378 ELECTRICAL ENGINEERING 



The power factor can be maintained approximately unity by 

 adjusting the excitation, but the power losses cannot be eliminated 

 and the values of alternating current in the table must be in- 

 creased by the small component required to supply the losses in 

 the machine. When the power factor is not unity the reactive 

 or wattless components of current must be added to the power 

 components. 



The ratios of e.m.fs. are the ratios of the generated e.m.fs. 

 and can only approximately represent the ratios of terminal 

 e.m.fs. since components of e.m.f. are consumed in the resistance 

 and reactance of the armature. The ratios also depend on the 

 assumption that the alternating e.m.f. wave is a sine wave. If 

 the wave is peaked, the ratio of the effective value to the maxi- 



mum value is less than -p. and the values of the alternating 

 e.m.fs. must be reduced. If the wave is flat topped the ratio of 

 effective to maximum value is greater than 7= and the values of 



the e.m.fs. must be increased. 



239. Wave Forms of Currents in the Armature Coils. The 



current in any armature coil is the difference between the alter- 

 nating-current input and the direct-current output. 



In Fig. 360, l\ and 1% are the two leads of one of the n-phases of 

 a converter, a is the coil next to one lead and c is the coil in the 

 centre of the phase. The alternating e.m.f. and the power com- 

 ponent of the alternating current in the phase li to lz are maximum 

 when this section of the winding is midway between the brushes 

 and they are both zero when the centre coil c is under the brush. 



Fig. 361 shows the resultant of the alternating and direct 

 currents in coil c during one revolution. The alternating current 

 is opposed to the direct current and is zero when the direct current 

 reverses as the coil passes under the brush. The current in the 

 centre coil is, therefore, less than the current in any other coil 

 in the phase when the power factor is unity. 



Fig. 362 shows the current in coil a next to one of the leads. 

 The alternating and direct currents are not directly opposing and 

 the resultant current is greater than in the centre coil c. 



The coils next to the leads, therefore, carry larger currents than 

 the coils in the centre of the phases and they rise to a higher 

 temperature. 



