Electrical Measuring Instrument

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15 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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Electrical Measuring Instrument
& Machines

Trim iii


A
-
Div

1




Introduction


Electrical measurements are used in physical, chemical,
and biological research and in the chemical industries.
television equipment, of aircraft, and of spacecraft.



Measurements
of electrical quantities, such as voltage,
impedance, current, AC frequency and phase, power,
electric energy, electric charge, inductance, and
capacitance
.



Electrical
equipment capable of converting nonelectrical
quantities into electrical
quantities.

2

Active electrical quantities


The
measurement of “active” electrical quantities
such
as
current and
voltage
.




Which
characterize the energy state of a measured
circuit, makes use of the direct action of these quantities
on the measuring instrument and generally draws some
amount of power from the circuit




AMMETER, VOLTMETER , WATTMETER,
ELECTRIC METER and

FREQUENCY METER

3

Passive electrical quantities


The

measurement

of

“passive”

electrical

quantities

such

as

impedance

and

its

complex

components,

inductance,

and

the

tangent

of

the

dielectric

loss

angle
.




which

characterize

the

electrical

properties

of

a

measured

circuit,

requires

excitation

of

the

circuit

by

an

outside

source

of

electric

energy

and

measurement

of

the

circuit’s

response




OHMMETER,

MEGOHMMETER,

INDUCTANCE

METERS,

CAPACITANCE

METER

and

QUALITY

FACTOR

METER


4

AC & DC Measuring instruments


The

techniques

and

instruments

used

for

electrical

measurements

in

DC

circuits

differ

substantially

from

those

used

in

AC

circuits
.




AC

Measuring

Instruments
:




In

AC

circuits,

the

choice

of

technique

and

instrument

depends

on

the

frequency,

on

the

nature

of

the

quantities’

variations,

and

on

which

values

instantaneous,

effective,

maximum,

or

average

of

the

varying

electrical

quantities

are

being

measured
.



measurements

in

AC

circuits

are

made

with

electromagnetic,

electrodynamics,

induction,

electrostatic,

rectifier,

and

digital

instruments

and

with

oscillographs
.



5

DC measuring instruments




Permanent
-
magnet instruments and digital measuring devices
are the instruments most widely used for measuring DC circuits.



whereas Some of these instruments are used for measurements in
both AC and DC circuits.




the values of measured electrical quantities the following ranges:



current, from 10

16

to 10
5

amperes


voltage, from 10

9

to 10
7

volts


resistance, from 10

8

to 10
16

ohms


power, from 10

16

watt to tens of
gigawatts
, and


AC frequency, from 10


3

to 10
12

hertz.




6

GENERALIZED MEASUREMENT SYSTEM
CONFIGURATION

Output
devices

Transmission
for telemetry

Data
processing

Signal
conditioning

amplifier

transducer

sensor

Input

parameter

Inverse transducer

Data display

Recorder

Data storage

LED ,LCD, CRT

Magnetic tap

7


Sensor
-

primary sensing element , sense the input parameter or variable


like pressure, velocity , acceleration etc and gives an output


signal to the input signal.



Transducer
-

which converts any non electrical input to an electrical signal.



Amplifier
-

the low level signal from the transducer has to be amplified


and brought to a useful level before it can be processed.



Signal conditioning
-

many times the transducer characteristics are




nonlinear, I.e. the relationship b/w
i
/p and o/p is



nonlinear especially with temperature transducer.



8


in

such

cases,

signal

conditioning

like

linearization

is

required
.

Thus

signal

conditioning

could

be

linearization,

scaling,

filtering
.



Data processing
-

the data is thus processed before it goes to




the final output device.



Transmission
-

data can be transmitted over long distances



by using a pair of wires , a coaxial cable, a fiber



optical link or radio wave.



Output devices
-

the data could be displayed using an




oscilloscope if it is a digital data ,we can have



LED or LCD display.


9

Performance Characteristics Of Measuring
Instruments

Static characteristics


Accuracy


Precision


Error


Repeatability


Reproducibility


Reliability


Linearity


Sensitivity


resolution

10

Accuracy


Accuracy is a measure of how close the measured value is to the true
value

How
close a measurement is to the actual or true
value????

good
accuracy





true value







poor
accuracy

true
value




11

Precision

The closeness of agreement between independent test results
obtained under stipulated conditions

How
well several measurements agree with each
other???


good precision







poor
precision


12

Accuracy and Precision


What can you say about the accuracy and
precision in each of the following:

Good
precision, poor accuracy



Good precision, good accuracy



13

Error
-


Accuracy of the measured value is of prime importance, hence a knowledge of
various likely error is essentials in order to obtain a reasonable estimate of the
uncertainty in measurement.






