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Electrodynamometer Type Instrument

Transfer Instrument:

Calibrated with a DC source and then used without modification

to measure AC

Electrodynamics instruments are capable of service as transfer

instrument. ,with the current in the movable coil if the field coil is connected in series

with the movable coil.

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Construction:

The field is produced by a fixed coil e co s v e n o wo sec ons o g ve a more un orm e near

the centre and to allow passage of shaft

to reduce eddy current loss in conductors

assembly This makes the construction rigid so that there is no shifting or change

in dimension

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Single element instrument has one moving coil

wounded either as a self-sustaining coil or on a non-metallic former

to avoid eddy current induced by the alternating field Light but rigid construction is used

ot : xe an mov ng co s are a r core .

Control: Controlling torque is provided by two control spring

Moving System

Moving coil is mounted on an aluminum spindle

Also carries the counter weight and truss type pointer

uspens on may e use n case o g sens v y

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Tor ue E uationi1 instantaneous value of current in fixed coil; A

i2 instantaneous value of current in moving coil; A

1

L2 self inductance of moving coil; H

MMutual inductance between fixed and moving coil

Detailed formula is explained on the board.

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Electrodynamometer Ammeter:

In this case, the fixed and moving coils are connected in series

and, therefore carry the same current.

= = = .

Formulae is explained on the board.

T e current s ou not excee 100mA. For ammeters of higher capacity, the moving coil is shunted

to limit the current through moving coil to 100mA.

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The electrodynamometer movement is used as a voltmeter

b connectin the fixed and movin coils in series with a hi h non-

inductive resistance.Formulae is explained on the board.

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Advantage

s t e co s are a r core , t ese nstruments are ree rom

hysteresis and eddy current errors

The have a recision rade accurac for fre uencies from 40

up to 500 Hz. Can be used on both AC and DC.

Disadvantage

They have a low torque/weight ratio hence, have a low sensitivity

They are more expensive than either the PMMC or the MI type

ens ve o over oa s an mec an ca mpac s Must be handled with great care

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Thermocouple Instrument

When two metals having different work functions are placedtogether, a voltage is generated at the junction.

s vo age s near y propor ona o e empera ure.

This junction is called a Thermocouple. convert heat energy to electrical energy.

The heat at the junction is produced by the electrical current

flowing in the heater element. ,current passing through the heater

Used for AC and DC applications

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Electrostatic Instrument

The deflecting torque is produced by the action of electric field

on charged conductors.

They are essentially voltmeters

But, may also be used with the help of external components to measure

current and power

Two ways in which the force act:

when two oppositely charged electrodes are considered, one of them is

fixed and other is movable.

due to the force of attraction, the movable electrode is drawn towards the

fixed one.

when there are forces of attraction or repulsion or both between theelectrodes which cause rotary motion of the moving electrodes.

,

force or torque is related to the moving system.