Eletrical_Measurements

CHRISTU JYOTHI INSTITUTE OF TECHNOLOGY AND SCIENCES
(Approved by AICTE, New Delhi & Affiliated to JNTU, Hyderabad)
ACADEMIC YEAR 2013-2014
ELECTRICAL MEASUREMENTS LAB
IV B.Tech I-Sem
Prepared by
Y.Vijay Jawahar Paul
Assistant Professor
for
IV B.Tech EEE
DEPARTMENT OF ELECTRICAL ELECTRONICS ENGINEERING
1
PREFACE
This Laboratory book in Electrical Measurements has been revised in order to be up to date with
Curriculum changes, laboratory equipment upgrading and the latest circuit simulation.
Every effort has been made to correct all the known errors, but nobody is perfect,
if you find any additional errors or anything else you think is an error, Please contact the
HOD/EEE at [email protected]
The Authors thanked all the staff members from the department for their valuable
Suggestion and contribution
The Authors
Department of EEE
2
INDEX
Sl. No
Laboratory Practice Safety Rules
4
List of experiments as per university
6
List of experiments to be conducted for this semester
7
Cycle indicate schedule and the batch size
8
Guidelines For Laboratory Notebook
11
Experiment Name
Page No
1.
Calibration and Testing of single phase energy Meter.
13
2.
Measurement of 3phase reactive power with single wattmeter.
16
3.
Schering bridge & Anderson bridge.
20
4.
Measurement of 3phase power with single wattmeter and 2
No‟s of C.T.
26
5.
30
7.
Measurement of parameters of a choke coil using 3voltmeter
and 3 ammeter methods.
Kelvin‟s double Bridge-Measurement of resistanceDetermination of Tolerance.
Calibration of dynamometer power factor meter.
8.
Resistance strain gauge-strain measurements and Calibration.
42
9.
LVDT and capacitance pickup- characteristics and
Calibration.
Calibration LPF wattmeter- by Phantom testing.
45
6.
10.
36
39
48
Additional Experiments
11.
12.
Crompton D.C Potentiometer- Calibration of PMMC ammeter
and PMMC voltmeter.
Measurement of % ratio error and phase angle of given C.T.
by comparison.
3
51
56
SAFETY RULES
1.SAFETY is of paramount importance in the Electrical Engineering Laboratories.
2. Electricity NEVER EXECUSES careless persons. So, exercise enough care and attention in
handling electrical equipment and follow safety practices in the laboratory. (Electricity is a good
servant but a bad master).
3. Avoid direct contact with any voltage source and power line voltages. (Otherwise, any such
contact may subject you to electrical shock)
4.Wear rubber-soled shoes. (To insulate you from earth so that even if you accidentally contact a
live point, current will not flow through your body to earth and hence you will be protected from
electrical shock)
5. Wear laboratory-coat and avoid loose clothing. (Loose clothing may get caught on an
equipment/instrument and this may lead to an accident particularly if the equipment happens to
be a rotating machine)
6. Girl students should have their hair tucked under their coat or have it in a knot.
7. Do not wear any metallic rings, bangles, bracelets, wristwatches and neck chains. (When you
move your hand/body, such conducting items may create a short circuit or may touch a live point
and thereby subject you to electrical shock)
8. Be certain that your hands are dry and that you are not standing on wet floor. (Wet parts of the
body reduce the contact resistance thereby increasing the severity of the shock)
9. Ensure that the power is OFF before you start connecting up the circuit.(Otherwise you will be
touching the live parts in the circuit)
10. Get your circuit diagram approved by the staff member and connect up the circuit strictly as
per the approved circuit diagram.
11. Check power chords for any sign of damage and be certain that the chords use safety plugs
and do not defeat the safety feature of these plugs by using ungrounded plugs.
12. When using connection leads, check for any insulation damage in the leads and avoid such
defective leads.
13. Do not defeat any safety devices such as fuse or circuit breaker by shorting across it. Safety
devices protect YOU and your equipment.
14. Switch on the power to your circuit and equipment only after getting them checked up and
approved by the staff member.
4
15. Take the measurement with one hand in your pocket. (To avoid shock in case you
accidentally touch two points at different potentials with your two hands)
16. Do not make any change in the connection without the approval of the staff member.
17. In case you notice any abnormal condition in your circuit ( like insulation heating up, resistor
heating up etc ), switch off the power to your circuit immediately and inform the staff member.
18. Keep hot soldering iron in the holder when not in use.
19. After completing the experiment show your readings to the staff member and switch off the
power to your circuit after getting approval from the staff member.
20. Some students have been found to damage meters by mishandling in the following ways:
i.
ii.
Keeping unnecessary material like books, lab records, unused meters etc. causing
meters to fall down the table.
Putting pressure on the meter (specially glass) while making connections or while
talking or listening somebody.
STUDENTS ARE STRICTLY WARNED THAT FULL COST OF THE METER
WILL BE RECOVERED FROM THE INDIVIDUAL WHO HAS DAMAGED IT
IN SUCH A MANNER.
Copy these rules in your Lab Record. Observe these yourself
and help your friends to observe...
I have read and understand these rules and procedures. I agree to abide by
these rules and procedures at all times while using these facilities. I understand that
failure to follow these rules and procedures will result in my immediate dismissal
from the laboratory and additional disciplinary action may be taken.
Signature
Date
Lab
5
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY
IV Year B.Tech EEE I Sem
Academic year 2013-2014
L T/P/D C
0 -/3/- 2
(57604) ELECTRICAL MEASUREMENTS LAB
The following experiments are required to be conducted as compulsory experiments:
1. Calibration and Testing of single phase energy Meter.
2. Calibration of dynamometer power factor meter.
3. Crompton D.C Potentiometer- Calibration of PMMC ammeter and PMMC voltmeter.
4. Kelvin‟s double Bridge-Measurement of resistance-Determination of Tolerance.
5. Dielectric oil testing using H.T. testing Kit.
6. Schering bridge & Anderson bridge.
7. Measurement of 3phase reactive power with single-phase wattmeter.
8. Measurement of parameters of a choke coil using 3voltmeter and3 ammeter methods.
In addition to the above experiments, at least any two of the experiments from the
following list are required to be conducted.
9. Calibration LPF wattmeter- by Phantom testing.
10. Measurement of 3phase power with single wattmeter and 2 No‟s of C.T.
11. C.T. testing using mutual Inductor- Measurement of % ratio error and phase angle of given
C.T.by null method.
12. P.T. testing by comparison-V.G.as Null detector-Measurement of % ratio error and phase angle
of the given P.T.
13. LVDT and capacitance pickup- characteristics and Calibration.
14. Resistance strain gauge-strain measurements and Calibration.
15. Transformer turns ratio measurement using A.C. bridge.
16. Measurement of % ratio error and phase angle of given C.T. by comparison.
6
Experiments Conducted by the Department:1. Calibration and Testing of single phase energy Meter.
