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The course web site is: http://users.encs.concordia.ca/~trueman/web_page_273.htm
ELEC273 Lecture Notes Set 10 Phasors and Impedance
Homework on complex numbers.
Final Exam: Friday December 15, 2017 from 9:00 to 12:00 (confirmed)
What is AC Circuit Analysis?
β’ AC circuit analysis uses complex numbers to find the forced response to a sinusoidal generator.
β’ AC circuit analysis is very similar to DC circuit analysis that we studied earlier in the course. DC Circuit Analysis AC Circuit Analysis
Constant voltage V and current I Sinusoidal voltage π£π£ π‘π‘ = π΄π΄ cos πππ‘π‘ + ππSinusoidal current ππ(π‘π‘) = π΅π΅ cos πππ‘π‘ + ππ
Real numbers for V and I Complex numbers called phasorsππ = π΄π΄ππππππ and πΌπΌ = π΅π΅ππππππ
Real number resistances R Complex number impedancesπππ π = π π , πππΏπΏ = ππππππ, and πππΆπΆ = 1
πππππΆπΆ
Real number node matrix or mesh matrix Complex number node matrix or mesh matrix
Given: β’ the frequency ππ Hz, hence ππ = 2ππππ rad/secβ’ The amplitude of the source, 10 voltsFind:β’ The amplitude π΄π΄ and the phase ππ of the output
voltage π£π£ π‘π‘ at βsteady stateβ after the transients have died out.
From Alexander and Sadiku.
10 cosπππ‘π‘+π£π£ π‘π‘ = π΄π΄ cos(πππ‘π‘ + ππ)β
π π 1π π 2πΆπΆ
ππ
PhasorGiven an AC voltage π£π£ π‘π‘ = π΄π΄ cos πππ‘π‘ + ππ , define the phasor representing voltage π£π£(π‘π‘) as the complex numberππ = π΄π΄ππππππ
where ππ = β1and ππ = 2ππππ is the frequency in radians per second. The magnitude of the phasor π΄π΄ππππππ is the amplitude of the cosine π΄π΄ cos πππ‘π‘ + ππ .The angle of the phasor, ππ, is the phase angle of the cosine π΄π΄ cos πππ‘π‘ + ππ .
It is sometimes convenient to visualize the phasor by drawing it on the complex plane.
The phasor ππ = π΄π΄ππππππ is an arrow of length A making an angle of ππto the real axis.
It can be quite useful to draw phasors on the complex plane!
We can use angle notation:ππ = π΄π΄β ππOr exponential notationππ = π΄π΄ππππππ
Imaginary
Real
πππ΄π΄
ππ
ππ = π΄π΄ππππππ
Rotating PhasorThe phasor representing AC voltage π£π£ π‘π‘ = π΄π΄ cos πππ‘π‘ + ππ is the complex number ππ = π΄π΄ππππππ .
It is sometimes convenient to visualize the relationship between two phasors by defining a rotating phasor as
π£π£ππ π‘π‘ = ππππππππππ
Henceπ£π£ππ π‘π‘ = π΄π΄ππππππππππππππ = π΄π΄ππππ(ππππ+ππ)
The AC voltage we started with is the real part of the rotating phasor.π£π£ π‘π‘ = Re π£π£ππ π‘π‘ = Re π΄π΄ππππ ππππ+ππ = Re π΄π΄cos πππ‘π‘ + ππ + jπ΄π΄sin πππ‘π‘ + ππ = π΄π΄cos(πππ‘π‘ + ππ)
Draw the rotating phasor on the complex plane at t=0.
At π‘π‘ = 0, π£π£ππ 0 = π΄π΄πππππππππππππ₯π₯π₯ = π΄π΄ππππππ
where the βcβ subscript on π£π£ππ π‘π‘ denotes a complex version of voltage π£π£ π‘π‘ .
