19ee301 measurements & instrumentation systems

19EE301

Measurements & Instrumentation

Systems

by

Sakthisudhursun B.

Assistant Professor

Department of Electrical and Electronics Engineering,

Mepco Schlenk Engineering College

Sivakasi

Methods of MeasurementDirect Method

• Unknown quantity is directly compared against a

standard

• Result is expressed as numerical number and a unit

• Common for measuring length, mass

Indirect Method

• Used when desired parameter to be measured is difficult to be

measured directly

• But the parameter got some relation with some other related

parameter which can be easily measured.

Standard

Measurand

ResultCompare

Direct Method Example

Classification of Instruments based on comparison

• Deflection Type

• Null Type

Deflection Type:

• Value of the quantity being measured is displayed in terms

of the amount movement of a pointer.

Example: PMMC meter, MI meter, multimeter

Example for Deflection Type Instrument:

Null Type Instrument

• Zero or Null indication leads to determination of

magnitude of measured quantity

• Null condition depends on other known conditions

Example for Null Type Instrument:

Null Type Instrument

Advantages:

• Null type instrument more accurate than deflection type

• Highly sensitive

Disadvantage:

• Time consuming since requires many manipulations before null

detection

Instruments

Primary (Absolute)

Instruments

Secondary Instruments

Indicating Instrument

Recording Instrument

Integrating Instrument

Classification of Instruments

• Absolute Instrument: Gives the magnitude of the quantity to be measured

in terms of the constants (dimensions like length , number of turns and etc.)

and fundamental units.

• Calibration & comparison are not required

• Example :A tangent galvanometer, Rayleigh current balance

• In Rayleigh current balance, current flowing through coils will exert

some force

• This Force is measured by the balancing weight in a balance

• Current is calculated from force and coil dimensions

• Used in standard laboratory for calibration of secondary instrument

• Takes more time since every time measurement takes lot of time to

compute magnitude of quantity


• Secondary Instruments: They give direct values of measured

quantity (with the help of pointer & scale or a digital display)

• These have to be calibrated by comparison with an absolute

instrument

• Without calibration deflection of such instrument has no

meaning

• CLASSIFICATION OF SECONDARY INSTRUMENTS:

i. Indicating instruments

ii. Recording instruments

iii. Integrating instruments


Indicating Instruments:

Indicating instruments indicate, generally the quantity to

be measured by means of a pointer which moves on a scale

Example: Ammeter, Voltmeter, Wattmeter

Classification of Secondary Instruments

Recording Instruments:

The instruments which keep a continuous record of the

variations of the magnitude of an electrical quantity to be

observed over a defined period of time.

Example: Graphic Recorder, X-Y Recorder, Paper less recorder


Integrating Instruments:

• These instruments totalize events over a specified

period of time

Example: Energy Meter, odometer

The Energy meter measure the total amount electrical

energy supplied over a period of time.


• Indicating instrument consist essentially pointer which moves

over calibrated scale & which is attached to moving system

• Moving system essentially subject to three torques

1. Deflecting Torque (operating torque)

2. Controlling Torque (Restoring torque)

3. Damping Torque

Essentials of Indicating Instrument

• Deflecting torque is required for moving the pointer from its

zero position

• Can be produced by one of the following effects

1. Magnetic effect

2. Heating effect

3. Electrostatic effect

4. Electromagnetic (or) induction effect

5. Hall effect

• Deflecting system of instrument converts electrical current or

voltage into a mechanical force

Deflecting Torque

• Deflection of moving system is indefinite if there is no control torque

• Controlling torque opposes the deflecting torque

• Pointer brought to rest position when controlling force is equal to deflecting

force

• The controlling torque developed in an instrument has two functions:

1. Limits movement of moving system & ensures that magnitude of

deflection always remains same for a given value of quantity to be

measured

2. Brings back moving system to its zero position where the quantity being

measured is removed or made zero

Controlling Torque

• Controlling torque is achieved by any one of following method

• Spring Control

• Gravity Control

Spring Control:

• In spring control the controlling torque is achieved by two spiral hair spring

attached to moving system

• With deflection of pointer, spring is twisted in opposite direction

• This twist in spring produces control torque proportional to angle of

deflection

• Springs are used as leads of current to the instrument

• Phosphor bronze is most suitable used material for making spring

Controlling Torque

radin deflectionAnular

spring oflength

spring of thicknesst

spring ofwidth b

spring of modulus sYoung'E where

l

kCT

lk

12

Ebt 3

constant spring where k

The spring material should also have following properties:

• It should have low resistance

• The temperature coefficient should also be low.

• The springs must be of non-magnetic material

Spring Control

• If deflecting torque is directly proportional to current then at steady

condition of pointer

Iθ

Ik

Kθ

kθIK

TT

d

d

cd

Spring Control

• Small adjustable weight called control weight is attached to spindle of

moving system such that deflecting torque produced by instrument has to act

against action of gravity

• Another adjustable weight attached to moving system for zero adjustment &

balancing purpose is called as balance weight

• When the control weight is in vertical position the controlling torque is zero

& hence the pointer must read zero

Gravity Control

Gravity Control

• Weight acts at a distance l from the center

• Component of weight trying to restore the pointer back to zero

position is W sin Ɵ

• Expression for control torque:

C

C

T =force×distance

T sin

sin

sing

W l

Wl

k

Gravity Control

Activity: Compare Gravity Control & Spring control

Compare based on

Scale (Relation between angle of deflection and current)

Position of usage

Aging problem

Cost

Performance variation with temperature

Advantages of Gravity Control:

• It is cheap and not affected by temperature variations.

