Can I shield the magnetic field from power cables?

[William_H]

As said try twisting the power supply wires to the windscreen wiper motor. I presume the wiper motor is a permanent magnet type so magnetic fields when running is unlikely to cause compass error. The field generated by the armature will be alternating at rotation rate and shielded by the field outer magnets and pole pieces.
Now much depends on how hard it is to get to and replace the supply wires. If this is difficult it might be worth running temporary wires well away from the compass to check for error. Twist these wires together as described. (make sure only the wiper motor is fed by these wires. There may be a wire from power on at all times to facilitate auto parking. Try disconnecting that wire if it has one. If there are 3 wires twist them all together and if possible run further away from the compass. If you give up on these fixes then you need another compass correction chart for wipers on.
For actual shielding while Mu metal is the best it is not likely to be available in tube form to shield your wires. Any iron or even stainless steel tube that is attracted by a magnet should help.
olewill

 

[Lon nan Gruagach]

I am guessing that when I said "star quad microphone cable" it was perhaps a bit specialist.
Microphones are often used in situations where there is a high level of interference, thus they need good immunity. As a result they also produce greatly reduced interference, even if they are used for ' high ' power signals.

The trick is not just 2 conductors twisted together, but 4 ( thus "quad"). The cable is constructed such that the conductors form an even square ( in cross section), when terminating the cable diagonally opposite conductors are used as a single conductor. The net result is to create 2 virtual signal paths in exactly the same place, along the centre of the cable. Since, from the outside world, the + and - signals are identical magnitude and effectively follow exactly the same route there is no effective radiation. Just for a laugh, and because manufacturing isn't perfect the quad assembly also has a very tight twist. As I said earlier of the pitch is in the order of 3 times conductor separation.
As with twisted cables it is entirely possible to roll your own, but unlike a pair you will need a central packing piece in order to maintain an even square instead of a diamond.

 

[anoccasionalyachtsman]

I am guessing that when I said "star quad microphone cable" it was perhaps a bit specialist.
Microphones are often used in situations where there is a high level of interference, thus they need good immunity. As a result they also produce greatly reduced interference, even if they are used for ' high ' power signals.

The trick is not just 2 conductors twisted together, but 4 ( thus "quad"). The cable is constructed such that the conductors form an even square ( in cross section), when terminating the cable diagonally opposite conductors are used as a single conductor. The net result is to create 2 virtual signal paths in exactly the same place, along the centre of the cable. Since, from the outside world, the + and - signals are identical magnitude and effectively follow exactly the same route there is no effective radiation. Just for a laugh, and because manufacturing isn't perfect the quad assembly also has a very tight twist. As I said earlier of the pitch is in the order of 3 times conductor separation.
As with twisted cables it is entirely possible to roll your own, but unlike a pair you will need a central packing piece in order to maintain an even square instead of a diamond.

That's blimmin clever.

 

[GregOddity]

I am guessing that when I said "star quad microphone cable" it was perhaps a bit specialist.
Microphones are often used in situations where there is a high level of interference, thus they need good immunity. As a result they also produce greatly reduced interference, even if they are used for ' high ' power signals.

The trick is not just 2 conductors twisted together, but 4 ( thus "quad"). The cable is constructed such that the conductors form an even square ( in cross section), when terminating the cable diagonally opposite conductors are used as a single conductor. The net result is to create 2 virtual signal paths in exactly the same place, along the centre of the cable. Since, from the outside world, the + and - signals are identical magnitude and effectively follow exactly the same route there is no effective radiation. Just for a laugh, and because manufacturing isn't perfect the quad assembly also has a very tight twist. As I said earlier of the pitch is in the order of 3 times conductor separation.
As with twisted cables it is entirely possible to roll your own, but unlike a pair you will need a central packing piece in order to maintain an even square instead of a diamond.

Elegant solution. I’m assuming the cables need to be fabricated separately then replacing the ones in use. That being the only quid por quo but a good trade of to get rid of the interference.
You will need a central packing piece and the 4 wires mounted at some distance sideways from the centre core, forming a square with the centre as the packing piece to be able to do the twisting.

 

[lw395]

What does the motor draw? 5A at a guess?
As a rough guide, the field at a distance is reduced proportionally to the area between the conductors, so changing to something like speaker wire where the two wires are as close as possible will be betterthan two single wires loosely strung together.
Going from speaker cable to the same thing twisted ought to reduce the field by typically another factor of 10.
That's the kind of improvement we'd expect when trying to get a circuit to pass interference tests.
Plain old twisted wires was always good enough for the compass light wiring, even before we reduced the current with LEDs.

Twisted wires and more advanced arrangements are theoretically perfect, but only while the cable is straight and symmetrical about the point where you measure the field.

 

[PaulRainbow]

The cable obviously goes up the roof pillar closest to the compass, can it be moved to the other pillar ?

 

[nimbusgb]

run the cables up a section of lead pipe ( if you could find some )

wrap them in lead flashing.

Foil wont make the blindest bit of difference.

An interesting exercise ..... take a beer can and place it next to your compass...... think it's aluminium? Guess again and watch the compass!

 

[PaulRainbow]

run the cables up a section of lead pipe ( if you could find some )

wrap them in lead flashing.

Foil wont make the blindest bit of difference.

An interesting exercise ..... take a beer can and place it next to your compass...... think it's aluminium? Guess again and watch the compass!

I've already said, lead won't make a difference to a magnetic field.

 

[macd]

An interesting exercise ..... take a beer can and place it next to your compass...... think it's aluminium? Guess again and watch the compass!

I think an aluminium beer can is made of aluminium. Steel ones less so. You've simply described a method of telling one from the other.

 

[lw395]

I've already said, lead won't make a difference to a magnetic field.

Some bloke called Maxwell said it a long time before....

 

[PaulRainbow]

Some bloke called Maxwell said it a long time before....

He did indeed, but i don't think he posts here

 

[Kukri]

I've seen one just like you’re describing from the Kriegsmarine, I do wonder if it works though.

I don't think it does. At least not on anything but the Voodoo level.

 

[starfire]

Twisting the cables i

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:

Had a similar problem on a delivery trip, once in a ehile the autopilot would veer off 40 odd degrees.
Eventually traced to a tin of beans rolling around in the bilge, bit of a roll & towards the gyro compass it went.....

 

[NorthUp]

gyro...beans?!

 

[GregOddity]

I don't think it does. At least not on anything but the Voodoo level.

yeah my thinking exactly

 

[GregOddity]

Had a similar problem on a delivery trip, once in a ehile the autopilot would veer off 40 odd degrees.
Eventually traced to a tin of beans rolling around in the bilge, bit of a roll & towards the gyro compass it went.....

LOL yeah a can rolling under it can be interesting.

gyro...beans?!

LOL +1 for that :encouragement:

 

[lw395]

gyro...beans?!

Electromagnetism is all Greek to some people.