ELECTROMOTIVE FORCE AND CURRENT. 9 



voltage given out by the machine for any position of the 

 armature if the position of maximum or minimum voltage 

 is known. 



The complete curve shown for a variation from to 360 

 corresponds to a rotation of the armature or field of the 

 alternator equal to the angular distance between the centre 

 of one N pole and the centre of the next N pole. In a 2-pole 

 machine this is one complete revolution. 



Definitions. The time taken by the electromotive force 

 to pass through the complete series of changes represented in 

 Fig. 4 is called a Period. The Periodicity of an alternating 

 circuit is the number of periods in one second. Thus an 

 alternator giving a periodicity of 50 would impress on the 

 circuit the series of changes shown in the figure, 50 times per 

 second. Frequency is another term for periodicity. 



The Amplitude is the maximum value of the variable 

 quantity which passes through the changes shown above. 

 Thus the length P is the amplitude in Fig. 4, corresponding 

 to 25 volts. 



The angle measured from the starting point Q to any 

 particular point P on the curve is called the phase of this 

 point; thus in Fig. 4 the phase of P is measured by the angle 

 QOP. 



Example. An alternator gives a maximum of 100 volts. 

 If its frequency is 50, what will be the phase and value of its 

 voltage after 3f seconds starting from the point of zero 

 electromotive force ? In three seconds it will have passed 

 through 360 150 times, and in f sec. it will further pass through 

 f x 50 of 360, i.e., 33 J times 360. Hence at the end of 

 3| sees, it will have passed through the complete cycle of 



*yf*f\ 



changes 183 times, and its phase will be -x ' or 120. The 



value of the electromotive force will then be V sin Q, or 

 100 sin 120 = 100 x -866= 86-6 volts. 



Effect of Increase in Number of Poles. If instead of two 

 poles only, the machine has a greater number, the rotating 

 armature might be provided with as many conductors as the 

 alternator has poles. By coupling them together suitably 

 the electromotive forces generated in them may be directed 

 along each successive conductor in the same sense from end to 

 end of the winding. The conductors must all be situated 

 symmetrically with respect to the poles, in order that they may 