2. Measurement of 3phase reactive power with single wattmeter.
3. Schering bridge & Anderson bridge.
4. Measurement of 3phase power with single wattmeter and 2 No‟s of C.T.
5. Measurement of parameters of a choke coil using 3voltmeter and3 ammeter methods.
6. Kelvin‟s double Bridge-Measurement of resistance-Determination of Tolerance.
7. Calibration of dynamometer power factor meter.
8. Resistance strain gauge-strain measurements and Calibration.
9. LVDT and capacitance pickup- characteristics and Calibration.
10. Calibration LPF wattmeter- by Phantom testing.
Additional Experiments
11. Crompton D.C Potentiometer- Calibration of PMMC ammeter and PMMC voltmeter.
12. Measurement of % ratio error and phase angle of given C.T. by comparison.
7
GUIDELINES FOR LABORATORY NOTEBOOK
The laboratory notebook is a record of all work pertaining to the experiment. This
record should be sufficiently complete so that you or anyone else of similar
technical background can duplicate the experiment and data by simply following
your laboratory notebook. Record everything directly into the notebook during the
experiment. Do not use scratch paper for recording data. Do not trust your memory
to fill in the details at a later time.
Organization in your notebook is important. Descriptive headings should be used to
separate and identify the various parts of the experiment. Record data in
chronological order. A neat, organized and complete record of an experiment is just
as important as the experimental work.
1. Heading:
The experiment identification (number) should be at the top of each page.Your name
and date should be at the top of the first page of each day's experimental work.
2. Object:
A brief but complete statement of what you intend to find out or verify in the
experiment should be at the beginning of each experiment.
3. Diagram:
A circuit diagram should be drawn and labeled so that the actual experiment
circuitry could be easily duplicated at any time in the future. Be especially
careful to record all circuit changes made during the experiment.
4. Equipment List:
List those items of equipment which have a direct effect on the accuracy of the
data. It may be necessary later to locate specific items of equipment for rechecks if
discrepancies develop in the results.
8
5. Procedure:
In general, lengthy explanations of procedures are unnecessary. Be brief. Short
commentaries alongside the corresponding data may be used. Keep in mind the
fact that the experiment must be reproducible from the information given in your
notebook.
6. Data:
Think carefully about what data is required and prepare suitable
data tables.
Record instrument readings directly.
Do not use calculated results
in place of direct data; however, calculated results may be recorded in the same table
with the direct data.
Data tables should be clearly identified and each data column
labeled and headed by the proper units of measure.
7. Calculations:
Not always necessary but equations and sample calculations are often given to
illustrate the treatment of the experimental data in obtaining the results.
8. Graphs:
Graphs are used to present large amounts of data in a concise visual form. Data to
be presented in graphical form should be plotted in the laboratory so that any
questionable data points can be checked while the experiment is still set up.
grid lines in the notebook can be used for most graphs.
The
If special graph paper is
required, affix the graph permanently into the notebook. Give all graphs a short
descriptive title. Label and scale the axes. Use units of measure. Label each
curve if more than one on a graph.
9. Results:
The results should be presented in a form which makes the interpretation easy.
Large amounts of numerical results are generally presented in graphical form.
Tables are generally used for small amounts of results.
Theoretical and
experimental results should be on the same graph or arrange in the same table in a
way for easy correlation of these results.
10. Conclusion:
This is your interpretation of the results of the experiment as an engineer. Be brief
and specific. Give reasons for important discrepancies.
9
1. CALIBRATION AND TESTING OF SINGLE PHASE ENERGY METER
Aim:
To calibrate the given Energy Meter by Direct Testing.
Apparatus Required:
Sl.
No.
Name of the Equipment
01
Energy Meter
02
Auto Transformer
03
U.P.F. Wattmeter
04
05
06
07
08
Voltmeter
Ammeter
Resistive Load
Stop Watch
Connecting Wires
Range
Type
Quantity
240V,(5-20)A,
50 Hz
230 / (0-270)V,
(0-10)A
(150/300/600)V,
(0-10)A
(0-300)V
(0-10)A
230V, (0-10)A
1- Φ, MI
01
1- Φ
01
------
Dynamometer
Type
MI
MI
1- Φ
Digital
------
01
01
01
01
01
As required
Theory:
Induction type energy meters are universally used for measurement of energy in domestic
and industrial ac circuits. Induction type of meters possesses lower friction and higher
torque/weight ratio. Also induction type meters are inexpensive and accurate and retain their
accuracy over a wide range of loads and temperature conditions.
There are four main parts.
1. Driving System.
2. Moving System.
3. Braking System.
4. Registering System.
The supply voltage is applied across the pressure coil. The pressure coil winding is highly
inductive as it has very large number of turns and the reluctance of its magnetic circuit is very
small owing to presence of air gaps of very small length. Thus the current Ip through the pressure
coil is proportional to the supply voltage and lag it by a few degrees less the 90 degrees. This is
because the winding has a small resistance and there are iron losses in the magnetic circuit.
Current input produces a flux. This flux divides itself into two pars фg and фp. The major portion
фg flows across the side gaps as reluctance of this path is small. The reluctance to the path of flux
фp is large and hence its magnitude is small. This flux фp goes across aluminum disc and hence
is responsible for production of driving torque.
Flux фp is in phase with current Ip and is proportional to it. Therefore flux фp is
proportional to voltage V and lags it by an angle a few degrees less than 90 degrees since flux фp
is alternating in nature, it induces an eddy emf in the disc which in turn produces eddy current,
10
The load current I flows through he current coil and produces a flux фg. This flux is
proportional to the load current and is in phase with it. This flux produces eddy current is in disc.
Now the eddy current is interacts with flux фp to produce a torques and eddy current is interacts
with фg to produce another torque. Here two torques are in the opposite direction and the net
torque is the different of these.
Circuit Diagram:
Procedure:
1. Connections are made as per the circuit diagram.
2. Set Auto Transformer at zero voltage position before switching on the supply.
3. Gradually increase the voltage using the auto-transformer till the voltmeter reads
230V.
4. Now apply the Load at certain value (i.e. 2A )
5. Time taken for 25 rev. of the disc of the energy meter in the forward direction is
noted
6. Record the Voltmeter, Ammeter, & Wattmeters are noted.
7. The experiment is repeated for different values of current
(i.e. 4A, 6A,8A) at constant voltage.
8. After noting the values slowly decrease the auto transformer till
Voltmeter comes to zero voltage position and switch off the supply.
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their ratings
3. Observe the ammeter reading. Apply the voltage slowly so that the current is
Within the limited range of ammeters, wattmeter and energy meter.
4. If the energy meter rotate in reverse direction, change either its current coil
terminals or pressure coil terminal or pressure coil terminal but not both
11
Theoretical Calculations:
CALCULATION:
E1=wt/3600 wh(actual or practical)
for E2
k=900 rev/kwh
900 rev 1000wh
0.9 rev  1wh
19 rev  19/0.9 = 11.11 wh
Energy consumed measured by meter for 10 rev is e2=11.1 (theoretical).
