Energy saving and energy efficiency of industry cannot be imagined without electrical measurements, since it is impossible to save what you do not know the account for.
Electrical measurements are performed in one of the following types: direct, indirect, cumulative and joint. Name direct view speaks for itself, the value of the desired value is determined directly by the device. An example of such measurements is the determination of power with a wattmeter, current with an ammeter, etc.
indirect view is to find the value on the basis of the known dependence of this value and the value found by the direct method. An example is the determination of power without a wattmeter. By the direct method, I, U, phase are found and the power is calculated by the formula.
Cumulative and Joint Views measurements consist in the simultaneous measurement of several similar (cumulative) or non-similar (joint) quantities. Finding the desired values is carried out by solving systems of equations with coefficients obtained as a result of direct measurements. The number of equations in such a system must be equal to the number of sought quantities.
Direct measurements as the most common type of measurement can be made by two main methods:
- direct evaluation method
- measure comparison method.
The first method is the simplest, since the value of the desired value is determined on the scale of the instrument.
This method determines the current strength with an ammeter, the voltage of voltmeters, etc. Dignity this method can be called simplicity, and the lack of low accuracy.
Measurements by comparison with a measure are performed according to one of the following methods: substitution, opposition, coincidence, differential and zero. A measure is a kind of reference value of a certain quantity.
Differential and null methods– are the basis for the operation of measuring bridges. With the differential method, unbalanced-indicating bridges are made, and with the zero method, balanced or zero ones.
In balanced bridges, the comparison takes place with the help of two or more auxiliary resistances, selected in such a way that they form a closed circuit (four-terminal network) with the compared resistances, fed from one source and having equipotential points detected by the equilibrium indicator.
The ratio between the auxiliary resistances is a measure of the relationship between the compared values. An indicator of balance in chains direct current a galvanometer acts, and in alternating current circuits a millivoltmeter.
The differential method is otherwise called the difference method, since it is the difference between the known and the desired current that affects the measuring instrument. The null method is a limiting case differential method. For example, in the indicated bridge circuit, the galvanometer shows zero if the equality is observed:
R1*R3 = R2*R4;
From this expression follows:
Rx=R1=R2*R4/R3.
Thus, it is possible to calculate the resistance of any unknown element, provided that the other 3 are exemplary. A constant current source should also be exemplary.
Contrasting method- otherwise, this method is called compensation and is used to directly compare voltage or EMF, current, and indirectly to measure other quantities converted into electrical ones.
Two oppositely directed EMFs that are not interconnected are switched on to the device, along which the branches of the circuit are balanced. In the figure: it is required to find Ux. With the help of an exemplary adjustable resistance Rk, such a voltage drop Uk is achieved so that it is numerically equal to Ux.
Their equality can be judged by the readings of the galvanometer. If Uki Ux is equal, the current in the galvanometer circuit will not flow, since they are oppositely directed. Knowing the resistance and the magnitude of the current, we determine Uх by the formula.
substitution method- a method in which the desired value is replaced or combined with a known exemplary value, equal in value to the substituted one. This method is used to determine the inductance or capacitance not known values s. An expression that determines the dependence of the frequency on the circuit parameters:
fo=1/(√LC)
On the left, the frequency f0 set by the RF generator, on the right side, the values of the inductance and capacitance of the measured circuit. By selecting the frequency resonance, one can determine the unknown values on the right side of the expression.
The resonance indicator is an electronic voltmeter with a large input resistance, the readings of which at the moment of resonance will be the largest. If the measured inductor is connected in parallel with the reference capacitor and the resonant frequency is measured, then the value of Lx can be found from the above expression. Similarly, the unknown capacity is found.
First, the resonant circuit, consisting of an inductance L and an exemplary capacitance Co, is tuned to resonance at a frequency fo; at the same time, the values of fo and the capacitance of the capacitor Co1 are fixed.
Then, parallel to the exemplary capacitor Co, the capacitor Cxi is connected by changing the capacitance of the exemplary capacitor to achieve resonance at the same frequency fo; accordingly, the desired value is equal to Co2.
Match method- a method in which the difference between the desired and the known value is determined by the coincidence of scale marks or periodic signals. A prime example application of this method in life is the measurement angular velocity rotation of various parts.
To do this, a mark is applied to the measured object, for example, with chalk. When the part with the mark rotates, a stroboscope is directed to it, the blinking frequency of which is known initially. By adjusting the frequency of the stroboscope, the mark is kept in place. In this case, the rotational speed of the part is taken equal to the flashing frequency of the stroboscope.
ELECTRICMEASUREMENTS IN
SYSTEMS
POWER SUPPLY
Lecturer: Ph.D., Associate Professor of the Department of EPP
Buyakova Natalya Vasilievna Electrical measurements are
a set of electrical and electronic measurements,
which can be considered as one of the sections
metrology. The name "metrology" is derived from two
Greek words: metron - measure and logos - word, doctrine;
literally: the doctrine of measure.
