406 ELEMENTS OF ELECTRICAL ENGINEERING. 



netism in each case in revolutions per second, the frequency of 

 the polyphase currents being 60 cycles per second. 



Sample answer for sf-pole j -phase connections. Phase A is 

 connected as follows : Positive main to -f (section i) to -j- (sec- 

 tion 2) to (section 7) + to (section 8) -f to -j- (section 13) 



to + (section 14) to (section 19) -f to (section 20) 

 -f to negative main. 



Phase B is connected as follows : Positive main to -f (section 3) 



to -j- (section 4) to (section 9) -f to (section 10) -f to 

 4- (section 15) to + (section 1 6) to (section 21) + to 



(section 22) -f to negative main. 



Phase C is connected as follows : Positive main to -f (section 

 5) to -f (section 6) to (section u) -f to (section 12) 

 -f to + (section 17) to -f (section 1 8) to (section 23) -f 

 to (section 24) + to negative main. 



Speed : Two-phase, four poles 30 revolutions per second ; six 

 poles 20 revolutions per second; twelve poles 10 revolutions per 

 second. Three-phase, two poles 60 revolutions per second ; four 

 poles 30 revolutions per second ; eight poles 1 5 revolutions per 

 second. 



1290. The armature of an ordinary direct-current dynamo is 

 short circuited between the brushes, and the resistance of the 

 armature between brushes is 0.036 ohm. The number of arma- 

 ture conductors is 260, and the flux through the armature is 

 1,500,000 lines, which is assumed to be invariable. The field 

 magnet is set rotating about the shaft as an axis at a speed of 25 

 revolutions per second, carrying the direct-current brushes with 

 it. Calculate the torque dragging upon the armature when its 

 speed is 24 revolutions per second and when its speed is 23 

 revolutions per second. What portion of the power expended 

 in driving the field is available at the armature belt, and what por- 

 tion is lost in heating the armature, friction losses being ignored? 



Ans. (a) Torque = 6.86 kg. -meters, 96 per cent, of the power 

 is available at the armature belt and 4 per cent, is lost in heating 

 the armature. 



