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Date: Thu, 24 Feb 2005 17:29:33 -0700
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
From: Bill Dube <bdube@al.noaa.gov>
Subject: Re: [FlyRotary] Re: Racemate alt/water pump
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At 03:03 PM 2/24/2005 -0500, you wrote:
>My premise may be way off, but isn't the open lead output voltage linear
>with RPM?  So my thinking goes, if you get charging voltage at idle of
>1000, it will be 6 times higher (over 70V) at a cruise RPM of 6000.

         This is true, but that is corrected with the electronics in the 
regulator. It is a lot like your laptop power supply that doesn't care if 
it is plugged into 120 volts or 240 volts.

>The Yellow Daisy it is! You are correct in your assumption that the 
>POTENTIAL output increases with RPM but it isn't really linear.

         Actually, at the same current draw, the output voltage of the 
unregulated PM alternator is a linear function of the RPM. It is called the 
"Motor Constant."

>Something to do with internal coil hysteresis or histowhatsit.

         Makes some difference, but it is generally not huge. It can be 
significant on really cheezey alternators, but they would overheat badly at 
sustained high RPM.

         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.