EI 34-ELECTRICAL MEASUREMENTS
Unit: I
1. Define Ballastic sensitivity?
Ballastic sensitivity sq =θ1/Q
2. State the advantage and disadvantage of rectifier type instruments?
Advantage
The frequency range extends from about 20 hz to high audio frequency
With the introduction of germanium and silicon rectifier rectifier
instrument may be built in half wave configuration with a sensitive
PMMC instrument at frequency in the region of 3000 MHZ.
Disadvantage
i) Effect of waveform
ii) Effect of rectifier Resistance
3. Why is the MI meter has non uniform scale?
The deflection in a MI instrument is given by
θ=1/2 *I2 /K*dL/dθ
Thus the angular deflection is proportional to the square of the operating
current and the instrument has a square law response. The deflection is in
terms of rms value of current or voltage as the deflection is proportional
to square of the current it is evident that the scale of such instrument is
non-uniform.
4. What is residual error?
The happenings or disturbances about which we are unaware are lumped
together and called Random or Residual .Here the errors caused by these
happenings are called Random error .Since these errors remain even after
the systematic errors have been taken care of we all these errors as
residual.
5. Why electrostatic instruments cannot be used for the measurement of
low voltage while electromagnetic instrument can be?
When the voltage being measured is small the two discs should be very
near together in order to get an appreciable force. In such cases the
measurement of distance between the plates is difficult to carry out. The
solution lies in increasing the voltage between fixed and moving plates
by using hetrostatic connection.
6. Define loading effect of an instrument?
The incapability of the system to faithfully measure record or control the
input signal in undistorted form is called the loading effect.
7. Define the term current sensitivity and voltage sensitivity?
The current sensitivity of a galvanometer is defined as the deflection
produced by unit current
Current sensitivity Si=Gi/K
Voltage sensitivity
Voltage sensitivity is the deflection in scale division per unit voltage
impressed on the galvanometer.
Sv=d/i*Rg*106
8. What precautions are to be observed when using an ammeter?
Ammeter which is connected in series with the circuit carrying the
current under measurement must be very low resistance. So that the
voltage drop across the ammeter and power absorbed from the circuit are
as low as possible.
9. State the advantage disadvantage of PMMC instrument?
Advantage
i) The scale is uniformly divided.
ii)The power consumption is very low as 25 μw to 200 μw.
Disadvantage
i) There instrument useful only for dc
ii) The cost of this instrument is higher than that of MI instrument.
10. Compare Ammeter and Voltmeter.
Ammeter
a. The ammeter carries the current to be measured or a definite
fraction of it and the current or its definite fraction produces the
deflecting torque.
b. Connected in series.
Voltmeter
a. Carries the current proportional to the voltage to be measured which
produces the deflecting torque
b. Connected in parallel
11. What is the basic principle of PMMC instrument?
Working principle is same as the d’Arsonoval type of galvanometer the
difference being that a direct reading instrument is provided with a
pointer and scale.
12. Compare and contrast moving iron and Electro dynamometer type
meters?
MI
a. Suitable for DC or AC
b. Inexpensive type used for rough indication of currents and voltages
widely used in indicator type applications such as on panels
Electrodynamometer type instrument
a. Suitable for DC or AC
b. Widely used for precise ac current or voltage measurement current
frequencies Used as standard meter for calibration and also as
transfer instrument.
13. A PMMC instrument has a 0.12T magnetic flux density in its airgaps.
The coil dimensions are D=1.5 cm and l= 2.25 cm .Determine the no of
coil turns required to give a torque of 4.5µN-m when the coil current is
100µA.
N= Kθ/BldI= 1111 turns
14. What is the use of Swamping resistor?
When swamping resistance of maganin of resistance 20 to 30 times the
coil resistance is connected in series with the coil and a shunt of
manganin is connected across this combination. Since copper forms a
small fraction of the series combination the proportion in which the
currents would divide between the mater and the shunt would not change
appreciably with the change in temperature.
15. Explain the term instrumentation and instruments?
An instrumentation system is a physical system which is a an collection
of physical objects connected in such a way to give the desired output
response.