Imaginary
Real
π΄π΄
ππ
π΄π΄ππππππAt π‘π‘ = 0π£π£ππ 0 = π΄π΄ππππππ
Draw the rotating phasor on the complex plane as time advances:
π΄π΄ππππ(ππππ+ππ)
Draw the rotating phasor at time π‘π‘ > 0
π£π£ππ π‘π‘ = π΄π΄ππππ(ππππ+ππ)
This amounts to adding angle πππ‘π‘ to the angle of phasor π΄π΄ππππππ.
As time increases, πππ‘π‘ increases. We add a full 2ππradians of angle in one AC period ππ = 1
ππ.
The phasor rotates by one full rotation for each T seconds of time.
Imaginary
Real
π΄π΄
ππ
For π‘π‘ > 0π£π£ππ π‘π‘ = π΄π΄ππππ(ππππ+ππ)
πππ‘π‘
πππ‘π‘ + ππ
Phasorβ’ The phasor representing sinusoidal voltage π£π£ π‘π‘ = π΄π΄ cos πππ‘π‘ + ππ is the complex number ππ = π΄π΄ππππππ
β’ The amplitude A of the cosine is the magnitude of the phasor: π΄π΄ = ππ = π΄π΄ππππππ
β’ The phase angle ππ of the cosine is the angle of the complex number βphasorβ.
β’ We can βrecoverβ the cosine from the phasor by multiplying the phasor by ππππππππ and then using Eulerβs Identity: ππππππ = cosππ + ππ sinππ
π£π£ π‘π‘ = Re ππππππππππ = Re π΄π΄ππππππππππππππ = Re π΄π΄ππππ(ππππ+ππ)
π£π£ π‘π‘ = Re π΄π΄ cos πππ‘π‘ + ππ + ππ sin πππ‘π‘ + πππ£π£ π‘π‘ = Re π΄π΄ cos πππ‘π‘ + ππ + ππ π΄π΄sin πππ‘π‘ + πππ£π£ π‘π‘ = π΄π΄ cos πππ‘π‘ + ππ
β’ To recover the cosine from the phasor:β’ The amplitude A of the cosine is the magnitude of the phasor V β’ The phase angle ππ of the cosine is the angle of the complex number
Examples of Phasors1.Find the phasor for π£π£1 π‘π‘ = 10 cos πππ‘π‘ + ππ
4Answer: ππ1 = 10ππππ
ππ4
2.Find the cosine represented by phasor ππ2 = 5ππβππππ6
Answer: π£π£2 π‘π‘ = 5 cos πππ‘π‘ β ππ6
3.Find the sum: π£π£3 π‘π‘ = π£π£1 π‘π‘ + π£π£2 π‘π‘Using trig identities is the hard way: π£π£3 π‘π‘ = 10 cos πππ‘π‘ +
ππ4
+ 5 cos πππ‘π‘ βππ6
Try it!The easy way: ππ3 = ππ1 + ππ2 = 10ππππ
ππ4 + 5ππβππ
ππ6 (see the next slide)
Evaluate the sum ππ3 = ππ1 + ππ2ππ3 = 10ππππ
ππ4 + 5ππβππ
ππ6
Showing ALL the steps:ππ3 = 10 cos
ππ4
+ ππππ sinππ4
+ 5 cosβππ6
+ πππ sinβππ6
ππ3 = 7.071 + πππ.071 + 4.330 β ππ2.5ππ3 = 11.401 + ππ4.571Magnitude 11.4012 + 4.5712 = 12.283Angle tanππ = 4.571
11.4π₯1= 0.4049 so ππ = 21.85 deg or 0.3814 rad
The phasor isππ3 = 12.283πππππ₯.3814
so the time function isπ£π£3 π‘π‘ = 12.283 cos πππ‘π‘ + 0.3814
Check the result by sketching the phasors on the complex plane:
ππ1 = 10ππππππ4 = 10 angle 45 degrees = 7.071+j7.071
ππ2 = 5ππβππππ6= 5 angle -30 degrees=4.330-j2.500
ππ3 = ππ1 + ππ2
ππ3 = 12.283ππππ21.85Β°= 12.283 angle 21.85 degrees
Drawing the phasors tip to tail in the complex plane shows that 12.283 angle 21.85 degrees is a reasonable answer.
