Amp load indicates 0.8 discharge when both alternators are online.

[dlmorris]

My K Model has two 80 amp alternators (two regulators as well). Amp loads are read from the digital Electronics International MVP-50. In flight, I'll get a warning a slight discharge (0.08 Amps) which is corrected when I take the #2 alternator off line. Both alternators test fine during run up when each is separately taken off line to test for charging ability. Any wisdom out these to pass along.   

[jlunseth]

Alternators are AC current and in the back of my mind I seem to remember this causes issues if they are out of phase, and they will invariably be out of phase.  AC current means essentially that the alternator is putting out burst of current at a cycling rate, usually 60 Hz, but it can be something else, and out of phase means the bursts are coming at different times.  Output is varied by a feedback loop to each alternator, and the two feedback loops interfere with each other.  Boy, it has been a long time since I dealt with that so not sure my memory is right, but I think that is it.  My guess is that if you read your POH, it probably says not to have the two on at the same time.  I also seem to remember that there are ways to phase two alternators and have them run in parallel, but I don't remember what that is.

[N201MKTurbo]

Most likely the #2 regulator is adjusted to a slightly lower voltage. I would need to see the schematic to see exactly where the shunt is in your system to know for sure. 

A couple of things, have you checked the system voltage with each and both alternators on line?

Are the regulators missing a sync wire between them to equalize their outputs?

[Piloto]

Check the current shunt connections.

José

[Piloto]

6 hours ago, jlunseth said:

Alternators are AC current and in the back of my mind I seem to remember this causes issues if they are out of phase, and they will invariably be out of phase.  AC current means essentially that the alternator is putting out burst of current at a cycling rate, usually 60 Hz, but it can be something else, and out of phase means the bursts are coming at different times.  Output is varied by a feedback loop to each alternator, and the two feedback loops interfere with each other.  Boy, it has been a long time since I dealt with that so not sure my memory is right, but I think that is it.  My guess is that if you read your POH, it probably says not to have the two on at the same time.  I also seem to remember that there are ways to phase two alternators and have them run in parallel, but I don't remember what that is.

Unlike AC generators on a power grid the ones in planes are easier to connect. The ones in Mooneys are 14/28VDC alternators with a set of diodes for each winding (three phase) for a DC output. When connected in parallel their outputs are routed though diodes to avoid current flow between alternators, even though the internal diodes would prevent this.

José

[terbang]

I have an MVP-50 as well and see exactly the same effect. Mine is a 252, so it's 28V system. Could it be that the MVP misreads the voltage drop over the shunt?

[carusoam]

 

depending on how they are wired together...

- phase issues (as mentioned previously)

- actual wiring details.  

If one alternator is wired In upstream of the shunt and the other is connected downstream of the shunt. That would lead to a smaller  amount of current actually getting accounted for going across the shunt.

check the plane's wiring diagram for details.  Compare current readings while operating alternators separately and a third value while operating both at the same time.  Make sure that operating at the same time is an acceptable procedure.

the dual battery switch is sort of funny, you can't operate from both batteries at the same time like a marine set-up.

Thoughts of a PP, not a mechanic...

Best regards,

-a-