When open, (E=V+IaRa), Ia is zero or E=V. As E=OZNP/60A, as flux is constant(permanent magnet) E proportional to N, and as H.P=2(pai)NT/4500, could find T the torque for input HP of prime mover. When shorted E/Ra=Ia or Ia would be proportional to N, as mech. input(H.P) is known could find torque T(as friction windage loss constant) using same equation, short ckt  speed would be Ra times less than open ckt speed for same input(mech.) torque(T)....

Prof Bhupendra thanks for the guidance, if N is proportional to Ia ....... will there be an opposite force according to lenz law and try to oppose the torque.....

Hence there would be instant reduction of speed before winding gets burned....