ImIm
Re
Re
ππ1 = 10ππππππ4
ππ2 = 5ππβππππ6
45Β°
30Β°
10
5
ππ1 = 10ππππππ4
ππ2 = 5ππβππππ6
ππ3 = ππ1 + ππ2
Impedance: the ratio of voltage phasor to current phasor.
β’ Consider a component in a circuit driven by a sinusoidal generator cosπππ‘π‘
β’ The voltage π£π£ π‘π‘ = π΄π΄ cos πππ‘π‘ + ππ is represented by phasor ππ = π΄π΄ππππππ
β’ The current ππ π‘π‘ = π΅π΅ cos πππ‘π‘ + ππ is represented by phasor πΌπΌ = π΅π΅ππππππ
β’ The impedance is defined as
ππ = πππΌπΌ
ohms
+π£π£(π‘π‘)β
ππ(π‘π‘)+ππβ
πΌπΌ
The Impedance is a Complex Number
β’ ππ = π΄π΄ππππππ
β’ πΌπΌ = π΅π΅ππππππ
β’ The impedance is a complex number:
ππ =πππΌπΌ =
π΄π΄ππππππ
π΅π΅ππππππ=π΄π΄π΅π΅ ππ
ππ(ππβππ) =π΄π΄π΅π΅ cos ππ β ππ + ππ
π΄π΄π΅π΅ sin ππ β ππ = π π + ππππ
ππ = π π + ππππ
R=the resistance is the real part of the impedanceX=the reactance is the imaginary part of the impedance.
+ππβ
πΌπΌ
Admittance: the reciprocal of the impedanceThe admittance is the ratio of the current phasor to the voltage phasor:ππ = πΌπΌ
ππSiemens
Note thatππ = 1
ππ
The admittance is a complex number:ππ = πΊπΊ + πππ΅π΅
G=the conductance is the real part of the admittanceB=the susceptance is the imaginary part of the admittance.
What is the impedance of a resistor?
π£π£ π‘π‘ = π π ππ π‘π‘π£π£ π‘π‘ = π΄π΄ cos πππ‘π‘ + ππ volts then the current is
ππ π‘π‘ =π£π£(π‘π‘)π π =
π΄π΄π π cos πππ‘π‘ + ππ
with amplitude π΄π΄π π
and phase ππThe voltage phasor is ππ = π΄π΄ππππππ
The current phasor is πΌπΌ = π΄π΄π π ππππππ; magnitude π΄π΄
π π and angle ππ
The impedance is ππ = πππΌπΌ
= π΄π΄πππππππ΄π΄π π ππ
ππππ= π π ohms.
The admittance is ππ = 1ππ
= 1π π
= πΊπΊ Siemens
Time domain Frequency Domain also called the βphasor domainβ
+π£π£(π‘π‘)β
ππ(π‘π‘) +ππβ
πΌπΌ
ππ = π π π π
What is the impedance of an inductor?π£π£ π‘π‘ = ππ
πππππ‘π‘ππ π‘π‘
If π£π£ π‘π‘ = π΄π΄ cos πππ‘π‘ + ππ volts then we can find the current usingππππππππ
= 1πΏπΏπ£π£ π‘π‘ so ππππ = 1
πΏπΏπ£π£ π‘π‘ πππ‘π‘ and by integrating both sides
ππ =1πποΏ½π£π£ π‘π‘ πππ‘π‘ =
1πποΏ½π΄π΄ cos(πππ‘π‘ + ππ)πππ‘π‘ =
π΄π΄ππππ
sin(πππ‘π‘ + ππ)
To write the phasor we need to change sine to cosine.Use the trig identity:sinπΌπΌ = coπ π πΌπΌ β
ππ2
With Ξ± = πππ‘π‘ + ππ, we have
ππ =π΄π΄ππππ sin(πππ‘π‘ + ππ) =
π΄π΄ππππ cos(πππ‘π‘ + ππ β
ππ2)
Which has amplitude π΄π΄πππΏπΏ
and phase ππ β ππ2
, so the current phasor is
πΌπΌ =π΄π΄ππππ ππ
ππ ππβππ2
The impedance is
ππ =πππΌπΌ =
π΄π΄ππππππ
π΄π΄ππππ ππ
ππ(ππβππ2)= ππππππππ
ππ2
Time domain
Frequency domain
+π£π£(π‘π‘)β
ππ(π‘π‘)
+ππβ
πΌπΌ
ππ = ππππππ
ππ
Impedance of an inductor, continued;ππ = ππππππππ
ππ2
We can see from the Argand diagram that ππππππ2 = ππ.
