In a shunt motor, the permanent magnet is replaced by an electromagnet activated by a field coil that shunts the armature. The shunt motor shown in Fig. 33-4 has an armature resistance of 0.050 and is connected to a 120 V line. 

(a) What is the armature current at the starting instant, i.e., before the armature develops any back emf? 

(b) What starting rheostat resistance R, in series with the armature, will limit the starting current to 60 A? 

(c) With no starting resistance, what back emf is generated when the armature current is 20 A? 

(d) If this machine were running as a generator, what would be the total induced emf developed by the armature when the armature is delivering 20 A at 120 V to the shunt field and external circuit?

Transcribed Image Text:

Field oooooo Armature 0.050 92 Rheostat Fig. 33-4 120 V