Lathe/VFD/DRO issue

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Rockingdodge on 24/06/2023 12:25:49:

Just got the cable delivered from Inverter Drive, downside was the postage cost about the same as the cable

If I were to go down the metal wall box route how do I go about cooling the vfd without comprising the shielding?

Roger

As long as the box is large enough to provide the clearnce in the manual it should be OK. Additionally holes or mesh with holes less than about 6mm will not affect the shielding.

Let us know how you get on with the new cable.

Robert.

[MacolmParticipant @macolm]

Screened cable can be useful in managing emissions, but be aware that high frequencies can propagate through it as a coaxial mode. Unlike ventilation holes which work as short sections of cut-off waveguide which have a lower frequency transmission limit, coax has no lower cut off frequency. Thus it should be used to provide distance between the interference and the susceptible circuit.

If things need to be in close proximity, consider feed through capacitors, failing which any filter needs to be connected with as short wires as possible. This should be as near the interference source as possible.

[Roger ClarkParticipant @rogerclark]

Posted by Macolm on 24/06/2023 17:30:25:

Screened cable can be useful in managing emissions, but be aware that high frequencies can propagate through it as a coaxial mode. Unlike ventilation holes which work as short sections of cut-off waveguide which have a lower frequency transmission limit, coax has no lower cut off frequency. Thus it should be used to provide distance between the interference and the susceptible circuit.

If things need to be in close proximity, consider feed through capacitors, failing which any filter needs to be connected with as short wires as possible. This should be as near the interference source as possible.

Sorry Malcom,

That kinda went way over my head, I'm aware of capacitors in electronic circuits and motor start and run ones but saying I should consider feed through caps. means nothing to me without an explanation of how they're connected and what values would be needed, I'm assuming the filter you mentioned is one on the mains input side which we will look at if required.

Is your paragraph about screened cable referring to my current cable, which seems to be the initial source of my interference or the cable which I will replace it with?

Thanks for your input but please expand on it for a non-expert in this field.

Roger

[Robert Atkinson 2Participant @robertatkinson2]

Macolm,
While you comments are not wrong, they do not really apply to Rodgers specfic case of a VFD motor connection.

Robert.

[MacolmParticipant @macolm]

Screened cable of itself does not attenuate interference much, interference entering the cable appears little reduced at the other end. Thus both ends should consist of closed metal boxes, and the screen be well connected at each end. This should confine the interference (provided it cannot escape somewhere else). This is why it is sensible to use some form of 360 degree termination of the screen rather than just a single wire to an earth point.

As others have suggested, the mains input should also be filtered to limit escape by that route. Again, the control pendant is another possible escape route, but hopefully the maker of the VFD will have taken some precautions here.

Finally, the motor might not constitute a particularly good screened box, in which case capacitors or other filter components connected at the VFD or within the motor might help. Ensure any such items are rated for the correct mains voltage at least. Be careful! Assuming 240 volts, examples;

https://www.bitsboxuk.com/index.php?main_page=index&cPath=65_81

Unfortunately it can be very difficult to work out what is going on, and many of us have had experience of intractable problems with interference and filtering. Carefully noting what happens when changes are made can build up some insight. Even if something makes matters worse, that information may prove useful.

[Robert Atkinson 2Participant @robertatkinson2]

Macolm,
Nothing personal, but you appear to have little knowlege of VFDs. You MUST NOT connect capacitors across the motor output of a VFD (unless the manufacturer specfically says otherwise). The output of a typical VFD is a series of high frequency pulses. Connecting a capacitor across the output will cause excessive current and probable destruction of capacitors and VFD. If output filtering is required it must be inductive. A simple ferrite sleeve can help with high frequences. Thes can be pplied without too much concern. For lower frequencies a inductor is required in each line. These are often called "reactors" by VFD manufacturers. You need to follow the mnufacturers guidelines when selecting them. They also need higher voltage insulation than you might expect due to the high dI/dt.
Simplified explanations of EMI/EMC are not adequate for complex systems like VFDs.

Robert.

[noel shelleyParticipant @noelshelley55608]

Robert, with the material the spiral is made of being a chromed brass strip and each turn interlocked with the next and provided the 2 ends have continuity with the fittings please explain how or why it will not provide screening. Each turn of the spiral is part of the strip and any gaps between turns are tiny and the interlocking is such that there is no line of sight to the conductors. Noel.

[Robert Atkinson 2Participant @robertatkinson2]

Posted by noel shelley on 25/06/2023 13:24:44:

Robert, with the material the spiral is made of being a chromed brass strip and each turn interlocked with the next and provided the 2 ends have continuity with the fittings please explain how or why it will not provide screening. Each turn of the spiral is part of the strip and any gaps between turns are tiny and the interlocking is such that there is no line of sight to the conductors. Noel.

Contact or electrical continuity do not make an adequate RF shield. additionally the material may have coatings. From practical experience with flexable conduit designed for electrical applications I know it does not work as an EMI screen. In the UK at least it is not permitted to use flexible conduit as an earth conductor. (BS7671 543.2.4 ). This video explains https://www.youtube.com/watch?v=1Wn7Xt8JAzQ If it not good enough as a power earth it cetainly won't work at RF.

[Roger ClarkParticipant @rogerclark]

Posted by noel shelley on 25/06/2023 13:24:44:

Robert, with the material the spiral is made of being a chromed brass strip and each turn interlocked with the next and provided the 2 ends have continuity with the fittings please explain how or why it will not provide screening. Each turn of the spiral is part of the strip and any gaps between turns are tiny and the interlocking is such that there is no line of sight to the conductors. Noel.

Noel,

Thanks for the idea but I have ordered and received the correct inverter cable which will be fitted as soon as there is a wet day to stop me working outside so this discussion re shower hose is kind or irrelevant.

But thank you anyway.

Roger

[Roger ClarkParticipant @rogerclark]

Today's (Monday) update:-

Fitted cable with shield grounded to motor, results were mixed, am radio was still swamped with noise, the DRO readout did show improved stability at times, seemed to depend on where the crosslide was.

Next stage is to order a box and fit an input suppressor unit (might as well as I need to rewire anyway) and see where that takes me.

Robert

Good idea on using a compression fitting as a gland/shield earth, worked a treat but had to cut the locknut down due to the curvature of the motor casing.

Time to ask for a pocket money raise from the missus( that'll cost me a clothes shopping trip )

Roger

[Roger ClarkParticipant @rogerclark]

As per Robert's request via pm this is the cable sent by Inverter Drive which we think is the wrong cable.

Regards

Roger

[Robert Atkinson 2Participant @robertatkinson2]

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

[Roger ClarkParticipant @rogerclark]

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!

[Roger ClarkParticipant @rogerclark]

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

[Roger ClarkParticipant @rogerclark]

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

[SillyOldDufferModerator @sillyoldduffer]

Posted by Rockingdodge on 03/07/2023 13:32:24:

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

[Roger ClarkParticipant @rogerclark]

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.

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

[EmgeeParticipant @emgee]

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

[Robert Atkinson 2Participant @robertatkinson2]

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.

[Robert Atkinson 2Participant @robertatkinson2]

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.

[Roger ClarkParticipant @rogerclark]

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

[EmgeeParticipant @emgee]

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

Emgee

[Robert Atkinson 2Participant @robertatkinson2]

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.

[SillyOldDufferModerator @sillyoldduffer]

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

…

The one thing that confuses me is the difference between the safety earth and shielding earth … then surely earth is earth and thus the mains cable carries the difinitive earth?

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

[EmgeeParticipant @emgee]

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

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