DIRECT-CURRENT MACHINERY 



215 



maximum and the constant losses and variable losses are nearly 

 equal ; above full load it decreases again due to the rapid increase 

 of the variable losses. The limit of the efficiency which can be 

 reached commercially depends on the output, the voltage and the 

 speed. A higher efficiency can be obtained with large machines 

 than with small machines. A higher efficiency can be obtained 

 with high- voltage or high-speed machines than with low-voltage 

 or low-speed machines. 



40 80 120 160 200 240 280 320 3CO 400 

 Amperes Output 



FIG. 181. Characteristic curves of a 200-kw. compound- wound 

 generator. 



For 220-volt direct-current motors the full-load efficiency ranges 

 from about 85 per cent for small sizes to 93 per cent for large 

 sizes. 



For 550-volt direct-current generators the full-load efficiency 

 ranges from about 90 per cent for small sizes to 95 per cent for 

 large sizes. 



Fig. 181 shows the characteristic curves of a 200-kw. compound- 

 wound generator, 550 to 625 volts. 



