electrical measurement pressntaion

8/17/2019 Electrical measurement pressntaion

1/22

Electrical Measurement


2/22

Learning Outcomes• Explain how a moving-coil movement

measures current.

•Calculate the internal resistance of a moving-

coil voltmeter.


3/22

Learning Outcomes• Calculate the loading effect of a voltmeter.

• Design an ohmmeter using a moving-coil

movement.

• Use a Wheatstone bridge for accurate

resistance measurement.

•


4/22

11-1 Moving-Coil Meters• Used for measuring current, voltage,

resistance.

• Depends on moving-coil movement ord’Arsonval movement!, which uses the

interaction of a current-carr"ing coil and a

permanent magnet.

• #perates on the motor principle, which

states that a magnetic field exerts a force on

a current flowing perpendicular to it.


5/22

11-1 Moving-Coil MetersWhen current flows through the coil, it becomes

an electromagnet. $s current increases, the

electromagnetic

force increases,

moving the

pointer acrossthe scale.

Moving coil —— —— Soft-iron core

Figure 11-1 Moving-coil meter


6/22

11-1 Moving-Coil Meters%wo &e" characteristics'

sensitivity

the value of current that causes full-scaledeflection of a d($rsonval meter movement)

determined b" the strength of the permanent

magnet and number of turns on the moving coil

resistance

comes from the resistance of the wire in the

coil) more turns on the moving coil means

higher resistance


7/22

11-2 The Ammeter • Used for measuring the current flowing in a

branch of a circuit.

• %o measure larger currents, it must beconnected in parallel to a shunt resistor.

shunt resistor

a resistor connected in parallel with anammeter movement so that the full-scale

reading corresponds to a current greater than

the current capabilit" of the movement


8/22

11-2 The Ammeter • *or a moving-coil movement with a sensitivit"

of +. m$ and a resistance of Ω,

• %herefore the calibrating

resistance is

Ω

5050

1.0mA

mVT

I

V R

Ω

232750C R

Figure 11-2 Moving-coil ammeter with

shunt resistor


9/22

11-2 The Ammeter • /ensitive 0-µ $ movements are used for

measurements in electronic circuits.

• 0.-m$ movements are used with electric

power circuits.

• $ calibrating or swamping! resistor is

connected in series with the moving coil to

bring the total resistance of the movement

up to some specific value.


10/22

11-2 The Ammeter %o use the +.-m$ movement to measure

current up to + $, we arrange the meter circuit

as shown in *ig ++-1,forming two parallel

branches with 222

times as much current

flowing through theshunt resistor as

through the meter

movement.Figure 11-3 Measuring current


11/22

11-3 The Voltmeter • $ useful instrument for chec&ing the operation

of a circuit.

• With suitable clips on the test leads, we canconnect a voltmeter across an" part of a circuit

without having to disconnect an" components.

Figure 11-5 Simple voltmeter


12/22

Key Termsvoltmeter sensitivity

a parameter indicating the load that a

given voltmeter adds to a circuit,

usuall" expressed as resistance per

volt of full-scale reading. When used

as a voltmeter, a 0-m$ movement

has a voltmeter sensitivit" of &Ω34.


13/22

11-4 Voltmeter Loading !!ect• $n ideal voltmeter should draw 5ero current

and have an infinitel" high internal resistance.

• 6owever, a moving-coil voltmeter shoulddraw some current in order to deflect the

pointer.

• %he smaller the current re7uired for full-scaledeflection, the more sensitive the voltmeter and

the smaller the loading effect it has on the

circuit tested.


14/22

11-" #esistance Measurement• 4er" low resistances ma" be measured b"

appl"ing #hm(s law to readings obtained

from a voltmeter and ammeter.

• 8uic& chec&s of circuit resistance can be

accomplished with an ohmmeter, whichmeasures resistance directl".


15/22

11-" #esistance Measurement• %he nonlinear scale of an ohmmeter ma&eslow resistances easier to read but high

resistances harder to read.• With a 0-µ $ move-

ment the total internal

resistance of the ohm-meter in *ig ++-2a! is

Ωk 50μ0

VT 90

5.4

I

E R

Figure 11- Simple ohmmeter


16/22

Key Termsgalvanometer

a sensitive moving-coil meter in which

the pointer is at the center of its range

when the current through the coil is

5ero) essentiall", a sensitive ammeter

that can measure current flowing

through it in either direction


17/22

.

Figure 11-13 !lectrodynamometermovement


18/22

•

Figure 11-1" !lectrodynamometer movementas a wattmeter


19/22


20/22

• /ince the deflection of the pointer in a

moving-coil movement is proportional to the

current through the coil, the movement canbe used as the measuring device in an

ammeter.

• $ voltmeter with a number of voltageranges can be constructed from a moving-

coil movement b" placing multiplier

resistors in series with it.

$ummary


21/22

• $n analog voltmeter can reduce the

voltage being measured.

• $n ohmmeter with a number of resistance

ranges can be constructed using a moving-

coil movement, a batter", shunt resistors,

and series resistors.• %he Wheatstone bridge can be used for

accurate measurements of resistances.

$ummary


22/22

•END

electrical measurement pressntaion

Documents