• It does not deteriorate with time.

• It is not subject to fatigue

Disadvantages of Gravity Control:

• Since controlling torque is proportional to sine of angle of deflection, scale is

not uniformly divided but cramped at its lower end.

• Gravity control instruments must be used in vertical position so that the

control weight may operate & also must be leveled otherwise they will give

zero error

Gravity Control

• Moving system of instrument will tend to move under the action of

deflecting torque.

• On account of control torque, it will try to occupy a position of rest when

two torques are equal & opposite.

• Due to inertia of moving system, the pointer will not come to rest

immediately but oscillate about its final deflected position

• Damping torque is stabilizing torque which brings the pointer to steady

state quickly

• The damping torque is proportional to the speed of rotation of the moving

system

Damping Torque

• Ideally the damping torque has to produced only when the

moving system is in motion

• To be effective damping torque should be proportional to

velocity of moving system & independent of operating

current

1.Air Friction Damping

2.Fluid Friction Damping

3.Eddy Current Damping

Damping torque

Air Friction Damping

• Light aluminum vane is attached to the moving system

• Consists of piston moves in fixed air chamber which is closed at one end

• Clearance between piston & wall is uniform throughout and very small

• When piston moves into the chamber the air inside is compressed

• Hence pressure of air builds up which opposes the motion of piston and whole

moving system

• When piston moves out of chamber vice versa action happens to develop

damping torque

• Used in moving iron & dynamometer type of instrument where the operating

magnetic field is weak

Advantages:

Simple and cheap

Suitable for meters with low operating magnetic field


• similar to air friction damping

• Mineral oil is used in place of air

• As the viscosity of oil is greater, the damping force is also much greater

Fluid friction Damping

Method 1:

• Disc is attached to the moving system is immersed

in the fluid

• When the moving system moves the disc moves in

oil and a frictional drag is produced.

Method 2:

• Number of vanes are attached to the spindle is

arranged to move in the damping oil

Eddy Current Damping

• Aluminum disc is connected to spindle

• Arrangement of disc is made such that, when

it rotates cut the magnetic field produced by

permanent magnet

• When pointer rotates aluminum disc cuts the magnetic field produced by magnet

• Hence as per Faraday’s law an EMF will be induced & since disc is closed path

current will flow.

• This current is called as eddy current and it opposes the cause that producing it

(i.e.,) movement of pointer

9/20/2021 31Basic Electrical and Electronics Engg.

Factors affecting force on a current-carrying conductor in a magnetic field:

• Strength of the magnetic field

• Current flowing through the wire

• Length of the wire

F=BIlsinθ, where

F is force acting on a current carrying conductor,

B is magnetic flux density (magnetic field strength),

I is magnitude of current flowing through the conductor,

l is length of conductor,

θ is angle that conductor makes with the magnetic field.

Basic Principle of PMMC:

Current carrying conductor experiences a force when placed in magnetic field

Principle of operation of PMMC

Construction of PMMC

Moving Coil:

• Wound with many turns of enameled or silk covered copper wire

• Coil is mounted on rectangular aluminum former which is pivoted on jewelled

bearing

• Coils move freely in the field of permanent magnet

• Magnetic former are used for voltmeter

• Non magnetic former are used for ammeter

Permanent Magnet:

• Olden days U-shaped magnet having soft iron pole pieces are used

• To make field radial and uniform

• To decrease the reluctance

• Flux density of permanent magnet varies from 0.1wb/m2 to 1Wb/m2

• Movement of coil is restricted (i.e.) no part of coil is allowed to move near pole

tips where there is a fringing

• Limitation is overcome by concentric type construction

PMMC Construction

Control Torque:

• Provided by two phosphor bronze hair springs

• Spring also serves as lead for in and out of coil

Damping Torque:

• Damping torque is provided by eddy current damping

• When aluminum former moves with moving coil in the field of permanent magnet,

induces a voltage in it

• This voltage causes eddy current to flow in it

• These current exerts force on former & thus damping torque is produced

• By Lenz’s law this force opposes the motion of the former

Pointer and Scale:

Pointer is carried by spindle & moves over graduated scale

Made from light weight aluminum

PMMC Construction

Errors in PMMC:

• Error due to aging

• Strength of spring changes with time

• Weakening of permanent magnet due to ageing & temperature effects

• Flux density of permanent magnet weakens with increase in temperature

• Weakening of springs due to temperature effects

• A 1⁰C rise in temperature reduced the strength of spring about 00.04%

• Change of resistance of moving coil with temperature

• Copper wire having a temperature co-efficient of 0.004/⁰C. Causes serious

error when used in micro and milli ampere range of current flows through

moving coil

PMMC meter errors

Advantages:

• Scale is uniform

• Power consumption is less

• High Torque-to-weight ratio which gives high accuracy

• Error due stray magnetic field are less due to high operating flux density

Disadvantage:

• Can be only used to measure DC voltage & current

Reason:

During positive half the pointer experiences force in one direction & in

negative half pointer experiences force in opposite direction

Pointer can’t follow rapid reversal and deflection corresponds to mean

torque which is zero

• Cost is high

PMMC Advantages &

Disadvantage

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