Error

can

be

classified

as
:


Gross

error
:

this

class

of

errors

mainly

covers

human

mistake

in

reading

instruments

and

recording

and

calculating

measurement

result
.

the

responsibility

of

the

mistake

normally

lies

with

the

experimenter
.


Systematic

error
:

these

type

of

error

are

divided

into

three

categories



Instrumental

error


Environmental

error


Observational

error


Random

error
:



it

has

been

consistently

found

that

experimental

result

show

variation

from

one

reading

to

another,

even

after

all

systematic

error

have

been

accounted

for
.


These

error

are

due

to

a

multitude

of

small

factors

which

change

or

fluctuate

from

one

measurement

to

another

and

due

to

surely

to

chance
.

14

Repeatability


Closeness
of the agreement between the results of
successive measurements of the same
measure and
carried
out under the same conditions of
measurement



Same (repeatability) conditions include:


the same measurement procedure


the same observer


the same measuring instrument, used under the same
conditions


the same location


repetition
over a short period of
time




15

Reproducibility


Closeness

of

agreement

between

the

results

of

measurements

of

the

same

measure

and

carried

out

under

changed

conditions

of

measurement



The

changed

conditions

may

include
:


principle

of

measurement


method

of

measurement


observer


measuring

instrument


reference

standard


location


conditions

of

use


time

16

Linearity


A
measure of how close is the output of an instrument to a
straight line


The closeness to which a curve approximates a straight line.

17

Sensitivity


A measure of the change in instrument output that occurs when the
measure and changes by a given amount



The ratio of the change in output magnitude to the change of the
input which causes it after the steady state has been reached.



18

Sensitivity to disturbance


Temperature


Humidity


Electromagnetic interference


Radiation


Acceleration


Shock


Vibration


19

Resolution


The least interval between two adjacent discrete details which can
be distinguished one from the other.




Classic
definition based on analog output instruments:


The smallest change of the magnitude of the
measure and
that
produces a minimum observable output of the instrument




the
instrument's output is
digital:


More and more modern instruments have digital outputs because of
the wide usage of computer


The "resolution" of the digital output can be a very small number,
but this is not the resolution of the instrument (e.g. what is the
"resolution" of a 32
-
bit IEEE floating point number?)


The resolution of an digital output instrument should be limited by
the front end rather than the digital computation


20

Dynamic characteristics


Input which are not static but are dynamic in nature
i.e

the input is vary
with the time.



Speed of response:
-

it is defined as the rapidity with which a
measurement system respond to change in the measured quantity.



Measuring lag:
-

it is the delay in the response of a measurement
system to changes in the measured quantity has occurred.



Fidelity:
-

it is defined as the degree to which a measurement system
indicate to change in the measured quantity without any dynamic error.



Dynamic error:
-

it is the difference between the true value of the
quantity changing with the time and the value indicate by the
measurement system if no static error is assumed.


21

Basic electrical principle


Electricity. It's everywhere. It's in your house, in the computer on
which you are reading this, in the air, in your body. You get the point.
It also plays an important integral part of sound systems. From the
microphone to the loudspeaker, everything is electrical, and you
need to know a few things about it.



Voltage is defined as the electric charge between two points.



In engineering terms, voltage is the potential energy of a bunch of
electrons.




Current, measured in Amperes, is the flow of electric charge; we
can say that current is the rate of flow of electrons, but technically
it is the opposite flow that we call current
--

as electrons move in
one direction, holes are left behind, and appear to flow in the
opposite direction.

22

Principle of
Voltmeter


A

voltmeter

is a device that measures the voltage passing between any
two points, typically used to check electric circuits for power.



A voltmeter, also known as a voltage meter, is an instrument used for
measuring the potential difference, or voltage, between two points in an
electrical or electronic circuit.



Some voltmeters are intended for use in direct current (DC) circuits;
others are designed for alternating current (
AC
) circuits.



A basic

analog

voltmeter consists of a sensitive galvanometer (current
meter) in series with a high

resistance
. The internal resistance of a
voltmeter must be high.


A

digital

voltmeter shows voltage directly as numerals. Some of these
meters can determine voltage values to several significant figures.

23


24

No Voltage and No
Current

Without the cell there
is no source of voltage
so current cannot flow.

25

Voltage and Current
The switch is closed

making a complete
circuit so current can
flow.

Voltage but No
Current

The switch is open so
the circuit is broken
and current cannot
flow.





26

Voltage:


Voltage is a measure of the

energy carried by the charge
.


Strictly: voltage is the "energy per unit charge".



The proper name for voltage is

potential difference

or
p.d
. for short, but
this term is rarely used in electronics.



Voltage is

supplied by the battery

(or power supply).