AT modern understanding science is called metrology
about measurements, methods and means of ensuring their
unity and ways to achieve the required accuracy.
AT real life metrology is not only a science, but also
region practical activities associated with
the study of physical quantities.
Subject
metrology
is
receiving
quantitative information about the properties of objects and
processes, i.e. measurement of properties of objects and processes with
required accuracy and reliability. Measurements are one of the most important ways knowledge
nature by man.
They give quantitative characteristic surrounding
of the world, revealing to man the acting in nature
patterns.
Measurement is understood as a set of operations,
carried out with the help of special technical
means that stores the unit of the measured value,
allowing to compare the measured value with its
unit and get the value of this quantity.
The measurement result of X is written as
X=A[X],
where A is a dimensionless number, called a numerical
the value of a physical quantity; [X] - unit
physical quantity.
ELECTRICAL MEASUREMENTS
Measurement electrical quantities such as voltage,resistance, current, power are produced with
help various means - measuring instruments,
circuits and special devices.
The type of measuring device depends on the type and size
(value range) of the measured value, as well as from
required measurement accuracy.
Electrical measurements use the basic
SI units: volt (V), ohm (Ohm), farad (F),
henry (G), ampere (A) and second (s).
STANDARDS OF UNITS OF ELECTRIC VALUES
Electricalmeasurement
this is
finding
(by experimental methods) the values of the physical
quantity expressed in appropriate units
(for example, 3 A, 4 B).
The values of the units of electrical quantities are determined
international agreement in accordance with the laws
physics and units of mechanical quantities.
Since the "maintenance" of units of electrical quantities,
defined
international
agreements
associated
With
difficulties
them
present
"practical"
standards
units
electrical
quantities.
Such
standards
supported
state
metrological laboratories of different countries. All common electrical and magnetic units
measurements are based on the metric system.
AT
consent
With
modern
definitions
electrical and magnetic units they are all
derived units derived from certain
physical formulas from metric units length,
mass and time.
Since most electrical and magnetic
quantities
not
so that
simply
to measure,
using
mentioned standards, it was considered that it is more convenient
install
through
relevant
experiments
derived standards for some of the specified
quantities, while others are measured using such standards.
SI units
Ampere, unit of force electric current, - one ofsix basic units SI systems.
Ampere (A) - the strength of a constant current, which, when
passing along two parallel straight lines
conductors of infinite length with negligible
circular cross-sectional area,
located in a vacuum at a distance of 1 m one from
another, would call on each section of the conductor
1 m long, an interaction force equal to 2 ∗ 10−7 N.
Volt, unit of potential difference and electromotive
strength.
Volt (V) - electrical voltage Location on
electrical circuit with a direct current of 1 A at
power consumption 1 W. Coulomb, unit of quantity of electricity
(electric charge).
Coulomb (C) - the amount of electricity passing
through transverse section conductor at
direct current with a power of 1 A for a time of 1 s.
Farad, unit of electrical capacitance.
Farad (F) - capacitor capacitance, on the plates
which, with a charge of 1 C, an electric
voltage 1 V.
Henry, unit of inductance.
Henry is equal to the inductance of the circuit in which
arises EMF self-induction at 1 V with uniform
change in the current strength in this circuit by 1 A in 1 s. Weber, unit of magnetic flux.
Weber (WB) - magnetic flux, while decreasing
which to zero in the circuit coupled to it,
having a resistance of 1 ohm, flows
electric charge equal to 1 C.
Tesla, unit of magnetic induction.
Tesla (Tl) - magnetic induction of a homogeneous
magnetic field in which the magnetic flux
through a flat area of 1 m2,
perpendicular to the lines of induction is equal to 1 Wb.
10. MEASURING INSTRUMENTS
Electrical measuring instruments are most often used to measureinstantaneous values of either electrical quantities, or
non-electric, converted to electrical.
All devices are divided into analog and digital.
The former usually show the value of the measured
values by means of an arrow moving along
graduation scale.
The latter are equipped with a digital display, which
shows the measured value as a number.
Digital instruments in most measurements are more
preferred as they are more accurate, more convenient
when taking readings and, in general, are more versatile.
11.
Digital multimeters("multimeters") and digital voltmeters are used
for medium to high precision measurements
DC resistance, as well as voltage and
AC power.
Analog
appliances
gradually
are forced out
digital, although they still find application where
low cost is important and high accuracy is not needed.
For the most accurate resistance and impedance measurements
resistance (impedance) there are measuring
bridges and other specialized meters.
To register the course of change of the measured value
in time, recording devices are used - strip chart recorders and electronic oscilloscopes,
analog and digital.