Another way to prove this is with Eulerβs Identity:ππππ
ππ2 = cos
ππ2
+ ππ sinππ2
= 0 + πππ = ππ
Hence the impedance of an inductor is
ππ = ππππππ ohms
The admittance of an inductor is
ππ = 1ππ
= 1πππππΏπΏ
SiemensArgand diagram
Re
Im
1 ππ2
ππ = 1ππππππ2
Time domain
Frequency domain
+ππβ
πΌπΌ
ππ = ππππππ
+π£π£(π‘π‘)β
ππ(π‘π‘)
ππ
What is the impedance of a capacitor?ππ(π‘π‘) = πΆπΆ
πππππ‘π‘π£π£ π‘π‘
If π£π£ π‘π‘ = π΄π΄ cos πππ‘π‘ + ππ volts then we can calculate the current as
ππ = πΆπΆπππππ‘π‘π΄π΄ cos(πππ‘π‘ + ππ) = βΟπΆπΆπ΄π΄ sin(πππ‘π‘ + ππ)
To write the phasor we need to change sine to cosine using the trig identity:sinπΌπΌ = βcoπ π πΌπΌ + ππ
2With Ξ± = πππ‘π‘ + ππ, we haveππ = βΟπΆπΆπ΄π΄ sin(πππ‘π‘ + ππ) = ΟπΆπΆAcos(πππ‘π‘ + ππ +
ππ2)
which has amplitude ΟπΆπΆπ΄π΄ and phase ππ + ππ2
, so the current phasor is
πΌπΌ = ΟπΆπΆπ΄π΄ππππ ππ+ππ2
The impedance is
ππ =πππΌπΌ =
π΄π΄ππππππ
ΟπΆπΆπ΄π΄ππππ ππ+ππ2=
1
ΟπΆπΆππππππ2
Since ππππππ2 = ππ we can write the impedance of a capacitor as
ππ = 1πππππΆπΆ
ohms.
The admittance of a capacitor is ππ = 1ππ
= πππππΆπΆ Siemens
Frequency domain
+ππβ
πΌπΌ
ππ =1
πππππΆπΆ
Time domain
+π£π£(π‘π‘)β
ππ(π‘π‘)
πΆπΆ
Component Time Domain Frequency Domain
Resistor
Inductor
Capacitor
Impedance
+π£π£(π‘π‘)β
ππ(π‘π‘)
π π
+π£π£(π‘π‘)β
ππ(π‘π‘)
πΆπΆ
+π£π£(π‘π‘)β
ππ(π‘π‘)
ππ
π£π£ = π π ππ
π£π£ = πππππππππ‘π‘
ππ = πΆπΆπππ£π£πππ‘π‘
ππ = π π πΌπΌ
ππ = πππππππΌπΌ
ππ =1πππππΆπΆ
πΌπΌ
Combining ImpedancesImpedances in series:
ππ = ππ1 + ππ2 = ππ1πΌπΌ + ππ2πΌπΌ=(ππ1+ππ2)πΌπΌSo the equivalent impedance is
ππ =πππΌπΌ
=(ππ1+ππ2)πΌπΌ
πΌπΌ= ππ1 + ππ2
Series impedances add.
