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Eric Page
Joined: 15 Feb 2017
Posts: 245
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 Posted: Mon May 22, 2023 3:38 pm Post subject: Overvoltage Protection for Rotax 9-Series Engines |
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In a forum thread that I started in October 2021 about using a Rotax 912iS on my Kitfox, there was some discussion about my use of crowbar overvoltage protection (OVP). In that thread, Joe Gores asked, "Can it be designed to just open the relay without blowing a 1 amp fuse?" It turns out that, yes, it can. After several prototypes, I have a non-crowbar OVP Module for Rotax 9-series engines. See the attached PDF document.
The oscilloscope screen capture shows its performance. Each horizontal division is half a millisecond (0.0005 sec) and each vertical division is 10V. The yellow trace is bus voltage (it starts at ~14.5V) and the purple trace is the output driving the relay coils (this trace is inverted; the relays are closed when the trace is at zero and open when it's above zero). When the bus voltage jumps to 40V, there's a ~3 millisecond delay (a time lifted from Bob's design, to prevent very short transients from causing nuisance trips), then the relay coil drive drops out (purple trace rising). Once tripped, the module latches in the tripped state, holding the relays open, which isolates the engine's stator feed from the regulator input. The module also flashes an indicator LED on the instrument panel and sends an alarm signal to an EFIS/EMS input. The trip voltage is adjustable and is set at 16.3V (another value lifted from Bob's design) but I used 40V for the test to demonstrate that the module can survive an overvoltage event (it's designed to survive 60V).
The module adds pilot control of the charging system, a feature lacking in the Rotax wiring diagrams. The 912/914 manuals specify a B-lead circuit breaker that also feeds the regulator's Control terminal, but without a switch. On the 912iS/915iS engines, the regulators are exclusively controlled by the Rotax "Fuse Box" and the only regulator terminals accessible to the installer are the AC inputs. To provide pilot control, the module includes a latching PUSH ISOLATE switch, allowing it to be held in the off state (e.g. in the event of smoke/fire) or to be reset in the event of a nuisance trip.
I'd love to hear feedback on this design from anyone who cares to offer it. As I've demonstrated repeatedly on this forum, I'm not an electrical engineer, so I may have overlooked something.
Eric
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| Description: |
| Oscilloscope screen capture showing OVP Module performance during 40V overvoltage event. |
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| Filesize: |
72.5 KB |
| Viewed: |
1606 Time(s) |
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| Description: |
| OVP Module schematic and description. |
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Download |
| Filename: |
OVP Module (Rev J).pdf |
| Filesize: |
319.95 KB |
| Downloaded: |
142 Time(s) |
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user9253
Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan
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| Posted: Mon May 22, 2023 5:50 pm Post subject: Re: Overvoltage Protection for Rotax 9-Series Engines |
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Eric, your schematic is way over my head. Are you sure that you aren't an electronic engineer. Good work.
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dj_theis
Joined: 28 Aug 2017
Posts: 56
Location: Minnesota
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| Posted: Tue May 23, 2023 10:38 am Post subject: Re: Overvoltage Protection for Rotax 9-Series Engines |
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Nice design Eric.
In particular, I appreciate your use and reasoning behind switching the AC side of the charge circuit. Reliable switching of higher current DC generally requires relays that are expensive and bulky.
I think it will require someone of Bob's caliber to catch any shortcomings in your work.
Like Joe implies, you are you are punching well above your weight class.
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Dan Theis
Scratch building Sonex #1362
Still working on the Revmaster Alternator improvement |
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Eric Page
Joined: 15 Feb 2017
Posts: 245
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| Posted: Wed May 24, 2023 3:39 pm Post subject: Re: Overvoltage Protection for Rotax 9-Series Engines |
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Thanks, fellas. I'm hoping someone with some electronics chops will cast a critical eye over it. It seems to test well on the bench, but airplanes ain't benches...
Eric
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