198 THE DIRECT-CURRENT MOTOR CH. IX 



assume that the parallel method necessarily takes a greater 

 current than the series-parallel method. We may adjust 

 the starting rheostat so as to take the same current in both 

 cases, but the parallel method would then occupy a longer 

 time in starting. Neither of these curves represents the best 

 start that might be made with the two different controllers, 

 but they afford a fair comparison and show the superiority 

 of the series-parallel method. 



In Fig. 50 a curve is given showing the current taken 

 by the motors on the Liverpool Overhead Railway. In 

 the test of which this curve is a record, the weight of 

 the train was 39 tons, and the average tension of the line 

 484 volts. For further particulars the reader is referred 

 to Mr. Thomas Parker's paper in the ' Proceedings of the 

 Institution of Civil Engineers,' Vol. CXVII. 



This railway is equipped with motor cars each carrying 

 two series- wound motors controlled on the series -parallel 

 method. No gearing is used, and the wheels have a 

 diameter of 33 inches. The curve gives the total current 

 from the line for one train. The notch in the curve repre- 

 sents the period during which the motors are speeding up 

 in series. When the current was cut off and the brakes 

 put on, the train came to a standstill in 19 seconds. 



We may notice that these motors have a very high 

 internal resistance, amounting to as much as 1 *04 ohms 

 each, made up as follows : armature resistance O67 ohm ; 

 magnet resistance 0'37 ohm. The object of making the 

 resistance so high appears to have been to get as much as 

 possible of the resistance at starting in the form of con- 

 volutions on the armature, the supposition being that if 

 the starting rheostat could be entirely dispensed with, a 

 much greater starting torque could be obtained. 



