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

  • Allen Inks
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2 weeks 6 days ago - 2 weeks 6 days ago #17443 by Allen Inks
Allen Inks created the topic: Why is external DC power needed for Alternators with Permanent Magnets on start
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
Last Edit: 2 weeks 6 days ago by Allen Inks.

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2 weeks 5 days ago #17444 by richard Boslaugh
richard Boslaugh replied the topic: Why is external DC power needed for Alternators with Permanent Magnets on start
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).
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2 weeks 4 days ago - 2 weeks 4 days ago #17451 by Allen Inks
Allen Inks replied the topic: Why is external DC power needed for Alternators with Permanent Magnets on start
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?
Last Edit: 2 weeks 4 days ago by Allen Inks.

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2 weeks 4 days ago #17452 by richard Boslaugh
richard Boslaugh replied the topic: Why is external DC power needed for Alternators with Permanent Magnets on start
Have never had an alternator failure in my 912iS so cannot answer that. Perhaps Roger can pull an answer out of his magical hat.

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2 weeks 2 days ago #17465 by Rob Seaton
Rob Seaton replied the topic: Why is external DC power needed for Alternators with Permanent Magnets on start
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.

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2 weeks 2 days ago #17470 by Allen Inks
Allen Inks replied the topic: Why is external DC power needed for Alternators with Permanent Magnets on start
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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2 weeks 2 days ago #17475 by Rob Seaton
Rob Seaton replied the topic: Why is external DC power needed for Alternators with Permanent Magnets on start
Hi Allen,
I don't know, I will have to pass this question to someone with a better knowledge of the EMU.
I was told that voltage can be present, despite a failure, so it is not a reliable indication.
The best solution would be a shunt on both gen outputs but this gets complicated and increases the parts/connections/failure points. It is also unnecessary as we already get a warning of discharge when the gens switch over.
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2 weeks 2 days ago - 2 weeks 2 days ago #17476 by Bill Hertzel
Bill Hertzel replied the topic: Why is external DC power needed for Alternators with Permanent Magnets on start
The Lane/Reg/Gen"A" Voltage is normally the voltage on the EMS internal Bus.
The Lane/Reg/Gen"B" Voltage is normally the voltage on the Airframe Systems. Battery, Panel, etc.
The Two Generators are not in parallel and will show separate but similar voltage.
If one generator fails the two voltage will differ significantly.
~ ~ ~

The X3 connector on the Fuse Box has three pins.

The BACKUP Power Pin(#1) ties directly to the EMS internal power Bus for emergency use only.
It bypasses everything.

The START Power Pin(#2) connects to the EMS internal power bus through a relay.
When the engine is stopped the relay closes and allow START Power to energize the EMS.
Once the engine starts and the Reg"A" comes online, The START Power relay Opens and isolates the EMS from the Airframe Main Bus (the big 12v Battery.) and Reg"A" takes over powering the EMS.

The Main Power Pin(#3) connects internally to Reg"B" when the engine is runnings normally. This is the power source to charge the battery and operate all the airframe loads.
When the Engine Stops or the Reg"A" fails, Reg"B"s Output is removed from the X3 Main Power Pin and transfers to the EMS internal bus to maintain the ignition.
The Airframe power is supplied by the Big 12v battery for as long as it will last. 30 minutes???

What indication is there of an Alternator A failure?


The Lane"A" Lamp will illuminate and the airframe power buss Amp Meter will show a DISCHARGE from the battery.

In the double rare event that BOTH REGs fail simultaneously, the engine will stop.
The emergency procedure is to activate the Emergency Bypass circuit that connects the big 12v Battery directly to the EMS internal buss and attempt a restart.
In this case, the main battery is the only source of power for everything and draining fast. Land Soon!
Leave the START Power Switch ON for extra confidence.

This is the reason that a lower capacity, higher current, Lithium Battery is NOT a good option for the 912is engine. It will start the engine just fine but if ever need, unlike the 912ULS engine that can run forever on its internal magnetos, the 912is engine will only continue to operate for as long as the battery lasts.
~ ~ ~

A seldom documented option in the case that the Reg "A" FAILS and Reg"B" takes over the EMS Bus leaving the Airframe loads to the Main Battery and you are in a position where the battery is not expected to last until touchdown, Or you need IFR Instruments, or it is Night, etc., ... try this...

The Reg"B" generator has much more capacity than the Reg"A" Generator that it is replacing internally.
Before the Main Battery gets too far discharged, turn OFF all electrical Airframe loads.
Then Activate The Emergency BACKUP Switch.
This will allow you to "Steal" some of the excess power from the EMS internal bus.

Watch the Amp Meter closely for a battery CHARGE indication.
One by One, turn ON any critically need equipment while watching the Amp Meter.
As long as it stays on the CHARGE Side of the indicator, The Generator is holding its own.
This will allow you to at least get some flight instruments and a radio working long enough to make an airport, and not be restricted by the battery's limited capacity.
Last Edit: 2 weeks 2 days ago by Bill Hertzel.
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2 weeks 2 days ago #17477 by Allen Inks
Allen Inks replied the topic: Why is external DC power needed for Alternators with Permanent Magnets on start
THIS is good and valuable information. Thanks a bunch!

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