Two days ago I had a ALT 1 failure, and I am still not sure what is wrong, although i have an idea.
After an eventless flight (through dry and sunny air) I landed and while leaving the rwy I had “LOW VOLTS” message and the ALT 1 Circuit Breaker had popped. I stopped on the taxiway with the engine idling at 1000 rpm and reset the breaker. When I switched on ALT 1 the CB immediately tripped again.
When i tried again today (engine off) the CB tripped immediately when I swicthed BAT 1 ON and ALT 1 ON. I took off the field wire from the alternator and tried again: CB tripped. I even took off ALL WIRES from the ALT 1 (FWIW ..) and still the CB tripped.
Now my best guess is that either the wire from the ALT 1 to the FCU 1 (Field Control Unit) has a short – or the FCU 1 itself say bye bye …
My PLAN is to go basck later this week and try to measure the Field Wire from ALT 1 to the FCU and see if it has a short (from what I saw today it looks very good) … and if that is not the case I’ll replace the FCU. I just think that if I replave the FCU with a short in the wire I’ll burn the NEW FCU aswell.
Any thoughts? Ideas?
Some pictures:
Have I got it right that the FCU regulates the alternator field by modulating the current from + (presumably the equivalent of D+ on automotive alternators) to the field? If so, and perhaps also if the configuration is different, replace it with some light bulb and see/measure what happens? To make sure about a short circuit in the wire from the FCU to the alternator, disconnect it at the alternator side and do the same test? If the bulb still glows, your suspicion is confirmed.
[ edited to add: I see you already did disconnect the field wire at the alternator end – not much doubt possible, then ]
In your first picture, which wire does what? The two big ones must be B+ and D+ (as these functions are called in automotive applications, meaning the positive outputs from the rectifiers) but what is the thin one? It looks rather meagre for feeding the field coil, but I cannot imagine what else it could be for.
There is no such thing as a “Field Control Unit” in the Cirrus MM. You probably meant to say MCU
[edited suitably]
FCU = Filed Control Unit, sorry! There’s two FCUs in the “MCU” (Master Control Unit), because the SR22 has two alternators.
The left big cable is the main, the small one below is the Field Wire that goes to the MCU. The right big cable is the ground.
@Jan,
thank you for your kind reply!
No need to say sorry, it was impeccable in your opening message!
from the ALT 1 to the FCU 1 (Field Control Unit)
@Jan,
BTW: I have very good technical support @ COPA and by some specialists there. I posted the case here since I thought it might be interesting for some visiting Cirrus pilot in the future.
@Peter: What is your opinion on this? It would interest me what you think about it.
@Jan
You know, inside the MCU there’s 2 FCU’s, Field Control Units. It’s a common failure that a) either a short in the field wire destroys one of the 2 FCUs, or (with older MCU’s of the type 100 and 120) a short in the generator kills the FCU aswell.
The NEWER type (MCU 130) has a different design, and Tim Timmermann, one of the people responsible of the design of the aircraft at Cirrus, wrote that with an MCU 130 the generator can not kill the FCU.
I don’t know enough about this plane’s electrics.
One should be able to debug an alternator, by rotating it at say 2000rpm, under some load, and putting a high wattage variable resistor (a “rheostat”) in series with the field winding.
One should be able to debug a voltage regulator by putting a variable voltage across the bus and seeing how much current the field winding terminal draws (with a suitable current limiting resistor, in case it is a switch-mode regulator).
BTW @Flyer59 if you want to reference somebody so they get a notification, there has to be
Thank you, Peter. Don’t worry about it. I will study the schematics, but it looks like the ALT 1 FCU just burned. Not that expensive … But before i install it I have to check if there’s a short in the wire or I’ll burn the next FCU aswell …
@Peter: Copied!