Observation Table:
Sl
No.
Voltage
(V)
(Volts)
Load
Current
(IL)
(Amps)
Wattmeter
Reading
W (Watts)
Time
for
10 rev
T(Sec)
Energy
meter
Reading
(E1)
Actual
Energy
(E2) = W X T
% Error =
E1-E2/E2 x 100
Result:
Viva Voce Questions
1. What is a phantom Load?
2. What is creeping?
3. What is Braking?
4. Define meter constant?
5. What is Friction? What are the different types of Friction, explain it?
6. Write short notes on driving system, Moving system, Braking systems, registering systems?
7. What are the errors occurred by the driving system?
8. How do you prevent the creeping?
9. What are the errors occurs in the energy meter?
10. What is the principle of 1-Phase energy meter?
12
2. MEASUREMENT OF 3- PHASE REACTIVE POWER WITH SINGLE
WATTMETER
Aim:
To measure the total reactive power of a three phase balanced load using single
Wattmeter method
Apparatus Required:
Sl.
No.
01
Inductive Load
02
Auto Transformer
03
U.P.F. Wattmeter
04
Name of the Equipment
Range
Type
Quantity
440V, 10A
3–Φ
01
415V/(0440)V
(0-20)A
(150/300/600)V
(0-5/10)A
3–Φ
01
Dynamometer
Type
01
Ammeter
(0-10)A
MI
01
05
Voltmeter
(0-600)V
MI
01
06
Connecting Wires
------
------
As required
Theory:
The reactive power in a circuit Q= VI Sinф. It is often convenient and even essential
that the reactive power be measured. For example, in load monitoring, such a measurement gives
the operator and load dispatcher information concerning the nature of the load. Also the reactive
power serves as a check on power factor measurements, since ratio of reactive and active power
is tanф = Q/P. Also the apparent power VI, which determines the line generator capacity, may be
determined from measurements of active and reactive power.
VI = √ P2 + Q2 In the case of balanced three phase circuits, it is simple to use a
single wattmeter to read the reactive power. The current coil of the wattmeters is connected in
one line and the pressure coil is connected across the other two lines. Wattmeter is connected in
one line and the pressure coil is connected across other two lines.
Current through the current coil = IR
Voltage across the pressure coil = VYB
Reading of wattmeter = W = VYB * IR Cos (90-Ө)
Total reactive power of the circuit Q = √ 3VI sinӨ
Phase angle, Ө = Sin-1 Q/(√ 3VI)
13
Circuit diagram:
Phasor Diagram:
14
Procedure:
1. Make the Connections as per circuit diagram.
2. Keep the 3-Phase Autotransformer is in minimum output position.
3. Switch on the supply and by slowly varying the autotransformer, rated value is applied to
motor.
4. Note down the readings of Ammeter, Voltmeter, Wattmeter‟s readings (Wr & Wa )
5. After noting the values slowly decrease the Auto Transformer till Volt meter comes to
zero voltage position, and switch of the supply
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their ratings
3. Readings of the meters must be taking without parallax error.
4. Ensure that setting of the Auto Transformer at zero output voltage
during starting.
Theoretical Calculations:
Ammeter reading = Iph =
Voltmeter reading = Vph =
Wattmeter reading (Wr) =
i.e. total reactive power =√ 3xWr
Total calculated reactive power = √ 3xVxIxSin ф
Total measured reactive power = √ 3xWr
Observation Table:
S.No
Voltage
V (Volts)
Line Current
IL (Amps)
Wr
(Watts)
I
Result:
15
Total
Reactive
power
(Watts)
Viva Voce Questions
1. How do you define reactive power
2. Distinguish between Leading VAR‟s and Lagging VAR‟s .
3. How many methods are represent to calculate 3-phase reactive power
4. What are the advantages of measuring 3-phase reactive power using single wattmeter
5. What is the disadvantage of this test.
6. In this method is applicable to Unbalance loads.
7. What is method to be calculate reactive power in Unbalanced loads.
8. Is it beneficial to have high reactive powers.
9. What are the different types of Wattmeter‟s are available.
10. What are the principles used in measurement of Reactive power
16
3. SCHERING BRIDGE & ANDERSON BRIDGE
a) SCHERING BRIDGE
Aim:
To find the capacitance of the unknown capacitor and it‟s dissipation factor.
Apparatus Required
Sl.
No.
1
Specification
Quantity
Schering bridge kit
01
2
LCR meter
01
3
Detector
01
4
Unknown Capacitance
01
5
Connecting wires
As per required
Theory:
The Schering bridge, one of the most important AC bridges, is used extensively for the
measurement of capacitors. It is particularly useful for measuring insulating properties. i.e., for
phase angles very nearly 90degree. The basic circuit arrangement is shown in figure. And
inspection of the circuit shows a strong resemblance to the comparison bridge. Notice that arm 1
now contains a parallel combination of a resistor and a capacitor, and the standard arm contains
only a capacitor. The standard capacitor is usually a high quality mica capacitor for insulation
measurements. A good quality mica capacitor has very low losses (no resistance) and therefore a
phase angle of approximately 90degrees.
The balance conditions require that the sum of the phase angles of arms 1 and 4 equals
the sum the phase angles of arms 2 and 3 since the standard capacitor is in arm 3, the sum of the
phase angles of arm 2 and arm 3 will be 0o + 90o = 90o In order to obtain 90o phase angle needed
for balance, the sum of the angles of arm 1 and 4 must equal 90o . Since in general measurement
work the unknown will
As can be seen from the circuit diagram from fig. The two variables chosen for
the balance adjustment are capacitor C, and resistor R2 There seems to be nothing unusual about
the balance equations or the choice of variable components, but consider for a moment how the
quality of a capacitor is defined.
17
Circuit Diagram:
Procedure:
1)
2)
3)
4)
5)
6)
Make the connections as shown in fig. using ac supply of frequency 1Khz&head
phones
Connect one unknown capacitor as shown .Set the capacitor dial C2 at zero
position &R also at zero position
Now introduce some resistance from decade resistance and dial R1 say 1000Ω and
adjust the decade resistance dial R2 to minimum the ground in the head phone
with alternate adjustment for decade resistance R1 &R2 say we can get the
minimum sound or no sound in the head phones
Note down the values of R1 R2&C2
Repeat the experiment with different values of unknown capacitor
C1=1………………0.1µF
C1=2………………0.2µF
Precautions:
1. There should not be any loose connections.
2. Handle the Bridge very carefully.
18
Theoretical Calculations
Unknown Capacitance Cx = C1R2/R1
Resistance Rx = C1R2/C
Dissipation factor Tan = Cx Rx
Observation Table:
s. no
C1(µF)
R1
R2
C=C1(R2/R1) µF
Result:
Viva Voce Questions
1. What is the other name of this Bridge
2. What are the types of capacitances which can be measured using. Schering Bridge.
3. What are the other bridges are used to find the capacitance.
4. What are the different types of D.C Bridges.
5. What are the different types of A.C Bridges.
6. Upto what value of capacitance this schering bridge is used?
19
(b) ANDERSON BRIDGE
Aim:
To measure the self-inductance of the given coil using Anderson‟s bridge.
