1.
Measuring Instrument Task
MEASURING INSTRUMENT TASK
DISUSUN OLEH
KELOMPOK
RAHMAWATI TH. DIAMANTI
PERNI KRISTA U. KARUNDENG
UNIVERSITAS NEGERI MANADO
FAKULTAS MATEMATIKA DAN ILMU PENGETAHUAN ALAM
JURUSAN FISIKA
2010
2.
Measuring Instrument Task
TO THE EFFECT LEARNING
For over know electricity and its component
BENEFIT
can apply concept to electrify at in various shooting problem and also in day-to-day life
PREFACE
In to electrify will result still electricity that aroused by menggosokkan as erect as glass,
look on it as contraption of object then there are many theory which grows and present cognitive
it is accepted and is called ‖ electron theory ‖ one arises around year 1900. At century final to
eighteen while electric source first times found by Voltaic Volta so maybe to be studied effect to
electrify it ruled by jurisdictional given so maybe to be accounted its effect.
Electric current can be equalled by liquid in one pipe if is jointed one introduction goes to
pole pole current source. Meaning electric current current of electric one flows to pass through
introduction at one particular enclosed series. Current electricing to evoke effect in introductory.
Instrument to electrify
Herein there is three instrumental types, namely
1. Moving is instrument's coil
2. Moving iron instrument
3. Moving is instrument's magnet
More explanation is as follows
a ) Moving instrument's coil
Moving instrument’s coil is length square that resident at one particular punk with bolster
so gets pivot on among magnetic poles, indicator needle is pasted on punk and if no voltage to
indicator needle instrument lies on course 0 (zero) because of roll spiral spring (spring's coil).
Current of positive pole goes to moving coil via bottom rolled spiral spring. Resulting magnet
field around moving coil concerning in style magnet field between magnetic poles so causes
moving coil moves. Instrument as it a lot of is utilized on vehicle tools testing. Moving is
instrument's coil one for voltmeter, resistor provedes with that instrument that linked quits its
prisoner one is accounted deep its relationship with moving's prisoner coil.
b ) Moving Iron Instrument
Moving iron instrument has coil that magnet field effect it to one vane of soft iron, vane
that was placed on needle punk and is pulled little farther if wax current, irregular scale because
its magnet situation. A part first of scale with short division distance, this instrument match for
current DC and AC.
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Measuring Instrument Task
c ) Moving instrument's Magnets
One vane of soft iron to be pasted on needle punk and is placed magnetic pole betwixt nail
rides on horseback. That armature's position determined by field of that magnet style and which
that magnet field resultant by current that passes through koil. If current is adrift pass through
koil vane that will revolve and deviates current. That instrument is utilized one for amperemeter
on electric system, it points out charge (fill) or not charge but that instrument not precision.
I. ELECTRODYNAMOMETER
The electrodynamometer can be used as a wattmeter, a VARmeter, a power-factor meter,
or a frequency meter. The electrodynamometer movement may also serve as a transfer
instrument, because it can be calibrated on dc and then used directly on ac, establishing a
direct means of equating ac and dc measurements of voltage and current.
Where the d'Arsonval movement uses a permanent magnet to provide the magnetic
field in which the movable coil rotates, the electrodynamometer uses the current under
measurement to produce the necessary field flux. Figure 5-1 shows a schematic
arrangement of the parts of this movement.
A fixed coil, split into two equal halves, provides the magnetic field in which the
movable coil rotates. The two coil halves are connected in series with the moving coil and
are fed by the current under measurement. The fixed coils are spaced far enough apart to
allow passage of the shaft of the movable coil. The movable coil carries a pointer, which
is balanced by counterweights. Its rotation is controlled by springs, similar to the
d'Arsonval movement construction. The complete assembly is surrounded by a
laminated shield to protect the instrument from, stray magnetic fields which may affect
its operation. Damping is provided by aluminum air vanes. moving in sector-shaped
charribers. The entire movement is very solid and rigidly constructed in order to keep its
mechanical dimensions stable and its calibration intact. A cutaway view of the
electrodynamometer is shown in Fig. 5-2.
