74663986 electrical measuring instruments

Electrical Measuring Instruments

(Industrial Instrumentation Assignment, Deflection Instruments & Potentiometers)

Submitted to:

Dr. Ing. Naveed Ramzan

Submitted By:

Muhammad Usman

(08-chem-02-B)

Date: November 5, 2011

Department of Chemical Engineering

University of Engineering & Technology, Lahore

He to Whom belongs the dominion of the heavens and the earth, and Who has begotten no son (children

or offspring) and for Whom there is no partner in the dominion. He has created everything, and has

measured it exactly according to its due measurements.

(Al-Quran, Chapter #25, Verse #2)

http://www.searchtruth.com/chapter_display.php?chapter=25&translator=5

http://www.searchtruth.com/chapter_display.php?chapter=25&translator=5#2

Acknowledgement

By watching this report, it apparently seems as the work is Complete but yet it isn’t. Until or unless you

acknowledge the people who have really helped you till the completion of your task, your work remains

incomplete. As Ludwig Wittgenstein’s said

“Knowledge is in the end based on acknowledgement”

So, I want to pay my gratitude & respect to Dr. Ing. Naveed Ramzan for his help. Library attendant Mr.

Shahid was also helpful during the work for arranging the scarcely found books in library but however,

this work is dedicated to his majesty Dr. Ing. Naveed Ramzan.

As Francis Bacon said,

“If a man will begin with certainties, he shall end in doubts; but if he will be content to begin with doubts,

he shall end in certainty”

I have tried my best to do the task but it will definitely be containing many deficiencies as it’s natural that

nothing from human being is perfect but effort doesn’t mean perfection. I have done it somewhat

approaching towards precision from my side but from your side it is a minute thing…

Your knowledge will be necessary to improve this little work…

Muhammad Usman

http://www.brainyquote.com/quotes/quotes/l/ludwigwitt165051.html

Abstract

Electrical Measuring Instruments are the requirement of the today’s process industry because of their

functions of indicating, recording & integrating values to be measured at remote locations & in central

Control Room. There are two main types of instruments. One which draw some current from the circuit

but show direct reading with the help of a pointer and dial arrangement & are easy to read and can be

calibrated for the desired variable by manipulating the measured variable into desired variable with the

help of five basic but different arrangements i.e. Moving Coil, Moving Iron, Induction type, Hot wire &

Electrostatic instruments. The inherent disadvantage associated with them is that they are not as much

precise as the other type of instruments which do not draw any current from circuit known as

“Potentiometer.” They are more accurate than indicating instruments. They are of two types classified as

A.C & D.C Potentiometers used with A.C or D.C supply.

Table of Contents

Chapter 1

1.1 Importance of Electrical Measuring Instruments:................................................................ 8

1.2 Classification of Electrical Instruments: .............................................................................. 8

1.3 Types of Secondary Instruments:......................................................................................... 9

1.4 Essentials of Indicating Instruments: ................................................................................. 10

Chapter 2

2.1 Types of Electrical Instruments: ........................................................................................ 13

2.1.1 Moving-iron instruments: ............................................................................................................... 14

2.1.2 Moving Coil Instruments: ............................................................................................................... 15

2.1.3 Hot-Wire Instruments: .................................................................................................................... 17

2.1.4 Induction-Type Instruments: ........................................................................................................... 18

2.1.5 Electrostatic Instruments: ................................................................................................................ 20

Chapter 3

3.1 Potentiometer: ................................................................................................................................. 23

3.4.1 D.C Potentiometers: ............................................................................................................ 23

3.4.2 A.C Potentiometers: ............................................................................................................ 24

Bibliography ................................................................................................................................. 25

Chapter 1

(Introduction To Electrical Measuring

Instruments)

Electrical Measuring Instruments:

In this modern era of technology when focus is automation in plant design and its steady state

operation the remote indication and controlling of process parameters through instrumentation is

of vital need. Almost every type of instrument generates some signal that after proper

Manipulation is displayed or recorded. The signal may be of two types.

