CH. XI DESIGN OF RAILWAY MOTORS 231 



Thus iii Example 54, an accelerating current of 10 

 amperes per motor gives an acceleration of - 421 f.p.s. per 

 second ; if the motors are in parallel, we require 80 amperes 

 from the line to accelerate the whole car. If, however, the 

 motors are in series, we only require an accelerating 

 current of 1<0 amperes from the line. Hence the series- 

 parallel arrangement enables us to get the same acceleration 

 as before with half the accelerating current from the line, 

 as long as the motors can be held in series. Our calcula- 

 tions will then be unaltered except in this, that during 

 the time that the motors are in series the accelerating 

 current is half that given by Equation 105. In the case 

 quoted, the motors will remain in series for 25 seconds, 

 during which time the current from the line will be 59'4 

 instead of 118'8 amperes as with the parallel connection. 



It is of course always possible to reduce the time 

 occupied in covering any distance by increasing the 

 accelerating current, without altering the general design. 

 Thus in Example 54, if the accelerating current were 

 doubled, we should cover the 400 yards in five-sixths of 

 the previous time, or 67 seconds, with JJ/=156 and 

 (=30 inches. The accelerating current would then be 72 

 amperes. We could, however, obtain an equally good 

 result with less expenditure of current, either by putting 

 on wheels 52 inches in diameter, or by inserting gearing, 

 t'=l'73, keeping M unaltered; in either case we should 

 cover the 400 yards in 67 seconds with only 62 amperes 

 of accelerating current. 



Thus far we have assumed that the induction factor 

 remains constant throughout the whole period ; we must 

 now consider the effect of the variation in the 

 value of M due to series winding. 



