Voltage Regulators and Over Voltage protection on a Rotax 912ULS

[FlyinFish N928M]

Hello everyone,

Wondering if anyone has any knowledge on the Rotax 912 power system with regards to over-voltage.

The way it has been explained to me is that with the Rotax's stator system, it can only put out a maximum of some 18 amps and is not prone to an over voltage. AC power is regulated into the Rotax supplied Ducati voltage regulator. The enemies to running this regulator is when you exceed more than 70% of it capacity or roughly (13.5 amps) of continuous duty. This creates excessive heat, and when not cooled properly, tend to fail over time. So in my mind installation location, and excessive loads are to be avoided.

If if this regulator is to fail, to my understanding, the onboard capacitor tied into the circuit is designed to absorb the excessive load, especially if the field wire or control wire function on the regulator become disabled. To my understanding, this is the only way the Ducattti voltage regulator could put out more voltage than needed. Any load or excessive voltage is absorbed by electrical system and capacitor, or the regulator will simply over heat and fail.

In addition, being that the AC power generated is slave to permanent magnet and coil assembly, power is ultimately governed by engine rpm, unlike an automotive alternator, there is no chance the rotax configuration can put out anything more than factory specked amperage.

All this being said, when running a glass panel, is there any reason to be concerned about an over voltage scenario destroying my expensive equipment?

If anyone has any extensive knowledge on this subject, I would greatly appreciate the feedback.

Thank you!

[Eric Page]

First, let me say that I'm no expert on Rotax charging systems. I've never owned one, and I've never even flown behind one.


...Rotax supplied Ducati voltage regulator. The enemies to running this regulator is when you exceed more than 70% of it capacity or roughly (13.5 amps) of continuous duty. This creates excessive heat, and when not cooled properly, tend to fail over time.
Yes, the Ducati regulator does have a reputation for poor thermal design leading to failure. See this study (http://contrails.free.fr/elec_ducati_en.php) of the Ducati regulator. Here is its schematic (http://aeroelectric.com/Pictures/Regulators/Rotax-Ducati_Rectifier-Regulator.jpg), for reference.


If this regulator is to fail, to my understanding, the onboard capacitor tied into the circuit is designed to absorb the excessive load, especially if the field wire or control wire function on the regulator become disabled. To my understanding, this is the only way the Ducattti voltage regulator could put out more voltage than needed. Any load or excessive voltage is absorbed by electrical system and capacitor, or the regulator will simply over heat and fail.
In the first sentence, I presume you mean "...absorb the excessive voltage...". The large capacitor might mitigate very brief (on the order of milliseconds) duration spikes in bus voltage, but it will be ineffective against a runaway regulator. The capacitor will simply charge to the new bus voltage very rapidly, and if that new voltage is well above the capacitor's rating, it's likely to fail. Electrolytic capacitors are well known for explosive failures (https://www.youtube.com/watch?v=0OIGz9gHCRI) [language warning] under overvoltage conditions. The ship's battery is much more effective at standing off an overvoltage event.


In addition, being that the AC power generated is slave to permanent magnet and coil assembly, power is ultimately governed by engine rpm, unlike an automotive alternator, there is no chance the rotax configuration can put out anything more than factory specked amperage.
See this study (http://contrails.free.fr/elec_alt_rotax_en.php) of the Rotax alternator, which agrees with your statement that the unit is inherently current limited.


All this being said, when running a glass panel, is there any reason to be concerned about an over voltage scenario destroying my expensive equipment?
If the equipment was designed to comply with DO-160 (https://en.wikipedia.org/wiki/DO-160) requirements, and the electrical system is prevented from creating overvoltage events that exceed those limits, then no. If it were my plane, I would not rely on the Ducati regulator alone to protect my avionics.

Here's an article (http://www.aeroelectric.com/articles/The_Truth_about_Crowbar_OV_Protection.pdf) by Bob Nuckolls about crowbar overvoltage protection, and here's his schematic (http://www.aeroelectric.com/DIY/DIY_Crowbar_OVP_F.pdf) for a DIY overvoltage protection module.

