VALLIAMMAI ENGINEERING COLLEGE SRM NAGAR, KATTANKULATHUR

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1 VALLIAMMAI ENGINEERING COLLEGE SRM NAGAR, KATTANKULATHUR DEPARTMENT OF ELECTRONICS AND INSTRUMENTATION ENGINEERING EI6301-ELECTRICAL MEASUREMENTS QUESTION BANK UNIT I MEASUREMNET OF VOLTAGE AND CURRENT PART A (2 MARKS) 1. A permanent magnet moving coil instrument has full scale deflection of 75μA and a coil resistance of 1KΩ. Calculate the required shunt resistance value to convert the instrument into an ammeter with full scale deflection of 75mA (B.T-3) 2. Give the torque equation for an electro dynamometer type wattmeter (B.T-2) 3. How do you define rectifier type of instruments? 4. Recommend the type of resistor connection to extend the range of voltmeter in PMMC instruments (B.T-5) 5. Interpret the term transfer instrument (B.T-2) 6. Identify the main sources of errors in PMMC instruments 7. List the methods of calibrating a ballistic galvanometer 8. How do you infer the term error? 9. Generalize on how the range of instruments can be extended in PMMC instruments (B.T-6) 10. Define calibration 11. List the basic requirements of any measuring instruments 12. Contrast the advantages and disadvantages of moving iron instruments 13. Examine why the scale in the MI instrument is non linear? (B.T-3) 14. Discuss few advantages of using D Arsonval galvanometer for measurement purpose (B.T-2) 15. Compare spring control and gravity control 16. Give the expression for deflection in MI ammeter (B.T-2) 17. Formulate to convert basic instruments in PMMC into higher ranger ammeter (B.T-6) 18. Show the basic constructional diagram of attraction type moving iron meter (B.T-3) 19. Is it possible to measure DC and AC using permanent magnet moving coil type instrument? Explain (B.T-5) 20. Describe the type of control system and damping system are used in dynamometer instrument PART B (16 MARKS) 1. i) Illustrate with neat diagram the construction and working principle of attraction and repulsion type MI instruments (B.T-3) Solve for the torque equation of MI instruments (B.T-3) 2. i) Explain the construction and operation of D Arsonval galvanometer

2 Obtain expression for deflection of D Arsonval galvanometer. Explain step by step 3. Sketch the basic construction of PMMC instrument. Develop the torque equation for a PMMC instrument and show that its scale is linear if spring control is employed (B.T-6) 4. i) Define the principle of operation of Ballistic Galvanometer and explain with neat diagram List out the advantages of PMMC instrument 5. i) Explain the operation of Dynamometer type ammeter with torque equation A moving coil instrument gives a full scale deflection of 1mA when the potential difference across its terminals is 10mV. Calculate both the shunt resistance for a full scale deflection corresponding to 10A and the series resistance for full scale reading with 100V and what do you infer from it 6. Describe the step by step procedure on Calibration of ballistic galvanometer (B.T- 1) 7. i) Describe the principle of operation of PMMC instrument and its constructional details with suitable diagrams (B.T-2) Discuss how the range of a PMMC voltmeter and ammeter be extended? (B.T-2) 8. Describe the construction and working of a ballistic galvanometer. How do you conclude that in a ballistic galvanometer, the charge is proportional to first swing of the moving coil (B.T-5) 9. i) Describe the working of a rectifier type instrument which uses a Half Wave Rectifier (B.T-2) Discuss the factors affecting the performance of Rectifier type instruments (B.T-2) 10. Define Thermo Electric Instruments. Classify and explain any two of them (B.T- 1) 11. Tabulate the features of PMMC, MI and Dynamometer type of instruments (B.T- 1) 12. Distinguish between Half Wave Rectifier type instruments and Full Wave Rectifier type of instruments using form factor, accuracy and other features also (B.T-2)

