362 



ELECTRICAL MACHINERY 



The power factor is a maximum for the high speeds 

 and gradually decreases with increase of load. The reason 

 for this may be seen by reference to the vector diagram 

 shown in Fig. 235. The impressed voltage on the motor 

 is constant and in Fig. 235 is given by the radius OD of 

 the circular arc CC'D. The phase of the current is assumed 

 as 01. There are three reactions balancing the impressed 

 e.m.f. of the motor, viz., the inductance reaction, which is 

 proportional to the current, the resistance reaction, also 

 proportional to the current, and the c.e.m.f. due to the 

 rotation of the armature conductors. 



B' B 



FIG. 235. Vector Diagram of the Series Single-phase Motor. 



For a current of 10 amperes the inductance drop of the 

 motor tested was OA and the power factor was cos cj>. 

 For 40 amperes the inductance drop was OA' and the 

 power factor was cos 4/, which is eveidently less than cos (}>. 



109. The Repulsion Motor. This motor is one employ- 

 ing a wound armature with commutator and brushes, 

 but the armature circuit is not connected electrically to 

 the power supply line. It was first developed by Elihu 

 Thomas in 1887, but has not yet come into very exten- 

 sive use. It has operating characteristics very similar 

 to those of the series a-c. motor. It has an advantage 



