252 ELECTRICAL MACHINERY 



From this example it is seen that the kv-a. capacity of a 

 three-phase machine is obtained by multiplying the prod- 

 uct of the safe current per line times the voltage between 

 lines by -\/3 and dividing by 1000. The reason for the use 

 of \/3 in this calculation will be given in Chapter XIV. 



Reason for Rating in Kilovolt-amperes. Why should 

 the capacity of an alternating current machine be given in 

 kilo volt-amperes instead of in kilowatts? We have shown 

 that the output of an electric machine is fixed by the safe 

 heating and that this heating in the conductors fixes, there- 

 fore, the safe current the machine may carry. Its voltage 

 is fixed by the saturation of 'the magnetic field, number of 

 conductors, speed, etc. and cannot safely exceed that for 

 which the machine was designed. The two limiting factors 

 of the output are thus the current and voltage, and it may be 

 that the power factor of the load to which the generator 

 is furnishing current is less than unity. 



Example. Let us consider the 2300-volt, 100-ampere, 

 single-phase generator; its rating was 230 kv-a. Now if 

 the load power factor were 0.5, when the machine was 

 delivering 100 amperes at 2300 volts the watts output 

 would be 2300X100X0.5 = 115000 watts = 115kw. So that 

 the generator would be working to its safe limit to .supply 

 115 kw., whereas if the power factor of the load were 

 unity the possible output would be 230 kw. Thus the 

 possible power output of a generator depends entirely upon 

 the power factor of the load; alternating current machinery 

 is therefore rated in terms of volts and amperes, and not 

 watts. If a machine should be rated in kilowatts a load 

 of power factor equal to unity would be assumed. 



62. Characteristic Curves. The tw r o curves to be con- 

 sidered here are the external characteristic and armature 

 characteristic. The latter is often called the field-compound- 

 ing curve. The efficiency curve is similar in shape to that of 

 a continuous current generator and, as the same factors 

 determine the form of the curve for both machines, they 



