98 SYNCHRONOUS ALTERNATORS. |K\r. 



conditions. In the preceding pages this was done by two tests, the 

 open-circuit test at normal voltage and zero current, and the short- 

 circuit test at normal current and zero voltage, in each test the power 

 output being zero. But, inasmuch as power output is the product of 

 current, voltage and power factor, E and / may simultaneously have 

 normal full-load values without involving expenditure of power if 

 the power factor is zero. This leads to the low power factor tests 

 ( 52) and split field tests ( 53), concerning which only a brief state- 

 ment will be made; for fuller information consult references. These 

 tests are used in heat runs and efficiency tests, as well as in test for 

 the determination of regulation. 



52. Tests at Low Power Factor. When operated at low power 

 factor, an alternator may have full-load current and normal voltage 

 with only a small expenditure of energy. If Eo and ET are thus 

 determined for one power factor, their values and the regulation can 

 be calculated (22) for unity or any other power factor. This cal- 

 culation is usually made either for the same terminal voltage or for 

 the same excitation (same o). The load may consist of react- 

 ances, unloaded induction motors or a synchronous motor with low 

 or no-field excitation. The power factor is known from readings of 

 ammeter, voltmeter and wattmeter. Any power factor less than 0.20 

 or 0.25 may be considered as zero, for between these limits (see Fig. 6) 

 there is practically no change in regulation. 



When a synchronous motor is used, the generator voltage is adjusted 

 by the field rheostat of the generator; the armature current by the 

 field rheostat of the motor. In this way a full-load saturation curve 

 for low power factor can be obtained (Fig. i) and compared with 

 the no-load curve ; or points can be plotted for an external charac- 

 teristic, as in Fig. 7. 



53. Split Field Method. When an alternator is operated at low 

 power factor with a synchronous motor load, as in the preceding 

 paragraph, electric energy is given out by the alternator to the motor 

 one quarter-cycle and is practically all returned the next quarter- 

 cycle; power circulates between the two machines. Circulation of 

 power in one machine was first proposed by Mordey*; this was 

 accomplished by dividing the armature coils in two parts, one opposed 

 to the other. In this way part of the armature acted as a generator 

 and part as a motor. This, however, proved open to objection. 



* W. M. Mordey, Journal Brit. Inst. of Elect. Eng'rs, Vol. II, 1893. 



