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Date: Sat, 10 Sep 2005 14:14:23 -0500
From: Jim Sower <canarder@frontiernet.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Overvoltage control (help Ed A)
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<... the alternator is capable of producing over 100 volts ...>
I don't think so ...
Years ago in another life I was in the auto repair business.  During that period, GM internally regulated alternators went south on a pretty regular basis.  Mostly it was the regulator.  Most folks swapped out the alternator.  I chose to troubleshoot the unit and replace the regulator if that's all that was wrong (which it typically was).  I made more money than if I replaced the Alt, and the customer paid less than half as much.  

Anyway, the failure modes on the regulator were no output (no power to the field) and full output (unregulated B+ to the field).  When the alternator "ran away", the charging voltage was 16V - 18V max.  It would boil all the water out of the battery in a day or two, but not hurt ANY of the electrics in the car.  If the charging voltage was high like that, I swapped out the regulator.  If the charging voltage was battery voltage, I grabbed a paper clip and shorted a tab on the regulator to ground.  If the charging voltage went to 16 or more, I swapped out the regulator.  If these things didn't happen (a rare event), I replaced the alternator.  

Bottom line, with the regulator failed (or bypassed) to send full B+ voltage to the field, the charging voltage never exceed 18V in my experience.  I would guess most avionics will withstand 28V.  I would guess that anything much beyond that would require extensive modification of the internal wiring of the rotor and stator.

NOT a catastrophic failure mode ... Jim S.


Mark R Steitle wrote:

>============================================
>Mark,
>
>The self destruct fire item is the only reason that I see to go beyond
>interrupting the B lead.
>
>Interesting my ND alterator does shut down when I shut the power field
>off and it is internally regulated. I thought that was normal, but there
>were potential internal failures that could trigger the runaway
>alternator and then I would not be able to shut it down. Is that
>confused
>thinking or why am I able to shut it down while running?
>
>Bernie 
>  
>
>>    
>>
>Bernie,
>If the internal regulator's I/C decides to act up, who knows what the
>results will be, depends on how it fails.  Ninety-nine percent of the
>time everything works as designed.  Its that other one percent that
>we're talking about.  With the internal regulator, you have no way of
>stopping the current from flowing into the field windings if/when the
>I/C malfunctions, therefore you can't shut it down.  
>
>So, what would happen to your electronics if this happens?  I don't know
>the maximum voltage limit on the EC-2 or EM-2, but the alternator is
>capable of producing over 100 volts.  I doubt the EC-2 or EM-2 can
>tolerate anywhere near this.  (Years ago when I was working as a fleet
>mechanic, we tried a device that hooked up to the alternator that would
>allow you to run 110v tools off the vehicle's charging system.  It
>worked, but was unreliable.  But that's another story.)  The specs on my
>SL-30 nav/com say 32v is max.  So, if the alternator malfunctions and
>starts over-charging, you will likely start letting the smoke out of
>your expensive electronics.  And it won't take long and things will get
>real quiet (after the smoke is out of the EC-2).  So, this is enough
>reason for me to go with the external regulator and OVP.  
>
>My system uses two ND alternators and two RG batteries, with a crossfeed
>relay between the batteries.  So, if one alternator self-destructs, the
>OVM will automatically shut it down, and turn on a warning light on the
>panel.  At that point I would activate the x-feed relay (connecting both
>batteries to the remaining "good" alternator and continue on my way.  I
>can repair/replace the alternator at my destination.    
>
>Mark S.
>
>--
>Homepage:  http://www.flyrotary.com/
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>
>
>  
>