1. Pneumatic signal

2. Electric signal

Hence, electrical measuring instruments play a vital role in process control & instrumentation.

1.1 Importance of Electrical Measuring Instruments:

The measurement of current, voltage & power is required to study the behavior of electrical

equipment or an electric circuit, under certain load conditions.(Singh, 2009)

1.2 Classification of Electrical Instruments:

They are broadly divided in two categories.

1.2.1 Absolute Instruments:

They are also called the Primary Instruments. They are not calibrated against anything

else.

Example:

Water Manometers Used for pressure Measurements are absolute or primary instrument

because water column differential is unchangeable due to its physical nature.

1.2.2 Secondary Instruments:

These instruments need to be calibrated against the primary Standards. As these

instruments are calibrated, they provide quick display of the variable of intent.

Example:

A bourdon type gauge used to measure the Pressure needs to be calibrated against some

primary standard e.g. water manometer.

Figure 1 shows the Classification of Electrical Instruments & Their Essentials.

1.3 Types of Secondary Instruments:

1.3.1 Indicating Instruments:

They display the instantaneous reading of the desired

variable. E.g. ammeter

Figure 2 shows an indicating instrument.

Figure 2: Ammeter (www.REUK.co.uk)

Electrical

Measuring

Instruments

Absolute/

Primary

Instruments

Secondary

Instruments

Indicating

Instruments

Recording

InstrumentsIntegrating

Instruments

Essentials of Indicating

Instruments

Deflecting/

Operating

Torque

Restoring/

Controlling

Torque

Damping

Torque

Spring

Control

Gravity

Control

Air-Friction Damping

Fluid-Friction Damping

Eddy-Current Damping

Figure 1: Classification of Electrical Instruments

1.3.2 Recording Instruments:

They mostly use an inked pen on chart arrangement

where pen moves according to the change in output

over a rolling chart thus recording the continuous

readings to be monitored or analyzed.

Figure 3 represent a recording instrument.

1.3.3 Integrating Instruments:

They use the set of dials & pointers to measure & record the quantity of Electricity. Figure

4 & 5 represent indicating instruments.

1.4 Essentials of Indicating Instruments:

1.4.1 Deflecting torque:

It is the torque which deflects the pointer on a calibrated scale according to the electrical

quantity passing through the instrument.

It is denoted by Td.

Figure 4: Gas Meter

(www.made-in-china.com)

Figure 3: Recording Instrument

(www.americanlaboratorytrading.com)

Figure 5: Electricity Meter

(www.made-in-china.com)

1.4.2 Controlling torque:

It is the torque which controls the movement of pointer on a particular scale according to

the quantity of electricity passing through it.

It is obtained by two methods.

a. Spring control

b. Gravity Control

1.4.3 Damping torque:

It is the torque which avoids the vibration of the pointer on

a particular range of scale. It is a stabilizing force which

brings the pointer to the rest or equilibrium position

quickly against its inertia.

Figure 6 represents a fluid friction damped instrument.

It is achieved using 3 methods.

a. Air-friction damping

b. Fluid-friction damping

c. Eddy-current damping

Figure 6: Pressure Gauge Filled with

Fluid to Damp the Pointer Inertia

(www.golbalw.com)

Chapter 2

(Direct Reading Instruments)

2.1 Types of Electrical Instruments:

Figure 7 shows the main & sub types of Electrical Measuring Instruments.

Types of

EMI’s

Moving-Iron

Instruments

Moving-Coil

Instruments

Hot Wire

InstrumentsInduction-Type

Instruments

Electrostatic

Instruments

Quadrant Type ES

Voltmeter

Attracted Disc Type

ES Voltmeter

Shaded Pole Type

Ferraris Type

Dynamometer Type

Permanent Magnet

Attraction Type

Repulsive Type

Figure 7: Types of Electrical Measuring Instruments

2.1.1 Moving-iron instruments:

In these types of instruments the deflecting element is made up of soft iron moving in the

magnetic field produced by a stationary coil.

