294 



PRINCIPLES OF ELECTRICAL DESIGN 



This step-by-step method of drawing the e.m.f . waves will yield 

 surprisingly accurate results, with the one exception that the 

 ripples known as "tooth harmonics" which are generally present 

 in oscillograph records, will not appear in the graphical work. 

 The effect of the distributed winding in smoothing out the irregu- 

 larities of the flux-distribution curve is very clearly shown by the 

 shape of the e.m.f. wave in Fig. 118. If the armature windings 

 are star-connected, the wave representing terminal voltage may 

 be obtained by adding the ordinates of two such e.m.f. waves 

 placed 60 electrical degrees apart. 



101. Form Factor. The ratio of the r.m.s. or virtual value to 

 the mean value of an alternating e.m.f. or current is the form 

 factor. The average ordinate of an irregular wave such as may 

 be obtained by the process represented in Figs. 117 and 118, is 

 readily obtained by measuring its area with a planimeter and 



Negative Lobe \ 



/ 



FIG. 120. Wave of alternating 

 e.m.f. plotted to polar coordinates. 



FIG. 119. Illustrating calculation 

 of r.m.s. value of variable quantity 

 plotted to polar coordinates. 



then dividing this area by the length of the base line, i.e., the pole 

 pitch. If another curve is plotted by squaring the ordinates of 

 the original curve, it is merely necessary to take the square root 

 of the average ordinate of this new curve in order to obtain the 

 virtual value of the alternating quantity. It will, however, 

 be more convenient to re-plot the original curve to polar co- 

 ordinates. The general case of a variable quantity plotted to 

 polar coordinates is illustrated in Fig. 119, where the radial 

 distance from the point represents the instantaneous value of 

 the variable quantity, while time (or distance of travel) is 

 measured by the angular distance between the vector considered 

 and the axis OX. 



