey bun this lunchtime, and quite a few people do these 3+3E VFD cables, however sadly no-one seems to make them in less than 1.5mm2, and they all seem to have a pretty massive minimum bending radius, typically 20x diameter, which given the large cable OD's (10...13mm for 1.5mm2) is not useful for the typically "small" machines the likes of us are making.

Plus the manufacturers of small machine spindles like ours would have to bring both ends of the three windings, plus a separate casing earth connection out to a seven pin plug.
Readers should also beware of some cable sizes quoted in diameter and others in cross-sectional area.

Kit

[Voicecoil]

Plus the manufacturers of small machine spindles like ours would have to bring both ends of the three windings, plus a separate casing earth connection out to a seven pin plug.
Readers should also beware of some cable sizes quoted in diameter and others in cross-sectional area.
Kit

I'm pretty sure there wouldn't be any problems with commoning up the 3 earth conductors to a single pin at the spindle, it's all about balancing inter-conductor capacitance along the length of the cable, a few cm at the far end is unlikely to upset things. And the outer screen would be best connected only at the VFD end to avoid HF earth currents, so likely you could do it with a standard 4 pin plug - if you could fit the rather fat cable into said plug of course.

[Kitwn]

I'm pretty sure there wouldn't be any problems with commoning up the 3 earth conductors to a single pin at the spindle, it's all about balancing inter-conductor capacitance along the length of the cable, a few cm at the far end is unlikely to upset things. And the outer screen would be best connected only at the VFD end to avoid HF earth currents, so likely you could do it with a standard 4 pin plug - if you could fit the rather fat cable into said plug of course.

I have to say I'm unconvinced that's worth the expense. Coming from a communications background, to avoid significant radiation of noise I'd want each winding fed with it's own balanced, twisted pair. Telephones have been doing it this way since Alexander Graham Bell was in charge, twisted pairs minimise both radiation and reception of noise, and Ethernet cables are done the same way. I don't know if these VD cables are constructed as three twisted pairs but for minimal noise radiation they would need to be.

While we're on about it, using a twisted pair rather than loose wires for connecting limit switches etc. to your BOB might help reject noise pickup even for non-balanced inputs.

Kit

[Voicecoil]

. I don't know if these VD cables are constructed as three twisted pairs but for minimal noise radiation they would need to be.

From what I read it's more of 2 interleaved twisted triplets - a bit like 1.5x the starquad cable the BBC OB guys I worked with were so fond of in high noise situations. You might be right in that at 3x twisted pair would work better as far as EMI goes, it would all depend on the switching points of the VFD. However it would make the cable even more bulky and/or increase copper losses, so that's probably why it's not been done. It would be nice to investigate this further, but as no-one makes a 1mm2 such cable and it probably won't matter much on a 2.5m run, I have more important things to concentrate on.

While we're on about it, using a twisted pair rather than loose wires for connecting limit switches etc. to your BOB might help reject noise pickup even for non-balanced inputs.
Kit

I've already done this. The leads on the proximity sensors were woefully short, so had to be extended anyway, so I chopped them back to a sensible minimum length and extended with my favourite Mogami screened twisted pair cable. Why they don't make such sensors with a screened cable to start with I don't understand.

[Kitwn]

........a bit like 1.5x the starquad cable the BBC OB guys I worked with were so fond of in high noise situations

Ha! Owning up to an Auntie connection now! I haven't heard about star quad since my earliest days as a trainee back at Wood Norton back when Noel Edmonds was a lad.
I'm sure the cable manufacturers are brainy enough to construct their products in a suitably 3-phase field-cancelling way.

Kit

[Voicecoil]

Ha! Owning up to an Auntie connection now!
Kit

Back in the day I worked for a company supplying audio equipment to the Beeb, mostly for OB trucks, got to know one or two of their engineers quite well during the acceptance testing. I got to see some of the syllabus from the training courses at Wood Norton, very good stuff: when it got watered down/shut down/hived off it was a great loss to the industry IMHO.