What is measurement? Explain measurement system with appropriate block diagram. (AKTU 2021, AU 2022, PTU 2019)
The process of measuring a quantity is known as measurement and the apparatus used to measure the quantities like voltage, current, power, energy, resistance and so on are called measuring instrument. The quantity to be measured using the measuring instrument is called measurand. The measurement of a measurand is the result of comparison between the unknown quantity to be measured and the standard quantity.
Mention the basic requirements of measurement. (HPTU 2016)
When the instrument is used in the circuit, its conditions should not be altered and therefore the quantity to be measured goes unaffected.
It should consume low power.
It should possess very high efficiency and high sensitivity.
The output should be linearly proportional to the input.
It should be less affected by the noise, modifiable and properly priced.
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List the functional elements of a measuring instrument. (AU 2023)
Primary Sensing Unit. The first unit in the measurement system, which detects the measurand, is known as the primary sensing unit. It helps in transferring the measurand to a variable-conversion unit for further processing. For example, liquid or mercury in glass thermometer acts as a primary sensing unit. Displacement or voltage is the output of the primary sensing unit.
Variable-conversion Unit. The conversion of primary sensing unit output to more suitable variables while preserving the information is achieved with the help of this unit.
Variable-manipulation Unit. It is the intermediate stage of the measuring system, where the numerical value of the signal gets modified. It helps in improving the output quality of the measurement system by removing the random signals like noise.
Data-Transmission Unit. When the functional units of the measuring system are spatially separated, the data-transmission unit acts as a communication link for transmitting the signals from one unit to another. Some of the common data-transmission units used are cables, wireless antennae, transducers, telemetry systems and so on.
Data Processing Unit. This unit is used to process the data obtained from either the variable-manipulation unit or data-transmission unit and to produce suitable output to be presented to the experimenter. In addition, it is used to compare the measured value with the standard value to produce the required output.
Data Presentation Unit. This unit is used for communicating the measured quantity to the experimenter, which could be used for controlling and analysing purposes.
Differentiate between Accuracy and Precision. (AKTU 2021, AU 2023, BPUT 2020, 2022, GTU 2022, PTU 2019)
Accuracy: Accuracy is a closeness with which the instrument reading approaches the true value of the variable under measurement. Accuracy is the degree to which instrument reading match the true or accepted values. It indicates the ability of the instrument to indicate the true value of the quantity. Accuracy refers to how closely the measured value of a quantity corresponds to its “true” value.
Precision: Precision is a measure of the reproducibility of the measurement, i.e., its measure of the degree to which successive measurements differ from one other. It is the degree of agreement within a group of measurements or instruments. For example, if any resistance has true value 3.385,695 Ω, it always read 3.4 MW in scale reading.
Discuss the different types of characteristics of instrument. (UTU 2023)
Resolution: Resolution is the smallest amount of input signal change that the instrument can detect reliably. If the input is slowly increased from some arbitrary input value, it will again be found that output does not change at all until a certain increment is exceeded. This increment is called resolution or discrimination of the instrument. Thus, the smallest increment in input which can be detected with certainty by an instrument is its resolution or discrimination.
Sensitivity: It is defined as the ratio of the change in the output of an instrument to a change in the quantity to be measured. Mathematically, it is expressed as,
Sensitivity = Change in output/Change in input
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Define following term. (GTU 2021, 2022)
(i) Drift
(ii) Reproducibility
(iii) Threshold
(iv) Fidelity
(vi) Linearity
Drift: It is the measure of deviation in the instrument output for a particular period. If an instrument has no drift, then it has the capability of producing the same reading at different times when there is a variation in the measured variable.
Reproducibility: It is the degree of closeness with which a given value may be repeatedly measured using the same instrument, under different conditions like changes in the method of measurement, observer, measuring instrument location, conditions of use and time of measurement. It is specified in terms of scale readings over a given period of time.
Threshold: The minimum value of input quantity required to change the output reading from zero is known as threshold. It is defined as the minimum value below which there exists no output signal or smallest measurable input.
Fidelity: It is the capability of the instrument to reproduce the output in the same form as the input is known as fidelity. It is also defined as the degree to which an instrument indicates a change in the input quantity without any dynamic error.
Linearity. It is normally desired that the output reading of the instrument is linearly proportional to the quantity being measured. An instrument is considered linear if the relationship between output and input can be fitted in a line.
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Explain the various standards of measurements. (HPTU 2016)
Primary standards
These are absolute standards having the highest degree of accuracy and are used as the ultimate reference standards.
These are maintained by National Standard Laboratories in different parts of the world.
They are independently calibrated by absolute methods at each of the national laboratory periodically.
These are not available for use outside the national laboratories.
They are used in the calibration of secondary standards.
These are highly stable over long periods of time.
Primary standards are very few.
Secondary Standards
There are reference standards in use in industrial and day-to-day works.
These are periodically calibrated and compared against primary standards. National Laboratories issue a certificate in regard to their measured values in terms of primary standards.
Responsibility for maintenance and calibration of these standards lies with the particular industry involved.
Working Standards
Working standards are the principle tools of a measurement laboratory. They are used to checking laboratory instruments for accuracy and performance. These standards are used to perform comparison measurements in industrial application. For example, manufacturers of components such as capacitors, resistors etc. use a standard called a working standard for checking the component values being manufactured, e.g. a standard capacitor for checking of capacitance value manufactured.
What is mean by working standards? (AU 2021)
Already explained.
Differentiate primary and secondary type of measuring instruments. (AKTU 2023)
Already explained.
