Dot convention in coupled circuits

Circuit Theory - Dot convention Rule

1. Dot Convention in Coupled
Circuits
T.Mrunalini

2. Coupled circuit
Anelectriccircuitissaidtobeacoupled
circuit,whenthereexistsamutualinductance
betweenthecoils(orinductors)presentinthat
circuit.Intheabsenceofresistor,coilbecomes
inductor.Sometimes,thetermscoiland
inductorareinterchangeablyused.

3. Dot Convention
Dotconventionisatechnique,whichgives
thedetailsaboutvoltagepolarityatthedotted
terminal.Thisinformationisuseful,whilewriting
KVLequations.
•Ifthecurrententersatthedottedterminalofone
coil(orinductor),thenitinducesavoltageat
anothercoil(orinductor),whichis
havingpositivepolarityatthedottedterminal.
•Ifthecurrentleavesfromthedottedterminalof
onecoil(orinductor),thenitinducesavoltageat
anothercoil(orinductor),whichis
havingnegativepolarityatthedottedterminal.

4. Classification of Coupling
Couplingis classified into the following two
categories.
•Electrical Coupling
•Magnetic Coupling

5. Electrical Coupling
•Electricalcouplingoccurs,whenthereexists
aphysicalconnectionbetweentwocoils(or
inductors).Thiscouplingcanbeofeither
aidingtypeoropposingtype.Itisbasedon
whetherthecurrententersatthedotted
terminalorleavesfromthedottedterminal.

6. Coupling of Aiding type
•Sincethetwoinductorsareconnected
inseries,thesamecurrentIflow
throughbothinductorshavingself-
inductancesL
1andL
2.
•Inthiscase,thecurrent,Ienteratthe
dottedterminalofeachinductor.Hence,
theinducedvoltageineachinductor
willbehavingpositivepolarityatthe
dottedterminalduetothecurrent
flowinginanothercoil.
•Therefore, the equivalent
inductanceofseriescombinationof
inductorsshownintheabovefigureis
Leff = L1+L2+2M

7. Coupling of Opposing type
Inthecircuit,thecurrentIentersatthedotted
terminaloftheinductorhavinganinductanceofL
1.
Hence,itinducesavoltageintheotherinductor
havinganinductanceofL
2.
So,positivepolarityoftheinducedvoltageis
presentatthedottedterminalofthisinductor.
Intheabovecircuit,thecurrentIleavesfrom
thedottedterminaloftheinductorhavingan
inductanceofL
2.Hence,itinducesavoltageinthe
otherinductorhavinganinductanceofL
1.
So,negativepolarityoftheinducedvoltageis
presentatthedottedterminalofthisinductor.
Therefore,theequivalentinductanceofseries
combinationofinductorsshownintheabovefigure
isLeff=L1+L2-2M
Inthiscase,theequivalentinductancehasbeen
decreasedby2M.Hence,theaboveelectricalcircuit
isanexampleofelectricalcouplingwhichis
ofopposingtype.

8. Magnetic Coupling
•Magneticcouplingoccurs,whenthereisno
physicalconnectionbetweentwocoils(or
inductors).Thiscouplingcanbeofeither
aidingtypeoropposingtype.Itisbasedon
whetherthecurrententersatthedotted
terminalorleavesfromthedottedterminal.

9. Coupling of Aiding type
Thecurrentsflowingthroughprimaryandsecondarycoilsare
i
1andi
2respectively.
Inthiscase,thesecurrentsenteratthedottedterminalof
respectivecoil.Hence,theinducedvoltageineachcoilwillbehaving
positivepolarityatthedottedterminalduetothecurrentflowingin
anothercoil.

10. Coupling of Opposing Type
Thecurrentsflowingthroughprimaryandsecondarycoilsarei
1and
i
2respectively.Inthiscase,thecurrent,i
1entersatthedottedterminalofprimary
coil.Hence,itinducesavoltageinsecondarycoil.So,positivepolarityofthe
inducedvoltageispresentatthedottedterminalofthissecondarycoil.
Intheabovecircuit,thecurrent,i
2leavesfromthedottedterminalof
secondarycoil.Hence,itinducesavoltageinprimarycoil.So,negative
polarityoftheinducedvoltageispresentatthedottedterminalofthisprimary
coil.

11. Thank You