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Old 25-08-2008, 10:15   #1
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GPS / Autopilot Compass Swings Off 10 Degrees

Any ideas?,my Trace 1500DR makes my A.P. electric compass go out by 10 degrees when I switch it on.It is 4ft. away from inverter.
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Old 25-08-2008, 10:39   #2
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you need to move the inverter or the compass.
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Old 26-08-2008, 04:12   #3
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The amount of Deviation, caused by your inverter (or other magnetic influences), will not be a constant 10 degrees, but will depend upon your heading, at any given time.

Deviation is caused by the magnetic influence of anything near the compass needle.

If you cannot eliminate the error by moving the inverter, then you must tabulate it on what is known as a deviation card.
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Old 26-08-2008, 04:24   #4
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Perhaps try a Mu metal shield around the inverter taking into account any potential cooling issues, but as others have posted, move the source of interference.
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Old 26-08-2008, 08:46   #5
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Probably easier to move the inverter than the compass but a quick question - what is the inverter powering?
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Old 26-08-2008, 12:03   #6
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Quote:
Originally Posted by GordMay View Post
The amount of Deviation, caused by your inverter (or other magnetic influences), will not be a constant 10 degrees, but will depend upon your heading, at any given time.

Deviation is caused by the magnetic influence of anything near the compass needle.

If you cannot eliminate the error by moving the inverter, then you must tabulate it on what is known as a deviation card.

Only problem with the deviation card is that it will not stop the autopilot from making an abrupt turn.
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Old 26-08-2008, 12:11   #7
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Sometimes the power feed to the electronic compass is routed alongside the Inverter AC line and causes radiation.

See if you can separate it or shield it in some way by wrapping in tinfoil as a test
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Old 26-08-2008, 12:23   #8
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You really just need to separate the inverter and the autopilot compass.
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Old 26-08-2008, 13:06   #9
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Sine or Square?

I think the clue here is in the distance apart the items are, suggesting its not magnetic!

You do not say what type of inverter you are using AC-DC (like a battery charger), AC-DC (shore supply) or maybe DC-AC 220/110 or even a DC-DC converter. As many people use the word inverter for convertor or invertor.

The efficency and performance is proportional to the cost of these devices as to produce a nice clean ripple free output (AC or DC) requires the removal of any variations produced in the conversion process. This does not mean a high price is the best though.

If you have a Battery Charger type device the output is often left a bit rough as it is relying on the battery to smooth the ripples (often good for wet cell type). Others chop the AC using thyristors into a rectified square wave DC but it often remains as a pulsed DC. This is cheap as it does away with the cost of a transformer (used for many household plug in AC-DC adaptors) and the capacitors for smoothing.

Then if its a DC-AC inverter, many produce a ramp up of the voltage in pulsed steps to simulate the production of the sine wave which to look at is like a stairway over the hill and down the valley. If this is put through a transformer it can still be transferred to the output.

A pure smooth sine wave output (as is recommended by supliers of TV and electronics gizmos) has so many steps and they are smoothed out before and after the transformer stages to produce a clean sine wave (or close to it).

So how does this help you all - well its the old story for every action there is an opposite reaction. If you are feeding, from your batteries, a pulsing device you will find a pulsing current drain from the battery which is acting like an AC ripple on the DC supply and seen by all connected devices. Hold that thought.

If you are feeding to your battery with a rippling DC from the inverter / convertor and especially if the battery voltage is low, it will be seen and felt by every device connected, i.e. its the same as above.

Your GPS, flux gate compass, etc. work by "sample - hold - display, sample - hold - display, sample - hold - display". otherwise how else would you be able to read them as you move along. Give them a rough and ripply DC and its like giving them AC, they are not designed for it. If the harmonics set up between the ripple and the sample hold dispay cycle fall right the sample point may end or start at the wrong time, the flux gate compass is a DC device put some AC in and those sensitive coils are going to do something dfferent.

How to rectify - feed the "GPS/Autopilot compass" (is that one or two devices?) with a separate 12V DC supply then switch on the inverter. If it behaves itself (them selves) then is probably a ripple. If the swing 10 degrees then back to the magnetic theory!

Good luck.

Martin
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Old 26-08-2008, 15:11   #10
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I think the clue here is in the distance apart the items are, suggesting its not magnetic!...Martin
Most ferro-magnetic items are safely located about 3 - 4 Ft (1m) away from a compass, but Electro-Magnetic devices (ie: Inverter/Converter) can affect a compass from a much greater distance.
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Old 26-08-2008, 19:10   #11
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Thanks Martin. A very nice short course on the subject. I only wish a decent O-scope was cheaper. People would be stunned at the amound of wobble on the average DC line. Little wonder there are so many issues.

Highseas, turn everything off, BC, Lights, Gen, Radios etc, and see what happens. I found a magnetic screwdriver in a drawer 8 in from my autopilot compass. Duh! felt like a ma-roon!
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Old 01-10-2008, 00:28   #12
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Inverter is Trace Dr 1500,with built in charger.Have recently discovered it only happens if there is a large AC load on inverter.Only Autopilot is affected.Cant move inverter,Fluxgate is in good location as when I swung compass,deviation for align heading came within 3 degrees.Took long time to get it that close,and potential locations are limited.May have to get used to compensating with dev. card.Power supply may be problem,will try switch to seperate 12v source.Wasnt aware of the rough and ripply effect.Thanks
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Old 01-10-2008, 07:51   #13
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Someone noted something important, don't pass it up.

If there is a power line running near the compass with direct current (DC or battery power) on the line, the compass WILL SWING OFF. Depending on the distance and the angle the wire takes nearby it might be as much as 90 degrees off!

(That's plain old physics)
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Old 01-10-2008, 07:57   #14
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Oh yeah, if the deviation is caused by a real magnetic field from the inverter (I find that unlikely because of the way they function) then you can shield the field with ferrous materials (sheet steel comes to mind, if the item is mounts on a bulkhead/wall nearby and the wall is between the inverter and compass, putting a piece of light-weight steel sheeting behind the item and remounting it to the wall should help. The steel needs to extend a good distance around the back of the unit.

Inverters take direct current and feed it into either a mechanical or electronic circuits called "choppers" or oscillators then the pulsating DC is fed into step up transformers to generate the 120v you need.

Since there are DC power cables involved, I think you might find the heavy wiring used to feel the inverter is the culprit, rather than the inverter itself.
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Old 01-10-2008, 08:53   #15
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Can I shield the battery cable?It passes by fluxgate in bilge about 1ft apart.
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