Describe the IEEE Standards. (BPUT 2022)
A slightly different type of standard is published and maintained by the Institute of Electrical and Electronics Engineers, IEEE, an engineering society headquartered in New York. These standards are not physical items that are available for comparison and checking of secondary standards but are standard procedures, nomenclature, definitions etc. These standards have been kept updated.
A large group of the IEEE standards is the standard test methods for testing, and evaluating various electronics systems and components. As an example, there is a standard method for testing and evaluating attenuators.
What are the types of errors in measuring instruments? (JNTUH 2023)
Gross errors
Systematic errors
Random errors
Define absolute and relative errors. (AU 2021)
Absolute Error
Measurement is the process of comparing an unknown quantity with an accepted standard quantity.
Absolute error may be defined as the difference between the measured value of the variable and the true value of the variable.
δA = Am – A
where
δA = absolute error
Am = expected value
A = measured value
Relative error
The relative error is the ratio of absolute error to the true value of the quantity to be measured.
Mathematically, the relative error can be expresses as,
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What are random errors? (PTU 2019)
Experimental results show variation from one reading to another, even after all systematic errors have been accounted for. These errors are due to the multitude of small factors, which change or fluctuate from one measurement to another, and are due surely to chance. The happening or disturbances about which we are unaware are called RANDOM and the errors caused by these happening are called RANDOM errors.
What is environmental error? (PTU 2019)
These are due to the measuring device, including conditions surrounding the instrument. These may include temperature, pressure, humidity, dust etc.
Environmental errors may be avoided by
Using the proper correction factor and information supplied by the manufacturer of the instrument.
Using the arrangement which will keep the surrounding condition constant like use of air condition, temperature controlled enclosures etc.
Making the new calibration under the local conditions.
Measurement of Voltage and Current
What is the difference between gravity control and spring control? (BPUT 2022)
Spring Control
Two helical springs of rectangular cross-sections are connected to the spindle of the moving system, as shown in following figure. With the movement of the pointer, the springs get twisted in the opposite direction, which affects the moving system.
The inner end of the spring is attached to the spindle while the outer end is attached to a lever or arm which is actuated by a set of screw mounted at the front of the instrument. So zero setting can be easily done. The controlling torque provided by the instrument is directly proportional to the angular deflection of the pointer.
Gravity Control
In this method, small weights which can be adjusted are added to the moving system as shown in following figure.
When the pointer deflects, this weight also takes a deflected position. The required controlling torque is produced by the gravitational force, which is acting on the moving weight.
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Distinguish between air friction damping and fluid friction damping. (AU 2024)
Air-Friction Damping
Following figure shows the arrangement where a piston, attached to the spindle of the moving system, moves inside the air chamber provided with a very small clearance between the piston and the chamber.
When the deflecting torque acts on the moving system, the suction and compression actions on the air inside the air chamber produce the necessary damping torque.
Fluid Friction Damping
It is like air friction damping with fluid instead of air.
Differentiate between null type and deflection type of measuring instruments along with suitable example. (AKTU 2022)
A deflection type of instrument must be larger, more rugged, and thus less sensitive if it is to measure a large magnitude of an unknown quantity.
Deflection types of instruments have a faster response than null type instruments.
The accuracy of null-type instruments is higher than that of deflection type. This is because the opposing effect is calibrated with the help of standards that have a high degree of accuracy.
What are the advantages of M. I instrument? (JNTUH 2023)
The instruments can be used for both a.c. and d.c. measurements.
It can be used in low frequency and high-power circuits.
Errors due to the friction are very less.
A single type of moving element can cover the wide range.
There are no current carrying parts in the moving system hence reliable.
Give good accuracy.
These can withstand large loads and are not damaged even under severe overload conditions.
The range of instruments can be extended.
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Can Moving Iron instruments be used for measurement of DC currents? Justify it. (BPUT 2020)
Moving Iron (MI) instruments are capable of measuring both Direct Current (DC) and Alternating Current (AC) quantities because of their operating principle. They work on the principle of magnetic effects of electric current, which is applicable to both DC and AC. The moving iron is attracted towards the fixed coil when current flows through it, regardless of the direction of the current. Therefore, these instruments can measure both DC and AC quantities.
List the possible causes of errors in moving iron instruments. (AU 2023)
Hysteresis error: Due to hysteresis effect, the flux density for the same current while ascending and descending values is different. While descending, the flux density is higher and while ascending it is lesser. We should use smaller iron parts which can demagnetize quickly or to work with lower flux densities.
Temperature error: The temperature error arises due to the effect of temperature on the temperature coefficient of the spring. The coil and series resistance must have low temperature coefficient.
Stray magnetic field error: The operating magnetic field in case of moving iron instruments is very low. Hence, effect of external i.e. stray magnetic field can cause error.
Frequency error: These are related to a.c. operation of the instrument. The change in frequency affects the reactance of the working coil and also affects the magnitude of the eddy currents.
Eddy current error: When the instrument is used for a.c. measurements, the eddy currents are produced in the iron parts of the instrument. The eddy current affects the instrument current, causing the change in the deflecting torque. This produces the error in the meter reading. As eddy currents are frequency dependent.
Why PMMC instrument cannot be used for AC measurement? (GTU 2021)
When the polarity is reversed, a torque is produced in the opposite direction. The mechanical stopper does not allow the deflection in the opposite direction. Therefore, the polarity should be maintained with PMMC instrument. If A.C. is supplied, a reversing torque is produced. This cannot produce a continuous deflection. Therefore, this instrument cannot be used in A.C.
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State the advantages of PMMC instruments. (HPTU 2016)
Scale is uniformly divided.
