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Old 26-07-2015, 05:15   #1
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Alternator interferes with electronic compass

We are having a problem I can't figure out a solution to. When we turn on our engine and the externally regulated 100A alternator kicks in, it causes the electronic compass heading to change 20-30*.

Now here is the strange part: we are a catamaran and that problem engine/alternator is in the other hull 20' away from the electronic compass.

There is no problem when the other engine located just 5' away from the compass is running. That engine only has a stock internally-regulated 60A alternator.

I know it is the alternator causing the problem because the issue does not occur if I disconnect the power to the regulator, or disconnect the field wire and leave the regulator powered up. The problem goes away when the batteries reach full and the alternator output drops below 10A.

Again, it does not occur at all when the stock alternator is running full output on the engine just 5' away from the compass - only when the externally regulated alternator is running >10A on the engine 20' away in a different hull.

The NMEA backbone cable that the compass is also connected to does run 6' away from that engine, but I don't see how that could be transmitting a magnetic field or any electrical interference.

I'm embarrassed to admit that I don't know if this has been occurring since install 3yrs ago or a new thing. I never really noticed it before - by the time I am usually paying attention to the compass heading, the engines are generally turned off.

Does anyone have a suggestion as to why the problem is occurring and what I could do to rectify it?

Mark
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Old 26-07-2015, 09:09   #2
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Re: Alternator interferes with electronic compass

Well, knowing your skills and knowledge of you boat if you can't resolve it then it will be pretty difficult for any of us to come up with an answer. However some things to think about...

Does the alternator cause other compasses to swing? Where are the batteries located? Can you move the compass to a different location and will that resolve the problem? Is there some type of ferrous metal structure that could transmit the magnetic field? Is the compass N2K? If so can you temporarily (or permanently) connect it to another N2K bus? Are there any cables carrying large current 12v (that could carry more current if the alternator is charging, like to a fridge) near the compass that could be the cause of the disturbance?
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Old 26-07-2015, 09:15   #3
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Re: Alternator interferes with electronic compass

A ground wire twisted around the alternator wire?
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Old 26-07-2015, 10:12   #4
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Re: Alternator interferes with electronic compass

You might try isolating from that alternator output the 12-volt source which powers your autopilot compass.
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Old 26-07-2015, 10:22   #5
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Re: Alternator interferes with electronic compass

Could it be the same problem that causes alternator noise in car stereos? If that is the case and I think it could be, the problem could be the AC voltage created by the alternator is not completely being converted to DC voltage. Some AC voltage is leaking past the rectifiers and causing interference. It could be a bad diode in the rectifier and could be tested without removal. Another cause could be poor connections at the battery or grounds for the alternator. Good luck, Greg
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Old 26-07-2015, 10:37   #6
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Re: Alternator interferes with electronic compass

Have you disconnected the alternator and battery from the boats wiring and run a test? Also my understanding of the field wire is once the alternator is producing current it no longer needs any external field wire current as it connected to it's own output.
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Old 26-07-2015, 11:52   #7
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Re: Alternator interferes with electronic compass

Thanks for the suggestions so far.

The alternator output is connected directly to the house battery bank, which also powers the N2K bus containing the compass. As a test, I can try to rewire the bus to a standalone battery, but that won't be a solution.

Turning off the alternator is the same as disconnecting it from the boat wiring. It can't be operated disconnected without damaging it. The external field wire controls the amount of current the alternator produces - if disconnected, the output stops.

I don't understand the ground wire twisted response. Are you saying to make them twisted or not to? Since they run 15' back to the house bank, I'm sure they probably have a twist or two somewhere in there.

This is the only electronic compass on the boat. I didn't notice the mechanical compass at the helm swinging, but will check on that.

The batteries are located in the saloon about halfway between the two engines, but a bit closer to the engine causing the issue.

There is no ferrous metal between the compass and alternator other than some odd bits and bobs scattered about like pots and pans, an AC compressor, etc.

The compass is N2K. I can try isolating that bus to just contain the compass and short past the length of backbone that runs near the engine.

There are cables carrying higher current near the compass - the other engine alternator output wires run to the house bank and there is a set of wires running from the house bank to the starting battery via a VSR combiner. This would be very difficult to change. However, the current going to the house bank is provided by the alternator next to the compass that has no effect on the compass. The current returning to the start battery is always negligible (<2A), and provided by the house bank. No other charging sources like the solar or mains charger causes an issue.

Moving the compass permanently would be a large challenge, if even possible. I can move it temporarily and plug it into another part of the N2K bus to test.