Measurement involves the use of instruments as a physical means of
determining quantities or variables.
16. What is the importance of static characteristics of systems?
It is used to check how good the measurements under steady state
conditions.
17. What is sensitivity of voltmeter?
Sdc=1/Ifs
18. What are the necessary forces to operate an indicating instrument?
Deflecting ,controlling ,Damping
19.Compare moving iron and moving coil type instruments?
Moving iron
o For AC and DC
o Inexpensive type used for rough indication of current and
voltages . Widely used in indicator type applications such as
on panels .
Moving coil
a. Suitable for DC
b. Most widely used meter for current and voltage and resistance
measurement in low and medium impedance circuit.
20.What is meant by measurement?
Measurement is an act or the result of comparison between the quantity
and a predefined standard.
21 Mention the basic requirements of measurement.
The standard used for comparison purpose must be accurately defined
and should be commonly accepted.· The apparatus used and the method
adopted must be provable.
22.What are the 2 methods for measurement?
·Direct method and
·Indirect method.
23.Explain the function of measurement system.
The measurement system consists of a transducing element
which converts the quantity to be measured in an analogous form. The
analogous signal is then processed by some intermediate means and is
then fed to the end device which presents the results of the measurement.
24.List the types of instruments.
The 3 types of instruments are
·Mechanical Instruments
·Electrical Instruments and
·Electronic Instruments.
25.Classify instruments based on their functions.
Indicating instruments
Integrating instruments
Recording instruments
UNIT: II
1. List the various types of registering mechanism used in single phase
energy meter?
Pointer type of register.
Dycometer register.
2. Derive the average deflecting torque equation for electrodynamometer
type of wattmeter?
Td= 1/T∫T0T1d(wt)
=VI/Rp*cos Ф.dM/dθ
3. Two wattmeter are connected to measure the input of a balance three
circuit indicates 1000w(+ve) and 125 (+ve ) respectively. Find the power
factor of the circuit?
Pf cosФ = cos [tan-1(√3(1000-125/1000+125)]=0.5960
4. What is the need for light load compensation in watt hour meter?
Despite every care taken in the design of both the jeweled pivot bearing
which forms the lower bearing for the spindle and of the simple sleeve
pin type bearing at the top of the spindle friction errors are liable to be
serious particularly at light loads .In order to ensure accurate registration
at low loads it is therefore necessary to arrange for small torque
practically independent of the load on the meter which acts in the
direction of rotation and which is nearly as possible equal in magnatitude
to the friction torque. This is usually obtained by means of small shading
loop.
5. Write a short note on adjustments required in energy meter?
Lag adjustment (LPF Adjustment)
The pressure coil is connected across rated supply voltage and rated full
load current is passed through the current coil at 0.5 pf lagging .The lag
device is adjusted till the meter runs at correct speed.
6. Define creep?
In some meters a slow but continuous rotation even when there is no
current flowing through the current coil and only pressure coil is
energized. This is called creeping.
7. List the different errors that occur in a Electro dynamometer type
wattmeter.
i) Pressure coil inductance
ii) Pressure coil capacitance
iii)Error due to Mutual Inductance Effect.
8. An induction type energy meter runs fast. How the speed can be
reduced?
The current coil flux Фs is made small as compared pressure with coil
flux Фp
9.List out the special features incorporated in an electrodynamometer
type of wattmeter for low power factor applications?
i) Pressure coil current
ii) Compensation for pressure coil current
iii) Compensation for Inductance of pressure coil.
10. What is phantom loading?
When the capacity of the meter to be tested is high considerable power
will be wasted if ordinary loading arrangement is made. For testing of
meter of higher capacities phantom or fictitious loading arrangement is
made.
11. State the disadvantage of wattmeter?
i) Even a small error in measurement of voltages causes serious errors
ii) Supply voltage higher than normal voltage is required.
12. What is the basic principle of energy meter?
Induction principle
13. What does systematic error of instrument mean?
Such errors that remain constant or change according to a definite law
on repeated measurement of the given quantity.
14. A three phase 500 v motor load has p.f 0.4 .Two wattmeter’s
connected to measure the input. They show the input to be 30 kw .Find
the reading of each instrument?
