10 COMMERCIAL ELECTRICAL TESTING 



differ, the piece will be strongly attracted; whereas, if 

 the poles are of like sign, a much less attraction will be 

 exerted. 



On direct current machines, the voltage drop per 

 spool with a given current flowing through the field 

 should not vary more than a few per cent (maximum to 

 minimum) from the average voltage drop per spool. 

 The field spools for alternating current apparatus are 

 assembled on the field spider in the Armature Depart- 

 ment; hence, it is only necessary to take a resistance- 

 measurement per spool before starting a test. The 

 variation between maximum and minimum drop per 

 spool should not differ materially from the average 

 drop per spool. In recording the drop on spools, the 

 spools should be numbered in a clockwise direction, 

 beginning at the spool next to the opening in the shunt 

 field when facing the commutator. When testing a 

 machine, a very careful record must be kept of all 

 winding resistances. Many armatures are fitted with 

 equalizing rings, impossible to obtain the true armature 

 resistance without disconnecting the equalizer taps. 



The shunt field resistance is obtained by the "drop 

 method," using an ammeter and voltmeter. This 

 measurement is required on each machine before a 

 test is started. For measuring the series field resistance 

 a special galvanometer measuring set is used, with 

 which the various testing sections are provided; this 

 set is described in a later article. As a considerable 

 amount of the resistance of a series field may consist 

 of the contact resistances of the joints between the 

 spools, all connections must be carefully cleaned and 

 clamped tightly together before measuring the 

 resistance. 



The measurement of air gaps on all apparatus is 

 important. Air gaps on direct current machines are 

 measured in the following manner: With the armature 

 stationary, the gap under all poles should be measured 

 at both the commutator and pulley ends, the measuring 

 scale being inserted under each pole tip, without in- 

 cluding the chamfer of the tip. A mark should then 

 be placed on the armature circumference under a given 



