Transcript Measurement - WordPress.com

By Velar H. Elias
Introduction
INTRODUCTION
Measurement: measurement means, to
monitor a process or a operation and using an
instrument, express the parameter, quantity or
a variable in terms of meaningful numbers.
 Measurement of a given parameter or quantity is
the act or result of a quantitative comparison
between a predefined standard and an unknown
quantity to be measured.

Evolution of Instruments I
Mechanical
b) Electrical
c) Electronic Instruments
a)
A. Mechanical:
these instruments are very reliable for static and
stable conditions. But their disadvantage is that they
are unable to respond rapidly to measurements of
dynamic and transient conditions.
Evolution of Instruments II
B. Electrical:
it is faster than mechanical, indicating the output are
rapid than mechanical methods. But it depends on the
mechanical movement of the meters. The response is
between 0.5 to 24 seconds.
C. Electronic:
it is more reliable then other system. It uses
semiconductor devices and weak signal can also be
detected.
Advantages of Electronic
Measurement






Most of the quantities can be converted by transducers
into the electrical or electronic signals.
Electronic signals can be amplified, filtered, multiplexed,
sampled and measured.
Measured signals can be transmitted over long distance
through cables or radio links, without any loss of
information.
Many measurements can be done simultaneously or in
rapid succession.
Electronic circuits can measure the events of very short
duration.
Higher sensitivity, low power consumption and a higher
degree of reliability are the important features.
Performance Characteristics

Static Characteristics: the set of criteria defined
for the instruments, which are used to measure
the quantities which are slowly varying with time
or mostly constant, i.e., do not vary with time is
called static characteristics.

Dynamic Characteristics: when the quantity
under measurement changes rapidly with time, it
is necessary to study the dynamic relations
existing between input and output which is
expressed as differential equations.
CALIBRATION

calibration is the process of making an adjustment or
making a scale so that the reading on an instrument
agree with the accepted and certified standard.

Calibration is a comparison between a known
measurement (the standard) and the measurement using
your instrument. Typically, the accuracy of the standard
should be ten times the accuracy of the measuring
device being tested. However, accuracy ratio of 3:1 is
acceptable by most standards organizations.

In practice, calibration also includes repair of the device
if it is out of calibration.
CALIBRATION

Calibration of your measuring instruments has two
objectives:
1.
2.
It checks the accuracy of the instrument
And it determines the traceability of the measurement.
Calibration improves the accuracy of the measuring
device. Accurate measuring devices improve product
quality.
 When should you calibrate your measuring device?

1.
2.
3.
According to recommendation of the manufacturer.
After any mechanical or electrical shock.
Periodically (Daily, monthly, before use)
Definitions
Instrument: a device for determining the value
or magnitude of a quantity or variable.
 Accuracy: closeness with which an instrument
reading approaches the true value of the variable
being measured.
 Precision: a measure of the reproducibility of the
measurement i.e., given a fixed value of a
variable, precision is a measure of the degree to
which successive measurements differ from one
another.

Definitions
Sensitivity: the ratio of output signal or response of
the instrument to a change of input or measured
variable.
 Resolution: the smallest change in measured value to
which the instrument will response.
 Error: deviation from the true value of the measured
variable. Several techniques may be used to
minimize the effect of error.
 the algebraic difference between the indicated value
and the true value of the quantity to be measured is
called an ERROR

Precision and accuracy comparison

Accuracy refers to the degree of closeness or
conformity to the true value at the quantity under
measurement. Precision refers to the degree of
agreement within a group of measurements or
instruments.

In critical work, good practice dictates that the observer
make an independent set of measurements, using
different instruments or different measurement
techniques, not subject to the same systematic errors.
He must also make sure that the instruments function
properly and are calibrated against a known standard,
and that no outside influence affects the accuracy of his
measurements.
Precision and accuracy comparison
Sources of Errors
Faulty design of instrument
2. Insufficient knowledge of quantity and design
conditions
3. Improper maintenance of the instrument.
4. Sudden change in the parameter to the measured.
5. Unskilled operator
6. Effects of environmental conditions.
1.
Errors

A study of errors is a first step in finding way to
reduce them.

Errors types:
1. Gross errors: Largely human errors, among them
misreading of instruments incorrect adjustment and
improper
application
of
instruments,
and
computational mistakes.
2. Systematic errors: Short coming of the instruments,
such as defective or wrong parts, and effective of
environment on the equipment or the user.
(Instrument Error)
3. Random errors: these errors are due to unknown
causes and occur even when all systematic errors
have been accounted for.
Gross Error: (Personal Errors)
o
o

Occurs due to carelessness of human while
reading, recording and calculating results.
Due to incorrect adjustments of instruments.
To eliminate error
o Take care while reading, recording and
calculating results.
o Take 3 or more reading with 3 or more persons.
Systematic Error

Types
1. Instrumental error
2. Environmental error
3. Observational error
Systematic-Instrumental Errors

Defined as shortcoming of the instrument, and
can be avoided by:
1. Selecting a suitable instrument for the particular
measurement application
2. Applying correction factors after determining the
amount of instrument error
3. Calibrating the instrument against the standard
Systematic-Environmental errors

Due to external conditions affection the
measurement, environmental errors are due to
conditions external to the measuring device,
including conditions in the area surrounding the
instrument, such as the effect of change in
temperature, humidity, barometric pressure, or of
the magnetic or electrostatic fields

Systematic errors can also be subdivided into
static or dynamic errors.
Systematic-Environmental errors

To eliminate the error:
1. Proper correction factors given by the manufacturer.
2. Make arrangements to keep surrounding constant .
3. Sealing the components to avoid dust, humidity
4. Providing magnetic or electrostatic shields
Observational errors
Errors made by observes
 Ex: parallax error while reading a meter, wrong
scale selection


To eliminate the error
o Use instruments with mirrors
o Knife edged pointers
Random errors
In well designed experiments, few random errors
usually occur, but they become important in high
accuracy work.
 The only way to offset these errors is by
increasing the number of readings and using
statistical means to obtain the best approximation
of the true value of the quantity under
measurement
 They cannot be corrected by any method.

UNITS

It is necessary to specify type and magnitude for
the reading. Where unit represents the type of the
physical quantity and reading on the instrument
represents its magnitude.

Different system of units are
 M.K.S
 C.G.S
 S.I (System International units)
System International Units (SI)
Quantity
Units
Symbol
Length
Meter
M
Mass
Kilogram
Kg
Time
Second
S
Electrical current
Ampere
A
Thermal temperature
Kelvin
K
Luminous intensity
Candela
cd
Amount of substance
Mole
mol
Table of units
Name
Symbol
Equivalent
Yotta
Y
10^24
Zetta
Z
10^21
Exa
E
10^18
Peta
P
10^15
Tera
T
10^12
Giga
G
10^9
Mega
M
10^6
Kilo
K
10^3
Hecto
h
10^2
Deca
da
10
Deci
d
10^-1
Table of Units
Name
Symbol
Equivalent
Centi
c
10^-2
Mili
m
10^-3
Micro
μ
10^-6
Nano
n
10^-9
Pico
p
10^-12
Femto
f
10^-15
Atto
a
10^-18
Zepto
z
10^-21
Yocto
y
10^-24
UNITS

Units categories
1. Fundamental Units
2. Supplementary Units
3. Derived Units
1)
Fundamental Units:
Units which are independently chosen and not dependent
on any other units are called fundamental units of
base units.
UNITS
Primary fundamental units
Auxiliary fundamental units
Length
Thermal
Mass
Electrical
Time
Illumination
Amount of substance
UNITS

Supplementary Units:
 Radian for the plane angle(θ,φ)
Plane angle subtended by an arc of a circle equal in
length to the radius of the circle.

