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