3H ELEMENTS OF ELECTRICAL ENGINEERING. 



by the combined * ohmic resistance of four of the choke coils 

 in series. 



144. Characteristics of the single-phase series motor. The 

 single-phase series motor is very much like the direct-current 

 series motor in its speed and torque characteristics. The chief 

 difference between the direct-current motor and the single-phase 

 motor grows out of the fact that an alternating voltage of a given 

 value gives less current than a direct voltage of the same value. f 



Advantages of single -phase series motor for railway service. 

 The operation of a railway by electric motors involves two 

 distinct problems, namely (a) the problem of transmitting power 

 from a central station to the motors and (&) the problem of con- 

 verting the electrical power into mechanical power. The first 

 problem is a problem in long distance transmission when the rail- 

 way exceeds a few miles in length and high voltage must be 

 used. The second problem seems to admit of no other practical 

 solution than by the use of the series motor which develops a 

 large starting torque and operates at a fairly high efficiency 

 throughout a wide range of speed. The series motor, however, 

 is essentially a low voltage machine, and indeed the single-phase 

 series motor cannot operate at as high a voltage as the direct- 

 current series motor on account of the greater tendency of the 

 single-phase machine to spark. Therefore the operation of a 

 railway by electric power requires high-voltage transmission and 

 step-down transformation, and the choice may be said to lie 

 between (a) the series motor supplied with direct current at a 

 voltage not exceeding about 1,000 volts, and (^) a series motor 

 supplied with single-phase alternating current at a voltage not 

 exceeding about 500 volts. If the direct-current motor is used, 

 synchronous-converter sub-stations have to be placed a few miles 

 apart all along the line of railway, and not only is the total cost 



* This is due to transformer action as may be readily seen by tracing the short- 

 circuit current in two armature sections. 



f A brief description of the performance diagram (clock diagram) of the single- 

 phase series motor is given by V. Karapetoff, Trans. Am. Inst. Elec. Engineers, Vol. 

 XXXIII, page 78, 1904. 



