96 



cent, of the available energy put into the compressor. In other cases the effici- 

 ency is as low as 20 per cent. 



There should be no comparison between the cost of the transmission of power 

 by compressed air and its so-called rival, electricity, since each has its own field 

 of usefulness, yet it may be interesting to note for our present purposes the effici- 

 ency of electric transmission. 



A modern generator, belted from an engine, will have an efficiency of about 

 90 per cent, when working under favorable conditions, but as the average load is 

 ordinarily not more than two-tliirds full load, and often much less, the efficiency 

 will not usually be more than 85 per cent. Since the engine friction was added 

 to the losses in compression, so also it should be considered here, in which case 

 the efficiency of generation will lie between 75 and 80 per cent. With a pressure 

 of 220 volts, which is very suitable for ordinary shop transmissions when both 

 light and power are to be taken oft" the same line, the loss in transmission need 

 not be more than 5 per cent, so that the efficiency at the motor terminals will not 

 be far from 75 per cent. With motors running under a nearly constant full load 

 the efficiency of motor may be 90 per cent., but with fluctuating loads this may 

 fall to 60 per cent, at quarter load. In numerous tests made by the writer the 

 average load on several motors in machine shops was only about one-third of the 

 rated capacity of the motor. 



It is interesting to note that in recent tests made at the Baldwin Locomotive 

 Works it was found that with a total motor capacity aggregating 200 horse-power, 

 a generator of only 100 horse-power was sufficient to furnish the current, and 

 ordinarily only 80 horse-power was required. 



Under these conditions when the driven machines are not greatly over- 

 motored we may assume a motor efficiency of 80 per cent., which may be less or 

 greater in individual cases. The combined efficiency, then, of generator and 

 motor working intermittently with fluctuating loads will be about 75 x S0=: 00 per 

 cent, of the power delivered to the engine. 



For greater distances than those which obtain in plants of this character the 

 loss in transmission will be greater, and higher voltage must be employed in 

 order to keep down the line loss ; while it is possible to put in conductors suffi- 

 ciently large to carry the current with any assumed loss, yet the cost of the line 

 soon becomes prohibitive with low voltage. In work of this kind it is well to 

 remember that while the efficiency may be very high the economy may be very 

 poor, and good engineering is primarily a question of good economy, all things 

 considered. It is not the most efficient plant which produces the greatest 

 economy. While it is interesting to know that a certain amount of power may 



