CHAP. V] EXCITING AMPERE-TURNS 85 



calculate the energy component of the exciting current separately, 

 from the core loss curves (Fig. 10). The magnetizing current 

 required for the joints is calculated separately, using y a = \/2, 

 according to eq. (41). The total effective magnetizing current is 

 found by adding together the values of to for the iron and for the 

 joints. The loss component, i\, is added to this value in quadra- 

 ture, to get the total no-load current. As is mentioned above, it 

 is preferred in practice to estimate the total exciting current of new 

 transformers from the curves of no-load volt-amperes per kilogram 

 of iron, the values being obtained from tests on similar trans- 

 formers. 



Prob. 7. The core of a 25-cycle cruciform type transformer (Fig. 14) 



Jis _'(..") kg.; the mean length of the magnetic path is 170 cm.; the 

 material is silicon steel. The 4400-v. winding of the transformer has 

 1100 turns in series. What is the reactive component of the no-load 

 current.' Ans. 8.4 amperes. 



Prob. 8. Check a few points on the curves in Fig. 22. 



Prob. 9. Show that in formula (42) the coefficient TT is a special 

 case of the more general factor 4.44/^ a , when the magnetizing current 

 does not follow the sine wave. 



Prob. 10. What are the reactive volt-amperes per kilogram of carbon 

 steel at 40 cycles and at a flux density of 16 kl./sq.cm.? Ans. 56.4. 



Prob. 11. Show how to calculate the exciting ampere-turns required 

 for a given flux in a thick and short core in which the flux density is 

 different along different paths. 



35. The Types of Magnetic Circuit Occurring in Revolving 

 Machinery. The remainder of this chapter and the next chapter 

 have for their object the calculation of the exciting ampere-turns 

 necessary for producing a certain useful flux in the principal types 

 of electric generators and motors. In direct-current machines, 

 in alternators, and in rotary converters it is necessary to know the 



it ing or field ampere-turns in order to plot the no-load satura- 

 t i u curve, to predict the performance of the machine under vari- 

 ous loads, and to design the field coils. In an induction motor one 

 wants to know the required excitation in order to determine tin* 

 no-load current, or to calculate the number of turns in the stator 

 winding, when the limiting value of the no-load current is i 

 scribed. The general procedure in determining the required 

 number of ampere-turns for a given flux is in many respects the 

 same in nil the types of electrical machinery, so that it is possible 

 to outline the general method before going into details. 



