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
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
