frequency meter

1. Mechanical frequency meter

3. Reed:- A reed is a thin strip of material
which vibrates to produce a sound on
musical Instrument.
youtube.com/watch?v=4t9E-YFnOWU

5. • When the electromagnet is connected with
the supply, its magnetism varies with the
frequency of the supply all reeds experience
attractive force and start vibrating.
• Only particular reed, whose natural frequency
become equal to the supply frequency comes
in mechanical resonance and vibrates with
maximum amplitudes.
• The frequency meter is connected across the
supply, whose frequency is to be measured.

6. • The current flows through the coil of the
electromagnet and the reeds feel force of
attraction.
• The current flows through the coil of the
electromagnet and the reeds feel force of
attraction.
• The force of attraction is proportional to the
square of the current, therefore it varies at
twice the supply frequency and a force acts on
the reeds at every half cycle.

7. • The reeds being free at their top hence start
vibrating at their natural frequency as
mentioned above.
• But a particular reed vibrates rapidly as this
reed attains the resonance.
• The frequency opposite to this reed which is
vibrating with maximum amplitude is read on
the scale.

8. Electrical frequency meter

9. • The electrical resonance type frequency meter
is an indicating type instrument. As the name
suggests its action depends upon the electrical
resonance.
• It mainly consists of a fixed coil and a moving
coil. There is a laminated iron core of varying
cross-section.

11. • The electrical resonance type frequency meter
measures the frequency of these supply mains.
• Now there is a moving coil which is so pivoted at
its top end that it can move along the extended
core of the fixed coil like a pendulum.
• The pointer of the instrument is so attached at
the top end of the moving coil that its tip moves
along the semicircular dial.
• Now, we connect on a capacitor across the two
leads of the moving coil.

13. Working Principle of Electrical
Resonance Type Frequency Meter
• Due to the current in the moving coil, the moving
coil produces a flux in phase with the current.
• This flux flows along with the extended core of
the fixed coil.
• Therefore the flux links the moving coil. Hence,
the flux induces an emf across the moving coil.
Obviously, this induced emf lags the flux by 90°.
• Since it is a coil; the moving coil will have some
inductive reactance. Again, as it is connected
across a capacitor, it will have some capacitive
reactance also.

14. Torque Equation
• I1 is the supply current of the fixed coil
• I2 is the induced current of the moving coil.
• Now, we have already mentioned that the phase angle
between the supply current I1 (current in the fixed coil)
and the emf induced in the moving coil is 90°.
• Again there is a phase difference between the induced
emf and the induced current I2 (current in the moving
coil).
• Let us consider the angle of this phase difference is α.
So, the actual phase difference between I1 and I2 will
be (90°-α).

15. • Therefore, we can write the expression of the
torque (T) as

16. Resonance
• That can only be possible when inductive reactance of the moving
coil becomes equal to its capacitive reactance.
• Again, the inductive reactance (2πfL) depends upon the angular
position of the moving coil on the extended core of the fixed coil.
• So, when we just switch on the supply, the fixed coil starts
attracting the moving coil towards it. This attraction due to the
torque acting on the moving system. Therefore, the moving coil
starts rotating along with the pointer attached to it.
• As a result, the inductive reactance of the moving coils changes.
• Then after certain angular rotation of the moving coil the inductive
reactance of this coil exactly becomes equal to the capacitive
reactance of the coil.
• At that point of time, there will be no torque acting on the moving
system of the electrical resonance type frequency meter.

17. Resonance
• Therefore the pointer of the instrument becomes stationary at that
point.
• If somehow the supply frequency changes, the value of inductive
reactance of the instrument also changes. Therefore the resonance
of the circuit gets disturbed. Therefore again the deflecting torque
appears on the moving system and tries to rotate it further.
• Hence, again the inductive reactance of the moving coils changes.
• And after a certain rotation again resonance occurs.
• So, here again, the torque becomes zero. Therefore the pointer
rests on a new position.
• So, we have seen how the position of the pointer on the dial of
electrical resonance type frequency meter changes with changing
the supply frequency.