Derived Units:
 These units are derived from fundamental and
supplementary units.
 Ex: velocity (m/s), acceleration (m/s2), force (N)
UNITS
Quantity
Table of some of the derived units
Units
Symbol
Electrical capacitance Farad (1C/1V)
F
Electrical charge
Coulomb (1A.1Sec)
C
Electric conductance
Siemens (1/ohm)
S
Electric potential
Volt (1J/1C)
V
Electric resistance
Ohm (V/A)
Ω
Energy
Joule (1N.1meter)
J
Force
Newton (1Kg*meter)/sec^2
N
Frequency
Hertz (1/sec)
Hz
Magnetic flux
Weber (Kg*meter^2)/(A*sec^2) = (1V*1sec)
Wb
Magnetic flux density
Tesla (V*sec/meter^2)
T
Plane angle
Radian (length of a corresponding arc of a unit
circle)
rad
Power
Watt (1J/1Sec) (V^2
/ Ω) = (A^2 * Ω)
W
Units
Radian =360/2π degree=180/π degree
 1 Coulomb = 6.28 *10^18 Electrons
 Light speed = 299792 458 m/sec


What is mean by Unitless value?

Conversion of UNITS
English to SI unit conversion
English Units
Symbol
Metric equivalent
Length
1 foot
1 inch
ft
in
30.48 cm
25.4 mm
Mass
1 pound
Lb
0.45359 kg
Density
1 lb/ft^3
16.0185 kg/m^3
Velocity
1 ft/sec^2
0.3048 m/sec^2
Force
1 poundal
Work, energy
Power
1 ft*pdl
1 horse-power
Temperature
pdl
0.138255 N
0.04214 J
hp
degree Fº Fahrenheit
745.7 W
5(t-32)/9 Cº
STANDARDS
a standard of measurement is a physical representation of a
unit of measurement.
 a unit is realized by reference to an arbitrary material
standard or to natural phenomena including physical and
atomic constant.


Type of standards
International standards
Primary standards
Secondary standards
Working standards
Just as there are fundamental and derived units of
measurement, we find different types of standards of
measurement, classified by their function and application
in the following categories.
1.
2.
3.
4.
International Standards
These standards are maintained at the (IBS)
international bureau of weights and measures and
are periodically evaluated and checked by
absolute measurements.
 These standards are not available for ordinary
users for calibration.
 For accuracy they are replaced by absolute units
which are more accurate than international
standards.

Primary Standards
They are maintained at national standard
laboratories in different countries (NBS).
 These standards represents fundamental units as
well as electrical and mechanical derived
calibrated by absolute measurements at each
national laboratories.
 Used for calibration and verification of secondary
standards.

Secondary Standards
Since primary standards are not available for
outside users, various industries need some
reference.
 They are used by measurement and calibration
laboratories and are maintained by the particular
industry to which they belong.
 Each industry has its own standards

Working Standards
These are the basic tools of a measurement
laboratory.
 Use to check and calibration for accuracy and
performance or to perform comparison
measurements in industrial applications.
 Ex: resistor industry maintains a standard resistor
for checking the values of manufactured resistors

IEEE Standards

Institute of Electrical and Electronic Engineering
standards are checking of secondary standards.

Used for testing and evaluating of electronic
systems and components.

The most important standards is (IEEE 488)
digital interface for programmable
instrumentation for test and other equipment.

IEEE classification (word file)
Analog and Digital Instruments
An analogue instrument are the instrument
that use analogue signal to display the
magnitude of quantity under measurement.
 An analogue instrument gives an output that
varies continuously as the quantity being
measured changes. The output can have an
infinite number of values within the range that
the instrument is designed to measure.

Analog and Digital Instruments
The digital instrument use digital signal to
indicate the results of measurement in digital
form.
 A digital instrument has an output that varies in
discrete steps and so can only have a finite
number of values. like binary signal which take
only two levels zero and one.

Analog to Digital Converter ADC
Analogue instruments must be interfaced to the
microcomputer by an analogue-to-digital (A/D)
converter, which converts the analogue output
signal from the instrument into an equivalent digital
quantity that can be read into the computer.
 This conversion has several disadvantages. Firstly,
the A/D converter adds a significant cost to the
system. Secondly, a finite time is involved in the
process of converting an analogue signal to a digital
quantity, and this time can be critical in the control
of fast processes where the accuracy of control
depends on the speed of the controlling computer.

Electromechanical Instruments vs
Digital measuring Instruments
The electromechanical indicating instruments,
cheap manually balanced bridge instruments or
induction type watt-hour meters are still present
everywhere.
 There are several advantages of traditional
electromechanical instruments: simplicity,
reliability, low price. The most important
advantage is that the majority of such instruments
can work without any additional power supply.

Electromechanical Instruments vs
Digital measuring Instruments

Drawbacks of electromechanical analogue
instruments:
1. they do not provide electrical output signal, thus
there is a need for operator’s activity during the
measurement (at least for the reading of an
indicated value).
2. instruments generally use moving mechanical
parts, which are sensitive to shocks, aging or
wearing out.
Electromechanical Instruments vs
Digital measuring Instruments

Moreover without understanding of the principles
of old analogue measuring methods it can be
difficult to understand usually more complicated
digital instruments which often use traditional
principles of operation.
Active and Passive Instruments

Active Instruments
 – the quantity being measured simply modulates
(adapts to) the magnitude of some external power
source.

Passive Instruments
 – the instrument output is entirely produced by the
quantity being measured

Difference between active & passive instruments
is the level of measurement resolution that can be
obtained.
Active and Passive Instruments
1. Active Instrumentse.g. Float-type petrol tank level indicator
Active and Passive Instruments
1. Active Instruments1. The change in petrol level moves a potentiometer
arm, and the output signal consists of a proportion of
the external voltage source applied across the two
ends of the potentiometer.
2. The energy in the output signal comes from the
external power source: the primary transducer float
system is merely modulating the value of the voltage
from this external power source.
Active and Passive Instruments
2. Passive Instrumentse.g. Pressure-measuring device
Active and Passive Instruments
2. Passive Instruments1. The pressure of the fluid is translated into a
movement of a pointer against scale.
2. The energy expanded in moving the pointer is derived
entirely from the change in pressure measured: there
are no other energy inputs to the system.
Active and Passive Instruments

Another different is in resolution:
1.
Passive instrument: Whilst it is possible to increase
measurement resolution by making the pointer
longer, such that the pointer tip moves through a
longer arc, the scope for such improvement is
clearly restricted by the practical limit of how long
the pointer can conveniently be.
2. Active instrument, however, adjustment of
the
magnitude of the external energy input
allows much greater control over
measurement resolution.
Active and Passive Instruments
In terms of cost, passive instruments are
normally of a more simple construction than
active ones and are therefore cheaper to
manufacture.
 Therefore, choice between active and passive
instruments for a particular application involves
carefully balancing the measurement resolution
requirements against cost.

Measuring Instruments

Classified measuring instruments in to two
groups:
1. Absolute Instruments (Standard instruments)
2. Secondary Instruments
Absolute Instruments

These instruments gives the magnitude of quantity
under measurement in terms of physical constants
of the instrument
 e.g. Tangent Galvanometer. These instruments do not
require comparison with any other standard instrument

These instruments give the value of the electrical
quantity in terms of absolute quantities (or some
constants) of the instruments and their deflections.

In this type of instruments no calibration or
comparison with other instruments is necessary.
Absolute Instruments

• They are generally not used in laboratories and
are seldom used in practice by electricians and
engineers. They are mostly used as means of
standard measurements and are maintained lay
national laboratories and similar institutions.