Apparatus Required:
Sl.
No.
1
2
3
4
5
5
6
Specification
Anderson‟s Bridge kit
LCR meter
Detector
Unknown Inductance
Head Phones
Connecting wires
Bridge Oscillator
Quantity
01
01
01
01
01
As per required
01
Theory:
This bridge is a modification of Maxwell Wein Bridge. In this method, the unknown
inductance is measured in terms of a known capacitance and resistance as shown. Tha equation
at balance is,
L= Rc(Q+m+Qm/P), where „L‟is the unknown inductance which can be determined by
substituting the values of other quantities in the above balance equation. The advantage of the
Anderson bridge is it is capable of precise measurements of inductance over a wide range of
values from a few micro henrys to several henrys & is one of the commonest and the best bridge
methods.
This bridge is very common for measurement of self-inductance in terms of standard
capacitance and non-inductive resistances
An audio-frequency oscillator of, say. 1000 CPS and a variable output of 10 volts is used
as a source of supply.
A pair of headphones of good sensitivity is used as a detector in the bridge network.
20
Circuit Diagram:
Procedure:
1. Connect the circuit as shown in the diagram.
2. Initially when bridge is not balanced amplitude proportional to the potential
difference is indicated by detector.
3. Connect the oscillator and headphones at the places marked in the
circuit diagram..
4. Keeping m = o vary ‘s’ to get null deflection in the galvanometer,
5. Choose appropriate value of capacitance {say 0.01 Farads)
6. Further balance is obtained by varying m so as to give minimum
sound in the headphone.
Precautions:
1. There should not be any loose connections.
2. Handle the Bridge very carefully.
21
Theoretical Calculations
L= C{PQ+{R+S}m} henries
P= Q=R=1000 ohms
Observation Table:
s. no
C(µF)
S(Ω)
M(Ω)
L(mh)
Result:
Viva Voce Questions
1. Can Anderson Bridge be used for frequencies higher than normal frequencies.
2 .What is the other name of this Bridge.
3. This Bridge is A.C or D.C?
4. What are the other bridges to be find the unknown inductance.
5. What is the range of Maxwell Bridge.
6. What is the difference between Maxwell bridge & Anderson Bridge.
22
4. MEASUREMENT OF 3- ф POWER USING SINGLE WATTMETER
AND 2 NO’S OF CT’S
Aim:
Measurement of a 3-phase active power and also power factor using 2 CT‟s and one wattmeter.
Apparatus Required:
Sl.
No.
Name of the Equipment
Range
Type
Quantity
01
Resistive Load
440V, 10A
3-Φ
01
02
Current Transformer
5/1A, 660V
50Hz
01
03
U.P.F. Wattmeter
Dynamometer
Type
01
04
Auto Transformer
3-Φ
01
05
Ammeter
(0-5)A
MI
02
06
Voltmeter
(0-600)V
MI
01
07
Connecting Wires
------
------
As required
(150/300/600)V
(0-5/10)A
415V/(0-440)V
(0-20)A
CIRCUIT DIAGRAM:
23
Theory:
In this method the power absorbed in a 3-φ balanced circuit is measured using a single wattmeter
in conjunction with 2 CT's. Usually 2-Wattmeter method is used measure the 3-φ power for both
balanced and unbalanced load, but method like this requires only one wattmeter. The CT's used
for this method should be of 1:1 ratio.
The primaries are connected in series with the 2 phases. The secondaries are connected to the
current coil of wattmeter such that the difference in the two phase currents will flow through the
current coil. The wattmeter pressure coil is connected between the same two phases.
Any wattmeter measures the product of

Voltage across pressure coil

Current through the current coil
Cosine of phase angle between voltage and current is
φ.
24
The circuit is assumed to be connected in star, for delta connection also the procedure is valid and the
wattmeter reads directly total power absorbed.
Procedure:
1.
2.
3.
4.
5.
Give the connections as per the circuit diagram.
Apply 415 V, 3-φ, 50Hz supply closing the TPST switch.
Vary the load in suitable steps.
At each load note down the wattmeter readings.
Tabulate the results.
Precautions:
1. Loose connections are to be avoided.
2. Circuit connections should not be make while power is on.
3. Reading of the meters must be taking without parallax error..
4. Voltage is to be varied gradually till rated current flows.
5. Ensure that the setting of the variac is at zero output voltage during starting
Expected example tabulated results are as follows:
Observation Table:
Sl.
No.
V0ltage (volts)
Current (Amps)
25
Wattmeter
(watts)
W calculated
(1.732 VI
Cosφ)
Result:
Viva Voce Questions:
1. What is a C.T?
2. Explain the working principle of C.T?
3. What is a P.T?
4. Explain the working principle of P.T?
5. What is the difference between C.T & P.T?
6. Where we use the C.T‟s & P.T‟s
26
5. MEASUREMENT OF PARAMETERS OF A CHOKE COILS USING
3-VOLTMETER AND 3-AMMETER METHODS
Aim:
To calculate the resistance and inductance of the given choke coil by
using (a) 3 Ammeters (b) 3 Voltmeters methods
(b) Three Voltmeter method
Apparatus Required:
Sl.
No.
Name of the Equipment
01
Choke coil
02
Auto Transformer
03
04
05
06
Rheostat
Voltmeter
Ammeter
Connecting Wires
Range
0.41A, 40W,
230)V,
230/(0-270)V,
(0-10)A
50/100Ω, 5A
(0-300)V,(0-30)V
(0-5)A(0-1)A
------
Type
Quantity
50 Hz
01
1- Φ
01
1- Φ
MI
MI
------
01
02,01
02,01
As required
Theory:
The parameters of a choke coil ( L & R ) can be measured using 3 Voltmeters. Out of 3Voltmeter is connected across supply, second voltmeter across a series known resistance and
third is connected across choke coil as shown in figure. The phasor diagram of the circuit is
shown below.
