TV/A' INDUCTION MnrnK 271 



the operation of the motor may be determined with a very fair 

 degree of accuracy by t he use of such a circle diagram. 



The motor is first run at rated voltage without load and the 

 line voltage V, the line current 7 , and the total watts P, an 

 measured. The no-load power-factor angle BO can then be deter- 



T> 



mined (cos = ~~5~ f r ;l three-phase motor). The voltage 



per phase V is laid off vertically, Fig. 251, and the no-load cur- 

 rent I (per phase' is laid off at an angle B from V and lagging. 

 The rotor is then blocked. In order that the current may be 

 kept within reasonable limits, the supply voltage per phase, V, 

 is reduced to voltage v', which should be of such value as to give 

 a short-circuit current approximately, equal to the rated motor 

 current. The phase current /*, the total power P f and the 

 phase voltage v' are measured under these conditions. Let V 

 be the rated phase voltage of the machine. V = V for a delta- 

 connected machine and V = V/\/3 for a Y-connected machine. 

 The measured current IB is increased in the ratio of the rated 

 motor voltage V (per phase) to the reduced voltage v'. 

 This gives I R = OH, the current per phase which would exist 

 ' lie rated line voltage V impressed across the motor when 

 blocked. This current lags V by an angle 6 fl . 



COS 



nI B v' 



V 



7 r ' 

 IB = In ~, 



where // is the number of pha 



oj. i- drawn making an angle of ( .Mf with OY' in a clockwise 

 direction. 1 H Oil is laid off making an angle <) with 0V. 

 Point- /' and // on the circle are therefore determined. 



Line /'// is drawn. /'A is drawn parallel to OL. It is not 

 know point A in order to construct the diagram. 



With /'A as a diameter, a semi-circle is drawn through points 

 I* and //. The center, .V, of this semi-cirele i> found by erect ing 

 a perpendicular MM' at the center of /'//. The intersection of 

 M M' with /'A gives the center U of the circle. With MP as a 

 radiu- and M ii-cirde /'A7/A is drawn. /'A' 



is the diameter of the .-enii-cird,' and it- length in amp- 