• Some of the examples of absolute instruments
are:

* Tangent galvanometer
* Raleigh current balance
* Absolute electrometer
Absolute instruments

Tangent galvanometer is an early measuring
instrument for small electric currents. It consists of a
coil of insulated copper wire wound on a circular nonmagnetic frame. Its working is based on the principle of
the tangent law of magnetism

An electrometer is an electrical instrument for
measuring electric charge or electrical potential
difference. The absolute electrometer was first proposed
by Lord Kelvin

Raleigh current balance (or ampere balance), a device
used to reproduce the unit of electric current, the
ampere. Made of nonmagnetic materials.
Secondary Instruments

There are direct reading instruments.

These instruments are calibrated by comparison with an
absolute instrument or another secondary instrument, they are
used in general for all laboratory purposes.

Therefore secondary instruments are most commonly used.

They are direct reading instruments. The quantity to be
measured by these instruments can be determined from the
deflection of the instruments.

Some of the very widely used secondary instruments are:
ammeters, voltmeter, wattmeter, energy meter (watt-hour
meter), ampere-hour meters etc.
Secondary Instruments
Secondary Instruments

Secondary instrument may be grouped on the
basis of various effects of electric current as
follows:
1. Magnetic effect (usually for Ammeters and Voltmeters)
2. Heating effect (for Ammeters and Voltmeters)
3. Electromagnetic effect (ammeter, voltmeter, wattmeter,
watt-hour meter)
4. Electrostatic effect (for Voltmeters only)
5. Chemical effect (DC Ampere-Hour meter
Secondary Instrument effects
Secondary instruments

Secondary instruments can be classified due
to the method of taking reading from the
instruments as follow:
1. Indicating Instruments
2. Recording instruments
3. Integrating instruments
1. Indicating Instruments
Indicating instruments are those which indicate the
instantaneous value of the electrical quantity being
measured at the time at which it is being measured.
 Their indications are given by pointers moving over
calibrated dials.
 The moving system is subjected to the following
three torque forces:
A. Deflecting (Torque) force
B. Controlling (Torque) force
C. Damping (Torque) force

1. A. Deflecting Torque (Td)

The deflecting or operating torque which causes the
moving system of the instrument to move from its
zero position by utilizing any one of the effects
already measured (magnetic, electrostatic, thermal or
inductive …).

The deflection of the moving system would be
indefinite if these were no controlling or restoring
torque.
1. B. Controlling Torque (Tc)
This torque is oppose the deflecting torque and
increases with the deflection of the moving system.
 This force control and limits the deflection of the
pointer on scale which must be proportional to the
measured value, and also ensure that the magnitude
of the deflection is always the same for the same
values of quantity.
 Without such at torque, the pointer would swing over
to the maximum deflected position irrespective of the
magnitude of the current to be measured.
 Absence of a restoring torque, the pointer one
deflected, would not return to its zero position

1. B. Controlling Torque

Function of controlling torque:
1. It balance the deflecting torque, and insures that the
magnitude of the deflection is always same for a
particular value of the quantity to be measured.
2. It brings back the moving system (pointer) to its zero
position.

Tc=Td (Equilibrium) at measured (position) value.

Controlling torque obtained be:
1. Controlling by Spring.
2. Controlling by Gravity.
1. B. I. Controlling Torque by Spring





A hair spring (usually of phosphor-bronze) attached to
the moving system from one end and second end of the
spring is fixed.
With the deflection of the pointer, the spring is twisted in
the opposite direction.
The twist in the spring produce restoring torque which is
directly proportional to the angle of deflection of the
moving system.
The pointer comes to a position of rest (equilibrium)
when the deflecting torque and controlling torque are
equal.
Note that the controlling torque is in opposite direction to
the deflecting torque.
1. B. I. Controlling Torque by Spring
1. B. I. Controlling Torque by Spring





Td α I
(Td=Kd * I)
And for spring control
Tc α ϴ (Tc=Kc * ϴ)
As Tc =Td at measured value
ϴ α I ; ϴ Kc=I Kd
ϴ= Kd / Kc * I
Then ϴ= K * I

Kd : deflecting constant
 Kc : controlling constant
 K : relation constant

Linear relation
1. B. I. Controlling Torque by Spring

Tc = Kc * ϴ

Tc
E
Td
L
b
t
ϴ
I







Controlling torque
Young’s modulus of spring material (Kg/m)
Deflecting torque
Total length of spring strip (m)
Depth of the strip (m)
Thickness of the strip (m)
Angular of deflection (rad)
Moment of inertia of spring (m^4)
1. B. I. Controlling Torque by Spring
The control torque is provided by
two hair springs, coiled in
opposite directions acting one
against the other.
 Equilibrium force of two springs
equal to zero, therefore the
pointer remains at zero position.

Springs are made of such material which
1. Are non-magnetic
2. Are not subject to fatigue
3. Have low specific resistance
4. Have low temperature-resistance coefficient
1. B. I. Controlling Torque by Spring
1. B. II. Controlling Torque by Gravity
In Gravity controlled instruments a small weight is
attached to the moving system such that the
deflecting torque has to act against the action of
gravity. Thus the controlling torque is obtained.
 The another weight is used for ZERO adjustment and
balancing of the moving system.


When the current flows through the instrument, the
pointer deflects through an angle ϴ. Controlling
mass also deflect from its original by an angle ϴ thus
providing a control-line torque equal to the product
of the controlling weight and specific distance.
1. B. II. Controlling Torque by Gravity
1. B. II. Controlling Torque by Gravity
Controlling torque is proportional to the sine of the
angle of deflection:
 Tc α sin ϴ
Tc = W L sin ϴ
 W =controlling weight
 L = deflecting distance
Kc=W L


Tc=Kc*sin ϴ
 At equilibrium
 Tc =Td
 Then

and
Td=Kd * I
Kc sin ϴ = Kd * I
1. B. II. Controlling Torque by Gravity

This is nonlinear relation

Gravity control is not suited for the indicating
modern instrument. Instead spring control is used in
almost all types of indicating instruments.
Comparison between spring and gravity control
In Gravity the controlling torque increases very
slow because it is proportional to the sine of angular
deflection. But in spring increases very fast because
the controlling torque is proportional to angle of
deflection.
2. Gravity instrument must used vertically, but spring
instrument can be used vertically and horizontally.
3. Gravity instrument must be leveled before being
used. Otherwise there will be a serious zero error.
4. Gravity instruments has lower cost compared to
spring instrument.
1.
1. C. Damping torque
Due to the Inertia of the moving system, the pointer
would oscillate about its final position for a long
time before it comes to rest position (steady state
position).
 To overcome these difficulties, damping torque is
essential.
 A damping torque is act on the moving parts of the
instrument only when it is moving and its always
opposed its motion.
 A damping torque is necessary to bring the pointer to
rest position quickly.

1. C. Damping torque

The degree of
damping should
be adjusted to a
value which is
sufficient to
enable the
pointer to rise
quickly to
deflected
position
without
overshooting .
1. C. Damping torque
If the instrument is under damping, the moving
system oscillates a lot before it finally settles down to
its steady value.
 In case of critically damped instruments, the pointer
reaches its final steady position rapidly and smoothly
(such an instrument is also called dead-beat
instrument).
 An over damped instrument produces damping
torque more than the required value, as such the
pointer moves slowly to its final steady value.
 There are three types of damping torque

1. C. I. Air Friction Damping system
1. C. I. Air Friction Damping system

In the first case, a thin aluminum vane moves in a
sector shaped box.
 The vane is contacted to the spindle of moving system.

