ALT Field wire and mag heading

[bkthomps]

Have you guys ran into any issues with the alternator field wire causing interference with the magnetic heading?

Any ideas what would cause this since it's just raw voltage...we have been able to get around this with simply doing the mag calibration while the field wire is engergised, it does not require the alternator to be turning in order for the interference to exist

 

[lolachampcar]

I am running a Plane Power accessory case mounted primary alternator.  The field draws roughly five amps just after starting (when the battery is discharged).  At 1300 rpm or above when the alternator will put out significant current, the ADAHRS and my whisky compass will swing nine to ten degrees.  I have roughly two feet of spacing between the back of the alternator and the ADAHRS/Compass combination (which are mounted about six inches from each other).

So, the short answer to your question is yes, I've had problems with field excitation affecting magnetic heading.  I do not believe it is field current in the wire to the alternator that causes the problem but rather the alternator magnetic field which is created by that field current.  There is a firewall and a fuel tank between the source and magnetic heading elements; both are carbon and thus permeable from a magnetic point of view.

My current draw once the battery is fully charged is roughly 3 amps.  I simply calibrated both the ADAHRS and whisky compass once the battery charge dropped to zero.  This was roughly three to four degrees.  The good thing was that my winds aloft vector started working nicely once the calibration was in place and the TAS/GS comparison was much better.  The only issue I face is night time operation when position and strobe lights cause about a three degree shift.

Hope this helps.

 

[bkthomps]

given everything that you just said- which is all valid, i can produce this issue without the alternator spinning- engine off.

It's not a big deal because we have calibrated with the alt ON, but, in the event of an alternator failure, i am going to be presented with a ADHARS failure screen and have to choose the correct one, etc, so i'd like to solve the problem, my wire is shielded between the regulator and the alternator

 

[dynonsupport]

Are you saying you have two ADAHRS units right on top of one another and only one is affected by this?

You'll only get an ADAHRS failure screen if they disagree with one another, which they shouldn't because they will react to the change in the same way.

 

[bkthomps]

when I turn the screens on or the master + avionics master on, but fail to turn the alt switch on (powering the field wire), one of the adahrs heading is about 20deg off from the other one, they are mounted about 6" apart on a level plane, so I'll have to do more research and see if it's the one nearer the circuit breakers/switches/voltage reg

so far like i said it's not a problem in normal flight, when all is working, but if I was in IMC, had to disable my alternator for any reason, then i'd have a heading mismatch and that concerns me if it's something i can fix

 

[dynonsupport]

If that's the case, you and don't want to move things around, do a test to see which one is correct without that alternator on (they're identified by serial number). Then, when you get the mis-match, you'll know which one to disable in that scenario.

 

[dynonsupport]

Also, you can set a preferred ADAHRS in PFD SETUP > ADAHRS SOURCE SELECTION, so that even if you do get the mismatch during a failure, there's a very high percentage chance that the unaffected ADAHRS is the one that's alredy being used.

 

[lolachampcar]

Please keep the thread posted with your results. I'd like to better understand how alt field current in a single wire run can affect the ADAHRs. I'd like to try to repeat your results in my installation.

 

[bkthomps]

I can tell you that we dismounted the voltage regulator, moved it around, and it didnt affect the heading, we took the alt field wire on the voltage regulator side (not the alternator->volt reg wire, but the switch -> volt reg wire) and moved it, that caused the interference

activating the standby b&c alt which is on the back of the engine, does not cause this interference....very odd

 

[lolachampcar]

I'll play with mine a bit as using a shielded wire from the dash mounted switch to the alternator (internally regulated) to remove my issue would allow the ADAHRS to be accurate at any charging current. That would be nice.