140 ELEMENTS OF ELECTRICAL ENGINEERING. 



always understood to be the power that it can deliver steadily to 

 a non-inductive receiving circuit (power factor unity) without 

 undue rise of temperature. That is, the power rating of an alter- 

 nator is understood to be the product of its rated voltage and its 

 rated current. This power rating of an alternator is sometimes 

 specified in terms of volt-amperes or kilovolt-amperes, to dis- 

 tinguish it from the actual power the alternator can deliver, for 

 the actual power depends upon the power-factor of the receiving 

 circuit. Thus an alternator rated at 2000 volts and 100 amperes 

 is called a 200 kilowatt or a 200 kilovolt-ampere alternator. 

 Such an alternator could in fact deliver 200 kilowatts to a non- 

 inductive receiving circuit, but with the same rise of temperature 

 it could deliver only 150 kilowatts to a receiving circuit of 0.75 

 power factor. 



The recommendations of the Committee on Standardization, of 

 the American Institute of Electrical Engineers, concerning over- 

 toad capacities, are given on page 1 7 1 of the first volume of this 

 treatise. 



67, Frequencies. The standard frequencies employed in Amer- 

 ican practice * are 2 5 cycles per second, 60 cycles per second, 

 and in older installations 125 or 133 cycles per second. 



Very low frequencies are not suitable for lighting on account 

 of the tendency of low frequency to produce flickering of the 

 lights. A frequency of about 60 cycles per second seems to be 

 the lower limit for satisfactory lighting. 



High frequencies are not well adapted for the operation of in- 

 duction motors, synchronous motors, or rotary converters, be- 

 cause high frequencies necessitate either great speed or a great 

 number of field poles, and in single-phase commutator motors 

 high frequencies involve excessive sparking and low power fac- 

 tors. For driving motors and rotary converters a frequency of 

 25 cycles per second is the standard in American practice. 



When an alternator is to supply current for lighting and also 

 for driving motors a frequency of 60 cycles per second is gen- 

 erally used in American practice. 



* See page 170 of the first volume of this treatise. 



