A 115 kW, 600 V dc series wound railway track motor has a combined, field and armature resistance (including brushes) of 0.155 Ω. The full-load current at rated voltage and speed is 216 A. The magnetization curve at 500 rpm is as follows.
(a) Neglecting armature reaction, calculate the speed in rpm at the rated current and voltage.
(b) Calculate the full-load internal (developed) torque.
(c) If the starting current is to be restricted to 290 A, calculate the external resistance to be added in the motor circuit and the starting torque.