INDICATING INSTRUMENTS (1) (4).pptx

7/30/2019 INDICATING INSTRUMENTS (1) (4).pptx

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INDICATING INSTRUMENTS


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MEASUREMENT

The process of measuring is essentially that of

comparing some unknown value with a value which

is assumed to be known.

1) latter one in standard.

2) And measuring system is instrument.


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Characteristics of INSTRUMENTS

Calibration:All the static characteristics areobtained in one form or another by a process calledCalibration.

Calibration procedures involve a comparison ofthe particular instrument with either

a) a primary standard.b) a secondary standard with a higher accuracy

than the instruments be calibrated.

c) an instrument of known accuracy.

Accuracy:Accuracy may be defined as the ability ofa device or a system to respond to a true value of ameasured variable under reference conditions.


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Precision: Precision is defined by the degree of exactnessfor which an instrument is designed or intended to perform.

Repeatability: It is the closeness of agreement among anumber of consecutive measurements of the output for thesame value of the input, under the same operatingconditions.

Reproducibility: It is the closeness of agreement amongrepeated measurements of the output for the same value ofthe input, made under the same operating conditions over aperiod of time.

Drift: It is an undesired change or a gradual variation inoutput over a period of time that is unrelated to changes ininput and operating conditions.


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Span: If in a measuring instrument the highest point ofcalibration is y units and the lowest point x units.

Then the instrument range y units

The instrument span is given bySpan=(x-y)units

Sensitivity: Sensitivity can be defined as the ratio of a

change in output to the change in input which causes it.

Resolution: The smallest increment in input (the quantitybeing measured) which can be detected with certainty by aninstrument is its resolution.

Dead zone: Dead zone is the largest range of values of ameasured variable to which the instrument does notrespond.


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ERROR

Types of Error

Static Error: the numerical difference between the truevalue of a quantity and its value as obtained bymeasurement.

Mistakes: These are errors due to human mistakes such ascareless reading, mistakes in observations, incorrectapplication of a correction, improper application ofinstruments and computational errors.

Systematic Error: 1)Instrumental Error: Instrumental errors are the errors

inherent in measuring instruments because of theirmechanical structure, such as friction in bearings of variousmoving components, irregular spring tension.


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2)Environmental Error: it is due to conditions external to

the measuring device including conditions in the areasurrounding the instrument such as the effect of change

in temp , humidity, barometric pressure , or magnetic or

electrostatic fields.

Random Error: the cause of such error is unknown or not

determinable in the ordinary process of making

measurements. Such errors are normally small and follows

the laws of chance.


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SOURCESOFERROR

Insufficient knowledge of process parameters and

design conditions.

Poor design.

Poor maintenance. Error caused by people who operate instrument

equipment.

Certain design limitations.


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TYPESOF INSTRUMENTS

Absolute Instruments:

Absolute instruments are those which give the value of theelectrical quantity to be measured, in terms of the constant ofthe instruments and their deflection only, e.g. tangentgalvanometer.

Secondary Instruments:

Secondary instruments are those which have been

precalibrated by comparison with an absolute instrument. Thevalue of the electrical quantity to be measured in theseinstruments can be determined from the deflection of theinstrument.

Without calibration of such an instrument, the deflection ismeaningless.

Different types of secondary instruments:

1)Indicating

2)Recording

3)Integrating


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1)Indicating:

Indicating instruments are those which indicate the instantaneousvalue of the electrical quantity being measured, at the time atwhich it is being measured. Their indications are given by pointersmoving over calibrated dials(scale), e.g. ammeters,voltmeters andwattmeters.

2)Recording:

Recording instruments are those which give a continuous record

of variations of the electrical quantity over a selected period oftime. The moving system of the instrument carries an inked penwhich rests tightly on a graph chart. E.g. recording voltmetersused in supply station.

3)Integrating:

Integrating instruments are those which measure and register thetotal quantity of electricity (in ampere-hour) or the total amount ofelectrical energy(in watt-hours or kilowatt-hours) supplied to acircuit over a period of time, e.g. ampere-hour meters, energymeters.


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ESSENTIALSOF INDICATING INSTRUMENTS

Deflecting Torque(Td):

It is the torque which deflects the pointer on a calibratedscale according to the electrical quantity passing through theinstrument. The deflecting torque causes the moving systemand hence the pointer attached to it to move from zeroposition to indicate on a graduated scale the value of

electrical quantity being measured.Controlling Torque(Tc):

It is the torque which controls the movement of the pointeron a particular scale according to the quantity of electricity

passing through it. If deflecting torque were acting alone, thepointer would continue to move indefinitely and would swingover to the maximum deflected position irrespective of themagnitude of current (or voltage or power) to be measured.


