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Earth Leakage issues

marcsO

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Mar 24, 2020
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Hi all I have a little problem and hoping for some advice.

I recently converted an old Hardinge HLV lathe which had bad electrics and an underpowered main drive motor, the lathe needed to run off my 240v 1ph supply. Following research I ended up with the following.

Main motor 3hp, 3ph running off a Mitsubishi VFD
Carriage motor 3/4hp, 3ph running off a Mitsubishi VFD

I have included in the electronics two line filters one for each VFD

I have around a 100m cable run to my workshop which is 10mm2 armoured cable, the armoured cable is terminated using proper SWA fittings to correctly earth and keep the continuity. In my workshop the cable terminates on a simple fuse panel with a 2 pole ON/OFF switch and two RCD's one for the main power ring rated at 32A and one for the lighting rated at 8A.

The supply from the house is via a 50A RCD which sits on a fuse board which is protected with a 30mA trip 64A breaker.

Everything is securely terminated and enclosures are waterproof where located on external walls etc.

I can run a 2kw heater, all the LED lights, a halogen lathe light, coolant pump and the main motor on the lathe all simultaneously without the house breaker tripping but as soon as the carriage motor is engaged and run up the trip goes down. I've been told the VFD's used to drive motors are notorious for earth leakage and that this is likely to be the cause along with a fairly long cable run, what I'm trying to understand is is there a simple device you can buy to add between the lathe and power supply to remove the spikes the earth leakage presents and remove the very annoying tripping?

For info I'm planning to put a more capable fuse panel in the workshop with a 40A RCD (30mA) to hopefully stop the house tripping and just leave the shed down in the event of a trip (is this worth doing btw?)

I'm planning a mill at some point and really don't want to have to spend stupid money bringing in a new service just for the workshop in from the grid.

Any advice and suggestions would be very welcome.

I've added a picture of the 'newly' built electrics cabinet for reference.

Thanks in advance

marc
IMG_3027.jpg
 
Hi all I have a little problem and hoping for some advice.

I recently converted an old Hardinge HLV lathe which had bad electrics and an underpowered main drive motor, the lathe needed to run off my 240v 1ph supply. Following research I ended up with the following.

Main motor 3hp, 3ph running off a Mitsubishi VFD
Carriage motor 3/4hp, 3ph running off a Mitsubishi VFD

I have included in the electronics two line filters one for each VFD

I have around a 100m cable run to my workshop which is 10mm2 armoured cable, the armoured cable is terminated using proper SWA fittings to correctly earth and keep the continuity. In my workshop the cable terminates on a simple fuse panel with a 2 pole ON/OFF switch and two RCD's one for the main power ring rated at 32A and one for the lighting rated at 8A.

The supply from the house is via a 50A RCD which sits on a fuse board which is protected with a 30mA trip 64A breaker.

Everything is securely terminated and enclosures are waterproof where located on external walls etc.

I can run a 2kw heater, all the LED lights, a halogen lathe light, coolant pump and the main motor on the lathe all simultaneously without the house breaker tripping but as soon as the carriage motor is engaged and run up the trip goes down. I've been told the VFD's used to drive motors are notorious for earth leakage and that this is likely to be the cause along with a fairly long cable run, what I'm trying to understand is is there a simple device you can buy to add between the lathe and power supply to remove the spikes the earth leakage presents and remove the very annoying tripping?

For info I'm planning to put a more capable fuse panel in the workshop with a 40A RCD (30mA) to hopefully stop the house tripping and just leave the shed down in the event of a trip (is this worth doing btw?)

I'm planning a mill at some point and really don't want to have to spend stupid money bringing in a new service just for the workshop in from the grid.

Any advice and suggestions would be very welcome.

I've added a picture of the 'newly' built electrics cabinet for reference.

Thanks in advance

marc
View attachment 309279

Do you have an Oscilloscope?

I'd be looking for (relatively) high-frequency switching artifacts as a probable contributor - not faulty insulation. You bought decent goods.

Among other things that defy basic and obvious, one can end-up with an inadvertently "tuned" resonance that generates a problem out of proportion to the modest power you THINK is involved.

Your relatively low actual load causing the trip hints at that. Do you have a trip-out problem if activating the 3/4 HP motor AND NOT the 3 HP one?

'Scope is your tool to make such doin's visible to the Mark One human eyeball.

Not new.

