66 ELECTRICAL MACHINERY 



one-half this number will be the proper number of ampere- 

 turns per coil. 



The B-H curves given on p. 18 are to be used in this 

 calculation. These curves are plotted between flux density 

 and ampere-turns per cm. length of the magnetic circuit. 

 Suppose that a ring of cast iron is to be magnetized to a 

 flux density of 5000 lines per.sq.cm. and that the average 

 length of the magnetic path in the ring is 75 cm. How 

 many ampere-turns will be required? 



By reference to the magnetization curve for cast iron 

 it is seen that to produce a flux density of 5000 lines per 

 sq.cm. 9 ampere-turns are required per cm. of length of the 

 path. As the length of the path is 75 cm. the number of 

 ampere-turns required is 75 X 9 = 675 ampere-turns. 



Field Coil Calculation. This same method may be used 

 for calculating the necessary ampere-turns for the magnetic 

 circuit of the machine shown in Fig. 34. 



Suppose the necessary flux through the armature core 

 is 1,600,000 lines. There must be through the poles and 

 yoke more flux than this because of the leakage lines. If 

 the leakage factor is taken as 1.25 the flux through the yoke 

 and poles is 1,600,000X1.25 = 2,000,000 lines. 



The cross-sectional area of the different parts of the 

 magnetic circuit would be given. The average length of 

 the magnetic path in each part may be estimated. From the 

 flux and cross-section the density in each part is calculated 

 and then from the magnetization curves the ampere-turns 

 per cm. are obtained; this quantity multiplied by the length 

 of path in that part of the circuit gives the required 

 ampere-turns for that part. The sum of the ampere-turns 

 required for each part gives the number required for the 

 complete magnetic circuit and one-half this number are 

 to be put in each field coil. 



The number of ampere-turns required for the air gaps 

 cannot be obtained from the curve sheet but is easily 

 calculated as follows; 