In the second method, a light piston made of
aluminum attached to the moving system, moves in
an air chamber closed at one end.
 When the piston moves rapidly into the chamber, the air
enclosed in the chamber is compressed and pressured, thus
developed, opposes the motion of the moving system.
 The motion of the moving system is again opposed by the
air on the open side to the piston, when it is moving out the
chamber as the pressure outside is greater than that on the
operate side.
1. C. II. Fluid Friction Damping system
1. C. II. Fluid Friction Damping system


In this case the disc or vane attached to the spindle
moves in a damping oil.
The oil used must fulfill the following requirements:
1.
2.
3.
4.


It should not evaporate quickly
It should not have any corrosive action upon metals.
Its viscosity should not change with temperature.
It should be good insulator.
In the first system, a disc is immersed in the oil. The
friction drag developed during the motion of the disc
attached to the moving system always opposes the
motion.
In second system, vans are used.
1. C. III. Eddy Current Friction
Damping system
1. C. III. Eddy Current Friction
Damping system





This form is most efficient form of damping.
The essential component in this method is a
permanent magnet and a light disc of conducting
materials, mostly of aluminum.
When a sheet of a conducting material moves in a
magnetic field, eddy currents are induced in it, and a
force is produced opposing the motion.
This principle has been applied to provide damping
torque in many instrument.
Damping torque is directly proportional to the
movement of the moving system.
Types of Indicating Instruments
2. Recording Instruments
these instruments record continuously the variations
of an electrical quantity or physical quantities such as
flow, pressure, temperature as a function of time.
 instruments like recording devices, X-Y plotter, and
oscilloscope.

3. Integrating Instruments
These instruments measure the total amount of
quantity of electricity (Ampere-Hour) or the total
energy (Watt-Hour) supplied to a circuit over
specified period.
 These type of energy meters is used both for AC and
DC.

Ammeters and Voltmeters

The operation principles of ammeters and
voltmeters are the same, and hence both these
meters are discussed together.

In all types of ammeter, the deflecting torque is
produced by the current to be measured.

For voltmeters, the deflecting torque is also
produced by the same current, which is
proportional to the voltage being measured.
Ammeters and Voltmeters

Ammeters are used to measure the current
flowing in a circuit, as such these are connected
in series with the circuit.

The voltage drop across the terminals of the
ammeter should be as low as possible, so that the
power consumed by the meter is small. Hence
the resistance of the ammeter should be very
low.
Ammeters and Voltmeters

Voltmeters are used in a circuit for the
measurement of voltage across any two points of
the circuit.

Thus these are connected in parallel with the
circuit.

When connected in a circuit, the voltmeter must
draw a very small current, so that the power
consumption of the meter is small. The
resistance of the voltmeters should be very high.
Permanent Magnet Moving Coil
Instrument
Permanent Magnet Moving Coil (PMMC)
ammeters and voltmeters are used for measuring
current and voltage respectively in dc systems.
 The PMMC type instrument uses two permanent
magnets in order to create stationary magnetic
field.
 And it consists of a moving coil suspended
between the poles of a horseshoe type permanent
magnet is called the D’Arsonval meter.

Permanent Magnet Moving Coil
Instrument

This design offers the largest magnet in a given
space and is used when maximum flux in the air
gap is required.

Also, Shoe poles are curved to have a uniform
magnetic field through the coil.

It provides an instrument with very low power
consumption and low current required for full
scale deflection (fsd).
Permanent Magnet Moving Coil
Instrument
PMMC construction

(a) Stationary part or magnet system: In the present
time we use magnets of high field intensities, high
coercive force instead of using U shaped
permanent magnet having soft iron pole pieces.

(b) Moving coil: The moving coil can freely moves
between the two permanent magnets as shown in
the figure given below. The coil is wound with
many turns of copper wire and is placed on
rectangular aluminum which is pivoted on jeweled
bearings.
PMMC construction

(c) Control system: The spring generally acts as
control system for PMMC instruments. The spring also
serves another important function by providing the
path to lead electric current in and out of the coil.

(d) Damping system: The damping force hence torque
is provided by movement of aluminum former in the
magnetic field created by the permanent magnets.


(e) Meter: Meter of these instruments consists of light
weight pointer to have free movement and scale which
is linear or uniform and varies with angle.
Error in PMMC instruments

(a) Errors due to permanent magnets: Due to temperature effects
and aging of the magnets the two magnet may lose their
magnetism to some extent. The magnets are generally aged by the
heat and vibration treatment.

(b) Error may appear in PMMC Instrument due to the aging of the
spring. However the error caused by the aging of the spring and
the errors caused due to permanent magnet are opposite to each
other, hence both the errors are compensated with each other.

(c) Change in the resistance of the moving coil with the
temperature: Generally the temperature coefficients of the value
of coefficient of copper wire in moving coil is very low. Due to
lower value of temperature coefficient the temperature rises at
faster rate and hence the resistance increases. Due to this
significant amount of error is caused.
Advantages of PMMC instruments

(1) High sensitivity

(2) The scale is uniformly divided as the electric current is
directly proportional to deflection of the pointer. Hence it
is very easy to measure quantities from these instruments.

(3) Power consumption is also very low in these types of
instruments.

(4) Higher value of torque is to weight ratio.

(5) These are having multiple advantages, a single
instrument can be used for measuring various quantities
by using different values of shunts and multipliers.
Disadvantages of PMMC instruments
(1) These instruments cannot measure ac
quantities.
 (2) Cost of these instruments is high as compared
to moving iron instruments.
 (3) develop errors due to ageing of control springs
and permanent magnets.

Moving Iron Instruments





Moving iron type instruments are of mainly two
types. Attraction type and repulsion type instrument.
Whenever a piece of iron is placed nearer to a magnet
it would be attracted by the magnet.
The force of this attraction depends upon the strength
of magnetic field.
If the magnet is electromagnet then the magnetic field
strength can easily be increased or decreased by
increasing or decreasing electric current through its
coil.
Accordingly the attraction force acting on the piece of
iron would also be increased and decreased.
Depending upon this attraction the moving iron
instrument was developed.
Moving Iron Instrument
Moving Iron Instruments





As well as measuring d.c. signals, the moving-iron
meter can also measure a.c. signals at frequencies up
to 125 Hz.
The signal to be measured is applied to a stationary
coil, and the associated field produced is often
amplified by the presence of an iron structure
associated with the fixed coil.
The moving element in the instrument consists of an
iron vane that is suspended within the field of the
fixed coil.
When the fixed coil is excited, the iron vane turns in a
direction
that increases the flux through it.
Moving Iron Instruments

Advantages
1. They can be used for both DC and AC circuits.
2. They robust and cheaper

Disadvantage
1. Effect of frequency variations
2. Power consumption is more
3. Low accuracy.

Note: the current is proportional to the voltage to
be measured.
Moving Iron (Attraction type)





A thin disc of soft coil iron (moving iron) is pivoted
at the end of the core of the coil (current carrying).
When the current being measured flows in the coil, a
uniform magnetic field is produced inside the coil and
in the direction of the axes of the coil.
the moving iron tends to move from the weaker
magnetic field outside the coil into the stronger field
inside it.
Whatever the direction of current through the coil, the
iron disc would always be magnetized in such a way
that it is pulled inwards.
And cause the pointer to deflect.
Moving Iron (Repulsion type)
If two pieced or (Vanes) of soft iron are mounted
close together inside a coil and current is passed
through the coil, the iron vane are magnetized,
with north poles at one end and south poles at the
other.
 Repulsion takes place between the two vanes,
since like poles are adjacent to one another.

Theory of operation of the moving
iron instruments
A general expression for the torque of a moving iron
instrument may be derived by considering the energy
relations when there is a small increment in current
applied to that instrument.
 Therefore, there will be a small deflection (dϴ) of the
pointer with mechanical torque (Td).


