8-9. For the separately excited motor of Problem 8-8:(a) What is the maximum no-load speed...

8-9. For the separately excited motor of Problem 8-8:

(a) What is the maximum no-load speed attainable by varying both and ?

(b) What is the minimum no-load speed attainable by varying both and ?

(c) What is the motor's efficiency at rated conditions? [Note: Assume that (1) the brush voltage drop is 2 V; (2) the core loss is to e determined at an armature voltage equal to the armature voltage under full load; and (3) stray load losses are 1 percent of full load.]