PROCEDURE IN DESIGN OF D.C. GENERATOR 235 



of the curve, the maximum efficiency of 0.92 will probably be 

 obtained with an overload of about 25 per cent. 



EFFICIENCY TABLE 



Referring to the usual efficiencies of commercial machines as 

 given on page 197, it is seen that the calculated value of 91.5 

 compares favorably with the average value of 91.2 for a 75-kw. 

 dynamo. 



64. Design of Continuous-current Motors. The dynamo 

 being a reversible machine, may be used as a generator to 

 convert mechanical into electrical energy, or as a motor to convert 

 electrical into mechanical energy. If the machine is to be used 

 as a motor, the efficiency should first be estimated by referring 

 to the figures on page 197. This efficiency, in the case of a 



,. output 



motor, is the ratio , whence 



input 



horsepower X 746 

 efficiency X 1,000 



and the design may be proceeded with exactly as if the machine 

 were to be used as a generator to give this particular kilowatt 

 output at the specified speed. 



It is even more important in a motor than in a generator 

 that the machine should work sparklessly at all loads without 

 change of the brush position. The specification usually calls for 

 operation without destructive sparking from zero load to 25 

 per cent, overload, with the brushes in a fixed position. If the 

 direction of revolution of the motor is to be reversible, it is neces- 

 sary for the brushes to be on the geometric neutral line, a con- 

 dition which is usually met by providing commutating interpoles. 



Kw. 



