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

TO THE EFFECT LEARNING

For over know electricity and its component

Measuring Instrument Task

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.

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

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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.

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

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T = B x A x I x JV

indicating that the torque, which deflects the movable coil, is directly propor tional

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 electro dynamometer 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

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 11R

watts. The average rate of producing heat by an ac of i amperes during one cycle in the

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same resistance R is 1T

∫o

T

i 2 R dt . By definition, therefore,

I2r = 1T

∫o

T

i 2 R dt

and I = √ 1T∫o

T

i 2dt = √average i2

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 fie ld

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 Ω/V

(compare this to the 20 kΩ/V of a d'Arsonval meter). The reactance and resistance of

the coils also increase with increasing frequency, limiting the application of the

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.

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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

write for the average deflection over one period:

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θav = K 1T

∫0

R

ic ip dt

Where θav = average angular deflection of the coil

K = instrument constant

ic = instantaneous current in the field 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

θav = K 1T

∫0

R

ie

Rp dt = K2

1T

∫0

r

ei dt

By definition, the average power in a circuit is

P a v = 1T

∫0

r

ei 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 ωt and i = im sin (ωt± θ), Eq. (5-9) reduces to

θav = K3EI cos θ

where E and I represent the rms values of the voltage and the current, and θ

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 potentia l coil should 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

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the amount of the 12R losses in the field windings. Choice of the correct connection

depends on the situation. Generally, connection of the potential coil at point A is prefe rred 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

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wattmeter I carries the line current , 1A1 A, which is the vector sum of the phase currents IAC and

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 – θ) = VI cos (30o – θ)

W2 = VBC IB1 B cos (30o + θ) = VI cos (30o – θ)

and W1 + W2 = VI cos (30o – θ) + VI cos (30o + θ)

= ( cos 30o cos θ + sin 30o sin θ + cos 30o cos θ - sin 30o sin θ )

= √3 VI cos θ

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

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be shown that the algebraic sum of the readings of the two wattmeters will give

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 50Ω 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 = 2π

Em = 2 √ 2

π 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 V

1mA = 9 KΩ

So, Rs = RT – Rm = 9000 Ω - 50 Ω = 8950 Ω

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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

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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_movement.htm

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