CHAPTER VI 



LOSSES IN ARMATURES VENTILATION 

 TEMPERATURE RISE 



31. Hysteresis and Eddy-current Losses in Armature Stamp- 

 ings. The loss due to hysteresis in iron subjected to periodical 

 reversals of flux may be expressed by the formula, 



watts per pound = KhB l -*f 



where KH is the hysteresis constant which depends upon the 

 magnetic qualities of the iron. The symbols B and / stand as 

 before for the flux density and the frequency. 



An approximate expression for the loss due to eddy currents 

 in laminated iron is, 



watts per pound = K e (Bft) 2 



where t is the thickness of the laminations, and K e is a constant 

 which is proportional to the electrical conductivity of the iron. 



With the aid of such formulas, the hysteresis and eddy-current 

 losses can be calculated separately and then added together to 

 give the total watts lost per pound. This method will give good 

 results in the case of transformers; but when the reversals of 

 flux are due to a rotating magnetic field, as in dynamo-electric 

 machinery, the losses do not follow the same laws as when the 

 flux is simply alternating; and moreover there are many causes 

 leading to losses in built-up armature cores which cannot easily 

 be calculated. These additional losses include eddy currents due 

 to burrs on the edges of stampings causing metallic contact 

 between adjacent plates. There are also eddy currents produced 

 in the armature stampings due to the fact that the flux cannot 

 everywhere be confined to a direction parallel to the plane of 

 the laminations. Some flux enters the armature at the two 

 ends and also into the sides of the teeth through the spaces pro- 

 vided for ventilation. Since this flux enters the iron in a direction 

 normal to the plane of the laminations it is sometimes account- 

 able for quite appreciable losses. For these reasons calculations 



100 



FACULTY OF APPLIED 







