Download 1-TRANSFER FUNDTION OF DC GENERATOR

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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)
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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)
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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.
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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
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CALCULATIONS
Field resistance Rf =1.2* Rf avg Ω
Impedance Zf
=
KΩ
Xf  [Z f  R f ]1 2
2
2
Xf
2f
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