12. DIGITAL INSTRUMENTS
All digital measuring instruments (exceptprotozoa) amplifiers and other electronic
blocks for converting the input signal into a signal
voltage, which is then digitized
analog-to-digital converter (ADC).
A number expressing the measured value is displayed on
light-emitting diode (LED), vacuum fluorescent or
liquid crystal (LCD) indicator (display).
The instrument is usually operated by a built-in
microprocessor, and simple appliances microprocessor
combined with an ADC on a single integrated circuit.
Digital instruments are well suited to work with
connection to an external computer. In some types
measurements such a computer switches measuring
device functions and gives data transfer commands for their
processing.
13. Analog-to-digital converters (ADC)
There are three main types of ADCs: integrating,successive approximation and parallel.
The integrating ADC averages the input signal over
time. Of the three listed types, this is the most accurate,
albeit the slowest. Conversion time
integrating ADC is in the range from 0.001 to 50 s and
more, the error is 0.1-0.0003%.
SAR ADC error
somewhat more (0.4-0.002%), but the time
conversion - from 10 ms to 1 ms.
Parallel ADCs are the fastest, but also
the least accurate: their conversion time is on the order of 0.25
ns, error - from 0.4 to 2%.
14.
15. Discretization methods
The signal is sampled in time by fastmeasuring it at individual points in time and
holding (storing) the measured values for a while
converting them to digital form.
The sequence of obtained discrete values
can be displayed in the form of a curve having
waveform; squaring these values and
summing up, we can calculate the root mean square
signal value; they can also be used for
calculations
time
rise,
maximum
value, time average, frequency spectrum, etc.
Time discretization can be done either for
one signal period ("real time"), either (with
sequential or random sampling) per row
recurring periods.
16. Digital voltmeters and multimeters
Digitalvoltmeters
and
multimeters
measure
quasi-static value of the quantity and indicate it in
digital form.
Voltmeters directly measure voltage,
usually DC, while multimeters can measure
AC and DC voltage, current strength,
DC resistance and sometimes temperature.
These most common test and measurement
appliances general purpose with measurement error from 0.2
up to 0.001% can have a 3.5 or 4.5 digit digital display.
The "half-integer" sign (digit) is a conditional indication that
the display may show numbers that are out of range
nominal number of characters. For example, a 3.5 digit (3.5 digit) display in the 1-2V range might show
voltage up to 1.999 V.
17.
18. Impedance meters
These are specialized instruments that measure and displaycapacitor capacitance, resistor resistance, inductance
inductors or total resistance (impedance)
connecting a capacitor or inductor to a resistor.
There are devices of this type for measuring capacitance from 0.00001 pF
up to 99.999 uF, resistances from 0.00001 ohm to 99.999 k ohm and
inductance from 0.0001mH to 99.999G.
Measurements can be made at frequencies from 5 Hz to 100 MHz, although neither
one device does not cover the entire frequency range. At the frequencies
close to 1 kHz, the error can be only 0.02%, but
accuracy decreases near the boundaries of the frequency ranges and measured
values.
Most instruments can also show derivatives
quantities such as the quality factor of a coil or the loss factor
capacitor, calculated from the main measured values.
19.
20. ANALOGUE INSTRUMENTS
For measuring voltage, current and resistance onpermanent
current
apply
analog
magnetoelectric devices with permanent magnet and
multi-turn moving part.
Such pointer-type devices are characterized
error from 0.5 to 5%.
They are simple and inexpensive (for example, automobile
instruments showing current and temperature), but not
used where there is a need for
significant accuracy.
21. Magnetoelectric devices
In such devices, the interaction force is usedmagnetic field with current in the turns of the winding movable
part, tending to turn the latter.
The moment of this force is balanced by the moment
generated by the counter spring, so that
each current value corresponds to a certain
pointer position on the scale. The moving part has
the shape of a multi-turn wire frame with dimensions from
3-5 to 25-35 mm and made as light as possible.
Movable
part,
established
on the
stone
bearings or suspended on a metal
ribbon, placed between the poles of a strong
permanent magnet.
22.
Two coil springs that balance the torquemoment, also serve as conductors of the winding of the movable
parts.
Magnetoelectric
device
reacts
on the
current,
passing through the winding of its moving part, and therefore
represents
yourself
ammeter
or,
more precisely,
milliammeter (because the upper limit of the range
measurement does not exceed approximately 50 mA).
It can be adapted to measure currents of greater
force by connecting parallel to the winding of the moving part
shunt resistor with low resistance to
the winding of the moving part branched off only a small fraction
total measured current.
Such a device is suitable for currents measured
many thousands of amperes. If in series with
connect an additional resistor with a winding, then the device
turn into a voltmeter.
23.
