So if you are in the UK and have a earth wire on pin 4 do you need to, or should you need to, earth the cable shield? If you do. Where and how do you do this?
this thread got a bit confusing to be honest. I'm just glad earth is earth in the UK.
Way back in the last century, I obtained my UK Industrial Electricians license, at that time it was a no-no for an installer to allow the Neutral and Earth to come in contact Anywhere in the installation, Earth was connected at the star point only.
I believe now, things have changed a lot.
In the UK you have 240v single phase which consists of one phase and a star point earth grounded neutral of a 3ph transformer, here in N.A. the domestic supply is a 240v 1phase transformer with centre tapped secondary, the C.T. being the earthed neutral for 120v-0-120v , as far as the input to the VFD which usually consists of a 3phase rectifer that is either fed with 3ph or 2 conductors of a 240v supply, it can tell no difference on the nature of the 240v 1ph supply.
One section of the 3ph rectifier is used as a 1ph bridge
Al..
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Last edited by Al_The_Man; 02-02-2019 at 12:04 PM.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
So if you are in the UK and have a earth wire on pin 4 do you need to, or should you need to, earth the cable shield? If you do. Where and how do you do this?
this thread got a bit confusing to be honest. I'm just glad earth is earth in the UK.
UK three conductors, L, N, E. 230v
The earth ground used to be connected at the supply transformer only, and you picked it up locally via a metalic water pipe or ground rod.
I believe things have changed now.
Just the same as N.A., where the earth is referenced to the neutral in the supply panel.
Otherwise the earth ground is treated the same.
I have been away from the UK for a few decades now.
Al..
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
Thanks Al.
So what do I do with the cable shielding?
Should that need connecting to the earth on the VFD?
it all about EM noise control.
Now shielding is normally connected to earth ground at each end.
Ensure the motor frame is also connected to GND as well as all metallic parts of the machine.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
That might be problematic.
Have you got any pictures on how you did yours.
I don't fancy drilling a hole into my concrete floor to instal a earth spike.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
All uk instals need a earth circuit. All MCB are earthed through ether a mains water system or in my case a earth spike.
The whole house circuit is feed from the MCB which receives the 230 100amp mains NG supply.
That is split up into the required circuits (I know you know this but this is for the benefit of people who don’t)
So the garage where my router is based has a supply from the MCB. The feed gets split by the garage MCB and we are all earthed happily.
What I’m not clear on is do I need to Earth the VFD cable shielding?
If I do. Where would be a good place to earth it.
If it’s a question of attaching the shielding to the motor housing and the earth on the VFD that’s fine.
I have a four core cable one of which is connected to earth on the VFD with the other end coming through pin 4 on the motor which gets attached to the motor top plate via a tapped hole drilled into the top cover.
Ok so if I have to earth the shield tha runsa along the other of the cable where do I earth that too?
Will connecting shielding to the machine and the earth on the VFD create a earth loop?
It’s not my garage.
The floor has a vaper layer under the slab which I do not want to penatrate.
Besides we have a earth circuit within our instals.
I just don’t want a earth loop to deal with.
If it’s fine to wire it through the VFD Earth (so by extension the main earth circuit) that’s fine.
As long as I don’t make issues for my self. That’s all I’m worried about.
Mactec54
I have my VFD mounted to a brick wall. Not on a metal plate.
This is the second time i have run this spindle. well its the second time I have installed in on a router.
The first time instal was replaced with a different system.
I never earthed the shielding before.
The VFD is also a plastic box.
Are you saying the mains earth and the out put earth are not connected?
This is old kit (five/six years) brining used again. In the "manual" it says not to connect a wire between the mains earth and the output earth. I assumed this was to prevent ground loops as they were connected internally.
I have to say I'm no closer to understanding this than before. I will call in a sparky and have them do it.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
Mactec54
Oh god!
The VFD is earthed via the three pin plug in the electrical circuit.
There are four cable and coming out of the VFD to the spindle.
One is a earth.
That earth wire is soldered into pin 4.
Pin 4 goes nowhere when you take the spindle top off. So I have drilled and tapped the spindle top so I can fix a wire from pin 4 and attach it to the spindle top.
Now the VFD is just screwed to the brick wall.
If I need to Earth the shield. Can I earth it through the same point that wire 4 comes out of the VFD?
Or what does that shielding actually do?
Because as far as I can see it’s there to screen out EM noise!
There is no point in grounding the shield if there is no point to ground too.
I have no metal plate the VFD is mounted too. So that’s out.
The VFD circuit is grounded/earthed via the mains circuit. As is the rest of the control circuits and motors.
Right? I should have no noise to contend with.
@ AL
I have a industrial guy sorted with one phone call today.
Beer money.
I would still like to understand the thing form a UK perspective.
Maybe I should not have bothered. Lol
You should of explained that from the start, the only thing you have to figure out is how to Earth the shield and yes it can go to the same Ground connection as the VFD is using, so I don't know what all your fuss was about, I think your electrical friend will tell you the same as I just have, and if he knows his stuff he will tell you the shield will need to be Earthed as well
Mactec54
I did explain that. Right at the start!
What I want I know is why are we earthing a shield?
What’s the point of the shield?
Is it to shield em leak?
Or to guard against it?
My 100amp mains supply cable has a shield to stop it being cut through by accident.
If your circuits are earthed properly I don’t understand what the point of the shield is.
After all your washing machine is a motor right? That’s create EM. But you don’t need a shielded cable to run it!
The shield should be grounded at both ends from the vfd to the motor. depending on the nature of your machine its also good to ensure an extra ground path from the motor to the frame of the machine, that way in the case of an electrical failure you don't send substantial current through the linear bearings or the ballscrews.
The 3 phases from the vfd are square waves of continuously varying duty cycle, they average out to get a sine wave current in the motor. the sharp rising and falling edges (and the ringing that occurs) is as high as 400 volts per microsecond and the frequency is as high as 1Mhz. the 350 volt bus of the vfd means there can be substantial current flowing at that frequency, as that +/-170 vdc square wave at the pwm frequency of the drive charges up and discharges the parasitic capacitance of the motor windings to ground, and that's the reason you need to ground the motor.. otherwise you can shock yourself pretty good on an ungrounded motor driven by a vfd, or the current will flow through something else and cause problems.
even the voltage on the cable can radiate through near field capacitance into other signal wires and mess your machine up, shielding it solves that problem, but only if the shield is grounded to the vfd.