From the phasor diagram,
V12 = V22 + V32 + 2 V2V3CosӨ
2 V2V3CosӨ = V12 -V22 -V32
V12 -V22 -V32
CosӨ = ------------------- = Power factor of the load
2 V2V3
And also the current flowing through the choke coil is
V2
I = ---- = or V2 = IR
R
By substituting V in the equation
2IR V3 CosӨ = V12 -V22 -V32
2RP = V12 -V22 -V32
P = V12 -V22 -V32
---------------= Power consumed by choke coil
2R
27
Circuit Diagram:
(Three voltmeter method)
Phasor Diagram:
Procedure:
3- Voltmeters method:
1. Connect as per the circuit diagram.
2. Initially the Auto Transformer should be in minimum output position.
3. By slowly varying the Auto Transformer the Voltmeter V1 is adjusted at different values
4. Note down the corresponding readings of V2 & V3
5. After noting the values slowly decrease the auto-transformer till the Voltmeter comes to
zero position, and Switch off the Supply.
28
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their ratings
3. Readings of the meters must be taking without parallax error.
4. Ensure that setting of the Auto Transformer at zero output voltage during starting.
Theoretical Calculations:
CosΦ=( V12-V22 -V3 2) / 2V2V3
I =V2/Rh
Z = V3/I
R =Z CosΦ
XL = √ (Z2 –R2 )
L =XL /2πf
Observation Table:
Sl.No. V1
V2
V3
cosΦ I
Z
R
XL
L
Average Inductance =
Average Resistance =
(b) Three Ammeter method
Theory:
The parameters of a choke coil (R & L) can also be measured using 3-Ammeters, out
of 3-Ammeters one is connected in series with the supply; second one is connected in series with
Standard resistance & third is in series with choke coil the phasor diagram of the circuit is shown
in the figure.
I12 = I22 + I32 + 2 I2I3CosӨ
2 I2I3CosӨ = I12 -I22 -I32
I12 -I22 -I32
CosӨ = ------------------- = Power factor of the load
2 I2I3
And also the voltage across the choke coil is
V2
29
I = ---- = or V2 = IR
R
By substituting V in the equation
I12 -I22 -I32
2I3 X CosӨ = ------------2 I1I2
2
2
2
(I1 -I2 - I3 )R
P = ------------------ = Power consumed by choke coil
2R
Circuit diagram:
Phasor Diagram:
30
Procedure:
1. Connections are made as per the circuit diagram
2. Initially the Auto Transformer should be in minimum output position.
3. By slowly varying the Auto Transformer, the Ammeter A1 is adjusted at different values
from 0 to 5A.
4. Note the corresponding readings of A2 & A3.
5. After noting the values slowly decrease the auto-transformer till the Voltmeter comes to
zero position, and Switch off the Supply.
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their ratings
3. Readings of the meters must be taking without parallax error.
4. Ensure that setting of the Auto Transformer at zero output voltage During starting.
Theoretical Calculations:
CosΦ=(I12-I22 -I3 2) / 2I2I3
V =I2Rh
Z = V/I3
R =Z CosΦ
XL = √ (Z2 –R2 )
L =XL /2πf
Observation Table:
Sl.No. I1
I2
I3
cosΦ V
Z
Average Resistance:
Average Inductance:
Result:
31
R
XL
L
Viva Voce Questions
1. What are the disadvantages of 3-Voltmeter & 3- Ammeter Method?
Parameters of choke coil.
2. What are main parts of a moving coil instrument?
3. What is known as M.I instrument?
4. What are the types M.I type instruments?
5. Which meter, the ammeter or voltmeter has high resistance?
6. Why a shunt is provided with an ammeter
7 . Name the device used for extension of voltmeter range.
8. If you connect the voltmeter in series with the load what will happen?
9. Can you use the M.C.type meter on A.C?
10. If an ammeter is connected across the A.C line what will happen?
32
In measuring the
6. KELVIN’S DOUBLE BRIDGE
Aim;
To measure the low value of unknown resistance and resistance of connecting leads using
a Kelvin‟s double bridge.
Apparatus Required:
Sl.
No.
01
02
03
04
Name of the Equipment
Portable Kelvin‟s double bridge Kit
Dry Cells -- (1.5Volts)
Unknown Resistance
Connecting wires
Quantity
01
02
01
As required
Theory:
This is a portable bridge with built in taut suspension galvanometer and a 1.5 Volts dry
battery ( 3 cells of 1.5 V each in parallels ) This bridge is useful for the measurement of low
resistance.
Main dial – There are 10 coils of 0.1 ohm each.
Slide wire:- 100 divisions of slide wire are equal to 0.1 ohm, Each main division is to
0.001 ohm. Each sub – division is to 0.0005 ohm, The reading to the left of zero is to be
subtracted from main dial reading and that to the right of zero is to be added to main dial reading.
Rang switch:- A range multiplier switch furnishers 5 range of X100, X10, x1, x0.1,x0.01.
The value of unknown resistance is given by sum of main dial and slide wire reading multiplied
by range used.
Method for Measurement of low resistance:The methods for measurement of low resistance are:1. Ammeter Voltmeter method.
2. Kelvin‟s Double bridge method.
3. Potentiometer method.
4. Ducter.
Kelvin’s Double Bridge
This bridge is a modification of the Wheatstone bridge and provides greatly increased
accuracy in measurement of low value resistance an understanding of the Kelvin‟s bridge
arrangement may be obtained by a study of the difficulties that arise in a Wheatstone bridge on
accoure of the leads and the contact resistances while measuring low valued resistors. Two actual
resistance units of correct ratio be connected between points m and n in the Galvanometer be
connected to the function of the resistors. This is the actual Kelvin bridge arrangement. The
Kelvin double bridge incorporates the idea of a second set of ratio arms-hence the name double
bridge and the use of four terminals resistors for the low resistance arms. The first of ratio arms.
P and Q is used to connect the galvanometer to a point C at the appropriate potential between
33
points M and N to eliminate the effect of connecting lead of resistance R between the known
resistance, R and the standard resistance. S as shown.
Circuit Diagram:
Procedure;
1. The connections are made as shown in fig.
2. Across the terminals X meant for the unknown resistance,
Connected whose shunt resistance can be measured
3. The ratio (P/Q) is adjusted to a particular value.
4. For the ratio, balancing resistance is varied until Galvanometer
Shows null deflection.
5. The balance is similarly obtained for different ratios of (P/Q)
6. The resistance since it includes the resistance of the leads.
7. The lead resistance is measured by shorting the leads.
8. To obtain ammeter shunt resistance, the resistance of the leads is
subtracted from the total resistance.
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their
3. Handle the Bridge very carefully
34
Ratings
Theoretical Calculations;
R= ( P/Q) x S
Where R= Unknown Resistance
P= Variable resistance
Q= Variable resistance
S= Standard resistance
Observation Table;
Resistance of the given connecting wire:
S.No
multiplier(m)
R ( resistance of
main dial)
r (resistance of
slide wire)
Unknown
resistance
X=M(R+r)
Result:
Viva- Voce Questions
1. Why it is called as double bridge.
2. What is the ranges of resistance that can be measured using The bridge.
3. What are the ranges of resistances for low, medium and high resistances.
4. What is sensitivity of bridge.
5. What are the detectors used for DC bridge.
6. Why the low resistances are four terminal resistances.
7. Why the methods used for medium resistances are not Suitable for measurement of low
resistances.