The voltage drop across such a seriesconnection
equals
work
resistance
resistor to the current shown by the device, so that it
the scale can be graduated in volts.
To
do
from
magnetoelectric
milliammeter ohmmeter, you need to attach to it
series measured resistors and apply to
this is
consistent
compound
permanent
voltage, such as from a battery.
The current in such a circuit will not be proportional
resistance, and therefore a special scale is needed,
corrective non-linearity. Then it will be possible
make a direct reading of resistance on a scale, although
and with not very high accuracy.
24. Galvanometers
Tomagnetoelectric
appliances
relate
and
galvanometers are highly sensitive instruments for
measurements of extremely low currents.
There are no bearings in galvanometers, their moving part
hung on a thin ribbon or thread, used
stronger magnetic field, and the arrow is replaced
a mirror glued to the suspension thread (Fig. 1).
The mirror rotates along with the moving part, and
corner
his
turn
evaluated
on
displacement
the light spot he throws off on the scale,
installed at a distance of about 1 m.
The most sensitive galvanometers are capable of giving
deviation on the scale, equal to 1 mm, with a change in current
only 0.00001 uA.
25.
Figure 1. A MIRROR GALVANOMETER measures the currentpassing through the winding of its moving part, placed in
magnetic field, according to the deviation of the light spot.
1 - suspension;
2 - mirror;
3 - gap;
4 - permanent
magnet;
5 - winding
moving part;
6 - spring
suspension.
26. RECORDING DEVICES
Recording devices record the "history" of changemeasured value.
The most common types of these devices are
strip chart recorders that record the change curve with a pen
values on chart paper tape, analog
electronic oscilloscopes sweeping the process curve
on the
screen
electron beam
pipes,
and
digital
oscilloscopes that store once or rarely
repetitive signals.
The main difference between these devices is speed.
records.
Tape
recorders
With
them
moving
mechanical parts are most suitable for registration
signals that change in seconds, minutes and even slower.
Electronic oscilloscopes are capable of recording
signals that change over time from parts per million
seconds to several seconds.
27. MEASURING BRIDGES
Measuringbridge
this is
usually
four-shouldered
electric
chain,
drawn up
from
resistors,
capacitors and inductors, designed for
determining the ratio of the parameters of these components.
To one pair of opposite poles of the circuit is connected
power supply, and to the other - a null detector.
Measuring bridges are used only in cases where
the highest measurement accuracy is required. (For measurements with
middle
accuracy
better
enjoy
digital
appliances as they are easier to handle.)
Best
transformer
measuring
bridges
alternating current are characterized by an error (measurements
ratio) of the order of 0.0000001%.
The simplest bridge for measuring resistance is named after
its inventor C. Wheatstone
28. Double DC measuring bridge
Figure 2. DOUBLE MEASURING BRIDGE (Thomson bridge) more exact variant Wheatstone bridge suitable for measurementresistance of four-pole reference resistors in the region
microohm.
29.
It is difficult to connect copper wires to a resistor without introducingwhile the resistance of the contacts is of the order of 0.0001 Ohm or more.
In the case of a resistance of 1 ohm, such a current lead introduces an error
of the order of only 0.01%, but for a resistance of 0.001 ohm
the error will be 10%.
Double measuring bridge (Thomson bridge), the scheme of which
shown in fig. 2, designed to measure
resistance of reference resistors of small denomination.
The resistance of such four-pole reference resistors
defined as the ratio of voltage to their potential
terminals (p1, p2 of the Rs resistor and p3, p4 of the Rx resistor in Fig. 2) to
current through their current terminals (c1, c2 and c3, c4).
With this technique, the resistance of the connecting
wires does not introduce errors into the measurement result of the desired
resistance.
Two additional arms m and n eliminate the influence
connecting wire 1 between terminals c2 and c3.
The resistances m and n of these arms are selected so that
the equality M/m = N/n was satisfied. Then, changing
resistance Rs, reduce the imbalance to zero and find Rx =
Rs(N/M).
30. Measuring AC bridges
The most common measuring bridgesalternating current are designed for measurements either on
mains frequency 50-60 Hz, or at audio frequencies
(usually around 1000 Hz); specialized
measuring bridges operate at frequencies up to 100 MHz.
As a rule, in measuring bridges of alternating current
instead of two shoulders that precisely define the ratio
voltage, a transformer is used. To exceptions
this rule includes measuring bridge
Maxwell - Wine.
31. Maxwell Measuring Bridge - Veena
Figure 3. MAXWELL MEASURING BRIDGE - VINA forcomparing the parameters of reference inductors (L) and
capacitors (C).
32.
Such a measuring bridge allows you to compare standardsinductance (L) with capacitance standards on the unknown
exactly operating frequency.