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Measuring Instrument Task
The operation of the instrument may be understood by returning to the expression for the
torque developed by a coil suspended in a magnetic field. We previously stated, Eq. (4-1).
that
T = B x A x I x JV
indicating that the torque, which deflects the movable coil, is directly proportional
to the coil constants (A and N), the strength of the magnetic field in which the coil
moves (B), and the current through the coil (I). In the electrodynamometer the flux
density (B) depends on the current through the fixed coil and is therefore directly
proportional to the deflection current (I). since the coil dimensions and the number of turns
on the coil frame are fixed quantities for any given meter, the developed torque
becomes a function of the current squared (I2
).
If the electrodynamometer is exclusively designed for dc use, its square-law scale
is easily noticed, with crowded scale markings at the very low current values,
progressively spreading out at the higher current values. For ac use, the developed
torque at any instant is proportional to the instantaneous current squared (I2
). The
instantaneous-value of i2
is always, positive and torque pulsations are therefore
produced. The movement, however, cannot follow the rapid variations of the torque
and takes up a position in which the average torque is balanced by the torque of the
control springs. The meter deflection is therefore a function of the mean of the
squared current. The scale of the electrodynamometer is usually calibrated in terms
of the square root of the average current squared, and the meter therefore reads the
rms or effective value of the ac.
The transfer properties of the electrodynamometer become apparent when we
compare the effective value of alternating current and direct current in terms of their
heating effect or transfer of power. An alternating current that produces heat in a given
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Measuring Instrument Task
resistance at the same average rate as a direct current (I) has, by definition, a value of I
amperes. The average rate of producing heat by a dc of I amperes in a resistance R is 11
R
watts. The average rate of producing heat by an ac of i amperes during one cycle in the
same resistance R is . By definition, therefore,
I2
r =
and I = =
This current, I, is then called the root-mean-square (rms) or effective value of the alternating
current and is often referred to as the equivalent dc value.
If the electrodvnamometer is calibrated with a direct current of I-A and a mark is
placed on the scale to indicate this I -A dc value, then that alternating current which
causes the pointer to deflect to the same mark on the scale must have an rms value of I
A. We can therefore "transfer" a reading made with dc to its corresponding ac value and
have thereby established a direct connection between ac and dc. The electrodynamometer
then becomes very useful as a calibration instrument and is often used for this purpose
because of its inherent accuracy.
The electrodynamometer, however, has certain disadvantages. One of these is its
high power consumption, a direct result of its construction. The current under
measurement must not only pass through the movable coil, but it must also provide the fiel
d
flux. To get a sufficiently strong magnetic field, a high mmf is required and the source must
supply a high current and power. In spite of this high power consumption, the magnetic field
is very much weaker than that of a comparable d'Arsonval movement because there is no
iron in the circuit, i.e., the entire flux path consists of air. Some instruments have been
designed using special laminated steel for part of the flux path, but the presence of metal
introduces calibration problems caused by frequency and vaveform effects. Typical values
of electrodynamometer flux density are in the range of approximately 60 gauss. This
compares very unfavorably with the high flux densities (1,000-4.000 gauss) of a good
d'Arsonval movement. The low flux density of the electrodynamometer immediately
affects the developed torque and therefore the sensitivity of the instrument is typically
very low.
The addition of a series resistor converts the electrodynamometer into a voltmeter, which
again can be used to measure dc and ac voltages. For reasons previously mentioned, the
sensitivity of the electrodynamometer voltmeter is low, approximately 10 to 30 /V
(compare this to the 20 k /V of a d'Arsonval meter). The reactance and resistance of
the coils also increase with increasing frequency, limiting the application of the
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Measuring Instrument Task
electrodynamometer voltmeter to the lower frequency ranges. It is, however, very
accurate at the powerline frequencies and is therefore often used as a secondary standard.
The electrodynamometer movement (even unshunted) may be regarded as an
ammeter, but it becomes rather difficult to design a moving coil which can carry
more than approximately 100 mA. Larger current would have to be carried to the
moving coil through heavy lead-in wires, which would lose their flexibility. A shunt,
when used, is usually placed across the movable coil only. The fixed coils are then
made of heavy wire which can carry the large total current and it is feasible to build
ammeters for currents up to 20 A. Larger values of ac currents are usually measured
by using a current transformer and a standard 5-A ac ammeter (Sec. 5-11).