8. What are the other instruments to measure the resistance?
9. What is a megger?
10. How megger is work?
35
7. CALIBRATION OF DYNAMOMETER TYPE 3-Ф POWER FACTOR METER
Aim:
To test and calibrate the given 3 phase electro dynamometer type power factor meter.
Apparatus Required:
Sl.
No.
Name of the Equipment
Range
Type
Dynamometer
Type
01
3-Φ
01
Dynamometer
Type
02
(0-600)V
MI
01
(0-10)A
MI
01
Resistive Load
440V, 10A
3-Φ
01
07
Inductive Load
440V, 10A
3–Φ
01
08
Capacitive Load
440V, 10 KVAR
3–Φ
01
09
Connecting Wires
------
------
As required
01
Power factor Meter
02
Auto Transformer
03
U.P.F. Wattmeter
04
Voltmeter
05
Ammeter
06
(300/600)V,
(5/10)A
415V/(0-440)V
(0-20)A
(300/600)V,
(0-5/10)A
Quantity
Theory:
It consists of a fixed coil which acts as the current coil. This coil is split up into two parts.
Therefore the magnetic field produced by this coil is proportional to the main current two
identical pressure coils A and B provided on a spindle constitute the moving system pressure coil
A has a non inductive resistance R connected in series with it and coil B has a supply highly
Inductive choke coil L connected in series with it. The two coils are connected across the voltage
of the circuit the values of R and L are so adjusted that the two coils carry the same value of
current at normal frequency i.e R= WL. The current through coil A is in phase with the circuit
voltage while that through coil B lags the voltage by an angle. Which is rear by equal to 90
degrees.
The angle between the phases of coil is also made equal. There is no controlling device
connections to moving coils are made through this silver or gold ligaments which are extremely
flexible and thus give a minimum control effect on the moving system.
36
Circuit Diagram:
Procedure;
1.
2.
3.
4.
Connect the circuit as per circuit diagram.
Check meter positions to be at zero.
Adjust the Auto Transformer at rated voltage ( i.e. 440V)
By adjusting the Resistive & Inductive load, adjust the reading of Ammeter to desired
value.
5. Note the readings of Wattmeters, and P.F meter.
6. Repeat 4 & 5 steps for various values of a power factor.
7. Calculate % Error.
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their ratings
3. Readings of the meters must be taking without parallax error.
4. Ensure that setting of the Auto Transformer at zero output voltage during starting.
Theoretical Calculations
% Error =
Calculated Value – Actual Reading
------------------------------------------------------Actual Reading
W1 & w2 are two readings of wattmeter then
37
X 100
Tan Ø= cos{tan-1[√3(w2-w1/w2+w1)]}
Observation Table;
Sl.
No.
Load
Voltage
Load
V{Volts Current
}
I L {Amps}
Wattmeter{
Watts)
W1
Wattmeter{
Watts)
W2
P.F
Theoretical
01
02
03
04
Result:
Viva Voce Questions
1. What are the different types of Power Factor Meters.
2. How is the operating force produced in power factor meter.
3. Why is the controlling force not present in a power factor meter?
4. What is Power factor angle?
5. What is the principle of electrodynamometer wattmeter?
6. What are the various types frequency meter?
7. What are the applications of Induction type instruments.
8. What type of coils are there in a dynamometer type instruments?
9. How many current coils are there in a single phase wattmeter?
10. How many potential coils are in a three phase wattmeter?
38
P.F Meter
Reading
%Error
8. MEASUREMENT OF STRAIN USING STRAIN GUAGE
Aim:
To measure the strain of strain gauge.
Apparatus Required:
Sl.
No.
01
Name of the Equipment
Dead weight
Range
Type
Quantity
100 gms
round
10
02
9 pin connector
----
-----
01
03
Contilever beam setup
----
-----
01
04
DMM
10 mv
IE123
01
Theory:
CIRCUIT DIAGRAM:
39
Procedure:
1. Check the connections made & switch on the instrument by toggle switch at the back at
the box. The display glows to indicate the instrument in ON.
2. Allow the instrument in ON position for 10 min for initial warm up.
3. Adjust zero potentiometer on the panel till the display reads 000.
4. Apply load on sensor using the loading arrangement provided in steps 100 gms upto 1kg.
5. The instrument display exact micro strain strained by contilever beam.
6. Note down the readings in tabular column. Percentage errors in reading, hysteresis &
accuracy of instrument can be calculated by with theoretical values.
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their ratings
3. Readings of the meters must be taking without parallax error.
Theoretical Calculations
Specimen calculation for contilever beam:Sth=6PL/BT2E
P
L
B
T
S
E
 load applied in kg(1kg)
 effective length of beam in cm(22cm)
 width of beam (2.8 cm)
 thickness of the beam (0.25 cm)
 micro strain
 Young‟s modulus (2*10-6 kg-cm2) or pascals
Observation Table;
S.NO
P (KG)
Sth µ strain
Sp µ strain
40
% error= (Sth -Sp/Sth)*100
Results:
Viva Voce Questions
1. What is Bridge used in strain gauge.
2. What is input &output of strain gauge.
3. Applications of strain gauge.
4.Define strain.
41
9. LINEAR VARIABLE DISPLACEMENT TRANSDUCER
Aim:
To measure the linear displacement.
Apparatus Required
Sl.
No.
1
2
3
4
5
Quantity
Specification
Power supply
01
Display
Frequency generator
Signal Conditioner
DMM
01
01
01
01
Theory:
Let's study the working of LVDT by splitting the cases into 3 based on the iron core position
inside the insulated former.
Case 1:On applying an external force which is the displacement, if the core reminds in the null
position itself without providing any movement then the voltage induced in both the secondary
windings are equal which results in net output is equal to zero
i.e., Esec1-Esec2=0
Case 2:When an external force is applied and if the steel iron core tends to move in the left hand
side direction then the emf voltage induced in the secondary coil is greater when compared to the
emf induced in the secondary coil 2.
Therefore the net output will be Esec1-Esec2
Case 3:When an external force is applied and if the steel iron core moves in the right hand side
direction then the emf induced in the secondary coil 2 is greater when compared to the emf
voltage induced in the secondary coil 1. Therefore the net output voltage will be Esec2-Esec1
42
Circuit Diagram:
Procedure:
1. Connect the power supply at rear panel to the 230 volts, 50 Hz supply. Switch on the
instrument by pressing down the toggle switch. The display glows to indicate the
instrument is on.