Very low power consumption (25μw to 200μw).
Torque weight ratio is high, giving high frequency.
Voltage range can be changed merely by connecting a suitable series multiplier.
Errors due to stray magnetic fields are small.
Useful in aircraft and aerospace applications due to self shielding magnets.
How the current range of PMMC instrument extended with the help of shunts? (GTU 2021)
The moving coil instruments can carry maximum current of about 50 mA safely and the potential drop across the moving coil is about 50 mV. However, in practice, heavy currents and voltages are required to be measured. Therefore, it becomes necessary that the current and voltage being measured be reduced and brought within the range of instruments.
Shunts are used for the extension of range of ammeters. A shunt is a resistance of small value, just like a strip having minimum temperature coefficient. It is always connected in parallel with the ammeter whose range is to be extended. The combination is connected in series with the circuit whose current is to be measured.
The current range of a DC moving coil ammeter is extended by connecting a shunt RS (low resistance) across the coil, the circuit is shown in the figure.
Rs = Rm/(N - 1)
Hence, for measurement of current N times, the current range of instruments, the shunt resistance should be 1/(N -1) times the meter resistance.
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Define sensitivity of voltmeter. (AKTU 2020)
When selecting a meter for certain voltage measurement, the sensitivity of DC voltmeter is an important factor. It is defined as the total resistance of the instrument per unit volt. The total resistance of the voltmeter is the sum of multiplier resistance and coil resistance.
What is the difference between an ammeter and a voltmeter? (AKTU 2021)
In an ammeter, the deflecting torque is produced by the current to be measured or by a definite fraction of it, whereas in a voltmeter, torque is produced by the current proportional to the voltage to be measured.
Thus, the real difference between the two instruments is in the magnitude of the current producing the deflecting torque.
An ammeter is connected in series with the circuit whose current is to be measured. Therefore, it should have a low resistance.
On the other hand, a voltmeter is connected in parallel with the circuit whose voltage is to be measured, and therefore, it must have high resistance.
So we see that the difference is only in the resistance of the instrument; in fact, an ammeter can be converted into voltmeter by connecting a high resistance in series with it. Similarly, a voltmeter can be converted into an ammeter by connecting a shunt across the voltmeter.
Why a voltmeter should have a high resistance value? (BPUT 2020)
A voltmeter is connected in parallel with the circuit whose voltage is to be measured, and therefore, it must have high resistance.
A PMMC meter has an internal resistance 200 Ω and the current required for its full scale deflection is 50 μA. What will be the maximum measuring voltage of the meter. (BPUT 2022)
Current sensitivity is given by
S = 1/If = Rint/Vm
Where
S = Sensitivity of ammeter.
If = Full scale deflection of an ammeter.
Rint = Internal resistance of an ammeter
Vm = Full range voltage.
Given Rint = 200 Ω, If = 50 μA
S = 1/(50 x 10-6) = 20,000 Ω/V
Vm = Rint/S = 200/20,000 = 10 mV
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Potentiometers
Define the term Standardization of a potentiometer. (AKTU 2020, BPUT 2020)
Standardisation of a potentiometer is a process of adjusting the working current supplied by the supply battery such that the voltage drop across a portion of sliding wire matches with the standard reference source.
After standardising a potentiometer, it is used as direct reading potentiometer as the voltage along the slide wire at any point is proportional to the length of the slide wire where the point is obtained by moving sliding contact along the wire to get null deflection in the galvanometer for any battery whose e.m.f. is to be measured.
What is meant by DC potentiometer and AC potentiometer and its applications? (PTU 2020, HPTU 2026)
In a d.c. potentiometer, the balance between voltage drop across the slide wire and the magnitude of unknown voltage is obtained. While in an a.c. potentiometer, the two voltages should be balanced in magnitude as well as phase.
Applications of DC potentiometer
The main application of d.c. potentiometer is measurement of voltage. But it may be also used for measurement of resistance, current and power The d.c. potentiometer is also useful in the calibration of voltmeters, ammeters, watt meters etc.
Calibration of Voltmeter
Calibration of Watt meter
Calibration of Ammeter
Measurement of Resistance
Measurement of Power
Applications of AC potentiometer
Watt meter and energy-meter Testing
Measurement of Self Reactance of a Coil
Calibration of voltmeter
Calibration of ammeter
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Measurement of Power and Energy
State Blondel's Theorem. (RTU 2016, 2017)
French electrical engineer Andre Blondel has said in his theorem that we must have at least (N — 1) number of Wattmeters to measure the power consumed by an N-phase system. This is known a Blondel's theorem.
According to the theorem, if we need to measure power consumed by a three phase circuit then we must have at least two (3 – 1 = 2) number of Wattmeters.
Two wattmeter method used to measure power in 3-phase balanced system, the readings of two wattmeters given same, what is the power factor of the load? (JNTUH 2023)
Two watt-meters used to measure three-phase power reads equal reading at unity power factor.
\(\begin{array}{l}{W_1} = \sqrt 3 {V_p}{I_p}\cos (30 - \phi )\\{W_2} = \sqrt 3 {V_p}{I_p}\cos (30 + \phi )\\For\phi = {0^0}\\\cos \phi = pf = 1\\{W_1} = \sqrt 3 {V_p}{I_p}\cos ({30^0}) = \frac{3}{2}{V_p}{I_p}\\{W_2} = \sqrt 3 {V_p}{Z_p}\cos ({30^0}) = \frac{3}{2}{V_p}{I_p}\end{array}\)
Hence, two watt-meters used to measure three-phase power reads equal reading at unity power factor.
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Define Meter Constant of single phase energy meter. (AKTU 2020)
Meter constant = number of revolutions/kWh
State, various errors present in 3-phase energy meter. (JNTUH 2023)
Some errors that can occur in a 3-phase energy meter include:
Phase angle error: The correct reading for the energy consumed is possible only if the phase difference between φsh and supply voltage is 90°. But due to the presence of winding resistances and iron losses, the phase angle error exists in the system. It can be rectified by adjusting the position of the copper band placed in the shunt magnet.
Speed error: The speed error exists if the disc rotates either slower or faster, which results in wrong reading of energy consumption. It can be eliminated by adjusting the position of the brake magnet.
Friction error: The error caused in the frictional forces existing at the bearings of moving system and in registering system at light loads is called friction error. This can be reduced by making the ratio of the φsh and φse large by using two shading rings in the outer limbs of the shunt magnet. Due to this strip an additional torque independent of load is produced which acts on the disc in the direction of rotation. This compensates for the friction and the meter can be made to read accurately. This is shown as L2 in the figure.
Creeping: It is the process in which the disc rotates slowly but continuously when no load is connected to the system and when the pressure coil is energised by the supply. It is due to excessive supply voltage, vibration, stray magnetic field, etc. It is prevented by drilling two holes or slots in the disc.
What is the creep error in a single phase induction type energy meter. (BPUT 2022)
It is the process in which the disc rotates slowly but continuously when no load is connected to the system and when the pressure coil is energised by the supply. It is due to excessive supply voltage, vibration, stray magnetic field, etc. It is prevented by drilling two holes or slots in the disc.
These holes lie on the opposite side of the spindle, causing the required distortion in the field and helps in making the disc stationary.
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Measurements of RLC Parameters
What are the applications of bridge circuits? (AKTU 2021)
Bridges are used to measure the values of the electronic components. For example, a Wheatstone Bridge is used to measure the unknown resistance of a resistor. However, they are also used to measure the unknown inductance, capacitance, admittance, conductance or any of the impedance parameters.
Besides this, bridge circuits are also used for the precision measurements in some circuits and for the interfacing of transducers. Actually, nowadays, fully automatic bridges which electronically null a bridge to make precision measurements are also used.
State different methods used to measure low, medium and high resistance. (AKTU 2020, GTU 2021)
Measurement of Low Resistance
Ammeter Voltmeter Method
Potentiometer Method
Kelvin Bridge Method
Kelvin Double Bridge Method
Measurement of medium resistance
Ammeter-Voltmeter Method
Substitution Method
Wheatstone Bridge Method
Carey-Foster slide-wire Bridge Method
Measurement of high resistance
Direct Deflection Method
Loss of Charge Method
Megohm Bridge
Megger
Give the uses of D.C. bridges. (JNTUH 2022)
Resistance Measurement
Calibration of Instruments
Temperature Measurement:
Strain Gauge Measurements
Testing of Electrical Components
Bridge Circuit Analysis
Detection of Faults
Discuss the difference between AC and DC Bridges. (UTU 2023)
What are the sources of errors in bridge circuits? (HPTU 2016)
Stray-conductance effects, due to imperfect insulation ;
Mutual-inductance effects, due to magnetic coupling between various components of the bridge ;
Stray-capacitance effects, due to electrostatic fields between conductors at different potentials ;
'Residues' in components - e.g., the existence of a small amount of series, inductance or shunt capacitance in nominally non-reactive resistors.
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State the condition for balance in a Wheatstone bridge. (AU 2023)
See following figure.
R1R4 = R2R3
Enlist the errors found in Wheatstone bridge. (AKTU 2022)
The Wheatstone bridge is widely used for precision measurement of resistance from approximately 1Ω to the low mega ohm range.
Measurement errors are given below:
The main source of measurement error is found in the limiting errors of three known resistors
Insufficient sensitivity of the null detector
Changes in resistance of the bridge arms due to the heating effect of the current through the resistors
Thermal e.m.f. in the bridge circuit or the galvanometer circuit can also cause problems when low-value resistors are being measured.
Discuss the limitations of Wheatstone bridge. (BPUT 2022, GTU 2023, HPTU 2021)
The effect of lead resistance and contact resistance is very much significant while measuring low resistances.
The bridge cannot be used for high resistance measurement, i.e. measurement in high mega ohm range. This is because while in such measurement, the resistance presented by the bridge becomes so large that the galvanometer becomes insensitive to show any imbalance.
Heating effect due to large current also plays a major role. The excessive currents may generate heat which may cause the permanent change in the resistance.
The resistance used must be very precise having tolerance up to 1 % or 0.1 %, hence cost is high.
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What conditions must be satisfied to make an AC bridge balanced? (AKTU 2020, AU 2022)
The two conditions for balance in an AC bridge circuit are:
Voltage Balance: The ratio of the voltages across the two branches of the bridge must be equal. This ensures that the voltage at the output of the bridge is zero.
Phase Balance: The currents in the two branches of the bridge must be in phase with each other. This ensures that the reactive components, such as capacitors and inductors, do not contribute to the output voltage.
Specify the purpose of Wagner Earthing device. (AU 2024)
This device is often used to eliminate errors due to electrostatic coupling.
Arm AB of Maxwell’s bridge comprises a 700 Ω resistor, CD has a 300 Ω resistor. In arm AD, a 1.2 kΩ resistor is in parallel with a 0.525 pf capacitor. Determine unknown inductance and resistance. (AU 2021)
See following figure.
Draw and explain circuit diagram of Maxwell’s bridge. (GTU 2022, 2023)
See figure.
