3H 



SYNCHRONOUS MACHINES. 



[Exp. 



CURRENT 

 LAGGING 



CURRENT 

 LEADING 



PART III. EXCITATION CHARACTERISTICS. 

 21. No Load Run. Connect a circuit breaker in the armature 

 circuit and an ammeter, voltmeter and wattmeter* to measure 

 current, voltage, power and power factor; in the field circuit 

 connect an ammeter and field rheostat. (It will often be con- 

 venient to determine I sin 6 directly by the method of 13, 43, 



Exp. 6-B.) After synchro- 

 '/// nizing, run the rhotor at no 

 load and vary the field current 

 through as wide a range as is 

 possible with the motor keep- 

 ing in step. Take simultane- 

 ous readings of all instru- 

 ments and, with field current 

 ^ CURREN ver - excited . as abscissae, plot as ordinates : 



FIG. 6. Change of armature current with armature current, power fac- 

 excitation for different loads. tor (cQS ^ ^^ ang]e ( ^ 



power component of current (7cos0) and wattless component 

 (/sin*). 



22. Load Runs. Similar runs may be made at various loads. 



23. V-curves. Fig. 6 shows the variation of the armature 

 current with field excitation for no load, full load and one inter- 

 mediate load. (These curves are often plotted with values of 

 motor electromotive force E' as abscissae instead of field current, 

 the values of E' corresponding to particular values of field 

 current being taken from the no-load saturation curve, 6, Exp. 

 IO-B.) Individual readings are likely to fall off the curves on 

 account of hunting. 



In an ideal case the curve for the wattless current / sin 6, would 

 fall to a minimum value of zero, as shown by the dotted curve, 



* (213). For a polyphase machine, determine power and power factor 

 as in Exp. 6-B ; or, make measurements on only one phase. In the labora- 

 tory, where the experiment is only illustrative, the motor may be operated 

 as a single-phase machine. 



