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I'm reading the Operator's Manual for Rotax Engine t7pe 912 i Series, Edition 1, Rev. 0, page 7-8, section 7.4) Electrical system

At the bottom of the page is a "text box" with a margin label "EMS power supply".


And it reads, "The EMS power supply primarily comprises 2 alternators with permanent magnets.......They are driven by the crankshaft and require no external supply once the engine has reached its idle speed. NOTE: Until the idle speed is reached, the EMS requires an external 12 V supply from the on-board system of the aircraft."


And I was wondering why the heck a permanent magnet alternator needed external power,.. it shouldn't need it to form a magnetic field on the armature to excite the coils in the stator...it's got PERMANENT magnets.

And so I said, well, I'll just go ask on the Rotax Owner website,. Surely someone there will know.

I searched and didn't find answer in previous questions.

So I started typing up this one. And about 4 lines ago it hit me. The external 12 v power isn't to create a field on the alternator.... it's to run the ECU, etc., to keep the fuel injectors and spark plugs operating properly so that the ENGINE can RUN to drive the alternators. Without the external power, even though the alternator was putting out something while the engine is turned by the starter motor, it wouldn't be enough voltage/amps to run the ECU., fuel pumps, etc., and it wouldn't be enough until a much higher engine RPM was reached (alternator output being related to speed). Which is to say, not until the engine starts and comes up to at least idle speed.

So then I said to myself (I talk to myself sometimes because sometimes I need expert advice) , "Self, you should erase this draft post from the forum. But I replied, "Nope, the post is almost all done, and who knows, maybe someone as similarly "slow" as I will come looking with the same question.

Of course, if I'm wrong, please let me know.

Allen
  • Re: Why is external DC power needed for Alternators with Permanent Magnets on start

    by » 7 years ago


    Very nice post, glad to see you figured it out. The only small thing you missed is that the A alternator , which is exclusive to the ECU, is not referenced to A/C ground but instead to ECU ground. So that s why the "start power" ,derived from the A/C battery, is applied to the ECU ,and thus to the fuel pumps and ignition system, by a MOMENTARY switch. Once the engine starts ECU power is now being supplied by the A alternator and the momentary switch is released. Meanwhile the B alternator is now supplying power to the the general aircraft system and its output can be monitored by an installed amp meter system. The only monitor for the A alternator is seen in whatever electronic system is being used to display the engine sensors. However only A volts is displayed not amps ( as in my Advances 5600 EFIS).

    Thank you said by: Allen Inks

  • Re: Why is external DC power needed for Alternators with Permanent Magnets on start

    by » 7 years ago


    Thanks for the reply. It actually brings up a point that I was wondering about. I'm trying to figure out: What indication there is of an Alternator A failure? On my Dynon SkyView display, there are indications for LANE A voltage, and LANE B voltage, as well as voltage at, and current through the shunt downstream of my Master solenoid. If Alternator A fails, does LANE A voltage go away for good, or does it come back when ECU automatically switches one-time over to the B alternator?

  • Re: Why is external DC power needed for Alternators with Permanent Magnets on start

    by » 7 years ago


    Have never had an alternator failure in my 912iS so cannot answer that. Perhaps Roger can pull an answer out of his magical hat.

  • Re: Why is external DC power needed for Alternators with Permanent Magnets on start

    by » 7 years ago


    voltage is tricky to use as a warning, your ammeter will give you a better indication.
    If the 16a gen fails it will switch to the 30a gen. you will see a discharge on your ammeter.
    Same with the 30a gen.

  • Re: Why is external DC power needed for Alternators with Permanent Magnets on start

    by » 7 years ago


    Thank you for this answer, Rob.

    I was an Electrician's Mate in the Navy. When I was running multiple DC generators to a common bus, I still had indication of voltage of each generator AND a bus voltage. If a generator failed, causing the output breaker to open, I could see it's output voltage go to zero, while bus voltage and the output voltage of the other generator would.. usually drop just a bit, and, similar to what you say, would start seeing battery discharge current if the one good generator could not carry all my DC loads by itself.

    It seems odd to me that we have both LANE A voltage and LANE B voltage indications, if they are always going to be the same. Perhaps I'm misunderstanding what LANE A voltage and LANE B voltage indications are: I thought that they represented the output voltages of the two alternator/generators.

    I do not recall seeing an electrical schematic showing where these voltmeters are sensing. Of course, I haven't super-scrutinized every manual available. Could you recommend a drawings showing this (something that would make it clear why a LANE voltage would stay up, even if an alternator/generator failed)? Like I said earlier, it seems odd to me, and now I'd like to know about this aspect of the systems I operate.

    Again, thank you very much for taking time to answer my question(s). If you are too busy to look up a drawing, I certainly understand.

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