introduction
measurement
measurement is the prosses through we can identify the magnitude of any physical parameter in the form of meaning full number value . the measurement of a given quantity is essentially an act or the result of comparing between the quantity ( whose magnitude is unknowns ) the numerical measures is meaning less unless followed by a unit used.
method of measurement may be two types such as direct method and indirect method .
direct method
in this method the unknows quantity ( also known by measured ) is directly is compared with a standard value like scale and watch . the result is expressed on a numerical number ans a unit . the direct method are common for the measurement of physical.
indirect method
measurement by direct method not always possible these method is in most of the case are incorrect because they involve human factor . they are less sensitive hence indirect method are used in some measurement system . indirect method is used to determine such quantity as velocity , power etc .
electrical instrument
the instrument which are to used measure the electrical value such as volt , current , power , frequency etc are called electrical measuring instrument . the main types of electrical measuring instrument are show in below
a) indicating type
b) recording type
c) integrating type
d) digital type
indicating type
the instrument which are indicates the value of electrical quantity by the scale of pointer are called indicating type instrument . the indicating type instrument also called deflecting type instrument such as ammeter and voltmeter
integrating type
the instrument which are record the total quantity with respects to time are called
integrating type instrument such as digital energy meter and amper hour meter .
recording type
the instrument which indicate the value on graph paper instead or pointer and scale are called recording type instrument . these instrument are mostly used in power house.
essential factors of indicating type instrument
a) deflecting torque
the force or torque which bring the needle in motion is called deflecting torque
this force can be obtained by magnetic effect a heating effect of electricity.
b)controlling torque
the force or torque which control the movement of pointer caused by deflecting forced
and bring the pointer at its original position when there is no deflecting forced this type of force provided by metal spring or mass of material.
c) damping torque
the force or torque which bring the needle in its proper position very quickly is known as damping forced.
type of measuring instrument
mainly there are three type of measuring instrument which are
a) permanent magnet moving coil (PMMC)
b) moving iron (MI)
c) electro dynamo
there three measuring instrument are mostly used during practical purpose but there are three more type of measuring instrument , they are point below
d) hot wire type
e) thermo couple
f) induction type
from these instrument PMMC type instrument is only used for DC supply , MI type instrument is used for AC supply but other instruments can be used for AC and DC both
construction and working principle of PMMC type instrument
permanent magnet moving coil PMMC instrument are the most accurate type for the measurement of dc current and voltage.
the permanent magnet moving coil is mounted on a rectangular aluminium former which is pivotal on jewelled bearing . the coil can move freely in the field of permanent magnet most voltmeter coil are wound om metal former to provide the required electromagnetic damping torque most ammeter coils are wound non magnetic former because coil turns are effectively shorted .the ammeter short therefore electromagnetic damping PMMC type instrument contains following main parts they are
a) magnet system
b) control
c) damping
d) pointer and scale
magnet system
old system magnet system consist of a relatively long V shaped permanent magnet having soft iron pole piece .the flux density used in PMMC instrument vary from 0.1 wb/M square to
1 wb/ M square thus in small instrument it is possible to used a small coil having small or less number of turn.
control
when the coil is supported between two jewel bearing , the control torque is provided by two spring.
damping
damping torque is produced by movement of the aluminium frames moving in the magnetic field of the permanent magnet .
pointer and scale
The pointer is carried by the spring and move over the scale . the pointer has light weight apart from t
those used in some in expensive instrument
Moving iron instrument
it is the most commonly used ammeter and voltmeter for switching panel board . it can be work for power frequency . this instrument is cheap and provides higher accuracy with the other types of AC instrument .
working principle of moving iron instrument
moving iron instrument are mostly used for measurement of alternating current and voltage through it.
the generally principle of moving iron instrument can be explained under
let a plate or vane or soft iron or high permeability steel forms the moving element of the system . the iron vane is situated so as it can move on a magnetic field produced by a stationary coil. the coil is excited by the current and voltage under measurement. when the coil is excited it become an electromagnet and the iron vane moves in a such way so as to increase the flux of the electromagnet thus the vane tries the occupy a position of minimum reactance thus the force produced is way in such a direction so as to increase the induction of the coil
for the development of torque { current flux
T{ QI
Td{I²
type of moving iron instrument
as this meter works on attraction and repulsion of iron plate . therefore its classification into two types
a) attraction type moving iron instrument
b) repulsion type moving iron instrument
a) attraction type moving iron instrument
in this type of instrument a single soft iron vane ( moving iron ) is mounted on the spindle and attracted towards the coil when operating current flow through it.
b) repulsion type moving iron instrument
in this two soft iron vanes are used and fixed and attached the stationary coil , while the other is moveable ( moving iron ) and mounted on the spindle of the instrument when operating current flow through the coil , the two vanes are magnetised developing similar polarity at the same end consequently , repulsion takes place between the vanes and the move able vanes case the pointer to move over the scale .
working principle of repulsion type moving iron instrument
when the supply is given to the fixed coil both the iron pieces will be magnetized pole due to the repulsion force of electromagnet it will produced a torque which movies the moving iron pieces in a particular direction this torque is kwon as deflecting torque
Advantage of moving iron type instrument
1) it is cheaper
2) it can be used in ac for the different value of supply frequency
3) simple in construction
4) it has more deflecting torque
Disadvantage of moving iron type instrument
1)its scale is not uniform
2)its consume more power
3) stray loss cause to affect its reading
Electro dynamo type instrument
Electro dynamo type instrument are can be used for ac and dc both , electro dynamo meter type instrument are used as ac voltmeter and wattmeter both in the range of power frequency they are also used as wattmeter , power factor and frequency meter .
Working principle
an electro dynamo instrument is a moving coil instrument in which the operating filed is produced by fixed coil not by permanent magnet , this instrument can be used either as as ammeter or voltmeter but its generally used as wattmeter
in this instrument the filed can be reverse with the current in the moving coil if the (fixed) field wire is connected in series with the moving coil.
construction
fixed coil
the field is produced by the fixed coil the fixed coil is divided into two section to give more uniform field . filed coil are wound with heavy wire which carries the main current ammeter and wattmeter therefor the the fixed coil also known as current coil
moving coil
the moving coil is wound either as coil self sustaining coil or pressure coil which over a non metallic form the nature of the coil is very thick wire having large number of turns and in most of the instrument a high value of resistor is connected in series with moving coil to drop the extra voltage.
control
the control torque is provides by two control spring mad by phosphor bronze material .
moving system
the moving coil is mounted on an aluminium spindle at the top of which needle is connected which move simultaneously with the moving of spindle or coil.
scale
the scale are hand drawn using machine sub dividing equipment
advantage
1) these instrument can be used for both ac and dc measurement.
2) such instrument are free from hysteresis and eddy current errors .
disadvantage
1) scale is not uniform
2) a good amount of screening of the instruments is required to avoid the effect of stray field .
3) since torque / weight ratio is small ,therefore such instrument have frictional errors which reduce sensitivity.
0 Comments