1892.] 



Dynamo-Electric Machinery. 



53 



loss in the core, when magnetised, is greater than can be accounted 

 for by the known value of hysteresis. 



Effects of the Civrrent in the Armature. 



Quoting from the Royal Society paper, p. 342, " The currents 

 in the fixed coils around the magnets are not the only magnetising 

 forces applied in a dynamo machine; the currents in the moving 

 coils of the armature have also their effect on the resultant field. 

 There are in general two independent variables in a dynamo machine, 

 the current around the magnets and the current in the armature, and 

 the relation of E.M.F. to currents is fully represented by a surface. 

 In well- constructed machines the effect of the latter is reduced to a 

 minimum, but it can be by no means neglected. When a section of 

 the armature coils is commutated it must inevitably be momentarily 

 short-circuited, and, if at the time of commutation the field in which 

 the section is moving is other than feeble, a considerable current 

 will arise in that section, accompanied by waste of power and destruc- 

 tive sparking. 



" Suppose the commutation occurs at an angle \ in advance of the 

 symmetrica] position between the fields, and that the total current 

 through the armature be C, reckoned positive in the direction of the 

 resultant EMF of the machine, i.e., positive when the machine is used 

 as a generator of electricity. Taking any closed line through 

 magnets and armature, symmetrically drawn as ABCDEFA, it is 



obvious that the line integral of magnetic force is diminished by 

 be current in the armature included between angle \ in front and 



