Lect No. Dynamometer Type Instrument
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Transcript of 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.