measuring instruments (1)

7/29/2019 Measuring Instruments (1)

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MEASURING INSTRUMENTS


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MEASURING INSTRUMENTS

The device used for comparing the unknown quantitywith the unit of measurement or standard quantity

is called a Measuring Instrument.


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CLASSIFICATION OF INSTRUMENTS

Electrical instruments may be divided into two categories,that are;

1. Absolute instruments,

2. Secondary instruments.- Absolute instruments gives the quantity to bemeasured in term of instrument constant & itsdeflection.

- In Secondary instruments the deflection gives themagnitude of electrical quantity to be measureddirectly. These instruments are required to becalibrated by comparing with another standardinstrument before putting into use.


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CLASSIFICATION OF INSTRUMENTS

Electrical measuring instruments may also be classifiedaccording to the kind of quantity, kind of current,principle of operation of moving system.

CLASSIFICATION OF SECONDARY INSTRUMENTS Secondary instruments can be classified into threetypes;

i. Indicating instruments;

ii. Recording instruments;iii. Integrating instruments.


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CLASSIFICATION OF SECONDARY INSTRUMENTS

- Indicating Instruments:It indicate the magnitude of anelectrical quantity at the time when it is beingmeasured. The indications are given by a pointer

moving over a graduated dial.- Recording Instruments:The instruments which keep acontinuous record of the variations of the magnitudeof an electrical quantity to be observed over a definedperiod of time.

- Integrating Instruments:The instruments whichmeasure the total amount of either quantity of electricityor electrical energy supplied over a period of time. For

example energy meters.


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ESSENTIALS OF INDICATING INSTRUMENTS

A defined above, indicating instruments are thosewhich indicate the value of quantity that is beingmeasured at the time at which it is measured. Such

instruments consist essentially of a pointer whichmoves over a calibrated scale & which is attached to amoving system pivoted in bearing. The moving systemis subjected to the following three torques:

1. A deflecting ( or operating) torque;

2. A controlling ( or restoring) torque;

3. A damping torque.


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DEFLECTING TORQUE

- The deflecting torque is produced by making one ofthe magnetic, heating, chemical, electrostatic andelectromagnetic induction effect of current or voltage

and cause the moving system of the instrument tomove from its zero position.

- The method of producing this torque depends uponthe type of instrument.

- CONTROLLING TORQUE- The magnitude of the moving system would be somewhat indefinite under the influence of deflectingtorque, unless the controlling torque existed to opposethe deflecting torque.


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CONTROLLING TORQUE

- It increases with increase in deflection of movingsystem.

- Under the influence of controlling torque the pointer

will return to its zero position on removing the sourceproducing the deflecting torque.

- Without controlling torque the pointer will swing at itsmaximum position & will not return to zero after

removing the source.- Controlling torque is produced either by spring orgravity control.


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Spring Control:

When the pointer is deflectedone spring unwinds itself while

the other is twisted. This twist in

the spring produces restoring

(controlling) torque, which isproportional to the angle of

deflection of the moving systems.


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Spring Control


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Gravity Control

In gravity controlled instruments, a small adjustableweight is attached to the spindle of the moving systemsuch that the deflecting torque produced by the instrumenthas to act against the action of gravity.

Thus a controlling torque is obtained. This weight is calledthe control weight. Another adjustable weight is alsoattached is the moving system for zero adjustment andbalancing purpose. This weight is calledBalance weight.


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Tc Sin

Td I

Tc = T

I Sin


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DAMPING TORQUE

We have already seen that the moving system of theinstrument will tend to move under the action of thedeflecting torque.

But on account of the control torque, it will try to occupya position of rest when the two torques are equal andopposite.

However, due to inertia of the moving system, thepointer will not come to rest immediately but oscillate

about its final deflected position as shown in figure andtakes appreciable time to come to steady state.

To overcome this difficulty a damping torque is to bedeveloped by using a damping device attached to the

moving system.


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DAMPING TORQUE

The damping torque is proportional to the speed ofrotation of the moving system, that is

Depending upon the degree of damping introduced in

the moving system, the instrument may have any one ofthe following conditions as depicted in above graph.


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DAMPING TORQUE

1. Under damped condition:

The response is oscillatory

2. Over damped condition:

The response is sluggish and it rises very slowly fromits zero position to final position.

3. Critically damped condition:

When the response settles quickly without anyoscillation, the system is said to be critically damped.

The damping torque is produced by the following methods:

1.Air Friction Damping 2.Fluid Friction Damping

3.Eddy Current Damping 4.ElectromagneticDamping


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Air Friction Damping


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Eddy current Damping


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TYPES OF AMMETERS & VOLTMETERS

1) Moving Iron Type Meters (AC & DC);

a) Attraction type,

b) Repulsion type.

2) Moving Coil Type Meters (AC & DC);

a) Permanent Magnet type,b) Electrodynamic or Dynamometer.

3) Induction Type (AC & DC);

a) Split phase,

b) Shaded Pole type.


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Moving-iron instrument

An attraction type of moving-iron instrumentis shown diagrammatically in Figure. Whencurrent flows in the solenoid, a pivoted soft-iron disc is attracted towards the solenoid andthe movement causes a pointer to move acrossa scale.

