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Unformatted text preview: kVA ≡ kW kVAR ≡ kW Srated := 75⋅ kVA Problem 5-14: Vprated := 208⋅ V Vsrated := 480⋅ volt Rp := 0.0132⋅ Ω Xp := 0.0189⋅ Ω Rfels := 101.5⋅ Ω Rs := 0.07⋅ Ω Xs := 0.10⋅ Ω Xmls := 37.15⋅ Ω Fp := 0.8 leading Solution
First find the turns ratio a := Vprated
Vsrated a = 0.433 The equivalent winding impedance referred to the high side (secondary) is: Rp + j⋅ Xp + ( Rs + j⋅ Xs)
a Zeqs := Zeqs = 0.14 + 0.201j Ω
θ := acos ( Fp ) θ = 36.87 deg The transformer delivers rated voltage so: angle is positive since leading
Vfl := Vsrated
Vfl = 480 V At full load, the current is: Is := Srated j ⋅ θ
Vfl Is = 125 + 93.75j A The primary voltage, referred to the secondary side at full load, is calculated by adding
the drop across the equivalent impedance (caused by the load current) to the secondary
full load voltage
Vreferredp := Vfl + Is⋅ Zeqs Vreferredp = 478.726 + 38.234j V Vreferredp = 480.25 V which is the no load voltage Vnl := Vreferredp VR := Vnl − Vfl
Vfl VR = 0.052 % ...
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- Spring '12