8-15. A 7.5-hp 120-V series dc motor has an armature resistance of 0.2 Ω and a series fiel...

8-15. A 7.5-hp 120-V series dc motor has an armature resistance of 0.2 Ω and a series field resistance of 0.16 Ω. At full load, the current input is 58 A, and the rated speed is 1050 r/min. Its magnetization curve is shown in Figure P8-5. The core losses are 200 W, and the mechanical losses are 240 W at full load. Assume that the mechanical losses vary as the cube of the speed of the motor and that the core losses are constant.

(a) What is the efficiency of the motor at full load?

(b) What are the speed and efficiency of the motor if it is operating at an armature current of 35 A?

(c) Plot the torque-speed characteristic for this motor.