PARTS OF A DYNAMO-ELECTRIC MACHINE 39 



plate. Then each armature core was in turn mounted 

 in the field of the experimental generator and the power 

 required to rotate these bare armature cores at 1200 r.p.m. 

 with a certain strength of magnetic field, was measured. 

 When the laminated core was tested it was found that 

 the required power (excluding friction) was 9 watts; the 

 core was left running for sometime but it remained cool. 

 The solid core was then substituted for the laminated one 

 and the power consumption was 350 watts; after running 

 for a few minutes it became so hot that it had to be stopped, 

 for fear of damaging the bearings. From this simple test 

 we conclude that a solid armature core cannot be used 

 in dynamo-electric machinery because, 1st, it requires so 

 much power merely to rotate the armature core in the 

 magnetic field, and 2d, the heat generated in the rotating 

 core is so great that any windings on the armature core 

 would be burned. 



Losses in Armature Core. The heat generated in any 

 piece of iron, rotating in a stationary magnetic field, is due 

 to two effects, namely, hysteresis and eddy currents. It is 

 evident that as the armature core rotates it is magnetized 

 first in one direction and then in the opposite direction, 

 as a point on the core moves from under a N pole to a S 

 pole. In Chapter I the question of hysteresis loss in a piece 

 of iron going through magnetic reversals was explained. 

 There is then this hysteresis loss in an armature core and 

 this loss may be kept small by using for the core iron 

 that has a very narrow hysteresis loop, such as a specially 

 treated alloy steel. After the sheet steel has been punched 

 out in proper shapes for the core construction, these lam- 

 inations are heated to a dull red heat and allowed to cool 

 slowly. This process, called annealing, results in a steel 

 having a very narrow hysteresis loop. 



Loss due to Eddy Currents. Eddy currents (local currents 

 in the material of the core itself) are produced in the rotating 

 armature core because, as it moves through the magnetic 



