Code.No 45007

R07

SET-1

JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD III.B.TECH - I SEMESTER REGULAR EXAMINATIONS NOVEMBER, 2009 ELECTRICAL MEASUREMENTS

( EEE) Time: 3hours

Max.Marks:80

1.a)

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Answer any FIVE questions All questions carry equal marks --What is a PMMC instrument? Explain with a neat sketch working of such an instrument when used as an ammeter. b)

The coil of a moving coil voltmeter is 40mm× 30mmwide and has 100turms wound on it.

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The control spring exerts a torque of 0.25×10-3Nm when the deflection is 50 divisions on the scale. If the flux density of the magnetic field in the air gap is 1 wb/m2, estimate the resistance that must be put in series with the coil to give 1V/division. Resistance of volt meter is 10,000 ohms. 2.

Draw the equivalent circuit and phasor diagram of a current transformer. Derive the

b)

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expressions for ratio and phase angle errors. 3.a)

Derive the expression for torque when the instrument is used on a.c. Explain why it is [8+8]

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With a neat figure, describe the construction, working principle and the theory of a single

phase induction type energy meter.

[8+8]

Explain the basic principle of operation of a D.C. potentiometer with a neat sketch.

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5.

[8+8]

Describe the constructional details of an Electro dynamometer type wattmeter. necessary to make the potential coil circuit purely resistive?

4.

[8+8]

Explain why a potentiometer does not load the voltage source whose voltage is being measured?

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-26.a)

Set 1

Classify the resistances from the point of view of measurements?

b)

Explain in brief the different methods used for measurement of medium resistances.

c)

A voltmeter of resistance 500Ω and a milli Ammeter of 1Ω resistance are used to measure a resistance by ammeter- voltmeter method. If the voltmeter reads 20V and

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milli Ammeter 100mA, calculate the value of measured resistance (i) if the

voltmeter is put across the resistance and milli Ammeter connected in series with

the unknown resistance (ii) if the voltmeter is put across the unknown resistance and ammeter connected on the supply voltage.

Derive the equations for balance in the case of Maxwell’s Inductance – Capacitance

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7.a)

[6+5+5]

bridge. Draw the phasor diagram for balance conditions. State the advantages of this bridge. b)

A 1000Hz bridge has the following constants:

Arm AB, R1= 1000Ω in parallel with C1 =0.5µF; Arm BC, R3=1000Ω in series with C3 = 0.5µF;

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Arm CD, L4 =30mH in series with R4 = 200 Ω.

Find the constants of arm DA to balance the bridge. Express the result as a pure resistance R in series with a pure inductance L or Capacitance C. 8.

[8+8]

Explain the construction and working principle of a ballistic galvanometer with a neat

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sketch?

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SET-2

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1.a) b)

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JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD III.B.TECH - I SEMESTER REGULAR EXAMINATIONS NOVEMBER, 2009 ELECTRICAL MEASUREMENTS Time: 3hours Max.Marks:80 Answer any FIVE questions All questions carry equal marks ---

What are electrostatic instruments? What is the basic principle over which they operate? Discuss the working of a repulsion type electrostatic instrument with a neat sketch.

2.

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[8+8]

Obtain the expression for power, in terms of correction factor, wattmeter reading, actual ratio of PT and CT, in case of power measurement along with instrument transformers.

3.a)

Explain how 3-phase power can be measured with the help of two wattmeters with suitable connections.

Prove that tan θ = 3 (W1 –W2) / (W1 + W2)

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b)

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Where W1 and W2 are the wattmeter readings and θ is the power factor angle. 4.a) b)

[8+8]

Explain the testing of energy meter using R.S.S. meter. The meter constant of a 230, 10A, watt hour meter is 1800 revolutions per Kwh. The meter is tested at half load and rated voltage and unity power factor. The meter is found

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to make 80 revolutions in 138 second. Determine the meter error at half load. 5.a)

[8+8]

Explain the term “standardization” of a potentiometer. Explain the procedure of

standardization of a D.C. potentiometer with help of a circuit diagram.

Give the constructional details of a potentiometer.

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b)

[8+8]

Contd -2-

-26.a)

SET-2

Explain the Ammeter- voltmeter method of measurement of resistances. There are two ways in which the circuit of Ammeter voltmeter method can be used (i) ammeter connected to the side of unknown resistance and (ii) voltmeter connected to the side of unknown resistance. Derive the condition which decides which resistance. Assume equal relative error in both the cases.

b)

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circuit is to be used for a particular set of Ammeter, voltmeter and unknown In a laboratory a voltmeter of 200Ω resistance and an ammeter of 0.02 Ω

resistances are available. Calculate the value of resistance that can be measured by the ammeter –voltmeter method for which the two different circuit measurement

7.a) b)

Derive the general equations for balance of an A.C. bridge?

Why is it preferable in bridge circuits, that the equations of balance are independent of frequency? Explain.

c)

[8+8]

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give equal errors.

A four arm A.C. bridge has the following impedances:

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Arm AB = Z1 = 200∟600 Ω (inductive impedance)

Arm AD = Z2 = 400∟-600Ω (capacitive impedance) Arm BC = Z3 = 300∟00 (purely resistive) Arm CD = Z4 = 600∟300Ω (inductive impedance) Determine whether it is possible to balance the bridge under above conditions? Give the reason. 8.

[6+5+5]

Prove that in a ballistic galvanometer the charge is proportional to first swing of the

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moving coil?

