Electrical Engineering ⇒ Topic : Synchronous Impedance

Alex
 
The synchronous impedance of an alternator can be determined using the opencircuit and shortcircuit characteristics of the alternator shown in Figure (a). For a given field current, say OS_{1}, the corresponding noload voltage is Q_{1}S_{1}, and the corresponding armature current is R_{1}S_{1}. Then, the synchronous impedance per phase, Z_{s} can be calculated as
FIGURE (a) Opencircuit characteristic, shortcircuit characteristic and synchronous impedance (Z_{s}) of an alternator
................. (1) The value of Z_{s} at different values of field current (I_{f}) can be determined using Eq. (1) and the data of the O.C. and S.C. characteristics shown in Fig. (a). The resultant data when plotted gives the Z_{s} vs. I_{f} characteristic, also shown in Fig.(a). The values of Z_{s} remain constant for the portion of the O.C. characteristic which is linear and decrease gradually thereafter, with increasing field currents. The decrease in the value of Z_{s} in more rapid corresponding to the flat (saturation) portion of the O.C. characteristic. Next, the value of Z_{s} which is to be used for computing the regulation of the alternator has to be determined. To do this, an ordinate OP equal to the rated voltage of the alternator per phase is fixed. A horizontal drawn from the point P cuts the O.C. characteristic at Q. The field current (I_{f}) corresponding to the point Q is OS. This is the field current required to generate the voltage QS when the alternator is on open circuit and to circulate a current RS when it is shortcircuited. The value of the synchronous impedance (Z_{s}) corresponding to the alternator rated voltage of OP is .........[using Eq. (1)] An alternate method to determine the value of the alternator synchronous impedance (Z_{s}) in a similar way is to determine it at a short circuit current equal to the fullload armature current. The synchronous reactance X_{s} can be obtained as
Knowing the values of Rs and Xs, the regulation can be computed at different power factors, usually at unity power factor 0.80 lagging and 0.80 leading, using Eq. (a). To estimate E_{0}, we use Eq. (b) which reads as .................. (a) .................. (b) .............. (2) In this equation, plus sign is used when the power factor is lagging and minusmsign is used when the power factor is leading  
 
Lalan
 
SYNCHRONOUS IMPEDANCE It is given as follows  
 
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