If you prefer an off-the-shelf solution, B&C Specialty Products offers a unit (https://bandc.com/product/avc1-advanced-voltage-controller-homebuilt/#installation-kit) specifically tailored to Rotax engines. It serves as both a regulator and overvoltage protection module.

[FlyinFish N928M]

Thanks for the valued input on this subject Eric. You must be an electrical engineer, you seem quite familiar with this subject. I'm sure many people have wondered the same and never put the question out there. I am familiar with the B&C's AVC1 which looks to be a very promising solution. I am actually at a crossroad as to which to purchase after testing out my current Ducatti regulator. B&C to my knowledge is known for their quality electrical components and feel pretty secure going with them on a new regulator purchase. However, not to create any doubt, the AVC1 Is still a fairly new product to market, the aviation community will actually help prove its' relibality. The AVC1 as I was told was purpose built for the Rotax, with that I'm sure they have fully tested their product before releasing this market. It has a 3 stage over voltage circuit protection built into the design.

As mentioned, the reason for my post is that my originally dated 2003 Ducatti regulator, never used, (sadly) was finally mounted on my firewall with connections made exactly as rotax has indicated. Sadly, this was shorting out my battery fuse link. I had direct connectivity from positive to negative via the regulator housing and capacitor ground. I had systematiaclly isolated my newly built wiring harness and everything is in the green. No miss-wired circuits until I connected the regulator back to service. A real head scratching moment for sure. I made a few calls to Rotax repair centers. As it turn out a new Ducattil regulator on the bench with no connections, when tested with an ohm meter found 8.9 Mega Ohms between R and +B and regulator case ground. This is what I was told a new regulator will test out as. Mine, when tested had zero ohm resistance. How could this be after just sitting around for so many years just hangin on the firewall with no connections?

Can anyone else bench test their (new) out of the box Ducatti regulator to see what measurements come from it? I would be very curious to hear these results!

Thanks!

[Eric Page]

Thanks for the valued input on this subject Eric. You must be an electrical engineer, you seem quite familiar with this subject.
No, I just play one on TV. :rolleyes: Actually, the links I provided were culled from the AeroElectric-List forum (http://forum.matronics.com/viewforum.php?f=3), which I've been following for years. I do dabble in electronics, but it's just another hobby.


As it turn out a new Ducattil regulator on the bench with no connections, when tested with an ohm meter found 8.9 Mega Ohms between R and +B and regulator case ground. This is what I was told a new regulator will test out as. Mine, when tested had zero ohm resistance. How could this be after just sitting around for so many years just hangin on the firewall with no connections?
I guess it's possible your regulator has been dead the whole time but it was never discovered since it wasn't used. Does it have a case that can be opened? If so, you might open it up and see if there's any obvious reason for the short to ground. Look for component leads that weren't cut short enough on the bottom of the board, or extra solder blobs where they shouldn't be, both of which could be touching the metal case. If you find and correct something like that, you might get it working.

[Delta Whisky]

FlyinFish -

Here's my go to link for the Rotax alternator/regulator/rectifier system: http://contrails.free.fr/elec_alt_rotax_en.php

Note that the open circuit voltage from a pure alternator is a function of rpm and the no load output in the case of the 912/914 can be 100V peak (See the "Discussion" paragraph). Reading it and posts here and elsewhere motivated me to purchase the B&C unit and keep the Ducati in the spares bag, just in case the B&C unit goes bad. (Admittedly and for full transparency, I'm a rabid B&C fan due to the fact that I've never had anything they make go bad.) There are other benefits in making the switch but the over voltage protection was enough for me.

The good news - although it appears that English isn't the author's first language, which means I might be reading one of the paragraphs incorrectly, it does appear to say that they ran one at full power (20A) "for weeks" and couldn't kill the test article.

Let us know what you decide to do and any additional knowledge you pick up along the way.

[JethTran]

HI....Insufficient information to reach an inference, but rather SLA batteries can ordinarily take a considerable amount of maltreatment without transforming into lights. In the event that the battery load had remained, it's improbable that the overvoltage would have at any point surpassed the flying's capacity to endure. On the off chance that the expert was pulled inside a couple of moments of OV discovery, it's practically sure that there would have been no inadvertent blow-back.