RJT> What comes to mind on this right off is the fact that motors,  as well
RJT> as transformers and other devices involving inductance and which are
RJT> made to operate off the 60 Hz power line,  are typically optimized for
RJT> that frequency in terms of their core design and such.
 
RJT> You'll see some stuff spec'd at 60 Hz only,  some at 50/60 Hz,  and very
RJT> seldom stuff at other,  lower frequencies like maybe 25 Hz.  (Come to
RJT> think of it,  I remember seeing a reference not all that long ago to
RJT> a system running at 25 Hz that ran *slow*.  Hmm...)
 
RJT> Anyhow,  you take something that's designed to run at 60 Hz or even
RJT> 50/60,  and you try to go as low in frequency as you're talking about
RJT> here and it's gonna run *hot*.  The lower the frequency,  the more
RJT> inductance that's needed to give a specified impedance to the current
RJT> in the circuit.  You go lower in frequency without changing anything
RJT> else and the impedance is going to be that much lower,  with the
RJT> resulting higher current flow.
 
Roy,
 
Most gear like motors and transformers are rated at 60hz because that is
normally all that they are used on.  Motors must be de-rated for use on
lower frequencies, but that is probably not a problem in a windmill
application because the power available in the wind goes down rapidly
with the windspeed.  VFD's are made to reduce voltage in proportion to
operating speed and to maintain a nearly constant Volts/HZ ratio.  Motors
operated on a VFD can produce constant torque but variable hp below
their design speed and constant hp variable torque at higher rpm's.
VFD's account for this in their control electronics and the motors do
just fine.
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