Jim Mcandrew wrote in a message to All:
 JM> My thought was: The frequency at which induction generators 
 JM> operate is set by the excitation frequency. In the example, 
 JM> that is the 60hz grid. But variable frequency AC drives to 
 JM> power motors are readily available.   So..
 JM>
 JM> Why not use a variable frequency drive to excite an induction 
 JM> generator? Motors w/ nominal 60hz rpm of 900 are a standard 
 JM> offering and variable frequency drives can run down to 20% of 
 JM> line frequency.  This would allow prop speeds down to 180rpm's. 
 JM> A tach on the prop shaft to send a control signal to adjust the 
 JM> excitation frequency of the VSD could control the system for 
 JM> maximum efficiency.
What comes to mind on this right off is the fact that motors,  as well as 
transformers and other devices involving inductance and which are made to 
operate off the 60 Hz power line,  are typically optimized for that frequency 
in terms of their core design and such.
You'll see some stuff spec'd at 60 Hz only,  some at 50/60 Hz,  and very 
seldom stuff at other,  lower frequencies like maybe 25 Hz.  (Come to think 
of it,  I remember seeing a reference not all that long ago to a system 
running at 25 Hz that ran *slow*.  Hmm...)
Anyhow,  you take something that's designed to run at 60 Hz or ever 50/60,  
and you try to go as low in frequency as you're talking about here and it's 
gonna run *hot*.  The lower the frequency,  the more inductance that's needed 
to give a specified impedance to the current in the circuit.  You go lower in 
frequency without changing anything else and the impedance is going to be 
that much lower,  with the resulting higher current flow.
Somehow this doesn't seem to be what you'd want out of this...
email: roy.j.tellason%tanstaaf@frackit.com 
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