THE MAGNETIC CIRCUIT 



189 



Referring to Fig. 49 on page 133, the curve (a) was plotted 

 by assuming different values of air-gap density; it shows the 

 relation between the ampere-turns required for air gap, teeth 

 and slots, and the air-gap density over the slot pitch at the 

 center of the pole face. We are now in a position to plot an 

 open-circuit saturation curve which shall include the ampere- 

 turns necessary to overcome the reluctance of all parts of the 

 magnetic circuit; but instead of giving the relation between total 

 ampere-turns per pole and the air-gap density, it will be more 

 convenient to plot a curve connecting field ampere-turns and 

 e.m.f. generated in the armature. This can easily be done since 

 we know the total flux per pole and the generated e.m.f. corre- 

 sponding to the value B a of the maximum air-gap density (Fig. 

 49). Thus, in Fig. 73 the distance OP a is the same as in Fig. 

 49, but the vertical scale has been altered so that the correspond- 

 ing value Eo as read off the dotted curve of Fig. 73 now stand for 

 the volts developed in the armature by the cutting of the flux, 

 instead of the air-gap density under the center of the pole face. 

 The full-line curve of Fig. 73 is the open-circuit characteristic 

 of the whole machine. It gives 

 the connection between field am- 

 pere-turns per pole and the termi- 

 nal voltage on open circuit, on the 

 understanding that the speed is 

 constant. The additional ampere- 

 turns required to overcome the -g 

 reluctance of the field poles, yoke, 

 and armature core, account for the p 

 space between the full-line and Q 

 dotted curves. This no-load sat- 

 uration curve for the complete 

 machine has been re-drawn in Fig. 74. 



Ampere Turns per Pole 



FIG. 73. 



Here OE is the termi- 

 nal voltage on open circuit; OE t is the terminal voltage at full 

 load (the machine is assumed to be over-compounded) ; and OE d 

 is the necessary developed voltage at full load, i.e., the voltage 

 that must be generated in the armature conductors by the 

 cutting of the flux in order that the terminal voltage at full 

 load shall be OE t . 



Draw a straight line connecting the origin, 0, of the curve and 

 the point F corresponding to the no-load voltage, and produce 

 this to G where it meets the horizontal line representing full- 