The applied voltage on the coil (e) = - induced emf
Theory of operation of the moving
iron instruments


Let the initial current is Io, the instrument inductance L,
and deflection ϴ.
If the current increases by dI, the deflection changes by
dϴ, and the inductance by dL.
ϴ



= IL/N
dϴ : change in the position of the moving iron due to the
changing in flux.
dt : time taken for the above change
N : number of turns of the coil
Theory of operation of the moving
iron instruments

Then :Multiplying both side
of equation with (I)
power drawn from the supply = e.I (W)
The deflection torque is proportional to the square of the
measured current.
And, the scale of instrument is non-uniform,
Theory of operation of the moving
iron instruments
For an excitation current I, the torque produced that cause the vane to turn is given by
Where L is the mutual inductance and ϴ is the angular deflection. Rotation is opposed by a
Spring that produced a backwards torque given by:
At equilibrium, Td=Tc, and ϴ is therefore given by:
From the last equation
I : in Amp.
L : in Henry
ϴ : in Rad
Theory of operation of the moving
iron instruments
Theory of operation of the moving
iron instruments

EX1/
 A 250-volt moving iron voltmeter takes a current of
0.05A when connected to a 250 volt DC supply. The
coil has an inductance of 1 Henry. Determine the
reading on the meter when connected to a 250 volt, 100
Hz AC supply.

Ex2/
 The change of the inductance for a moving-iron
ammeter is 2μH/degree. The control spring constant is
(5*10^-7 N.m/degree). The maximum deflection of the
pointer is 100 ͦ , what is the current corresponding to
the maximum deflection
Introduction
Early measurements of Direct
Current (DC) required a
suspension galvanometer.
 This instrument was the former
of the moving coil instrument,
basic to most DC indicating
movements currently used.
 A coil of fine wire is suspended
in a magnetic field produced by a
permanent magnet.
 The coil will rotate in the
magnetic field when it carries an
electric current.

Introduction

the coil deflection is a
measure of the
magnitude of the
current carried by the
coil.

The coil continue to
deflect until its
electromagnetic
torque balances the
mechanical counter
torque of the
suspension.
Introduction

The suspension galvanometer is
still used certain high sensitivity
laboratory measurements when the
accuracy of the instrument is not
objectionable and portability is not
required.

Galvanometer are used to indicate
or measure small current in bridge
circuits, potentiometers and other
measuring equipment.
Torque and Deflection of the
Galvanometer
Steady-State Deflection
 Dynamic Behavior

Steady-State Deflection
 The principle working of galvanometer represented by




PMMC .
The basic movement often called the d’Arsonval
movement after its inventor.
When the current flows in the coil, the developed
electromagnetic (EM) torque causes the coil to rotate.
The EM torque is counterbalanced by the mechanical
torque of control springs attached to the movable coil.
The balance of torque and therefore the angular
position of the movable coil, is indicated by a pointer
against a fixed reference, called a scale.
Steady-State Deflection

The equation of developed torque is:

T=BAIN
 T: torque in N.M.
 B: flux density in the air gap in weber/m (Tesla)
 A: effective coil area in m^2.
 I: current in the movable coil in amperes A.
 N: turns of wire on the coil.
Steady-State Deflection

The developed torque is a
direct indication of the
current in the coil.

This torque causes the
pointer to deflect to a
steady-state position where
it is balanced by the
opposing control spring
torque.
Dynamic Behavior

If current is passed through the coil to give it a
deflection and then the circuit is opened, the coil
swing back toward the zero position.

The oscillation decrease slowly and last for a
considerable time unless something is done to
provide a damping effect.
Dynamic Behavior

The motion of a moving coil in a magnetic field is
characterized by three quantities:
 The moments of inertia (J) of the moving coil about its axis
of rotation.
 The opposing torque (S) developed by the coil suspension.
 The damping constant (D).

If the coil is deflected to an initial ϴ, and then
allowed to swing freely, we may write for the torque
acting on it:

Acceleration torque + damping torque + suspension
torque =0
Dynamic Behavior
Quadric equation
Dynamic Behavior

There are three possible cases, depending on the
quantity under the root:
 Case I:
Roots real and unequal
 Case II:
Roots conjugate-complex
 Case III:
Roots real and equal
Dynamic Behavior
Working of suspended galvanometer











T=nIAB sinϴ
When ϴ =90 degree, the field is called radial field
Then T=nIAB
And this torque twist the suspension strip
Restoring torque (of twisted strip)Tc=Kϴ
K is the restoring torque per unit twist
When at equilibrium restoring torque =deflection
torque.
nIAB=Kϴ
I=Kϴ / nAB
I=Gϴ
G is the galvanometer constant G=K/nAB
Then
Iαϴ

Ex/
 A PMMC instrument has a coil of dimension
(10mm*8mm). The flux density in the air gap is
0.15Wb/m² . If the coil wound for 100 turns, carrying a
current of 5mA . Then calculate the deflection torque .
Calculating the deflection if the spring constant is
(0.2*10^-6 N.m/degree)
Ballistic Galvanometer


if the control springs of such an instrument are purposely
made of large moment of inertia, then it can be used as
ballistic galvanometer.
Constructionaly, it is similar to a moving-coil
galvanometer except that:
1.
2.
It has extremely small electromagnetic damping
Has long period of undamped oscillation (several second).
These conditions are necessary if the
galvanometer is to measure electric charge rather
than current.


for the large moment of inertia permits the passage of a
quantity of charge before the coil moves significantly.
Ballistic Galvanometer

The passage of the charge produces an impulse, a
momentary torque, which causes the coil then to swing
slowly to some maximum position.

Such a galvanometer was often used to standardize
capacitors. Ballistic galvanometer is a type of mirror
galvanometer.

Unlike a current measuring galvanometer, the moving
part has a large moment of inertia, thus giving it a long
oscillation period.

It is really an integrator measuring the quantity of charge
discharged through it.
Ballistic Galvanometer
In fact, the moment of inertia of the coil is made so
large that whole of the charge passes through the
galvanometer before its coil has had time to move
sufficiently.
 In that case, the first swing of the coil is proportional
to the charge passing through the galvanometer.
 After this swing has been observed, the oscillation
coil may be rapidly brought to rest by using eddycurrent damping.
 i.e. the coil moves after the charge to be measured
has passed through it. Obviously, during the
movement of the coil, there is no current flowing
through it.

Ballistic Galvanometer
Classification of magnetic material

All materials possess magnetic properties to a
greater or lesser degree and these are determined
by the facts that
1. Magnetic field exerts forces and torques on the
bodies
2. A body placed in a magnetic field distorts the field.

The magnetic properties of the materials are
characterized by relative Permeabilities. In
accordance with the value of relative
permeability the materials may be classified in
the following three ways:
1) Ferromagnetic materials:
The relative permeability of these materials are
much greater than unity and are dependent on the
field strengths. They attract the lines of force
strongly fig1. The principal ferromagnetic
elements are iron, cobalt, nickel. However, also
comes under this classification. These have high
susceptibility.
 Binside=Km *Bexternal added
 Km is material constant
 Km >>>1 near to 1000-10000

2) Paramagnetic materials:
These have relative permeability slightly greater
than unity and are magnetized slightly. They
attract the lines of force weakly fig1. Aluminum,
platinum, and oxygen belong to this category.
 Km >=1

3) Diamagnetic materials:

The relative permeability of these materials is
slightly less than unity. They repel the lines of
force slightly fig1. The examples are bismuth,
silver, copper, and hydrogen.

Km<=1
Diamagnetic, paramagnetic,
Ferromagnetic
Classification of magnetic measurements
Measurements of various magnetic phenomena are
called magnetic measurements.
 The magnetic measurements and a thorough
knowledge of characteristics of magnetic materials
are of significant importance in designing and
manufacturing electrical equipment.