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1) Spring Control:In the spring control method, a hair-spring, usually of phosphor-bronze, attached to the moving system is used. With the deflection ofthe pointer, the spring is twisted in the opposite direction. This twist inthe spring produces a restoring torque which is directly proportionalto the angle of deflection of the moving system. The pointer comes toa position of rest (or equilibrium) when the deflecting torque (Td) andcontrolling torque (Tc) are equal.

Tc

To give a controlling torque which is directly proportional to the angle ofdeflection of the moving system, the number of turns of the springshould be fairly large so that the deformation per unit length is small.The stress in the spring must be limited to such a value that there is nopermanent set. Springs are made of materials which are

Non magnetic

Not subject to much fatigue

Low in specific resistance

Have low temperature coefficient of resistance.


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2) Gravity Control:Gravity control is obtained by attaching a small weight to the movingsystem in such a way that it produces a restoring or controlling torquewhen the system is deflected.

Tc SinThus, controlling torque in a gravity control system is proportional to the sine

of the angle of deflection.

The degree of control is adjusted by screwing the weight up or down on thecarrying system.

Advantages:

The advantages of gravity control system, as compared to spring control,are given below:

It is cheap

It is unaffected by temperature

It is not subjected to fatigue or distortion, with time.

Disadvantages:The disadvantages of gravity control system, as compared to spring control,

are given below:

It gives a cramped scale.

The instrument has to be kept vertical.


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Damping Torque:

If the moving system is acted upon by deflecting and controlling torques alone,then pointer, due to inertia, will oscillate about its final deflected position for quitesometime before coming to rest. This is often undesirable because it makesdifficult to obtain quick and accurate readings. In order to avoid these oscillations

of the pointer and to bring it quickly to its final deflected position, a dampingtorque is provided in the indicating instruments.

There are three types of damping:

Air friction damping:

air friction damping uses either aluminium piston or vane, which is attached to ormounted on the moving system and moves in an air chamber at one end.

Fluid friction damping:In fluid friction damping, a light vane (attached to the moving system) is dippedinto a pot of damping oil. The fluid produces the necessary opposing (ordamping) force to the vane. The vane should be completely submereged in theoil.

The disadvantage of this type of damping is that it can only be used in the verticalposition.

Eddy Current Damping:

Eddy-current damping uses a conducting material which moves in a magneticfield so as to cut through the lines of force, thus setting up eddy currents. Forcealways exists between the eddy current and magnetic field which is alwaysopposite to the direction of motion. This is most efficient type of damping and islargely used in permanent magnet moving coil instruments.


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TYPESOFINDICATINGINSTRUMENTS

PMMC(permanent magnet moving coil)

MI(moving iron)


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PMMC PRINCIPLE:

This type of instrument based on the principle thatwhen a current carrying conductor is placed in a

magnetic field, a force acts on the conductor,which tends to move it to one side and out of thefield.

CONSTRUCTION:

It consists of a powerful U-shaped permanentmagnet made of Alnico, and soft iron pole pieces

bored out cylindrically.A soft iron core is fixed between the magneticpoles whose functions are i) to make the fielduniform, and ii) to decrease the reluctance of theair path between the poles and hence increasethe magnetic flux.

Surrounding the core is a rectangular coil of manyturns wound on a light aluminium or copperframe, supported by delicate bearings.

A light pointer fixed to the frame moves on thecalibrated scale according to the amount ofelectricity passed through the coil.

The aluminiun frame provides not only support forthe coil but also a damping torque by the eddycurrents induce in it.


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WORKING:When the instrument is connected in the

circuit to measure current or voltage,the operating current flows through the

coil. Since the coil is carrying currentand is placed in the magnetic field ofthe permanent magnet , a mechanicalforce acts on it. As a result , the pointerattached to the moving system movesin a clockwise direction over thegraduated scale to indicate the value of

current or voltage being measured.If the current in the coil is reversed, the

deflecting torque also be reversedsince the direction of the permanentmagnet is same. Consequently, thepointer will try to deflect below zero.Deflection in this direction is prevented

by a spring stop. Since the deflectingtorque reverses with the reversal ofcurrent in the coil, such instrument canbe used to measure direct current andvoltage only.


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ADVANTAGE:

Low power consumption.

Uniform scale extendable over an arc of 270 or so.

High torque weight ratio.

No hysteresis loss. Very effective and efficient eddy current damping.

