Terminating a shielded cable with a VFD & spindle?

[sploo]

I think it's important to be clear on the words "shielded" and "earth".

Shielding may be mechanical protection - e.g. steel wire armour, or may be to reduce incoming (or more likely in this case) outgoing electromagnetic 'noise'. Was the person who advised you clear on which of these they meant (I presume mechanical protection - what with 415V AC).

Earthing similarly, can be used to provide protection from electrocution and the earth wire is usually referred to as the CPC - circuit protective conductor. All bits of metal that you can touch should (almost certainly) be connected to a known good protective earth conductor.

Earth loops occur because all shielding/earthing conductors have resistance - and any current flowing in them can result in unwanted voltage appearing on one end of the conductor (this can have implications for safely and for EM interference). This is the consideration that will help decide whether one end or both ends of the conductor need to be 'earthed'.

The context on the other advice was about electrical noise not mechanical protection - so hopefully on the same "subject".

My "easy" solution would be to connect the earth core on the VFD-to-spindle cable to the mains earth connector in the VFD, and also bond the shield on that cable to the same earth point. If nothing else it does guarantee that the spindle can and CNC machine body would definitely be grounded. My concern is that may render the cable shield "useless" from the point of view of noise protection (or may even cause problems), and as this cable is the one carrying the variable frequency supply, would I cause problems by having that cable's earth core going to the 50Hz mains earth?

I.e. doing the below; using the mains earth as the "star" point; to which both the VFD-to-spindle earth core and that cable's shield are grounded:

 

[guineafowl21]

Firsty I should say that I used to be involved in the design of switched mode power supplies using various topologies and a good understanding of electromagnetics.

So what is a rectifier, it only passes in ONE direction and nothing in reverse. If this was a linear circuit then the output response would be directly proportional to the input and in the case of an AC circuit that would mean a sinusoidal voltage resulting in a sinusoidal current because the instantanious current will rise and fall in proportion to the voltage, therefore the current waveform is also a sine wave. Once you introduce rectification and filters this no longer applies, the output current is no longer proportional to the input current. This is also evident when performing electrical test and inspection, with motors directly connected to an installation you test loops down to the motor junction box, once a VFD or softstarter is used then you can only test upto the input of the electronic device, there is no continuation of the loop through the device.

Not doubting your credentials, of course. I think we might be talking of different things.

I mean galvanic isolation - by analogy with a switching supply, eg phone charger, you mustn’t connect your scope probe ground to the 330V DC bus, because there is a reference to mains despite the rectifier. You must either float the scope (dodgy) or run the device under test through an isolation transformer.

Maybe your example of motor loop testing is complicated by possible damage to the VFD by offering an unbalanced load?

I would be interested in @NetBlindPaul ’s and @Myfordman ’s take on this and also the OP’s problem. I find these discussions very interesting.

 

[Sideways]

On several Invertek and yaskawa VFDs, i can confirm the input side and output side earth terminals are bonded solidly together through the chassis metalwork. I would expect nothing less.
I recognise the comments about single end only connection of the protective outer braid on the feed to the motor, it echoes common practice in hifi to avoid earth loops and hum. However hifi doesn't work at hazardous voltages or have to consider electrical safety.
I always connect motors to VFDs using SY cable (4 core = 3 phases plus earth, plus the outer braid) with both the internal earth conductor and the braided outer connected in parallel - i.e. connected together at both ends using ring crimps on the earth terminals of the vfd and motor chassis.
Belt and braces.
Pedantically, because SY is often the best choice for this application but isn't approved to any british standard, to earth using it's braid alone could be questioned.

I've yet to have any issues as a result of this.
If you do choose to ground the braid at one end only, do it at the VFD end and obviously only do this if you have a separate earth conductor connected at both ends as well.

Lastly, if you have remote control switches for your vfd (slightly more likely to be susceptible to interference), connect them using CY screened cable and connect the screen to the signal ground terminal at the VFD end only.
Those voltages are low, and the screen is acting as a shield. It is not a protective electrical earth.

 

[sploo]

I just went into the garage to take another look at the VFD: terminals shown below:

Ignore the 2 core cable in the foreground - I'm working on the relay control to switch the spindle on and off from the CNC Controller.

On the left you can see the 240VAC mains input, and on the right you can see the U, V and W terminals going out to the spindle (currently just using some 3 core flex).

As far as I can work out, there's no continuity between either of the two earth terminals at the bottom of the image, or terminal 9 on the right hand side.