Rx = R2R3/R1
Lx = R2R3C1
Quality factor of coil
Q = ωC1R1
How Hay’s Bridge is suitable for the measurement of inductance of large Q coils. (AKTU 2023, GTU 2022)
The Hay bridge will not give correct results for Q < 10 because, at balance
for Q > 10, Lx = R2R3C because 1/Q2 can be neglected compared to 1.
If Q is small, this equation will not be valid.
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State the advantage of Hay's bridge over Maxwell's Inductance-Capacitance bridge. (BPUT 2022)
Hay's bridge circuit can be used for high Q coils with a Q factor > 10.
Draw the circuit for Anderson’s bridge. (JNTUH 2023, AU 2023, GTU 2021)
This method is one of the commonest and best bridge method for precise measurement of inductance over a wind range. In this method, the unknown self-inductance is measured in terms of a known capacitance and resistance by comparison. It is actually a modification of the Maxwell‑Wien bridge and is an example of a more complicated bridge network.
Write advantages and disadvantages of Anderson’s bridge for the measurement of inductance? (AKTU 2023, AU 2022)
Advantages
In case adjustments are carried out by manipulating control over rx and r, they become independent of each other. This is a marked superiority over sliding balance conditions met with low Q coils when measuring with Maxwell's bridge. A study of convergence conditions would reveal that it is much easier to obtain balance in the case of Anderson's bridge than in Maxwell's bridge for low Q-coils.
A fixed capacitor can be used instead of a variable capacitor, as in the case of Maxwell's bridge.
This bridge may be used for accurate determination of capacitance in terms of inductance.
Disadvantages
The Anderson's bridge is more complex than its prototype Maxwell's bridge. The Anderson's bridge has more parts and is more complicated to set up and manipulate. The balance equations are not simple and in fact are much more tedious.
An additional junction point increases the difficulty of shielding the bridge.
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Draw circuit of Owen’s bridge. Write its applications. (GTU 2023)
This bridge is used to measure inductance in terms of capacitance. The main application of the Owen's bridge is to measure incremental inductance.
L1 = R2R3C4
R1 = R3C4/C2
Explain the disadvantage of De Sauty Bridge and how this disadvantage is overcome by modified De Sauty Bridge? (JNTUK 2020)
The De Sauty's bridge is used to measure the unknown capacitance by comparing it with two capacitances. The bridge provides reasonable accuracy for measuring capacitance over an extensive range. It is the simplest AC bridge circuit.
The De Sauty Bridge is simple and easy to use but provides inaccurate results for capacitors with dielectric losses.
If we neglect the dielectric loss in the bridge circuit, then this bridge provides us with the most appropriate method to compare two values of capacitors.
C1 = C2R4/R3
The following figure shows the circuit diagram of modified De-Sauty’s bridge.
It shows that electrical resistances r1 and r2 were introduced on the 1-2 and 4-1 arms, respectively, for including the dielectric losses and resistors R1 and R2 are also connected across those arms.
\(\frac{{{C_1}}}{{{C_2}}} = \frac{{{R_2} + {r_2}}}{{{R_1} + {r_1}}} = {R_4}{R_3}\)
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What are the applications of Scheming Bridge? (JNTUH 2023)
Scheming bridge is particularly useful for measuring the capacitance, associated dissipation factors, and the loss angles.
Explain any one method for measurement of high resistance. (GTU 2021)
Loss of Charge Method
In this method, the insulation resistance (high resistance) to be measured is connected in parallel with a capacitor and electrostatic voltmeter, as shown in the figure. The capacitor is charged by means of a battery to a suitable voltage and is then allowed to discharge through the insulation resistance and the change in terminal voltage across the capacitor during discharge is observed for a considerable period.
This method is used for measuring the insulation resistance of very high value where C is a known capacitance, V is an electrostatic voltmeter, and Rleak is the total leakage resistance of the capacitor and voltmeter. Rx is the unknown resistance to be measured.
\[(frac{1}{{{R_x}}} = \frac{1}{{{R_{eff}}}} + \frac{1}{{{R_{leak}}}}\)
where Reff = Effective/equivalent resistance of Rx and Rleak connected in parallel.
Discuss the loss of charge method for high resistance measurement. (AKTU 2021, GTU 2023, RGPV 2020)
Already described.
What are difficulties for the measurement of high resistance? (HPTU 2016, JNTUK 2022)
The insulation resistance of the resistor may be comparable with the actual value of the resistor. Thus, leakage currents are produced. These leakage currents are of comparable magnitude to the current being measured and must be eliminated from the measurement.
Due to electrostatic effect, stray changes can appear in the measuring circuit, causing errors. Alternating fields can also affect the measurements considerably.
In measurement of insulation resistance, the specimen often has considerable capacitance. On application of a direct voltage, a large charging current flows initially, which gradually decays down after a short interval. Further, insulating materials possess the property of dielectric absorption, i.e., after the main charging current has decayed down, further charge is slowly absorbed over a considerable period of time, perhaps for minutes or even hours. Thus, measurement of true conduction current should be delayed until after the cessation of the charging and absorbing currents.
Resistance of low conductors changes drastically with temperature and humidity.
Fairly high voltages are used in tests in order to raise the currents to reasonable values in order to be measured. So normally a sensitive galvanometer or micro-ammeter is required and adequate steps have to be taken to prevent damage to these delicate instruments.
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Explain the working principle of LCR meter. (GTU 2022, PTU 2019)
These meters are used to measure values of inductance (L), capacitance (C) and resistance (R) directly. LCR meters are provided with switching arrangement for the measurement of L, C and R.
The following figure illustrates a typical LCR meter with a block diagram and important controls on the face panel of the meter. It consists of a permanent magnet moving coil (PMMC) instrument. The scale is calibrated in terms of R, L and C.
When the component under test (R, L or C) is connected across the "test terminals" of the meter, the pointer of the meter indicates the value of the component directly.
Basically, the meter is an electronic instrument containing various electronic components fabricated on a PCB (printed circuit board) which needs D.C. supply. For this purpose, the meter also has a step down transformer and a rectifier. However, a sinusoidal single-phase supply is given to the meter from outside.
Working
The meter is connected to the supply and ON/OFF switch is made 'ON'.
The glow of the indicator confirms that the meter is properly connected with the supply.
Now the position of function selector is adjusted to L, C or R depending upon the component to be tested. Also, a suitable range is selected by a range selector.
The unknown component is connected across the test terminals of the meter and the value is read on the scale.
What is a Q-meter? Draw the diagram of Q meter. (BPUT 2020, AU 2021)
The Q-meter is an instrument which is designed to measure of the electrical properties of the coils and capacitors by measuring the Q-value of an R-L-C circuit.
Working Principle
The Q meter is based on the characteristics of a series circuit. The series resonant has characteristics the voltage across the coil or capacitor is equal to the applied voltage times the Q factor of the circuit.
The Q-factor is called quality factor or storage factor. It is defined as the ratio of power stored in the element to the power dissipated in the element.
If a fixed voltage is applied to the circuit, a voltmeter across the capacitor can be calibrated to read Q directly. The voltage and current relationships of the series-resonant circuit are shown in the following figure.
We get the value of Q from the following equation
\(Q = \frac{{{X_L}}}{R} = \frac{{{X_C}}}{R} = \frac{{{E_C}}}{E}\)
Where,
EC = voltage across the capacitor
XC = capacitive reactance
XL = capacitive reactance
R = coil resistance
In a Q meter, at 1.5 MHz, C1 = 550 pf. At 3 MHz, C2 = 110 pf. Determine the unknown values of self-capacitance and inductance. (AU 2022)
Hint:
\(\frac{{{f_2}}}{{{f_1}}} = \sqrt {\frac{{{C_1} + {C_d}}}{{{C_2} + {C_d}}}} \)
Instrument Transformers
Write applications/features of Instrument Transformers. (AKTU 2020, 2023, HPTU 2026, PTU 2020)
To measure the high value of electric current i.e. Current Transformer (CT)
For measure high value of electric voltages or potential difference i.e. Potential Transformer (PT)
To measure electric power with uses of both CT and PT
What is meant by burden of current transformer? (AKTU 2020, 2021, 2022)
The nominal ratio of an instrument transformer, does not remain constant in practice as the load on the secondary changes. It changes because of effect of secondary current, power factor and magnetizing as well as core loss components of current and this causes errors in the measurements. For the particular class of transformers, the specific loading at rated secondary winding voltage is specified such that the errors do not exceed the limits. Such a permissible load is called burden of an instrument transformer.
Thus, the permissible load across the secondary winding expressed in volt-amperes at the rated secondary winding voltage or current, such that errors do not exceed the limits is called burden of an instrument transformer.
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Differentiate between CT and PT. (AKTU 2021, JNTUH 2020, 2021, 2023)
Define Nominal ratio and Transformation ratio for CT and PT. (AKTU 2022, 2023)
Transformation Ratio or Actual Ratio (R)
It is the ratio of the magnitude of the primary phasor to the secondary phasor.
It is the ratio of rated primary winding current (or voltage) to the rated secondary winding current (or voltage).
Why the secondary of a CT is never left open circuited? (GTU 2023, BPUT 2020)
The following are the reasons for secondary of CT to be closed (i.e. not opened):
If the secondary of CT is open, the current through the secondary winding, I2 = 0. Therefore, the counter mmf produced by the secondary ampere turns is zero. Due to this, the mmf produced in the primary winding is undisturbed and produces more flux with high magnitude in the core. Therefore, there will be more core losses and heat produced in the core will cross its limits.
Also, the emf induced in the primary and secondary windings will be more and hence the insulations gets damaged.
Therefore, the secondary of CT should be either grounded or connected to a low resistance coil like ammeter and wattmeter while energising the primary winding.
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Magnetic Measurements
Why in a D’Arsonval galvanometer, an iron core is usually used between the permanent magnet pole faces? (BPUT 2020):
Flux density in the air gap becomes high, thereby a large deflecting torque is produced.
Advantages of placing iron core between the permanent magnet pole faces:
Provides the uniform air gap between the coil sides and pole faces.
Reduces the gap between coil sides and pole faces of the coil. Therefore, it increases the magnetic linkage with the coil.
The sensitivity of the D’Arsonval galvanometer increases.
The deflection torque of the coil is directly proportional to the current through it. Therefore, we can obtain a uniform scale of the instrument.
Explain the dynamic behaviour of Galvanometer. (HPTU 2016)
Answer: When we pass current through a galvanometer it does not reach its steady state deflection immediately, but there is a time interval or period of transition during which the moving system of the galvanometer deflects
from its initial position to the final steady state position. The dynamic behaviour of the galvanometer during this period is examined by the equation of motion.
This equation helps us to study the problems relating to speed of response, overshoot and damping.
This equation is
J(d2θ/dt2) + D(dθ/dt) + Kθ = Gi
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Electronic Measurements
Explain the advantages of digital instruments over their analog counterpart. (AKTU 2021, AU 2022)
easy readability
greater accuracy
better resolution
automatic polarity and zeroing
What are the advantages of electronics voltmeters in comparison to electromechanical type voltmeters? (BPUT 2020)
Analog multimeter
No power supply required.
No use of electronic components such as diodes, transistors etc.
Suffer less from electric noise.
The isolation problems are less.
The accuracy is less.
The input impedance is less.
Possibility of ambiguous reading, which is invisible from distance.
The output cannot be interfaced with external devices.
Size is more and bulky.
Simple in construction.
Digital multimeter
Power supply is required.
It uses electronic components such as diodes, transistors etc.
Suffer more from electric noise.
The isolation problems are more.
The accuracy is more.
The input impedance is high.
The reading is unambiguous at greater viewing distance.
The output can be easily interfaced with external devices.
It is compact and light in weight.
Construction is complicated but, due to modern integrated technology, becoming simple from production point of view.
Mention any two applications where vector meter is used for measurement. (AU 2022)
A vector voltmeter measures the amplitude of a signal at two points in a circuit and simultaneously measures the phase difference between the voltage waveforms at these two points. This instrument can be used in a wide variety of applications, especially in situations where other methods are very difficult or time-consuming. The vector voltmeter is useful in VHF applications and can be used successfully in such measurements as
Insertion Losses
Complex impedance of mixers
S parameters of transistors
Radio frequency distortion
Amplitude modulation index
Amplifier gain and phase shift
Filter transfer functions
Two port network parameters
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Write principal of operation of digital frequency meter. (AKTU 2022, AU 2022, GTU 2023)
The signal whose frequency is to be measured is converted into a train of pulses, one pulse for each cycle of the signal. Then the number of pulses appearing in a definite interval of time is counted by means of an electronic counter. Since the pulses represent the cycles of the unknown signal, the number appearing on the
counter is a direct indication of the frequency of the unknown signal. Since the electronic counters are extremely fast, the frequency of high frequency signals may be known.
The block diagram of the basic circuit of a digital frequency meter is shown below.
What are the errors in measurements with frequency counters? Explain any one of them. (GTU 2023)
Gating Error
Time-base Error
Trigger Level Error
Gating Error
Frequency and time measurements made by an electronic counter are subject to several inaccuracies inherent in the instrument itself. One very common instrumental error is the gating error, which whenever frequency and period measurements are made. For frequency measurement, the main gate is opened and closed by the oscillator output pulse. This allows the input signal to pass through the gate and be counted by the decade counters. The gating pulse is not synchronized with the input signal; they are, in fact, two totally unrelated signals.
In the figure, the gating interval is indicated by waveform (c). Waveforms (a) and (b) represent the input signal in different phase relationships with respect to the gating signal.
Clearly, in one case, six pulses will be counted; in the other case, only five pulses are allowed to pass through the gate. We have therefore count ambiguity in the measurement.
Mention the various types of signal analyser. (JNTUH 2023)
Heterodyne Wave Analyser (Wave Meter)
Measurements in the megahertz range are usually done with another wave analyser that is particularly suited to the higher frequencies. The input signal to be analysed is heterodyned to a higher intermediate frequency (IF) by an internal local oscillator.
Heterodyne Harmonic Analyser or Wave Meter
The output of a variable-frequency oscillator is mixed (heterodyned) successfully with each harmonic of the input signal, and either the sum or the difference frequency is made equal to the frequency of the filter. Since now each harmonic frequency is converted to a constant frequency, it is possible to use highly selective filters of the quartz-crystal type.
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Draw the diagram of Q meter. (AU 2021, BPUT 2020)
The Q meter is an instrument designed to measure some of the electrical properties of coils and capacitors.
The operation of this useful laboratory in instrument is based on the familiar characteristics of a series-resonant circuit, namely, that the voltage across the coil or the capacitor is equal to the applied voltage times the Q of the circuit. If a fixed voltage is applied to the circuit, a voltmeter across the capacitor can be calibrated to read Q directly.
Q = XL/R = XC/R = EC/E
In a Q meter, at 1.5 MHz, C1 = 550 pf. At 3mHz, C2 = 110 pf. Determine the unknown values of self-capacitance and inductance. (AU 2022)
What are the objectives of DAS? (GTU 2023)
Data acquisition systems are used to measure and record signals obtained in basically two ways
signals originating from direct measurement of electrical quantities; these may include DC and AC voltages, frequency, or resistance.
signals originating from transducers, such as strain gauges and thermocouples.
What is the need of ADC in DAS? (AKTU 2021)
The objective of an analog-to-digital converter (ADC) is to convert analog signals into digital data:
Digital representation. ADCs convert analog signals into digital numbers that can be processed mathematically.
Speed. The conversion rate of an ADC determines how quickly it can convert an analog value to a digital value.
Accuracy. The resolution of an ADC determines how close the digital number is to the original analog value.
Oscilloscope
State various application of oscilloscope. (AKTU 2020, AU 2021, BPUT 2022)
Television: A CRT is used along with sweep circuits.
Radar: A CRT screen is used to provide visual displays.
To measure voltage or current, the drop across a resistor is measured.
To study waveforms: A visual display is seen.
Measurement of frequency and phase angle.
Write the application of sweep/base generator in CRO. (AKTU 2023, JNTUH 2023)
Oscilloscopes are generally used to display a waveform that varies as a function of time. If the waveform is to be accurately reproduced, the beam must have a constant horizontal velocity. Since the beam velocity is a function of the deflecting voltage, the deflecting voltage must increase linearly with time.
During the sweep time, Ts, the beam moves from left to right across the CRT screen. The beam is deflected to the right by the increasing amplitude of the ramp voltage and the fact that the positive voltage attracts the negative electrons. During the retrace time, or flyback time. Tr, the beam returns quickly to the left side of the screen.
We can change the sweep rate in steps by switching different capacitors into the circuit. The sweep rate can be adjusted in minor ways by making the resistor, R, in the following figure a variable resistor.
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Mention any four special oscilloscopes. (AU 2022)
Dual Beam Oscilloscope
Dual Trace Oscilloscope
Storage Oscilloscopes
Sampling Oscilloscope
What are the types of CRO Probes? (JNTUH 2023)
The different types of CRO probes arc as follows:
Direct probe.
High-impedance probe.
Detector probe.
High-voltage probe.
Briefly explain, Lissajous patterns on an oscilloscope. (BPUT 2020)
Lissajous patterns are formed when two sine waves are applied simultaneously to the vertical and horizontal deflecting plates of a CRO. The shape of the Lissajous pattern depends on the frequency and phase relationship of the two sine waves.
Draw Lissajous pattern with frequency ratio 2:1. (AKTU 2020)
Draw the Lissajous pattern for frequency ratio 3:2. (AKTU 2022, 2023)
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How far the Lissajous patterns are effective in frequency measurement. (AU 2024)
There are two practical limitations on the use of Lissajous patterns. One, of course, is the precision with which the oscillator is calibrated. In many instances, the oscillator frequency calibration is quite good, while in others it is poor. It is often recommended that the oscillator frequency be measured on a frequency counter, but if a counter is available, then the use of Lissajous patterns is an exercise in the absurd.
The other limitation involves the inability to lock the oscillator on the unknown. The Lissajous pattern will probably rotate at a frequency equal to the difference between the two frequencies. Trying to make the pattern stand still long enough to make a measurement is a good idea.
Describe the basic principle of CRT. (GTU 2023)
Answer: A CRT is a vacuum tube that displays images by directing an electron beam onto a phosphor-coated screen. This beam, when striking the phosphor, excites the phosphor atoms, causing them to emit light.
Transducers
Differentiate active and passive type of transducers with suitable example. (AKTU 2022, 2023)
This classification of transducers is based upon the methods of energy conversion used.
Active transducers
Self-generating type transducers i.e. the transducers, which develop their output in the form of electrical voltage or current without any auxiliary source, are called the active transducers. Such transducers draw energy from the system under measurement. Normally such transducers give very small output and, therefore, use of an amplifier becomes essential. Thermocouples used for measurement of temperature, piezoelectric crystal used for measurement of force fall in this category.
Passive transducers
Transducers, in which electrical parameters i.e. resistance, inductance or capacitance changes with the change in input signal, are called the passive transducers. These transducers require external power source for en
ergy conversion. In such transducers electrical parameters i.e. resistance, inductance or capacitance causes a change in voltage, current or frequency of the external power source. These transducers may draw some energy from the system under measurement.
Differentiate primary and secondary Transducers with the help of examples. (AKTU 2023)
This classification of transducers is based upon the methods of application.
Primary transducer. When input signal is directly sensed by the transducer and physical phenomenon is converted into the electrical form directly, then such a transducer is called the primary transducer. Thermistor, which senses the temperature directly and causes the change in resistance with the change in temperature, falls in this category.
Secondary transducer. When the input signal is sensed first by some detector or sensor and then its output, being of some form other than the input signal, is given as input to a transducer for conversion into electrical form, then such transducers fall in the category of secondary transducers.
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Write principle of operation of Thermocouple. (AKTU 2022, 2023)
A temperature transducer, which converts thermal energy into electrical energy, is known as a thermoelectric transducer. A thermocouple is the most commonly used thermoelectric transducer. A thermocouple, a type of primary transducer, is used for measuring temperature, where the change in temperature arising from two dissimilar metals is converted into an electrical energy. Thermoelectric phenomena like Seebeck effect, Peltier effect and Thompson effect are used to describe the thermocouple behaviour.
Two dissimilar metals, when joined together to form two junctions T1 and T2, form a thermocouple, as shown in the following figure.
Why reference junction compensation is necessary in thermocouple? (AKTU 2021)
The voltage produced at the reference junction depends on the temperatures at both the measurement junction and the reference junction. Since the thermocouple is a differential device rather than an absolute temperature measurement device, the reference junction temperature must be known to get an accurate absolute temperature reading. This process is known as reference junction compensation (cold junction compensation.)
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Explain the different principles of working of capacitive transducers for measurement of displacement. (GTU 2022)
The variable capacitive transducer comprises a capacitor, the capacitance of which varies with the non‑electrical quantity being measured.
As the capacitance of a capacitor depends upon the area of its electrodes, the distance between them and the permittivity of the dielectric material, hence such transducers can be used for measuring non‑electrical quantities affecting any one of the above-mentioned parameters.
Capacitive transducers are analog passive transducers. In such transducers, the capacitance of the capacitor is varied by any of the following methods :
By varying overlapping area of plates.
By varying distance between plates.
By varying permittivity of dielectric material between the plates.
Capacitive Transducer ‑ By Overlapping Area of Plates
Such a transducer operates on the fact that capacitance of any capacitor is proportional to the over lapping area of plates. Let us consider a capacitive transducer having parallel plates of constant width, as shown below.
In such a transducer, capacitance is proportional to the length where x is the length of overlapping parts of the plates and varies according to displacement which is to be measured. So any change in displacement causes a change in capacitance which is measured by a.c. bridge circuits.
This type of capacitance transducer is suitable for measurement of linear displacements.
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