Voltage is

used up in components
, but not in wires.



We say

voltage across

a component.



Voltage is measured in

volts
,

V
.Voltage

is

electric potential energy

per
unit

charge
, measured in joules per coulomb ( = volts).



Voltage is measured with a

voltmeter
, connected in

parallel
.



27

Types:


An

oscilloscope

can be used to measure low voltages; the
vertical displacement corresponds to the instantaneous voltage.



Oscilloscopes are also excellent for the measurement of peak
and peak
-
to
-
peak voltages in AC and RF applications.



Voltmeters for measuring high potential differences require
heavy
-
duty probes, wiring, and insulators.



In computer practice, standard lab voltmeters are adequate
because the voltages encountered are moderate, usually between
1 V and 15 V.



Cathode
-
ray
-
tube (
CRT
) monitors operate at several hundred
volts. A typical lab voltmeter can indicate these voltages, but CRT
units should be serviced only by qualified technicians because the
voltages are high enough to be lethal.


28

Current principle


An

ammeter

is a

measuring instrument

used to
measure the

electric current

in a

circuit
.


Electric currents are measured in

amperes

(A),
hence the name. Instruments used to measure
smaller
currents,in

the
milliampere

or
microampere range, are
designatedas

milliammeters

or
microammeters
.



It must be placed in series with the measured
branch, and must have very low resistance to
avoid significant alteration of the current it is to
measure.

29

Types:


The

D'Arsonval

galvanometer

is a

moving coil

ammeter. It
uses

magnetic

deflection
, where current passing through a coil
causes the coil to move in a

magnetic field
.






30

Linear Scale due to

Moving Coil

Moving iron

ammeters



Moving iron

ammeters use a piece of

iron

which moves when
acted upon by the electromagnetic force of a fixed coil of wire. This
type of meter responds to both

direct

and

alternating

currents (as
opposed to the moving coil ammeter, which works on

direct
current

only).







Non
-
linear Scale

Due to

Moving Iron

Continue


In a

hot
-
wire ammeter
, a current passes through a wire
which expands as it heats. Although these instruments have
slow response time and low accuracy, they were sometimes
used in measuring radio
-
frequency current
.



Digital

ammeter designs use an

analog to digital
converter

(ADC) to measure the voltage across the shunt
resistor; the digital display is calibrated to read the current
through the shunt.




A

picoammeter
, or
pico

ammeter, measures very low
electrical current, usually from the
picoampere

range at the
lower end to the
milliampere

range at the upper end.
Picoammeters

are used for sensitive measurements where the
current being measured is below the theoretical limits of
sensitivity of other devices, such as

Multimeters
.

32

Current:


Current is the

rate of flow of charge
.


Current

is not used up
, what flows into a
component must flow out.


We say

current through

a component.


Current is measured in

amps (amperes)
,

A
.


Current is measured with an

ammeter
,
connected in

series
.


To connect in series you must break the circuit
and put the ammeter
acoss

the gap, as shown in
the diagram.


The symbol

I

is used for current in equations.



33


34

35

What is Current?



Current is the
flow of charge

from a voltage source



1 Ampere (“Amp”) = Flow of 1 Coulomb/sec

+++

36

How Does Current Flow?

Current can only flow through
conductors

+++

Metal wires (conductors)

Current

flow

37

When Does Current
NOT

Flow?

+++

Plastic material (insulators)

Current cannot flow through
insulators

No current

flow

38

What is Current?



Electricity flows
when electrons

travel through


a conductor.



Only some materials have free electrons inside.


Conductors:

YES!


Insulators:

NO!


No free electrons = No current



We call this flow “
current
.”


silver

copper

gold

aluminium

iron

steel

brass

bronze

mercury

graphite

dirty water

concrete

glass

rubber

oil

asphalt

fiberglass

porcelain

ceramic

quartz

(dry) cotton

(dry) paper

(dry) wood

plastic

air

diamond

pure water

Application of Ammeter/Voltmeter


Wattmeter:


Power in an electric circuit is the product (multiplication) of
voltage

and

current,



A wattmeter is a device used to measure how much electrical power a circuit
is producing, expressed in watts.



Wattmeters

measure electrical currents in three different values. These are
watts, volts and amperes.



A wattmeter is hardly over required in a
d.c

circuit because power (P = VI) can
be easily determined from voltmeter and ammeter readings.



However, in an
a.c

circuit, such a computation is generally speaking impossible.
It is because in an
a.c

circuit, power (P = VI Cos
θ
)



depends not only on voltage and current but also on the phase shift between
them.



40


Therefore, a wattmeter is necessary for
a.c

power
measurement.The

wattmeter shows a reading which is proportional to the product of
the current through its current coli, the
p.d

across its potential or
pressure coil and cosine of the angle between this voltage and
current.