2. Allow the instrument in on position for 10 mins for initial warm-up.
3. Rotate the micrometer till it reads “20.0”
4. Adjust the CAL potentiometer at the front panel so that the display reads”10.0”
5. Rotate the core of micrometer till the micrometer reads “10.0” and adjust the zero
potentiometer till display reads “00.0”
6. Rotate back the micrometer core up to 20. 0 and adjust once again CAL potentiometer
till the display reads 10.0. Now the instrument is calibrated for +/- 10.0 mm range. As the
core of LVDT moves, the display reads the displacement in mm
7. Rotate the core of micrometer in steps of 1 or 2mm and tabulate the readings. the
micrometer will show the exact displacement given to the LVDT core. The display will
read the displacement sensed by the LVDT.
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their ratings
3. Readings of the meters must be taking without parallax error.
43
Theoretical Calculations
%error,D = (B-C)/C *100
Observation Table:
s. no
Actual meter reading
B(vm)
Indicator reading
(LVDT) C(mm)
% Error D= (B-C)*100/C
Result:
Viva Voce Questions
1. What is the i/p and o/p of lvdt?
2. What is the o/p of a secondary transformer in lvdt?
3. Lvdt convert ________ type of signal to ________ type of signal?
4. Applications of lvdt?
44
10. CALIBRATION OF LPF WATTMETER - BY PHANTOM TESTING
Aim:
To calibrate a given LPF Wattmeter by phantom testing Method.
Apparatus Required:
Sl.
No.
Name of the Equipment
Range
Type
Quantity
230/(0-270)V,
(0-5)A
(150/300/600)V
(2.5/5)A
1- Φ
02
Dynamometer
Type
01
01
Auto Transformer
02
L.P.F. Wattmeter
03
Voltmeter
(0-300)V
MI
01
04
Ammeter
(0-5)A
MI
01
05
Resistive Load
50 ohm, 5A
Wire wound
01
06
Connecting Wires
------
------
As required
Theory:
Measurement of power in circuits having low power factor by ordinary electro dynamometer
wattmeters is difficult and inaccurate because
1. The deflecting torque on the moving systems is small even when the current and pressure
coils and fully excited.
2. Errors introduced because of inductance of pressure coil tend to be large at low power
factors; special features are incorporated in an electro dynamometer wattmeter to make it
a low power factor type of wattmeter. These features are discussed in detail below.
(a) Compensation for pressure coil current.
(b) Compensation for Inductance of pressure coil.
(c ) Small control torque.
(d) Pressure coil current.
Thus from the advantages of low power factor wattmeters it is calibrated using Phantom loading.
45
Circuit Diagram:
Procedure:
1. Connections are made as per circuit diagram.
2. Kept the Auto Transformer ( 1 & 2 ) in minimum position.
3. The Auto Transformer 2 is varied in pressure circuit the voltmeter reading is adjusted
to rated value i.e 150V.
4. Slowly the Auto Transformer 1 is varied in current coil circuit the Ammeter reading
is adjusted at different valued in steps from 0-5Amps.
5. The experiment is repeated for different values of current
at constant voltage.
6. After noting the values slowly decrease the auto transformers till Ammeter and
Voltmeter comes to zero position and switch off the supply.
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their ratings.
3. Readings of the meters must be taking without parallax error.
4. Ensure that setting of the Auto Transformer at zero output voltage
during starting.
46
Theoretical Calculations:
True Power (Wt) = VI Cosф
% Error =
Wattmeter Reading – True Power
----------------------------------------------------- X100
True Power
Observation Table:
Sl.
No.
Voltage
V{Volts}
Load Current
I L {Amps}
Wattmeter
{Watts)
True Power
(Wt) = VI Cosф
Result:
Viva Voce Questions
1. What is the difference between moving coil & Fixed coil.
2. What type of control is used for electrodynamometer?
Type wattmeter.
3. What is damping.
4. What type of scales & pointers used for electro
Dynamometer wattmeter.
5. What are the different types of errors occur as the Wattmeter.
6. What is the power factor?
7. Explain the shape scale.
8. How the current is related with the voltage in current coil?
9. On What principle does the power factor meter work?
10. How the torque is developed in case of power factor meter?
47
%Error
Additional Experiments
11. CROMPTON DC POTENTIOMETER – CALIBRATION OF PMMC
AMMETER & PMMC VOLTMETER
Aim:
To measure the unknown voltage using DC Crompton Potentiometer.
Apparatus required;
Sl.
No.
1
Quantity
Specification
D.C Potentiometer
01
2
Standard cell{ 1.018 v}
01
3
Sensitive Galvanometer
01
4
5
RPS
{0-30V}
{say dry cell}
Standard Resistance
5
D.C Ammeter - (0-5)A
01
6
D.C Voltmeter - (0-300)V
01
7
Rheostat - 5A,50Ω
01
8
Connecting Wires
As required
or
unknown
emf
01
01
Theory:
A potentiometer is used for measuring and comparing the emf of different cells and for
calibrating and standardizing Voltmeter, Ammeters, and Wattmeters etc. in its simplest form, it
consists of a German silver or Manganin wire usually one meter long, stretched between two
terminals.
In its commercial form, D.C. Potentiometer is no calibrated that the potentiometer
reading gives the voltage directly, thereby eliminating tedious arithmetic calculation no saving
time.
Such a direct reading potentiometer consists of some equal resistances joined in series the
resistance of each unit being equal to that of the whole slide wire, which is in turn divided into a
known number of equal parts. The battery current is controlled by slide wire resistance.
48
Circuit Diagram:
Procedure;
Initial calibrations {Fixing working current}
1. Connect the battery, galvanometer in the kit as shown in the circuit diagram.
2. Put the switch „S‟ in the standard cell position and connect the standard cell to the
standard knob.
3. Rotate the main dial and slide wire in order to get the same voltage of the standard
emf.
4. Now press the key and observe the galvanometer deflection.
5. If the galvanometer does not show the balance position, adjust the rheostat and make
it to read zero.
49
To find unknown emf:
1.
2.
3.
4.
Connect the unknown battery across the knob marked „X‟
Put the switch „S‟ in the unknown emf position.
Adjust the main dial slide wire to get the null position in the galvanometer.
The reading in the main dial and slide wire gives the voltage of the unknown cell.
CALIBRATION OF AMMETER:
1. Made the connections as per the circuit diagram.
2. A standard resistance of suitable value and sufficient current carrying capacity is
placed in series with the Ammeter under calibration.
3. The voltage across the standard resistance is measured with the help of potentiometer
and the current through the standard resistance can be compound current I= Vs / S amps.
4. Since the resistance of standard resistor is accurately known and the voltage across the
standard resistor is measured by a potentiometer, this method of calibrating an Ammeter
is very accurate.
5. A calibration curve indicating the errors at various scale reading of the ammeter may
be plotted
CALIBRATION OF VOLTMETER;
1. Made the connections as per the circuit diagram.
2. The first and foremost requirement in this calibration is that a suitable stable DC
voltage supply is available since any changes in the supply voltage will cause a
corresponding change in the voltmeter calibration.
3. The figure given is a potential divides, consisting of two rheostats, one or course and
the other for fine control of calibrating voltage.
