Voltmeters
A voltmeter is an electronic
instrument used to measure the volts of a circuit or various electric
drives used in electric circuits. These voltmeters are also
used for the measurement of electronic components. These instruments are
available in many ranges from milli volts to kilovolts. These
voltmeters are available for AC and DC.
Types of Voltmeters
These instruments are available in so many
types in market and in various ranges available for the measurement of voltage
of different circuits. These types of voltmeters are explained below:
- Moving Iron Type Voltmeters (AC and DC)
Attraction
Type
Repulsion
Type
- Moving Coil Type Voltmeters
Permanent
Magnet Type (DC only)
Electrodynamic
or Dynamometer Type (AC and DC)
- Hot wire Type Voltmeters (AC and DC)
- Induction Type Voltmeters (AC)
- Electrostatic type (AC and DC)
Moving Iron Type Voltmeters
Construction and Working Principle
Moving iron voltmeters
consists of a moving iron and a moving pointer when its coil generates flux,
a torque is produced in the instrument and the force is attracted to the moving
iron to move from its position according to the amount of the torque produced
in it. The pointer is connected to the moving iron and it moves and shows
deflection on its scale and a reading of voltmeter is observed. Moving Iron
type voltmeters are divided into its two types explained below:
Attraction Type Voltmeters
Working Principle
Attraction Type Voltmeters consists of the
same parts that a moving iron voltmeter consists but the working principle is
different. It depends on the
attraction of soft iron into a magnetic field. The working principle of these
instruments is according to this: “If a piece of a non-magnet soft iron is
brought near, either of the two ends of a current-carrying
coil, it would be attracted into the coil in the same way as it would be
attracted by the pole of a bar magnet.” Hence, if we attach a disc of soft iron
on a spindle the iron disc will rotate into the
coil and the electric current will pass through it and the pointer
will deflect.
Repulsion Type Voltmeters Working
Principle
The repulsion type instruments depend on the repulsion of the two
adjacent pieces of iron into a magnetic field. In these instruments, magnetic
field is produced by the current
carrying coil.
These instruments consist of a
fixed coil which is placed between the two soft iron rods and are parallel to
each other and one rod is fixed and the other is movable and it carries a
pointer. When the current is passed through the coil, it makes its own magnetic
field and magnetizes the two rods which are parallel to each other and these
two rods repel each other and as a result the pointer deflects opposing the controlling torque of a spring.
Moving Coil Voltmeters Working
Principle
Moving Coil Voltmeters consists of a moving
coil and a pointer is attached on it. When magnetic flux is produced in
a moving coil, it produces a force in it and starts moving according to the
amount of flux and a pointer also moves as it is attached to the moving coil
and shows its reading on a scale. Moving coil voltmeters are of two types explained
below:
Permanent Magnet Moving Coil
Voltmeters Working Principle
These instruments consist of a permanent
magnet and a rectangular coil of many turns wound on a light aluminum or
copper. A U-shapped magnet is fixed and a soft iron cylinder is fixed
between it. When current is passed through the coil, a force is produced
on both sides and a deflecting torque is produced and pointer starts moving on
a scale and shows its reading.
Electrodynamic Moving Coil
Voltmeters Working Principle
These instruments consist of a
fixed coil having two equal sections which are parallel to each other. These
two fixed coils are air-cored to avoid from hysteresis losses. When alternating current passes through the fixed coil, it
produces magnetic effect and moving
coil starts moving and a
pointer is attached to the moving coil with a spring
and it shows it reading on its scale.
Hot - wire Type Moving Coil Voltmeters
Working Principle
Hot - wire Instruments are based on heating
effect of current. It consists of platinum iridium wire which is
stretched between a fixed end and tension adjusting screw. When current is
passed through the stretched wire, it expands and a sag is
produced and slack in phosphor bronze wire which is attached on the middle of the
stretched wire. This slack is taken up by the silk fiber and
is passing around the pulley. The pulley is attached to the spring.
As the silk thread is pulled by spring, the pulley moves by deflecting
the pointer. The deflection of the pointer is proportional to the
points of the stretched wire which is itself proportional to I2.
Hence deflection is α I2. If spring control is used then Tc α
Θ and Θ α I2
These instruments work with rms value of current
their readings are independent from its form and frequency.
Induction Type Voltmeters Working
Principle
Induction type instruments are used for the
measurement of alternating current. These instruments depend upon the
production of torque due to the reaction between flux and eddy current induced
in a metal disc by another flux. The magnitude of the eddy current
depends on the flux and the instantaneous value of the torque is produced and
is proportional to the square of the voltage and a force acts on a pointer and
shows its reading.
Electrostatic Voltmeters Working
Principle
Electrostatic voltmeters are widely used as industrial
instruments. These instruments work on a force of attraction between
electric charges on in line plates. This force gives rise to a deflecting
torque. The potential difference is very large and are used for the measurement
of very high voltages. These instruments consist of its two types:
Quadrant Type Working Principle
These instruments are used to measure up to
20KV.These instruments consists of a light aluminum vane on a spindle and is
situated with a hollow metal quadrant. When the voltage is under measurement
the vane is attracted inwards into the quadrant and pointer starts rotating.
Attracted Disc Type Voltmeters
Working Principle
These instruments consist of the two metal
discs which are mounted parallel to each other. The first metal disc is fixed
and is connected to the earth connection and the other is attached to the
spring and is movable. The first plate is connected to the positive terminal of
the supply. When a potential difference is applied between the two discs then
the first disc is attracted towards the second disc. This movement of first
disc is balanced by a control device which actuates a pointer attached to it.
