Introduction
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I
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Sinusoid means both cosine and sine waveforms.
Sinusoidal voltage and current are referred to ac voltage and
ac current respectively.
The voltage available at our home, office and everywhere is
sinusoid.
The single phase voltage
vi = Vm sin(ωt)
where vi - instantaneous voltage
Vm - maximum voltage. ω = 2πf - angular frequency (rad/sec).
However, the voltage available at our outlet is 230 V (rms).
Vm
V = √ (for sinusoid)
2
It is root mean squared (rms) voltage. It is also a DC equivalent
voltage.
RC Circuit
R
+
−
+
vi
vc
C
−
vi = Vm sin(ωt)
By KVL,
dvc
+ vc = Vm sin(ωt)
dt
Let us solve this differential equation using the same steps.
I Homogeneous solution: To find this, set vi to zero.
dvcH
RC
+ vcH = 0
dt
As we have already solved in DC Transients,
RC
VcH = Ae (−t/RC )
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Particular solution:
RC
dvcP
+ vcP = Vm sin(ωt)
dt
Guess any solution that satisfies this equation.
1. First try. Let vcP = A. It will not satisfy.
2. Second try. Let vcP = A sin(ωt). It will not also satisfy.
3. Third try. Let vcP = A sin(ωt + φ). It will work.
On substituting this in the above equation,
RCAω cos(ωt + φ) + A sin(ωt + φ) = Vm sin(ωt)
RCAω(cos(ωt) cos φ − sin φ sin(ωt))+
A sin(ωt) cos φ + A sin φ cos(ωt) = Vm sin(ωt)
..
.
Vm
A= p
1 + (ωCR)2
φ = − tan−1 (ωCR)
Vm
∴ vcP = p
sin(ωt − tan−1 (ωCR))
2
1 + (ωCR)
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Total solution:
Vm
vc (t) = Ae (−t/RC ) + p
sin(ωt − tan−1 (ωCR))
1 + (ωCR)2
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To find constant: Use an initial condition.
First part is the transient response. Second part is the steady state
response.
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Transient response is also called as natural response.
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It is always decaying exponential irrespective of DC or AC
excitation.
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Forced ( steady state) response is now sinusoid with a phase
shift because excitation is sinusoid.
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If you give a sinusoid of one frequency to a linear circuit, the
stead state response will be sinusoid of the same frequency
with or without phase shift.
Since the transient response dies out, we are interested in
sinusoidal steady state response.
vc (t) = VC sin(ωt + φ)
Vm
and φ = − tan−1 (ωCR).
where VC = p
1 + (ωCR)2
v
Vm
VC
vc
vi
φ
Figure: Sinusoidal steady state response - RC Circuit
ωt
Simple Approach
Let us a simple approach to find the sinusoidal steady state
response1 of a linear circuit.
Vm sin(ωt)
A sin(ωt + φ)
Vm cos(ωt)
A cos(ωt + φ)
Since the circuit is linear,
Vm sin(ωt)
Vm cos(ωt) + Vm sin(ωt)
A sin(ωt + φ)
A cos(ωt + φ) + A sin(ωt + φ)
By Euler’s identity,
Vm e ωt
Ae (ωt+φ)
If your input is sine, then take imaginary component of the
response.
1
Steady state response follows superposition
Let us consider the same circuit.
R
+
−
+
vi
vc
C
−
Let us apply vi = Vm e ωt (mathematically possible).
By KVL,
dvc
+ vc = Vm e ωt
RC
dt
If we apply this voltage, the steady state response vc would be
Ae (ωt+φ) .
RCAωe (ωt+φ) + Ae (ωt+φ) = Vm e ωt
(RCAω + 1)Ae φ = Vm
Ae φ =
Vm
(RCAω + 1)
Vm
−1
Ae φ = p
e (− tan (ωCR))
2
1 + (ωCR)
It can be written in polar form as follows.
Vm
∠ − tan−1 (ωCR)
A∠φ = p
1 + (ωCR)2
Since the excitation is sine, we have to take imaginary part of the
above complex response.
