CHAPTER XVII 



161. Single-phase commutator motors 162. Series motor. 

 Conditions determining power factor of series motor 

 motor 165. Methods of obtaining spitrkless running 



Circle diagram 163. 

 .. 164. Compensated series 



g obtaining spitrkless running 166. Repulsion motor 



167. Compensated repulsion motors 168. Atkinson's repulsion motors 169. 

 Mixed- action commutator motors. 



161. Single-phase Commutator Motors 



WITH the rapid growth of electric railway and tramway systems, the 

 necessity of high-voltage transmission on economical grounds soon 

 became apparent. It was realized that beyond a certain distance the 

 usual 500- or 600-volt continuous-current systems could no longer be 

 successfully employed, as the cost of the conductors required to secure 

 reasonable regulation and losses became a very heavy item in the 

 total cost of the system. This led to the development of the rotary 

 converter system, in which we have three-phase generation and trans- 

 mission to converter sub-stations at a high voltage, combined with 

 low-voltage (500 volts) continuous-current distribution. But the 

 cost of rotary converter sub-stations is still very heavy. Attempts 

 were then made, in the case of very long lines, to use the three-phase 

 system pure and simple, the current being generated and transmitted 

 at a very high voltage, and then transformed to a lower voltage 

 before being supplied to the three-phase induction motors employed 

 for driving the cars. 



Apart from the complication and increased cost of overhead con- 

 struction necessitated by the use of two overhead conductors, the 

 three-phase system of electric traction suffers from another defect 

 the unsuitability of induction motors for variable speed work. The 

 induction motor is, as we have seen, essentially a constant-speed 

 motor, and is extremely wasteful during the period of acceleration. 

 Such motors can only be satisfactorily employed for traction work on 

 lines where stoppages are infrequent, and where long runs at constant 

 speed are the rule. 



In view of the disadvantages of the tliree-phase system of traction, 

 and the heavy cost of rotary converter sub-stations, the single-phase 

 system of traction, in which both transmission and distribution are 



