1 Measuring Instruments Basics

8/10/2019 1 Measuring Instruments Basics

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Measuring Instrument : instrument that shows the amount or quantityor degree of something.

Classification of Measuring Instruments according to use :(i) Galvanometer(ii) Ammeter(iii) Voltmeter(iv) Watt meter(v) Energy Meter(vi) Ohm Meter(vii) Megger(viii) Avometer(ix) Maximum Demand Indicator(x) P.F Meter(xi) Frequency Meter


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(B) Secondary Instrument :The instrument which measures an electrical

quantity on a calibrated scale is termed as secondary instrument. Most ofthe measuring instruments which are generally used are secondaryinstruments

They are often calibrated by comparing them with either some absoluteinstruments or with those which have already been calibrated.


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Secondary instruments are further classified as:

Indicating instrumentsRecording instrumentsIntegrating instruments

Indicating instruments: Indicating instruments indicate, generally the quantity to be measured by means of a pointer which moves on a scale. Examples are ammeter, voltmeter, wattmeter etc.

Recording instruments:

These instruments record continuously the variation of anyelectrical quantity with respect to time. In principle, these are indicating instruments but soarranged that a permanent continuous record of the indication is made on a chart or dial.

The recording is generally made by a pen on a graph paper which is rotated on a dice or drum ata uniform speed.

The amount of the quantity at any time (instant) may be read from the traced chart. Anyelectrical quantity like current, voltage, power etc., (which may be measured lay the indicatinginstruments) may be arranged to be recorded by a suitable recording mechanism. Ex-Recordingvoltmeter, Ammeter,

p.F meters


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Integrating : These instruments record the consumption of the total quantity ofelectricity, energy etc., during a particular period of time. That is, theseinstruments totalize events over a specified period of time. No indication of therate or variation or the amount at a particular instant are available from them.Some widely used integrating instruments are: Ampere-hour meter: kilowatt hour(kWh) meter, kilovolt-ampere-hour (kVARh) meter


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According to Working Principle:

1.Electromagnetic Instrument: Moving coil and moving iron instruments belong to this category.

2.Electro dynamic Instrument: Dynamo type ammeter, voltmeter and wattmeter belong to this category

3.Electro thermal Instrument: Hot wire ammeter and thermocouple meters belong to this category.

4.Electro induction Instrument: Watt meters and energy meters belong to this category.

5.Electrochemical Instrument: D.C. ampere-hour meter and watt-hour meters belong to this category.

6.Electrostatic Instrument: Certain voltmeters belong to this category.

FORCES ACTING IN AN INSTRUMENT

Following forces are necessary for working of an instrument1.Deflecting force

2.Controlling force

3.Damping force.


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Deflecting force:The force acting on the moving system of an instrument

produce a torque which moves the moving system, is called deflecting force or deflecting torque.

The deflection of the pointer is proportional to the deflectingtorque, and which itself is proportional to the current passed through theinstrument.

Controlling Force:This force opposes the action of deflecting force, and

the pointer will rest in a position where the two force will neutralize eachother.

The controlling force stops the pointer to move

beyond the actual value of electrical quantity under measurement. It also brings the pointer back to zero position after disconnecting the meterfrom the circuit. Thus no indicating instrument can work without acontrolling force.


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The controlling force can be developed in the following two ways:

1.Spring control :

2.Gravity control :1.Spring Control :

The spindle of the moving system rests on two pivots.A hair spring made of phosphor bronze wire is attached to the spindle.


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. The spring is wound in a direction which is opposite to the rotation ofpointer.

.Under the influence of deflecting force, when the pointer moves, then

the spring gets twisted in the opposite direction.

.As the deflecting force becomes zero, the twist of the spring bringsback the pointer to its zero position.

A spring used in instruments should have the following characteristics:

(i)It should be non-magnetic

(ii)It should have a low resistance

(iii)It should have a low temperature coefficient.

Materials used for spring Phoshporous Bronze, Berillyum Copper

Advantages of Spring Control :


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Gravity Control:In this system, the pointer should rotate in a vertical plane.

The control is obtained by attaching a control weight to the pointer insuch a way that it produces a controlling torque because of change in its

position,

A balance weight is also attached to the pointer. Both the weights shouldbe adjustable.

The controlling torque is proportional to the sine of the angle of

deflection and hence the scale will not be a uniform scale.


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The various methods adopted for damping are:1.Air friction damping

2.Fluid friction damping

3.Eddy current damping


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In air friction damping a light aluminum piston is attached to the movingpart of the instrument.

The piston moves in an air chamber which is closed at its one end. It willhave a small clearance with the air chamber.

The system works on the compression and suction of the air. Now days,two aluminum vanes are attached to the moving part, which rotate in

sector shaped air-chambers.

1.Air Friction Damping:


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Fluid Friction Damping:

This system is similar to the air friction damping, but it employs a liquid in place ofair.The liquid should have a high viscosity so as to produce sufficient damping force.