Vm
vc (t) = p
sin(ωt − tan−1 (ωCR))
2
1 + (ωCR)
If the excitation were cosine, we would take the real part of the
complex response.
Phasor Approach
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This is simpler than previous approach.
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We found e ωt as redundant since the frequency of sinusoidal
steady state response is same as the supply frequency in a
linear circuit.
Vm e ωt
Vm e ωt e 0
VC e ωt+φ
VC e ωt e φ
Since e ωt appears in both terms, it can be removed.
Let us apply.
Vm e 0
VC e φ
Vm ∠0
Vm
VC ∠φ
VC
where Vm and VC are complex numbers and called as phasors.
Phasor is a complex number that contains magnitude and phase
angle information.
Resistor
In time domain,
v (t) = i(t)R
Let us apply complex current through resistor.
i(t) = Im e ωt
The voltage response will be
v (t) = Vm e (ωt+φ)
Vm e (ωt+φ) = Im e ωt R
Vm e φ = Im e 0 R
Vm ∠φ = Im ∠0 R
In frequency domain,
Vm = Im R
Since φ = 0, voltage and current are in phase.
v
Vm
Im
i v
Figure: Voltage and current waveforms of Resistor
Im
Vm
Figure: Phasor diagram of Resistor
ωt
Inductor
In time domain,
di
dt
Let us apply complex current through inductor.
v (t) = L
i(t) = Im e ωt
The voltage response will be
v (t) = Vm e (ωt+φ)
Vm e (ωt+φ) = ωLIm e ωt
Vm e φ = ωLIm e 0
Vm ∠φ = Im ∠0 ωL
In frequency domain,
Vm = ωLIm
Since φ = 90, voltage leads current by 90◦ in inductor.
v
v
Vm
Im
i
Figure: Voltage and current waveforms of Inductor
Vm
Im
Figure: Phasor diagram of Inductor
ωt
Capacitor
In time domain,
dv
dt
Let us apply complex voltage across capacitor.
i(t) = C
v (t) = Vm e ωt
The current will be
i(t) = Im e (ωt+φ)
Im e (ωt+φ) = ωCVm e ωt
Im e φ = ωCVm e 0
Im ∠φ = Vm ∠0 ωC
In frequency domain,
Vm =
Im
ωC
Since φ = 90, current leads voltage by 90◦ in inductor.
v
Vm
Im
i
v
Figure: Voltage and current waveforms of Capacitor
Im
Vm
Figure: Phasor diagram of Capacitor
ωt
1. In Resistor, there is no significant advantage in frequency
domain.
2. In Inductor and Capacitor, the voltage and current are related
by an algebraic equation in frequency domain. (This is great).
3. The ratio of complex voltage to complex current is
impedance (Z).It’s unit is Ω.
ZR = R,
ZL = ωL,
ZC =
1
ωC
In general,
Z = R + X
4. Admittance is the reciprocal of impedance and denoted by
Y. It’s unit is f.
YR =
1
,
R
YL =
1
,
ωL
In general,
Y = G + B
YC = ωC
RC Circuit - Frequency domain
R
+
−
+
Vm ∠0
VC
−
1
ωC
By voltage division,
1
Vm
ωC
VC = Vm ∠0
=p
∠ − tan−1 (ωCR)
2
1
1 + (ωCR)
R+
ωC
That’s it. How simple it is.
Vm
|VC | = p
,
1 + (ωCR)2
∠VC = − tan−1 (ωCR)
If you want, the response can be written in time domain.
RLC Circuit - Example
Find VI and draw the phasor diagram. Assume ω = 1 rad/sec.
1Ω
2H
−
+
VI
1 ∠0
1F
In Frequency domain,
1Ω
−
+
VI
1 ∠0
2Ω
1
Ω
1
By KVL, ( KVL and KCL are equally applicable in frequency
domain too.)
1
VI = IR + IωL + I
ωC
1
VI = (1 + 2 + ) × 1∠0

√
VI = 2∠45◦ V
IXL
VI
45◦
I
IXL − IXC
IR
IXC
Figure: Phasor diagram taking I as reference