They are of two types.

2.1.1.1 Attraction Type:

In this arrangement the magnetic field

produced at the centre of the coil or

solenoid attracts an eccentrically

mounted disc or vane towards itself

causing the motion in the pointer

moving over a calibrated scale

whenever the current is passing through

the coil.

Figure 8 represent an attraction type

instrument.

2.1.1.2 Repulsion Type:

Two parallel rods of soft Iron are arranged along

the axis of the fixed coil. One vane is fixed to the

coil frame while the other is free & is attached

with a pointer that moves over a calibrated scale.

When current is passed through the coil the

magnetic field induced in both the coils is of same

polarity causing repulsion between both the vanes

resulting in the deflection of pointer

Figure 9 represents a Repulsion type instrument.

Advantages of Moving-Iron Instruments:

The various advantages are following.

1. The instrument can be used for both A.C & D.C Measurements because a reversal in current

direction will affect the assembly in such a way that force of repulsion or attraction lies still in the

same direction.

2. Errors due to friction are very less due to high torque to weight ratio.

3. A single type of moving element can cover a wide range hence these instruments are cheaper than

other types of instruments.

Figure 8: Moving Iron Attraction Type Instrument

(U.A.Bakshi A. &., 2008)

Figure 9: Moving Iron Repulsion Type Instrument

(U.A.Bakshi A. &., 2008)

4. Extremely Rugged & reliable due to absence of any current carrying parts.

5. The range of instrument can be extended.

6. These can withstand large loads and are not damaged even under severe overload conditions.

7. These are capable of giving good accuracy. Modern Moving-Iron Instruments have a D.C error of

2% or less.

Disadvantages of Moving-Iron Instruments:

The various disadvantages are following.

1. Have non-linear scale.

2. Cannot be calibrated with a high degree of precision for D.C due to hysteresis in Iron cores.

3. If damping torque is due to gravity control than instrument needs to be placed in vertical position

always.

4. Deflection of only 240ocan be obtained.

Errors with Moving-Iron Instruments:

The various errors are following.

1) Hysteresis Error:

Due to hysteresis effect, the flux density for the same current while ascending & descending

Values is different.(U.A.Bakshi A. &., 2008)

2) Stray Magnetic field error:

External stray magnetic fields are liable to affect adversely the accurate functioning of

instrument.(Rajput, 2003)

3) Temperature Error:

Self heating of the coil can cause change in the resistance of the coil. The error is of the order

of 0.02% per 0C change in temperature.(U.A.Bakshi A. &., 2008)

4) Frequency Error:

Change in frequency of current causes the change in the reactance of the coil thus causing

error in the instrument.(U.A.Bakshi A. &., 2008)

5) Eddy Current Error:

With A.C current, the generated eddy currents in iron core cause error in the deflecting torque

thus changing the meter reading.(U.A.Bakshi A. &., 2008)

6) Wave form error:

In A.C current, change in the waveform of signal cause error in reading.(Singh, 2009)

2.1.2 Moving Coil Instruments:

In these types of instruments the deflecting element is the current carrying coil itself.

There are two types of moving-coil instruments.(Singh, 2009)

2.1.2.1 Permanent Magnet type:

A coil wound on aluminum or copper frame is placed in the radial magnetic field

produced by a Permanent u-shaped magnet provided with soft-iron pole pieces provided

with to reduce the reluctance of magnetic field by air gap.

Figure 10 shows Construction of a typical Permanent Magnet type Moving Coil

instrument.

Advantages of Permanent Magnet type Moving-Coil Instrument:

The various advantages are following.