3 13. Illustrate how MI instruments can be used for both AC and DC measurement using construction diagram with different vane arrangements. Justify why MI instruments cannot be accurate for DC measurement (B.T-3) 14. (i) Explain the construction and operation of Thermocouple type instrument ( Explain the different types of Rectifier circuits and obtain the voltage equation for each type UNIT II- MEASUREMENT OF POWER AND ENERGY PART A (2 MARKS) 1. Define Creeping 2. Define Phantom Loading 3. Two watt meters connected to measure the input of a balanced 3 phase circuit indicate 2000W and 500W respectively. Calculate the power factor of the circuit (B.T-3) 4. Express Energy, Power and Power Factor of an electrical signal (B.T-2) 5. List the errors in Electrodynamometer Wattmeter 6. Interpret the necessity to make the Potential Coil circuit purely resistive (B.T-2) 7. Summarize the coils employed in Dynamometer type wattmeter (B.T-2) 8. Describe Pressure Coil in meters 9. List any four adjustments in the energy meter 10. Identify the errors due to the mutual inductance effects in a wattmeter 11. Explain Pressure Coil inductance error in Electrodynamometer wattmeter 12. Analyze the circuit difference between LPF Wattmeter and UPF Wattmeter with neat sketch 13. Differentiate between current coil and pressure coil of Electrodynamometer wattmeter 14. The disc of an energy meter makes 600 revolutions per unit of energy. When a 1000 W load is connected, the disc rotates at 10.2 rpm. If the load is on for 12 hours, measure how many units are recorded as an error? (B.T-5) 15. Generalize methods to prevent creeping (B.T-6) 16. A load draws 10A current from 230V AC mains at 0.75 power factor for half an hour calculate the energy consumed. (B.T-3) 17. Describe the term Motor meters (B.T-2) 18. Support the need for Lag adjustment devices is single phase energy meter (B.T-5) 19. An Energy meter is designed to make 100 revolutions of disc for one unit of energy. Calculate the number of revolutions made by it when connected to load carrying 40 A at 230V and 0.4 power factor for an hour (B.T-3) 20. Develop the need for lag adjustment devices in single phase energy meter (B.T-6) PART B (16 MARKS) 1. i) With a neat sketch describe the construction and working principle of dynamometer type wattmeter. Develop its torque equation

4 An energy meter is designed to make 100 revolutions of disc for one unit of energy. Calculate the number of revolution made by it when connected to load carrying 40 A at 230V and 0.4 PF for an hour. Calculate the percentage of error if it actually makes 360 revolutions (B.T-3) 2. Describe construction and operation of electrodynamometer wattmeter to measure single phase A.C power 3. A 220V 5A DC energy meter is tested at its marked ratings. The resistance of the pressure circuit is 8800Ω and that of current coil is 0.1 Ω. Calculate the power consumed in when testing the meter with (i) ( Direct loading arrangements Phantom loading with current circuit excited by a 6V battery (B.T-3) 4. (i) List out various type of errors in dynamometer wattmeter ( Explain the Procedure of Calibration of wattmeter 5. Develop the step by step procedure for calibrating LPF wattmeter by phantom loading (B.T-6) 6. Explain how power can be measured in a 3 phase circuit with the help of two watt meters 7. A 230V single phase watt-hour meter has a constant load of 4A passing through it for 6 hours at unity pf. If the meter disc makes 2208 revolutions during this period, what are the meter constants in revolutions per KWh. Express the power factor of the load if the no of revolutions made by the meter are 1472 when operating at 230V and 5A for 4 hours (B.T-2) 8. i) Describe the lag adjustments and friction compensation made in single phase induction type energy meters Describe phantom loading testing of energy meter 9. With neat sketch and phasor diagram discuss the construction and operation of induction type single phase energy meter (B.T-2) 10. Assess the various adjustments available in single phase energy meters to get the accurate reading (B.T-5) 11. Explain the operation of three phase energy meter with neat sketch 12. (i) List the different types of errors occurring in wattmeter and write short notes on it

