External alternator kit from Rotec Aerosport

[bbs428]

1. "Does the "field" connection on their alternator allow its output to be turned OFF remotely?"
2. "If so, is it connected to the alternator's actual field winding, or is it an ON/OFF control signal to the regulator?"

I have no idea. Did some fishing around and the consensus is that the Nippondenso alternators field goes to the windings but until you know for certain, you don't really know.
Guess I'll just have to check it out after startup! Will post a reply when I have the results.

Agreed about the ammeter. I'll use the G3x to monitor its output.

Thanks for the insight, Eric!

[Eric Page]

Guess I'll just have to check it out after startup! Will post a reply when I have the results.

Yes, please do! The answers will be useful to anyone considering an external alternator.

Unfortunately, on-engine testing can only answer question #1 if switching the "field" input turns the unit ON/OFF, but without asking Rotec or reverse engineering the unit, we can't answer #2.

You may know all of the following -- if so, I apologize -- but let me try to explain why the answer to my second question is so important.

1. If the "field" terminal is directly connected to one end of the field winding and the regulator controls the other end -or- if the "field" terminal supplies power to the regulator which controls both ends of the field winding, then an external switch, either manual or automatic, can shut down the alternator regardless of which component of the unit has failed.

2. However, if one end of the field winding is connected to the "B" terminal and the other end is controlled by the regulator -or- the regulator is powered from the "B" terminal and controls both ends of the field winding, -and- in either case the "field" terminal is an ON/OFF signal to the regulator, then there are regulator failure modes (e.g. short circuit of the field drive transistor) that can cause the regulator to apply maximum field current with no way to turn it off.

In paragraph 1, above, an external OVP circuit can trip the field breaker, ending the overvoltage event in a few tens of milliseconds.

In paragraph 2, neither an OVP circuit nor pilot switch action can stop it. With a failed regulator applying full field current sourced from the "B" terminal and the alternator spinning, it becomes self-sustaining and stopping its rotation is the only way shut it off.

Either of the examples in paragraph 1 is preferred, ideally with manual control of the field via a panel-mounted switch and an external OVP circuit. The examples in paragraph 2 aren't great choices for aircraft.