ARMATURE LAMINATIONS 21 



When an armature has been built of iron of which samples 

 have been thus tested, core loss tests on the completed machine 

 will show a loss, in watts per kg., greatly in excess of the 

 values derived as above described from measurements on the small 

 samples. This is not an indication that anything is wrong, but is 

 in accordance with well-known experience. By a careful system of 

 recording the Ewing readings, and the results on test of the com- 

 pleted machine, interesting values of the ratio of increase which is 

 to be expected may be obtained. For this purpose it is well to 

 keep records, for which, in Table VI., a convenient arrangement is 

 suggested. It is assumed that four samples, A, B, C, and D, have 

 been tested, and machines built from material of which A, B, C, 

 and D are samples. 



Then in the left-hand section of Table VI. should be entered 

 the mean results of tests by the Ewing hysteresis tester on each 

 of these samples. On the right-hand section should be recorded the 

 losses observed on machines with armatures built of iron, of which 

 A, B, C, and D are samples, and the mean values (M) entered up. 



In an intermediate section of the table, the Ewing tests are 

 reduced to the densities and periodicities corresponding to the 

 machine tests. In the last column, the ratios of the machine tests 

 to the Ewing tests are entered. It will be found that these ratios 

 are often of the value of 2 to 3, or more. The excess is partly due to 

 eddy current losses, which will be greater the thicker the sheets 

 and the less carefully they are insulated from one another. The 

 eddy current loss also varies with the degree of conductivity of 

 the material of the sheets. In many cases, a very large part of 

 the eddy current loss is attributable to wasteful circuits formed by 

 the touching of the adjacent rough edges at the sides of the slots, 

 due to poor die-.work, or to too much filing of the sides of the slots. 

 Such losses cannot be closely predetermined ; but the keeping of 

 the records above described enables a close control to be kept over 

 them, and it should be the aim of the manufacturer to gradually 



reduce the ratio of Table VI. by the gradual improvement of 



methods of construction. 



In France, an instrument similar to the Ewing hysteresis 

 tester is employed, except that it is the magnet which is rotated. 1 



1 Hysteresis testers with rotating magnets were built by Holden in 

 1895 (see Electric Machine Design, Parshall and Hobart, Engineering, 190C, 

 pp. 11-13). Holden, however, employed electromagnets, whereas Ewing and 

 Blondel employ permanent magnets. 