So I have some experiments to try.

Mark
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Old 26-07-2015, 12:35   #8
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Re: Alternator interferes with electronic compass

Did this alternator go thru your lightning strike?

If so, we're ALL the diodes replaced?

I'm leaning toward noise on the power bus more than a magnetic problem.

The reason I ask about the diodes, of the six there could be only 1 leaking and the alternator would still put out 80+% of rating plus cause noise. That's something easy to overlook.
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Old 26-07-2015, 12:52   #9
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Re: Alternator interferes with electronic compass

Mark,

Most likely this is caused by the circuit from alternator + output all the way back to the alternator case. You want the + and return wires to travel together and keep them close to one another.

If I had to guess my guess would be there are multiple return paths through the engine block and the "old" path has become corroded. The "new" path makes a bigger loop and thus higher magnetic field at the compass. Put a hand bearing compass next to the electronic compass and see if same change in swing happens.
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Old 26-07-2015, 13:32   #10
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Re: Alternator interferes with electronic compass

DotDun: The alternator was a new one to replace the one that did go through the lightning strike. Interesting thought on a bad diode. This is a 120A alternator that I have running at 95A using the belt manager on the regulator. I do see up to 95A, but that is only 80% of rated max. Further interesting, is that I have a problem with the tach going screwy when that alternator is charging. It is a new tach, and I have thought there was a problem with it (but have not been in the country until now to send it back). The tach jumps around a lot, which could be AC noise from the alternator.

Dan: The alternator is an isolated ground, so the +/- wires run together to the batteries and the case should be isolated from any ground loop. I will try the compass test - I actually have both mechanical and electronic hand bearing compasses.

However, the compass is located 20' away from that alternator and any of its wires. It would have to be a whopping magnetic field to effect it.

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Old 26-07-2015, 15:47   #11
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Re: Alternator interferes with electronic compass

I highly doubt it's causing magnetic interference in the bus itself. This is digital data. It's not going to change vaules. Problems would effect all data or more likely cause all sensors to itermitatly start and stop working. It's not like the magnetic inference knows which pgn string in the buss to attack.

So it's 2 things. The alt is causing mag interface to the compass location itself. (You'd probably see this with a mag compass held in same spot)

Or the interference is getting into the cables, reaching the compass (probably on the power line). Enough to effect it working. Maybe a power filter in the n2k power bus worth trying.

But I doubt it's from noise getting into the bus in the engine room and effecting the data in the bus
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Old 26-07-2015, 19:47   #12
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Re: Alternator interferes with electronic compass

Re twisted wires: I'm referring to a recommendation I heard years ago to twist the positive and negative wires of a DC circuit around each other to avoid interference with a compass. Does anybody here know if this is valid, or an old wives' tale? Might be easy to construct a test.
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Old 26-07-2015, 21:26   #13
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Re: Alternator interferes with electronic compass

Well, it seems that there are two possible classes of interference: magnetic and electronic noise issues. If it is magneticc, then putting a small magnetic compass (ie HBC) next to the electronic compass's sensor and running the alternator will show a similar deviation.

If it passes that test, then the interference must be some sort of electrical noise getting into the unit. The usual filters, shielding and ferrite approaches should then help. Some alternators and some PWM regulators can generate wide band noise, so this would not be too surprising.

More investigation is needed, so good luck and good hunting!

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Old 27-07-2015, 00:31   #14
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Re: Alternator interferes with electronic compass

Quote:
Originally Posted by Steve Bean View Post
Re twisted wires: I'm referring to a recommendation I heard years ago to twist the positive and negative wires of a DC circuit around each other to avoid interference with a compass. Does anybody here know if this is valid, or an old wives' tale? Might be easy to construct a test.

This is not an old wive's tale. But big wires suitable for 120A are hard to twist together. The best alternative is to keep the + & - wires close together. The further they are apart the farther the magnetic field will spread out.
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Old 27-07-2015, 06:23   #15
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Re: Alternator interferes with electronic compass

I had a similar problem when I had my C-36...


I changed the stock alternator to a high output one.


The fluxgate compass was fine when we left the dock, but if we discharged the batteries, say at anchor, the electronic chart heading would be off as much as 90 degrees.


The fluxgate compass was not located near the alternator, but then I realized that the Battery cables were.


When the batteries were discharged the alternator was kicking out about 75 or 80 amps of charging power it would crate a electronic field around the cables and throw the fluxgate compass off. This only occurred when the batteries were discharged and the alternator was in high output.


I moved the fluxgate compass and had no further problems.
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