COMMUTATING ALTERNATING CURRENT MOTORS 361 



of the c-c. series motor. A small series motor tested in the 

 laboratory gave results as shown in Fig. 234. 



The torque, or tractive effort, increases nearly as the 

 square of the current, being a maximum for the lowest 



FIG. 234. Operating Characteristics of a Small Series Single-phase 



Motor. 



speeds; this is one of the characteristics that make the 

 motor suitable for tractive work. The efficiency is fairly 

 high for all loads in the working range of the motor, but 

 is lower for all loads than that of a corresponding c-c. 

 motor. The extra iron and copper loss in the a-c. motor 

 account for this fact. 



