M20k alternator field breaker trips

[FloridaMan]

The alternator field breaker tripped today in flight so I returned to base. With the engine off, the breaker trips immediately when I turn the master on. It’s a hartzell direct drive (no belt) alternator. The alternator is a bit noisy and on a recent flight I noticed it stopped charging briefly. My IA inspected the brushes and noted there was no filter on the output. I suspect a bad diode. 

I think all of this is related:

Batteries seem weak and prop spins slowly with the starter. 

Cricket in the headset from alternator noise. It went away when the breaker tripped. 

Now the breaker trips as soon as the master goes on. 

 

[carusoam]

Very likely to be diodes... rpm related whine...?

Search on alternator and diodes...

https://mooneyspace.com/search/?q=Alternator diodes&sortby=relevancy&search_and_or=and

You might be able to access them and test them for continuity... ?  And check to see if they look cooked?

PP thoughts only, not a mechanic...

Best regards,

-a-

[N201MKTurbo]

If the field breaker trips on the ground without the engine running, it is not the diodes. The field breaker just runs the regulator which powers the spinning electromagnet on the armature through the brushes.

The first thing I would do is disconnect the field wires from the alternator and see if it still blows the breaker. If it does then the regulator is bad. If it doesn’t then the alternator has a shorted armature.

It is possible that bad diodes caused the problem. If diode(s) opened, the regulator would have to put out more current to the field to get the required voltage. This could have blown the regulator or overheated the field (armature).

Edited July 11, 2018 by N201MKTurbo

[RobertGary1]

+1 or a chaffed wire grounding out. Disconnecting things and repeating is the way to go

 

-Robert

[FloridaMan]

May also mean it’s time for two new Concorde batteries. I suspect the root cause could have been a single cell failure in one battery that would put a constant deep cycle on the other one and excessive charging load.