4. These controls are connected to the supply source and with the help of these controls it
is possible to adjust the voltage so that the pointer coincides exactly with a major division
of the voltmeter.
5. The voltage across the voltmeter is stepped down to a value suitable for application to
a potentiometer with the help of a volt-Ratio Box for accuracy of measurement, it is
necessary to measure voltages near the maximum range of the potentiometer, as for as
possible.
6. Thus the potentiometer has a maximum range of 1.6V. To achieve high accuracy we
will have to use low voltage ranges for voltages less than 1.6V and use appropriate
tapping‟s on volts ratio box for voltages higher than 1.6V.
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their ratings
3. Observe the ammeter reading. Apply the voltage slowly
So that the current is within the limited range of ammeters
4.Handle the Bridge carefully
50
Observation Table;
S.No.
True Value of Unknown EMF
(Volts)
Measured Value of Unknown
EMF (Volts)
% Error
CALIBRAION OF AMMETER:
Sl.
No.
Current (Iin) Amps
Current (Iout) Amps
% Error
((Iout - Iin ) / Iou ) x 100
CALIBRATION OF VOLTMETER;
Sl.
No.
Voltage (Vin) Volts
Voltage (Vout) Volts
51
% Error
(Vout - Vin ) / Vou ) x 100
Result:
Viva Voce Questions
1. Explain Standardization
2. What precautions have to be followed in the case of Standard cell
3. What is the application of D.C Potentiometer
4. What are the precautions to be observed in connecting and using the Galvanometer
5. Is it absolutely necessary to standardize the potentiometers for resistance measurements.
6. How do you choose the standard resistance to be connected in series with the Ammeter to
be calibrated
7. What is the potentiometer?
8. Can a potentiometer be used on A.C?
9. What are applications of A.C potentiometer?
10. What are the errors and precautions for the potentiometer?
52
12. C.T. Testing by Silsbee’s method - Measurement of % ratio error and
phase angle of given C.T. by comparison
Aim:
To test the given current transformer by Silsbee‟s method and measure percentage ratio error,
phase angle of given C.T by comparison.
Apparatus Required:
Sl .
No.
Name of the Equipment
Range
Type
Quantity
50Hz
01
50Hz
01
(5/10/15/20)A,
50Hz, 660V
(0-5/10/)A, 50Hz,
660V
(150/300/600)V
(0-5/10)A
(150/300/600)V
(0-5/10)A
Dynamometer
Type
Dynamometer
Type
Ammeter
(0-10)A
MI
02
06
Voltmeter
(0-600)V
MI
01
07
Phase-Shifting Transformer
440V, 500VA
3-ф
01
08
Rheostat
100Ω,10A,
1-ф
01
09
Resistive Load
230V. (0-5)A
1-ф
01
09
Connecting Wires
------
------
As required
01
Precision C.T
02
Commercial C.T
03
U.P.F. Wattmeter
04
L.P.F. Wattmeter
05
01
01
Theory:
When large currents are to be measured, it is usual to use low range ammeters with suitable
shunts, in DC circuits but since this is neither convenient nor practical to use this method with
AC current, current transformers are employed, which are specially constructed accurate ratio
instruments in conjunction with standard low range AC instruments. There purpose is to reduce
the line current to a value small enough to be measured with meters of moderate rise and
capacity. In other words CT‟s are used for extending the range of AC ammeters. The current
transformer has a primary winding of one of more turn of thick wire connected in series with the
line carrying the current to be measured. The secondary consists of a large no. of turn of fine
wire & feeds a standard 5A ammeter or he current coil of a wattmeter or watt hour meter. Since
the ammeter resistance is extremely low, a CT operates with it secondary under nearly short
circuit condition.
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Ratio error:
For satisfactory and accurate performance, it is necessary that the ratio of transformation of the
CT should be contact within close limits. However in practice it is found that current.
Transformation ratio do not remain constant, which is found to depend on the exciting current &
the current and Power factor of the secondary current. This fact leads to an error called RATIO
ERROR of the transformer, which depends on the working component of primary,
Phase angle error:
From the vector diagram, it is seen that the phase angle between the primary secondary currents
is not exactly 180 degrees but slightly less. This difference angle „β‟ may be found by reversing
vector I2. He angular displacement between I1 & I2 reversed is called the PHASE ANGLE
ERROR of the current transformer. This angle is reckoned =ve if the reversed secondary current
leads the primary current. However on very low pf‟s. the phase angle may be negative.
Silsbee‟s method is a comparison method. There are two types of Silsbee‟s methods deflection
and null deflection method is used in this experiment two transformer are connected with this
primaries in series. Two transformer an adjustable burden is put in the secondary circuit of the
transformer and which is under test an ammeter is included in the secondary circuit of the
standard transformer so the current may be set to desired value. W1 is the wattmeter whose
current coil is connected to carry the standard secondary current of standard transformer.
The current coil of is connected to carry the secondary current of standard transformer. The
current coil of wattmeter W2 carries a current I which is the difference between the secondary
currents of the standard and test transformer the voltage circuits of the wattmeter‟s are supplied
in parallel from a phase shifting transformer at a constant voltage.
Circuit Diagram:
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Phasor Diagram:
Procedure:
1. The circuit is connected as shown in figure.
2 The 3- Phase main supply is switched ON.
3. The single phase A.C supply is switched ON
4. The burden (150 Ohms/5A) Rheostat is adjusted at approximately middle position.
5. The Phase shifter is rotated so that Ws (UPF wattmeter) reads zero.
6. The various values of V, Ip, Is, Wr, & Ws are note down in Z.P.F method.
7. Now the knob is unknocked and the phase shifter is adjusted so that the pointer coincides
with 90 degrees. ( Cos 90 = 0) then the knob is tightened
8. The phase shifter is rotated through 90 degrees, so that U.P.F wattmeter reads maximum.
9. The various values of V, Ip, Is, Wr, & Ws are note down in U.P.F method.
Precautions:
1. There should not be any loose connections.
2. Meter readings should not be exceeded beyond their ratings
3. Readings of the meters must be taking without parallax error.
4. Ensure that setting of the Auto Transformer at zero output voltage during starting.
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Observation Table;
Specifications
Z.P.F
Voltage (Volts)
Standard C.T (Amps) (Is)
Precision C.T (Amps) (Ip)
U.P.F Wattmeter (Watts) (Wr )
L.P.F Wattmeter(Watts) (Wa )
Result:
Viva Voce Questions
1. Why a phase shifting transformer used in this experiment.
2. Explain the operation of phase shifting Transformer
3. What is power factor angle.
4. What is the main condition for phase shifting of transformer
5. Explain the principle of current transformer
6. Explain the principle of potential transformer
7. Why it is called Silsbee‟s method.
8. Explain the Principle & operation of Auto Transformer.
9. What is the difference between Transformer & Auto Transformer.
10. What is the difference between C.T & P.T.
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U.P.F

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