148 ELEMENTS OF ELECTRICAL ENGINEERING. 



Estimation of ratings from dimensions and speed. It is often 

 important to be able to make an approximate estimate of the 

 proper rating of a machine from its specified speed, and its dimen- 

 sions ; for example in the purchase of machines which are not 

 fully guaranteed, such as second-hand machines. There is of 

 course a considerable difference between the legitimate ratings of 

 well designed and poorly designed machines of the same size, so 

 that no accurate general rule can be given for finding ratings. 

 The following formula is as good, perhaps, as can be devised. : 



in which / is the gross length of the armature core in inches, d 

 is the external diameter of the armature core in inches, n is the 

 speed in revolutions per minute, Pis the power rating of a motor 

 in horse-power, or of a generator in kilowatts, and k is a factor 

 the approximate values of which are as follows : For motors, to 

 give P in horse-power, k ranges from about 0.00002 for a 5 horse- 

 power motor to about 0.00004 for a 50 horse-power motor. The 

 rating of an enclosed motor is about 65 per cent, of that of an open 

 motor of the same size and speed. For generators, to give P in 

 kilowatts, k ranges from about 0.000015 for a 5 kilowatt genera- 

 tor, to about 0.000030 for a 50 kilowatt generator, to about 

 0.000032 for a 200 kilowatt generator. Ratings obtained from 

 the above equation may very properly be slightly increased or 

 decreased according to a general estimate of the degree of per- 

 fection of design of a given machine. 



74. Heating. The ultimate steady running temperature of 

 any part of a dynamo is the temperature at which the part gives 

 off heat (by radiation, convection and conduction) to its surround- 

 ings as fast as heat is generated in the part by friction, by eddy 

 currents and hysteresis, or by the heating effect of the electric 

 current. This ultimate steady running temperature is not reached 

 in less time than from six to eighteen hours of steady running 

 under full load, according to the size of the machine. Small 



