CHAPTER IX 



DIRECT CURRENT GENERATORS PRE- 

 LIMINARY TESTS AND EFFICIENCIES 



Preliminary Tests 



Preliminary tests on direct current generators consist 

 in drop on spool, polarity, hot and cold resistance 

 measurements, air gap, potential curve, rheostat data, 

 brush shift, running light and equalizing ring tests. 

 With the exception of potential curve, rheostat data 

 and equalizing ring, the tests have all been previously 

 described. 



On multiple wound armatures of self-contained 

 machines not equipped with equalizing rings, a potential 

 curve may be taken. All the brushes except those on 

 two adjacent studs are raised from the commutator, 

 the voltage is raised to normal and the field current 

 noted. This field current and the speed must be held 

 constant for all other points on the curve. The brushes 

 on stud No. 3 should now be lowered, those on No. 1 

 raised and the voltage read between studs No. 2 and 

 No. 3. This procedure should be continued until voltage 

 readings have been taken between every pair of studs. 

 The test should be made with the field current rising. 

 The maximum voltage variation permissible is 4 per 

 cent of the average value. This test, although similar 

 in nature, should not be confused with the bar potential 

 curve taken to determine the wave form of a direct 

 current machine. 



Equalizers consist of rings or cross connections 

 tapping into equi-potential points on the winding of 

 multiple wound armatures between each pair of poles. 

 These rings prevent inequalities in voltage between 

 brushes of similar potential, due to inaccurate centering 

 of the armature. The rings allow alternating currents 



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