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Thread: External alternator kit from Rotec Aerosport

  1. #1
    Senior Member bbs428's Avatar
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    Default External alternator kit from Rotec Aerosport

    Received my external alternator from Rotec Aerosport.

    They applied a nice discount due to the strong dollar overseas. It's a one wire Alt. Nippon-Denso type, 45amp. It can be easily modified to an external regulator if you desire. I had to purchase the plastic excitation plug separately. Would have been nice to have been included. Wiring diagram on their web site is for Jabaru 2200 not 912ul. I also had to mark and drill the six propeller hub bolt holes in the aluminum pulley. Would have been nice if this was done at the factory. But it's all simple stuff, so nbd. Everything bolted up fine. All hardware was included.

    Been debating if I should keep the internal regulator or go with a more sophisticated external regulator. In my 40 year's experience, I have had only one regulator failure that destroyed the battery and that was an external ford regulator on a 1969 mustang. Since I have a backup Alt. it makes no sense to add weight and spend more $$

    One thing to note that unlike the Rotax external Alt. kit, this kit the aluminum pulley will push your propeller spacer out 5mm. So if your cowling is all measured and cut it might not work for you. I took my prop. spacer to a machine shop and took 5mm off the thick end so all ended up the same.

    I saved over a thousand dollars vs the Rotax kit, so the inconveniences were worth it to me. We shall see if the internal regulator holds up or not. Given the time and resources, I would make my own bracket and buy the Alt. separately. These Alternators are priced between $83.00 - $200.00 on the internet.
    B&C has a balanced, heavy-duty, 40-amp Alt. at a substantially higher price.

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    rotec5.jpg
    Somebody said that carrier pilots were the best in the world, and they must be or there wouldn't be any of them left alive. Ernie Pyle

    Brett Butler
    Building: 1998 Model 5 Outback, 912ul 110hp, G3x and 7 mods

  2. #2
    Senior Member Eric Page's Avatar
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    Default Re: External alternator kit from Rotec Aerosport

    Most true 1-wire alternators cannot be shut off remotely. Does the Rotec alternator kit offer external control of the field so it can be shut off by the pilot or by an overvoltage sensing circuit?
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

  3. #3
    Senior Member bbs428's Avatar
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    Default Re: External alternator kit from Rotec Aerosport

    It has 3 wires - two 18ga. wires that plug into the back and the 14v 45-amp output cable. So, it's not a true one wire, sorry for the confusion.
    Of the two 18 ga wires that plug into the back, one is for an Alt. warning light, and one is for 12+ field. No overvoltage sensing circuit. You would have to purchase an external regulator. If my amp meter showed a failure and/or the Alt light was illuminated, I would pull the 5-amp field c/b and switch to the secondary stock internal alt. If all I had was the external alternator, I think I would opt for the better external regulator imho.

    The LR3D regulator controller from B&C offers more protection.
    07-06742.jpg

    LR3D-14 Alternator Controller/Regulator, 14V (Homebuilt) - B&C Specialty Products (bandc.com)
    Last edited by bbs428; 11-21-2022 at 01:08 PM.
    Somebody said that carrier pilots were the best in the world, and they must be or there wouldn't be any of them left alive. Ernie Pyle

    Brett Butler
    Building: 1998 Model 5 Outback, 912ul 110hp, G3x and 7 mods

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    Senior Member Eric Page's Avatar
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    Default Re: External alternator kit from Rotec Aerosport

    OK, that's good info.

    Some internally regulated automotive alternators have a "field" connection too, but in many of them it's not really tied to the field winding; it's a control line from the car's computer to the internal regulator. In those units it works to turn the alternator on, but pulling it to ground will
    not turn the alternator off.

    I looked at the Rotec website but couldn't find any technical details. I was looking for the answers to two important questions:

    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?

    Regarding panel indicators, a low voltage warning light is definitely useful to the pilot. It tells you that the battery is not being charged and allows you to troubleshoot or load-shed and find the nearest runway.

    A high voltage warning light is difficult to implement (a severe overvoltage condition can destroy the sensing circuit) and of little value since the voltage rate-of-rise can be extremely fast -- much faster than the pilot's recognition and reaction time.


    An ammeter is also of little value, for several reasons:

    • Alternators are inherently current-limited by the physics of their design
    • Current from the battery is inherently limited by the total load of all installed equipment
    • Wiring is protected against excessive loads by fuses or circuit breakers
    • System load is constantly changing, making ammeter readings difficult to interpret
    • Detecting anomalies demands that the pilot monitor or at least frequently scan the ammeter

    Sensing battery voltage and including active notification of low volts is a more useful way to monitor charging system function.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

  5. #5
    Senior Member bbs428's Avatar
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    Default Re: External alternator kit from Rotec Aerosport

    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!







    Somebody said that carrier pilots were the best in the world, and they must be or there wouldn't be any of them left alive. Ernie Pyle

    Brett Butler
    Building: 1998 Model 5 Outback, 912ul 110hp, G3x and 7 mods

  6. #6
    Senior Member Eric Page's Avatar
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    Default Re: External alternator kit from Rotec Aerosport

    Quote Originally Posted by bbs428 View Post
    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.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

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