1. Low power consumption(Wadhwa, 2006)

2. The sensitivity is high.(U.A.Bakshi A. &., 2009)

3. No hysteresis loss(Singh, 2009)

4. Uniformity of the scale and the possibility of a very long scale (Wadhwa, 2006)

5. It consumes low power, of the order of 25W to 200µW.(U.A.Bakshi A. &., 2009)

6. Very efficient eddy current damping(Singh, 2009)

7. High torque/weight ratio(Wadhwa, 2006)

8. Not affected much by stray magnetic fields due to strong operating fields(Singh, 2009)

9. Operating current is small(U.A.Bakshi A. &., 2009)

10. It has high accuracy.(U.A.Bakshi A. &., 2009)

Disadvantages of Permanent Magnet type Moving-Coil Instrument:


1) They are costly as compared to the moving iron instruments.(Singh, 2009)

2) Ageing of permanent magnet and control springs introduces the errors.(U.A.Bakshi A. &., 2009)

3) Use limited to D.C only(Singh, 2009)

4) Scale length of meter can be increased from 120o and 240

o or even 270

o or 300

o only(Singh,

2009)

5) Friction due to jewel pivot suspension.(U.A.Bakshi A. &., 2009)

Errors with Permanent Magnet type Moving-Coil Instrument:

The various errors are following.(Singh, 2009)

Figure 10: Permanent Magnet Moving Coil Instrument

(Rajput, 2003)

1. Due to friction of moving parts and temperature

2. Due to weakening of permanent magnet with the passage of time but can be eliminated by

carefully ageing the magnet during its manufacture.

3. An account of thermoelectric EMF when they are shunted for current measurement.

2.1.2.2 Dynamometer type:

If the permanent magnet in the moving-coil instrument is replaced by an electromagnet

the type is called the dynamometer type instrument. They are generally air-cored coils to

avoid any eddy current or hysteresis errors while using A.C current.

Advantages of Dynamometer type Moving-Coil Instrument:

The various advantages are following.(Singh, 2009)

1. No hysteresis loss

2. No eddy currents due to the absence of iron or aluminum core

Disadvantages of Dynamometer type Moving-Coil Instrument:

The various disadvantages are following.(Prasad, 2009)

1. These instruments are costlier than the permanent instruments.

2. The scale is not uniform.

3. Because of the heavy moving system the frictional error is more. The internal heating is also

more.

4. Because of weak operating field these instruments are very sensitive to stray field.

5. Torque to weight ratio is high.

6. Moving system is heavy and power loss becomes high.(Singh, 2009)

7. Such instruments have low sensitivity.(Singh, 2009)

Errors with Dynamometer type Moving-Coil Instrument:

The various errors are following.(Singh, 2009)

1. Frictional errors due to heavy moving parts

2. Temperature errors due to internal heating

3. Errors due to stray magnetic field.

2.1.3 Hot-Wire Instruments:

When the current to be measured passes through the hot wire, the wire gets heated and then

expands. Since the wire is fixed between two points, it sags due to expansion. This expansion

is taken up by the spring and the silk thread, which causes the pulley to rotate, moving pointer

attached to it. Figure 11 shows a hot wire instrument’s working principle.

Advantages of Hot wire Instrument:


1. It can be used for both A.C & D.C

2. Instruments are free from any wave form and frequency errors.

3. They are free from external stray magnetic field errors.

Disadvantages of hot wire Instrument:

The various disadvantages are following.(Singh, 2009)

1. As the wire takes time to heat up, response is very slow.

2. High power consumption as compared to moving coil instruments.

3. Changes in room temperature demand the adjustment in zero position of pointer.

4. Overloading may result in the melting of fine wire.

5. Fragile wire cannot withstand Mechanical shocks.

2.1.4 Induction-Type Instruments:

It works on the principle of induction i.e. on the production of eddy currents in a moving

system by the alternating fluxes. These eddy currents induced in a moving system interact with

each other to produce a driving torque due to which disc rotates to record the energy.

(U.A.Bakshi, A.V.Bakshi “Electrical Measurements And Instrumentation”)

Figure 11: Hot Wire Instrument

(www.dc146.4shared.com)

Induction type instruments are suitable for A.C circuits only, as their working depends on

induced current due to an alternating flux.(Singh, 2009)

They are mostly used as integrating meters to the amount of energy. There are two general

types of induction instruments.

