I think the total rotor resistance and the reactance has to be taken into consideration.
If rpm[mec]=855 wsync=900 then sn=0.05
Since , Pgap=3*Irotor^2*Rrotor/s then Rrotor=Pgap*sn/3/Irotor^2
Pgap=Pmec/(1-sn)=15789 w
Irotor=55 A will get Rrotor=0.08699 ohm.
But Irotor=Erot*sn/sqrt(Rrotor^2+Xrotor^2*sn^2) then Xrotor=sqrt(Vrot^2*sn^2/3/Irot^2-Rrot^2)/sn=0.9089 ohm
This wound rotor resistance and reactance usually does not present big difference from start up to rated rpm. Then for each step the rotor current will be:
Irotor=s*Vrot/sqrt(3)/sqrt((Rrot+Rext)^2+Xrot^2*s^2)
Pgap=3*Irot^2*(Rrot+Rext)/s
Let's take Rext total=1.7 ohm and let's take 5 steps as follows:
rotor current will be:
Irotor=s*Vrot/sqrt(3)/sqrt((Rrot+Rext)^2+Xrot^2*s^2)
Pgap=3*Irot^2*(Rrot+Rext)/s
Let's take Rext total=1.7 ohm and let's take 5 steps as follows: