CHAPTER 2 instrumentation

8/16/2019 CHAPTER 2 instrumentation

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CHAPTER 2

Direct –Current Indicating Instruments

1 Suspension Galvanometer

It is a device used to detect and measure small current in the circuit.

Principle: -A current carrying coil placed in a magnetic feld

Experiences a torque

Construction: - It consists o a narrow rectangular coil having a large number

o turns o copper wound on a non-metallic rame. It is suspended between the

semicircular poles pieces o powerul horseshoe magnet by mean o a

suspension wire made o phosphor bronze with a needle or pointer attached toit. A cylindrical sot iron core is fxed within the coil. his ma!es the magnetic

feld lin!ed with coil to be radial. At the lower end" coil is attached to a fne

spring. #amp and and $cale arrangement is used to measure de%ection

&ig'

2 Torue and De!lection o! t"e Galvanometer

(hen the current I is passed through the coil" it experiences a torque

which is

given as

T

d

(

d

e

f

e

c

ti

n

g

t

o

r

q

u

e

)=

N

I

A

B

(

h

e

r

e

)

*

n

o

o

t

ur

n

s

A *

area

coil

+ * magnetic feld" and

,ue to this torque" coil starts rotating. $o the suspension wire gets twisted and a restoring

torque is developed in the suspension wire made o hosphor bronze. he restoring torquecontinues to increase and stage reaches when restoring torque becomes equal to the

de%ecting torque.

At equilibrium Defecting Torque = Restoring Torque

or controlling torque

Td=Tc

N I A B = k θ


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(here θ is the angle by which the coil is rotated and ! is the restoring torque per unit

angle .

hereore I = k θ NBA = ! θ

(here ! = k NBA is called galvanometer constant.

I ∝ θ

ence current is directly proportional to the angle by which the coil rotated. Also light

rom lamp alls on the mirror gets de%ected and alls on the scale. (hen current passes

the mirror moves and light spot moves on the scale.

# Permanent $%agnet moving –coil mec"anism &P%%C'

In //0 meter or 1,2Arsonval3 meter or galvanometer all are the sameinstrument" a coil o fne wire is suspended in a magnetic feld producedby permanent magnet. According to the&undamental law o electromagnetic orce" the coil will rotate in themagnetic feld when it

0arries an electric current by electromagnetic 1E/3 torque e4ect. A pointerwhich attached the /ovable coil will de%ect according to the amount o current to be measured which applied to the coil. he 1E/3 torque iscounterbalance by the mechanical torque o control springs attached to themovable coil also. (hen the torques are balanced the moving coil willstopped and its angular de%ection represent the amount o electrical currentto be measured against a fxed reerence" called a scale. I the permanentmagnet feld is uniorm and the spring linear" then the pointer de%ection isalso linear.

( Galvanometer Sensitivit)

"ensiti#it$' - A galvanometer is said to be sensitive" i it give largede%ection" even when a small current is passed through it or a small voltageis applied across its coil.Current "ensiti#it$:-It is defned as the de%ection produced in the

galvanometer on passing a current o one ampere. i.e.0urrent $ensitivity *%olt&ge "ensiti#it$: - It is defned as the de%ection produced in thegalvanometer when a potential di4erence o one volt is applied its coil.

*oltage Sensitivit) +

, DC Ammeters


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Indirect D.C Ammeter:


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Have to determine values o! Ra-R. and R. to measure di!!erent range o!

currents


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/ DC *oltmeters


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0 *oltmeter Sensitivit)

8 Voltmeter- Ammeter method

This is very popular method for measurement of medium resistances since instrumentsrequired for this method are usually available in laboratory. The two types of connections

employed for ammeter voltmeter method are shown in figure. In both the methods if

readings of ammeter and voltmeter are taken then we can measure value of resistance by

using formula:

The measured value of resistance Rm, would be equal to the true value R, if the ammeter

resistance is zero and the voltmeter resistance is infinite, so that the conditions in the circuit are

not disturbed. ut in actual practice this is not possible and hence both methods give inaccurateresults.

Consider circuit of figure (a):


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In this method ammeter measures the true value of current flowing through resistance but

voltmeter does not measures the true value of the voltage across the resistance. the voltmeter

indicates the sum of the voltage across resistance and ammeter.!et Ra be the resistance of the ammeter

It is clear from the above equation that the error will be small if the value of the measuring

resistance is large as compare to the internal resistance of the ammeter .therefore circuit should

be used when measuring resistances are high.

Consider circuit of figure (b):

voltmeter ammeter method

In this circuit the voltmeter measures the true value of the voltage across the measuring

resistance but the ammeter does not measures the true value of the current flowing through theresistance .the current through the ammeter is the sum of the current through the voltmeter and

resistance.


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!et Rv be the resistance of the voltmeter.

rom t"e a.ove euation it is clear t"e true value o! t"e resistance ill .e eual to t"e measured

value onl) "en t"e voltmeter resistance is eual to t"e in!inite$ Hoever- i! t"e resistance o! t"e

voltmeter is ver) large as compared to t"e resistance under measurement3


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4 Series –t)pe 5"mmeter


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16 S"unt –t)pe 5"mmeter

11 %illimeter or *5% Cali.ration o! Dc Instruments3t"is is a instrument is used to measure

current -voltage and Resistance$

CHAPTER 2 instrumentation

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