"Extra" Earthed Copper conductor pulled in with the power bundle often needed with VFD even in metallic conduit. PM is not the only place all this s**t is covered, and "covered" it surely HAS been as VFD makers have steadily improved their solutions.

Decent research and analysis, the "hardware" side of a fix should be a cheap one and durable.

Might be load-side dv/dt or "sine wave" filters could be even more useful than your line-side filters? Might be better-off with NEITHER, too.
 
Hi thanks for the fast reply, unfortunately I don't have an oscilloscope nor an in depth electrical knowledge, I can work through stuff like the electronics mainly as the motor and VFD's came ready to roll from a company who supplies lathe upgrade kits (Newton Tesla a UK based company) If I had an oscilloscope I would have no clue what to do or look for unfortunately. I am waiting for some crimp terminals to also run the earth connections to a ground spike by the workshop, again no clue if this is advisable and good practice or will make no difference - any more suggestions?

marc
 
Sorry missed the suggestion on trying the smaller motor and not the main, will see what happens tomorrow and report.
 
Hi thanks for the fast reply, unfortunately I don't have an oscilloscope nor an in depth electrical knowledge, I can work through stuff like the electronics mainly as the motor and VFD's came ready to roll from a company who supplies lathe upgrade kits (Newton Tesla a UK based company) If I had an oscilloscope I would have no clue what to do or look for unfortunately. I am waiting for some crimp terminals to also run the earth connections to a ground spike by the workshop, again no clue if this is advisable and good practice or will make no difference - any more suggestions?

marc

Better bonding and grounding is "almost" always a good move. Also usually redundant spend as makes no discernable difference.

For known-problem solving, yah can't hit what yah can't see. Have to hope to "get lucky" at BFBI when throwing solutions - AKA "money" at the general area.

'Scope will further yer education. Nobody was BORN knowing how to use one, but the easy stuff comes right away. It's even fun!

I was nine years old when I twigged to electricity no longer being "invisible" once I got my first donated old 'scope! Never been without one if not two or three 'scopes ever since.

My newest is a benchtop "Rigol" 4-channel. Fours set of reliable, US-made leads safe at the high DC & spike Voltages I work with cost a LOT more than the 'scope itself!

But VFD's operate in the dirt-common "audio" passband, don't have the the high-energy spikes massive Dinosaur Current motors can kick-up as they briefly become right effective generators.

So the absolute cheapest ones made will do yah.

Some are hand-held meter size, but a tad larger is usually cheaper.
 
Sounds like you may have ground loops. At what points are the services earthed and what is the length of run from the VFD's to the lathe and are you switching the VFD's for on/off and not the mechanical controls?

Tom
 
RCD is the same thing as what is the US, called a "ground fault interrupter?" That is, a device which compares the neutral and hot leg currents, and opens the circuit if those disagree by a small amount. And your house unit is not tripping on an overcurrent, correct?

VFDs will tend to trip GFI protection. In my case the Hitachi drive in the lathe I have in my garage would do this reliably every time I went to run the motor - although not when first powering up the drive. To the point where I had to replace the GFI breaker on that circuit with a regular one.

The line filters you have probably are pi section whiich means they have four capacitors between lines and ground. You might consider as an experiment, bypassing the line filter on the feed motor drive to see if this mitigates the issue to some degree.

Another possible solution could involve a small isolation transformer for that drive, with some careful analysis of where the troublesome trip currents are flowing.
 
There are several possible causes and related solutions for this.

1) some RCDS/GFCIs are sensitive to high frequency signals, and may trip just on account of them, regardless of the actual ground current.
That problem can often be solved by a good filter in-line to the VFD, and/or an inductor in series with the VFD input.

2) Most VFDs have a significant common-mode output, which is generally at high frequency. That signal can go to ground via capacitance of the motor windings and cause ground current that may trip the RCD. Often it is spike current, and may trip the RCD at a lower actual current.
If there is a filter on the VFD output, it may have significant capacitance to gound causing the same thing.
That may be reduced by an inductor in the output of the VFD, often a smaller one than the typical big iron core inductor, which may have the same issue of capacitance to ground. A high current "common mode choke" is a typical part to use.

Also, having a good ground path back to the VFD from the motor can help. Even though the motor and VFD are connected to the machine frame, the return path may not be good. Running a ground wire with the VFD wires to the motor, from VFD case to motor case, may help. Same for any in-line filter unit.