The magnetic measurements are more inaccurate
than other types of measurements in electrical
engineering due to the following reasons:
1. The magnetic flux cannot be measured directly,
because it didn’t have a definite path
2. The magnetic materials are not homogeneous.
In magnetic measurements, the
principal requirements are
Measurement of magnetic field strength in air
II. Determination of B-H curve and hysteresis
loop for soft ferromagnetic materials
III.Determination of eddy current and hysteresis
losses of soft ferromagnetic materials
IV.Testing of permanent magnets
I.
Difinitions
B: is flux density in a spacemen of ferromagnetic
material
 H: is magnetizing force produce the flux density

The flux density B is measured by a ballistic
galvanometer or a fluxmeter which is a special
type of ballistic galvanometer. (ballistic
galvanometer and fluxmeter does not measure B
in magnetic material directly, instead measures
the changes in the flux)
 Magnetic force H is measured by permeameter

Difinitions
Magnetomotive force: it is that force which
drives or tends to drive the flux through a
magnetic circuit of number of conductors N and
current I passing through them (mmf=NI)
 Similar to the way that electromotive force (EMF)
drives a current of electrical charge in electrical
circuits, magnetomotive force (MMF) 'drives'
magnetic flux through magnetic circuits.

EMF
Types of tests:
The following tests are normally carried out on
ring specimens of the ferromagnetic materials
although bar specimens are much easier to
construct.
 The study of magnetic measurements has been
divided into three categories:

1. Dc tests
2. Ac tests
3. Steady state tests
DC. Tests

These tests are often termed as “ballistic tests”
1. These are used for determining of G-H curves and
hysteresis loop of ferromagnetic materials.
2. These tests provide an adjustable MMF on the
magnetic circuit and a ballistic galvanometer of flux
meter for measurement of flux density.
AC. Tests

These tests may be carried at power, audio or
radio frequencies.
 AC . tests are used to determine iron losses or core
losses in strip (sheet) material when it is subjected to
alternating field
 These losses can be separated into hysteresis losses and
eddy current losses
 The strip material is assembled as a close magnetic
circuit in the form of square.

There are two types of squares: (1) Epstein square
and (2) Lloyd Fisher square
Steady state tests

These are used to measure the steady state value
of flux in the air gap of magnetic circuit.
1) Ballistic Tests (or DC. tests)

These test are used for :
1. Determination of (flux / flux density) in specimen
2. Determining of B-H curves
3. Plotting of hysteresis loop
A) Measuring of Flux / Flux density
A) Measuring of Flux / Flux density
The ring specimen is wound with a magnetizing
winding carries a current I
 A search coil (known as B coil) of convenient
number of turns is wound on the specimen and
connected through a resistance and calibrating
coil, to a ballistic galvanometer as shown.
 The current through the magnetizing coil is
reversed and therefore the flux linkages of the
search coil change inducing an e.m.f. (e) in it.
This e.m.f. sends a current through the ballistic
galvanometer causing it to deflect.

A) Measuring of Flux / Flux density
Φ = flux linking with search coil
R = resistance of the ballistic galvanometer circuit
N = number of turns in the search coil
t = time taken to reverse the flux
average emf induced in the search coil
And the average current through the ballistic
galvanometer
Measurement of value of magnetizing
force (H)


The value of H inside a specimen can be
calculated from the formula, given by:
i.e., ampere .turns / meter
N = number of turns in the specimen
 I = current flowing through the specimen
 l = mean circumference of the ring (m)

Measurement of value of magnetizing
force (H)
H can also be determined from measurements made
outside the specimen from the equation

Where

B (flux density)

A is cross section aria of the specimen
Measurement of value of magnetizing
force (H)

Flux
ϴ1 is the throw of the galvanometer
 Gq is the constant of galvanometer expressed in
coulomb per unit deflection
 Gq * ϴ1 is the charge indicated by the
galvanometer

That is H cannot be measured directly
Measurement of value of magnetizing
force (H)

Thus the magnetising force of a constant magnetic field can be
measured indirectly by a ballistic galvanometer and a search
coil as shown in fig.

The search coil as positioned in figure, measures the value of
flux density Bo in air.
B) Determination of B-H curve

The following two methods are available for the
determination of B-H curve of a specimen:
1. Method of reversal
2. Step-by-step method
B-1) Method of Reversal
 For determination of B-H curve, a ring shaped
specimen whose dimensions are know is used
 A layer of thin tape is put on the ring and a search
coil insulated by wax is wounded over the tape.
Another layer of taps is put over the search coil and
the magnetizing winding is uniformly wound over
this tape
 The

circuit 11.8
The B-H curve may be plotted from the measured values of B
corresponding to the various value of H.
B-1) Method of Reversal

Procedure:
 First of all the specimen is demagnetized and then magnetizing
current I is set to its lower value .
 The ballistic galvanometer key K is closed and the reversing
switch S is operated about twenty times backward and forward.
This is done to bring the specimen into a Reproducible cyclic
magnetic state
 Key K is now opened and the value of flux corresponding to this
value of H is measured by reversing the switch S and noting the
throw of galvanometer. The value of flux density corresponding
to this H can be calculated by dividing the flux by the area of the
speciment
 The above procedure is repeated for various values of H upto the
maximum testing point.
B-2) Step-by-step method


The special feature of this method of determining of B-H
curve is that there is no reversal of magnetizing current.
Connection diagram is 11.8, except that the direct current is
now supplied to the magnetizing coil through a potential
divider having a number of tapping as shown
B-2) Step-by-step method

Procedure:
 The tapping switch S2 is set on tapping 1 and switch S1 is closed.
The throw of galvanometer corresponding to this increase in flux
density in the specimen, from zero to some value B1 is observed.
The value of B1 can be calculated from the throw of the
galvanometer. The value of the corresponding magnetizing force
H1 may be calculated from the value of current flowing in the
magnetizing winding at tapping 1.
 The magnetizing force is then increased suddenly to H2 by
suddenly changing the position of switch S2 from tapping 1 to
tapping 2 and the corresponding increase the flux density is
determined from the galvanometer throw observed. The flux
density B2 corresponding to magnetizing force H2 will be equal
to B1 +ΔB (increase in flux density), determined from the
galvanometer throw.
 The process is repeated for other value of H to the maximum
point and complete B-H curve is obtained as figure below:
B-2) Step-by-step method
C) Determination of Hysteresis loop

Similar to determination of B-H curve, there are
two methods for determination of hysteresis loop
for a magnetic material specimen:
1. Method of reversals
2. Step-by-step method
C-1) Method of reversals:

R1, R2, and R3 are the variable resistances for adjusting the
resistances in the ballistic galvanometer and magnetizing coil
circuits, R4 is a variable shunting resistance, which can be
connected across the magnetizing coil by means of switch S2
thus reducing the magnetizing current from its maximum value
down to any desired value depending upon the value of R4.
C-1) Method of reversals:

Procedure: refer to figures 11.12 and 11.13
 The value of magnetizing force Hmax required to procedure flux density
Bmax to be used during the test is obtained from the B-H curve of the
specimen
 The resistance R2 and R3 are adjusted to give such a current in the
magnetizing coil that magnetizing force Hmax (determined from B-H
curve) is produced with S2 in ‘off’ position. The resistance in the
galvanometer circuit R1 is adjusted to obtain suitable deflection in
ballistic galvanometer on reversing the maximum magnetizing current.
The shunting resistance R4 is adjusted to give required reduction in
magnetizing current when connected across magnetizing winding.
 The reversing switch RS2 is placed on contacts (1,1’) and ballistic
galvanometer is connected to the circuit by opening short-circuiting key
K. the value of Bmax is determined corresponding to Hmax from the
deflection of galvanometer observed on reversing switch RS2 and point
A on the hysteresis loop in obtained Fig. 11.13
 The switch S2 is then thrown from off position to contact b in order to
connect resistance R4 across the magnetizing winding and reduce the
magnetizing force to HK. the corresponding reduction in flux density ΔB
is obtained from the galvanometer throw and thus point K is obtained on
the loop.
C-1) Method of reversals:

The galvanometer is
then short circuited
by closing key K and
reversing switch RS2
is reversed to
contacts (2,2’).
Switch S2 is moved
to the ‘off’ position
and reversing switch
S2 is moved back to
contacts (1,1’). This
procedure passes the
specimen through the
cycle of
magnetization and
back to point A.
Determination of leakage factor in Dynamoelectric machinery

In dynamo-electric machine, the leakage factor is defined as:

Where “useful flux” is the flux in the ARMATURE (i.e., flux
crossing the air gap)
And “total flux” in the pole bodies which in turn is equal to the
(useful flux + leakage flux) existing on the pole body at its root.
Thus in order to measure the leakage factor, we have to measure
the flux in the pole bodies and flux in the armature.
For measurement of leakage factor the (Flux meter) is used,
Ballistic galvanometer is unsuitable due to high inductance of the
field system of the machine.




