Download Set No: 1

Set No: 1
Code No-: V3109/R07
III B.Tech I Semester Supplementary Examinations, April/May - 2012
ELECTRICAL MEASUREMENTS
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
*****
1. a) Derive the torque equation for the D’Arsonval Galvanometer with necessary assumed
constants.
b) The coil of a 250 V moving iron voltmeter has a resistance of 500 Ω and inductance of
1 H. The current taken by the instrument when placed on 250 V, Dc supply is 0.05 A.
Determine the percentage error when the instrument is placed on 250 V, Ac supply at
100 Hz.
[8+8]
2. a) Derive the expressions for actual transformation ratio and phase angle error for the
current transformer using a neat phasor diagram
b) A 500/5A, 50 Hz current transformer has a one turn primary and an output of 20 VA.
The number of secondary turns is 99, and the resistance and leakage reactance of the
secondary winding are 0.4 Ω and 0.35 Ω respectively. At rated load with non- inductive
burden, the in- phase and quadrature components (referred to the flux) of the exciting
mmf are 6 AT and 8 AT respectively. Calculate the current and phase angle errors.
[10+6]
3. a) What are the errors in induction type wattmeter, and how are they compensated?
b) The inductive reactance of the pressure coil circuit of a dynamometer wattmeter is
0.35 percent of its resistance at normal frequency and the capacitance is negligible.
Calculate the correction factor and percentage error due to reactance for load having 0.5
power factor lagging
[8+8]
4. a) Explain the construction and theory of operation of induction Type Single phase
Energy meter
b) A 50 A, 230 V energy meter in a full load test with unity power factor makes 61
revolutions in 37 seconds. If the normal speed of the disc is 520 revolutions/KWH.
Compute the percentage error.
[8+8]
5. a) Explain how an unknown voltage can be measured by using a polar type
potentiometer?
b) Calculate the inductance of a coil from the following measurement on an a.c.
potentiometer. Voltage drop across a 0.1 standard resistor connected in series with the
coil = 0.613 ∟120 6′. Voltage across the test coil through a 100/1 volt-ratio box = 0.781
∟500 48′.V. Frequency is 50 Hz.
[10+6]
6. Explain what do you mean by low, medium and high resistances? Suggest various
suitable methods for measuring them giving justification. Explain any one method to
measure a low resistance with accuracy using a neat diagram?
[16]
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Set No: 1
Code No-: V3109/R07
7. a) With the help of circuit diagram explain how capacitance can be measured by the use
of a Schering bridge.
b) The four arms of an ac bridge network are as follows: arm AB: an Unknown
impedance;
arm BC: a standard capacitor of 1000 pF; arm CD: a non-inductive
resistor of 100Ω in parallel with a capacitor of 0.01µF; arm DA: a non-inductive resistor
of 1000Ω. The A.c supply is connected across terminals B, D and the supply frequency is
50 Hz. If the bridge is balanced with the above values, calculate the components of the
unknown impedance.
[8+8]
8. a) Explain the Lloyd Fisher square method of measuring iron losses in ferromagnetic
materials.
b) The iron loss in the sample is 300 watts at 50 Hz with the eddy current component 5
times as big as the hysteresis loss component. At what frequency will the iron loss be
doubled if the flux density is kept same?
[8+8]
*****
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Code No-: V3109/R07
Set No: 2
III B.Tech I Semester Supplementary Examinations, April/May - 2012
ELECTRICAL MEASUREMENTS
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
*****
1. a) Explain Attraction type moving iron instrument with a neat diagram.
b) The coil of a moving coil voltmeter is 40 mm long and 30mm wide and has 100 turns
on it. The control spring exerts a torque of 240 × 10 -6 Nm when the deflection is 100
divisions on full scale. If the flux density of the magnetic field in the air gap is 1.0
Wb/m2, calculate the resistance that must be put in series with the coil to give 1
V/division. The resistance of the voltmeter coil may be neglected.
[8+8]
2. a) Explain the construction and working of a Dynamometer type single – phase power
factor meter.
b) A current transformer with a bar primary has 250 turns in its secondary winding. The
resistance and reactance of the secondary circuit are 1.4 Ω and 1.1 Ω respectively
including the transformer winding. When 5A current flows in the secondary winding, the
magnetizing mmf is 80 AT and the iron loss is 1.1 W. Determine the ratio error and
phase angle error.