Impedances in parallel:
πΌπΌ = πΌπΌ1 + πΌπΌ2 =ππππ1
+ππππ2
=1ππ1
+1ππ2
ππ
The equivalent impedance is
ππ =πππΌπΌ =
ππ1ππ1
+ 1ππ2
ππ=
ππ1ππ2ππ1 + ππ2
Parallel impedances combine using product over sum.
+
ππ
β
+ππ2β
πΌπΌ
+ππ1β
πΌπΌ2πΌπΌ1
+
ππ
β
πΌπΌ
ππ2ππ1
ππ2
ππ1
Combining Admittances: πΌπΌ = ππππ so ππ = πΌπΌππ
Admittances in series:
ππ = ππ1 + ππ2 =πΌπΌππ1
+πΌπΌππ2
=1ππ1
+1ππ2
πΌπΌ
So the equivalent admittance is
ππ =πΌπΌππ
=I
πΌπΌππ1
+ πΌπΌππ2
πΌπΌ=
ππ1ππ2ππ1 + ππ2
Series admittances combine like parallel resistors: product over sum.
Admittances in parallel:πΌπΌ = πΌπΌ1 + πΌπΌ2 = ππ1ππ + ππ2ππ = (ππ1 + ππ2)ππThe equivalent admittance is
ππ =πΌπΌππ
=(ππ1 + ππ2)ππ
ππ= ππ1 + ππ2
Parallel admittances add.
+
ππ
β
+ππ2β
πΌπΌ
+ππ1β
πΌπΌ2πΌπΌ1
+
ππ
β
πΌπΌ
ππ2ππ1
ππ2
ππ1
Series-Parallel Combinations Find the input impedance ππππππ at an operating frequency of 60 Hz.The radian frequency is ππ = 2ππππ = 2ππππππ = π2πππ = 376.99 rad.sec
Change the components to impedances:
πππΏπΏ1 = ππππππ1 = πππππ2πππππ2.ππππ10β3 = πππ
πππ π 1 = π π 1 = 2
πππ π 2 = π π 2 = 2
πππ π 3 = π π 3 = 3
πππΏπΏ2 = ππππππ2 = πππππ2ππππππ.3πππ10β3 = ππ2
πππΆπΆ =1πππππΆπΆ =
1ππππ120ππππ1326ππ10β6 =
2ππ =
2ππβππβππ = βππ2
Method:1.Change the components into impedances.2.Combine impedances in series and in parallel.
ππππππ
π π 1 = 2 Ξ©
π π 2 = 2 Ξ© π π 3 = 3 Ξ©ππ1 = 2.65 mH
ππ2 = 5.30 mH
πΆπΆ = 1,326 Β΅F
π π 1 = 2 Ξ©
π π 2 = 2 Ξ©
π π 3 = 3 Ξ©ππ1 = 2.65 mH
ππ2 = 5.30 mH
πΆπΆ = 1,326 Β΅F
Find the input impedance, continued:
ππ1is j2 ohms in parallel with 3 ohms:
ππ1 =ππ2ππ3ππ2 + 3 =
πππ3 + ππ2
3 β 2ππ3 β ππ2 =
ππππ + 123ππ3 + 2ππ2 =
12 + ππππ13
ππ2is -j2 ohms in parallel with 2 ohms:
ππ2 =βππ2ππ2βππ2 + 2 =
βπππ2 β ππ2
2 + 2ππ2 + ππ2 =
βπππ + 82ππ2 + 2ππ2 =
8 β πππ8 = 1 β ππ
ππππππ2 Ξ©
2 Ξ© 3 Ξ©ππ2 Ξ©j1 Ξ©
βππ2 Ξ©
ππ1 ππ2 Ξ© 3 Ξ©
ππ2
2 Ξ©
βππ2 Ξ©
ππππππ2 Ξ©
j1 Ξ© ππ1ππ2
Combine ππ1 and ππ2 in series:
ππ3 = ππ1 + ππ2ππππππ2 Ξ©
j1 Ξ© ππ1ππ2 ππ1
ππ2
ππ1 =12 + ππππ
13ππ2 = 1 β ππ
ππ3 = ππ1 + ππ2 =12 + ππππ
13 + (1 β ππ) =12 + ππππ
13 +13 β πππ3
13ππ3 =
25 + ππ513
Find the input impedance, continued:Define ππ4as j1 ohms in parallel with ππ3:
ππ4 =ππππ 25 + πππ
13ππ + 25 + πππ
13
=β5 + ππ2π25 + ππππ
25 β ππππ25 β ππππ
=β125 + ππππ + πππ2π + 450
2πππ2π + ππππππ=
325 β πππππ949
ππππππ is 2 ohms in series with ππ4:
ππππππ = 2 + ππ4 = 2 +325 β πππππ
949 =1989 + 325 β πππππ
949=
2223 β πππππ949
ππππππ = 2.342 β πππ.7534 Ξ©
This is our final answer!