Capacitance standards are used in measurements of high
precision,
because the
they
constructively
easier
precision standards of inductance, more compact,
they are easier to shield, and they practically do not create
external electromagnetic fields.
The equilibrium conditions for this measuring bridge are:
Lx = R2*R3*C1 and Rx = (R2*R3) /R1 (Fig. 3).
The bridge is balanced even in the case of "impure"
power supply (i.e. a signal source containing
harmonics of the fundamental frequency), if the value of Lx is not
frequency dependent.
33. Transformer measuring bridge
Figure 4. TRANSFORMER MEASUREMENT BRIDGEalternating current for comparison of the same type of complete
resistance
34.
One of the advantages of AC measuring bridges- ease of setting the exact ratio of stresses by means of
transformer.
Unlike voltage dividers built from
resistors, capacitors or inductors,
transformers for a long time retain
constant set voltage ratio and rarely
require recalibration.
On the
rice.
4
presented
scheme
transformer
measuring bridge to compare two similar complete
resistance.
To the disadvantages of the transformer measuring bridge
can
attributed
then,
what
attitude,
given
transformer, to some extent depends on the frequency
signal.
it
leads
to
need
design
transformer
measuring
bridges
only
for
limited frequency ranges in which guaranteed
passport accuracy.
35. AC SIGNAL MEASUREMENT
In the case of time-varying AC signalsusually it is required to measure some of their characteristics,
related to the instantaneous values of the signal.
More often
Total
desirable
know
rms
(effective) values of the electrical quantities of the variable
current, since the heating power at a voltage of 1V
direct current corresponds to the heating power at
voltage 1 V AC.
In addition, other quantities may be of interest,
e.g. maximum or average absolute value.
RMS (effective) voltage value
(or AC strength) is defined as the root
square of the time-averaged squared voltage
(or current strength):
36.
where T is the period of the signal Y(t).The maximum value Ymax is the highest instantaneous value
signal, and the average absolute value of YAA is the absolute value,
time averaged.
With a sinusoidal form of oscillation Yeff = 0.707Ymax and
YAA = 0.637Ymax
37. AC voltage and current measurement
Almost all voltage and force measuring instrumentsalternating current show the value that
it is proposed to consider as an effective value
input signal.
However, in cheap devices often in fact
the mean absolute or maximum is measured
signal value, and the scale is graduated so that
indication
corresponded
equivalent
effective value under the assumption that the input
the signal is sinusoidal.
It should not be overlooked that the accuracy of such instruments
extremely low if the signal is not sinusoidal.
38.
Instruments capable of measuring true effectivevalue of AC signals, can be
based on one of three principles: electronic
multiplication, signal sampling or thermal
transformations.
Devices based on the first two principles, as
usually respond to voltage, and thermal
electrical measuring instruments - for current.
When using additional and shunt resistors
all devices can measure both current and
voltage.
39. Thermal electrical measuring instruments
The highest measurement accuracy effective valuesvoltage
and
current
provide
thermal
electrical measuring instruments. They use
thermal current converter in the form of a small
evacuated glass cartridge with heating
wire (0.5-1 cm long), to the middle part of which
a tiny bead attached to the hot junction of the thermocouple.
The bead provides thermal contact and at the same time
electrical insulation.
With an increase in temperature, directly related to
effective
meaning
current
in
heating
wire, at the output of the thermocouple there is a thermo-EMF
(DC voltage).
Such transducers are suitable for measuring force
alternating current with a frequency of 20 Hz to 10 MHz.
40.
On fig. 5 shows a schematic diagram of a thermalelectrical measuring instrument with two matched
according to the parameters of thermal current converters.
When an AC voltage is applied to the input circuit
Vac at the output of the thermocouple of the converter TC1 occurs
DC voltage, amplifier A creates
constant
current
in
heating
procrastination
converter TC2, in which the thermocouple of the last
gives the same DC voltage as the conventional
A DC instrument measures the output current.
41.
Figure 5. THERMAL ELECTRIC METER formeasurement of effective values of voltage and AC power
current.
With the help of an additional resistor, the described current meter can be
turn it into a voltmeter. Since thermal electrical meters
devices directly measure currents only from 2 to 500 mA, for
current measurements greater strength resistor shunts are required.
42. Measurement of AC power and energy
Power consumed by the load in the AC circuitcurrent, is equal to the time-average product
instantaneous values of voltage and load current.
If voltage and current vary sinusoidally (as
this usually happens), then the power P can be represented in
P = EI cosj, where E and I are the effective values
voltage and current, and j is the phase angle (shift angle)
sinusoids of voltage and current.
If voltage is expressed in volts and current in amps,
the power will be expressed in watts.
The factor cosj, called the power factor,
characterizes
degree
synchrony
hesitation
voltage and current.
43.
