96 COMMERCIAL ELECTRICAL TESTING 



auxiliary pole may be placed on either the leading or 

 the trailing side of the main field, depending upon the 

 conditions under which the machine is to operate. If 

 it is to operate as a straight rotary, the auxiliary pole 

 is to be placed on the trailing side ; while if the machine 

 is to float on the line to take fluctuations of load through 

 a storage battery, and hence run inverted part of the 

 time, the auxiliary pole should be on the leading side. 

 The reason for this is as follows: The auxiliary pole 

 influences commutation when on the leading side, as- 

 well as regulates the direct current voltage, and will 

 be of correct polarity for commutation if the machine 

 inverts at a direct current voltage corresponding to no 

 excitation of the auxiliary poles. 



In wiring a split pole rotary for test, the transformers 

 used must be exactly alike. The best results are obtained 

 by using transformers with two secondaries excited 

 by one primary. Care should be taken to see that the 

 cables from the transformers to the rings do not differ 

 in length or cross section, and that all switches in these 

 circuits have their contact surfaces well cleaned with 

 sandpaper. These precautions are necessary to prevent 

 any unbalancing of the current in the alternating 

 current circuits outside of the armature. 



The testing instructions should specify the manner 

 in which the transformers are to be connected, both 

 primary and secondary; the alternating current volts 

 to be held across corresponding rings; and the range 

 through which the direct current volts are to be varied 

 by means of the auxiliary field. The following no load 

 readings should be taken : 



Current per phase. (Must be balanced.) 



No load phase characteristic. 



Ratio of voltage. 



Volts between adjacent collector rings with main 

 field only. 



A set of readings of alternating current amperes 

 while varying the direct current volts by means of 

 the auxiliary field through the total voltage range, the 

 main field being held at minimum input value, the 

 alternating current volts constant, and the brushes 