2.1.4.1 Ferraris-type induction instrument:

A rotating magnetic field is produced by

two pairs of coils wound upon a

laminated magnet system. These pairs

of coils are supplied from the same

source, but a phase displacement of

approximately 90o is produced in the

current flowing in them by connecting

an inductance in series with one pair

and a high resistance with the other, to

produce a rotating magnetic field. This

rotating field induces currents in an

aluminum drum and causes this drum to

follow its rotation. If the drum is free to

rotate, it will rotate at a speed slightly

less than that of the rotating field but

in the same direction of the

field.(Singh, 2009)

Figure 12 shows construction of a Ferraris type instrument.

2.1.4.2 Shaded pole type induction instruments:

It consists of a laminated electromagnet having

an air gap in it in which an edge of a thin

aluminum disc rotates. This disc is mounted on

a spindle having a pointer and equipped with a

control spring. About one third of the pole of

the electromagnet is encircled by a copper strip

which provided a phase shifting arrangement.

When A.C current is supplied to the

electromagnet eddy currents are induced in the

shading ring, and the flux inside the shaded

portion of the ring lags behind the main flux by

an angle of 40-50o. This phase displacement

produces a torque on disc which rotates and

measures the value of current or voltage.

Figure 13 shows a shaded pole type instrument.

Figure 12: Ferraris-type Induction instrument

(S. Kamakshaiah)

Figure 13: Shaded Pole Type Induction Instruments

(S. Kamakshaiah)

Advantages of Induction type Instrument:


1. A full scale deflection of about 3000 obtainable, giving a long and open scale.

2. Stray magnetic fields having limited effect upon their readings.

3. Good and very efficient damping.

Disadvantages of Induction type Instrument:

The various disadvantages are following. (Singh, 2009)

1. These instruments are costlier.


3. Power loss becomes high.

2.1.5 Electrostatic Instruments:

In these types of instruments the deflecting torque is produced by a force of attraction between

two or more electrically charged conductors between which a potential difference is maintained.

There are two types of these instruments.

2.1.5.1 Quadrant-type Electrostatic instrument:

It consists of two sets of metal plates, one

movable and other fixed. A pointer is attached

to the movable spindle that in turn is attached

to the movable plates. When voltage to be

measured is applied between plates they act

like a capacitor having different charges on two

plates whose amount is proportional to the

applied voltage or current. Change in

capacitance is shown on a scale with pointer

movement.

Figure 14 shows a quadrant type instrument.

Figure 14: Quadrant type Electrostatic Instrument

(A.V.Bakshi, U.A.Bakshi , “Electrical Machines And

Instruments”)

2.1.5.2 Attracted-Disc type:

It consists of two discs, one fixed and the other

moving, mounted parallel to each other. When

the potential difference to be measured is

applied between two discs the moving disc is

attracted towards the fixed disc. The movement

of the moving disc is balanced by a control

device which actuates a pointer attached to it

that moves over a calibrated scale.

Figure 15 is an attracted disc type instrument.

Advantages of Electrostatic Instrument:


1. Very high accuracy

2. Work for both A.C & D.C

3. No Calibration needed for A.C & D.C separately

4. Free from hysteresis

5. Power loss is negligible

6. Unaffected by stray magnetic field

Disadvantages of Dynamometer type Moving-Coil Instrument:


1. These instruments are costlier & are not durable.


3. In low voltage applications the frictional error is more.

4. These instruments are very sensitive to stray Electrostatic field.

5. They are inherently laboratory type rather than industrial type instruments.

Figure 15: Attracted Disc Type Instrument

(A.V.Bakshi, U.A.Bakshi , “Electrical Machines

And Instruments”)

Chapter 3

(Indirect Reading Instruments)

3.1 Potentiometer:

Potentiometer is an instrument used to measure an unknown E.M.F which is compared with

known E.M.F. known E.M.F is obtained from a standard cell or reference voltage

source.(U.A.Bakshi K. A., 2008)

3.2 Requirements & Applications of Potentiometer:(Singh, 2009)

1. Precision required is very high as compared to that can be obtained by deflection instruments.

2. It is important that no current be drawn from the source under measurements.

3. The current must be limited to a small value.

3.3 Advantages of Potentiometer:

1. It has very high accuracy because the result obtained does not depend upon the actual

deflection of a pointer. Its result depends upon the accuracy with which the voltage of the

reference source is known.