In the repulsion type moving-iron instrumentshown diagrammatically in Figure, two pieces

of iron are placed inside the solenoid, onebeing fixed, and the other attached to thespindle carrying the pointer.


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Moving-iron instrument


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Moving-Coil instrument

There are two types of moving coil instruments

namely, permanent magnet moving coil type

which can only be used for direct current,voltage measurements.

The dynamometer type which can be used on

either direct or alternating current, voltage

measurements.


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PERMANENT MAGNET MOVING COIL

The principle operationof PMMC is basedupon the principle

of current carryingconductor is placedin a magnetic field itis acted upon by forcewhich tends to moveit.


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DYNAMOMETER

This instrument is suitable for the measurement of directand alternating current, voltage and power.

The deflecting torque in dynamometer is relies by theinteraction of magnetic field produced by a pair of fixed

air cored coils and a third air cored coil capable ofangular movement and suspended within the fixed coil.


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HOT WIRE TYPE

It is based on the heating effect of current.

It consist of platinum-iridium (it can withstand

oxidation at high temperatures) wire. When current is through wire, it expands

according to I2R formula.

This produces sag in the wire and pointer is

attached with this wire which in result deflects.


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INDUCTION TYPE INSTRUMENT

Such instruments are suitable for ac measurements only inthese instruments the deflecting torque is produced by theeddy currents induced in an aluminum or copper disc ordrum by the flux created by an electro-magnet.

The main advantages of such instruments are that

(i) a full scale deflection can be obtained giving long andopen scale

(ii) the effect of stray magnetic field is small;(iii) damping is easier and effective.


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These instruments have got some serious disadvantages(i) The greater deflection causes more stresses in the controlsprings.

(ii) Variation in supply frequency and temperature maycause serious errors unless compensating device is

employed.

(iii) These instruments are costlier and consume morepower

Such instruments are mostly used as watt-meters orenergy meters.


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Induction type wattmeter consists of two laminateelectromagnets known as shunt electromagnet andseries electromagnet respectively.

Shunt magnet is excited by the current proportional to

the voltage across load flowing through the pressurecoil and series magnet is excited by the load currentflowing through the current coil.

A thin disc made of Cu or Al, pivoted at its centre, isplaced between the shunt and series magnets so that itcuts the flux from both of the magnets.


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The deflection torque is produced by interaction ofeddy current induced in the disc and the inducing fluxin order to cause the resultant flux in shunt magnet tolag in phase by exactly 90 behind the applied voltage.

One or more copper rings, known as copper shadingbond are provided on one limb at the shunt magnet.

Correct disappointed between shunt and series magnetfluxes may be attained by adjusting the position ofcopper shading bonds.The pressure coil circuit of induction type instrumentis made as inductive as possible so that the flux of theshunt magnet may lag by 90 behind the applied

voltage.


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Block Diagram of Multimeter


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Multimeter 1) To measure resistance connect an unknown resistor across its input

probes. Keep rotary switch in position1. Some current flows through theresistor, from constant current source. Now according to Ohms law

voltage is produced across it. This voltage is directly proportional to itsresistance. This voltage is buffered by the buffer amplifier and then fed to

A/D converter, to get digital display in Ohms.

2) To measure AC voltage connect an unknown AC voltage across inputprobes. Keep rotary switch in position2. The voltage is attenuated, if itis above the selected range and then rectified to convert it intoproportional DC voltage. It is then fed to A/D converter to get the digital

display in Volts. 3) To measure AC current this circuit measures the current indirectly.

Because the circuit can measure only voltage and the A/D converter canconvert voltage into proportional digital signals. So the current isconverted into proportional voltage first and then measured. Connect anunknown AC current across input probes. Keep the switch in position3.

The current is converted proportionally into voltage with the help of IVconverter and then rectified. Now the voltage in terms of AC current is


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The current is converted proportionally into voltage withthe help of IV converter and then rectified. Now thevoltage in terms of AC current is fed to A/D converter toget digital display in Amperes.

4) To measure DC current here also the circuit measuresthe current indirectly. Connect an unknown DC currentacross input probes. Keep the switch in position4. Thecurrent is converted proportionally into voltage with thehelp of IV converter. Now the voltage in terms of DCcurrent is fed to A/D converter to get the digital display in

Amperes. 5) To measure DC voltage connect an unknown DC

voltage across input probes. Keep the switch in position5.The voltage is attenuated, if it is above the selected rangeand then directly fed to A/D converter to get the digitaldisplay in Volts.


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MEGGER

It is a portable instrument used for testing the insulationresistance of a circuit and for measuring resistances of theorder of megaohms


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Parts: 1) Control and Deflecting coil: They are normally mounted at

right angle to each other and connected parallel to the generator.The polarities are such that the torque produced by them is inopposite direction.

2) Permanent Magnet: Permanent magnet with north and southpoles to produce magnetic effect for deflection of pointer.

3) Pointer and scale: A pointer is attached to the coils and end ofthe pointer floats on a scale which is in the range from zero toinfinity. The unit for this is ohms.

4) D.C generator or battery connection: Testing voltage issupplied by hand operated D.C generator for manual operatedMegger and a battery and electronic voltage charger for automatictype Megger.

5) Pressure coil and current coil: Provided for preventingdamage to the instrument in case of low external source resistance.

measuring instruments (1)

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