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LAB MANUAL
Measurement Lab (EEE-496)
Department of Electrical Engineering
Zakir Husain College of Engineering and Technology Aligarh Muslim University
Aligarh
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COURSE DESCRIPTION FORM EE-496 Course Title Measurement Lab
Course number EEE-496 Credit Value 2.0 Course Category DC
Pre-requisite EE-251N Electrical Measurement, EE-352N Electrical & Electronic Instrumentation Contact Hours
(L-T-P)
0-0-3 Type of Course Practical Course
Objectives
For the enhancement of theoretical knowledge and to give the practical exposure of different transducers, ac bridges, instruments and measurement procedures including calibration and standardization of instruments.
Course
Outcomes After completing the lab course, the students
1. Should be able to know performance of various transducers, ac bridges and instruments.
2. Should be able to know about the need and basics of calibration and standardization procedure.
3. Should be able to do calibration of different instruments.
4. Can use the calibration and standardization procedures effectively in the field work.
5. Can develop skills for handling more complex measurement system and instruments.
Syllabus 1. Determination of the capacitance of unknown capacitor by Schering bridge method.
2. To study of Linear Variable Differential Transformers (LVDT).and to draw its characteristics.
3. Determination of the phase angle and ratio error of a current transformer by Petch-Elliot method.
4. Measurement of strain in a bar specimen using strain-gauge method.
5. Calibration for Wattmeter by D.C. potentiometer using Phantom method of loading.
6. Determination of the B-H curve of a ring specimen of cast iron by ballistic galvanometer.
7. Separation of iron losses in magnetic sheet steel by Lloyd-Fisher square method.
8. To measure THD and Harmonics components of different light loads using NI-DAQ device and LabVIEW.
Books*/
References
1. G.W. Golding & F. C. Widdis, Electrical and Electronic Measurement and Instruments, Pitman/ A.H. Wheeler, Allahabad.
2. D. Bell, Electronic Instruments and Measurement, PHI Learning.
3. A.K. Sawhney, A course of Electrical and Electronic Measurement and Instrumentation, Dhanpat Rai & Co. Pvt. Ltd., Delhi.
Course Assessment/
Evaluation/
Grading Policy
Sessional
Evaluation of each lab reports and viva-voce held every week on each lab report.
60 Marks
Sessional Total 60 Marks
End Semester Examination (2 Hours) 40 Marks
Total 100 Marks
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GENERAL INSTRUCTIONS FOR PERFORMING EXPERIMENTS IN LAB
1. All students shall carry out experiments in groups.
2. Each group will carry out a particular experiment, assigned to them in each term.
3. The next experiment to be performed is the next experiment mentioned in the list of experiments (cyclic order).
4. The student should come prepared and should go through the experiment sheet provided to them and the relevant theory.
5. After completion of the connection of circuit, get the connection checked by Instructor or lab staff.
6. After performing the experiment, get the observation signed by a teacher.
7. Submit the Report, complete in all respect, on the consecutive next turn. Provide sample calculation, graph, comment on result etc.
8. No student will be allowed to proceed to next experiment, unless he/she submits the report of previous experiment. In such case no attendance will be marked for the defaulter student.
9. The lab report will be checked and viva-voce will be held at the time submission on each tern.
10. The Instructors and lab staff are available to assist the students in their work.
11. In case of any accident while performing experiments, turn off the power supply immediately. Use fire extinguisher, if anything catches fire.
12. In case of any injury, use the first aid kit provided in the lab.
13. It is prohibited to smoke, eat or drink in the laboratory.
14. The class room discipline has to be maintained in the laboratory.
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ELECTRONIC DEVICES & EQUIPMENT
Generally the equipment and devices discussed are:
1. Nature and types of measuring instruments 2. Basic measuring instruments
3. Power supplies
4. Digital/electronic Instruments
1. Nature of Measured Quantity and Number of Measuring Elements i. Direct current circuit and/or dc responding measuring element ii. Alternating current circuit and/or ac responding measuring element
iii. Direct and/or alternating current circuit and/or dc and ac responding measuring element Safety
Chassis (casing or body) of instrument is normally connected to ground terminal of the input ac power supply, for the safety of working personnel.
In case of CRO used in power electronics lab, it could give an electric shock to working personnel and cause short-circuit condition with COMMON terminal of probe at the negative terminal of the ac-to dc converter, which is a floating point (in the negative half- cycle, it is at input phase (negative) level i.e. -2340V). Thus remove GROUND of input ac power supply of CRO and be careful. The COMMON terminal is now floating which is protected from short circuit but it still gives an electric shock while touching it.
High voltage flash may appear to working personnel during measurement (e.g. using DMM), which is due to the transmitted effect of lighting stroke through power supply wires/ conductors.
