288 INDUCTION MOTORS. [Exp. 



APPENDIX II. 



MISCELLANEOUS NOTES. 



31. Basis of the Circle Diagram. In any circuit or apparatus with 

 constant reactance and variable power consumption the current will 

 have a circle locus if the supply voltage is constant. Exp. 4-6 shows 

 this experimentally for a particular case. This was first shown by 

 Bedell and Crehore in 1892. That the induction motor nearly fulfills 

 these conditions and that its current locus is practically the arc of a 

 circle, was first shown by Heyland* in 1894. 



32. Accuracy. No circle diagram for an induction motor is 

 exactly correct, either in theory or in practice, for there are various 

 factors that cannot be accurately taken into account; for example,f 

 the effect of changes in the reluctance of parts of the iron under 

 changing conditions, the effect of wave distortion, the uncertainty 

 as to load losses, etc. The circle diagram, therefore, is theoretically 

 correct only in case certain assumptions are made. 



In practice, however, the circle diagram is found to give results 

 that are approximately correct and within the usual range of engineer- 

 ing accuracy. This is partly due to the fact that some of the errors 

 tend to cancel each other. The variations in the methods given by 

 different writers for constructing and using the circle diagram have 

 arisen from a difference in the selection of the errors to be eliminated 

 or minimized, the remaining errors being neglected. If the no-load 

 current 7 were, zero and there were no primary resistance loss, most 

 of the errors would disappear and some of these different construc- 

 tions would become identical. 



Generally speaking the effect of errors in using the circle diagram 

 becomes less as the size of motor increases, so that the method is 

 reasonably accurate on motors larger than, say, 5 or 10 H.P. For 

 motors under 5 H.P. the results, although not so accurate, are fairly 



* Elektrotechnische Zeitschrift, Oct. u, 1894; published later in book 

 form and translated into English by Rowe and Hellmund. 



t (32a). Behrend shows, for example, that there is a departure from a 

 true circle locus when the slots are bridged or closed; the short-circuit 

 characteristic between volts and amperes, under these conditions, is a curve 

 and becomes a straight line only if the leakage path contains no iron. 

 See pp. 20, 21, The Induction Motor, by B. A. Behrend. 



