102 



COMMERCIAL ELECTRICAL TESTING 



excessive line drop or speed changes of the driving 

 unit. In many cases the line drop alone will start a 

 rotary pulsating and once started the pulsating generally 

 increases rapidly until the rotary falls out of step or 

 flashes over. To prevent pulsation, copper or brass 

 bridges, which act as short circuited secondaries and pre- 

 vent sudden changes of the input armature current, 

 are placed between the poles. Rotaries of new 

 design are tested for pulsation by inserting a definite 

 resistance in each phase between the machine itself 

 and the driving alternator. The drop through this 



Dynamometer 

 Board 



Alternator 



Resistance 



Resistance 



Fig. 30. Connections for Pulsation Test on Rotary Converter 



resistance corresponds to the line drop which will 

 probably occur in practice. Usually 15 per cent 

 drop is assumed and the resistance per phase neces- 

 sary to produce this is determined from the formula 

 (a-c. voltage) 2 



kw.XlOOO 



If two rotaries are tested together each machine should 

 have 15 per cent drop between it and the driving alter- 

 nator, or 30 per cent between the two rotaries, as shown 

 in Fig. 29. 



With the two machines running in synchronism, 

 self-excited, and with fields adjusted to give minimum 

 input, observe the direct current voltmeters on the 