[8+8]
3. a) Show that the power in a 3-phase system is measured by the use of
i. Only one watt meter and
ii. Two wattmeters.
Indicate how the power is determined. Comment on the accuracy of the measurements
when the load is unbalanced and the supply is a four-wire system.
b) Two wattmeters used to measure the power input in a 3-phase circuit indicate 1000w
and 500w respectively. Find the power factor of the circuit.
i. When both wattmeters readings are positive
ii. When the latter is obtained by reversing the current coil connections.
[10+6]
4. Explain the working of a single-phase induction type energy meter with the help of
connection diagram. Give the various sources of errors and their compensation.
[16]
5. Explain with the help of suitable diagrams, how a d.c potentiometer can be used for:
(i) Calibration of a voltmeter.
(ii) Calibration of an Ammeter
(iii) Calibration of wattmeter
(iv) determination of an unknown resistance.[16]
6. a) Why is Kelvin’s double bridge superior to the wheat-stone bridge for the purpose of
low resistance measurement?
b) How can the difficulties associated with the measurement of a very high resistance are
over come?
c) How the effects of contact resistance and resistance of the connecting leads are
eliminated in the measurement of resistance by Kelvin’s double bridge?
d) Why is the Voltmeter-Ammeter method unsuitable for the precise measurement of the
low resistance?
[4x4]
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Set No: 2
Code No-: V3109/R07
7. a) Explain with the help of neat diagram the loss of charge method for determining the
insulation resistance of a length of a cable
b) The value of a high resistance is measured by loss of charge method. A capacitor
having a capacitance of 2.5 µF is charged to a potential of 500V d.c and is discharged
through the high resistance. An electrostatic voltmeter, kept across the high resistance,
reads the voltage as 300 V at the end of 60 seconds. Calculate the value of high
resistance
[8+8]
8. Derive an expression for equation of motion of a ballistic galvanometer with a neat
sketch
[16]
*****
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Code No-: V3109/R07
Set No: 3
III B.Tech I Semester Supplementary Examinations, April/May - 2012
ELECTRICAL MEASUREMENTS
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
*****
1. a) Explain the use of a shunt with respect to a PMMC instrument. How it is employed in
extending the range of an Ammeter?
b) With a neat circuit diagram explain how temperature effect is corrected in the shunts
[8+8]
2. Derive the expressions for actual voltage transformation ratio and phase angle error in
case of a potential transformer with the help of neat phasor diagrams.
[16]
3. a) What are electrostatic instruments? Explain the basic principle over which they
operate?
b) Explain with a neat diagram of a Repulsion type Electrostatic instrument
[8+8]
4. a) How will you test the single phase energy meter? Explain the procedure of
adjustments of different compensation systems.
b) A 230V, 50Hz single phase energy meter has a constant of 200 revolutions per kWh.
While supplying a non-inductive load of 4.4 A at normal voltage, the meter takes 3
minutes for 10 revolutions. Calculate the percentage error of the instrument.
[8+8]
5. a) Explain the Crompton potentiometer with a neat connection diagram and bring out its
salient features. How it is standardized?
b) A crompton’s potentiometer consists of a resistance dial having 15 steps of 10 Ω each
and a series connected slide wire of 10 Ω which is divided in to 100 divisions. If the
working current of the potentiometer is 10 mA and each division of slide wire can be
ଵ
read accurately up to ହ of its span, calculate the resolution of the potentiometer in volt.
[8+8]
6. a) Name the different methods used for measurement of medium resistances. Explain any
one method to measure medium resistance with accuracy using a neat diagram?
b) A voltmeter of resistance 500 Ω and a milli-ammeter of 1Ω resistance are used to
measure a resistance by Ammeter-Voltmeter method. If the Voltmeter reads 20V and
milli-Ammeter100 mA, calculate the value of measured resistance.
i. If the Voltmeter is put across the resistance and the milli-Ammeter connected in
series with the unknown resistance.
ii. If the voltmeter is put across the resistance with ammeter connected on the supply
side.
