Lathe/VFD/DRO issue

[Robert Atkinson 2Participant @robertatkinson2]

Posted by SillyOldDuffer on 04/07/2023 17:47:31:

Posted by Rockingdodge on 03/07/2023 17:33:45:

…

BIG SNIP

I see Robert supplied a Ferrite: I have no experience with these and VFD interference, but in principle they 'choke' wires, making it hard for EMC to get further whilst having no effect on normal function. Not as effective as a properly designed filter, but might be good enough, especially in conjunction with other measures, In that vein, experiment with earth points on the machine. Sometimes adding another earth point will successfully short out the EMC. In other cases, a new earth point creates a loop that makes it worse. Finding the best combination can be hard work, and maybe there isn't one.

Professionals approach the problem rather differently. When money is no object, EMC is conquered simply by following best practice from end to end. Everything is comprehensively shielded and filtered to stop electrical muck getting in or out. Not cheap…

Dave

 

EMC ferrites while they do increase inductance they are different to a conventional choke. A inductor (coil, choke) limits AC current flow and develops a voltage across it. A good inductor has very low loss. EMC ferrites are designed to be lossy at high frequencies. As a result they absorb energy converting the interference to heat (a tiny amount). A coil of wire with the same inductance would be less effective.
They do work. See post by Tony Pratt 1 near the bottom of this (rather contraversial in other areas of the thread) page: https://www.model-engineer.co.uk/forums/postings.asp?th=176304&p=4
A sleeve cured interference emitted by a DRO.

On EMC earths Emgee is correct. I would add that for TNC-S (PME) earthed supplies there is a particular hazard with connecting additional "earths" to the mains earth. In the case of a failure of the neutral conductor outside your premises the load current for all the houses affected by the fault can try to flow through your extra earth connection. This can result in damage or a fire. It's a particular problem for radio amateurs who tend to like large earths.. Don't do any creative earthing without consulting a competent electrician who has seen your installation.

While professionals will comply with the regulations, certainl for production designs you don't throw everything at the system. There will be a systematic approach to meet the emission and susceptability requirements. This may meen repeated iterations of measure, modify, measure… If you are producing thousands of an item you dont want to use a £3 filter if a £2 will do the job. The EMC components do not affect the noramal operation of the item which is why a lot of the cheap items leave them out.

Robert.

Edited By Robert Atkinson 2 on 05/07/2023 07:56:26

Edited By Robert Atkinson 2 on 05/07/2023 07:57:01

[SillyOldDufferModerator @sillyoldduffer]

Posted by Emgee on 04/07/2023 20:09:50:

Posted by SillyOldDuffer on 04/07/2023 17:47:31:

. Single phase has two wires, both of which would be 'hot', except one is earthed by the supplier at the transformer. Thus homes get two wires, Line (or Live), and Neutral. Although Neutral carries current, it's safe because it and anyone likely to touch it are all at earth potential. Also, each house has it's own separate earth (details vary). The house earth protects against failure of the supplier earth and also allows anything in the building that might accidentally become hot to be grounded. So if a bare Live wire accidentally touches a hot-water pipe, fuses blow rather than allow the plumbing to become shock hazard. House earths are designed and implemented in a particular way, affordably fit for purpose, but not suitable for everything.

Dave

I believe some points in your statement need more discussion.

The neutral conductor cannot be termed safe as if the neutral conductor is broken between the load of a piece of equipment on the circuit and the consumer unit it will be capable of giving a full mains voltage shock to a person or animal if they provide an earth path for the voltage to travel down.
The circuit may or may not be protected by an RCD to detect the fault to earth, so current flow would only be limited by the body resistance and eventual circuit protective device operating to isolate the circuit live.

Many properties rely entirely on the suppliers earth (PME) and have no independent earthing arrangement, it is only in mainly rural areas where no supplier earth is provided where the installation will be provided with it's own earthing system, usually copper rod/s depending on soil resistance.
These usually overhead service (TT) installations to comply will always be protected by an RCCB (RCD) which can be 30ma or 100ma operating current so still no guarantee of the neutral being a safe conductor.

Emgee

Agreed – I was trying to keep the example simple! I hinted it was more complicated, by saying 'Also, each house has it's own separate earth (details vary).' Here's an example in which an 'Extraneous-conductive-part' is connected, often not present. But note the 'PEN Conductor' is earthed by the supplier at the transformer, and also along the cable. One of these earths is likely to be nearish to the house. The Neutral wire is 'safe' because it cannot normally be above ground potential. Perhaps 'safer' is more accurate than 'safe' because there are faults that cause 'Neutral' to become 'hot'.

My main point though is that a mains safety earth is very unlikely in any configuration to be a good radio earth. The wires are too long.

Dave

[Robert Atkinson 2Participant @robertatkinson2]

Dave is correct, the mains "earth" is not a good radio earth. That is why the VFD and any filters screens should be mounted on a conductive surface with the cable screens connectd 360 degrees to this surface. A good, typical design uses a bare metal plate mounted at the back of a metal enclosure. The low impenace plate causes interference currents to flow through the smalleas possible area which limits radiation and generation of voltage.
The outer metal case helps with interference but is mainly for electrical safety and mechanical protection.

Robert.

[not done it yetParticipant @notdoneityet]

Any resolution to this thread? Soon be a month since started and seemingly no working fix. Or is the warning of not buying cheap (possibly sub-standard) kit, from the likes of vevor, the best advice – for all those reading this thread and considering purchasing similar kit?

[SillyOldDufferModerator @sillyoldduffer]

Posted by not done it yet on 16/07/2023 08:31:39:

Any resolution to this thread? Soon be a month since started and seemingly no working fix. Or is the warning of not buying cheap (possibly sub-standard) kit, from the likes of vevor, the best advice – for all those reading this thread and considering purchasing similar kit?