FROMeconomic
points
vision,
the most
important
electrical quantity - energy.
The energy W is determined by the product of the power and
time of consumption. AT mathematical form this is
is written like this:
If time (t1 - t2) is measured in seconds, voltage e is in volts, and current i is in amperes, then the energy W will be
expressed in watt-seconds, i.e. joules (1 J = 1 W*s).
If time is measured in hours, then energy is measured in watt hours. In practice, it is more convenient to express electricity in terms of
kilowatt-hours (1 kWh = 1000 Wh).
44. Induction electricity meters
The induction meter is nothing butas a low power AC motor with
two windings - current and voltage winding.
A conductive disk placed between the windings
revolves
under
action
torque
moment,
proportional to power consumption.
This moment is balanced by the currents induced in
disk with a permanent magnet, so that the rotational speed
drive is proportional to the power consumption.
45.
The number of revolutions of the disk in a given timein proportion to the total electricity received for
it's time by the consumer.
The number of disc revolutions is counted by a mechanical counter,
which shows electricity in kilowatt-hours.
Devices of this type are widely used as
household electricity meters.
Their error, as a rule, is 0.5%; they
have a long service life under any
allowable current levels.
ON THE TOPIC:
"ELECTRICAL MEASUREMENTS"
Introduction
The development of science and technology has always been closely linked with progress in the field of measurements. Great importance measurements for science were emphasized by some scientists.
G. Galileo: "Measure everything available to measurement and make accessible everything that is inaccessible to it."
DI. Mendeleev: "Science begins as soon as they begin to measure, exact science unthinkable without measure.
Kelvin: "Every thing is known only to the extent that it can be measured."
Measurements are one of the main ways of understanding nature, its phenomena and laws. Each new discovery in the field of natural and technical sciences preceded big number various measurements. (G. Ohm - Ohm's law; P. Lebedev - light pressure).
An important role is played by measurements in the creation of new machines, structures, and the improvement of product quality. For example, during testing of the world's largest bench turbine generator 1200 MW, created at the Leningrad Association "Elektrosila", measurements were made at 1500 of its various points.
especially important role play electrical measurements of both electrical and non-electric quantities.
The world's first electrical measuring instrument electrical force"was created in 1745 by Academician G.V. Rokhman, associate of M.V. Lomonosov.
It was an electrometer - a device for measuring the potential difference. However, only from the second half of XIX century in connection with the creation of generators electrical energy the question of the development of various electrical measuring instruments arose sharply.
The second half of the 19th century, the beginning of the 20th century, - Russian electrical engineer M.O. Dolivo-Dobrovolsky developed an ammeter and a voltmeter, an electromagnetic system; induction measuring mechanism; fundamentals of ferrodynamic devices.
At the same time, the Russian physicist A.G. Stoletov - the law of change in magnetic permeability, its measurement.
At the same time, Academician B.S. Jacobi - devices for measuring the resistance of an electrical circuit.
Then - D.I. Mendeleev - the exact theory of weights, an introduction to Russia metric system measures, organization of a department for checking electrical measuring instruments.
1927 - Leningrad built the first domestic instrument-making plant "Elektropribor" (now - Vibrator - production of counters).
30 years - instrument-making plants were built in Kharkov, Leningrad, Moscow, Kyiv and other cities.
From 1948 to 1967, the volume of instrument-making output increased 200 times.
In the subsequent five-year plans, the development of instrument-making proceeds at an invariably outstripping pace.
Main achievements:
– Analog devices for direct evaluation of improved properties;
– Narrow profile analog signaling control devices;
– Precision semi-automatic capacitors, bridges, voltage dividers, other installations;
– Digital measuring instruments;
– Application of microprocessors;
– Measuring computer.
Modern production is unthinkable without modern means measurements. Electrical measuring equipment is constantly being improved.
In instrumentation, the achievements of radio electronics are widely used, computer science, and other achievements of science and technology. Increasingly, microprocessors and microcomputers are being used.
The study of the course "Electrical measurements" aims to:
– Study of the device and the principle of operation of electrical measuring instruments;
– Classification of measuring instruments, familiarity with symbols on instrument scales;
– Basic measurement techniques, selection of certain measuring instruments depending on the measured value and measurement requirements;
– Acquaintance with the main directions of modern instrumentation.
1 . Basic concepts, measurement methods and errors
by measurement is called finding the values of a physical quantity empirically with the help of special technical means.
Measurements must be carried out in general accepted units.
Means of electrical measurements called technical means used in electrical measurements.
Distinguish the following types electrical measuring instruments:
– Electrical measuring instruments;
– Measuring transducers;
– Electrical measuring installations;
– Measuring Information Systems.
measure called a measuring instrument designed to reproduce a physical quantity of a given size.
electrical measuring instrument is a means of electrical measurements, designed to generate signals of measuring information in the form of an accessible direct perception observer.
measuring transducer called a means of electrical measurements, designed to generate signals of measuring information in a form convenient for transmission, further transformation, storage, but not amenable to direct perception.
