8-A] 



OPERATION AND LOAD TEST. 



2/3 



ever, more satisfactory on account of its convenience and the 

 fact that belt loss is properly included in the results. 



47. Curves. With power output of the motor as abscissae, 

 plot curves showing power input, voltage (if variable), primary 

 current,* speed in per cent, of synchronous speed, efficiency, pri- 

 mary power factor and torquef in pounds at one foot radius or 

 in synchronous watts. Plot also the primary copper losses, 44. 

 (Compare Fig. 3, Exp. 8-B.) 



48. Apparent power input of the motor is equal to the real 

 power input divided by power factor. Apparent efficiency is 

 power output divided by apparent power input; or it is equal to 

 the real efficiency multiplied by power factor. These quantities 

 may be computed when desired. 



49. Circle Diagram. It is instructive likewise to compute the 

 in-phase or power component of the pri- 

 mary current and the quadrature or watt- 

 less component of the current. The power 

 component divided by the total current is 

 equal to the power factor. 



These results may best be shown as in 

 Fig. 5, in which values of quadrature cur- 

 rent are plotted as abscissae and values 

 of power component are plotted as ordi- 

 nates. The curve obtained in this way 

 forms the arc of a circle. The circle dia- 

 gram for an induction motor, usually pre- 

 determined as in Exp. 8 B, is thus found 

 by actual load test. 



The increase of wattless current with the load is due to leakage 

 reactance, for it can be shown that the diameter of the circle locus 



* Plot also the secondary current, if measured, and secondary copper 

 loss. 



t Torque is calculated from power output and speed, as in 3b, Exp. 

 2-A. In a brake test, torque is read directly. 



19 



20 



010 



ull Load 



Load 







2 4 6 8 10 12 

 Wattless Cuxreot 



FIG. 5. Change in pri- 

 mary current from no load 

 to full load ; the curve is 

 the arc of a circle. 