[8+8]
7. a) Explain the common sources of error in a.c bridges. How are they eliminated?
b) Explain with the help of neat diagram the measurement of iron loss using Campbell
bridge Method.
[8+8]
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Set No: 3
Code No-: V3109/R07
8. a) Explain the method of reversals for determination of hysteresis loop of a magnetic
material specimen
b) A moving coil ballistic galvanometer of 150 Ω resistance gives a throw of 75
divisions when the flux through a search coil to which it is connected is reversed. Find
the flux density, if the galvanometer constant is 110µC per scale division and the search
coil has 1400 turns, a mean area of 50 cm2 and a resistance of 20 Ω.
[8+8]
*****
2 of 2
Code No-: V3109/R07
Set No: 4
III B.Tech I Semester Supplementary Examinations, April/May - 2012
ELECTRICAL MEASUREMENTS
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
*****
1. a) Explain the principle of working of a repulsion type moving iron instrument with a
neat diagram?
b) Discuss the various errors in moving iron instruments and suggest methods to
compensate these errors.
[8+8]
2. a) Draw and explain the equivalent circuit and phasor diagram of a potential transformer.
b) A potential transformer with a nominal ratio of 2000/100V, a RCF of 0.995 and a
phase angle (Vs lags Vp) of 25 min is used with a current transformer with a nominal
ratio of 100/5A, an RCF of 1.005 and a phase angle error (Is leads Ip) of 10min to
measure the power in a single phase inductive load. The meters connected to these
transformers give correct readings of 1.02V, 4A and 375watts. Determine the true values
of the voltage, current and power supplied to the load.
[8+8]
3. a) Explain how the measurement of power with wattmeter in conjunction with
instrument transformers can be used in single- phase ac circuits
b) A 3- phase, 440V motor load has a power factor of 0.6. Two wattcmeters connected to
measure the power show the input to be 25 KW. Find the reading on each instrument.
[8+8]
4. a) Explain the working of a Trivector meter with a neat diagram
b) 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 amps
at 230 volts and 0.4 power factor for an hour. If it actually makes 360 revolutions, find
the percentage error.
[8+8]
5. a) Explain the principle of operation a rectangular type co-ordinate A.C.Potentiometer
with a neat diagram and scheme of connections, and describe how the potentiometer is
standardized.
b) The following readings were obtained during the measurement of power by a polar
potentiometer: Voltage across 0.2 Ω standard resistance in series with the load = 1.52 V
with an angle of 350. Voltage across 200:1 potential divider across the line = 1.43 V with
an angle of 530 .Find the current, voltage, power factor of the load.
[10+6]
6.
a) Explain the procedure for measuring low resistance with the help of Kelvin double
bridge. Derive the relation for finding unknown resistance.
b) The value of a high resistance is measured by loss of charge method. A Capacitor
having a capacitance of 2.5 µF is charged to a potential of 500 V d.c and is discharged
through the high resistance. An electrostatic voltmeter, kept across the high resistance,
reads the voltage as 300 V at the end of 60 Seconds. Calculate the value of high
resistance.
[8+8]
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Set No: 4
Code No-: V3109/R07
7. a) What are the limitations of L.V. schering bridge?
b) Define dissipation factor? Derive the equation for dissipation factor in case of L.V.
Schering Bridge?
c) In a Low-voltage Schering bridge designed for the measurement of permittivity, the
branch ab consists of two electrodes between which the specimen under test is inserted;
arm bc is a non-reactive resistor R3 in parallel with a standard capacitor C3; arm CD is a
non-reactive resistor R4 in parallel with a standard capacitor C4; arm da is a standard air
capacitor of capacitance C2.Without the specimen between the electrodes, balance is
obtained with the following values, C3 = C4 = 120 pF, C2 = 150 PF, R3 = R4 = 5000 Ω .
With the specimen inserted, these values become C3 = 200 PF; C4 = 1000 pF; C2 = 900
pF, and R3 = R4 = 5000 Ω. In each test ω = 5000 rad/sec. Find the relative permittivity
of the specimen.
[4+4+8]
8. a) Why are magnetic measurements more in accurate than other types of measurements?
b) Why a ballistic galvanometer is usually light damped?
c) How does a flux meter differ from ballistic galvanometer?
d) Flux measurement using flux meter.
[3+3+5+5]
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