Position of use
i. Instrument to be used with the dial vertical ii. Instrument to be used with the dial horizontal
iii. Instrument to be used with the dial inclined (for example 60o) from the horizontal plane Principal of Operation of Instruments (type)
i. Permanent –magnet moving–coil instrument ii. Permanent-magnet ratio-meter (quotient-meter) iii. Moving-iron instrument
iv. Polarized moving-iron instrument v. Moving-iron ratio-meter
vi. Induction instrument vii. Electrostatic instrument viii. Vibrating-reed instrument
ix. Non-insulated thermo-couple (thermal converter) x. Insulated thermocouple (thermal converter) xi. Electronic device in a measuring circuit
4 Use & Application of Instruments
Type of Instrument Suitability Major uses PMMC (d’Arsonval
type)
D.C. Most widely used meter for d.c. current, voltage and resistance measurements. Good accuracy.
Moving Iron A.C. (D.C. also possible with correction)
Inexpensive type used for currents and voltages at power frequency. Also used in indicators/ panels.
Not very accurate.
Rectifier D.C. or A.C. It is combination of rectifier and PMMC instruments.
Good for variable frequency measurement including power frequency. Good for low impedance circuits.
Electrodynamometer Both D.C. and A.C.
Widely used for precise power measurements. Used as standard meter for calibration (AC/DC) and called as transfer instrument. Also used for precise a.c.
current and voltage measurements at power frequencies.
Heating effect Both A.C. &
D.C.
Used for ac current, voltage and power measurement at variable or radio frequency. Used for distorted or non-sinusoidal waveforms..
Thermocouple (Heating effect)
D.C. or A.C. Measurement of voltage, power etc. at variable or radio frequency. Used for distorted or non-sinusoidal waveforms.
Induction type A.C. only Current, voltage, power & energy measurement.
Electrostatic D.C. (or A.C. at one frequency)
Measurement of high voltages.
Hall effect A.C. & D.C. For power and current measurement, AC or DC current probes and also used as transducers for current & power.
2. Basic Measuring Instruments: The Basic measuring instruments commonly used in the laboratories are the instruments needed to measure the basic electrical quantities such as currents and voltages and include ammeters, voltmeters and multi-meters.
(a) Ammeters and Voltmeters: The ammeters, used to measure electric currents with very low internal resistance are used in series with the load. Voltmeters have high internal impedance and are connected in parallel with the load to measure voltages. Both instruments have their terminals marked with + & - polarities and should be carefully used with correct polarities. These instruments are available in different ranges and are accordingly named e.g., micro-ammeters, milli-ammeters, milli-voltmeters etc. The voltmeters draw negligible amount of current from the circuit under measurement and its sensitivity is expressed in ohms per volt. Typically a meter movement having a 1 ma full-scale current has a sensitivity of 1000 Ohms/Volt whereas if the full- scale current is 100 microampere, the sensitivity is designated as 10,000 Ohms/Volt. The higher the ohms-per-volt rating, the more sensitive is the meter and the smaller is the loading effect on the circuit. A good voltmeter has sensitivity between 20,000 – 100,000 Ohms/V.
5 Precautions:
(i) Higher voltages/currents should never be measured on low-range meters.
(ii) The meters must always be connected to the circuits with correct polarities.
(iii) For accurate measurement, the meters must be kept stationary in horizontal position.
(b) Multi-meters: It incorporates voltmeter, ammeter and ohmmeter into one case, and same meter movement is utilized for all of them. Generally it has three switches – the function switch which selects the type of measurement viz. current, voltage or resistance, the range switch, which selects different ranges, and the mode switch which selects the ac or dc mode of operation. Two types of multi-meters are generally available in the laboratories – Analog and Digital. In digital multi- meters, digital panel meter consisting of LCD or LED display with A-to-D converter and some processing circuitry replaces the meter movement. Batteries are used as a power supply in most solid-state multi-meter.
(i) Analog multi-meter: A typical AMM, which is widely used in laboratories, is SIMPSON Model 260-6M. It has 8 ranges for dc voltage measurement, 6 ranges for ac voltage measurement, 5 ranges for dc current, 3 ranges for resistance measurement, and 1 for power measurement in dBs as marked on the front panel. It has a single switch serving the functions of both mode and function switches.
(ii) Digital Multi-meters: Generally DMMs available in the laboratories are hand-held and auto- ranging having a power ON/OFF switch and separate jacks for measurements and are so simple to use that needs no explanation. DMMs are becoming more common and are replacing the analog multimeters (AMMs) in the laboratories today.
3. Power Supplies: The electronic circuits employing active devices need dc voltages for their operation, which are derived from ac mains and should be free from ripples and independent of any variation in the ac mains voltage or in the load current drawn by the circuit itself. The equipment providing such voltages are called Regulated Power Supply (RPS) Units. Fixed, Variable and Dual RPS are needed in the laboratories and therefore, Multi-output RPS is commonly available in various laboratories. As a sample RPS, the APLAB Transistorised RPS Unit is described as follows:
The APLAB Multi Output RPS Model 7711: This power supply delivers three outputs.
(i) 0-30V dc output continuously variable with 2 Amps capacity. This output is suitable for general purpose.
(ii) 5V pre-set dc output with 5 Amps. Capacity suitable for digital integrated circuits.
