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Re: G31 alternative
« Reply #10 on: August 21, 2016, 04:44:22 AM »
Oh, and another common error with power supplies of differing output voltages is to join the 0v lines together.... be careful.... the 0v is zero volts relative to the other output line (5v 12v 24v 36v etc).... don't join the 0v or sometimes marked gnd connections together.... or you end up with a ground loop problem as the zero volt lines can be offset from the earth and can be offset from other 0volt outputs from other power supplies.

Rob
Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”
Re: G31 alternative
« Reply #11 on: August 21, 2016, 08:26:12 AM »
Hi Guys.
thanks for your replies, welcome and instructive.

Rob, when I meggered (is that a term you use?) the motors were certainly disconnected from the drives.
I went thru much of the procedure you outlined, disconnected every circuit that goes to the machine and
bought each one online one at a time. The culprits are the steppers, not identical but all near enuf to 10mA
a piece.

Heres the odd bit, the leakage occurs with small variation only wether the axis is driving or not. Even when
stationary the drives still PWM current to the motors.

I've been thru the wiring again and again. All circuits from the BOB to the drives are opto types, I had the
limits and home switches disconnected, in fact removed, the spindle was dismounted and on the other side
of the room!

Vexta specify a 500v isolation result and max rated input voltage of 150V for the motors. The drives themselves
are 230V input. Their size tells me that they have inverter powered DC link. I have not pulled one to bits yet but am
thinking that the DC link may not be isolated from earth. Certainly most AC servo drives are not isolated (what
a surprise that was!!!). I was guessing that the leakage was in the drives but each drive frame is earthed back
to the common rail and I could not for the life of me see why leakage would flow in the mill frame earth.

Since I did that testing I have bought a Tectronrix current probe. One day soon I'll apply it to this problem,
I promise!

In absence of anyother explanation I concluded it was induced current, aka homopolar current in defiance of
Kirchoff. I thorougly agree it way too high but I'm not throwing these things out... they're superb in every
other respect.

I am very careful about power supplies fighting one another. I repair welding equipment or a living and have
seen numerous occasions where weld voltage, a very beefy power supply, blow up boards and their power
supplies should it inadvertently get connected.

Craig
My wife left with my best friend...
     and I miss him!
Re: G31 alternative
« Reply #12 on: August 21, 2016, 08:52:13 AM »
Craig, seems like you've done the insulation test the right way (yeah, used to be called meggered as the instrument was an AVO Megger (after megaohm I suspect... I've still got and use my old but prestine AVO CM500).

I unfortunately don't know your drives and motors well enough, but I'll try to spend a bit of time reading up on them (you haven't got any links to datasheets have you). (Unfortunately I'm a cheap stepper man not servos, some of the more experianced servo users (like hood I suspect) may have a view)

The other bits are if you're using screened cable... only ground (earth) one end or you end up with eddy currents (but not 30mA floating about of imbalance).

Back when I used usb motion controllers i found out that some usb leads have s grounded shielding cable that created a real pia ground loop between motion controller and pc.

Seems like you've got a good knowledge and ain't going to fall foul of some of the common pitfalls for new cnc users.

I need to have a think.

Rob
Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”
Re: G31 alternative
« Reply #13 on: August 22, 2016, 08:23:33 AM »
http://www.cncitalia.it/upload_ele/50VEXTASTEP.pdf

Having a look at page c90 here, and the insulation tests, I would have said that what you did was fine if your drives are the same, but I would not have expected any leakage current.

Suggest digging a little deeper to see if one, some or all are leaking to earth by removing and fuctionally testing them in operation, and measuring the current at the earth terminal.

Rob
Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”
Re: G31 alternative
« Reply #14 on: August 23, 2016, 05:26:11 AM »
Hi Rob,
you're right of course, roll up my sleeves and get stuck in, I certainly have the gear, inductive current probe
to 5Mhz, differential voltage probe to 25Mhz with common mode range of 1400V....

The point is I just don't want to!  I'm happy making chips and have a new coil winding script to try out
and then there's the board for my 'you beaut' servo drive not to mention finish grinding the micrometer
head mounts then there's that straight edge and end clamps for a measurement jig and..... You know
how it goes interesting projects and must do jobs always come up and solving this leakage problem
is mere safety, I sniff at it!!!

