ELEMENTS OF ELECTRICAL ENGINEERING. 



98. A two-ring converter with negligible armature resistance 

 and reactance takes current over a o.2-ohm line through a reac- 

 tion coil having 0.3 ohm reactance. The effective alternating 

 electromotive force at the generator terminals is kept constant at 

 1 20 volts. The excitation of the converter can be varied at will. 

 Find greatest value of alternating electromotive force of converter 

 for which it can run as a synchronous motor, and find the corre- 

 sponding value of the direct electromotive force of the converter. 

 Ans. 216.25 volts and 306 volts, respectively. 



99. Plot the curve of which the ordinates represent the instantaneous values of the 

 current in an armature conductor of a four-pole, three-ring converter with multicircuit 

 winding, the value of the direct current output of the machine being 250 amperes : 

 (a) When the conductor is adjacent to the connection of a ring, and (b] when the 

 conductor is midway between the connections of two rings. 



Note. It is to be remembered that the discussion given in Arts. 91 to 95 of 

 Chapter IX refers explicitly to the t\o-pole machine with two current-paths between 

 its direct-current brushes. The necessary modifications of the formula given in Arts. 

 91 to 95 are explained in the footnoote on page 1 86. 



100. If a loo-kilowatt direct current generator were to be provided with five col- 

 lector rings forming a five-ring converter, what would its power rating be as based on 

 average armature heating ? 



Note. The specifications in this problem are purposely chosen so that the answer 

 to the problem will not be given in the table on page 188. 



CHAPTER X. THE TRANSFORMER. 



101. An electromotive force of no volts, from a battery for 

 example, is applied during -^- second intervals in reversed direc- 

 tions to a coil of 100 turns of wire wound on an iron core. Plot 

 the curve representing the above electromotive force and, neglect- 

 ing the resistance of the coil, plot the curve representing the core 

 flux produced. The iron core is supposed to have a constant 

 magnetic reluctance of 0.00014. Plot the curve representing the 

 magnetizing current. 



A secondary coil of 50 turns is wound on the above core. 

 This secondary coil supplies current to a non-inductive receiving 

 circuit having 200 ohms resistance. Plot curves representing (a) 

 secondary electromotive force, (b) secondary current, and (c) 



