PRACTICAL-3
AIM: - IDENTIFY AND
CONNECT VARIOUS ELECTRICAL MEASURING INSTRUMENTS AND MEASURE VARIOUS ELECTRICAL
PARAMETERS LIKE CURRENT, VOLTAGE, POWER.
Voltmeter, Ammeter, Watt meter and Energy meter are
four pillars of electrical energy and are most important devices used in
electrical engineering field. You can't imagine any application in electrical
engineering without voltage, current, energy and power measurement. Voltmeter,
Ammeter, Watt meter and Energy meter connections and basics are discussed here.
If you are an electrical engineer than you must know how to connect various electrical instruments in a circuit, specially measuring instruments. The most common instruments or measuring apparatus you must know about are Voltmeter, Ammeter, Energy meter and Watt meter.
If you are an electrical engineer than you must know how to connect various electrical instruments in a circuit, specially measuring instruments. The most common instruments or measuring apparatus you must know about are Voltmeter, Ammeter, Energy meter and Watt meter.
AMMETER:
-
An
ammeter is a measuring
instrument used to measure the electric current in a circuit. Electric currents
are measured in amperes (A), hence the name. Instruments used to measure
smaller currents, in the milli ampere or microampere range, are designated as millimeters
or micro ammeters. Early ammeters were laboratory instruments which
relied on the Earth's magnetic field for operation. By the late 19th century,
improved instruments were designed which could be mounted in any position and
allowed accurate measurements in electric power systems.
TYPES
OF AMMETER:-
(1)Moving-coil
ammeters
The
D'Arsonval galvanometer is a moving
coil ammeter. A moving coil meter indicates the average (mean) of a
varying current through it, which is zero for AC. For this reason moving-coil
meters are only usable directly for DC, not AC.
This
type of meter movement is extremely common for both ammeters and other meters
derived from them, such as voltmeters and ohmmeters. Although their use has
become less common in recent decades, this type of basic movement was once the
standard indicator mechanism for any analogue displays involving electrical
machinery.
(2)
Moving-iron ammeters
Moving iron
ammeters use a piece of iron which moves when acted upon by the electromagnetic
force of a fixed coil of wire. This type of meter responds to both direct and
alternating currents (as opposed to the moving-coil ammeter, which works on
direct current only). Consequently such meters would normally have a non linear
scale, but the iron parts are usually modified in shape to make the scale
fairly linear over most of its range. Moving iron instruments indicate the RMS
value of any AC waveform applied.
(3)
Digital ammeters
Digital
ammeter designs use a shunt resistor to produce a calibrated voltage
proportional to the current flowing. This voltage is then measured by a digital
voltmeter, through use of an analog to digital converter (ADC); the digital
display is calibrated to display the current through the shunt. Such
instruments are generally calibrated to indicate the RMS value for a sine
wave only but some designs will indicate true RMS (sometimes with
limitations as to wave shape).
An
ammeter must always be connected in series, i.e. the measured
current must pass through the meter.
current must pass through the meter.
VOLTMETER:
-
Voltmeter
is a device that is used to measure voltage or potential difference across two
given points. Essentially a voltmeter is nothing but a galvanometer with
infinite resistance connected in series. This makes the resistance of an ideal
voltmeter infinite. Voltmeter is connected in parallel,
A
voltmeter is always connected in parallel, i.e., across the resistor that
causes the voltage drop.
TYPES OF VOLTMETER:-
(1) Analog voltmeter
A moving coil galvanometer can be used as a
voltmeter by inserting a resistor in series with the instrument. The
sensitivity of such a meter can be expressed as "ohms per volt", the
number of ohms resistance in the meter circuit divided by the full scale measured
value.
Moving-coil instruments with a permanent-magnet
field respond only to direct current. Measurement of AC voltage requires a
rectifier in the circuit so that the coil deflects in only one direction.
Moving-coil instruments are also made with the zero position in the middle of
the scale instead of at one end; these are useful if the voltage reverses its
polarity.
(2)
Digital voltmeter
Digital
voltmeters (DVMs) are usually designed around a special type of
analog-to-digital converter called an integrating converter. Voltmeter accuracy
is affected by many factors, including temperature and supply voltage
variations. To ensure that a digital voltmeter's reading is within the
manufacturer's specified tolerances, they should be periodically calibrated
against a voltage standard such as the Weston cell.
WATT METER: -
A watt meter Measures power consumed by an apparatus in a given time. Normally watt meter gives reading in watt or kilowatt. A Wattmeter has two coils one is current coil and other is voltage coil. The power is product of Voltage across the load and current flowing through the load i.e.
P=V/*I
The
Current coil is connected in series as current remains same in series (concept
of Ammeter) and potential coil is connected in parallel as voltage remains same
in parallel (concept of Voltmeter)
TYPES OF WATTMETER:-
(1)Electrodynamics
The
traditional analog wattmeter is an electrodynamics instrument. The device
consists of a pair of fixed coils, known as current coils, and a movable
coil known as the potential coil.
The
current coils connected in series with the circuit, while the potential coil is
connected in parallel. The result of this arrangement is that on a dc circuit,
the deflection of the needle is proportional to both the current and
the voltage, thus conforming to the equation
W=VA
or P=VI...
(2) Electrodynamometer
An early current meter was the electrodynamometer.
Used in the early 20th century, the Siemens electrodynamometer, for example, is
a form of an electrodynamics ammeter that has a fixed coil which is surrounded
by another having its axis at right angles to that of the fixed coil.
(3)Electronic
wattmeter
Electronic watt meters are used for direct, small
power measurements or for power measurements at frequencies beyond the range of
electrodynamometer-type instruments.
(4)Digital
wattmeter
A modern digital electronic wattmeter/energy meter
samples the voltage and current thousands of times a second. For each sample,
the voltage is multiplied by the current at the same instant; the average over
at least one cycle is the real power. The real power divided by the apparent
volt-amperes (VA) is the power factor. A computer circuit uses the sampled
values to calculate RMS voltage, RMS current, VA, power (watts), power factor,
and kilowatt-hours. The readings may be displayed on the device, retained to
provide a log and calculate averages, or transmitted to other equipment for
further use. Wattmeter vary considerably in correctly calculating energy
consumption,
CONCLUSION:-
