72 COMMERCIAL ELECTRICAL TESTING 



field coils forces the instantaneous heavy overload 

 current through the non-inductive german silver shunt 

 and leaves the commutating field without sufficient 

 excitation to neutralize armature reaction. The elec- 

 trical neutral immediately shifts and bad commutation 

 results. To eliminate this trouble, an inductive shunt 

 is used across the terminals of the commutating field 

 winding, and must always be in circuit when the machine 

 in test is under load. If a short circuit occurs, the 

 inductance of this shunt, being greater than that of 

 the commutating field winding, forces the heavy line 

 current through the field winding and tends to keep 

 the compensation normal for all conditions of load. 



Inductive Shunt 



An inductive shunt is used on all machines of 500 

 volts or more, of a normal current rating of 400 amperes 

 or greater. As a test is necessary to determine exactly 

 how much current must be shunted from the com- 

 mutating field, the inductive shunt is designed with 

 an inductance greater than that of the commutating 

 field winding and with low resistance and ample 

 carrying capacity. Any additional resistance necessary 

 is obtained by connecting german silver in series with 

 the inductive shunt, the length and resistance of which 

 is varied till an adjustment is obtained that gives 

 practically perfect commutation throughout the whole 

 load range for which the machine was designed. 



Location of Electrical Neutral 



After a commutating pole machine has been brought 

 to normal voltage at no-load, the no-load electrical 

 neutral must be located. To do this, a fibre brush of 

 the same size as the carbon brushes on the generator 

 in test must be produced. This brush should have 

 two holes drilled through it, each of which will take a 

 No. 12 bare wire; the spacing between the holes being 

 equal to the distance between adjacent commutator 

 bars. The wires should be small enough to move freely 

 through the holes, otherwise they may stick and make 

 poor contact on the commutator, or become wedged 