The “wattmeter” is an indicating type instruments, generally used for
power measurement of the electrical circuit .



A wattmeter consists of


(1) a low resistance current coil which is inserted in series with the
line carrying the current and


(ii) a high resistance pressure coil which is connected across the two
points whose potential difference is to be measured.



The wattmeter require polarity markings so that the current in the
stationary coils will be in the correct direction relative to the
current in the movable coil

41


42

Wattmeter


The fixed coil of the wattmeter is connected in series with the circuit
(load), and the moving coil is connected across the line.


When line current flows through the fixed coil of a wattmeter, a field is set
up around the coil. The strength of this field is in phase with and
proportional to the

line current
.


The moving coil of the wattmeter generally has a high
-
resistance resistor
connected in series with it. The purpose for this connection is to make the
moving
-
coil circuit of the meter as purely resistive as possible.


As a result, current in the voltage circuit is practically

in phase with

line
voltage
.

Therefore, when voltage is impressed on the voltage circuit,
current is proportional to and in phase with the line voltage.



V

I

.
..cos
W V I
W V I
q
q
= Ð
=
ur uuur
Types:


There are two principle types of wattmeter
viz
:


i
.
Dynamometer Wattmeter


for both
d.c

and
a.c

power


ii.
Induction Wattmeter


for
a.c

power only.



A special meter movement designed especially for power measurement is
called the

dynamometer

movement
, and is similar to a
D'Arsonval

or
Weston movement

in that a lightweight coil of wire is attached to the
pointer mechanism.


However, unlike the
D'Arsonval

or Weston movement, another
(stationary) coil is used instead of a permanent magnet to provide the
magnetic field for the moving coil to react against.



The moving coil is generally energized by the voltage in the circuit, while
the stationary coil is generally energized by the current in the circuit.



A dynamometer movement connected in a circuit looks something like
this:


44

Electrodynamometer


Electrostatic Voltmeter


One

of

the

direct

methods

of

measuring

high

voltages

is

by

means

of

electro
-
static

voltmeters
.


For

voltages

above

10

kV,

generally

the

attracted

disc

type

of

electrostatic

voltmeter

is

used
.


When

two

parallel

conducting

plates

(cross

section

area

‘A’

and

spacing

‘s’)

are

charged

q

and

have

a

potential

difference

V,

then

the

energy

stored

in

the

is

given

by










46

Newton
s
V
A
2
1
F
s
A
ds
dC
s
A
C

e,
capacitanc
field
uniform
For
Newton
ds
dC
V
2
1
F
Force,
ds

F
dC
V
dW
CV
W
2
2



























2
2
2
2
ε
ε
ε
2
1
2
1

It is thus seen that the force of attraction is proportional to the square of the potential
difference applied, so that the meter reads the square value (or can be marked to read the
rms

value).

Electrostatic


Voltmeter



Electrostatic

voltmeters

of

the

attracted

disc

type

may

be

connected

across

the

high

voltage

circuit

directly

to

measure

up

to

about

200

kV,

without

the

use

of

any

potential

divider

or

other

reduction

method
.

[The

force

in

these

electrostatic

instruments

can

be

used

to

measure

both

a
.
c
.

and

d
.
c
.

voltages]
.


The

right

hand

electrode

forms

the

high

voltage

plate
.


The

centre

portion

of

the

left

hand

disc

is

cut

away

and

encloses

a

small

disc

which

is

movable

and

is

geared

to

the

pointer

of

the

instrument
.


The

range

of

the

instrument

can

be

altered

by

setting

the

right

hand

disc

at

pre
-
marked

distances
.


The

force

of

attraction

F(t)

created

by

the

applied

voltage

causes

the

movable

part
-
to

which

a

mirror

is

attached
-
to

assume

a

position

at

which

a

balance

of

forces

takes

place
.


An

incident

light

beam

will

therefore

be

reflected

toward

a

scale

calibrated

to

read

the

applied

voltage

magnitude
.




47


Advantages:

i.
Low loading effect

ii.
Active power losses are negligibly small

iii.
Voltage source loading is limited to the reactive power needed to
charge the system capacitance.(i.e., For 1V Voltmeter
-

Capacitance
is few Pico farad)

iv.
Voltages
upto

600kV can be measured.




Disadvantage:

i.
For constant distance ‘s’, F
α

V
2
, the sensitivity is small. This can be
overcome by varying the gap distance d in appropriate steps.

48

Electrostatic Voltmeter

Absolute Electrostatic Voltmeter


Energy meter


An

electricity meter

or

energy
meter

is a device that measures the
amount of

electric energy

50