Electrical measuring installation consists of a number of measuring instruments and auxiliary devices. It can be used to produce more accurate and complex measurements, verification and calibration of instruments, etc.
Measuring information systems are a set of measuring instruments and auxiliary devices. Designed to automatically receive measurement information from a number of its sources, for its transmission and processing.
Measurement classification :
a). Depending on the method of obtaining the result, direct and indirect :
Direct called measurements, the result of which is obtained directly from experimental data (measurement of current with an ammeter).
Indirect are called measurements in which the desired value is not directly measured, but is found as a result of calculation by known formulas. For example: P=U·I, where U and I are measured by instruments.
b). Depending on the totality of methods of using the principles and means of measurement all methods are divided into methods direct evaluation and comparison methods .
Direct evaluation method– the measured value is determined directly from the reading device of the measuring device direct action(measurement of current with an ammeter). This method is simple, but has low accuracy.
Comparison Method- the measured value is compared with the known one (for example: measuring resistance by comparing it with a measure of resistance - an exemplary resistance coil). The comparison method is divided into zero, differential and substitution .
Null- the measured and known value simultaneously act on the comparison device, bringing its readings to zero (for example: measurement electrical resistance balanced bridge).
Differential- a comparator measures the difference between the measured and the known value.
substitution method– the measured value is replaced in the measuring setup by a known value.
This method is the most accurate.
Measurement errors
The results of measuring a physical quantity give only its approximate value due to a number of reasons. The deviation of the measurement result from the true value of the measured quantity is called the measurement error.
Distinguish absolute and relative error.
Absolute error measurement is equal to the difference between the measurement result Au and the true value of the measured quantity A:
Correction: yes=A-Ai
Thus, the true value of the quantity is: A=Au+dA.
You can find out about the error by comparing the readings of the device with the readings of the exemplary device.
Relative error measurement g A is the ratio absolute error measurements to true value measured value, expressed in %:
%Example: The device shows U=9.7 V. The actual value of U=10 V determines DU and g U:
ДU=9.7–10=–0.3 V g U =
%=3%.Measurement errors have systematic and random components. First remain constant during repeated measurements, they are determined, and its influence on the measurement result is eliminated by introducing a correction . Second change randomly and they cannot be identified or eliminated .
In the practice of electrical measurements, the concept is most often used reduced error r p:
This is the ratio of the absolute error to the nominal value of the measured quantity or to last digit according to the instrument scale:
%
Example: DU = 0.3 V. The voltmeter is designed for 100 V. g p \u003d?
g p \u003d 0.3 / 100 100% \u003d 0.3%
Measurement errors may be due to :
a). Incorrect installation of the device (horizontal, instead of vertical);
b). Incorrect accounting of the environment (external humidity, tє).
in). Influence of external electromagnetic fields.
G). Inaccurate readings, etc.
In the manufacture of electrical measuring instruments, certain technical means are used that provide one or another level of accuracy.
The error due to the quality of the manufacture of the device is called - basic error .
In accordance with the quality of manufacture, all devices are divided into accuracy classes : 0,05; 0,1; 0,2; 0,5; 1,0; 1,5; 2,5; 4,0.
The accuracy class is indicated on the scales of measuring instruments. It denotes the Basic maximum allowable reduced error of the instrument:
%.Based on the accuracy class when checking the device, it is determined whether it is suitable for further operation, i.e. whether it corresponds to its accuracy class.
Electrical measuring instruments are designed to measure the parameters that characterize: 1) processes in electrical systems: currents, voltages, powers, electrical energy, frequencies, phase shifts. For this, ammeters, voltmeters, wattmeters, frequency meters, phase meters are used; electric meters...()
(GENERAL ELECTRICAL ENGINEERING)
Basic information about electrical measuring instruments and electrical measuring instruments
The means of electrical measurements include: measures, electrical measuring instruments, measuring transducers, electrical measuring installations and measuring information systems. Measures called measuring instruments designed to reproduce a physical quantity of a given size ....(AUTOMATED CONTROL OF TECHNOLOGICAL PROCESSES OF DRILLING OIL AND GAS WELLS)
A. Electrical measurements
The development of science and technology is inextricably linked with measurements. D. I. Mendeleev wrote: “Science begins as soon as they begin to measure, exact science is unthinkable without measure.” W. T. Kelvin said: "Every thing is known only to the extent that it can be measured." It is quite natural that electrical engineering ...(ELECTRIC CIRCUITS THEORY)
Electrical measurements, classification of measuring instruments
Measurement - finding the values of physical quantities empirically using special means, called measuring instruments, and the expression of these values in accepted units Fridman A. E. Theory of metrological reliability of measuring instruments // Fundamental issues theory of accuracy. St. Petersburg: Science,...(THEORETICAL INNOVATION)
Basic methods of electrical measurements. Instrument errors
There are two main methods of electrical measurements: direct evaluation method and comparison method. In the method of direct assessment, the measured value is read directly on the scale of the instrument. In this case, the scale of the measuring device is pre-calibrated according to the reference device ...(GENERAL ELECTRICAL ENGINEERING)
Objects electrical measurements are all electrical and magnetic quantities: current, voltage, power, energy, magnetic flux, etc. Determining the values of these quantities is necessary to evaluate the operation of all electrical devices, which determines the exceptional importance of measurements in electrical engineering.
