Frequency Meter - Construction - Working Principle - Types of Frequency Meter - Advantages of Frequency Meter - Disadvantages of Frequency Meters
Frequency Meter
Frequency meters are the
indicating instruments which measures the frequency of electrical energy. This electrical
energy may be AC or DC or in a form of various signals or waves produced by
various circuits. The working principle of the moving coil frequency meter is the
variations in an electric current drawn by inductive and non-inductive
circuits are connected in parallel. The current flows from these
circuits and the currents of both circuits changes their value then the frequency
is measured.
Types of Frequency Meters
Frequency meters are of
three types which are as follows:
- Moving Iron Frequency Meter
- Electrodynamic Frequency Meter
- Vibrating reed Frequency Meter
Moving Iron Frequency Meter
Moving iron frequency meters are the meters in which the two coils
are fixed and a moving iron is attached to the spindle. This moving iron
frequency meters depend on the following working principle.
Working Principle
This meter depends on the variations in an electric
current drawn by inductive and non-inductive
circuits connected in parallel. The current flows from these circuits
when the frequency changes its value.
Construction
This meter consists of two fixed coils A and B that their magnetic
axes are perpendicular to each other. A long and soft iron needle in pivoted at
their centers. This circuit remains balanced at the supply frequency to
be measured. Coil A consists of a series resistance Ra and
a reactance La in
parallel and the coil B consists of a series reisitance Rb and
a reactance Lb in parallel. The series inductance helps to suppress higher
harmonics in the current waveform which helps to minimize the waveform errors
in the indication of the instruments.
Working of Moving Iron Frequency Meter
When the supply is connected to the meter, the current pass
through the coils A and B and these two coils produce opposing torques.
When the supply frequency increases then the current of the coil A increases
and decreases in the coil B. The iron needle lies more nearly to the magnetic
axis of the coil A. For low frequencies, the current of coil B increases and
the current of the coil A decreases.
Range
These instruments are designed to measure large amount of
frequency and also for measuring very small amounts.
Electrodynamic Frequency Meter
Electrodynamic frequency meter/moving coil frequency meters are
ratiometer type of instruments. These meters are used to measure the frequency
of high voltage ranges and for too low voltage ranges. These frequency
meters provide accurate frequency readings. These frequency meters
consist of two moving coils and a rectifier circuit.
Construction
This frequency meter consists of the two moving coils connected to
each other at right angles on a shaft and a pointer is
also connected with this shaft. These two moving coils are connected with
their bridge rectifiers. This circuit consists of a capacitor which
is connected in series with the bridge rectifier of the first moving coil C1
connected to DC Supply. The direct current (rectified current)
flows through a series resistance to the bridge rectifier and to the second
coil C2.
Range
These meters depend on the value of resistance and a capacitor.
These frequency meters have ranges of 40 to 60 Hz, 1200 to 2000Hz and 8000 to
12000Hz.
Working of Electrodynamic Frequency Meter
When the frequency is
connected to the supply, the rectified currents I1 and I2 pass through the
moving coil C1 and C2. When these coils come to rest position, their torques
becomes equal but in opposite direction.
Advantages
These frequency meters have linear frequency scales.
The readings are independent from voltages.
These frequency meters can be used over a wide range of voltage
and at too low voltage range.
Disadvantage
The rectifiers produce distortions which results as an inaccurate
reading of frequency.
Vibrating-reed Frequency Meter
A vibrating-reed frequency meter is measuring
instrument which is used to measure the frequency of
various electric circuits. It consists of 7 vibrating reeds and
each vibrating reed has a specific value. These reeds vibrate when this
frequency meter is connected to supply for the measurement of frequency. A reed
which vibrates more as compared to the other reeds, the more vibrating reed is
considered as frequency reading of a supply or electric circuit.
Construction
This frequency meter consists of thin flat steel reeds which are
arranged alongside and these reeds are near to the electromagnet.
The electromagnet consists of a laminated armature and a winding connected
with a resistance in series on it. This winding is connected
to the AC supply whose frequency is to be measured. The metallic reeds are 4mm
wide and 0.5 mm thick. These metallic reeds are arranged in a row and consists
of flexible base and these bases carries armature of the electromagnet. These
reeds are colored with white color. These reeds are arranged in ascending order
of frequency on a scale.
Range
These
vibrating reed frequency meters consists of small readings from 47 Hz to 53 Hz
and 57 Hz to 63 Hz.
Working of Vibrating-reed Frequency Meter
To measure the frequency of the circuit, it is compulsory to
connect the frequency meter to a supply. The electromagnet is connected to the
supply for which frequency is to be measured. The magnetism of the
electromagnet alternates with the same frequency and the electro-magnet exerts
the attracting force on each reed once every half cycle. All
reeds start vibrating but the reed whose frequency is double, vibrates with
maximum amplitude due to mechanical resonance. The vibration of the other reeds
is so small and these are unobservant.
Advantages
Its indications are independent.
Its readings are reliable.
These are cheap.
Disadvantages
The reading is not be observed properly if the frequency
difference is half cycle between the adjacent reeds.
These instruments are not accurate to provide the exact reading on
its scale.