192 



ALTERNATORS. 



The exact wave form of the electromotive force can only 

 be ascertained by some curve-tracing device such as that 

 described on page 84, or by the use of an oscillograph. 



Another factor affecting the voltage of the armature is the 

 distribution of the conductors connected in series. If each 

 winding consists of a number of conductors side by side, 

 the individual conductors will not come under the influence 

 of the poles simultaneously. The maximum and minimum 

 voltage will be induced in the conductors successively and not 

 simultaneously, thus tending to make the changes of voltage 

 more gradual. A winding in which each coil consists of a 

 number of conductors concentrated in a single pair of arma- 

 ture slots, will consequently tend to give a wave form passing 



J IT 



L_t_ 



FIG. 93. ILLUSTRATION OF WAVE FOBMS WITH SAME AVERAGE VALUES 

 AND DIFFERENT VIRTUAL VALUES. 



steeply from zero to its maximum value, whereas a winding 

 distributed in a number of slots would produce a voltage 

 rising and falling more gradually. 



As pointed out on page 89, the virtual or effective value 

 of the alternating current depends upon the average of the 

 squares of the instantaneous values, and not upon the average 

 values. In consequence of this a voltage of a given average 

 value will have a virtual value which is not constant for all 

 wave forms, but will depend upon the wave form, and will 

 in general be low for a flat wave and higher for a wave form 

 possessing a high maximum value, or peak. As an example 

 of this, two very simple wave forms are given in Fig. 90, 

 drawn to such a scale that they both have the same average 

 value. The virtual (root of mean squares) value of the 

 rectangular curve in Fig. 90 is the same as its average value, 

 and equal to its maximum value, while the virtual value of 



