74663986 electrical measuring instruments
Post on 28-Nov-2015
46 Views
Preview:
DESCRIPTION
TRANSCRIPT
Page 1
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
Page 2
Page 3
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)
Page 4
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
Page 5
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.
Page 6
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
Page 7
Chapter 1
(Introduction To Electrical Measuring
Instruments)
Page 8
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.
Page 9
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
Page 10
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)
Page 11
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)
Page 12
Chapter 2
(Direct Reading Instruments)
Page 13
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
Page 14
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)
Page 15
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.
Page 16
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:
The various disadvantages are following.
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)
Page 17
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.
Page 18
Advantages of Hot wire Instrument:
The various advantages are following.(Singh, 2009)
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)
Page 19
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)
Page 20
Advantages of Induction type Instrument:
The various advantages are following.(Singh, 2009)
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.
2. The scale is not uniform.
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”)
Page 21
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:
The various advantages are following.(Singh, 2009)
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:
The various disadvantages are following.
1. These instruments are costlier & are not durable.
2. The scale is not uniform.
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”)
Page 22
Chapter 3
(Indirect Reading Instruments)
Page 23
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
Page 24
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.
Page 25
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.
top related