58 CONSTANT-VOLTAGE TRANSMISSION 



85 per cent. A customer can probably meet this condi- 

 tion by keeping the induction motors approximately 

 fully loaded. Accordingly, when a supply of direct cur- 

 rent is needed, there will be little incentive to install a 

 synchronous motor-generator set. The amount of power- 

 factor correction, though it is small, furnished by a syn- 

 chronous converter, may determine the choice of that 

 type of apparatus. Other factors influencing the choice 

 will be the higher efficiency and lower first cost of a 

 synchronous converter compared with a motor-generator 

 set, and the fact that first-class operation is now ob- 

 tained with converters by the use of commutating poles. 

 Synchronous converters are best operated with a fixed 

 rheostat setting and are not suitable for giving power- 

 factor control which would necessitate changes in their 

 excitation. 



Another form of contract of frequent occurrence is 

 one which gives a bonus increasing in proportion as the 

 power-factor is raised until 100 per cent is reached. 

 This will tend to increase the purchase of synchronous 

 motors. It has been proposed at times to give this 

 bonus by charging for the total Kva.-hours instead of 

 the Kw.-hours, but this is hardly to be - recommended, 

 owing to the inadvisability of using a type of meter not 

 now commonly manufactured, and to the doubt whether 

 even good commercial accuracy of metering could be 

 obtained. The bonus might be given according to the 

 minimum readings of a graphic power-factor meter, but 

 it is not always easy to set the rate definitely and pre- 

 cisely by this means. Perhaps the fairest and most ac- 

 curate way of avoiding the disadvantages of low power- 

 factor is to meter the lagging reactive Kva.-hours and 



