THE CONTINUOUS CURRENT MOTOR 179 



Comparison of Field Control and Multiple Voltage Methods. 

 The field-control method of speed variation results in the 

 efficient operation of the motor and the wiring of the power 

 supply to the motor is so much simpler than that of the 

 multiple voltage system that it is much preferred to the 

 latter. The number of running speeds in the multiple- 

 voltage scheme shown in Fig. 107 is limited to six, while 

 the number obtainable when using field control depends 

 only upon the number of contact points in the field rheostat 

 of the motor; as many as 40 or 50 speeds are thus available 

 with the rheostats ordinarily employed. 



The multiple-voltage control may employ more or less 

 field control, however, in which case the number of running 

 speeds obtainable is greater than when field control alone 

 is used. The chief advantage of the multiple-voltage 

 control over the straight field control lies in the smaller 

 size of motor required for a given horsepower capacity. 



38. Speed control of Railway Motors. The speed 

 control of railway motors is accomplished by the variation 

 of the voltage impressed across the motor terminals by what 

 is called the series-parallel control. There are always at 

 least two motors on each electric car and sometimes there 

 are four, all of the same rating. When the car is first 

 started, all motors are connected in series, through a start- 

 ing resistance, to the 600-volt line, the trolley or third rail 

 being one side of the line and the track the other side. 

 The controller (which is simply a rotating switch) has 

 marked on its face various points, the number of these 

 points depending upon the type of motor equipment. 



The Various Steps Used in Starting. The first three 

 points are generally " resistance points," i.e., while the 

 controller handle is pointing to any one of them more or 

 less of the starting resistance is connected in series with 

 the motors. On the fourth point the two motors are con- 

 nected in series with each other and directly to the line. 

 This is called a " running point " because no power is 