Determination of leakage factor in Dynamoelectric machinery
Determination of leakage factor in Dynamoelectric machinery

Procedure
I.
The yoke carries HALF of the total flux and therefore
it is possible to measure the value of “Total Flux per
pole” by using two “search coils” on the yoke and
connecting them in series across the fluxmeter.
II.
The armature is kept stationary and another “search
coil” is put on it. the coil is so positioned that it
embraces the “useful flux per pole”. The search coil
is then connected to the fluxmeter and this way the
“Useful Flux” is measured.
Testing BAR specimens of magnetic
materials
The preparation of ring specimens is difficult while
bar specimens are easier to construct.
 The difficult in bar specimen are in inaccuracies and
encountered. Because the return circuit for the flux is
through air whose resistance is very high.





Magnetizing force H=Ha-Hd
Ha = applied magnetizing force
Hd = the magnetizing force due to selfdemagnetisation or end effect
Testing BAR specimens of magnetic
materials
Lloyd-fisher square for measuring iron
loss
AC magnetic testing
 The AC magnetic testing is carried out for the
following purposes:

1) To determine the iron losses in magnetic materials at
different values of flux density and frequency.
2) To separate two components of iron losses i.e. eddy
current losses and hysteresis losses
Lloyd-fisher square for measuring iron
loss
Iron losses

When ferromagnetic materials are subjected to an
alternating field, power loss due to hysteresis effects
and eddy currents occurs.


Hysteresis loss:
This loss depends upon the
frequency, and maximum flux density of the
magnetic field to which the specimen is subjected.
2. Eddy current loss:
this loss depends upon
the waveshape of the variation of flux with time,
frequency and maximum flux density of the
magnetic field to which the specimen is subjected.
1.
Lloyd-fisher square for measuring iron
loss

Factors affecting premeability and hysteresis loss:
 Generally if the initial permeability is high, the hysteresis loss is
low and vice versa.
The permeability and the hysteresis loss depend upon the following
conditions:
○ Physical condition of the sample
○ Chemical purity of the sample
 When the crystrals of a ferromagnetic material are cold worked,
they experience deformation as a result of which the material has
very poor magnetic properties.
 The impurity content of the material exercises a limit on the
highest magnetic permeability and the lowest hysteresis loss that
can be obtained.
 The main impurities in the magnetic materials used for
transformer cores and electrical machinery are carbon, oxygen, ..
Lloyd-fisher square for measuring iron
loss
Methods of measurement of iron losses:
 The following methods are used to measure iron
losses in ferromagnetic materials

1. Wattmeter method..
2. AC bridge methods
3. AC potentiometer method
4. Oscillographic method

Lloyd-fisher square for measuring iron
loss
 Wattmeter method
 This is perhaps the method of measuring the total loss
is sheet steel with alternating current
1.
2.
3.
The sheet material to be tested is arranged in the form of a
magnetic square [of which there are several forms, Epstein
being the originator of the arrangement] with fixed
magnetizing and search coils into which the strips of sheet
material to be tested are slipped and clamped.
The clamping arrangement should be such that air gaps in
the circuits are reduced to a minimum possible.
The windings extend as far as possible along the whole
length of the side and reasonable approximation to uniform
flux density is obtained.
Lloyd-fisher square for measuring iron
loss
A--Wattmeter method-Epstein square
 It consists of four stacks of strips. These
stacks are bound and then taped
 The individual strips are insulated from each
other and each strip is in the plane of the
square
 The stacks are slipped into four magnetising
coils with the strips projecting beyond the
coils
 The ends of the strips are interleaved (as in
the construction of transformer core) and
clamped at corners.
Lloyd-fisher square for measuring iron
loss
A--Wattmeter method-Epstein square
Lloyd-fisher square for measuring iron
loss
B--Wattmeter method-Lloyd-fisher square
Each strip (usually 0.25mm long and 50 to 60 mm wide) is
perpendicular to the plane of the square. The strips are built up into
four stacks which are placed inside four similar magnetizing coils of
large cross-sectional area. These four coils are connected in series to
form the primary winding.
 Each magnetizing coil has two similar single layer coils underneath
it; these are called secondary coils. These secondary coils are
connected in series in group of four, one from each core, to form
two separate secondary windings.
 The magnetic circuit is completed by bringing the four stacks
together in the form of a square and joining them at the corners. The
corner joints are made by a set of standard right angled corner
pieces.
 There is an overlapping of corner pieces and strips at the corners due
to which cross-section of iron is doubled at the corners; therefore, a
correction must be applied for this. Also the measured losses has to
be corrected for loss in the corner pieces.

Lloyd-fisher square for measuring iron
loss
B--Wattmeter method-Lloyd-fisher square
Lloyd-fisher square for measuring iron
loss
B--Wattmeter method-Lloyd-fisher square
 Advantages:
1. This square gives rather more reliable
than Epstein square, in case allowance
for corner pieces is known with
adequate accuracy
2. The use of corner pieces in this type of
square makes it superior for testing
anisotropic material.
Lloyd-fisher square for measuring iron
loss

Setup for the test

Figure below shows the connection diagram for
finding the total iron loss by wattmeter method:
The test specimen is weighed before assembly and its
cross-sectional area is determined.
2. The primary winding, which contains the current coil of
the wattmeter, is connected to an AC supply.
1.
3.
4.
The test specimen has two secondary windings s1 and s2, s1 is
connected to the pressure coil of the wattmeter through switch
k2. S2 is connected to an electrostatic voltmeter or an
electrodynamics voltmeter of very high impedance. The supply
frequency is adjusted to the correct value.
The voltage applied to the primary winding is adjusted, till the
magnetizing current adjusted to give the required value of Bmax .
the readings of the wattmeter and voltmeter are observed.
Lloyd-fisher square for measuring iron
loss
Hibbert’s magnetic standard:

A is a circular bar permanent magnet. B is an iron
Yoke. the Narrow gap between A and B is about 2
mm. A brass tube containing known number of
turns is allowed to fall though the air gap. The
ends of the coil are brought to the terminals of
brass tube which are connected to the ballistic
galvanometer. The brass tube is allowed to gall by
a trigger under gravity vertically downwards by
means of a guide.
Hibbert’s magnetic standard:
Hibbert’s magnetic standard:

As the coil passes through the air gap it is cut by
lines of force and emf will be induced in the coil.
This emf will drive current through the ballistic
galvanometer B.G. giving a throw on the scale
depending on the quantity of electricity
discharged. The number of turns on the brass tube
can be varied from 3 to 100.
Hibbert’s magnetic standard:
Hibbert’s magnetic standard:

