%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%  excreq.m - plots required field current for rated terminal
%             voltage vs. load current with fixed PF for a RR
%             3-phase alternator
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
clear; clf;
VLR=24000; S=400e6;        % Rated line voltage, apparent power
Xl=0.24; Ra=0.08;          % Leakage reactance, phase resistance
Xsu=1.6;                   % Unsaturated synchronous reactance  
% Any 3 arbitrary PF angles(radians), +'ve for lagging PF
theta=[acos(0.8) acos(1) -acos(0.8) ];  
IR=S/sqrt(3)/VLR;          % Rated current
% OC line voltage data
Voc=[0 17.39 19.3 21.12 22.78 24.39 25.84 27.13 28.45 29.66 ...
30.78 31.65 32.52 33.66 34.08 34.43 34.78 34.95]*1000;
% Field Current
Ifoc=[0 350 400 450 500 550 600 650 700 750 800 850 900 ...
1000 1050 1100 1150 1200];
Ia=linspace(0,1.25*IR,100);     % Current range
m=length(Ia);
for n=1:3                       % PF loop
  for i=1:m                     % Current loop
  Er=abs(VLR/sqrt(3)+Ia(i)*exp(-j*theta(n))*(Ra+j*Xl));
  Ifs=interp1(Voc/sqrt(3),Ifoc,Er); Ifg=Ifoc(2)/Voc(2)/sqrt(3)*Er;
  Xs=(Xsu-Xl)*Ifg/Ifs+Xl;
  Ef=abs(VLR/sqrt(3)+Ia(i)*exp(-j*theta(n))*(Ra+j*Xs));
  If(n,i)=interp1(Voc/sqrt(3),Ifoc,Er)*Ef/Er;
  end
end
plot(0,0,Ia,If(1,1:m),Ia,If(2,1:m),'--',Ia,If(3,1:m),'-.'); grid
title('Alternator excitation requirements');
xlabel('Line current, A'); ylabel('Field current, A');
legend('PF1','PF2','PF3',2)