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1-1 1. TRANSFER FUNDTION OF DC GENERATOR AIM: To determine the transfer function of dc generator. NAME PLATE DETAILS: Rated voltage = Rated current = Speed = Output Power = APPARATUS: Dc motor –generator set, motor starter, field rheostat, rheostat as potential differentiator for excitation of generator, ammeters –MC&MI, voltmeters–MC&MI, tacho generator, variac, connecting wires. CIRCUIT DIAGRAM: MC- Mechanical coupling Fig(i) 1-2 Fig(ii) Fig (iii) THEORY: The transfer function of a dc generator is derived as follows Vg(t) KgIf(t) dIf(t) dt generated voltage generator constant field current Vf(t) RfIf(t) Lf Vg(t) Kg If(t) 1-3 Applying L.T with initial conditions as zero Vg(s) KgIf(s) Vf(s) RfIf(s) Lf sIf(s) Vf(s) [Rf sLf]If(s) Vf(s) If(s) [Rf sLf] Kg=generated EMF constant Block diagram representation for the above two equations Vf(s) 1 [Rf+sLf ] If(s) Kg Vg(s) Kg [Rf sLf] In this equation, the values of Kg, Rf & Lf have to be obtained. Transfer Function PRECAUTIONS: 1. The motor should run at rated speed while conducting OCC test. 2. During the conduct of field impedance and resistance drop tests only the field windings should be energized. PROCEDURE: 1. Connect the circuit as per the circuit diagram shown in fig (i).Ensure that the motor field rheostat and the P.D in the generator field circuit should be minimum. 2. Start the motor with the help of a starter and adjust the speed of the motor to the rated value by varying the field rheostat. 3. Vary the P.D in the generator circuit in small steps and note down the field current of the generator (If) and generator emf (Eg) and tabulate it in table (i). 4. To determine Kg ,magnetization characteristics Eg Vs If of a separately excited dc generator has to be plotted as shown in fig(iν) and use the straight line position to determine Kg= Eg / If. 1-4 5. The field resistance of the generator Rf is determined by the field resistance drop test as shown in figure in fig(iii)Vary the P.D in steps ,note down the Ammeter and voltmeter values and tabulate it in table(ii). 6. To determine inductance Lf of a generator first the impedance Zf has to determined by field impedance drop test as shown in fig (ii). 7. Vary the variac in fig (ii). Note down the ammeter and voltmeter values and tabulate it in table (iii). OCC Characteristic: Field Current If(A) Generated Voltage Eg(V) Table (i) Field resistance drop test: Sl.No Vf (V) Rf=Vf/If (Ω) If (A) Average Rf = Ω Table (ii) Field Impedance drop test: Sl.No Vf (V) If (mA) Zf=Vf/If (Ω) Average Zf = Table (iii) KΩ 1-5 CALCULATIONS Field resistance Rf =1.2* Rf avg Ω Impedance Zf = KΩ Xf [Z f R f ]1 2 2 2 Xf 2f From graph Eg Kg If Lf Transfer Function Vg ( s ) Kg T.F Vf (s) [ R f sL f ] MODEL GRAPH: Fig (iν) RESULT: The transfer function of dc generator was determined by conducting OCC test on the given dc generator and the T.F of the system is found to be T.F Kg R L f f