It consists of a block of hard steel with a cylindrical slot
cut in it. The block is magnetized so that the field is
radial. A brass tube has a single layer of coil of n turns
wound upon it with its ends connected to the terminals
T1 and T2. The brass tube which can freely slide into the
slot is always dropped into slot from a certain fixed
height. The magnetic flux between the north and south
poles of the magnet is determined using a solenoid
inductor.
The two terminals T1 and T2 are conducted to a B.G.,
through a resistance box R and commentator. A resistance
R1 is introduced in the resistance box. The brass tube is
now dropped into the slot between the north and south
poles. The change in magnetic flux linked with the coil is
Φn where Φ is the magnetic flux in the Hibbert’s
standard.
Hibbert’s magnetic standard:

Let G = resistance of the B.G., and the coil.
Induced charge in the coil} = Φn / (R1 +G)
Let θ1 be the corrected first throw in the B.G., due to
the flow of this charge. Then
= Φn / (R1 +G) = Kθ1
… (1)
Now, another resistance R2 is included in the
resistance box and the experiment is repeated. The
mean corrected throw θ1 is found. Then,
Φn / (R2 +G) = Kθ2
… (2)
From (1) and (2),
K = Φn / (R2 - R1) (1/θ1 – 1/ θ2)
… (3)
DC Ammeters (Shunt resistor)
DC Ammeters (Shunt resistor)
The basic movement of a DC ammeter is a
PMMC galvanometer, since the coil winding of a
basic movement is small and light, it can carry
only very small currents.
 When a large currents are to be measured, it is
necessary to bypass the major part of the current
through a resistance, called a shunt.

DC Ammeters (Shunt resistor)

Calculation of shunt resistance





Rm = internal resistance of the movement (the coil)
Rs=resistance of the shunt ?
Im=full-scale deflection current of the movement
Is=shunt current
I = full-scale current of the ammeter including the shunt
 Vshunt
=Vmovement
 IsRs=ImRm

Since Is=I-Im

-
DC Ammeters (Ayrton Shunt)

The current range of the DC ammeter may be
extended by a number of shunts, selected by a Range
Switch. Such a meter called a Multirange Ammeter.
DC Voltmeter (Multiplier Resistaor)
DC Voltmeter (Multiplier Resistaor)

The addition of a series resistor, or multiplier.
Convert the basic d’Arsonval movement into a
DC voltmeter. The multimeter limits the current
through the movement so as not to exceed the
value f the full-scale deflection current (Ifsd).
 V=Im(Rs-Rm)
DC Voltmeter (Multiplier Resistaor)
Shunt type Ohmmeter
Shunt type Ohmmeter
E = internal battery voltage
 R1 = current-limiting resistor
 Rm = internal resistance of the movement

1)
Shunt type Ohmmeter
2) For any value of Rx: the meter current decrease
and is given by:
Shunt type Ohmmeter
3) The meter current for any value of Rx, expressed as a
fraction of the full-scale current is:
The meter can be calibrated by calculating S in terms of
Rx, R1, and Rm.
Shunt type Ohmmeter
4) H.M
Calculate the external resistance that causing half-scale
deflection of pointer (shunt type Ohmmeter).
Multimeter or VOM

This instrument, which contains a function switch to connect the
appropriate circuits to the d’Arsonval movement, is often called a
multimeter or volt-ohm-milliammeter (VOM).
AC Indicating Instruments

The d’Arsonval movement responds to the average or DC value
of the current through the moving coil. If the movement current
carries an AC with positive and negative half-cycles, the driving
torque would be in one direction for the positive alternation and
in the other direction for the negative alternation.

For very low frequency, the pointer would swing around the zero
point on the meter scale.
For high frequency, the inertia of the coil is so great that the
pointer cannot follow the rapid reversals of the driving torque
and hovers around the zero mark, vibrating slightly.


To measure AC on a d’Arsonval movement some means must be
devised to obtain a unidirectional torque. First is rectification of
the AC, and second is using heating effect of the AC to produce
an indication of its magnitude.
Rectifier –Type instruments



Rectifier  converts AC into a unidirectional DC and
then to use a DC movement to indicate the value of the
rectified AC.
This method is very attractive, because a DC movement
generally has a higher sensitivity than either the
electrodynamometer or moving-iron instrument.
Rectifier type use PMMC movement combined with
rectifier.
Rectifier –Type instruments
Rectifier –Type instruments


Because of the inertia of the moving coil, the meter
steady deflection proportional to the average value of the
current.
Since alternating current and voltage are expressed in rms
values, the meter scale is calibrated in terms of the rms
value of a sinusoidal waveform.
Rectifier –Type instruments

Form-Factor

For a sinusoidal waveform

The form is therefore also the factor by which the
actual (average) dc current is multiplied to obtain the
equivalent rms scale marking.
Rectifier-type Instrument
The idea rectifier element should have zero
forward and infinite reverse resistance.
 In practice, the rectifier is a nonlinear device.
 At low values of forward current, the rectifier
operate in extremely nonlinear part of its
characteristic curve, and the resistance is large as
compared to the resistance at higher current value.
 The lower part of the AC scale of a low-range
voltmeter is therefore often crowded, and most
manufacturers provide a separate low-voltage
scale, calibrated especially for this purpose.

Rectifier-type Instrument
Rectifier-type Instrument






The resistance of the rectifier element changes with
varying temperature.
The meter accuracy is usually satisfactory under normal
operating conditions at room temperature and is generally
on the order of ±5% of full scale reading for sinusoidal
waveforms.
If large temperature variations are expected, the meter
should be enclosed in a temperature-controlled box.
Frequency also affects the operation of the rectifier
elements.
The rectifier exhibits capacitive properties and tend to
bypass the higher frequencies.
Meter reading may be in error as much as 0.5% decrease
for every 1KHz rise in frequency.
Electrodynamometer





Electrodynamometer is the one of the most important AC
movement, used in accurate AC voltmeters and ammeters,
not only at the powerline frequency but also in the lower
audio-frequency range.
With some slight modification, can be used as a
wattmeter, a VARmeter, a power-factor meter, or a
frequency meter.
May also serve as a transfer instrument, because it can be
calibrated on DC and then used directly on AC.
The d’Arsonval movement uses a permanent magnet to
provide the magnetic field in which the movable coil
rotates.
The electrodynamometer uses the current under
measurement to produce the necessary field flux.
Electrodynamometer
A fixed coil, split into two equal halves, providing the
magnetic field in which the movable coil rotates.
 The two halves are connected in series with moving
coil and are fed by current under measurement.

 Like d’Arsonval
movement, its rotation is
controlled by a springs.
 And damping is
provided by aluminum air
vanes, moving in sectorshaped chambers.
Electrodynamometer






Torque = B*A*I*N
1- (T α A) and (T α N) “torque depend on design”
But A and N are constants
2- (T α I) “directly depend on current”
3- but (B α I) “because the flux is generated by
current”
4- then ( T α I² )
Electrodynamometer

If the electrodynamometer is designed for DC
use:
 Crowded scale markings at very low current values.
 Spreading scale markings at higher current values.

For AC use:
 Developed torque at any instant is proportional to the
instantaneous current square (i²)
 (i²) is always positive
 The meter deflection is function to the mean of the
square current.
Electrodynamometer
The scale of the electrodynamometer is usually
calibrated in terms of the square root of the
average current square:
 Therefore, the meter reads the rms or effective
value of the AC.


The transfer properties of the electrodynamometer
becomes apparent when we compare the effective
value of alternative current and direct current in
terms of their heating effect or transfer of power.
Electrodynamometer

The average rate of producing heat by an AC of i
amperes during one cycle in resistance R: is

The average rate of producing heat by a DC of I
amperes in a resistance R: is
Thermo-instruments