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Old 19-10-2015, 13:57   #1
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Join Date: Aug 2012
Posts: 811
Instrument interference.

For a few years I had needed to recalibrate my wind gauge direction ( no problem with digital wind speed) every time I start sailing. It would go 90*/ 180* out. To cut a long story short, at last I have found the problem and fixed it after trying a lot of possibilities. It was caused by radiated interference from the cables connecting the engine alternator, 80 amp, to my Next Step regulator.

What I'd really like to know is the exact nature of that interference (is it electro magnetic or RF? or something between?) and why is it generated by that cable which in my case is roughly 4 feet long? What is the nature of the current in that cable that it can have this effect? It must be quite a strong field of interference. I understand the general principles of induction, shielding, and earthing. There is no electrical connection between the alternator charging system and the instrument power system.

I fixed the problem after finding an online article by a professor from the University of Oregon. It was about EMI and interference in general. In passing it mentioned that cables connecting alternators and smart regulators can be a cause of interference. I haven't been able to find that article again.

So I wrapped the regulator cable (along with the, running parallel, alternator to diode power cables.) in tinned copper braid earthed to the engine, for the whole 4' length of the loom. (at a cost of $200 for 10 metres of 3/4' flat woven tinned copper braid 2.5 mm thick; so I am glad it worked)

That immediately fixed the problem, so I'm not looking for a solution now, just an explanation. The wind gauge is a Navman 3150 with an analogue type pointer.

Presumably that has a rotating permanent magnet attached to the pointer with a surrounding wound field magnet system. The instrument and its wiring is no closer than 3' / 4' to the regulator cable.

My other unassociated instrument problem was that I blew about 3 Navman chartplotters. The plotter, other instruments and the Autohelm were all wired to the house bank. The Autohelm was causing alternating voltage spikes in both + & - directions that couldn't be filtered out, and that blew the plotters causing a jumbled screen readout.

To fix that, I installed a dedicated instrument battery charged by solar only, not connected to the alternator system (with an option to connect with the house batteries if necessary) That solved the Navman plotter / Autohelm problem but not the above "Wind" problem until I shielded the regulator cable.

Navman replaced the plotters free of charge. Incidentally and later, the new agents Navico, wouldn't replace its non rechargeable memory battery when it eventually failed as "it was more than 10 years old and there were no parts". I did that successfully myself for less than $5 taking under 30 minutes to solder in a larger capacity lithium battery.
GrahamHO is offline   Reply With Quote

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