I've also now come across a seller in the US who distributes these VFDs, and on the page BuildYourCNC - 2.2 kW (kilowatt) Water Cooled Spindle they have this image:

That's pretty clearly showing the 4th core of the spindle cable going to the mains earth; so unless they don't know what they're doing I'll take it that this is the right (and safe) thing to do.

Interestingly the image mentions "use shielded cable", but it doesn't say anything about how/if/where the shield should be bonded. I'll send them an email and ask.

 

[sploo]

On several Invertek and yaskawa VFDs, i can confirm the input side and output side earth terminals are bonded solidly together through the chassis metalwork. I would expect nothing less.
I recognise the comments about single end only connection of the protective outer braid on the feed to the motor, it echoes common practice in hifi to avoid earth loops and hum. However hifi doesn't work at hazardous voltages or have to consider electrical safety.
I always connect motors to VFDs using SY cable with both the internal earth conductor and the braided outer connected in parallel - i.e. connected together at both ends using ring crimps on the earth terminals of the vfd and motor chassis.
Belt and braces.
I've yet to have any issues as a result of this.

That's useful to know. If Invertek (which I understand to be a decent brand) have a simple bond between the input and output earth terminals that tells me it's OK to connect the earth line coming from the spindle motor to the mains earth (as it's effectively the same path).

On shielding, the article at Cable Shield Grounded At One End Only - EMC Standards, posted earlier in this thread, appears to indicate that the shield should not be connected to earth; merely mechanically bonded at either end. I suppose I could try that, and if I have problems then I could try connecting the shield to earth too.

 

[Sideways]

Shielded, in common useage means protection from electrical noise, usually implies a braid around the working conductors.
Unless connected at least at one end it won't do that.
A strong wire braid like in SY cable is intended for physical protection ie. it is an "armour", and can do that role without being connected at all. If you connect it at one end, it will provide some shielding agaist interference, but the braid is a little sparse and may have limitations in extreme cases that we probably never need to worry about.
SY has plenty of metal in the braid but isn't well documented to be able to design and justify it's use as a protective conductor if you might find yourself in court following a fatal electrocution.
Ground loops and interference issues, I prefer my way. Start as safe as possible, if you have a problem, only then disconnect the braid at the motor end only provided you still have a primary earth conductor in place.
The emc article is fine but could be talking about noise in a pice of electronics running off a 9v battery.
Electrical safety comes first. Make sure you have an adequately sized earth connection (circuit protective conductor) in place at all times.

 

[Spectric]

you mustn’t connect your scope probe ground to the 330V DC bus, because there is a reference to mains despite the rectifier

Why not just use a differential probe which provides inputs not referenced to ground. The only time I have been involved with galvanic isolation is in systems and plant that has been deemed to have to comply with ATEX and DSEAR regulations. I suppose you could count opto isolation as a form of separation and has been used in SMPS's to keep the electronics safe from the higher voltage sections.

 

[guineafowl21]

Why not just use a differential probe which provides inputs not referenced to ground. The only time I have been involved with galvanic isolation is in systems and plant that has been deemed to have to comply with ATEX and DSEAR regulations. I suppose you could count opto isolation as a form of separation and has been used in SMPS's to keep the electronics safe from the higher voltage sections.

Yes, I could, but what I meant was, by analogy with the SMPS, the bridge rectifier doesn’t remove the mains reference. Here’s a crude diagram:

Only single phase output but illustrates the principle. This is why I would consider the output of a VFD to be mains referenced with respect to earthed objects.

Out of interest, I might see what voltage exists between my VFD output and real earth next time I have the chance.

 

[Spectric]

If the negative side of the rectifier diodes were earthed then this would be correct, but it is the Ac Neutral that is earthed. Once rectified the outputs whether single phase or three phase will be positive or zero.

 

[porker]

The AC neutral and line on the input side to the rectifier are referenced to earth. On the DC side, the dc voltage is also with respect to earth. There is no isolation provided by this circuit. If there was a transformer then yes the secondary side of the transformer would float unless the 0V line were referenced to ground.

I agree with @guineafowl21.

On the photo provided by sploo, it looks like the incoming supply earth is not terminated to an earth terminal but rather a screw that may be isolated. I would say the incoming earth should also go to the earth on the terminal shared with the spindle. with the few VFDs I have installed they have had a common earth between the incoming and the motor but there may be exceptions.

 

[guineafowl21]

If the negative side of the rectifier diodes were earthed then this would be correct, but it is the Ac Neutral that is earthed. Once rectified the outputs whether single phase or three phase will be positive or zero.

On the positive half wave, in the diagram, the man is touching the line conductor (with a diode and transistor in series). I think we’ll have to respectfully disagree - I would not consider the output of a VFD to be isolated. One of us will have to touch an output to be sure (joke).