(iii) A symmetrical dual supply +15V, 0, -15 pre-set dc output with 500mA capacity, suitable for linear IC circuits. All the outputs of Model 7711 are floating (i.e., neither any of the +ve output neither terminals nor any of the –ve output terminals nor any point within the regulator circuitry is connected to ground).
Description:
Input & output termination: The unit works from 230V ac supply through a mains cable with a 3- pin plug, with a ground terminal for the safety of working personnel. All the output terminals are provided on the front panel and are marked clearly.
6 Metering & panel controls:
+30V/2A Section: Two separate front panel meters continuously monitor the output voltage &
load current. The least count of the voltmeter is 0.5V on the scale 0-30V and that of the ammeter is 50ma on 0-2.5A scale. Coarse and Fine controls are provided on the front panel for setting the output voltage & current within the specified ranges.
Symmetrical Dual PS & +5V Sections: A single panel meter monitors, either +15V or –15V or +5V section output, with the help of selector push switch provided on the front panel. The meter has two scales. One is 4 to 6V and the other is12-18V. The least count of the 4-6V scale is 0.2V and that of 12V to 18V is 0.5V. One sepwerate control is provided to adjust the output voltage of +5V section from 4.5V to 5.5V and another one to adjust output voltage (+) & (-) 15V supply section from ±12V to ±18V. The maximum load-current supplied by 5V section is 5.0Amps. And that for Symmetrical Dual Supply section is 500mA.
Protection & Indication:
The outputs of all the three sections are fully protected against over loads & short circuits by means of fold back characteristics. The outputs automatically reset after removal of over load. A built-in
“Crowbar Circuit” operates and reduces the output voltage below 2V in case output voltage tends to exceed the crowbar limit (approx.6.2V) to protect the supply from over voltage.
The availability of the output voltages are indicated by the three red LED’s marked 5V, +15V & - 15V provided on the front panel.
The regulation is less than 0.1% and the ripples are less than 1mV. This supply can be used as CV or CC supply. For setting the current limit of the 30V section, short-circuit the output terminals and adjust the Current (COARSE & FINE controls) until the panel meter reads the desired value.
Leave the pots in this adjusted mode. The supply will now operate within the set voltage & current limits and will crossover from voltage mode to current mode when the load increases or vice-versa.
The current limit of the other sections is pre-set, both output voltage & load current will start falling simultaneously.
4. Digital and electronic instruments: These instruments are being widely used for general purpose (e.g. digital multimeters) and special applications (e.g. digital energy meters). Now, it is available at low cost and with good accuracy. Sometimes many measuring features are clubbed together without any additional cost.
Digital Multimeter (DMM 4011) Introduction
Hand-held
Compact Light-weight
High class Engineering & rugged design ensures very reliable performance.
DC/AC Volts.
DC/AC Currents.
Resistance, Capacitance, and Frequency Measurement.
Diode Testing & Continuity Testing.
Transistor hFE measurement.
10A Fuse protected.
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Palm size.
Technical Specifications
Function Range Accuracy Input
Impedance
Max.Input
DC Voltage 200mV
2V-20V-200V 1000V
±0.5% of rdg ± 1D
±0.6% of rdg ±1D
10MΩ 1000 VDC or Peak
AC AC Voltage (50-
500Hz)
200mV
2V-20V-200V 750V
±1.0% of rdg ± 4D
±1.5% of rdg ±4D
10MΩ 750 V rms
300V rms at 200 mV
DC Current 2-20-200mA
10A ±1.2% of rdg ± 1D
±2% of rdg ± 4D Burden Volts Protection
0.7 V 0.5A/250V Fuse
10A/250V Fuse 10A/60Sec. Max.
Test Condition 500
VDC or AC Peak
Resistance 200Ω
2V-2000KΩ 20MW
±1.0% of rdg ± 3D
±0.8% of rdg ± 3D
±3.0% of rdg ± 3D
3V DC 0.3 V DCV Diode Test Voltage 2V
Test Current (1 ± 0.6mA) Capacitance 2000pF
20-200nF 2-20µF
±5% of rdg ± 10D
400Hz-50mV
Frequency (Auto
Range) 2KHz-15MHz ± 0.5% of rdg ±
1D Trig-Lo-1Vrms
Trig-Hi-2Vrms HFE 0-1000 Ib=10µA approx. Vca<3.5V Continuity: 200Ω Beeper Sounds < 40Ω + 20Ω
Resolution: For DC V & AC V: 100µV For DC & AC A: 1µA
Res: 0.1Ω Freq: 10Hz
Large size LCD Display: 3½ Digit – 1999 counts Auto zero/Auto Polarity
Over Range indication 2.5 Measurement/Sec.
Power – 9V Battery: typical – 225 Hours. Low Battery Indication Operating Condition: 0-50oC – 75%RH
Dimensions (mm): 151H x 70W x 38D
Weight: 200g with Battery. (Subject to change) OPERATING INSTRUCTIONS
Voltage Measurements
1. Connect the red test lead to the “VΩA” jack and the black test lead to the “COM” jack.
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2. Set the Function/Range switch to the desired voltage range and slide the “AC/DC” selector switch to the desired voltage type. If magnitude of voltage is not known, set switch to the highest range and reduce until a satisfactory reading is obtained.
