Rêbeya Rezistansî Siner kêravetan
Rêbka Daraşîna Heyvî yên Hesteke, an jî Metoda EMF, çetkewtina armature reaction di navbera bi reactance imajînî ya hêmîngire derbasdar de. Ji bo hesabkirina regulation voltage pêşniyarihê, data ku hewce ne dike: resistance per phase, Curve Open-Circuit Characteristic (OCC) ku wekheviya nisbenda voltage bi current field da, û Curve Short-Circuit Characteristic (SCC) ku wekheviya nisbenda current bi current field da.
Ji bo generatorên sengavî yên taybetmend, heqeqe dike:
Ji bo hesabkirina synchronous impedance Zs, pêşniyarên parastnirin hatine kirin, û bêja Ea (EMF armature-induced) hatine cihaz kirin. Bi karîn Ea û V (terminal voltage), regulation voltage destape hate hesabkirin.
Pêşniyariya Synchronous Impedance
Synchronous impedance bi têstên sê yekemîn hate cihaz kirin:
Têsta DC Resistance
Di vê têstê de, alternator star-connected hatine qebûl kirin ji bilî field winding DC open-circuited, wêke li ser diagrama circuit dibe:
Têsta DC Resistance
DC resistance di her yek pair terminal de hatine pêşniyarkirin bi karîn ammeter-voltmeter method an Wheatstone’s bridge. Average ya têr lêkolîna resistance values Rt hatine hesabkirin, û per-phase DC resistance RDC hatine cihaz kirin bi divide kirdan Rt bi 2. Ji ber effect skin, ku effective AC resistance zêde kirin, per-phase AC resistance RAC hatine pelandin bi multiply kirdan RDC bi factor 1.20–1.75 (typical value: 1.25), bi taybetmendiyey makina.
Têsta Open Circuit
Bi têsta open-circuit, alternator îsereşîn di speed synchronous rated de, bi terminal load open (load disconnected) û field current initially set to zero. Diagrama circuit wêke li ser dibêje:
Têsta Open Circuit (Davam)
Pas set kirdana field current bi zero, hatine gradually increased in steps, ji bilî measurement terminal voltage Et at each increment. Field current typically raised until terminal voltage reaches 125% of the rated value. Graph plotted between open-circuit phase voltage Ep = Et/sqrt 3 and field current If, yielding O.C.C. curve. This curve mirrors standard magnetization curve, with its linear region extended to form air gap line.
O.C.C and air gap line illustrated in figure below:
Têsta Short Circuit
Di têsta short circuit de, armature terminals shorted via three ammeters, as illustrated in figure below:
Têsta Short Circuit (Davam)
Before starting alternator, field current reduced to zero, and each ammeter set to range exceeding rated full-load current. Alternator operated at synchronous speed, with field current increased in gradual steps—similar to open-circuit test—while measuring armature current at each increment. Field current adjusted until armature current reaches 150% of rated value.
For each step, field current If and average of three ammeter readings (armature current Ia) recorded. Graph plotting Ia against If yields S.C.C, which typically forms straight line, as shown in figure below.
Calculation of Synchronous Impedance
To calculate synchronous impedance Zs, first overlay OCC and SCC on same graph. Next, determine short-circuit current ISC corresponding to rated alternator voltage per phase Erated. Synchronous impedance then derived as ratio of open-circuit voltage EOC (at field current that yields Erated to corresponding short-circuit current ISC, expressed as s = EOC / ISC.
Graph shown below:
From above figure, consider field current If = OA, which produces rated alternator voltage per phase. Corresponding to this field current, open-circuit voltage represented by AB.
Assumptions of Synchronous Impedance Method
Synchronous impedance method assumes synchronous impedance (determined from ratio of open-circuit voltage to short-circuit current via OCC and SCC curves) remains constant when these characteristics are linear. It further assumes 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 modeled as 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 OCC's linear region (saturated impedance). This method yields higher voltage regulation than actual loading, earning it term pessimistic method.