@sploo I had a look at the connections of my ‘far-Eastern’ VFD, installed according to instructions. The mains earth and motor earth are tied to the same terminal. The motor cable is non-screened.

 

[EddyCurrent]

I used to work on small and large inverters driving single and multiple motors over long distances and what I read here is how I understand things should be; Problems VFDs cause and cable types that help solve them
Essentially the motor cable screen, inverter metal backplane and motor casing make a Faraday cage when they are all connected and because metal is involved a tail is connected from the screen to earth at the inverter end. This does not constitute the earthing system which means a separate earth wire must be used for that purpose.

 

[sploo]

On the photo provided by sploo, it looks like the incoming supply earth is not terminated to an earth terminal but rather a screw that may be isolated. I would say the incoming earth should also go to the earth on the terminal shared with the spindle. with the few VFDs I have installed they have had a common earth between the incoming and the motor but there may be exceptions.

I did see a comment on another forum from someone claiming to have the same Huanyang VFD where the two bottom screw points (marked with an earth symbol) were connected, but didn't provide any other details. In my unit they're definitely not connected; but it does appear that the "quality" units do have a physical connection between the terminal for the incoming mains earth wire and the outgoing terminal for the spindle earth. That seems logical to me; my only concern was the potential of introducing noise into the mains supply if you weren't supposed to connect the earth from the spindle to mains earth, but clearly that is the right thing to do. Obviously on my VFD I'll just have to wire the incoming and outgoing earth cores to a single terminal, but that's no problem.

Having spoken with BuildYourCNC (ref the spindle wiring diagram in post #24; Terminating a shielded cable with a VFD & spindle?) they've confirmed that the shield should be connected to the same earth terminal at the VFD end.

So, in summary I think where I'm going is:

1. 240VAC mains L to VFD R terminal
2. 240VAC mains N to VFD S terminal
3. U, V, W cores of a 4 core shielded VFD-to-spindle cable going to the VFD U, V, W terminals
4. Earth core of the 4 core VFD-to-spindle cable going to one of the earth screw terminals in the VFD
5. 240VAC mains earth core going to the same earth screw terminal in the VFD
6. Shield on the VFD-to-spindle cable bonded near the VFD with a clamp, and an earth wire run from that clamp to the same earth screw terminal in the VFD

The spindle was delivered with a 4 pin socket, but the 4th pin (for earth) is not connected to anything. I've already added a wire to connect it to the spindle can; so that it will be earthed then plugged in.

The only outstanding issue is the fact the spindle socket (and the plug that will be connected to it) are isolated from the spindle can by virtue of the fact the socket is mounted on plastic. As such I'll see if there's a way I can also wire the earth pin in the socket to the socket itself. Given that the cable shield will be grounded I could of course just expose the shield at the spindle end and use the clamp on the plug to create a connection to earth, but then I'm obviously using the shield as an earth path; which seems to be inadvisable.

 

[NetBlindPaul]

I have to say this thread concerns me somewhat.
From some of the advice given, through the questions asked, the data provided by the drive OEM, and the design and manufacture of the unit itself.
Having worked for several years within the UK Service divisionn for a global name in industrial drives and controls, in service, applications and sales, been a freelance machinery service engineer, and now a machinery and electrical safety consultant and expert witness.
I am concerned that this drive and its installation could pose a hazard to the user and others using the electrical installation to which it is connected, and may even result in the buyer breaching the terms of their contract with the electricity supply company, and the machine never being legal to sell after this even on the used market.
There is a reason some drives are more expensive than others and it’s not always greed, or overheads of the OEM.

 

[porker]

I would tend to agree. Calling out specifics I would say there is a current risk that there is no CPC protection as it appears that the incoming earth connection is currently connected to what looks like an isolated screw. The diagram provided would also bear this out. This potentially means it is possible that if a live conductor touches any metal part of the drive it could become live and present a hazard.

Having said that - if the diagram shown relates to the equipment being discussed then the earthing is fairly clear.

As far as electrical isolation, there is nothing shown that indicates any isolation of the outputs to the input supply. A rectifier does not provide isolation.

 

[guineafowl21]

As far as electrical isolation, there is nothing shown that indicates any isolation of the outputs to the input supply. A rectifier does not provide isolation.

Yes, that was the root of my little thread diversion about what constitutes isolation. If, as I contend, the VFD output is not isolated, then the motor and associated metalwork is an exposed conductive part that may become live under fault conditions, hence should be earthed.

We are a bit stuck, trying to reverse-engineer the VFD without knowing exactly what’s inside, and without the catch-all ‘install according to manufacturer’s instructions’.