Not buying cheap as a way of fixing problems is always dubious. The money might be blown on something that's 'reassuringly expensive' rather than 'fit for purpose'.

Unlikely that a Vevor VFD arrives fitted with a built-in filter, but neither do much more expensive VFDs. Filters are usually an optional extra because whether they are needed or not is installation dependent. VFDs are components rather than ready to go consumer items, and – if there's trouble, EMC is a complicated subject.

Model Engineers are awkwardly placed when buying stuff. Our purchases often require technical understanding but we're not fully informed professionals. Smarter than the average consumer and able to sort many problems out, but liable to get horribly out of our depth when a misbehaving technology is alien to us. When buying anything that's not a commodity, reading and understanding the specification is usually more important than making general assumptions based on price or brand-name.

I'm not surprised Rockingdodge's bad boy is taking time to fix: EMC problems are often right pigs, and there are always at least two different equipments involved. (In this case the VFD and DRO, either or both of which may need attention.)

If spending money is no object, the answer is to start again. Contract a firm to choose and install a VFD and DRO that work together. As this approach is costly, I'd go DIY because there's a good chance a home installed VFD and DRO won't interfere, or a mild case is easily fixed. But it's a risk.

What's the correct Model Engineering attitude to risk? As my bulging junk box proves, rather a lot of Model Engineering goes wrong despite my best efforts! Either we're all masochists, or fixing stuff is part of the fun.

Dave

[Roger ClarkParticipant @rogerclark]

Posted by not done it yet on 16/07/2023 08:31:39:

Any resolution to this thread? Soon be a month since started and seemingly no working fix. Or is the warning of not buying cheap (possibly sub-standard) kit, from the likes of vevor, the best advice – for all those reading this thread and considering purchasing similar kit?

SOD, well said,

Dave, in answer to your query it is just 10 days since the last post and in regard to my purchases and 'timely fixes' I'll just say this – I have no problems or issues with the quality of my purchases and have no issue in recommending them, when I bought my vfd to run my 1937 Denham lathe it did so with no issues. When I bought my dros' for my Warco mill they worked as expected and with the accuracy I expected, the only issue arose when I fitted my lathe with the same brand of dros, it happens!

Robert and others in his profession are well paid (I hope) to plan/fix/avoid this very issue with, I assume, much more expensive and complicated equipment than we are likely to use so why do you point the finger at my purchases rather than accept that this kind of issue can happen? It is quite definite that my installation left a lot to be desired hence my reaching out to this forum and the experts within to help me understand and fix the issue.

As to the length of time this thread has been running I do sincerely apologise but beg forgiveness as in this time I have also built a tool store according to SWMBOs requirements, battled with my own health issues as well as my wifes and done a number of trips helping my son to move house stuff into storage prior to their impending house move, as they say life always gets in the way but I promise to try and do better!

Roger

Edited By Rockingdodge on 16/07/2023 11:40:03

[Robert Atkinson 2Participant @robertatkinson2]

While I woulld not reccomend any Vevor mains powered equipment (the fact thay can't get it past UK import inspections supports this), it is not just the lower end of the market that can have EMI / EMC issues.
Some years ago a major international drive producer, based in the USA, replaced an established line of motor drives with a new and impoved model. As usual for a component they were not CE marked but they had done tests to assure customers that their finished products could be compliant.
Unfortunatly we, and a number of other customers, found our products failed emissions with the new drives fitted. The drives can be used in multiple configurations and the OEM had not tested all configurations. I managed to sort our machines out with custom filters on control lines. These lines carried high speed signals so "brute-force" filtering would not work. The OEM were still having troublesolving the issue. They paid for us to do testing with my filters at a UK test house and having seen that they worked, paid for me to go to their development lab in the USA and work on an internal fix. I think they chose us because we were working the drives at the edge of the performance envelope and also were testing to the laboratory emissions standard rater than industrial that covered most of their customers. Together we engineered a internal fix for production drives. I had to visit them because for IP protection they would let me see their design data but not have a copy. I believe we got a substantial discount on all the drives of that range that we bought in the future.

This shows why VFDs are not (or should not) be UKCA or CE marked. The OEM cannot possibly test all configurations a VFD can be used in never mind different cable and enclosure designs.
Legally, even as hobbyists if we build our own system with a VFD we are obliged to ensure it meets the essential requirements ofr various legisaltion including machine safety, electrical safety and EMC. Of course in practice theis is seldom done. Many small businesses don't do it either.
Machine and electrical safety tend to ony affect you and others in your workshop so are less of an issue. You are allowed to judge your own risk.
EMI/EMC however ann affect others, often when everthing is working fine. As hobbyists our best defence is to use quality, branded equipment that comes with comprehensive documentation that includes guidance on EMC and follow the guidance. Most usres of VFDs are not EMC experts so reputable suppliers give good advice to help their customers.

Robert.

[Roger ClarkParticipant @rogerclark]

Well, hopefully, a final update.

The vfd is now in a cabinet along with the filkter kindly donated by Robert also fitted the donut, the cable is grounded to the cabinet and the 3 earths taken to one of the fixing bolts on the mounting plate and the filter is grounded as well. I have grounded the cable screen to the motor as well as it is a pain to access for the wiring so decided to do it all once and once only.

The good news is that the dro display is now rock steady with the lathe running so many thanks for all the advice and support I've received, especially from Robert who went above and beyond.

Regards

Roger

[Robert Atkinson 2Participant @robertatkinson2]

Hi Rodger,

Great news. I'm glad you got it sorted.

Robert.

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