460 ELEMENTS OF ELECTRICAL ENGINEERING. 



61. The ring armature of a bipolar direct-current dynamo has 

 260 turns of wire upon it, the armature is driven at a speed of 

 1,200 revolutions per minute, and the magnetic flux from one 

 pole face into the armature core is 3,500,000 lines. Calculate 

 the electromotive force of the dynamo in volts. Ans. 182 volts. 



62. The armature specified in problem 61 has 400 feet of 320 

 mil copper wire wound upon it. What is the resistance of the 

 armature from brush to brush? Ans. 0.01055 onm at 2 5 C. 



63. The armature specified in problems 61 and 62 is used with 

 a six-pole field magnet with six sets of brushes, (a) What is the 

 resistance of the armature between positive and negative brush 

 sets? (b) The flux per pole of this six-pole field magnet is 

 600,000 lines. What is the electromotive force of this six-pole 

 dynamo? Ans. (a) 0.00117 ohm at 25 C. ; (b) 31.2 volts. 



64. The field winding of a shunt generator has a resistance of 

 40 ohms and the electromotive force between its brushes is 110 

 volts, (a) What is the value of the field current ? (b) What 

 amount of power is expended in field excitation ? (c) The full- 

 load current output of the generator being 50 amperes (at no 

 volts), express the field current in per cent, of the full-load cur- 

 rent output, and express the power consumed in field excitation 

 in per cent, of the full -load power output of the generator. Ans. 



(a) 2.75 amperes; (b) 302.5 watts; (c) 5.5 per cent, 5.5 per 

 cent. 



65. The field winding of a series generator has a resistance of 

 o. 121 ohm. The full-load current output of the generator is 50 

 amperes at no volts between the generator terminals, (a) 

 What electromotive force is lost in the field winding at full-load ? 



(b) How much power is consumed in field excitation at full-load ? 

 Express the volts lost in the field winding in per cent, of the full- 

 load terminal voltage of the machine, and express the power con- 

 sumed in field excitation in terms of the full-load power output 

 of the generator. Ans. (a) 6.05 volts or 5.5 percent. ; (b) 302.5 

 watts or 5.5 per cent. 



66. The motor of a motor-generator set takes 100 amperes at 



