LOSSES IN ARMATURES 101 



of core losses should be based on the results of tests conducted 

 with built-up armatures rotated in fields of known strength. 

 Such tests are. made at different frequencies, and the results, 

 plotted in graphical form, give the total watts lost per pound of 

 iron stampings at different flux densities, a separate curve being 

 drawn for each frequency. The reader is referred to the hand- 

 books of electrical engineers for useful data of this sort; but for 

 approximate calculations of core losses, the total iron loss per 

 cycle may be considered constant at all frequencies. This 

 assumption allows of a single curve being plotted to show the 

 connection between watts lost per pound and the product kilo- 

 gausses X cycles per second. This has been done in Fig. 34 

 which is based on experiments conducted by MESSRS. PARSHALL 

 and HOBART and confirmed lately by PROFESSORS ESTERLINE 

 and MOORE at Purdue University. The curve gives average 

 losses in commercial armature iron stampings 0.014 in. thick. 

 Great improvements in the magnetic qualities of dynamo 

 and transformer iron have been brought about during the last 

 20 years, and the introduction of 3 to 4 per cent, of silicon in 

 the manufacture of the material known as silicon steel has 

 given us a material in which not only the hysteresis, but also 

 the eddy-current losses, have been very considerably lowered. 

 There are great variations of quality in armature stampings, 

 and values obtained from Fig. 34 would not be sufficiently 

 reliable for the use of the commercial designer of any but small 

 machines. By taking pains in assembling the stampings to 

 avoid burrs and short-circuits between adjacent plates, the 

 total iron loss may be considerably reduced. In large machines, 

 with a surface which is small in proportion to the volume, 

 the losses will usually be less than would be indicated by Fig. 

 34. In the absence of reliable tests on machines built with a 

 particular quality of iron punchings, it is suggested that the 

 values obtained from Fig. 34 may be reduced as much as 50 per 

 cent, in cases where extra care and expense with a view to re- 

 ducing losses are justified; and for silicon steel (a more costly 

 material than the ordinary iron plates) the reduction may be as 

 much as 70 per cent. 



When calculating the watts lost in the armature core, it is 

 necessary to consider the teeth independently of the section 

 below the teeth. This is because the flux density in the teeth 

 is not the same as that in the body of the armature. 