II. ELECTRODYNAMOMETERS IN POWER MEASUREMENTS
II.1 Single-phase Wattmeter
The electrodynamometer movement is used extensively in measuring power. It may
be used to indicate both dc and ac power for any waveform of voltage and current and it
is not restricted to sinusoidal waveforms. As described in Sec. 5-2, the
electrodynamometer used as a voltmeter or an ammeter has the fixed coils and the movable
coil connected in series, thereby reacting to the effect of the current squared. When used as
a single-Phase power meter, the coils are connected in a different arrangement (see Fig. 5-18).
The fixed coils or field coils, shown here as two separate elements, are
connected in series and carry the total line current (ic). The movable coil, located in the
magnetic field of the fixed coils, is connected in series with a current-limiting resistor across
the power line and carries a small current (ip). The instantaneous value of the current in the
movable coil is ip = e/Rp, where e is the instantaneous voltage across the power line, and P, is
the total resistance of the movable coil and its series resistor.
The deflection of the movable coil is proportional to the product of these two currents, ic. and
ip, and we can
7.
Measuring Instrument Task
write for the average deflection over one period:
av = K c ip dt
Where av = average angular deflection of the coil
K = instrument constant
ic = instantaneous current in the fiel
d coils
ip= instantaneous current in the potential coil
Assuming for the moment that ic is equal to the load current, i (actually, ic = ip + i,) and using the
value for ip = e/Rp, we see that Eq. (5-8) reduces to
av = K dt = K2 dt
By definition, the average power in a circuit is
Pa v = dt
which indicates that the electrodynamometer-movement, connected in the configuration of
Fig. 5-18, has a deflection proportional to the average power. if e and i are sinusoidally
varying quantities of the form e = Em sin t and i = im sin ( t ), Eq. (5-9) reduces to
av = K3EI cos
where E and I represent the rms values of the voltage and the current, and
represents the phase angle between voltage and current. Equations (5-9) and (5-10) show that
the electrodynamometer indicates the average power delivered to the load.
Wattmeters have one voltage terminal and one current terminal marked ― .‖ When the
marked current terminal is connected to the incoming line, and the marked voltage
terminal is connected to the line side in which the current coil is connected, the meter will
always read up-scale when power is connected to the load. If for any reason (as in the two-
wattmeter method of measuring three-phase power), the meter should read backward, the
current connections (not the voltage connections) should be reversed.
The electrodynamometer wattmeter consumes some power for maintenance of its
magnetic field, but this is usually so small, compared to the load power, that it may be
neglected. If a correct reading of the load power is required, the current coil should
carry exactly the load current, and the potential
coil s
hould be connected across the
load terminals. With the potential coil connected-,
to point A, as in Pig. 5-18, the load
voltage is properly met- -d, but the current through the field coils is greater by the amount I
The wattmeter therefore reads high by the amount of additional power loss in the potential
circuit. If, however, the potential coil is connected to point B in Fig. 5-18, the field coils
meter the correct load current, but the voltage across the potential coil is higher by the
amount of the drop across the field coils. The wattmeter will again read high, but now by
the amount of the 12
R losses in the field windings. Choice of the correct connection
8.
Measuring Instrument Task
depends on the situation. Generally, connection of the potential coil at point A is prefer
red for
high-current, low-voltage loads. connection at B is preferred for low-current, high-voltage
"Loads.
The difficulty in placing the connection of the potential coil is overcome in the compensated
wattmeter, shown schematically in Fig. 5-19. The current coil consists of two windings, each
winding having the same number of turns. One winding uses heavy wire that carries the load
current plus the current for the potential coil. The other winding uses thin wire and carries
only the current to the voltage coil. This current, however, is in a direction opposite to the
current in the heavy winding, causing a flux that opposes the main flux. The effect of ip is
therefore canceled out, and the wattmeter indicates the correct power.
