8-BJ CIRCLE DIAGRAM. 281 



5. Construction of Circle Diagram. As lines of reference, in 

 Fig. I or 3, lay off line voltage in a vertical direction and, at 

 right angles to it, an indefinite line OJ, neither line being to scale. 



Lay off to scale the no-load current, 7 =0^ = 5.9 amperes; 

 this is to be laid off in its proper phase position, so that cos BO A 

 = no-load power factor = 0.185. 



6. In a similar manner, lay off to scale the locked or short- 

 circuit current at full voltage, 7s = OP" = 56.6 amperes, in its 

 proper phase position so that cos BOP" = power factor = 0.375. 



7. Two points, A and P", of the circle diagram are thus loca- 

 ted. The center of the circle lies on the diameter AP'" drawn 

 parallel to OJ, and is located at O' by dropping a perpendicular 

 from the middle of AP". With the center located, the circle 

 diagram is readily drawn ; it is seen to be the same as the diagram 

 Fig. 5, Exp. 8-A, found by loading the motor and to correspond 

 to the circle diagram for a transformer, Fig. n, Exp. 5~C, in 

 which the lettering is similar. Compare, also, Fig. 2, Exp. 4-6. 



8. As the load increases, the primary current is increased 

 from its no-load value, 7 , by the added current 7 (2) due to the 

 load; 7 (2) is equal to the secondary current expressed in terms 

 of the primary (2Oa, Exp. 5~C). The total primary current is, 

 accordingly, 7 1 = OP. The point P moves around the semi- 

 circle, as the load increases, until it reaches P", which corre- 

 sponds to such an over-load that the rotor is held at standstill. 

 The full-load position of P is shown at P', maximum power 

 factor at P 4 , maximum output at P 5 , maximum torque at P 6 , 

 maximum input at P 7 , as will be seen later from the method of 

 calculating these quantities. 



9. Results. The diagram enables us to readily determine 

 the various operating characteristics of the motor, as shown in 

 Fig. 2. 



Current values are scaled directly from the diagram. Power 

 may be calculated from the current values or measured by a 

 special scale. 