P1+P2=30 kw
P1-P2=39.7 kw
P1=34.85 kw
P2=-4.85 kw
15. Mention two advantage and disadvantage of analog instrument
composed to digital instrument?
Advantage
.Varied over continuous range of values
.Easily manipulated
Disadvantage
i) Difficult circuit design
ii) More error.
16. What is transfer type instrument? Why is an electrodynamometer a
transfer instrument?
A transfer instrument is that which may be calibrated with dc source and
the used on ac without any modification .Since it is used for both ac and
dc measurement with same calibration it is called as transfer type
instrument.
17. Why are instruments calibrated?
Some instruments which gives the deflection of magnatitude of the
quantity to be measured directly. These instruments are to be calibrated
by comparison with absolute instrument or secondary instrument which
are already calibrated and put in use.
18. What are the constructional parts of dynamometer type wattmeter?
Fixed coil
Moving Coil
Current limiting resister
Helical spring
Spindle attached with pointer
Graduated scale
19 How the errors caused by pc inductance is compensated.
By connecting a capacitor in parallel to the resister.
20. How the errors caused by methods of connection is compensated
By using compensating coil.
21. Name the methods used for power measurement in three phase
circuits.
(i) Angle wattmeter method
(ii) Wattmeter method
(iii) Wattmeter method.
22. What are the types of energy meters?
Electrolytic meters
Motor meters.
Clock meters
23. Name the constructional parts of induction type energy meter.
Current coil with series magnet
Voltage coil with shunt magnet
Al disc braking magnet
Registering mechanism.
24. Why Al disc is used in induction type energy meter
Aluminum is a nonmagnetic metal.
25. What is the purpose of registering mechanism?
It gives a valuable number proportional to the rotations.
Unit: III
1. Define nominal ratio and ratio correction factor in instrument
transformer?
Nominal Ratio=rated primary winding current /rated secondary winding
current for a CT.
Ratio correction factor of the transformer is transformation ratio divided
by nominal ratio.
2. Define the turn’s ratio for PT?
N= no of turns of primary winding /no of turns in secondary winding for
a PT
3. Write down the applications of Dc potentiometer?
i)Measurement of resistance
ii)Measurement of current
4. How is polar potentiometer standardized?
The slid wire is switched to dc supply the detector is taken out of circuit
the standard cell and D’Arsonval galvanometer is connected to the
terminals marked as test terminals the slide wire contact is set for
standard cell emf and the current control resistance R is adjusted until
balance is obtained. The reading of the ammeter A then indicates the
values of alternating current required in the ac measurements .The
galvanometer and standard cell are then removed from the circuit and dc
supply is replaced by that from the phase shifter and the rheostat R is
readjusted until the ammeter reads the same as before.
5. Explain how true zero is obtained a Crompton’s potentiometer?
When the contact P1 is in a position such that r1/r3=r2/r4 then there is no
potential difference between the zero stud and contact P1 and so the slide
wire has a true zero.
6. How do DC potentiometer differ from ac potentiometer in working
principle?
The difference between the working of two instrument is that in dc
potentiometer only the unknown emf is balanced against a known
potential drop of the potentiometer wire where as in case of ac
potentiometer there two voltages are balanced in magnatitude as well as
in phase .
7.Define Burden of an instrument transformer?
Total secondary winding burden = (Secondary winding induced
voltage)2/impedance of secondary winding circuit including impedance
of secondary winding.
=(secondary winding current)2*impedance of secondary winding circuit
including secondary winding
8.What is watt hour?
W =∫t0Vi dt
If v is expressed in v, I in A and t in s the unit of energy is joule or watt
second which is 1 watt over an interval of one second. If the unit of time
is taken as hour energy is then expressed in watt hour.
9.Compare and contrast Ac and Dc potentiometer ?
Dc
i)Comparision of EMF’s of two cells
ii)Calibration of wattmeter
Ac
i)Measurement of self inductance
ii)Calibration of wattmeter.
10. What is meant by standardization of a potentiometer?
Standardization of a potentiometer is known as calibration with help of
standard cell to get voltage drop as Vo.