II.2 Polyphase Wattmeter
Power measurements in a polyphase system require the use of two or more
wattmeters. The total real power is then found by algebraically adding the readings of the
individual wattmeters. Blondel's theorem states that real power can be measured by one
less wattmeter element than the number of wires in any polyphase system, provided that
one wire can be made common to all the potential circuits. Figure 5-20(a) shows the
connection of two wattmeters to measure the power consumption of a balanced three-wire
delta-connected three-phase load.
The current coil of wattmeter I is connected in line A, and its voltage coil is connected
between line A and line C. The current coil of wattmeter 2 is connected in line B, and its
voltage coil is connected between line B and line C. The total power, consumed by the
balanced three-phase load, equals the algebraic sum of the two wattmeter readings.
The phasor diagram of Fig. 5-20(b) shows the three phase voltages VAC, VCB, and VBA
and the three phase currents 1AC, 1CB, and 1BA. The delta-connected load is assumed to
be inductive, and the phase currents lag the
phase voltages by an angle 0. The current coil of
wattmeter I carries the line current , 1A1 A, which is the vector sum of the phase currents IAC and
9.
Measuring Instrument Task
IAB. The potential coil of wattmeter I is connected across the line voltage VAC. Similarly,
the current coil of wattmeter 2 carries the line current IB1 B, which is the vector sum of the
phase currents IBA, and IBC, while the voltage across its potential coil is the line voltage VBC.
Since the load is balanced, the phase voltages and phase currents are equal in magnitude and we
can write
VAC = VBC = V and IAC = ICB = IBA = I
The power, represented by the currents and voltages of each wattmeter is
W1 = VAC1A1 A cos (30o
– ) = VI cos (30o
– )
W2 = VBC IB1 B cos (30o
+ ) = VI cos (30o
– )
and W1 + W2 = VI cos (30o
– ) + VI cos (30o
+ )
= ( cos 30o
cos + sin 30o
sin + cos 30o
cos - sin 30o
sin )
= VI cos
Equation (5-14) is the expression for the total power in a three-phase circuit, and the
two wattmeters of Fig. 5-20(a) therefore correctly measure this total power. It may
be shown that the algebraic sum of the readings of the two wattmeters will give
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Measuring Instrument Task
the correct value for power under any condition of unbalance, power factor, or
waveform.
If the neutral wire of the three-phase system is also present, as in the case of a
four-wire star-connected load—according to Blondel's theorem—three wattmeters
would be needed to make the total real power measurement. In Prob. 12 the reader
is asked to prove that three wattmeters measure total power in a four-wire system.
Problem :
one alternating current voltmeter utilize series 4a, where is power d.' arsonval (PMMC) having
prisoner in 50 and need direct currents as big as 1mA for deflection heaving full. if diode was
looked on by ideal (zero forward prisoner and prisoner turns back not get until) and tension as
big as 10rms linked to entry terminals, prisoners appreciative determinative Rs who result
deflection heaving full.
Answer :
for full wave rectifier
Edc = Em = Erms = 0,9 Erms
Edc = 0,9 x 10 V = 9 V
serieses totaled prisoner neglectfully diode prisoner in tenor forward :
RT = Rs + Rm = = 9 K
So, Rs = RT – Rm = 9000 - 50 = 8950
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Measuring Instrument Task
GLOSARIUM
Calibrate : a comparison between measurements - one of known magnitude or correctness
made or set with one device and another measurement made in as similar a way as possible with
a second device
Damping : any effect that tends to reduce the amplitude of oscillations in an oscillatory system,
particularly the harmonic oscillator
Density : defined as its mass per unit volume
D’arsonval : A commonly used sensing mechanism used in DC ammeters, voltmeters, and ohm
meters is a current-sensing device
Equivalent : a unit of amount of substance used in chemistry and the biological sciences
a measuring tool that will show the potential difference in a series
Quantities : a kind of property which exists as magnitude or multitude
Voltmeter : a measuring tool that will show the potential difference in a series
Wattmeter : Measure of Electric Power
12.
Measuring Instrument Task
LITERATURE
www.google.com
http://alifis.wordpress.com/2010/01/18/bocoran-soal-uas-instrumentasi-elektronika/#more-1191
http://www.engineersedge.com/instrumentation/electrical_meters_measurement/darsonval_mov
ement.htm
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