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Date: Wed, 04 Jun 2008 10:30:33 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: combined starter alternator
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Al Gietzen wrote:
> For this to work as a starter, I'd need it to generate as much power as 
> a typical starter.  During cranking, I believe a typical starter will 
> draw 200A at around 11 volts.  I'm sure someone will correct me if that 
> isn't typical.  That's 2200 watts, or around 3HP.  One way to go would 
> be to adapt one of "hub motors", but the largest of the flat motors that 
> I've seen are rated at only 500W (<1Hp). 
> -------------------------------
>
> I'm going from memory here, but 2-3 years ago I did some kind of measurement
> on the starter current (don't even remember what I did, but I think the
> initial surge of current was way up there, but once spinning I was surprised
> that it was down around 60 -80 amps.  This was during initial engine tests,
> and perhaps the compression was low.
>
> FWIW,
>
> Al
>
>   
That's fairly typical of electric motors, Al.  A big inrush to get 
things moving, then dropping off to something less demanding.  If you 
put a big load on the motor, such that it bogs down, the current will 
shoot back up.  Unless regulated, it will burn itself up trying to do 
what you ask of it, ie, the typical burning smell from your wife or 
naive son trying to make a worn out engine run with the starter.

The numbers I've seen for the flat motors are running about 700W peak.  
350W continuous.  Still only 1/3rd of where what would be needed for our 
purposes.  I expect some motors that will fit the bill will be on the 
market very soon now if gas prices stay where they are. 3 or 4 Hp would 
make a reasonable bicycle for commuting to work.