CHAPTER VI. 

 RATINGS AND GUARANTEES. 



72, Dynamo speeds. The capacity (power output) of a given 

 sized dynamo, whether it is to be used as a generator or as a 

 motor, depends upon the speed at which the machine is to be 

 run. Roughly speaking the capacity of a dynamo is proportional 

 to its speed. Thus, a given sized dynamo might be rated as a 

 lO-horse power motor or a 15 -horse power motor according 

 to whether it is to be run at a speed of 1,000 revolutions* per 

 minute or at a speed of 1,500 revolutions per minute; and an- 

 other dynamo might be rated as a 5O-kilowatt generator or as a 

 7 5 -kilowatt generator according to whether it is to be driven 

 at a speed of 500 revolutions per minute or at a speed of 750 

 revolutions per minute. 



It is of prime importance in making a fair comparison of prices, 

 from different manufacturers, on generators or motors of a given 

 rated output, to consider the speeds at which the machines are to 

 be driven. One manufacturer may very properly charge, for a 

 large, slow-speed machine, a higher price than that charged by 

 another manufacturer for a small, high-speed machine of the 

 same rated output. 



The speed at which a dynamo may be driven depends partly 

 upon mechanical conditions, such as completeness of balance of 

 the rotating part, strength of the rotating part to withstand the 

 stresses due to rotation, and satisfactory running of the bearings 

 and brushes ; and if a dynamo is to be directly connected to an 

 engine as a generator, or to a printing press or other machine as 

 a motor, the speed at which the dynamo must run is fixed by the 

 speed of the machine to which it is to be connected. 



The speed at which a dynamo may be driven depends also to 

 some extent upon electromagnetic conditions as follows : 

 10 145 