3. Connect the test leads to the device or circuit being measured.
4. For DC a (-) sign is displayed for negative polarity: positive polarity is implied.
Current Measurements
1. Set the function/Range switch to the desired current range and slide the “AC/DC” selector switch to the desired current type.
2. For current measurements less than 200mA connect the red test lead to the “VΩA” jack and the black test lead to the “COM” jack.
3. For current measurements of 200mA or greater, connect the red test lead to the 10A jack
& the black test lead to the “COM” jack. (10A for max. 60 sec.).
4. Remove power from the circuit under test and open the normal circuit path where the measurement is to be taken. Connect the meter in series with the circuit.
Resistance and Continuity Measurements
1. Set the Function/Range switch to the desired resistance range or continuity position.
2. Remove power from the equipment under test.
3. Connect the red test lead to the “VΩA” jack and the black test lead to the “COM” jack.
4. Touch the probes to the test points. In ohms, the value indicated in the display is the measured value of resistance. In continuity test, the beeper sounds continuously, if the resistance is less than 40Ω±20Ω.
Diode Tests
1. Connect the red test lead to the “VΩA” jack. And the black test lead to the “COM” jack.
2. Set the Function/Range switch to the “├” position.
3. Turn off power to the circuit under test.
4. Touch probes to the diode. A forward voltage drop is about 0.6V (typical for a silicon diode).
5. Reverse probes, if the diode is good. “OL” is displayed, if the diode is shorted, “000” or another number is displayed.
6. If the diode is open “OL” is displayed in both directions.
7. If the junction is measured in a circuit and a low reading is obtained with both lead connections, the junction may be shunted by a resistance of less than 1kΩ. In this case the diode must be disconnected from the circuit for accurate testing.
CAPACITANCE MEASUREMENTS
1. Set the Function/Range switch to the desired F (capacitance) range.
2. Never apply an external voltage to the Cx sockets. It may result in damage to the meter.
3. Insert the capacitor leads directly into the Cx socket.
4. Read the capacitance directly from the display.
Frequency Measurements
1. Set the Function/Range switch to the Hz position.
2. Connect the red test lead to the “VΩA” jack and black test lead to the “COM” jack.
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3. Connect the test leads to the point of measurement and read the frequency from the display.
Transistor HFE Measurements
1. Set the Function/Range switch to the desired hFE range (PNP or NPN type transistor).
2. Never apply an external voltage to the hFE socket. Damage to the meter may result.
3. Plug the transistor directly into the hFE socket. The sockets are labeled E,B, and C for emitter, base & collector. Read the transistor hFE (dc gain) directly from the display.
Safety Instructions:
Don’t use DMM if it is or test leads look damaged, or if meter not operating properly.
Take caution while working above 60VDC or 30VACrms.Such voltages pose a shock hazard.
When using the probes, keep your fingers behind the finger guards on the probes.
Measuring voltage, which exceeds the limits of the multimeter, may damage the meter and expose the operator to a shock hazard. Always recognize the meter voltage limits as stated on the front of the meter.
Remove test leads before changing battery or performing servicing.
Fuse replacement: If no current measurement possible, check for a blown overload protection fuse. There are two fuses: F1 for the “VΩA” jack and F2 for the 10A jack. For access to fuses, remove the two screws from the back of the meter and lift off the fuses.
Replace F1with the original type 0.5A/250V, fast acting fuse. Replace F2 only with the original type 10A/250V, fast acting fuse.
Battery Replacement: Power is supplied by a 9 volt battery. To replace the battery, remove the two screws from the back of the meter and lift off the front case. Remove battery from battery contacts.
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Experiment Sheets
Measurement Lab (EE-496)
List of Experiments:
Experiment 1: Measurement of capacitance by Schering Bridge.
Experiment 2: To study of Linear Variable Differential Transformers (LVDT).
And to draw its characteristics.
Experiment 3: Testing of a current transformer for ratio and phase angle errors by Petch-Elliot method.
Experiment 4: Measurement of strain by a strain-gauge.
Experiment 5: Calibration for Wattmeter by D.C. potentiometer using Phantom method of loading.
Experiment 6: Determination of B-H curve of magnetic material by the Ballistic Galvanometer method.
Experiment 7: Separation of iron losses in magnetic sheet steel by Lloyd-Fisher Square method.
Experiment 8: Data Acquisition and measurement using NI DAQ device and LabVIEW.
Object: Study of Photovoltaic Transducer using NI-DAQ Device and LabVIEW.
Experiment 9: To measure THD and Harmonics components of different light loads using NI-DAQ device and LabVIEW.
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Experiment 1
Experiments: Measurement of capacitance by Schering Bridge.
Object: To determine the capacitance of unknown capacitor by Schering Bridge at different frequencies.
Theory: Schering Bridge is widely used for measurement of dielectric losses and power factor of lossy capacitors at high voltage. Moreover, this method is one the best methods of measurement of small capacitance at low voltages with high precision.
