Stray voltages from VFDs affecting cows

[Marke]

Some time ago I started a thread about stray voltages in dairy sheds, looking for definative information that can be used to establish acceptable limits. To date I have not found anything and am having to rely on my own experience.
I have some data at

http://www.lmphotonics.com/farm/dairy-shed.php

I have a situation where people wearing wigs are disputing the existance of these stray voltages and wanting to battle things out in court. The particular case is one where the farm had a very poor year with very high cell counts (low grade milk) low production and had to cull many cows due to severe mastitis. He also faced very high vet bills.
The problem started when a VFD was incorrectly installed in the milking shed and went away when the installation was corrected. There is very good anecdotal evidence in this case, but the insurance company is paying top dollar for "experts" to refute the theory and evidence.
Any authorative papers etc would be useful. Stray voltage from the mains power supply is clearly understood and accepted, it is the additional HF stray voltages from VFDs that is in dispute. - you can not measure it with a moving iron meter, therefore it does not exist!!

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[Skogsgurra]

Hello Mark!

I had a case a few years ago where a whole lake was supposed to be "electric". That time, all were convinced thar VFDs were the culprits. All except me.

Turned out to be little animals living in slime blobs that irritated people in the lake. But before that was proved, the authorities "earthed" the lake by putting ground rods at the shore and connecting copper wires that were put in the lake to them.

I did some research then about human being's sensitivity to stray voltages and I remember that I found some useful documents. That was several years and several computers ago, so not sure if I can find it again. Will keep on trying.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[roydm]

I know the electrical regulations are very stringent in NZ regarding a milking shed grounding.

https://www.msu.edu/user/hillman/elecshok.htm

 

[jraef]

Mark,
I have a customer who builds milking machines. He gave me a couple of references. One was an old old article he had on this subject referencing Nestle' having this problem. It referenced work done by this person, Alison Dewes, who apparently worked for them in Oz, but has since moved back to NZ. Might be worth contacting her.

http://www.agfirst.co.nz/consultant-alison-dewes.php

He also gave me a link to this page and he said he had heard the author speak authoritatively on the subject once in Canada.

http://www.agromedia.ca/ADM_Articles/content/strayvol.pdf

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[sreid]

I've never seen anything written on this but as an engineer who's worked R/D with PWM amplifiers driving motors for years, it's not uncommon to have an ungrounded motor C clamped to an ungrounded bench for test purposes. One quicly learns not to touch the motor with one hand and a "Water Pipe" with the other hand. Your knees buckle and your mouth gets a metalic tast. The electrical shock is from distributed capacitive coupling from the motor windings to the motor case the driving source being the PWM fast edges.

This doesn't really hurt [like say, touching 220 VAC] but is unplesant enough that you avoid repeating it.

In court, bring the rig I've described and have the Insurance Company's Attorney's see what they think. Sometines a little urine comes out.

 

[Skogsgurra]

That's a great idea, sreid!

And, if that doesn't help, set the carrier frequency as high as possible and connect an incandescent lamp (I use 230 V 15 W) between motor casing and nearest water pipe instead of the PE wire. The lamp usually shines quite brightly. That should convince the rest of the crowd.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dpc]

Stray voltages are a real problem with dairies.

Here's an old paper from US Rural Electrification Agency (now RUS) on the topic. RUS should have additional data available - but no specifically related to VFDs.

http://www.mrec.org/pubs/ARSpub696/introduction.pdf

Regards,

Dave

 

[davidbeach]

Try donaldwzipse.com. Take what you read with a grain of salt, but he's been working on grounding issues for decades.

 

[itsmoked]

sreid; Some thing happens when you forget to hook the return cable from a TIG welder to your work and you trigger the torch. The high frequency arc initiation comes thru everything including gloves to show you the error of your ways.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Some thing happens when you forget to hook the return cable from a TIG welder to your work and you trigger the torch. The high frequency arc initiation comes thru everything including gloves to show you the error of your ways.

Been there, done that, got the t-shirt and the hat...