2. It has no power consumption from the circuit containing unknown E.M.F, when it is

balanced.

3. The determination of voltage by potentiometer is quite independent of the source resistance.

3.4 Types of Potentiometer:

There are two types of potentiometers used in the industry.

3.4.1 D.C Potentiometers:

They are operated by D.C Current. There are many types of D.C Potentiometers.

Some of them are following.

3.4.1.1 Slide wire Potentiometer:

It consists of a slide wire AB

of uniform section and

higher resistance. An

adjustable and steady current

I, regulated by Resistance R

is maintained by a constant

E.M.F source of known

voltage B2. The unknown

E.M.F source i.e. battery B1

is attached in parallel to the

circuit so that when E.M.F’s

are equal galvanometer will

show zero deflection because

of absence of current flowing in the circuit as the two sources are providing the

current in opposite direction. From the slide wire length we can calculate the

unknown E.M.F.

Figure 16 shows a slide wire Potentiometer.

Figure 16: Slide Wire Potentiometer (Electrical Measurements and

Measuring Instruments)

By U.A.Bakshi, A.V.Bakshi

3.4.1.2 Crompton Potentiometer:

Crompton Potentiometer is the modified version of the slide wire potentiometer

wherein calibrated dial resistor with a small circular wire of one or more turns are

used thereby reducing the size of the instrument. The effect of a very long slide

wire is achieved by connecting a number of resistance coils in series with a

comparatively short slide-wire.

3.4.1.3 Vernier potentiometer:

They are also known as Duo-Range Potentiometers. They are designed to

overcome the disadvantages of Crompton Potentiometers by further increasing

the reading accuracy. It can read values up to 1µV.

3.4.2 A.C Potentiometers:

A.C Potentiometers works on the same principle as D.C potentiometer except that in

A.C potentiometer both Magnitude & phase of the unknown E.M.F and

potentiometer voltage drop have to be made equal to obtain balance.(Singh, 2009)

They are operated by A.C Current. They are of following types.

3.4.2.1 Polar type potentiometer:

It is also known as drysdale polar potentiometer. It measures the unknown

E.M.F in polar form i.e. in terms of its magnitude and relative phase.

3.4.2.2 Rectangular co-ordinate type:

They measure the unknown E.M.F in terms of its regular co-ordinates. They are

also called the gall co-ordinate Potentiometer.

Bibliography

Prasad, R. (2009). Fundamentals Of Electrical Engineering (Vol. 1). New Delhi: PHI Learning (P) LTD.

Rajput, R. (2003). A Textbook of Electrical Engineering (Vol. 1). New Delhi: Laxmi Publications (p)

LTD.

S. Kamakshaiah, J. A. Electrical Measurements and Measuring Instruments (Vol. 1). New Delhi: I.K.

International Publishing House.

Singh, S. K. (2009). Industrial Instrumentation & Control. New Delhi: Tata McGraw-Hill.

U.A.Bakshi, A. &. (2009). Electrical And Electronics Measuring Instruments (Vol. 1). pune: Technical

Publications Pune.

U.A.Bakshi, A. &. (2008). Electronic Measurements & Instrumentation (Vol. 1). Pune : Technical

Publications Pune.

U.A.Bakshi, A. Electrical Machines And Instruments.

U.A.Bakshi, K. A. (2008). Electrical Measurements (Vol. 1). pune: Technical Publications Pune.

Wadhwa, C. (2006). Basic Electrical Engineering (Vol. 1). New Delhi: New Age International (P) LTD.

74663986 electrical measuring instruments

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