I solved it in this way
function xdot=diagrama_bloques_nuria(t,x)
Ka=10;
Kb=20;
Km=10;
Ra=10;
Ks=0.75;
Kf=1;
Jm=0.01;
Ke=0.25;
A=5;
Vp=5;
Vr=Vp*square(2*pi*t)+5;
Va=(Vr-Kf*x(1, :))*Ka;
xdot = [
((x(2, :)*Ke)-(x(1, :)*Ks))*1/A;
x(3, :);
(Km/(Ra*Jm))*(Va-(Kb*x(3, :)));
];
i multiplied the xdot(1) by 1/A.