Lect No. Dynamometer Type Instrument

7/25/2019 Lect No. Dynamometer Type Instrument

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INSTRUMENTATION &

MEASUREMENTS

electrodynamometer type

instruments

1/23/16 1

5thterm BE (Electrical)


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ELECTRODYNAMOMETER TYPE INSTRUMENT

The electrodynamometer type instrument is a transfer

instrument.

A transfer instrument is one hich is cali!rated ith a d.c.

source and used ithout any modifications for a.c.

measurements. "uch a transfer instrument has same

accuracy for a.c. and d.c measurements.

#f an electrodynamometer instrument is cali!rated ith d.c

current. #f pointer indicates 1 A d.c. on scale then on a.c.$

the pointer ill deflect upto the same mar% (1 Amp r.m.s&alue).

Thus as it is a transfer instrument$ there is direct

connection !eteen a.c. and d.c. ence the instrument isoften used as a cali!ration instrument.


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The electrodynamometer type instruments

are often used in accurate a.c. &oltmeters

and ammeters$ not only at the poer line

freuency !ut also in the loer audiofreuency ran*e.

+ith some little modifications$ it can !e

used as a attmeter for the poer

measurements.


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!Y PMMC INSTRUMENTS CAN NOT "E USED

#OR A$C$ MEASUREMENTS %

The ,-- instrument cannot !e used on a.c. currents or

&olta*es !ecause$

#f a.c. supply is *i&en to these instruments$ an alternatin*

torue ill !e de&eloped. ue to moment of inertia of

mo&in* system$ the pointer ill not follo rapidly chan*in*

alternatin* torue and ill fail to sho any readin*.

#n order that the instrument should !e a!le to read a.c.

uantities$ the ma*netic field in the air *ap must chan*e

alon* ith the chan*e in current.

This principle is used in the electrodynamometer type

instrument. #nstead of a permanent ma*net$ the

electrodynamometer type instrument uses the current under

measurement to produce the necessary field flu0.


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OPERATIN& PRINCIPLE O#

ELECTRODYNAMOMETERS

or A. it ould ha&e a torue in one direction durin* one half ofthe cycle and an eual effect in the opposite direction durin* the

other half of the cycle.

#f the freuency ere &ery lo$ the pointer ould sin* !ac% and

forth around the ero point.

oe&er$ for an ordinary meter$ the inertia is so *reat that on

poer freuencies the pointer does not *o &ery far in either

direction !ut merely stays (&i!rates sli*htly) around ero.

#f$ hoe&er$ e ere to re&erse the direction of the flu0 each time

the current throu*h the mo&a!le coil re&erses$ a unidirectional

torue ould !e produced for !oth positi&e and ne*ati&e hal&es of

the cycle and this is the !asic or%in* principle for the

dynamometer instruments.


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ELECTRODYNAMOMETER TYPE INSTRUMENT

i*ures shos the construction of electrodynamometer

type instrument


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Construction

#i'ed Coils( The necessary field reuired for the operation of the instrument

is produced !y the fi0ed coils. A uniform field is o!tained near thecenter of coil due to di&ision of coil in to sections.

These coils are air cored. i0ed coils are ound ith fine ire for

usin* as &oltmeter$ hile for ammeter and attmeter it is ound

ith hea&y ire. The coils are usually &arnished. They are clamped in

place a*ainst the coil supports. This ma%es the construction ri*id.

eramic is usually used for mountin* supports. #f metal parts

ould ha&e !een used then it ould ea%en the field of the fi0ed

coil.

Mo)in* CoilThe mo&in* coil is ound either as a self4sustainin* coil or else on

a non4metallic former.

#f metallic former is used$ then it ould induce eddy currents in it.

The construction of mo&in* coil is made li*ht and coil is air cored.


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Controllin* The controllin* torue is pro&ided !y sprin*s. These

sprin*s act as leads to the mo&in* coil.

Mo)in* System The mo&in* coil is mounted on an aluminium

spindle. #t consists of counter ei*hts and pointer.

Dampin* The dampin* torue is pro&ided !y air friction$ !y a pair

of aluminium &anes hich are attached to the spindle at the

!ottom. They mo&e in sector shaped cham!ers. As operatin* fieldould !e distorted !y eddy current dampin*$ it is not employed.

S+ieldin* The field produced !y these instruments is &ery ea%.

E&en earths ma*netic field considera!ly affects the readin*. "o

shieldin* is done to protect it from stray ma*netic fields. #t is done!y enclosin* in a casin* hi*h permea!ility alloy.


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ELECTRODYNAMOMETER AS

AMMETER

or usin* electrodynamometer instrument asammeter$ fi0ed and mo&in* coils are connected

in series and carry the same current. A suita!le

shunt is connected to these coils to limit

current throu*h them up to desired limit.


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ELECTRODYNAMOMETER AS ,OLTMETER

The electrodynamometer instruments can !e usedas a &oltmeter !y connectin* the fi0ed and mo&in*

coils in series ith a hi*h non4inducti&e resistance.

#t is most accurate type of &oltmeter.


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ELECTRODYNAMOMETER AS ATTMETER or usin* electrodynamometer instrument as a attmeter to

measure the poer$ the fi0ed coils acts as a current coil and

must !e connected in series ith the load.

The mo&in* coils acts as a &olta*e coil or pressure oil and

must !e connected across the supply terminals. The

attmeter indicates the supply poer.

+hen current passes throu*h the fi0ed and mo&in* coils$ !oth

coils produce the ma*netic fields. The field produced !y fi0ed

coil is proportional to the load current hile the field

produced !y the mo&in* coil is proportional to the &olta*e.

As the deflectin* torue is produced due to the interaction of

these to fields$ the deflection is proportional to the poer

supplied to the load.


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AD,ANTA&ES O# ELECTRODYNAMIC INSTRUMENTS

1) As the coils are air cored$ these instruments are

free from hysteresis and eddy current losses.

2) These instruments can !e used on !oth a.c. and d.c.

They are also used as a transfer instruments.

3) Electrodynamometer &oltmeter are &ery useful

here accurate r.m.s &alues of &olta*e$ irrespecti&e

of a&eforms$ are reuired.

) ree from hysteresis errors.

5) 7o poer onsumption.

6) 7i*ht in ei*ht.


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DISAD,ANTA&ES O#

ELECTRODYNAMIC INSTRUMENTS

1) These instruments ha&e a lo sensiti&ity due to a

lo torue to ei*ht ratio. Also it introduces

increased frictional losses. To *et accurate results$

these errors must !e minimied.

2) They are more e0pensi&e than other type of

instruments.

3) These instruments are sensiti&e to o&erload and

mechanical impacts. Therefore care must !e ta%en

hile handlin* them.

) They ha&e a non4uniform scale.

5) The operation current of these instruments is lar*e

due to the fact that they ha&e ea% ma*netic field.

Documents

Lect No. Dynamometer Type Instrument