Lathe/VFD/DRO issue

That is just standard screened 3 core + earth 3 phase screened cable. It's not the VFD special triple earth shown here:
https://inverterdrive.com/group/AC-Motor-Power-Cables/3-Phase-symmetrical-shielded-EMC-motor-cable-2m/

If that is what you ordered you need to have a word with the supplier. They might have been given the wrong cable.

Robert.

Edited By Robert Atkinson 2 on 01/07/2023 13:19:06

Just sent an email with a photo of the cable, I wait with interest to see what their reply is and how fast they reply!

Update on the cable :-

Inverterdrive got back to me at 9:45 am and agreed it's the wrong cable, seems their supplier sent the wrong reel and my cut was the first off! They are sending a replacement overnight for me, first rate service in my eyes.

The filter from Robert arrived this morning with an included 'donut' as well so I'm praying for a stormy wet day so that I can argue for having a day in the workshop.

Regards

Roger

Now that I will have the necessary bits to 'hopefully' remove the interference from my vfd/motor/mains in cabling I just have a couple of queries:-

Is it bad practise for the mains in and the vfd to motor cable to cross over or should I route the cables away from each other?

Can I tie all the earths onto the mounting plate of the enclosure and then take one into the vfd?

So the final question is - do I earth the screen earth both ends or not?

Regards

Roger

Keeping cables apart helps. Try to run them so they're not parallel. If they have to cross do it at a right angle.

Earthing - try both. I'd start by earthing the VFD to the mounting plate, and if that doesn't work try earthing it separately.

Which end to earth, or both is much debated. The only printed advice I've seen was for double shielded wire, which suggested the inner be earthed at the noisy end, and the outer at both ends. I'd start by earthing only at the VFD end, and if that didn't fix the problem, both ends. Note that an earthed shield shouldn't be used as a safety earth. The safety earth has it's own wire, preferably inside the shield.

The cure is uncertain because EMC travels by 3 different mechanisms and what needs to be done varies by frequency. A Radio Engineering problem, Electrical, and the way wires behave when high-frequency power is applied to them is counter-intuitive and unpredictable. So expect to try different combinations.

If Robert RPI reads this, would Ferrites help? My feeling is not.

Dave

The one thing that confuses me is the difference between the safety earth and shielding earth, unless I'm supposed to ground the shield earth/s (there are 3 more in the main cable) to some other 'earth' other than the mains earth then surely earth is earth and thus the mains cable carries the difinitive earth?

I'm old and confused

It is important to connect the motor earth conductors to the incoming earth supply at the VFD, and as said start by connecting the screen at the vfd end only and test.

If you are using the correct glands then if the motor terminal box is metal you will need to insulate the screen from the gland, or use a plastic TRS gland in the first instance, revert to the correct gland for connecting the screen to the motor earth conductor when/if you want to connect the screen at both ends.

In your case because you have only a 2M run and a small load the reduced size of the 3 earth conductors will no doubt be satisfactory to pass an EFLI test, however they may need a suplementary earth conductor if used with large motors and/or long cable runs.

Emgee

Hi,
On earthing, the mains input earth should go directly to a stud / bolt on the bare metal plate. The VFD and filter both need a wire to taht stud/bolt or another one next to it on the plate AND be bolted directly to the plate. The cable screens need to go to a 360 degree gland either where it enters the box or to a angle bracket bolted to the metal plate and grommets or plastic strain reliefs where they enter the box. For the "triple ground" wires In the motor cable I wound take all three to a bolt on the metal plate close to the VFD terminals. This could be one of the VFD mounting bolts. At the motor end take all three to the earth terminal in the motor.

On screen grounding, it must be connected to the plate at the VFD end. At the motor end it is a choice as SOD says. Personally I'd try ungrounded at the motor end first. If still having trouble try 360 degree grounding. There is no right or wrong way.

Robert.

Just noticed Dave's question on ferrites. Yes, ferrite chokes on the VFD cable can help. They mostly help with high frequencies. There are different grade grades of ferrite for different applications and they can be a ring or sleeve. Unless the VFD manufacturer makes a specfic recommendation you have to use judgement for initial choice and then make measurements to check on effectiveness.
I sent Rockingedge a ferrite ring (donut) of a type that may help. The 3 motor wires are passed through this with it located at the VFD output before they go into the screened part of the cable.

Robert.

Should I / can I bring the dro panel earth into the enclosure and earth it there as well?

Not a good idea IMO, use the power source earth.

Emgee

I concur with Emgee. keep VFD and DRO as separate as possible. or instance don't plug them int the same extension lead. Take each back to the socket on the wall.
Best to do one thing at a time. If you are still having issues with the fancy motor cable, filter and box on he VFD we can look at improving the immunity of the DRO.

Robert.

Earthing is complicated and done for different reasons.

• In an electrical supply system, generators produce a 3-phase output which is distributed over three wires, with no earth. 3-phase is inconvenient for most domestic purposes: a 3-phase light bulb or TV would be bonkers. Also more dangerous in a home than need be because all three wires are 'hot'. Instead, 3-phase goes to a sub-station, where a transformer outputs 3 single-phase supplies, each of which serves different properties. 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.
• The movement of clouds often charges them with static electricity. The charge can become enormous because they're big and well insulated. Eventually the potential difference between cloud and ground breaks the insulator (air), and a lightning bolt jumps, often via tall objects like chimneys and towers. Several hundred amps flow causing considerable explosive damage. Bricks and mortar are blown apart when moisture inside flashes into steam; wood catches fire; and people are electrocuted or hurt by flying debris. The chance of a strike is reduced by fitting a lightning conductor, the wire and earth of which are much beefier than an electrical safety earth, which is inadequate for lightening protection. To avoid a lightening strike getting into a building's electrical system, lightening protection and supply safety earths are not connected together.
• In a long or medium wave radio transmitter, the planet acts as one side of a dipole antenna, BBC R4 LW transmits on 1500 metres. An efficient dipole at 198kHz would be 750 metres high. Instead, the planet is electrically balanced against a smaller aerial wire. Efficiency depends on achieving a very good earth at 198kHz, miles of wire spread over a few hundred acres. Although considerably more elaborate than a lightening earth, it isn't a good lightening earth, so at least one of those is required as well. Plus an electrical safety earth for the transmitter building. None of these earths are connected together.

EMC is yet another case, albeit closer to the radio example than the others. EMC uses shielding to stop inductive, capacitive, and electromagnetic coupling. As the coupling is usually electrically unbalanced, shielding won't be effective unless it too is earthed. And because shielding often consists of bare metal, it's necessary to ground it for electrical safety reasons. Unfortunately electrical safety earths are rarely good Radio earths. One problem is the earth wire on the lathe connects through a longish lead to a power socket. Then the supply earth wire runs around the building until it reaches the Consumer Unit, probably tens of metres away. From there, it's connected to the supply earth, probably some distance away. To a high frequency wave, all this wire looks more like an antenna or transmission system than an earth. It becomes a good way of transferring EMC further, not a way of stopping it. And when it gets to the actual safety earth, perhaps just a few feet of copper rod banged into the ground, 1ohm at 50Hz can behave like thousands of ohms at Radio Frequency.

Ideally, there should be an EMC earth near the VFD. However, as this could be difficult, expensive, or break electrical safety rules, it's usual to use the electrical safety earth and hope it will do. The safety earth may need help! 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

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

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

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

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.

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

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