The test capacitor is represented by either series or parallel combination of equivalent-series resistor (ESR) or equivalent-parallel resistor. It can be represented by r and C1, where r is the loss component of capacitor. The dissipation factor or tan δ or D=ω C4R4= C1r1. If the angular frequency, ω is fixed, the variation (dial) of capacitor C4 can be calibrated to read the dissipation factor directly.
C1 = (C2 X R4) /R3, r1 = (R3 X C4)/C2 Procedure:
1. Connect the circuit shown in figure.
2. Adjust R4 and C4 to obtain the balance condition.
3. Change the frequency from audio oscillator and check the gain for the balance condition. The electronic null detector indicates the balance condition where its deflection is minimum.
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Observations:
Frequency r1 C1 C2 R3 R4 C4
Sample calculations:
Results: C1 = tan δ = r1 =
Reports:
1. Draw the phasor diagram of Shering Bridge.
2. Why this bridge is used for measurement of small capacitance?
3. What are the other methods of measurement of capacitance?
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Experiment No.2
Experiment: To study of Linear Variable Differential Transformers (LVDT).and to draw its characteristics.
Object: To study the construction of LVDT and to draw its input output characteristics and find its linear range of operation.
Theory: A linear variable differential transformer is a transducer for linear displacement measurement. It is transformer with movable core single primary and two secondary coils. When core is at mean position, flux linkage with two secondary coils remains same, since the two secondary coils are connected in phase opposition the output voltage is zero. When core is moved in one direction the induced any flux of one secondary winding will be more than the other the output voltage increases either in positive or negative direction.
It can be said that the magnitude of output voltage is proportional to the displacement of the core away from the Centre and polarities are determined by direction of the core
.
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Observations:
Position of core (mm)
Output voltage (volt) (Forward)
Output voltage (volt) (Reverse)
Graph: Plot the graph between core position and output voltage.
Result: The linear range of LVDT is:
Report:
1. Comments on the graph.
2. Write applications of LVDT.
3. What are other transducers available for displacement measurement?
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Inst-4/1
Experiment 3
Testing of a current transformer for ratio and phase angle errors by Petch-Elliot method.
Object:
1. To study the construction, circuit diagram and operation of Petch Elliot Testing set.
2. To determine the phase angle and ratio errors of the current transformer at different currents and burdens.
Theory: When a current transformer is used for measuring current and in order that it shall not introduce an error into the measurement, it is essential that secondary current shall be definite and known fraction of primary current. The current ratio of the transformer, however, differs from the turns ration by an amount which depends upon the magnitude of the exciting current of the transformer, and upon the current, and power factor of the secondary circuit. The current ratio is, therefore, not constant under all conditions of load and frequency. In general this error is known as ratio error which is given by
Normal Ratio -- Actual Ratio Actual Ratio
It largely depends upon the value of the iron loss component Ie of the exciting current Io, where Ie =Io sin Where = angle between Io and the working flux. If the phase angle of the total burden is assumed zero, the actual ratio, R, is given by
R = n + (Ie/ Is) where n = turn ratio and Is = secondary current
The ratio error is considered to be positive when the actual ratio of the transformer is less than the nominal ratio. While measuring power, it is necessary that the phase angle of the secondary current shall be displaced by exactly 180o from that of the primary current. In general, this condition is not fulfilled but that the transformer has a phase angle error which may introduce appreciable error in power measurements.
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The phase angle error depends upon the magnetising component Im, where Im = Io.cos ά and the error is expressed as Im/NIs. This error is considered to be positive when the reversed secondary current leads the primary current.
Procedure:
i) Connect S and X CTS together with any necessary burdens and vibration galvanometer
ii) Set knife selector switch to suit secondary current of transformer under test.
iii) Set ratio and phase rheostats to zero, set Galvanometer sensitivity control to minimum and range key to “check” position.
iv) Bring up primary current to about 10% full rating. If polarity of connections is correct, there will be no appreciable galvanometer deflection. If there is marked deflection, connections of S or X should be reversed.
v) Increase primary current to required value, and move range key “X5” position.
Move galvanometer sensitivity control away from zero until a minimum deflection is obtained. If the movement increases the deflection, throw the reversing key. Then repeat with the phase error rheostat which can be moved until no deflection indicates balance.
vi) The galvanometer sensitivity control can be progressively moved from minimum towards maximum when the balance position will be more sharply defined.
vii) If the position of the two rheostats is such that the error is within the lower range, the range key be moved to “X1” and the balance operation repeated.
viii) The above procedure may then be repeated for other values of primary current.
Observations:
S.No. Current Range Switch
Ratio Switch
Ratio-dial
%
Phase angle switch
Phase angle Dial (min)
Burden
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Circuit Diagram:
G S
"Spill" Winding
Phase Error Winding Condenser
Calibrated Slide Wires
Phase Error
Spill Current Sec.
Current of
"X"
B D
A Ratio error
Secondary Current of "S"
S Primary Current
'S' C.T 'X' C.T
Ratio error Winding
Galvo Winding AutoTransformer
Galvo Winding
Test C.T.