Truth be told when I first discovered this problem is when I demounted the spindle and promptly got
quite a tingle from the mill frame. Additionally I laid the remote pendant down on the bed and managed
to take out one buffer channel in the BOB. Fortunately the fault didn't progress back into the PC but still
damage was done.

The solution is of course to demount each stepper so I can measure earth return current but I have just
not got around to it yet. Pretty slack and cavilier of me...

Craig
My wife left with my best friend...
     and I miss him!

Offline Tweakie.CNC

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Re: G31 alternative
« Reply #15 on: August 23, 2016, 06:04:36 AM »
Quote
solving this leakage problem is mere safety, I sniff at it!!!

It's your business but something to perhaps keep in mind...

"Even a small leakage current can mean a risk of harm or death due to electric shock if the leaking electric current passes through a human; a current of around 30 mA (0.030 amperes) is potentially sufficient to cause cardiac arrest or serious harm if it persists for more than a small fraction of a second".  (extract from Wikipedia).

Tweakie.
KEEP SAFE !
Re: G31 alternative
« Reply #16 on: August 23, 2016, 06:11:41 AM »
Craig,

I know how it goes, and whilst I don't advocate it... I've had a few shocks in my time (230v here), don't recommend 415v ph-ph.... 230 has kept me sharp.

Can you not unplug the drives or motors... remove a fuse to the drives or turn off a circuit breaker?


Only slaved drives will be difficult

Rob
Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”
Re: G31 alternative
« Reply #17 on: August 23, 2016, 06:24:48 AM »
Hi Rob,
great find on the info, I have bits and pieces but nothing as complete as this.

Supposedely these steppers can run at 3000 rpm with gear reduction 300 rpm. When I first
set the mill up I experimented and got axis speeds of 2200mm/min which equates to 440rpm
at the coupler and 4400 rpm at the motor. I ran everything at max to do it and of course they
got hot and were prone to stall. In more recent times I run 1200mm/min for a motor speed of
2400 rpm at 75% current output and they run comfortably and reliably pretty much irrespective
of whats loaded on the table.

The drives I have I think must be an earlier model as they don't have microstepping, full and half step
yes but otherwise look identical. Thanks again for the info.

Craig

PS just read your latest post and yes I've had a few close encounters with 230, and about 3 months
ago while at work I slipped probing a 400V circuit, it hurt!
We have a sign up saying "DANGER this will kill you and really, REALLY hurt while you die"

I can disconnect each motor and drive individually but when mounted I can't break the common
earth. Provided  I do one at a time I could measure return current in each, will do it and report
back, not this weekend as I have to work but maybe the next.
My wife left with my best friend...
     and I miss him!
Re: G31 alternative
« Reply #18 on: August 23, 2016, 06:38:24 AM »
Hi Tweakie,
I've always regarded LD50 (lethal dose 50% of the time) as 20mA.

In this case the open circuit voltage I measured was 130V but any moderate path to
earth drops the voltage bigtime. I can only guess but theres no way I could have stood
30mA and still write this to you...

You are correct that I should be much more concerned about this than I am or perhaps
appear to be.

Your concern is noted and appreciated.

Craig
My wife left with my best friend...
     and I miss him!
Re: G31 alternative
« Reply #19 on: August 23, 2016, 06:54:00 AM »
... sorry off topic, the 30mA current flow at the bodies"normal" impedance I believe is fairly difficult to achieve.

Note... normal is skin surface to skin surface ... left hand finger tip to right hand fingertip or feet.

Impedance more than resistance I would have said as the resistance is fairly easy to measure with a high resistance reading ohmmeter... think it's about the 1mohm range from memory (not read mine for a while) but this doesn't take into account the bodies surface capacitance, which will probably help current flow under ac (alternating current) conditions (mains voltage).

However... if you're in water, or wet... that is a different animal.. or break the skin (as I did once with a minor shock, tensed muscles, and inserted arm onto a sharp bit of electrical trunking (solid cable tray...) that was lights out for me, but thanks to a quick acting electrician working along side me who pounced on me to get me off, I'm still here... probably why I'm no longer on the tools day job 20 yrs later... as a consulting engineer (aka them what are not actually good at producing the end job... better at telling someone else what it should look like and not how to do it)

Yeah, it bites, and you can't see it.

Rob
Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”