CHAP. Ill] HYSTERESIS AND EDDY CURRENTS 43 



of electrical machinery and apparatus: First, because they bring 

 about a loss of power and hence lower the efficiency of a 

 machine; Secondly, because they heat up the iron and thus 

 limit the permissible flux density, or make extra provisions for 

 ventilation and cooling necessary; Thirdly, because they affect 

 t he indications of measuring instruments. The effects of hystere- 

 sis and eddy currents in the principal types of electrical machinery 

 are as follow 



(a) In a transformer an alternating magnetization of the iron 

 causes a core loss in it. The power thus lost must be supplied from 

 the generating station in the form of an additional energy com- 

 ponent of the primary current. The core is heated by hysteresis 

 and by eddy currents, and the heat must be dissipated by the 

 oil in which the transformer is immersed, or by an air blast. 



(6) In a direct-current machine the revolving armature is 

 subjected to a magnetization first in one direction and then in 

 the other; the heating effect due to the hysteresis and eddy 

 currents is particularly noticeable hi the armature teeth in which 

 the flux density is usually quite high. The core loss, being sup- 

 plied mechanically, causes an additional resisting torque between 

 the armature and the field. In a generator this torque is sup- 

 plied by the prime mover; in a motor this torque reduces the 

 available torque on the shaft. 



(c) The effect of hysteresis and of eddy currents in the armature 

 of an alternator or of a synchronous motor is similar to that in a 



ct-current machine. 



GO In an induction motor the core loss takes place chiefly 

 in the stator iron and teeth, where the frequency of the magnetic 

 cycles is equal to that of the power supply; the frequency in the 

 rotor corresponds to the per cent slip, so that even with 

 high flux densities in the rotor teeth the core loss in the rotor is 

 comparatively small. At speeds below synchronism t he necessary 

 power for supplying the iron loss is furnished electrically as part 

 <>f the input into the stator. At speeds above nism this 



power is supplied through the rotor from the prime m<> 



(e) In a direct-current ammeter, if it 1ms a piece of iron as 

 its moving element, residual magnetism in thin iron cause* inac- 

 ii..!i. ;.:!,,: \\iththesminerurrent the indi 



of tip "lent is .smaller when the cur than 



when it is decreasing; this can be understood with reference to 



