Can I shield the magnetic field from power cables?

salar

salar

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Like the title says, I have a run of power cables up some trunking in the windscreen pillar which is close to the compass. When I switch the wipers on I notice it affects the compass heading by about 10 degrees. Is there anything I can do - twisting the cables (if so how?) or is there a magnetic shield I can fit?
 
salar said:
Like the title says, I have a run of power cables up some trunking in the windscreen pillar which is close to the compass. When I switch the wipers on I notice it affects the compass heading by about 10 degrees. Is there anything I can do - twisting the cables (if so how?) or is there a magnetic shield I can fit?
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There are cables that minimise electro magnetic interference, and yes, the trick is in the twist.
Look for star-quad microphone cable. While it is intended for signal as opposed to power, one requirement is strength which means it can also handle significant currents. Coax will do the job too, but has lower power capability, although I have heard of TV aerial fitters that use the aerial cable as a mains extension for a hand drill!

Or, you could just pre twist a twin cable or a run of paired + and -, use a drill on a pre cut length. You need a twist pitch of approximately 3 times the spacing between conductors.


Ready made or diy, it is vital that the send and return use their own pair of conductors, no common - feed for multiple circuits or (heaven forbid) chassis return.
 
Surely the likely cause of the deflection is the magnetic field from the wiper motors? That will be far greater than the magnetic field from the cables. If that's the case, then it will be much harder - and probably impossible - to shield effectively. Moving the compass is the best solution - I have a remote fluxgate compass that is far from and potential source of interference.
 
AntarcticPilot said:
Surely the likely cause of the deflection is the magnetic field from the wiper motors? That will be far greater than the magnetic field from the cables. If that's the case, then it will be much harder - and probably impossible - to shield effectively. Moving the compass is the best solution - I have a remote fluxgate compass that is far from and potential source of interference.
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Very possibly, but twisting the cables in not hard to try.
A lot of motors are not that bad for stray magnetic field.
 
When I was the Master of a survey vessel one of our clients was getting bad interference on their multi beam data whenever we used the VHF.

They were well know as a cheapskate company... My engineer and the Mate worked with them and found the cable shielding was defective. Instead of replacing the cable they went to Asda and brought numerous rolls of aluminum foil. We rediculed them in jest ( the actual on board team were a good crowd), they spent about 6 hrs wrapping the cable in foil. We went for a sea trial and much to our amazement and their victory cries it worked and lasted for the remaining 4/5 weeks of the job.

W.
 
A silly but true story: I worked for an excellent British company which owns a lot of aeroplanes and a lot of ships, but at the date of this story the Chairman had been persuaded into buying an East German built bulk carrier. The word "lemon" would be unfair to lemons. From the dreadful East German MAN main engine to the miserable VP propeller to the unspeakable SKL generators everything was built on the well known principles of Communist Engineering - it has to look pretty, the materials are always dreadful and no attention is to be given to the needs of the poor so-and-sos who have to maintain it (the SKL for example needs you to take the cylinders off to get to the camshaft..)

Anyway, the ship always stopped dead at about 16.56 ship's time, every afternoon. Eventually this was traced to the Fourth Engineer (a "career" Fourth) starting the fuel transfer pump. The control circuitry passed down the same duct and the pump motors' starting surge tripped it every time. The Fourth Engineer never put two and two together.
 
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You ask a good question and the answer is “to some extent”.
Shielding magnetic fields is a Science on its own. Think of it in this way, if you oppose a certain force with the same amount of opposite force it should in theory cancel itself. In some applications ( i.e. Ethernet) the cables are twisted to a high grade so that the turns are as close to a curve equal to the other.
The problem with that is that currents suffer a string of effects because of it and the magnetic curves tend to not match each other which is impolite to say the least but a problem that many Labs and teams of scientists try to solve.
In most real-world applications Passive shielding is achieved using metal plates, for high end applications active shielding is used but read mitigation and you’re talking big systems like electron beam devices and electron microscopy. The results I believe are only about 65% to 95% and I never heard of 100 % even on Lab tests
The problem with miracle cures is that a magnetic field has both a strength and direction. If it is not met with the opposite strength or precise direction then deflection happens and a big percentage of cancelation goes south.
Keep them wires away from the compass, twist them preferably by pinning them on a portable drill, stretch them and slowly wind them to achieve the best equalization on the turns. Alternatively use roofing lead and wrap them inside.
Hope it helps some.
 
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salar said:
Great tips thank you.
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No problem, let me know if it solves it as it may help others with similar problems. I have solved a few problems like that in GPS RT Systems as TFT monitors get affected a lot by magnetic fields. But it is not a precise solution. For that I believe there are specialist companies that specialize in calibrating compasses affected by magnetic fields. Altough in your case, as it is only under power up inflence that may be the best solution for you, twist or lead.
 