11.Write the difference between CT and VT?
i)CT connected in series with line carrying the current to be measured .
PT are connected to parallel.
ii)Primary winding of CT is independent of secondary while the primary
current in a PT certainly depends upon the burden.
12.What is volt Ratio Box?
If higher voltage ranges have to be measured a precision potential divider
called volt ratio box is used.
13. What is the basic principle used in potentiometer. In potentiometer the unknown emf is measured by comparing it with a std known emf.
14.Name the potentiometer material used. German silver, Manganin wire
15. State the applications of potentiometerUsed for m/s of unknown emf
Used for ammeter calibration Used for Voltmeter calibration Used for wattmeter calibration
16. Classify ac potentiometersPolar potentiometer Coordinate potentiometer.
17.How the phase angle is measured in polar type potentiometers. It is measured from the position of phase shifter.
18.Name some ac potentiometers. Drysdale Tinsley potentiometer Gall Tinsley potentiometer
19. State the applications of ac potentiometers. M/s of self inductance. Ammeter calibration Voltmeter calibration Wattmeter calibration 20.State the advantages of instrument transformers. Used for extension of range Power loss is minimum High voltage and currents can be measured. 21.State the disadvantage of instrument transformers. Cannot be used for dc measurements. 22. What are the constructional parts of current transformer?
o Primary winding o Secondary winding o Magnetic core.
23.Name the errors caused in current transformer. Ratio error Phase angle error
24. How the phase angle error is created. It is mainly due to magnetizing component of excitation current. 25.State the use of potential transformer. Used for m/s of high voltage Used for energizing relays and protective circuits.
Unit : IV
1.List out the various factors which affect the precision measurement of
medium resistance with Wheatstone bridge.
i)Resistance of connecting leads
ii)Thermo electric effects.
2. Why series type ohm meters are preferred over shunt type ohm meter?
Since shunt types are used for low value resistors
3. Define the following terms low resistance and high resistance?
Low resistance is defined as the resistance of 1 ohm and under is
included in this class.
High resistances are resistance of 1000 Kohm and above are usually
termed as high resistance.
4. Classify the resistance from the point of view measurements?
1) Low resistance
2) High resistance
3) Medium resistance
5. What if the importance of the value of Earth’s resistance?
Importance of earth’s resistance to stabilize the potential of the circuit
with respect to earth.
6. Why low resistances are provided with four terminals?
Since there would certainly be a trouble in determining the correct point
for galvanometer connections low resistances are provided with four
terminals.
7.Define ground and short circuit faults?
Ground fault: The insulation of the cable may breakdown causing a
flow of current from the core of the cable to lead sheath or to the earth.
This called Ground fault.
Short circuit fault : If the insulation between two conductors is faulty a
current flows between them. This is called a short circuit fault.
8.What are the difficulties encountered in measurement of high
resistance?
In order to obtain definite ratios in the potential distribution with respect
to surroundings .One point of the circuit may be connected to earth for
accuracy in measurements.
9. Differentiate volume and surface resistivities?
Volume : Resistance between two faces of units distance apart the unit
distance with all conduction from face to face being through the body of
the material.
Surface resistivity: The resistances between two lines of unit length and
unit distance apart the lines being on the surfaces of the material and all
conduction being on the surface.
10.Classify the cables according to their sheathing. i) Armoured cables ii) Unarmoured cables.
11 Name the leads present in megger.a. Earth lead b. Line lead c. Guard lead.
12How resistance is measured by using ohm meter method.
a. Series ohm meter method b. Shunt ohm meter method.
13 How resistance is measured in loss of charge method. In this method a capacitor is charged and discharged for a specific time period and from this resistance is measured. 14State the balance equation used in bridge methods. The products of opposite branch resistances are equal.
15. State the advantages of price’s guard wire method. In this method leakage current does not flows through the meter and therefore it gives accurate reading
16. Name the methods used for low resistance measurement. i) Ammeter – voltmeter method
ii) Potentiometer method iii) Kelvin double bridge method iv) Ohm meter method.
