52 COMMERCIAL ELECTRICAL TESTING 



synchronous impedance curves. The method of making 

 this calculation is more fully treated under the subject 

 of alternating current generators. In making this 

 test the machine is subjected to normal load at normal 

 voltage. Holding the same field excitation, the load 

 is suddenly thrown off and the armature voltage 

 observed. The difference between this and normal 

 voltage, divided by normal voltage, is the per cent 

 voltage regulation. 



When a compound wound generator is compounded 

 hot, a compounding curve should be taken after the 

 german silver shunt is properly adjusted. Starting 

 with no-load voltage, readings of volts armature, 

 amperes armature, volts field and amperes field should 

 be taken at J, J, J and full load. The load should then 

 be reduced to zero by the same increments, and the 

 same readings taken. A curve should be plotted with 

 amperes line as abscissae and volts as ordinates. The 

 variation of this curve from a straight line will not 

 usually exceed 5 per cent. 



Input-Output Tests 



It is sometimes required to measure the efficiency 

 of a machine or set by the input-output method. The 

 measurement of the 'power input to the motor and out- 

 put from the generator is then required. The efficiency 



r ,- Total output of generator 



of the set = _ ^ . . K 



Total input to motor 



The efficiency of the generator = 



Total output of generator 

 (Input to motor) (motor losses) 

 The efficiency of the motor = 



(Output of generator) + (generator losses) 

 Input to motor 



In the case of induction motors, input-output test 

 is sometimes taken by the string brake method, which 

 will be discussed more fully under the heading of 

 induction motors. 



