External Alternator

I believe you are supposed to run one or the other not both.

Titus,

Typically when there is a manual switch for the external alternator it is only used when the engines internal alternator B cannot keep up with the load, or in the case of a failure of either of the 915s internal alternators.  I say either because no matter which one fails the result is the same, which is no alternator power to airframe.

If you switch the external alternator on in normal conditions, both internal alternator B and the external alternator will be connected to the airframe buss, and the alternator providing the higher voltage will carry the load. This can unload the internal Alternator B, and that will force its regulator to shunt more power.  Ideally you want a load on internal alternator B because the regulator does not work as hard and will say cooler, as will the stator. On these manual systems, you really don’t have any control of how the loads are shared. Essentially, the pilot is required to control it in lieu of a more complex load sharing scheme. 

As a side note, Internal alternator B can produce 28 amps, which is about 420 watts.  So if it’s not keeping up at cruise RPM you might want to do some math (or testing) to see if your are really using more power than that, or the alternator is not producing its full rated output.  These alternators are three phase, and its possible to lose a phase and still operate at a reduced output. If the alternator lost a phase, it would only produce about 240 watts.  

Thanks for the replies! I was concerned that generator B would stop producing much power as the external alternator would possible take over the majority of the load and it sounds like that is what happens. When you say that generator b will shunt more power and that this is makes it work harder can you explain why? Also why it will get hotter and the stator also get hotter?

Thanks Titus

Titus,
Unlike a car alternator where the field voltage is controlled, the internal alternators on the Rotax injected engines are permanent magnet type, that use a shunt regulator. The alternator will produce a constant current, and that energy has to go somewhere. 

Shunt regulators use fast switching transistors (SCRs) to create a variable resistance that shunts the coils together and to ground in order to regulate the voltage. You can go way down a rabbit hole learning about this, but the simple explanation is that the regulator works harder to keep the voltage in range when the load on the alternator is low. And there is a complex interaction between the shunt regulator and stator that heats the stator windings when the regulator is shunting more current.

Alternator “A”, which powers the engine functions, is deliberately smaller so that it is right-sized to the engine loads. This is likely why there are very few failures of that stator and regulator. This is harder to achieve for alternator “B” because Rotax can’t anticipate the airframe loads on every aircraft. As you have stated, 70-80% in normal operation is about ideal. After starting the engine, I find it a good practice is to turn on all the avionics and navigation lights before advancing the throttle past 2500 rpm. This puts a load on the electrical system before alternator B is producing its full output.  

The 912iS engine has had several SBs regarding regulator and stator failures. Rotax claimed the problems were mostly associated with lightly loaded electrical systems, which has merit, but I don’t think tells the whole story. Rotax progressive upgrades to correct this included a new larger wire connector between the regulator and stator, an optional heat sink for the regulator, a new oil spray nozzle to cool the stator, and an upgraded stator. The 915 12v system is essentially the same as the 912iS system, but I think by time the 915 was out most of these upgrades had been included. I don’t know the precise timeline, but I’m sure Rotax Wizard does 😁

There is at least one company developing (or now offering) a regulator that uses a more efficient type of transistor called a MOSFET, which is said to reduce the resultant heat, but those have not made their way to factory production at Rotax. I have an earlier 912iS engine (2016) and I’ve had both stator and regulator failures. After that, I did all the Rotax upgrades and have had no additional issues. I would also like to give a shout-out to Rotax for refunding me for all the parts for these upgrades, when they were under no obligation to do so. I did the work myself so did not request any labor cost. I filled out all their documentation properly, and a few months later I got a surprise check in the mail.