Std.
C.T.
P
S
P
S M L
M L M L
M L
K
Burden
Load 230
V
115 V
Result:
S.No .
Current Ratio error Phase angle error
Set Reading
Std.
error
Net error
Set Reading
Std error
Net error
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Experiment 4
Experiment: Measurement of strain by a strain-gauge.
Object: To measure strain in a bar specimen using electrical resistance gauge and verification of the same by measuring the deflection in the bar. By
Theory: The stain developed in a body, experiencing an elongation in the length due to application of force is given by
modules
s Young'
Stress Strain
The unit of strain is defined as the ratio of change in length in a particular direction to the original length in that direction. It can be measured by straining a fine resistor wire (or fine mesh of resistor wire) known as resistance strain gauge. Due to strain, the resistance of the gauge changes. This change in resistance is a measure of the strain. If G is the gauge factor of the strain gauge then the strain is given by
R G
ΔR
A mesh type strain-gauge (fine wire resistor) can be placed with and sticking to the bar specimen. In this way, the strain in a bar specimen, placed on two supports, can be found by measurement of the resistance.
Procedure A bar specimen is placed on two supports. A mesh type strain-gauge (fine wire resistor) is placed with and sticking to the bar specimen. The strain in the bar, between two edges (supports) is varied with the help of variation of loading of known weights on the bar. A Wheatstone bridge is used to determine the resistance of the given resistance (strain-gauge).
Support the bar specimen between two edges and strain it by means of known weights as shown in figure. Keep the resistance of three arm same i.e. R2 = R3 = R4. Vary R`2 which is a high resistance connected in parallel with R2 such that the bridge is balanced. The galvanometer deflection is zero again.
If R = Resistance of strain gauge (unstrained) and R1 = Resistance of strain gauge (strained)
Then, R2 R`2
R1 = --- R2 + R`2
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Knowing R1, the change in resistance (R1-R) can be calculated from where strain can be determined. Measure also the deflection of the bar specimen when strained using the dial gauge and calculate the strain using the expression.
b d
Strain = --- where = deflection of bar specimen, L2
d = thickness of bar specimen, and L = Distance between the supports Observations:
Resistance of the strain gauge = R = Gauge factor = Width of the bar specimen = Thickness of the bar specimen =
Inst-5/2
S.No. Wt.
Distance between supports
R2 to bring Galvanometer Deflection to zero
Deflection in the bar specimen shown by dial gauge.
Circuit Diagram:
G
2 V a
c
d b R2'
R4 R3 Strain
Gauge
R2
Figure: Wheat stone bridge for the measurement of strain.
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Experiment 5
Experiment: Calibration for Wattmeter by D.C. potentiometer using phantom method of loading.
Object: To draw curves of percentage calibration vs wattmeter readings.
Theory: For the calibration of dynamometer type wattmeter an accurately known amount of dc voltage and dc current may be applied to the load and their product may be compared with the reading of wattmeter. However, if this method is used enormous power would be wasted at rated voltage and rated current. Alternatively phantom method of loading is used where instead of one (with rated voltage and rated current), two independent power supplies are used. Here no actual load is used. High-voltage, low-current source energized the pressure coil and a high current, low-voltage source energizes the current coil. Thus, the wattmeter reading corresponds to the voltage of the pressure coil and current of the current coil.
Sufficient accuracy of the measurement may be achieved by the measurement of current and voltage using accurate digital multimeter and bench type digital voltmeter and wattmeter. Here high accuracy is achieved by the used of
potentiometer by the measurement of voltage and the current. The voltage drop across a standard resistance gives the amount of current through it. Similarly a voltage –ratio box (potential divider) is used for the measurement of actual voltage (high) across the pressure coil.
Incorrect reading – Correct reading
% age calibration = --- x 100%
Incorrect reading W-W1
= --- x100%
W1
Incorrect reading = W, watts (Taken from wattmeter)
Correct reading, W1 = Vp x Ic watts, (Taken from potentiometer)
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Procedure:
(a) Make connection of the potentiometer as shown in fig.1
(b) Complete the connections of both current coil and pressure coil circuits as shown in fig.2
(c) Put selector switch of the potentiometer at standardization mode.
(d) Set the knobs of the different potentials (100, 10, ---mv) such that its voltage is equal to the voltage of the standard cell (neglect the effect of temperature variation on the voltage of the cell).
(e) Find the balances condition with the help of galvanometer and by varying the coarse and fine resistances of the potentiometer box.
(f) Now, put the selector switch at emf 1 or emf 2 for the measurement of these voltages.
Measurement steps;
(g) Energize the current coil circuit such that a current 0.2A flows in it.
(h) Energize the potential coil circuit such that the voltage across the pressure coil would be 50v. A reduce voltage will appear at the output of V. R. box (E2) which depends upon the ratio of V.R.
(i) Measure E1 andE2by using different knobs of different potential of the potentiometer.