The system is not suitable for portable instruments, hence it is out dated now.


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Eddy Current Damping:

In this system, a circular thin disc of copper or aluminum is mounted onthe spindle. A damping magnet is placed on side of the disc.

The magnet is place in such a way that the rotating disc will cut themagnetic lines of force of the magnet.

Thus, eddy currents will be induced in the disc which will oppose itsmotion, and thus a damping force is produced in the instrument.


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Permanent Magnet Moving Coil (PMMC) Meter

A coil of fine wire is suspended in a magnetic field produced by permanentmagnet.

According to the fundamental law of electromagnetic force, the coil will rotatein the magnetic field when it carries an electric current by electromagnetic (EM)torque effect.

A pointer which attached the movable coil will deflect according to the amountof current to be measured which applied to the coil.


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Uniform scale.

Eddy current damping, so very effective & reliable.

No hysteresis loss.

Low power consumption as the driving power is small.

No effect of stray magnetic field.

High torque/weight ratio, require small operating current.

Very accurate & reliable.

The scale covered upto 270 degree.


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Can not be used for AC measurement.

Costlier in comparison to moving iron instruments.

Friction & temperature might introduce some error.

Errors due to ageing of control springs & permanentmagnets.


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1- D.c Ammeter:

An Ammeter is always connected in series with a circuit branch and measures the

current flowing in it.


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Extension of Ammeter Range: Since the coil winding in PMMC meter is small and light, they can carry only smallcurrents (A -1mA). Measurement of large current requires a shunt external r esi stor toconnect with the meter movement, so only a fraction of the total current will passesthrough the meter.

Vsh = Vm

IshRsh= ImRm

But Ish = IT Im

Ish

RmImshR

mT

RmImsh

IIR


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Example:If PMMC meter have internal resistance of 10 and full scale range of 1mA. Assume we wish to increase the meter range to 1A.

we must connect shunt resistance with the PMMC meter of Rsh

ImTI

RmImshR


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Example 1:- A moving coil ammeter has a fullscale deflection of 50 Amp and a coil resistanceof 1000 . What will be the value of the shuntresistance required for the instrument to beconverted to read a full scale reading of 1 Amp.

0.05 Example 2 :- The full scale deflection current ofan ammeter is 1 mA and its internal resistance is

100 . If this meter is to have scale deflection at5 A, what is the value of shunt resistance to beused.

R sh = 0.020004 ohm


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VOLTMETER


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ExampleConvert a 50 micro A, 3k PMMC into a 0-10VDC voltmeter.

The voltmeter looks like this:

A PMMC meter with a coil resistance 100 and a full scale deflection currentof 100 A is to be used in the voltmeter circuit as shown in Fig.42.6. Thevoltmeter ranges are to be 50. Determine the required value of resistances

for each range.(i) For 50V - 0.. 4999 M(ii) For 100V - 0.9999 M (iii)For 150V - 1.4999M


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A moving coil ammeter gives full scale deflection with 15 mA and has aresistance of 5 . Calculate the resistance to be connected in (a)Parallel to enable the instrument to read upto 1 A (b) Series to enableit to read up to 10 V.

Rsh = 0.07614 Rse = 661.6667


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Moving-iron InstrumentsThe deflecting torque in any moving-iron instrument is due to forces on a smallpiece of magnetically soft iron that is magnetized by a coil carrying the

operating current.(i) Attraction Type (ii) Repulsion Type


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REPULSION TYPE

Repulsive forces will act when two similarly magnetized iron pieces are placed near to each other.


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The change in energy of magnetic field,

dw = T d d

Since change in energy dE = workdone, dw

Td d = I 2dL

Td = I 2dL/ d

This is non- linear relationship


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Moving iron Instruments

Advantages The instruments are suitable for use in AC and DC circuits. The instruments are robust, owing to the simple construction of the moving parts. The stationary parts of the instruments are also simple. Instrument is low cost compared to moving coil instrument.

Torque/weight ratio is high, thus less frictional error.

Disadvantages Error due to friction is quite small as torque-weight ratio is high in moving coil

instruments. Error due to variation of frequency causes change of reactance of the coil and

also changes the eddy currents induced in neighboring metal. Deflecting torque is not exactly proportional to the square of the current due to

non-linear characteristics of iron material.


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Sensitivity: It is defined by the change in the output or response of theinstrument for a unit change of input or measured variable.

Resolution: Resolution is the smallest change in a measured variable (ormeasurand ) to which the instrument will respond.

Terms used :

Error: It is the deviation of the measured (or indicated) value from thetrue (or expected) value of a quantity.

In other words, error is the difference between the measured value andthe true value of the unknown quantity. It is also called absolute error aremaximum possible error.

A = Am At

where Am = measured value of the quantity

At = true value of the quantity


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