5 ( Examine how two numbers of wattmeter are sufficient for three phase power measurement instead of three numbers of wattmeter. Show it with an example 13. Discuss the registering mechanisms used in energy meters with neat sketches (B.T- 2) 14. (i) Illustrate how phasor diagram can be drawn in three phase circuits (B.T-3) ( Explain the terms active power, reactive power and apparent power in three phase circuits. Write short note on Power triangle UNIT III- POTENTIOMETERS AND INSTRUMENT TRANSFORMERS PART A (2 MARKS) 1. Explain how to calibrate DC Voltmeter using potentiometer? 2. Define Transformation ratio of CT and PT 3. Contrast the advantages of instrument transformers over ammeter shunts and voltmeter multipliers (B.T-2) 4. Conclude the reasons why current transformers must never be operated on open circuit (B.T-5) 5. Give the applications of potentiometers (B.T-2) 6. What do you infer by standardization? 7. Differentiate C.T and P.T (B.T-2) 8. Classify A.C potentiometers (B.T-3) 9. Define ratio error 10. Formulate to obtain true zero in a Crompton s potentiometer (B.T-6) 11. Explain the term volt-ratio box 12. Illustrate with neat sketch the diagram for a basic slide wire potentiometer (B.T-3) 13. Summarize the errors in CT (B.T-2) 14. Summarize the advantages C.T. Why C.T is used more than shunts in instrument range extension? (B.T-5) 15. How to modify a DC potentiometer to be used for AC applications? (B.T-6) 16. Define burden of an instrument transformer 17. A simple slide wire is used for measurement of current in a circuit. The voltage drop across a standard resistor of 0.1 Ω is balanced at 75 cm. Calculate the magnitude of the current if the standard cell emf of 1.45 V is balanced at 50 cm (B.T-3) 18. Define nominal and turns ratio of an instrument transformer 19. List the applications of C.T and P.T 20. List the parts of Drysdale potentiometer PART B (16 MARKS) 1. Develop a circuit for measuring high voltage and high current with the help of PT and CT. What purpose do they serve? (B.T-6)

6 2. Explain the use of instrument transformer for the measurement of power with a neat diagram (B.T-5) 3. Draw and explain the circuit of a basic potentiometer. Explain how the potentiometer should be calibrated and how it is used for the precise measurement of DC voltage 4. i) List the functions of transfer instrument and phase shifting transformer List two applications of D.C potentiometers and write short notes on it 5. A potential transformer rated 33kV/220V and current transformer of 100/5A is used to measure the voltage and current in transformer line, if the voltmeter shows 220V and the ammeter shows 4A, calculate the voltage and current in the line (B.T-3) 6. i) Explain the operation of Gall-Tinsley AC potentiometer Summarize the applications of A.C potentiometers with neat sketch (B.T-2) 7. Discuss the constructional features of C.T and P.T and explain how they are used to extend the range of instruments (B.T-2) 8. i) Summarize the following in relation to a P.T: (B.T-2) (1) Effect of change in secondary burden. (2) Effect of power factor of secondary burden. A current transformer rated as 1000/5A, 25 VA has a turn ratio of 1:200. The core loss and magnetizing components of the primary currents are 5A and 8A under rated conditions. Calculate the ratio and phase angle errors for the rated burden and rated secondary current at 0.8pf lagging (B.T-3) 9. With neat sketch describe the principle and explain construction & operation of polar type Drysdale A.C potentiometer 10. Describe the basic circuit construction and operation of Crompton laboratory type D.C potentiometer

