154 



ALTERNATING CURRENTS 



same value of field current as before, the coil voltage in the open- 

 circuit test is now 2,180 volts, and the corresponding coil current 



1,400 

 in the short-circuit test is /= = 808 amp. 



The synchronous impedance per coil 



Am, 



Res. 



or three times its previous value. 



Figure 155 shows the circuits 

 of the delta when the ohmic 

 resistance is measured with direct 

 current. Let the resistance per 

 coil be R and the resistance meas- 

 ured between any two terminals 

 be RQ. The circuit consists of 

 two parallel branches, one of R 

 ohms and the other of 2R ohms. 



D.C. 



Supply 



FIG. 155. Measurement of delta coil 

 resistance with direct current. 



Therefore, -;J + gj 



Therefore, the ohmic resistance per coil 



R = (3/2) X 0.12 = 0.18 ohm, 



or three times its previous value. This must be increased 50 

 per cent., in order to obtain the effective resistance. 



1.5 X 0.18 = 0.27 ohm effective resistance. 

 The rated coil current of the machine, 



= = amp 



Applying equation (39), page 139. 

 E = 



V[(2,300X0.85) + (217X0.27)] 2 +[(2,300X0. 

 = 2,700 volts 



which checks the result obtained when assuming that the machine 

 was Y-connected. 



Therefore, a machine may be assumed to be either Y- or delta- 

 connected when it is desired to calculate the regulation. 



