WT_VAWTTach:

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   Default Wind Turbine Tachometer Model. For use in WTSim.m.
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   Forms:
     
     ioData = WT_Tach;                 % Obtain i/o data 
     output = WT_Tach( input );        % Update
     output = WT_Tach( input, time );  % Update with current sim time
     output = WT_Tach( input, -1 );    % initialize
   
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   ------
   Inputs
   ------
   input       (.)  Input data structure
   time        (1)  Simulation time

   -------
   Outputs
   -------
   output      (.)  Output data structure

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