PROBLEMS. 463 



76, Given a two-pole dynamo having a ring armature 20 centi- 

 meters long and 34 centimeters outside diameter with 250 turns 

 of wire (= Z) t and 50 commutator bars which are connected to 

 the armature windings by radial connectors. The resistance of 

 the armature between brushes is 0.4 ohm. A double pilot brush 

 is made by tying two light copper strips to the sides of a stick 

 of which the thickness is equal to the width of a commutator bar, 

 and these two copper strips are connected to a voltmeter. The 

 dynamo is driven as a generator at a speed of 1,200 revolutions 

 per minute at zero-load, and the voltmeter reads 8. 1 volts when 

 the double pilot brush is held against the commutator anywhere 

 within the limits of a pole face. Find : (a) the mean intensity of 

 the magnetic field in the air gap between the pole face and the 

 armature core. 



The dynamo, driven as a generator at the same speed as 

 before, now delivers a current of 40 amperes from its armature, 

 and the voltmeter is observed to read 2.47 volts and 1 1.56 volts 

 when the double pilot brush is held against the commutator 

 under the leading tip and the trailing tip respectively of a pole 

 piece. Find : (b) the field intensity in the air gap, under a leading 

 pole tip, and (c) ditto under a trailing pole tip. Ans. (a) 3,791 

 gausses; (b) 1,456 gausses; (c) 5,711 gausses. 



NOTE. The voltmeter reading (6) or (c ) is the induced electromotive force minus 

 the ri drop in a section of the armature winding. This problem illustrates a point 

 which is discussed in Art. 77. 



77. A series generator has a total internal resistance of 0.2 

 ohm. When the generator is driven at a speed of 1,000 revolu- 

 tions per minute it gives a terminal voltage of 99 volts with a 

 current output of 5 amperes. Increasing the current output to 

 10 amperes increases the useful flux, <3>, by 50 per cent. Find 

 the terminal voltage of the generator when, driven at a speed of 

 1,200 revolutions per minute, it delivers a current of 10 amperes. 

 Ans. 178 volts. 