Circuit Diagram:
22
Observation Table:
23 Standard Resistance =
Standard Cell = Voltage Ratio =
Multiplying Factor = (Rated Voltage X Rated Current) / Rated Power
S. No. Voltage (v)
Current (A)
Power (W)
Voltage across Potential
Coil (mv)
Voltage across Current
Coil (mv)
Actual Voltage
(V)
Actual Current
(A)
Actual Power (W)
% Calibration
Reports:
1. What are the sources of errors in the experiment and how minimized or Eliminated?
2. Why is the phantom leading method employed for the calibration of Watt meter?
24
Experiment 6 Inst-6/1
Experiment: Determination of B-H curve of magnetic material by the Ballistic Galvanometer method.
Object: To determine the B-H curve of a ring specimen of cast iron by the Ballistic Galvanometer method.
Theory: The ‘throw’ of a ballistic galvanometer is proportional to the quantity of electricity passed through it and hence to the change in flux linkage of the search coil connected to the galvanometer terminals. When the current in the magnetizing winding is reversed the ‘throw’ of the ballistic galvanometer is a measure of the flux in the specimen. The calibration of the galvanometer is carried out by means of the Hibbert’s Magnetic standard (HMS).
Procedure: Connect the magnetizing winding to variable D.C. supply through a reversing switch and an ammeter and the search coil through a resistance box and Hibbert’s Magnetic standard (Fig.1).
First demagnetize the magnetic ring specimen to its cyclic state. To do so the short circuiting key is left closed and the current is increased in the magnetizing winding to its maximum rating say 3A. This current is reduced gradually to zero and simultaneously it is reversed quickly again and again by DPDT. Now increase the current and note the ‘throw’ of the ballistic galvanometer when the current is reversed. Increase the current further and observe ‘throw’ of the galvanometer for this current. Continue the process until the current in the magnetizing winding reaches the maximum value. For the calibration of the ballistic galvanometer release the trigger of the Hibbert’s Magnetic standard and note the ‘throw’ of the galvanometer. The constant of the latter can be calculated from the ‘throw’.
a) The scale of the lamp and scale arrangement should be so adjusted that deflection on either side of the scale is the same for the same steady current through the galvanometer.
b) The position of the galvanometer and the setting of the resistance-box must not be altered during the experiment.
c) A key which can short-circuit the galvanometer should be provided to bring the spot of light quickly to the zero position.
25
For the determination of the hysteresis loop, use step by step method. After reaching the point of maximum H, the magnetizing current is next reduced in steps by moving switch S2 (Fig.2) down through the tapping points. As the magnetizing force is reduced the ‘throw’ of the galvanometer gives the corresponding decrease in flux density, B. After the reduction of the magnetizing force to zero, negative values of H are obtained by reversing the switch S: (Fig.1) and then switch S2 (Fig.2) is then moved in steps as before.
Observations:
S.No. Ammeter reading Galvanometer deflections
Circuit Diagram:
A
G c
c' 12 V
S
P S R
Key HMS
Fig - 1
S1 c
c' S2
Fig - 2
Results:
Draw the B-H curve of the given specimen.
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Experiment 7
Inst-7/1 Experiment: Separation of iron losses in magnetic sheet steel by Lloyd-Fisher Square method.
Object: To separate hysteresis and eddy current losses by the above method.
Theory: The total iron loss in a magnetic material is sum of hysteresis loss and eddy current loss. Thus,
Total Iron Loss = Hysteresis Loss + Eddy Current Loss.
Wi = K1f + K2f2 where Bmax is constant Wi/f = K1 + K2f
If the graph is drawn between Wi/f and f would, therefore, be a straight line.
Knowing K1 and K2, the hysteresis and eddy-current losses at any frequency can be determined separately for a particular maximum flux density (Bmax)
(Ref. Golding and Widdis: Electrical Measurements and Measuring Instruments) Procedure: Lloyd -Fisher square accompanies standard size of test strips (25cms×7cms) formed into equal bundles of 7 in each, inserted into the square and clamped firmly with the help of non-magnetic clamps with L-shaped corner pieces;
the overlap of the corner pieces being uniforms of about 3mm. The magnetizing winding of the LIoyd-Fisher square consists of 1000 turns, divided into four coils each of 250 turns, connected in series and uniformly distributed over the length of each strip. There are four search coils B1, B2, B3, B4 having 640, 320, 160 and 80 turns respectively.
Identification of the search coil should be made so that the measured voltage across the voltmeter may be about 30-40 volts. The frequency of the applied voltage is adjusted to 20 Hz using function generator. The current in the magnetizing winding is adjusted, by adjusting the amplifier output, so that the flux density in the specimen is 0.75 Tesla. This can be verified by the voltmeter, connected to a search coil with the reading, E = 4Kf BmaxAN2 f. The magnetizing current and corresponding deflection in the voltmeter, are noted. The frequency is varied from 20 Hz to 80 Hz in the steps of 20 and a set of readings is taken.
27
Inst-7/2 Observations & Calculations:
Iron Loss: Grade of Iron:
Cross-sectional areas of test strips (specimen) A2 = 2.49 cm2 Form Factor = 1.11
Length of overlap = 8 × 0.3 = 2.4 cm.
