136 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
much more than that found in properly designed direct-current mo- 
tors of equal weight and like physical limitations. * * * 
It is customary to adopt a single-phase alternating-current motor 
rating which is based upon the performance of some direct-current 
motor. For example, a 125-horsepower single-phase machine is sup- 
posed to do the same work, that is, handle the total number of 
tons on some specified service, as a 125-horsepower direct-current 
motor. This may be an ingenious comparison, but it 1s misleading. 
The fact is that such a motor equipment, including its transformer, 
will be much heavier than the motor equipment with which it is com- 
pared, and consequently the net load which it can handle will be 
much less. 
What és of vital consequence is a comparison of capacities for 
equal weights, not only of motors but of total apparatus which must 
be carried on a car, and also to compare the speed-relations and the 
polar-clearances, in other words, the allowable wear of bearings, all 
of which is quite aside from gear and commutator brush considera- 
tions, which are of themselves serious. 
Valatin and others have indicated one measure of comparison be- 
tween motors of different makes, types, and capacities—the “ weight- 
coefficient,’”—which for convenience may be expressed by the follow- 
ing equation: 
Nominal rated horsepower. 
Revolutions weight in tons. 
Weight-coeflicient = 
This is a factor of the greatest importance, and it should be con- 
sidered not only for the one-hour 75°-rise load, but throughout the 
whole thermal curve. 
Let us investigate two standard modern machines. 
An initial comparison is as per this table: 
1-hour 
Machine! ype. : A ¥ rating, | Weight, 
yp Voltage.| Air gap. horse- | pounds. 
| power. 
De oes eae Direct wurtventss- ae ees a Roe ee eee! 550. 0.25 240 | 5,478 
ABs ea ER 2b-alternatinge currents 22. seetee ele 225 10 125 | 5,274 
The weights are minus pinions, gear, and gear cases. There is a 
difference of less than 4 per cent in net weights, or about 2.5 per cent 
in total weights. * * * 
The accompanying curves (fig. 1) show graphically, almost start- 
lingly, the comparative speeds, capacities, and weight coefficients of 
these machines, all referred to the time required to rise 75° in tem- 
perature when operating at full normal potential under varying loads 
and with natural ventilation. 