Not effected much by stray and magnetic fields due to strong operating field.

DISADVANTAGE:

Costlier compared to moving iron instruments, due to delicate construction and

accurate machining and assembly of various parts. Some error arise due to the ageing of control springs and the permanent magnet.

Use limited to d.c. only.

Scale length of meter can be increased from 120 and 240 or even 270 or 300.

APPLICATION: PMMC instruments can be used as dc ammeter. And its range can be increased

by using a large number of turns in parallel with the instrument.

The range of this instrument, when used as a dc voltmeter, can be increased byusing a high resistance in series with it.


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MOVING IRONINSTRUMENTS

Moving Iron instruments depend for their action uponthe magnetic effect of current, and are widely used as

indicating instruments. In this type of instrument , the

coil is stationery and the deflection is caused by a

soft-iron piece moving in the field produced by thecoil.

There are two types of moving iron instruments:

i) Attraction type

ii) Repulsion type


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Attraction type:Construction:Fig shows the constructional

details of an attraction typeMI instrument. It consistsof a cylindrical coil orsolenoid which is keptfixed. An oval-shaped soft-iron piece. The controllingtorque is provided by analuminium vane, attachedto the spindle, which

moves in a closed airchamber.


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Working:When the instrument is connected in the circuit to measure

current or voltage, the operating current flowing through

the coil sets up a magnetic field. In other words, the coilbehaves like a magnet and therefore it attracts the soft iron

piece towards it. The result is that the pointer attached to

the moving system moves from zero position. The pointer

will come to rest at a position where deflecting torque isequal to the controlling torque. If current in the coil is

reversed, the direction of magnetic field also reverses and

so does the magnetism produce in the soft iron piece.

Hence, the direction of the deflecting torque remains

unchanged. For this reason, such instruments can be used

for both d.c. and a.c. measurements.


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Repulsion typeConstruction:

Fig shows the constructional details

of repulsion type moving-ironinstrument. It consists of two soft-ironpieces or vanes surrounded by afixed cylindrical hollow coil whichcarries the operating current. One ofthese vanes is fixed and the other isfree to move. The movable vane is

cylindrical shape and is mountedaxially on a spindle to which a pointeris attached. The fixed vane, which iswedge-shaped and has a largerradius, is attached to the stationerycoil. The controlling torque is providedby one spiral spring at the top of the

instrument. It may be noted that inthis instrument, springs do notprovide the electrical connections.Damping is provided by air frictiondue to the motion of a piston in an airchamber.


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Working:When current to be measured or current proportional to the

voltage to be measured flows through the coil, a magnetic field is

set up by the coil. This magnetic field magnetises the two vanesin the same direction i.e. similar polarities are developed at the

same ends of the vanes. Since the adjacent edges of the vanes

are of the same polarity, the two vanes repel each other. As the

fixed vane cannot move, the movable vane deflects and causes

the pointer to move from zero position. The pointer will come torest at a position where deflecting torque is equal to controlling

torque provided by the spring. If the current in the coil is

reversed, the direction of deflection remains unchanged. It is

because reversal of the field of the coil reverses the

magnetisation of both iron vanes so that they repel each other

regardless which way current flows through the coil. For this

reason, such instruments can be used for both d.c. and a.c.

applications.


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o Advantages:

i) Cheap, robust and give reliable service.

ii) Usable in both a.c. and d.c. circuits.

o Disadvantages:i) Have non-linear scale.

ii) Cannot be calibrate with high degree of precision for d.c. on

account of the affect of hysteresis in the iron vanes.

iii) Deflection up to 240 only may be obtained with this instrument.iv) This instrument will always have to be put in the vertical position

if it uses gravity control.

o Errors with MI instruments:

i) Due to hysteresis when used in a.c. and d.c.ii) Due to stray magnetic fields when used both in a.c. and d.c.

iii) Due to frequency variation when used in a.c.

iv) Due to waveforms effect when used in a.c.


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Applications of MI instruments:As an ammeter:

It may be constructed for full-scale deflection of 0.1 to 30Awith out the use of shunts or current transformers. To

obtain full-scale deflection with currents less than 0.1A, it

requires a coil with a large number of fine wire turns, which

results in an ammeter with a high impedance.

As an voltmeter:

The MI voltmeter is a fairly low impedance instrument,

typically, 50/V for a 100V instrument. The lowest full

scale is of the order of 50V.The range of the instrument,

when used as a voltmeter, can be extended by using a

high non-inductive resistance R connected in series with it.

This series resistance is known as multiplier.


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INDICATING INSTRUMENTS (1) (4).pptx