RATINGS AND GUARANTEES. 149 



machines require a short time, and large machines a long time, to 

 reach this ultimate temperature. 



On account of the time required for a dynamo to reach its final 

 steady running temperature, it is evident that a machine may be 

 run for a short time under excessive load without causing undue 

 rise of temperature. 



The rated capacity of a machine which is intended for continu- 

 ous service is always based upon ultimate steady running tem- 

 perature, and operating engineers usually exact from the manu- 

 facturers a definite short period over-load guarantee, for example, 

 a 5O-per cent, over-load for one hour, or a 2 5 -per cent, over-load 

 for three hours, without exceeding a prescribed rise of temperature. 



The rated capacity of a machine which is intended for a specific 

 kind of intermittent service may properly be based upon a short 

 period run. Thus street car motors are usually rated on the 

 basis of a one-hour run, and a street railway motor rated at 25- 

 horse power would overheat if it were operated for much more 

 than one hour at full load. 



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

 from different manufacturers on generators or motors of a given 

 rating, to consider this time element which should always be 

 specified by the manufacturer. One manufacturer may very 

 properly charge for a large machine which will give I oo kilowatts 

 output steadily (or say 125 kilowatts for three hours) a higher 

 price than that charged by another manufacturer for a generator 

 which he rates at 100 kilowatts, if the second manufacturer bases 

 his rating on a three-hour run at full load. 



75. The heat run. Inasmuch as dynamo ratings and overload 

 guarantees are usually based upon rise of temperature of the 

 machine, it is evidently important to check these guarantees by 

 actual test. This test is called the heat run. 



Temperatures of armature and field windings, as specified in 

 rating and overload guarantees, are understood to be calculated 

 from the measured resistances, cold and hot, of these windings. 



