Kathmandu University
Department of Electrical and Electronics Engineering
BASIC ELECTRICAL LAB (ENGG 103)
EXPERIMENT 4:- MEASUREMENT OF REACTANCE OFFERED BY
CAPACITOR IN DIFFERENT FREQUENCY FOR R-C CIRCUIT
Objectives: i) Familiarization with function of function generator and oscilloscope.
ii) Measurement of Reactance and power factor of R-C Circuit
Materials and Equipments Required:
1.
2.
3.
4.
5.
6.
7.
Digital multi-meter
Function Generator
Breadboard
Probes [3]
Resistors ( 1K)
Capacitor (1µF)
Wattmeter
Theory:
Reactance is opposition of a circuit element's to a change of current, caused by the buildup of
electric or magnetic fields in the element. In vector analysis, reactance is the imaginary
part of electrical impedance, used to compute amplitude and phase changes of sinusoidal alternating
current going through the circuit element. It is denoted by the symbol . The SI unit of reactance is
the ohm. Both reactance and resistance are required to calculate the impedance , although in
some circuits one of these may dominate.
Both the magnitude
and the phase
of the impedance depend on both the resistance and the
reactance.
A positive reactance implies that the circuit is inductive, where phase of the voltage leads the phase
of the current; while a negative reactance implies that the circuit is capacitive, where phase of the
voltage lags the phase of the current. A reactance of zero implies the current and voltage are in
phase and conversely if the reactance is non-zero then there is a phase difference between the
voltage and current
Capacitive reactance
Inductive reactance
is inversely proportional to the signal frequency
is proportional to the signal frequency
and the capacitance
.
and the inductance .
The power factor of an AC electric power system is defined as the ratio of the real power flowing to
the load to the apparent power and is a dimensionless number between 0 and 1.
Circuit Diagram:
1 KΩ
B
A
2*Sin(2πft + φ)
1 μF
C
Fig 1. Circuit connection
Procedure:
1. Connect the circuit as shown in the circuit diagram of fig. 1 on the breadboard.
2. Set the value of frequency in 50 Hz in the function generator.
3. Adjust the ground of channel 1 and 2 of Cathode Ray Oscilloscope and then set it
into DC mode.
4. Observe the waveform of voltage across capacitor and compare it with input
waveform. Measure the phase difference between input and output waveform.
5. Observe the waveform across resistor. Remember that the phase of current is same
as that of voltage across resistor.
6. Set the Multimeter into AC mode and measure input voltage and voltage across
capacitor and Resistor and verify the ohm’s law in AC circuit. This value gives RMS
value of sinusoidal AC.
7. Repeat experiment for different frequency with step size of 100.
8. Observe the waveform across capacitor for triangular and square input.
9. Set the frequency to 500 Hz and repeat experiment for the capacitor value 470nF,
4.7µF, 10 µF, 47 µF and 100 µF.
Observations:
i)
Draw the input voltage waveform and waveform of voltage across capacitor 1µF for
(Sinusoidal) wave input of 1 KHz with peak to peak voltage 6V.
T/4
T/2
3T/4
T
5T/4
3T/2
7T/4
2T
Figure 1 ii)
Draw the input voltage waveform and waveform of voltage across capacitor 1µF for
(Square) wave input of 1 KHz with peak to peak voltage 6V.
T/4
T/2
3T/4
T
5T/4
3T/2
7T/4
2T
Figure 2 iii)
Draw the input voltage waveform and waveform of voltage across capacitor 1µF for
(Triangular) wave input of 1 KHz with peak to peak voltage 6V.
T/4
T/2
3T/4
T
5T/4
3T/2
7T/4
2T
Figure 3 Input peak voltage (V) = 3V
Input RMS voltage= …………….
Frequency
in Hz
Voltage
across
Resistor
(VR)
Voltage
across
Capacitor
(Vc)
Reactance of Capacitor
(XC)
)
XC=(VC*R/(√
Reactance
from
calculation
(XC=1/(2*π*f*C))
Power factor from
measurement
Cos(
phase
difference
between resistor
and input)
Power factor from
Calc
Cos (Tan-1( /
Voltage
across
Resistor
(VR)
Voltage
across
Capacitor
(Vc)
Reactance of Capacitor
(XC)
)
XC=(VC*R/(√
Reactance
from
calculation
(XC=1/(2*π*f*C))
Power factor from
measurement
Cos(
phase
difference
between resistor
and input)
Power factor from
Calc
Cos (Tan-1( /
50
100
300
500
700
1000
5000
Capacitor
value
C
1 µF
10 µF
100 µF
Conclusions and Inference:
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Exercises:
1) Sketch and interpret the graph of Output voltage across of capacitor versus input frequency.
2) Sketch and interpret the graph of Output voltage across of capacitor versus capacitor value.
3) Verify the Ohm’s law for AC circuit, i.e., VTotal =
4) What happens when the output is taken across resistor?
5) Draw the phasor diagram for f=1KHz and show that V=VR+VC (Bold quantities show
phasor quantity)
Instructor’s Signature
Performance Date
Submission Date