3. Power factor measurement in R-L and R

FR. CONCEICAO RODRIGUES COLLEGE OF ENGINEERING
Department of Electronics Engineering
3. Power factor measurement in R-L and R-C circuits
1.
Course, Subject & Experiment Details
Academic Year
2014 – 2015
Course &
Semester
F.E. (ALL) – Sem. I
Experiment No.
Experiment Type
3
Hardware
Performance
2.
Estimated
02 Hours
Time
Subject Name Basic Electrical and
Electronics Engineering
Laboratory
Chapter Title AC Circuit
Subject Code FEC105
Aim & Objective of Experiment
To study R-L and R-C circuits and to obtain the power factor of the circuits by observing the
voltage waveforms.
3.
Expected Outcome of Experiment
1) Students understand the concepts of phase difference and power factor in a.c. circuits.
2) Students learn to use CRO to measure amplitude, time period, phase differences of
waveforms.
4.
Brief Theoretical Description
R-L series circuit:
In a circuit consisting of resistance and inductance connected in series across an a.c.voltage,
the supply voltage gets divided into two parts. The voltage across resistance (VR) is in phase
with the circuit current while the voltage across inductance (VL) leads the current by 90o. Supply
voltage V which is the vector sum of VR and VL leads the current by 0o < Ф < 90o. (figure 1)
VR = IR ( in phase with I)
VL = IXL ( leads current by 90O)
V = VR + jVL
The magnitude of supply voltage is
and the phase angle difference between voltage and current is
Basic Electrical and Electronics Laboratory- Experiment 3
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FR. CONCEICAO RODRIGUES COLLEGE OF ENGINEERING
Department of Electronics Engineering
In the practical circuit a choke coil is used to provide inductance. It has high value of
inductance and small resistance (RL). Therefore the coil voltage leads the current by an angle
close to but less than 90o.
The power factor of the circuit is cos ø which is an important factor that decides the power
consumption of the circuit.
Power= VIcos ø
The voltage and current waveforms can be observed as shown in the figure 1.
Figure 1. Voltage and current waveforms of R-L circuit
R-C series circuit:
In a circuit consisting of resistance and capacitance connected in series across an a.c.voltage,
the supply voltage gets divided across the two. The voltage across resistance (VR) is in phase
with the circuit current while the voltage across capacitance (Vc) lags the current by 90o. Supply
voltage V which is the vector sum of VR and Vc lags the current by 0o < Ф < 90o. (figure 2 )
VR
Vc
= IR is in phase with I
= IXc lags current by 90 degrees
Basic Electrical and Electronics Laboratory- Experiment 3
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FR. CONCEICAO RODRIGUES COLLEGE OF ENGINEERING
Department of Electronics Engineering
V = VR - j VC
The magnitude is
and the phase angle difference between current and voltage is
The voltage and current waveforms can be observed as shown in the figure 2.
Figure 2. Voltage and current waveforms of R-C circuit
Basic Electrical and Electronics Laboratory- Experiment 3
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FR. CONCEICAO RODRIGUES COLLEGE OF ENGINEERING
Department of Electronics Engineering
5.
Circuit Diagram & Experimental Setup
5.
R-L circuit:
R=
L=
RL=
R-C circuit:
Basic Electrical and Electronics Laboratory- Experiment 3
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FR. CONCEICAO RODRIGUES COLLEGE OF ENGINEERING
Department of Electronics Engineering
R=
6.
Apparatus and components required
1.
7.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
C=
Circuit board
2.
Transformer
3.
Multimeter
1 No.
4.
CRO
1 No.
5.
Resistors
6.
Capacitor
7.
Choke coil
8.
Connecting wires
230V/15V, 50 Hz
1 No.
100 Ω, 5 W
1 No.
220 Ω, 5 W
1 No.
500 Ω, 5 W
1 No
47 µF
1 No.
1 No.
Experimental Procedure
Connect the R-L circuit as shown in circuit diagram.
For R= 100Ω, observe the Vcoil, VR, V waveforms on CRO.
Measure the amplitude of the three waveforms
Measure phase angle difference between VR and Vcoil that is coil power factor angle Фcoil.
Also measure the phase angle difference between VR and V that gives circuit power factor
angle Ф (because VR is in phase with current).
Calculate the power factor (cos Ф) of coil and of the circuit from the observed waveforms.
Verify those with theoretical values.
Repeat the same procedure with different values of R.
.
Now connect R-C circuit as shown in circuit diagram.
Measure the three voltages VC, VR, V. Also measure phase angle difference between VR
and VC, VR and V(power factor angle Ф) to calculate the power factor (cos Ф).
Verify the observed power factor with the theoretical value for two different values of
resistance.
Draw the observed waveforms on graph paper clearly indicating different phase angles
measured.
Basic Electrical and Electronics Laboratory- Experiment 3
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FR. CONCEICAO RODRIGUES COLLEGE OF ENGINEERING
Department of Electronics Engineering
Observation Table
8.
R-L circuit:
Load
VRmax
Vcoil-max
V
(volts)
(volts)
(volts)
Фcoil
Ф
Coil Power
Factor
Circuit
Power
Factor
R=
(observed)
XL =
Rcoil=
(observed)
(theoretical)
(theoretical)
(observed)
(observed)
(theoretical)
(theoretical)
R=
XL =
Rcoil=
R-C circuit:
Load
VRmax
Vcmax
V
(volts)
(volts)
(volts)
Ф
Circuit Power
Factor
(observed)
Circuit Power
Factor
(theoretical)
R=
XC=
R=
XC=
Basic Electrical and Electronics Laboratory- Experiment 3
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FR. CONCEICAO RODRIGUES COLLEGE OF ENGINEERING
Department of Electronics Engineering
9.
Conclusions & Inferences
10.
Post Lab Questions
1. Draw to the scale phasor diagrams showing experimental values of the voltages.
2. Give the formulae, unit of three types of power in R-L-C circuits. Calculate the same for
the experimental circuits.
3. Explain the importance of power factor. Discus a power factor improvement method.
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Basic Electrical and Electronics Laboratory- Experiment 3
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FR. CONCEICAO RODRIGUES COLLEGE OF ENGINEERING
Department of Electronics Engineering
Basic Electrical and Electronics Laboratory- Experiment 3
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