270 



ALTERNATING CURRENTS 



braking, such as is required by direct-current motors operating 

 under similar conditions. 



Figure 250 shows the variation of current, efficiency, slip, and 

 torque of an induction machine as it passes from motor to gen- 

 erator. It will be noted that the current does not pass through 

 zero, although the power does. The point of minimum current 

 is the exciting current of the machine, shown by 7 . At syn- 

 chronous speed, the line supplies the core losses, and the friction 

 losses are supplied mechanically. The generator must be driven 

 somewhat above synchronous speed before it supplies its own 

 core losses. 



111. The Circle Diagram. The operating characteristics of 

 an induction motor may be determined experimentally without 



o A J 



FIG. 251. The circle diagram for an induction motor. 



actually loading the motor, just as was done in the case of the 

 alternator and of the transformer. It can be shown that with 

 constant impressed voltage and constant frequency, the locus of 

 the primary current 7, Fig. 251, as the load on the induction motor 

 is varied, is an arc of a circle. That is, with change of load, the 

 end E, of the current vector 7, moves along the arc of a circle 

 PEHK. This diagram is approximate in that it neglects the 

 impedance drop and the copper loss in the stator due to 

 the magnetizing and core-loss currents. To obtain data for the 

 construction of this diagram an open-circuit and a short-circuit 

 (or blocked) run are made, as is done with the alternator and 

 the transformer. Using the data obtained from these two tests, 



