DIRECT CURRENT GENERATORS 63 



generator to pick up, the usual method is to put the 

 water box blades in and short circuit one of the boxes 

 with a fuse wire and then close the circuit breaker and 

 switches. If the machine then starts to pick up, and 

 the voltage decreases as soon as the fuse wire burns 

 away, there is too much resistance in the water boxes. 

 They should therefore be salted (to decrease the resist- 

 ance) and the operation repeated. Should the resist- 

 ance in the boxes be too small the load will increase very 

 rapidly and the breakers may have to be opened to 

 prevent the machine arcing over between brushes. 



Fig. 21. Three-Wire Generator 



After the brushes are set the german silver shunt 

 should be adjusted to give the required voltage. 



A series characteristic is taken on all series wound 

 generators. This is done by increasing the load by 

 small steps until full load is obtained, amperes line 

 and volts machine being recorded at each step. The 

 load is then reduced by small steps to no load, the same 

 readings being taken. A curve is then plotted between 

 amperes as abscissae and volts machine as ordinates. 

 (Fig. 20.) 



In the case of series machines which form part of 

 booster sets, the guarantee sometimes does not allow 

 this curve to deviate by more than a certain percentage 

 from a straight line. The curve should be taken in 

 all cases with the german silver shunt in place, if the 

 latter is necessary. 



Some direct current generators are provided with 

 collector rings for three- wire operation. If there are 