@sploo can you remove the front cover of the VFD and see what’s behind those two ‘earth’ screws in the black plastic? It’s unusual to see an earth point in plastic.

Be aware, the drive will hold a charge for a long time. Leave it unplugged overnight or for a good few hours first and touch nothing in there.

 

[EddyCurrent]

There's loads of information here that might throw some light on things ; Generic Chinese Spindles

 

[Sideways]

I just went into the garage to take another look at the VFD: terminals shown below:

View attachment 106500

Ignore the 2 core cable in the foreground - I'm working on the relay control to switch the spindle on and off from the CNC Controller.

On the left you can see the 240VAC mains input, and on the right you can see the U, V and W terminals going out to the spindle (currently just using some 3 core flex).

As far as I can work out, there's no continuity between either of the two earth terminals at the bottom of the image, or terminal 9 on the right hand side.

I've just taken a proper look at this photo.
If you match the white terminal label up to the series of connections below it, the connection arrangement LOOKS blindingly obvious.
There is one earth terminal identified - perfectly clearly with the electrical symbol for earth- and one only.
It's terminal 9 on the right hand end with the splash of red varnish on it.
Both the incoming and outgoing (to the motor) earth connections should be fitted to that. Use ring crimp terminals not fork type, you'll have to take the terminal screw out to thread them on then replace it
If there is an exposed metal heatsink or a metal casing on the back of this, use a meter to check that there is effectively ZERO (certainly less than one Ohm) resistance between this terminal and the exposed metalwork. These should be bonded inside the drive. Commonly there is a sizeable track on the printed circuit board that surrounds the holes where the circuit board is physically bolted to the heatsink and makes a good connection when the drive is assembled.
The two screws at the bottom left and right look to be nothing but screws holding the plastic shell to something behind. I would say they are nothing to do with the earth and the connection of an earth conductor to the left hand one looks to be a dangerous mistake.

I do agree with the posters above me. A device like this is not something that should be sold without proper, legible, understandable instructions. Perfectly safe if you know what you're doing, potentially lethal if not. The internet is not a reliable source of advice and you can't sue it if you are unable to recognise dangerous advice. Doubt everything - even this.

 

[Spectric]

I think the best thing is to totaly avoid these asian invertors and use something like ABB or Siemens where you get quality and proper literature, at the moment it feels like people are assisting in a suicide mission. The only invertors I have seen that have gone into meltdown have been from a well known online selling site and not only do I not recomend I also used to refuse to install, setup or have anything to do with them.

Two synarios, a motor supplied by a DOL from a wall mounted fused isolator, correctly installed and maintained. A short between live-Neutral or live-CPC will cause the protective device in the isolator to disconnect the supply. An overload condition will cause the DOL overload to disconnect.

Now stick a decent VFD or softstarter into the system that has been correctly setup and now a short between live-Neutral or live-CPC will not cause the protective device in the supply to operate because the device itself provides the protection, both for the motor and itself. An exception to this is with a softstarter and bypass contactor. Again a decent device may have the ability to accept the input from a motor thermister to monitor overload through increased winding temperature, but on a basic level it will provide over current protection and ground fault protection amongst others like over/under voltage loss of phase and overload, these parameters will all need setting up.

 

[Spectric]

Some other peoples thoughts: Quote Does a frequency drive need external overload protection (Per UL)?

" There is a slight glitch in this, not commonly understood.

IF the manufacturer was honest enough to get the UL listing for the VFD as a "Motor Controller", then they must provide not only the motor thermal OL protection in the VFD, but also the motor Short Circuit protection as well. Because a VFD can be programmed to control a much smaller motor than it is rated for, the fuses AHEAD of the VFD often end up incorrectly sized for the motor anyway, even if you ignore the fact that there is no longer a direct connection to input and output current. So years ago, UL began requiring VFD mfrs to provide both. This will be the case with all of the major VFD mfrs here in the US and most of the larger European and Japanese ones.

But, some "less than scrupulous" bottom feeder 2nd and 3rd tier VFD suppliers in Asia have found a loophole in that they do NOT get UL listing as a "motor controller", but rather as a "power converter", same as a UPS or a ballast. That listing does NOT have the same load circuit protection requirements, leaving the end user to have to provide that themselves. Their manual MUST state this, so if you are suspicious, look in the manual. If it tells you that you must provide an external OL relay, that is why.

Then the lowest of the low bottom feeders, the cheap-o Chinese drives making it onto Fleabay lately who's name sounds like vomiting, are not UL listed AT ALL, so even if they SAY they have it, I would not trust it. But then again, those drives are the worst POS things out there so I would never be in that position any"