EDIT: High frequencies will tend to take the lowest impedance path. Any return path that is separated from the source wires will have "loop area", and will have a higher impedance that a wire in closer proximity to the source wires. An overall shield covering the source wires is nearly as good as you can get.

3) In some cases, the total capacitance to ground through wiring, filter units, etc, may trip an RCD, although usually a higher current type (30 mA or more) will not trip on that.
Probably not your issue, since there is no problem until the motor is running.

4) there may actually be a problem. However, the VFD should trip on excess current if there really was a problem.

I suspect it is either #1 or #2.
 
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VFDs will tend to trip GFI protection.
MANY things in a shop will. If one is FORCED to use them? Different trip sensitivity can be had.
The line filters you have probably are pi section whiich means they have four capacitors between lines and ground. You might consider as an experiment, bypassing the line filter on the feed motor drive to see if this mitigates the issue to some degree.
Given what is common in UK supply? They are more LIKELY to be plain inductive reactances. NO caps at all.
Another possible solution could involve a small isolation transformer for that drive, with some careful analysis of where the troublesome trip currents are flowing.

Throwing money at a edge-case fault that can probably be corrected FAR more cheaply.

Job ONE is to identify the actual, not theoretical "it could be..." "you could TRY.." CAUSE.

Otherwise it just a question as to what you run out of first.

Patience.

or MONEY!

:(
 
Hi the house panel is earthed via the utilities supply, believe its via the Neutral leg, then there is a ground spike at the garage about 30m from the house and then a further earth spike at my workshop which is around 60m from the garage. The VFD's are housed in the lathe control box which is on the side of the lathe, VFD to motor is less than 2 foot. The VFD's are controlled by a push button panel controlled by low voltage links to the VFD's control terminals.
 
Hi the house panel is earthed via the utilities supply, believe its via the Neutral leg, then there is a ground spike at the garage about 30m from the house and then a further earth spike at my workshop which is around 60m from the garage. The VFD's are housed in the lathe control box which is on the side of the lathe, VFD to motor is less then 2 foot. The VFD's are controlled by a push button panel linked to low voltage links to the VFD's control terminals.

Sounds like "Best Current Practice" already. Or near-as-dammit.

You said "line" filters. Are they line side?

Or load side?

And what make, model, PN, or specs?
 
There are several possible causes and related solutions for this.


2) Most VFDs have a significant common-mode output, which is generally at high frequency. That signal can go to ground via capacitance of the motor windings and cause ground current that may trip the RCD. Often it is spike current, and may trip the RCD at a lower actual current.
If there is a filter on the VFD output, it may have significant capacitance to gound causing the same thing.
That may be reduced by an inductor in the output of the VFD, often a smaller one than the typical big iron core inductor, which may have the same issue of capacitance to ground. A high current "common mode choke" is a typical part to use.

Also, having a good ground path back to the VFD from the motor can help. Even though the motor and VFD are connected to the machine frame, the return path may not be good. Running a ground wire with the VFD wires to the motor, from VFD case to motor case, may help. Same for any in-line filter unit.

That is a good explanation.

I characterize these currents as stray currents - currents flowing in undesired paths. In the motor they flow through stator to rotor (SR) capacitance and stator to frame (SF) capacitance. Generally SF is around 100x SR capacitance and is the one you worry about most. The stray currents are common mode currents generated by the drive common mode voltage which is defined as voltage between neutral and ground.

The stray currents WILL make their way back to the drive - you need to control their path. To control the path, you need a good shielded motor lead cable that is very well bonded to the motor frame and to the drive back plane. Many times need to use high frequency straps to bond the shield properly.

To check the installation integrity, you should put a high frequency Rogowski coil (mine is 5 - 8 mhz bandwidth depending on coil length) around the motor leads. You really want to have zero high frequency (defined as over 100 khz) current. I helped design the Service Wire Service Drive cable, and I routinely achieve zero (or very close to it) on even large motors and long runs with this cable system.

Properly installed, drives don't cause earthing issues. I do not know all the ways that a properly installed drive affects a GFCI - I have never run a drive on a GFCI, but if a drive is not properly installed it can definitely trip a GFCI.

I have been trying to finish up an IEEE paper on proper drive installation, but can't seem to finish it. Its 8 years in the making.......
 
Hi the filters are in the pic I posted and are between the supply feed and the VFD's, if the specs are not visible on the [ics let me know and I will try to dig out the order and the part details.
 
