Dalilin Haɗin Iya Tattalin Kasa
An Synchronous Impedance Method, wanda ake kira EMF Method, yana ƙara matsalolin gurbin armature da sabon matsaloli mai hagu. Don in taka shiga cikakken tsari na fayuwarsa game da wannan hanyar, zai bukata cikakken bayanan da ke: rukunin armature per phase, Open-Circuit Characteristic (OCC) curve wanda ya nuna inganci daga fayuwar ta hanyar zuwa field current, da Short-Circuit Characteristic (SCC) curve wanda ya nuna inganci daga fayuwar ta hanyar zuwa field current.
Don sync generator, yawancin lissafin da aka bayyana su ne:
Don in taka shiga cikakken synchronous impedance Zs, ana yi takardun da aka samu, sannan an samu abin da ke Ea (armature-induced EMF). Tana amfani da Ea da V (terminal voltage), sannan an yi cikakken tsari na fayuwarsa.
Takarda na Synchronous Impedance
Synchronous impedance yana haɗa da uku takarde:
DC Resistance Test
A nan, ana ɗauka cewa alternator yana ci gaba da star-connected tare da DC field winding open-circuited, kamar yadda aka bincike a cikin diagrammin circuit:
DC Resistance Test
Rukunin DC da ke bayan duk ɗaya daga terminal yana taka shiga don amfani da ammeter-voltmeter method ko Wheatstone's bridge. An samu adadin ɗaya daga takarden rukunin Rt, sannan an samu rukunin per-phase DC resistance RDC tare da ƙarfin Rt ta 2. Amma saboda skin effect, wanda yake sa ƙarfin AC resistance, an samu rukunin per-phase AC resistance RAC ta ƙarfin RDC da factor of 1.20–1.75 (amfani da 1.25), tare da tsarin ƙarfin machine.
Open Circuit Test
Don in taka shiga cikakken synchronous impedance ta hanyar open-circuit test, ana yi alternator a rated synchronous speed tare da load terminals open (loads disconnected) da field current initially set to zero. Diagrammin circuit ta hakan yana cikin bayanin:
Open Circuit Test (Continued)
Ba a yi field current zuwa zero, an yi ta gradually in steps tare da maimaita terminal voltage Et a ɗaya increment. Excitation current tana ƙarfin har zuwa lokacin da terminal voltage yake ƙarfin 125% of the rated value. Ana kawo graph bayan open-circuit phase voltage Ep = Et/sqrt 3 da field current If, wanda ya ƙarfin Open Circuit Characteristic (O.C.C) curve. Wannan curve yana tasirin shape na standard magnetization curve, tare da linear region ta extended zuwa air gap line.
O.C.C da air gap line suna cikin bayanin:
Short Circuit Test
A nan short circuit test, armature terminals suna shorted ta hanyar uku ammeters, kamar yadda aka bincike a cikin bayanin:
Short Circuit Test (Continued)
Kadan ba a yi alternator, an yi field current zuwa zero, sannan an set ɗaya daga ammeters zuwa range wanda yake ƙarfin rated full-load current. Ana yi alternator a synchronous speed, tare da field current ƙarfin gradually in steps—kamar yadda ake yi a open-circuit test—tare da maimaita armature current a ɗaya increment. Field current tana ƙarfin har zuwa lokacin da armature current yake ƙarfin 150% of the rated value.
Har ɗaya step, field current If da average of three ammeter readings (armature current Ia) suna record. Ana kawo graph plotting Ia against If wanda ya ƙarfin Short Circuit Characteristic (S.C.C), wanda yana ƙarfin straight line, kamar yadda aka bincike a cikin bayanin.
Calculation of Synchronous Impedance
Don in taka shiga cikakken synchronous impedance Zs, first overlay the Open-Circuit Characteristic (OCC) and Short-Circuit Characteristic (SCC) on the same graph. Next, determine the short-circuit current ISC corresponding to the rated alternator voltage per phase Erated. The synchronous impedance is then derived as the ratio of the open-circuit voltage EOC (at the field current that yields Erated to the corresponding short-circuit current ISC, expressed as s = EOC / ISC.
The graph is shown below:
From the above figure, consider the field current If = OA, which produces the rated alternator voltage per phase. Corresponding to this field current, the open-circuit voltage is represented by AB.
Assumptions of the Synchronous Impedance Method
The synchronous impedance method assumes that synchronous impedance (determined from the ratio of open-circuit voltage to short-circuit current via OCC and SCC curves) remains constant when these characteristics are linear. It further assumes that flux under test conditions matches that under load, though this introduces error as short-circuited armature current lags voltage by ~90°, causing predominantly demagnetizing armature reaction. Armature reaction effects are modeled as a voltage drop proportional to armature current, combined with reactance voltage drop, with magnetic reluctance assumed constant (valid for cylindrical rotors due to uniform air gaps). At low excitations, is constant (linear/unsaturated impedance), but saturation reduces beyond the OCC's linear region (saturated impedance). This method yields higher voltage regulation than actual loading, earning it the term pessimistic method.
