504 ELEMENTS OF ELECTRICAL ENGINEERING. 



(&) Derive and plot the total characteristic of the machine, the 

 resistances of armature, shunt field winding and series field wind- 

 ing being 0.09 ohm, 100 ohms, and 0.04 ohm, respectively. Ans. 

 Coordinate of one point, E a = 576.5 volts, 7 o = 205.58 amperes. 



210. Given the following data specifying the armature charac- 

 teristic of a shunt generator, the voltage between the brushes being 

 kept at 220 volts. 



7 2.7 20 40 60 80 100 120 amp. 



7 2.7 2.84 3.05 3.28 3.57 3.9 4.26 

 (a) Plot this curve and also plot the curve between I x and I f 

 The shunt field winding of the machine has 2,000 turns, (b) Find 

 the number of turns of wire required in a series field winding (short- 

 shunt connection) to bring the voltage between the brushes to 

 220 volts when I x is 100 amperes, (c) What fraction of the 100 

 amperes would have to be shunted round the series winding to 

 give exactly 220 volts between the brushes when I x is 100 am- 

 peres ? (d) What fractional part of I x would have to be shunted 

 round the series winding to give exactly 2 20 volts between the 

 brushes when I x is 50 amperes? Ans. (b) 26 turns ; (c) 2.3 per 

 cent. ; (d) 22.9 per cent. 



211. The generator specified in problem 209 requires a total 

 of 6, 500 ampere-turns to give 500 volts at zero load, and a total of 

 9,750 ampere-turns to give 550 volts at full load (I x = 200 am- 

 peres). How many turns of wire are there in the series field 

 winding? Ans. 13 turns. 



212. The curve in Fig. 49, Appendix B, gives the relation be- 

 tween speed-of-car and amperes-per-motor when the two motors 

 on the car are connected in parallel between the 5oo-volt trolley 

 wire and the track with rheostat resistance all cut out. Derive from 

 this curve the speed-ampere curve of the two motors when they 

 are connected in series to the 500- volt supply, the resistance of 

 each motor being 0.50 ohm. Ans. Coordinates of one point, 

 speed 6.85 miles per hour, current 80 amperes. 



Note. For any given current i in a motor the Ri drop in the motor is 0.5 X *> 

 and the Ri drop in the two motors connected in series is 2X'5X Z> Therefore 