Hi the filters are in the pic I posted and are between the supply feed and the VFD's, if the specs are not visible on the [ics let me know and I will try to dig out the order and the part details.

"Mea Culpa", I should have zoomed it earlier. Those ones will have caps in them. They are very nice as to keeping trash out of your - or your neighbour's - electronics gear. Leave 'em be, as-built, do not go messing about inside them.

"In general.." line-side .. better-yet "drive isolation" transformers - are important on SCR-class DC Drives. Rude as can be as they directly switch the line. Abruptly!

VFDs not so much. That side is simply looking into a recifier matrix charging caps.

WHEN .. a VFD needs a filter at all, it is more likely to be output or LOAD side.

Major-maker vendors will specify what suits their drive. Right in the manual.

Typically 3%, 5% - seldom a lot more, "dv/dt" filter, ELSE a TCI/Emerson o/e "Sine-Guard" where critical.

My Sine-Guard is on a 10 HP Phase-Perfect output. Input has two FAT single-phase chokes. Not because the P-P needs either one. Nor the load MOTORS. Schaffner on the "watch list", no need so far.

Because the REST of the residence is FULL of "computerish" goods. And the P-P has "been KNOWN to" exhibit a potential partial failure-mode that can be nasty.

Your "remaining" application is NOT "critical". Most needs are covered. Mitsubishi VFD are decent, not junk.

FIND the SPECIFIC problem?

It might be resolvable with nothing more complicated than a few feet of wire or more appropriate termination hardware.

Or JUST NOT using a GFCI breaker!

UK being as fussy as it is about their mostly mired-in-Edwardian-age electricals? You'll probably have to just find one with a higher trip sensitivity.

:D
 
Filters seem to be pi section on each line. Four capacitors to ground in each housing. Mystery here is the main spindle drive does not trip the drive, the smaller one for the feed motor does. I suggest the filter on that drive be bypassed as an experiment. Simple, cheap, quick, reversible.

A common-mode choke in the power leads for this drive is another simple experiment, as is a small isolation transformer. Without the proper measurement capabilities, one is reduced to the 'optomotrist' approach. "Which is better - this, or this?"

This can be suprisingly effective, and when the correct change is found that resolves the problem then the cause is thus identified.

As a former boss once put it - when you don't know what to do, do SOMETHING.
 
So, an experiment and the results:

Garage lathe was plugged into another circuit there, one with a GFI breaker (20 amp ckt).

Drive powers up fine, but attempting to start the spindle motor, trips the GFI.

I then put a small isolation transformer between the lathe and the receptacle it had been connected to. The green wire ground from the machine was of course still connected across the isolation, to the correct place on the receptacle. The secondary of the transformer however was *not* bonded.

The spindle motor then starts fine, without tripping the GFI.
 
Does the UK require ground fault protection on submains and hard-wired machinery? Here in NZ it doesn't.
This assumes a TN-S or TN-C-S (PME) earthing system; I believe you need an RCD for everything on TT.
 
So, an experiment and the results:

Garage lathe was plugged into another circuit there, one with a GFI breaker (20 amp ckt).

Drive powers up fine, but attempting to start the spindle motor, trips the GFI.

I then put a small isolation transformer between the lathe and the receptacle it had been connected to. The green wire ground from the machine was of course still connected across the isolation, to the correct place on the receptacle. The secondary of the transformer however was *not* bonded.

The spindle motor then starts fine, without tripping the GFI.

Helluva "garage" you can enter by the door in Peekskill, NY, USA and work on a Hardinge in the UK...
 
One thing I've seen that causes this immediately is when the neutral and ground are unintentionally bonded at the machine, or the two connections are swapped. The idiot in the yurt next to the barn I used to live in like to mess this up and let me dig through all of his crap to see why the steel shop building had an AC bias.

Given how it only occurs on powerup of the spindle drive and not on connection or closing of a contactor, it does seem like noise to me.

Isolation transformers are bad at passing high frequency (generally) and this could be helping you.

I'd be curious to see whether a line filter module installed in the machine fixes that (I put one on all of my stuff just out of good practice) or if the full isolation is needed).

Another way to test this is to connect a jumper across one phase matched side of the two windings and see if the RCD trips. If so, you need the transformer.



I've seen the opposite issue where a motor ran with a winding shorted to the rotor. The hole air compressor became sort of live but wasn't grounded so it didn't trip any breakers, until someone saw the arcing smoldering mess and shut it off.
 








 
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