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[16]

SET-3

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JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD III.B.TECH - I SEMESTER REGULAR EXAMINATIONS NOVEMBER, 2009 ELECTRICAL MEASUREMENTS Time: 3hours Max.Marks:80 Answer any FIVE questions All questions carry equal marks --How moving iron instruments are classified? Explain with the neat sketches the working of an attraction type moving iron instrument. b) 2.

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1.a)

Discuss the advantages and limitations of a moving iron instrument.

[8+8]

In a 11KV,100 A,3 phase (balanced) supply, voltmeter of 0-110V Ammeter 0-25A and

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wattmeter with pressure coil rating of 110V and current coil rating of 5A are to be used

for measuring the voltages, currents and power. Draw wiring diagram using necessary instrument transformers. What types of errors are expected in such measurements? And how to minimize these errors? 3.a)

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Describe the method of measurement of reactive power in three phase circuits using angle dynamometer type wattmeter.

If the current in a pressure coil of a wattmeter lags 2o behind the voltage, and the

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b)

instrument is accurate when Cosф = 1, find the percentage error when Cos ф = 0.8. 4.a)

[8+8]

Explain the various types of errors occurring in an energy meter and also the method of compensation to overcome these errors.

b)

Explain with a neat sketch the construction and working principle of a single phase

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energy meter and derive an expression to show that number of revolutions/sec is proportional to power.

Draw the circuit diagram of a D.C. Crompton’s potentiometer and explain its working.

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5.

[8+8]

[16] Contd -2-

-26.a)

SET-3

Explain the substitution method of measurement of medium resistances. List the factors on which the accuracy of the method depends.

b)

In a measurement of resistance by the substitution method a standard resistance of 100KΩ is used. The galvanometer has a resistance of 2000Ω , and gives the [8+8]

(i) With unknown resistance: 46 divisions (ii) With standard resistance: 40 divisions Find the value of unknown resistance?

In a Maxwell inductance – capacitance bridge the dial of variable capacitor can be

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7.a)

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following deflections.

made to read the value of unknown inductance directly. How is it done? b)

Explain why Maxwell inductance- capacitance bridge is useful for measurement of inductance of coils having quality factor between 1 and 10.

c)

The 4 arms of a Maxwell’s capacitance bridge at balance are:

Arm AB, an unknown inductance L1 having an inherent resistance R1;

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Arm BC, a non inductive resistance of 1000Ω;

Arm CD, a capacitor of 0.5µF in parallel with resistance of 1000Ω; Arm DA, a resistance of 1000 Ω.

Derive the equations of balance for the bridge and determine the value of R1 and L1. Draw the Phasor diagram of the bridge under balance conditions.

Explain the construction and working principle of Flux meter with a neat sketch?

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8.

[6+5+5]

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SET-4

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Kelvin absolute electrometer. b) 2.

Explain with the diagram Kelvin multi cellular voltmeter.

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JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD III.B.TECH - I SEMESTER REGULAR EXAMINATIONS NOVEMBER, 2009 ELECTRICAL MEASUREMENTS Time: 3hours Max.Marks:80 Answer any FIVE questions All questions carry equal marks --1.a) What are portable electro static instruments? Explain with necessary theory, working of [8+8]

Describe how high currents and voltages are measured with help of instrument

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transformers. Draw diagrams to illustrate your answer. Describe the advantages of instrument transformers as regards extension of range of current and voltage on high voltage A.C systems. 3.a) b)

[16]

Explain the construction and working principle of Electro-dynamometer type wattmeter. In the calibration of wattmeter the following readings were obtained. The voltage across the low voltage tap of a 300:1 potential divider is (0.8+j0.15) volts, and voltage across a

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0.1 ohms standard resistor is (0.35-j0.1) volts. Determine the reading of wattmeter and load power factor. 4.a) b)

[8+8]

Explain the working of a trivector meter. The meter constant of a 5A, 220V DC watt hour meter is 3275 revolutions KWh. Calculate the speed of the disc at full load. In a test run at half load, the meter takes 59.5s [8+8]

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to complete 30 revolutions. Calculate the error of the meter. 5.a)

Describe the steps when D.C. Crompton’s potentiometer is used to measure an

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unknown resistance?

Contd -2-

-2b)

SET-4

A basic slide wire potentiometer has a working battery voltage of 3V with negligible internal resistance. The resistance of slide wire is 400Ω and its length is 200cm. A 200cm scale is placed along the slide wire. The slide wire has 1mm scale division and it is possible to read upto 1/5th of a division. The instrument is standardized with 1.018V standard cell with sliding contact at the 101.8cm mark on

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the scale. Calculate (i) Working current (ii) The resistance of a series rheostat (iii) The measurement range and (iv) The resolution of the instrument. 6.a)

Explain the substitution method of measurement of medium resistances. List the factors on which the accuracy of the method depends.

In a measurement of resistance by the substitution method a standard resistance of

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b)

[8+8]

100KΩ is used. The galvanometer has a resistance of 2000Ω , and gives the following deflections

(i) With unknown resistance: 46 divisions (ii) With standard resistance: 40 divisions

7.a)

[8+8]

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Find the value of unknown resistance?

Explain the working of Hay’s bridge for measurement of inductance with a circuit diagram. Derive the equations for balance and draw the phasor diagram under balanced conditions.

b)

The 4 arms of Hay’s bridge are arranged as follows: AB is a coil of unknown impedance;

BC is a non reactive resistor of 1000Ω;

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CD is a non reactive resistor of 833Ω in series with standard capacitor of 0.38µF;

DA is a non reactive resistor of 16800 Ω. If the supply frequency is 50Hz, determine the inductance and the resistance at the [8+8]

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balanced conditions.

8.

Prove that change in value of flux is directly proportional to the change in the deflection in case of Flux meter?

[16]

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