THE MAGNETIC CIRCUIT 



195 



near the iron pole core. The cooling coefficient will necessarily 

 depend upon the type and size of the machine, and it should, if 

 possible, be determined from tests made on machines generally 

 similar to the one being designed. The modern tendency in 

 design is all toward increased output by improvements in the 

 qualities of materials and in methods of ventilation. Field 

 coils are now frequently built with sectionalized windings so 

 arranged that the air has free access, not only between the sub- 

 divisions of the winding, but also between the inside of the coils 

 and the pole core. The gain is not always proportionate to the 

 total cost and space required; but the cooling coefficients given 

 in Fig. 76 would not be applicable to such designs without modi- 

 fication. Each manufacturer has his own data to guide him in 

 his calculation of new designs; but even if such data were 

 available for publication, it would be of little value without the 

 experience which enables the designer to apply it intelligently 

 to a practical case. 



60. Efficiency. The efficiency of a dynamo is the ratio of 

 power output to power input, or, 



output 

 Efficiency == output + losses 



In computing the total losses, an estimate has to be made 

 of the power lost through windage and bearing friction. It is 

 almost impossible to predetermine these quantities accurately. 

 The loss due to air friction will depend upon the design of the 

 armature and arrangement of poles and frame, apart from the 

 actual surface velocity; while the bearing friction will depend 

 upon the number and size of the bearings, the method of lubri- 

 cation, the weight of the rotating parts, and the method of 

 coupling to the prime mover. The factors to be taken into 

 account are so numerous and so difficult to determine that, in 

 the case of new designs or departures from standard types, it is 

 usual to group these losses together and make a reasonable 

 allowance for them in the calculations of efficiency. The fric- 

 tion losses will increase with the surface velocities; but since 

 the volume, and therefore the weight, of the rotating armature 

 of a machine of given output will decrease with increase of speed, 

 it is found that the total friction losses may be expressed as a 

 percentage of the total output, and this percentage will not 

 vary greatly in machines of different outputs and speeds. The 



