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There have been some issues with the 912iS stators, regulators and the wiring between, all to do with heat. It's something I have been interested in, so I did an experiment today and the results made me realize I made a misleading statement on this forum a while back.  I could not find the old post, so I will make the correction here. 

What I said was that the max wattage the shunt regulator can use (dissipate) is about 60 watts, or less than 15% of alternators rated load. This is true. But what I neglected to consider is the disproportionally high current (amps) associated with those watts. Amps is a measurement of electrical stress on components, the higher the voltage (pressure) is, the easier it is to push the same watts through a system. In other words, amps and volts are inversely proportional.  

Today I put an inductive amp meter on one of the 3-phase leads that run from the stator to the regulator. I ran the engine at 3000 RPM and turned off all airframe loads.  The amp meter indicated 21 amps!  I then started adding loads to the airframe and the amps on the stator leads decreased progressively until everything was turned on.  I have about 10 amps of avionics and lighting in the aircraft, and with it all on the meter on the stator lead was reading 13 amps. 

I was initially surprised that there was 21 amps of shunting load with zero connected load downstream of the regulator. But then I realized this does reconcile with my understanding that 60 watts is the maximum the shunt regulator will ever need to dissipate. The reason is that those 60 watts are being delivered at a very low voltage during the shunting process (about 1.5 volts across the SCR's in the shunt). 

Applying OHM's law for a 3-phase AC circuit:

21 amps x 1.5 volts x1.73 = 55.5 Watts. 

Watts are power, so the engine is not working as hard to spin the alternator when no loads are connected, but the stress on the wiring (amps) due to the shunt regulator is higher.  So, while 55 watts is only about 13% of the alternators rated power, the fact that it's imposing a high amperage on the wiring and stator is more significant when preservation of the components is the concern. Current (amps) create heat in the wiring, so I can see how a very lightly loaded airframe could contribute to heat damage to the regulator and those items upstream.  I did not have a way to test this today, but I believe that with the alternator fully loaded at 420 watts (at 13.8 volts) the amps between the stator and regulator would be lower than with no load, about 18 vs 21.  Also, it looks like with 10 amps (or more) of airframe load, the shunting load is not much of a factor. 

  • Re: 912iS Shunt Regulator Experiment

    by » 5 months ago


    Hello Jeff

    The real solution in my view is to switch to a Mosfet regulator for the B side.  this solves the heat problem without question.  I know that very shortly RS Flight Systems will release one for the iS series engines.  This has become common practice on many motorcycles with shunt regulators are failing stators. 

    Cheers

    metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which determines the conductivity of the device.


  • Re: 912iS Shunt Regulator Experiment

    by » 5 months ago


    Hello RW,
    Yes I agree that a MOSFET based shunt regulator would be an improvement, and I have heard rumors one is in the works at RS. They operate with the same shunt principle, but with a lower resistance across the junction. RMstator makes them for motorcycles and claim they are lower heat. However, they are still constructed with a heat sink casing, and some have over temperature protection circuits.  So it would seem they still produce significant heat.

    On my aircraft I went 350 hours with no issues, then suddenly I had melted wire insulation between the regulator and the grey connector to the the Stator. The connector did not look heat damaged but I suspect it was contributing to the wire heating. I installed the new larger Amphenol connector as per the Rotax SI, and I’m hoping that helps for now.  

    I wrote the long OP hoping it would help those who are interested understand the issue, and possibly mitigate it. I also realize not everyone cares to go that deep in the weeds.  

    37787_2_71814419861__92DE87BC-DE0C-494C-A328-CC5728AFF657.jpeg (You do not have access to download this file.)
    37787_2_IMG_2830.jpeg (You do not have access to download this file.)
    37787_2_IMG_2839.jpeg (You do not have access to download this file.)

  • Re: 912iS Shunt Regulator Experiment

    by » 5 months ago


    hi again Jeff

    I have had a talk with Micheal Stock and the testing they have done has completed bench testing and flight hours have been accumulated for more than 1 year now.  it is almost ready for release is what I was told.  I am aware that both Lockwood and LEAF in the USA will start to stock them as an aftermarket replacement for the one currently in use on the iS series.  Given that the stator is a split system only the B regulator needs to have it. 

    The major difference is in the fact that the diodes currently in use do not switch fast enough and the transistor switching (MOFSET) is so fast they reduce the heat.  I know from my own experience with stator failure on newer motorcycles 3 phase systems is exactly the same issue.  Some aircraft I know that have extensive issues with the iS series could really use the different design of regulator in my opinion.  I am unable to share some of the data as it was given to me in pre-release by RS Systems.  Feel free to check online and you will see extensive articles on upgrades to using MOFSET type regulators to solve that heat issue in the wiring in the motorcycle world.  

    I am sure that they will do a release on the new part as soon as it is available. 

    Cheers


    Thank you said by: Jeff Blakeslee

  • Re: 912iS Shunt Regulator Experiment

    by » 5 months ago


    Thanks for the information RW.  I will keep an eye out for this product release.  It’s good to know there is a fix in the works, even if it doesn’t come from Rotax.  Meanwhile we can mitigate the issue best as possible with what we have to work with.  I’m told the new large connector actually acts a a heat sink for the wires as well as providing a better connection.  


  • Re: 912iS Shunt Regulator Experiment

    by » 5 months ago


    Jeff, I noticed something in your third photo (of the new connector) that concerns me.  There are several places in the picture where plastic zip ties are secured directly around engine mount tubes.  That practice has led to engine mount damage.  First, the zip tie loses its tightness over time due to: (a) plastic deformation of the tie itself; (b) collapse of a hose sidewall; or, (c) cold flow of wire insulation.  Second, an oil film reaches the tube/tie interface and wicks between them.  Third, dust/grit reaches the tie and is absorbed by the oil film.  Fourth, vibration "wiggles" the tie, causing the trapped dust/grit to grind on the engine mount tube.

    You may want to consider replacing zip ties forward of the firewall with MS21919-WCJ Adel clamps, which have a silicone lining that's resistant to petroleum based fluids.  Once tightened with a bolt and AN363 all-metal stop nut (no elastic stop nuts under the cowling), Adel clamps cannot move.

    https://www.aircraftspruce.com/catalog/eppages/ms21919wcj.php

    https://www.aircraftspruce.com/catalog/hapages/an363.php


    Thank you said by: Jeff Blakeslee

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