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Synchronous ImpedanceAuthor: E.E. Kimberly The voltage E_{z} which forces the current I_{1} through the impedance of the motor windings produces a voltage drop I_{1}Z_{s}. The impedance Z_{s} is called the synchronous impedance and is composed of two parts. One part is R_{a}+jX_{a}, which is the resistance and leakage reactance of the winding. The other part is X_{r}, which is not a reactance in the usual sense but is the result of the effect of the stator ampereturns on the total flux of the motor. This effect is called armature reaction and its magnitude depends almost altogether on the angle by which the rotor poles lag the stator poles in space. It is apparent then that X_{r} depends on both the magnitude of the motor load and the motor power factor. If the mmf of the armature reaction tends to decrease the total flux, its effect is the same as additional reactance drop; and, therefore, X_{r} and X_{a} are commonly combined and the result is called X_{s}. The value of Z_{s} is composed of both parts. Thus,
To determine Z_{s} experimentally, the machine is driven at synchronous speed without excitation. The polyphase winding is shortcircuited through three ammeters, one in each lead, to measure the current I_{p} per phase. A small direct current is then used to magnetize the field poles to the point where the three ammeters show fullload rated current. From the opencircuit saturation curve of the machine, the voltage E_{p} per phase is found corresponding to the direct current used. The synchronous impedance, in ohms, is then Z_{s} = E_{p}/I_{p}. With the polyphase winding shortcircuited, all of E_{p} is used to force I_{p} through the total impedance Z_{s} of the machine. Refinements of this procedure are necessary if Z_{s} must be determined accurately for some particular condition of loading.


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