GregOddity said:
No problem, let me know if it solves it as it may help others with similar problems. I have solved a few problems like that in GPS RT Systems as TFT monitors get affected a lot by magnetic fields. But it is not a precise solution. For that I believe there are specialist companies that specialize in calibrating compasses affected by magnetic fields. Altough in your case, as it is only under power up inflence that may be the best solution for you, twist or lead.
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Lead will not have any effect, it cannot sheild anything from a magnetic field.
 
Another magnetic shielding silly story. I have a very fine deck watch which was originally supplied to the Royal Navy, by the Swiss firm Ulysse Nardin, in 1940. Unusually it has a stainless steel case with a stainless steel back and stainless steel inner dust cover. This was intended to offer some shielding to the balance spring against the magnetic fields present in conventional submarines, with their huge battery banks and DC motors. I have seen a precisely identical example, apart from the motif on the back, supplied to the Kriegsmarine for the very same purpose at the very same time by the very same firm!
 
PaulRainbow said:
Lead will not have any effect, it cannot sheild anything from a magnetic field.
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My answer was given on a lay basis without delving into the intricacies of the science as to keep it simple. The use of lead was demonstrated by Cabrera in 1983 as a thin bag of lead inside a cylinder of layered MU metal cylinder cooled to 4k Lead is used for deflection of forces as even MU will in time react to the magnetic fields. Lead will react slower than most materials readily available like aluminium or copper, which evidence a more pronounced movement
The best field results I’ve had was twisted cabling wrapped in lead. Not just lead alone. But nothing is there to say that the twisting of the wires is not the only factor and the lead had no effect as I could not measure any parameters of the forcefields involved. But having said that it is what I use as my “go to” solution based on the view that shunting magnetic field lines which the turns on the cables help reduce, then dampening or deflecting them seems the best low tech available. I may be wrong of course. It works for me.

Theory and Detection of Magnetic Monopoles in Gauge Theories: A Collected Set of Lecture Notes Hardcover – 1 Apr 1986
by Giorgio Giacomelli (Author), Werner Nahm (Author), Qaisar Shafi (Author)
https://www.amazon.co.uk/Theory-Detection-Magnetic-Monopoles-Theories/dp/9971966948
 
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GregOddity said:
My answer was given on a lay basis without delving into the intricacies of the science as to keep it simple. The use of lead was demonstrated by Cabrera in 1983 as a thin bag of lead inside a cylinder of layered MU metal cylinder cooled to 4k Lead is used for deflection of forces as even MU will in time react to the magnetic fields. Lead will react slower than most materials readily available like aluminium or copper, which evidence a more pronounced movement
The best field results I’ve had was twisted cabling wrapped in lead. Not just lead alone. But nothing is there to say that the twisting of the wires is not the only factor and the lead had no effect as I could not measure any parameters of the forcefields involved. But having said that it is what I use as my “go to” solution based on the view that shunting magnetic field lines which the turns on the cables help reduce, then dampening or deflecting them seems the best low tech available. I may be wrong of course. It works for me.
Click to expand...

Twisting the cables is known to work, but the lead will not have any effect, neither will aluminium.

If a material is successfully used to block a locally generated magnetic field then it will also block the effect of the Earths magnetic field, so will in itself cause the compass to read incorrectly. The answer is to either cancel the local magnetic field or to fit a fluxgate compass in a location free from local interference (as someone else suggested).

Siting of fluxgate sensors requires some consideration too, obviously. I recently worked on a boat where the sensor was fitted under the cockpit seating. I went past it with a screwdriver in my pocket and the auto pilot had a fit :ambivalence:
 
PaulRainbow said:
Twisting the cables is known to work, but the lead will not have any effect, neither will aluminium.

If a material is successfully used to block a locally generated magnetic field then it will also block the effect of the Earths magnetic field, so will in itself cause the compass to read incorrectly. The answer is to either cancel the local magnetic field or to fit a fluxgate compass in a location free from local interference (as someone else suggested).

Siting of fluxgate sensors requires some consideration too, obviously. I recently worked on a boat where the sensor was fitted under the cockpit seating. I went past it with a screwdriver in my pocket and the auto pilot had a fit :ambivalence:
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Hehe I know EXACTLY where you coming from I had that problem with a little magnet in my pocket and took me forever to realize what was going on. But the lead is not to block as it certainly cannot, but simply to dampen and deflect to some extent the eddy currents on the twisting of the cable. I do admit that my readings on magnetic fields predate the 90’s and are a bit long in the tooth. So I may be completely of the mark, but my best results are indeed using both.
But yes getting the cables away from any compass would be the optimal solution. I would not bother with aluminium tho…
 
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