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jrtens 02-11-2021 09:04 PM

If you are building a 12iS, don't skip the lights
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You will need them on hot days to keep the voltage regulator from overheating. (Yep - the exact opposite of the 12)

See the attached explanation from the 12iS FTS about one of the big electrical differences from the 12.

After the battery is topped off - and that happens rather quickly - I regularly see 7 amps without the lights.

Piper J3 02-12-2021 06:34 AM

Thatís really weird. You would think the voltage regulator circuit would modulate the field windings to match output with load.

What if you had to drive your car around all the time with headlights, radio, and turn signals on?

Mich48041 02-12-2021 09:05 AM

The Rotax alternators are permanent magnet type. Thus they do not have field windings.
I can not believe that, "These regulators reject any unused electrical power
as heat
." Van's must have been misinformed about this. What Rotax
document is this statement based on? Suppose that 21 amps are available but
not used by the aircraft. If those 21 amps were turned into 300 watts of heat,
how is that heat dissipated? Where are the cooling fins?

jrtens 02-12-2021 10:02 AM


Originally Posted by Mich48041 (Post 1502683)
Van's must have been misinformed about this.

That's possible of course but it seems unlikely.

Mich48041 02-12-2021 01:54 PM

It will be easy enough to test. Put a thermocouple on the voltage regulator and go fly.
Observe the thermocouple temperature first with a light electrical load, and then with
a heavy electrical load. Repeat a few times to be sure the numbers are consistent.

emsvitil 02-12-2021 07:10 PM

Permanent magnet alternators will either have a shunt type VR or PWM VR.

The shunt type WILL dump the excess amps as heat.

Mich48041 02-13-2021 03:32 AM

It is true that there are shunt type voltage regulators. But a shunt type can not
possibly be used with the Rotax iS. The Rotax iS generator output is rated at 420 watts.
A shunt type would need to be capable of dissipating 400 watts of heat. How is
that possible when the regulator is not much bigger than a pack of cigarettes?
The regulator is rated at 80 degrees C. If it had to dissipate 400 watts, it would
get as hot as a milk house heater. Why would Rotax design the voltage regulator
with old shunt technology instead of the new switching type? Well, they didn't.
The smaller the electrical load is, the cooler the regulator will be.
The voltage regulator does not care if the landing lights are on or not.

jrtens 02-13-2021 11:06 AM

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The fact that Vans guidance goes against conventional thinking, as you have pointed out, makes it hard to imagine that they put it in the FTS without some research and testing.

With a quick search, I found a 2016 Sling LSA safety advisory recommending the same - to turn on the lights to keep the B regulator from overheating.

Here is a note from that SA.

jrtens 02-13-2021 12:20 PM

BTW, it is obvious that you know far more about electrical systems than I do.

I am just a pilot who wants to know how to best operate my plane.

My only reason for bringing this up is to make 12iS owners aware of this important operational difference between the 12 and the 12iS.

Mich48041 02-13-2021 05:06 PM

Greg and Scott at Van's Aircraft monitor these forums. Perhaps they will do some
flight tests with different electrical loads while recording the regulator temperature.
Then we will have the facts instead of rumors and old wives tales.

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