Also, when that happens if you stand up from the welding table to try to figure out what's wrong, wearing a metal case tape measure who's belt clip is making contact with your skin, don't allow said tape measure case to come close to the welding table when you rest your hand on another grounded object...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Mike2468]

A Canadian company developed a high speed scope for dairy farm stray voltage monitoring. They are at:

http://www.strayvoltage.ca

A news article was published in 2007. A copy is at:

http://www.strayvoltage.ca/Larry.htm

The farmer had VFDs and other items in his parlor. He spent money to analyze his installation and to eliminate a number of possible stray voltage problems. He believes that it made a massive improvement in his production and profits.

 

[jraef]

Something is blocking me from that Canadian site.

But I found this because of it.
Strayvoltage.org


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Marke]

Thanks everyone for your input to date.
There is plenty of information around about the effects of line frequency and transient induced stray voltages along with a wide variance of opinions on threshold levels etc.
Essentially, this is overcome largely by adequate bonding of all the steelwork as the building is constructed, generally the use of single point earthing is important.
The issue that I have, is totally different and is nothing to do with the electrical supply to the dairy shed (parlor).
In New Zealand, we use a lot of VFDs in these facilities, usually one to drive the turntable (rotary cow sheds over here), one to drive the vacuum pump(s) and one to drive the milk pump. There are commonly additional VFDs to control water pumps as well.

A problem with VFDs, is that the output waveform is a PWM waveform with fast rising and falling voltage edges. The motor has capacitance between the windings and the frame and so we get high impulse currents flowing into the frame of the motor. This current needs to find it's way back to the DC bus on the VFD to complete the loop.
There are two ways back to the DC bus, one is via the frame of the VFD and the DC bus decoupling capacitors that are connected between the DC bus and the frame of the VFD, and the other path is back via the AC input (single or three phase).
Provided that the impedance of the decoupling circuit is very low to the transient currents, and the impedance between the motor frame and the VFD frame is very low, the majority of the transient current will return to the DC bus and not cause any issues. Current that does not return directly to the DC bus will flow through the structure of the building finding paths back to the input of the VFD and return that way.
If we connect the VFD to the motor using unscreened cable, we will have an equal impedance between the VFD output and the motor, and between the motor frame and the VFD frame.
The motor capacitance appear close to a short circuit to the transients and so we have a potential divider where the transient voltage on the motor can be as high as 50%. As the motor is mounted on the structure, there are multitudes of earth paths and the noise current flows in all directions.
To eliminate the transient current spread, you either have to totally isolate the motor from ALL other paths, conductive and capacitive, or you reduce the impedance between the motor frame and VFD frame considerably so that the transient voltage on the motor frame is small. As the transient voltage on the motor frame is reduced, the stray currents are also reduced.
Using a screened cable correctly bonded between the VFD frame and the motor frame is an easy way to reduce the earth return impedance dramatically with the screen impedance being 1/20 - 1/50 of the impedance of the phase conductor. This reduces the stray noise voltage from the VFD by a factor of 20 to 50.

Pretty easy in theory and in practice. It is easy to prove that this works, have done it many times, but I can not find supporting documentation to show that it overcomes problems in dairy sheds (parlors) and the only reference to VFD related problems that I can ind is the reference at

http://www.strayvoltage.ca/Larry.htm

This leaves the "facts" open to attack and the claims that VFDs obviously do not cause stray voltage problems because no one else is reporting them.
It has been suggested that the screened cable issue is not relevant and only the structure earthing is of any importance. - there is plenty of documentary evidence to support this.

It has also been suggested that because the noise voltage is very transient in nature, (repeated at a high rate) the RMS value will be very low and therefore not relevant to behavioral modification.

See my problem??
Wigs love to prove things with piles of paper and words. It is easy to show the impact of the HF stray voltages by looking at the time line for the site in question, but of course there must be something else that has been done to the earthing that we are not reporting because that is the only possible cause.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[Mike2468]

The

http://www.strayvoltage.ca

site is functional via Internet Explorer 7, but not Mozilla. Whatever.

The engineer at Strayvoltage was David Rogers, who created the high speed scope so he could monitor farms remotely.