Frequency Meter : Working principle

A frequency meter is an instrument that displays the frequency of a periodic electrical signal. Various types of frequency meters are used. Many are instruments of the deflection type, ordinarily used for measuring low frequencies but capable of being used for frequencies as high as 900 Hz.

1. Frequency Meter

2. Electrical Resonance
Electrical resonance occurs in an electric circuit at a particular
resonance frequency where the imaginary parts of circuit element
impedances or admittances cancel each other. In some circuits this
happens when the impedance between the input and output of the
circuit is almost zero and the transfer function is close to one.

3. Electrical Resonance
Since the inductive reactance and the
capacitive reactance are of equal magnitude,
ωL= 1/ωC, so:
where ω = 2πf, in which fis the resonance frequency
in hertz, Lis the inductance in henries, and Cis the
capacitance in faradswhen standard SI unitsare used.
The quality of the resonance (how long it will ring
when excited) is determined by its Q factor, which is a
function of resistance

4. FREQUENCY METERS
Typesoffrequencymeters:
1.Mechanicalresonance(reed)typefrequencymeter
2.Electricalresonancetypefrequencymeter
a)Ferro-dynamictypefrequencymeter
b)Electro-dynamometertypefrequencymeter
3.Westonfrequencymeter
4.Ratiometertypefrequencymeter
5.Saturablecoretypefrequencymeter

5. Electrical resonance type frequency meter
(a)Ferro-dynamic type frequency meter:
Thistypeofinstrumentoperatesontheprincipleof
electricalresonance,wheninductivereactanceXLand
capacitivereactanceXcbecomeequal,electrical
resonancesaidtobeoccurred.Themovingcoilis
furtherconnectedtoasuitablecapacitor“C”whereas
themagnetizingcoilisconnectedacrossthesupply
terminalwhosefrequencyistobemeasured.Ifwe
neglecttheresistanceofthecoilandtheironlossesin
thecore,fluxɸisinphasewithcurrentI.Sincefluxɸ
isanalternatingflux,itwillinducee.m.finthemoving
coil.

6. Construction
MagnetizingCoil
-Consistsofafixedcoilcalledmagnetizingcoil,whichis
connectedacrossthesupplywhosefrequencyistobemeasured.Itis
mountedonalaminatedironcore
Ironcore
Crosssectionofironcorevariesgraduallyoverthelength,being
maximumneartheendwheremagnetizingcoilismountedand
minimumattheotherend.
MovingCoil
Movingcoilispivotedovertheironcore
Apointerisattachedtomovingcoil
TerminalsofmovingcoilareconnectedtoasuitablecapacitorC
Nosupplyforcontrollingforce

7. Principle of Operation
Magnetizing coil carries a current I and produces flux f in phase with
current I. Flux f induces emf E in the moving coil lagging behind it by 90
o
.
Emf E circulates current I
min the moving coil. Phase of current I
m
depends upon inductance L of the moving coil and capacitance C.

8. Principle of Operation..

9. Vector Diagram

10. Circuit of moving coil is inductive & therefore current I
mlags
behind emf E by an angle a. The torque acting on the moving
coil is,
T
dis proportional to I
mI cos (90
o
+ a)
Circuit of moving coil is capacitive & therefore current I
m
leads emf E by an angle b. The torque acting on the moving
coil is,
T
d=k I
mI cos (90
o
-b)
Inductive reactance of the circuit of moving coil is equal to its
capacitive reactance & therefore current I
mis in phase with
emf E. The torque acting on the moving coil is,
T
d=k I
mI cos 90
o
= 0
Principle of Operation……

11. Principle of Operation……
At normal frequency ,At this position, inductive
reactance is equal to the capacitive
reactance.Position of the moving coil is at mean
position
Suppose the frequency increases above its normal
value then, X
L> X
C& therefore torque is produced.
This torque pulls the moving coil to an equilibrium
position i.e., moving coil deflects towards the
section of iron core having minimum cross section.
So inductive reactance decreases and moving coil
comes to rest at a position where X
L= X
C.

12. Principle of Operation……
• Suppose the frequency decreases below its normal value then, X
L
< X
C& therefore torque is produced. This torque pulls the moving coil to
an equilibrium position i.e., moving coil deflects towards the section of
iron core having maximum cross section. So inductive reactance
increases and moving coil comes to rest at a position where X
L= X
C.