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Date: Fri, 25 Feb 2005 09:37:42 -0700
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
From: Bill Dube <bdube@al.noaa.gov>
Subject: Re: [FlyRotary] Re: Racemate alt/water pump
In-Reply-To: <list-759045@logan.com>
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At 10:53 AM 2/25/2005 -0500, you wrote:
>WRJJRS@aol.com wrote:
>
>>We should look at the raw, open circuit, stator voltage as a function of 
>>RPM, just to be sure that the stator will NOT overheat at sustained high RPM.
>>
>
>Bill, I'd like an electron pump on both sides of the engine, just because 
>it would be so easy.  I've been looking at the Harley motorcycle 
>alternators for the gearbox end as a off-the-shelf solution, but it may 
>require rewinding to avoid this problem.

         Actually, this "problem" (if it is a problem at all) is caused by 
cheesey laminations (core material) in the stator, not by the windings. 
Rewinding will have no effect.

         If the laminations are of high quality, they have the low 
electrical conductivity in one direction (or more). This reduces eddy 
currents in the core material and the heating that goes with it.

         The very thing that you want in the coils is what you don't want 
in the core material.

         Just like the voltage in the coils, the voltage diving the eddy 
currents goes up with increasing RPM. Thus, a stator that will work just 
fine at 4000 RPM may overheat at 6000 RPM. Also, a stator that works fine 
in a motorcycle that only briefly runs at 8000 RPM, may overheat in an 
airplane that runs continuously at 6000 RPM.

         I don't see this as a deal killer, just something to watch out for.


>OTOH, there is no reason not to consider the Racemate unit for both 
>sides.  The projected cost difference is insignificant and going all 
>Racemate would mean only one development track to follow.

         The advantage of using the same unit on both side would be in 
manufacture. You only have to design one unit and you would sell twice as 
many of them.

         The disadvantage would be having identical failure modes. If you 
had a voltage spike of X magnitude, it would be likely to wipe out both of 
the identical systems. Conversely, if you had two distinctly different 
systems, one would be more likely to survive the event that killed the 
other. If you think in terms of heat, age, vibration, and other "wear" 
elements, you can see that two different  designs would be less likely to 
fail simultaneously than would two identical designs.

         This is why they have two isolated teams develop two separate 
control systems for fly-by-wire aircraft.