17. Name the methods used for medium resistance measurement
a. Ammeter – voltmeter methodb. Substitution method c. Wheatstone bridge method d. Carey foster bridge method.
18. What is the range of medium resistance? Resistance of about 1 ohm to 100 kilo ohms is called medium resistance.
19. State the advantages of Wheatstone bridge method. Free from errors The balance is quit independent of source emf 20.State the advantages of Kelvin double bridge method. Errors owing to contact resistance, resistance of leads can be eliminated by using This Kelvin double bridge.
21. Explain the calibration procedure. Calibration procedure involves a comparison of the particular instrument with either. ·A primary standard ·A secondary standard with a higher accuracy than the instrument to be calibrated or an instrument of known accuracy
22. Name the different essential torques in indicating instruments. a. Deflecting torque b. Controlling torque c. Damping torque
23.Name the types of instruments used for making voltmeter and ammeter
a) PMMC type b) Moving iron type c) Dynamometer type d) Hot wire type e) Electrostatic type f) Induction type.
24.How the range of instrument can be extended in PMMC instruments. 1) In ammeter by connecting a shunt resister 2) In voltmeter by connecting a series resister.
25. State the advantages of Hot wire type instruments a. Can be used for both dc and ac b. Unaffected by stray magnetic fields c. Readings are independent of frequency and waveform
Unit: V
1.State the use of ac bridges. i) AC bridges are used for the m/s of ii) self and mutual inductance and capacitance.
2.State the balance equation used in ac bridges. The product of opposite branch impedances are equal. 3.Name the bridge circuits used for the m/s of self inductance. a. Maxwell’s bridgeb. Maxwell-Wein Bridge c. Anderson bridge d. Hay’s bridge.
4. Name the bridge circuits used for the m/s of capacitance.a. De Sauty’s bridge b. Schering Bridge c. Wein bridge
5. Name the bridge circuits used for the m/s of mutual inductance.
a. The Heaviside Campbell bridgeb. The Campbell bridge
6.Which type of detector is used in ac bridges? Vibration galvanometers are used.
7.Name the ac sources used in ac bridges.
a. AC supply with step-down transformer b. Motor driven alternator c. Audio frequency and radio frequency oscillator.d.
8.In which cases audio frequency oscillators are used as ac source. a. For high frequency ac requirement b. Audio frequency oscillators are used.
9.Name the sources of errors in ac bridge machines.
a. Errors due to stray magnetic fields b. Leakage errors c. Eddy current errors d. Residual errors e. Frequency and waveform errors.
10.State the advantages of Maxwell-wein bridge. The balance equation is independent of frequency and therefore more accurate.
11.State the disadvantage of Maxwell-wein bridge. This method needs a std variable capacitor. Variable Capacitor is costliest. 12State the disadvantages of Hay’s bridge. The balance equation is dependent of frequency and therefore any changes in frequency will affect the machines. 13. State the use of Wein bridge. It is used for the m/s of unknown capacitance and frequency. 14.What is the use of Campbell bridge? This is used for the m/s of mutual inductance. 15.What is meant by inductometer? The standard variable mutual inductance meter is called as inductometer.
16.Define Q-factor of the coil. It is the ratio between power stored in the coil to the power dissipated in the coil. 17.Name the components of iron loss.
a. Eddy current loss b. Hysterisis loss.
18. Name the faults that occurs in cables. a. Break down of cable insulation b. Short circuit fault c. Open conductor fault
19.Name the loop test methods used in location of fault. a. Murray loop test b. Varley loop test.
20.How leakage errors are minimized in ac bridge circuits. By using high grade insulation.
21. What is the purpose of braking mechanism. It provides necessary braking torque.
22.State the reason why holes are provided in Al disc. To avoid creeping holes are provided on both sides of Al disc.
23. What are the practical difficulties in ac potentiometersa. More complicated b. Accuracy is seriously affected c. Difficulty is experienced in standardization.
24. State the applications of PMMC instruments a. Machines of dc voltage and current b. Used in dc galvanometer.
25. Write down the deflecting torque equation in dynamometer type wattmeter. Td α VICosΦ
Top Related