%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%  RRaltTC.m - plots the terminal characteristics ( VL vs. IL )of
%              an isolated, RR, 3-phase alternator with constant
%              field excitation maintained at the value for rated
%              voltage & current with load PF as a parameter.
%              Constant Xs is assumed to avoid iterative solution.
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
clear; clf;
VLR=24000; S=100e6;        % Rated line voltage, apparent power
Xs=6.94; Rs=0.15;          % Syn. reactance, phase resistance
% 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
VL=[]; IL=[];
for n=1:length(theta);     % PF loop
Ef=abs( VLR/sqrt(3)+IR*exp(-j*theta(n))*(Rs+j*Xs));
ZLR=VLR/sqrt(3)/IR*exp(j*theta(n)); % Load @ rated condition
ZL=linspace(0.5*ZLR,2*ZLR,1000); % Range of phase load
  for i=1:length(ZL)         % Voltage loop
  Ia(i)=Ef/abs(Rs+j*Xs+ZL(i)); 
  Va(i)=Ia(i)*abs(ZL(i)); 
  end
VL=[VL;sqrt(3)*Va]; IL=[IL;Ia];
end
n=length(ZL);
plot(IL(1,1:n),VL(1,1:n),IL(2,1:n),VL(2,1:n),'--',IL(3,1:n),...
VL(3,1:n),'-.', IR,VLR,'o',0,0); grid
title('Alternator terminal characteristics');
xlabel('Line current, A'); ylabel('Line voltage, V');
legend('PF1','PF2','PF3',1)