Cross-sectional areas (mean) of search coils, As in sq. cm B4 = 12.8, B3 = 15
B2 = 17.20, B1 = 19.6
Resistance of pressure coil of wattmeter, rpc = 13,100 Ω Resistance of current coil of wattmeter = 0.2 Ω
Resistance of search coils in ohms:
B4 = 0.49, B3 = 1.07 B2 = 2.24, B1 = 4.61
Number of turns in magnetizing winding, Nh = 1000 S.No. Frequency Search
coil, B1 voltage (E)
Supply Voltage
(V)
Magnetizing current (I)
Power
1. Magnetizing force corresponding to maximum flux, Hm = Nh×I/l Where, l is the effective length of the Lloyd-Fisher square.
2. No. of turns in search coil for voltmeter 3. No. of turns in search coil for p.c.
4. Max-flux density Bmax (apparent)
5. Correction for flux density:
b = µ 𝐻
As is the cross-sectional area of search coil. Thus, true max flux density:
Bmax = B´max – b
6. No. of turns in search coil connected to p.c. of wattmeter (NB2)
7. Actual power, W = power measured×
8. Power loss in p.c. = Wp =
28
Where rpc is the resistance of pressure coil, Vpc = E×
Inst-7/3 9. Power loss in search coil, B2 (power loss in search coil B1 is negligible) = Ws =
Where, rC is the resistance of search coil B2 10. Power loss in current coil = Wcc = I2 rcc
Where, rcc is the resistance of the current coil of wattmeter.
11. Corrected Wattmeter reading
Wc = W – Wp – Ws – Wcc
12. True Power loss taking overlap into account Wi = Wc/ (1 – 1.4C) 8 × mean overlap in cm
Where, C = --- 100
13. Wi/f 14. K1 15. K2
16. Hysteresis loss 17. Eddy current loss
29
Circuit Diagram:
Results:
Frequency Hz 20 40 60 80 Hysteresis Loss
Eddy current Loss
30
Experiment No-8
Object: To measure THD and Harmonic components of different light loads using NI-DAQ device and LabVIEW
Apparatus Used: NI-9221 DAQ device, NI cDAQ chasis -9174, LabVIEW 2012 SP1
Theory: Harmonic distortion is caused by the loads which draw nonlinear current from the supply. Harmonics are the integral multiple of the frequency for which the instrument is designed to operate. THD depends upon the kind of load connected.
The more the waveform deviates from the normal sinusoid, the greater the THD value. For a nonlinear load (CFL, LED lamp) the THD value is much more as compared to the measured THD value for a linear load (incandescent lamp).
Mathematically, the THD is obtained as in equation 1.
1 2
2 1 2
2
V V THD
I I THD
n n
V
n n I
The subscript 1 is for the fundamental component and rest corresponds to the various harmonic components.
The operation of customer’s sensitive loads which have nonlinear behavior, produce current harmonics during their operations. The current harmonics in addition to the network disturbances generate different distortions in the power supply voltage waveform at the consumer’s POC and their by cause poor PQ of the electric supply. The domestic consumers use diverse susceptible electronic devices i.e. Personal computer (PC), digital video recorder (DVD), micro-oven, digital LCDs, LEDs etc. These devices are susceptible and responsive to various PQ disturbances. All these domestic appliances have nonlinear current features that generate current harmonics in the network and distort the supply voltage waveform in addition to the network disturbances. The wide use of energy efficient lamps, such as compact fluorescent lamp (CFL) with electronic ballast, is generally promoted as it promotes usage of lesser active power when
… Eqn 1
31
compared to the conventional incandescent lamps. The drawback with the CFL/LED is that they tend to produce the reactive power and harmonic distortions much more than the incandescent lamps.
Circuit Diagram:
Procedure to develop a VI
1. Acquire real time data from available physical channels using DAQ Assistant
Path: Measurement I/O NI-DAQmx DAQ Assist.
2. Split the acquired signal at different nodes for measurement.
Path: Express Sig Manip Split Signals
3. Scale the acquired signal to its rated value
Path: Electric power Power Quality Scaling & calibration Scaling
4. Create graph indicator at each nodes
5. Add indicator at each nodes to measure RMS voltage, current, THD &
harmonic component
Path: Signal Processing Wfm Measure Basic DC/RMS Harmonic Analyzer 5. Add a delay as per required
32 Path: Programming Timing Wait (ms) 6. Put the developed VI in a while loop
Path: Programming Structure While loop
Block Diagram:
Observation
Fig 3(c)
Fig 3: Front panel in LabVIEW for (a) incandescent lamp; (b) CFL; (c) LED lamp
33
Conclusion:
Report:
1. What is virtual instrumentation? Differentiate between vendor and user defined instruments
2. Explain (a) THD (b) Harmonics, under harmonics and inter harmonics.
3. Discuss the effects of harmonics on electrical power components
4. Explain how commercial and industrial loads are responsible for harmonic distortion.