7 11. (i) List the different types of ratios present in instrument transformers and write how it is calculated ( Describe the method of construction and operation of C.T and P.T 12. Summarize briefly about the property of materials used in Instrument transformers construction (B.T-2) 13. (i) Explain the disadvantages of using shunts and multipliers while measuring high values of current and voltages ( A 1000/5A Current Transformer has the magnetizing current of 1A, it is being operated at full load primary current and with a secondary burden of non-inductive resistance of 1ohm at power factor of 0.4. Calculate i. the phase displacement between primary and secondary currents ii. The ratio error at full load. Assuming that there are no compensation (B.T-3) 14. (i) A 500/100 V potential transformer has the following constants: Primary winding resistance= ohm, secondary winding resistance= 0.43 ohm, primary winding reactance= 33.1 ohm. Secondary winding reactance is negligible. No load primary current= 0.1A at 0.6 power factor. Analyze the above data and calculate: i. The no load angle between primary winding and reversed secondary winding voltage ii. The value of secondary winding current at unity power factor when the phase angle is zero ( An AC potentiometer is used for the determination of impedance of a coil and the following results are obtained. The voltage across a 1 ohm resistor in series with a coil is j V. The voltage across a 10:1 potential divider used with the coil is j V. Analyze the above data and assume proper values to calculate the resistance and reactance of the coil UNIT IV- RESISTANCE MEASUREMENT PART A (2 MARKS) 1. List the difficulties in high resistance measurement and method to overcome them

8 2. Describe the use of a Megger (B.T-2) 3. Generalize how to measure resistance by ammeter and voltmeter method (B.T-6) 4. Discuss briefly the measurement of resistance by direct deflection method (B.T-2) 5. Interpret the significance of Kelvin s double bridge from Kelvin Bridge (B.T-2) 6. Identify the range of values of resistance measured by a Megger 7. Develop the condition for Wheatstone bridge balance (B.T-6) 8. Classify the resistance from the point of view of measurements (B.T-3) 9. Summarize the limitations of Wheatstone bridge (B.T-2) 10. Support the necessity to measure high resistances (B.T-5) 11. Define bridge sensitivity 12. Point out the need for a shunt resistor in a series type ohmmeter 13. Deduce the expression for unknown resistance connected in Wheatstone bridge (B.T- 5) 14. List the modifications required in a DC potentiometer to be used for AC applications 15. The value of a high resistance I measured by loss of charge method. A capacitor having a capacitance of 2.5μF is charged through the high resistance. An electrostatic voltmeter, kept across the high resistance, reads the voltage as 300V at the end of the 60 seconds. Calculate the value of high resistance (B.T-3) 16. A series ohmmeter is made up of a 1.5 V battery, a 100 μa meter and a resistance R, which makes R1 + Rm= 15kΩ. Determine the instrument indication when Rx = 0. And calculate the value of at 0.5 FSD (B.T-3) 17. Identify the range of resistance which a Kelvin s double bridge can measure accurately 18. Explain about the measuring scale employed in ohmmeters. 19. List the methods of measurements of medium resistance 20. Explain ground fault PART B (16 MARKS) 1. i) List the difficulties in the measurement of high resistance and direct deflection method for measurement of high resistance Describe the construction and working of shunt type ohmmeter 2. Describe measurement of resistance using kelvin double bridge method and obtain expression for unknown resistance 3. With neat sketch express the measurement of high resistance using price s guard wire method (B.T-2) 4. i) Interpret the principle of operation of series ohmmeter (B.T-2) Discuss the construction and working of a Megger (B.T-2)

9 5. (i) Design a series type ohmmeter. The movement to be used requires 0.5 ma for full scale deflection and has an internal resistance of 50Ω. The internal battery has a voltage of 3V. The desired value of half scale resistance is 3000 Ω. Differentiate and explain the method to obtain (a) the value of series and parallel resistances (b) The range of values of parallel resistance, if the battery voltage may vary from 2.7V to 3.1V. Use the value of the series resistance calculated above A shunt type ohmmeter uses a 10mA basic D Arsonval movement with an internal resistance of 5Ω. The battery emf is 3V. It is desired to modify the circuit by adding appropriate shunt resistance across the movement so that its instrument indicates 0.5Ω at the midscale. Separate the value of shunt resistance and value of current limiting resistor 6. i) List the sources of measurement errors in Wheatstone bridge Describe the circuit of Kelvin double bridge used for measuring low resistance 7. i) Explain how Earth Tester used for measurement of resistance of earth? Explain the Loss of charge method for Measurement of insulation resistance of cables (4) i Draw the circuit of a Wheat stone bridge and deduce the conditions for balance (B.T-5) (4) 8. i) Differentiate shunt type ohmmeter from series type ohmmeter with neat sketch (B.T-2) Explain how the earth resistance can be measured using three point fall of potential method 9. i) Describe the ammeter-voltmeter method of measurement of resistance

