CHAPTER XIII 

 ROTARY CONVERTERS 



Preliminary Tests 



The cold resistance of the armature of a rotary 

 converter is measured between the collector rings, 

 as follows: 



For a three-phase machine, between rings 12, 1-3, 

 2-3. 



For a two-phase machine, between rings 1-3, 2-4. 



For a six-phase machine, between rings 1-4, 2-5, 3-6. 



The resistance of the various phases should be the 

 same and it is immaterial whether the rings are numbered 

 from the inside or from the outside for this measurement. 



Running light on a rotary is taken with the machine 

 running from the direct current end. With the brushes 

 set on the neutral point, the direct current voltage is 

 held constant and the shunt field varied until the 

 rated speed of the machine is obtained. The input to 

 both field and armature is then read. Since there is 

 very little armature reaction in a rotary converter, 

 the brushes are set on the neutral point before the 

 machine is started. It often happens, however, that 

 better commutation can be secured by shifting the 

 brushes away from the neutral point very slightly. 

 In case of unsatisfactory commutation, the brushes 

 should be shifted in each direction, since some machines 

 require a forward and some a backward shift from the 

 mechanical neutral. 



The determination of the ratio of the alternating 

 current to the direct current voltage is one of the im- 

 portant tests on a rotary, and care should be taken 

 to secure accurate results. The converter may be 

 driven from either the alternating or the direct current 

 end and, in order to check the accuracy of the instru- 

 ments, two alternating current voltmeters, two potential 



[93] 



