Rotax 912is sport over voltage

[rocketman2tm]

So towards the end of my last flight, I had an over voltage event from the alternator. It would spike up above 15 volts and then drop back down to normal. I pulled the alternator breaker and witnessed another spike about a minute later. The measurement point would be on the alternator side and separate from the battery and anything else in the cockpit.

Today I inspected the the phase lines from the generator to the regulators. They seemed secure and didn't have any damage. The plugs seemed fine with no indication of damage. I checked resistance across all the phases on both generators but didn't find anything out of tolerance from what the manual says.

Any ideas?

[Av8r_Sed]

Do you have an external alternator or are you using the stator charging coils? What instrument are you measuring voltage with? Do you have a wiring diagram of the charging circuit?

[Eric Page]

There's no failure mode of the stator windings that could cause an overvoltage. The culprit is almost certainly a failing voltage regulator. The intermittent nature of the event suggests that it's heat related. If the charging system works normally when the engine is started cold but the overvoltage appears after you've been flying for awhile, that would tend to confirm a heat-related issue.

Eventually it will fail completely and you'll have an overvoltage at all times.

There's a Rotax Service Instruction (SI-PAC-019) about installing a heatsink on the voltage regulator to extend its service life. However, if you're able to pipe a significant flow of cooling air over the regulator, the heatsink may not be necessary.

If you do replace your voltage regulator, I would be VERY interested in getting my hands on the failed unit to see if I can figure out what happened. Depending on how it's built I may or may not be able to get inside of it, but I'm happy to pay for postage to give it a try.

[rocketman2tm]

I did end up changing out the voltage regulator, which solved the problem.

[Delta Whisky]

Does your new regulator have over-voltage protection? The no load voltage coming out of the alternator can be 100V so over voltage protection is a good idea.