Electrical measuring devices are also widely used for measuring non-electric quantities (temperature, pressure, etc.), which for this purpose are converted into proportional ones. electrical quantities. Such measurement methods are known as common name electrical measurements of non-electric quantities. The use of electrical measurement methods makes it possible to relatively simply transmit instrument readings over long distances (telemetry), control machines and apparatus (automatic control), perform automatic mathematical operations on measured quantities, simply record (for example, on tape) the progress of controlled processes, etc. Thus, electrical measurements are necessary in the automation of a wide variety of industrial processes.
In the Soviet Union, the development of electrical instrumentation goes hand in hand with the development of the electrification of the country, and especially rapidly after the Great Patriotic War. The high quality of the equipment and the necessary accuracy of the measuring devices in operation are guaranteed by the state supervision of all measures and measuring devices.
12.2 Measures, measuring instruments and methods of measurement
The measurement of any physical quantity consists in its comparison by means of a physical experiment with the value of the corresponding physical quantity taken as a unit. AT general case for such a comparison of the measured value with the measure - the real reproduction of the unit of measure - you need comparison device. For example, an exemplary resistance coil is used as a measure of resistance in conjunction with a comparison device - a measuring bridge.
The measurement is greatly simplified if there is direct reading instrument(also called an indicating instrument), showing the numerical value of the measured quantity directly on the scale or dial. Examples are ammeter, voltmeter, wattmeter, electric energy meter. When measuring with such a device, a measure (for example, an exemplary resistance coil) is not needed, but the measure was needed when graduating the scale of this device. As a rule, comparison devices have higher accuracy and sensitivity, but measurement with direct reading devices is easier, faster and cheaper.
Depending on how the measurement results are obtained, there are direct, indirect and cumulative measurements.
If the measurement result directly gives the desired value of the investigated quantity, then such a measurement belongs to the number of direct measurements, for example, current measurement with an ammeter.
If the measured quantity has to be determined on the basis of direct measurements of other physical quantities with which the measured quantity is related by a certain dependence, then the measurement is classified as indirect. For example, it will be indirect to measure the resistance of an electrical circuit element when measuring voltage with a voltmeter and current with an ammeter.
It should be borne in mind that with indirect measurement, a significant decrease in accuracy is possible compared to the accuracy with direct measurement due to the addition of errors in direct measurements of the quantities included in the calculation equations.
In some cases final result measurement was derived from the results of several groups of direct or indirect measurements of individual quantities, and the quantity under study depends on the measured quantities. Such a measurement is called cumulative. For example, cumulative measurements include determining the temperature coefficient of electrical resistance of a material based on measurements of the material's resistance at various temperatures. Cumulative measurements are typical for laboratory studies.
Depending on the method of application of instruments and measures, it is customary to distinguish the following main methods of measurement: direct measurement, zero and differential.
When using by direct measurement(or direct reading) the measured value is determined by
direct reading of the reading of a measuring instrument or direct comparison with a measure of a given physical quantity (measuring current with an ammeter, measuring length with a meter). In this case, the upper limit of the measurement accuracy is the accuracy of the measuring instrument, which cannot be very high.
When measuring null method the exemplary (known) value (or the effect of its action) is regulated and its value is brought to equality with the value of the measured value (or the effect of its action). With the help of a measuring device in this case, only equality is achieved. The device must be of high sensitivity, and it is called zero instrument or null indicator. As zero instruments for direct current, magnetoelectric galvanometers are usually used (see § 12.7), and for alternating current, electronic zero indicators. The measurement accuracy of the zero method is very high and is mainly determined by the accuracy of the reference measures and the sensitivity of the zero instruments. Among the zero methods of electrical measurements, bridge and compensation methods are the most important.
Even greater accuracy can be achieved with differential methods measurements. In these cases, the measured value is balanced by a known value, but the measuring circuit is not brought to full equilibrium, and the difference between the measured and known values is measured by direct reading. Differential methods are used to compare two quantities whose values differ little from each other.