10 b a P G Q c R d S A wheatstone bridge is shown in the fig: P=1KΩ, R=1KΩ and S=5KΩ, G=100 KΩ. The Thevenin s source generator voltage Eo =24 mv and Ig= 13.6 μa. Deduce the value of Q. (B.T-5) 10. i) The ratio arms of a kelvin bridge are 100Ω each. The galvanometer has an internal resistance of 500Ω and a current sensitivity of 200mm/μ A. The unknown resistance is Ω and the standard resistance is set at Ω. DC current of 10 A is passed through the standard and the unknown resistance form a 2.2V battery in series with a rheostat. Calculate the deflection of the galvanometer. Neglect the resistance of the link. Also substitute the unbalance resistance value to produce a deflection of 1mm. (B.T-6) List the categories of resistances based on their values and list the methods of measuring it. Describe the necessity for various methods employed in measuring the resistance values 11. i) Describe the substitution method of measurement of medium resistances. List the factors on which the accuracy of the method depends In a Wheatstone bridge circuit, the values of resistances are P= 1000 Ω, Q= 100 Ω, R= 2005 Ω and S= 200 Ω. The battery has an emf of 5V and negligible internal resistance. The galvanometer has a current sensitivity of 10mm/microAmp and an internal resistance of 100 Ω. Connect the deflection of galvanometer and the sensitivity of the bridge in the terms of deflection per unit change in resistance 12. i) A cable of length 1000m is tested for insulation resistance by the loss of charge method. An electrostatic voltmeter of infinite resistance is connected between the cable conductor and the earth forming a joint capacitance of 8x10-4 µf. It is observed that after charging, voltage falls from 400 to 200V in 50secs. Calculate insulation resistance of the cable of length 500m (B.T-3)

11 Calculate insulation resistance of a cable in which the voltage falls from 100 to 80V in 20secs. The capacitance is 300pF (B.T-3) (4) i A high resistance of 300M Ω has a leakage resistance of 600M Ω between each of its main terminals and the guard terminal. Calculate the percentage error in measurement if the above resistance is measured by an ordinary Wheatstone bridge without providing guard circuit (B.T-3) (4) 13. i) What are the different problems associated with measurement of low resistance? Describe the procedure to eliminate the effects of contact resistance and lead resistance in Kelvin Double Bridge (B.T-2) The ohm meter has E b = 1.5V, resistance R 1 =15K Ω in series with R x, R m = 50 Ω, parallel resistance R 2 = 50 Ω and meter FSD = 50micro ampere. Calculate the ohm meter scale at 0.5 FSD and determine the new resistance value that R 2 must be adjusted to when E b falls to 1.3V. Also rearrange the value of R x at 0.5 FSD when E b = 1.3V(B.T-6) 14. Discuss the various methods involved in high resistances measurement. Explain any one of them (B.T-2) UNIT V- IMPEDANCE MEASUREMENT PART A (2 MARKS) 1. Describe Q factor of the coil. (B.T-2) 2. List the errors in AC bridge methods and their compensation. 3. Identify the circuit for Schering Bridge. 4. Classify some commonly used detectors in A.C and D.C bridges (B.T-3) 5. List out the advantages of Schering Bridge. 6. List the detectors used in AC bridges. Illustrate any one detector (B.T-3) 7. Generalize the uses of vibration galvanometer (B.T-6) 8. What can we infer from the use of Campbell bridge equation? 9. Show the two conditions must be satisfied to make an AC bridge balance. (B.T-3) 10. Define the term null as it is applies to bridge measurement. 11. Discuss the applications of A.C bridges (B.T-2) 12. Point out the use of the detector. 13. Explain the two factors that cause errors in Q meter 14. Explain two balance equations required for AC bridges (B.T-5) 15. Give the circuit diagram of Maxwell Inductance Bridge. (B.T-2) 16. List the advantages and disadvantages of Hay s bridge. 17. Tabulate the sources and detectors used in AC galvanometer. 18. Generalize about tuning in Vibration Galvanometer. (B.T-6) 19. Compare the advantage of Hay s bridge over Maxwell s Inductive Capacitance bridge (B.T-5)

