电工电子技术第3章电路的暂态分析ENG.ppt

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transient state暂态 steady state稳态 transient process暂态过程 law of switch换路定理
first-order circuit一阶电路three-factor method三要素法 time constant时间常数 integral circuit积分电路differential circuit 微分电路
Terms:
2Grasplawofswitchandcomputeinitialvalue;
3ConceptsofZero-inputResponse,Zero-stateResponse,CompleteResponse;
1Understandtheconceptsoftransientstate、steadystateandphysicalsignificanceoftimeconstant;
Outline:
4Applicationofthree-factormethod.
Forthedirectcurrentcase,Lisshort-circuited
Forthedirectcurrentcase,Cisopen-circuited
+
-
L
inductor
uC
i
C
+
_
capacitor
R
u
+
_
resistor
R
u
+
_
resistor
R
u
+
_
resistor
resistor
resistor
R
u
+
_
resistor
BeforethemovementofK
i=0,uC=0
i=0,uC=Us
Transitionprocessofcircuit
K
+
–
uC
Us
R
C
i
t=0
LongafterKisclosed
Transitionprocess:theprocessbeundergonebythecircuittransientingfromonesteadystatetoanothersteadystate.
Switching:thecircuit(structureorparameter)changes.
i
+
–
uC
Us
R
C
Kisclosed

Differencesbetweensteady-stateanalysisandtransientanalysis
SteadystateTransient
;
;
、uCtime-varying;
;
;
、UCkeepunchanged;
Reasonsfortransitionprocess
-storageelementsL、C

RCcircuit:
Note:lawofswitchisjustusedtodeterminetheinitialvalueuC、iLatthemomentofswitching。
Assumptions:t=0—switchinginstant
t=0–—beforeswitchinginstant
t=0+—afterswitchinginstant
RLcircuit:
LawofSwitch:capacitorvoltageandinductorcurrentcannotchangeabrubtlyattheinstantofswitching.
Determinationofinitialvalue:
Outlineofsolution:
Initialvalue:thevalueofeachu、iatt=0+inthecircuit。
(0-)oriL(0-)withtheaidofthecircuit(steadystate)beforeswitching.
(0+)oriL(0+)withlawofswitch.
=0+.
(1)ifuC(0-)=0,Replacecapacitorbyshortcircuit;
iL(0-)=0,Replaceinductorbyopencircuit.
=0+.
(2)ifuC(0-)0,Replacecapacitorbyvoltagesource;
iL(0-)0,Replaceinductorbycurrentsource.
Getthevalueatt=0+,thedirectionisidenticaltoassumeddirectionofcapacitorvoltageandinductorcurrent.
=0+within0+circuitdiagrambyknownmethods(Ohm’slawetal.).
Example1
Solution:
(1)Solvefor
Accordingto
theconditions:
Bylawofswitch:
ForthecircuitinFig(a).,andUC=0、IL=0.
S
(a)
C
U
R2
R1
t=0
+
-
L