12 20. Differentiate the merits and demerits of Anderson s bridge (B.T-2) PART B (16 MARKS) 1. i) Draw and explain the working of Maxwell s bridge. Derive the condition for the balance of the bridge A Maxwell bridge is used to measure an Inductive impedance. The bridge constants at balance are: C F, R1 470 K, R2 5.1 K, R3 100 K.. Calculate Lx and Rx (B.T-3) 2. What are the conditions for balancing the AC bridge? Describe the working of Schering bridge for the measurement of capacitance with neat diagram. Derive the equations for balance (B.T-2) 3. Explain in detail the procedure of measurement of inductance and capacitance (B.T- 4) 4. i) Describe with a neat sketch, the Vibration galvanometer (B.T-2) The arms of a 4 arm bridge abcd supplied with the sinusoidal voltage, have the following values :Arm ab has 200Ω in parallel with 1 µf, arm bc has 400Ω, arm cd has 1000Ω and arm ba has resistance R 2 in series with 2µF. Calculate the value of R 2 and the frequency at which the bridge will balance. (B.T-3) 5. i) Identify the circuit diagram and phasor diagram for Anderson bridge. Examine the expression for unknown inductance in Anderson Bridge 6. Explain the measurement of inductance using maxwell-wein s bridge circuit (B.T- 5) 7. What are the different sources of errors in AC bridges? Explain the precautions taken and the techniques used for elimination of these errors 8. i) Describe the working of Q-meter (B.T-2) List the special features of a high voltage Schering bridge 9. i) List the source of errors in bridge circuit and discuss the precautions and techniques used for reducing errors

13 Examine how to measure relative permittivity with Schering bridge. 10. An bridge has 4 arms arm ab is 0.2µF, arm bc is 500Ω, arm cd consists of unknown impedance and arm da has a 300Ω in parallel with 0.1 µf. Calculate the R and C or L constants of arm cd considering as a series circuit. (B.T-3) 11. i) Identify and draw the phasor diagram of Maxwell Inductance Bridge Identify how to measure mutual inductance using bridge circuit 12. i) A capacitor bushing forms arm ab in a schering bridge and a standard capacitor of 500pF with negligible loss forms arm ad. Arm bc consists of a noninductive resistance of 300 Ω. When the bridge is balance, arm cd has a resistance of 72.6 Ω parallel with a capacitance of µf. The supply frequency is 50Hz. Develop the capacitance and dielectric loss angle of capacitorusing appropriate assumption (B.T-6) List the errors in AC bridges and their compensation 13. i) Describe Campbell Bridge with neat sketch (B.T-2) Describe and derive the equation in Hay s Bridge (B.T-2) 14. i) Why Maxwell s inductance-capacitance bridge is useful for measurement of inductance of coils having storage factor between 1 and 10? Support the answer with necessary data and figures (B.T-5) The four arms of an AC bridge contain the following elements: PQ, a coil of unknown impedance; QR, a non-inductive resistor of 1000Ω; RS, a noninductive resistor of 85 Ω in series with a standard capacitor of 0.35 µf; SP, a pure resistor of 16,500 Ω. The supply frequency is connected to points P and R, and a detector to point Q and S. Separate and identify the inductance and resistance values of the coil.