ππ3ππππππ2 Ξ©
j1 Ξ©
ππ4ππππππ2 Ξ©
Spice with a Sinusoidal Generator
To calculate the input impedance with LTSpice:β’ Drive the circuit with a 1 volt generator at 60 Hzβ’ Find the current in π π 1 flowing into the circuit πΌπΌππππβ’ Calculate the input impedance as
ππππππ = 1πΌπΌππππ
Construct the circuit.
How do we specify a sinusoidal excitation?
ππππππ2 Ξ©
2 Ξ© 3 Ξ©ππ2 Ξ©j1 Ξ©
βππ2 Ξ©
To specify that the generator is an AC source, right-click on the generator circle to pop up a menu:
Specify AC 1, meaning that the generator is a sinusoidal or βACβ source of amplitude 1 volt.
Choose AC Analysis and give the frequency: Click on Stimulate and then Edit Stimulation Card.
Choose AC Analysis:
β’ Specify the number of points per octave as 1.β’ Specify both the starting and the stopping frequency as 60 Hz (we only want one frequency).
We are ready to run the simulation:
The system adds the βdot commandβ .ac oct 1 60 60.Click on the βrunβ button to solve the circuit.
Spice calculates the amplitude and phase of the voltages and currents:
The current in R1 is reported as I(R1)=0.4064 angle -17.8 degrees
Find the input impedance with LTSpice:
The input current is the current in π π 1, soπΌπΌππππ =I(R1)=0.4064 angle -17.8 degrees
ππππππ =1πΌπΌππππ
=1
0.4064β β 17.8Β° = 2.342 + πππ.7522
which agrees with our calculation.
We calculatedππππππ = 2.342 β πππ.7534
To calculate the input impedance with LTSpice:β’ Drive the circuit with a 1 volt generator at 60 Hzβ’ Find the current in π π 1 flowing into the circuit πΌπΌππππβ’ Calculate the input impedance as
ππππππ = 1πΌπΌππππ
ππππππ2 Ξ©
2 Ξ© 3 Ξ©ππ2 Ξ©j1 Ξ©
βππ2 Ξ©
The course web site is: http://users.encs.concordia.ca/~trueman/web_page_273.htm
ELEC273 Lecture Notes Set 10 Phasors and Impedance
Homework on complex numbers.
Final Exam: Friday December 15, 2017 from 9:00 to 12:00 (confirmed)
Input Impedance Example (from a final exam)
Find the value of capacitance C such that the input impedance ππππππ is real. The frequency is ππ = 4 rad/sec.Solution
Convert the circuit elements to impedances at ππ = 4 rad/sec : πππΏπΏ = ππππππ and πππΆπΆ = 1πππππΆπΆ
.
ππππππ is 1 ohm in parallel with ππ2ππππππ=1||ππ2 = ππ2
1+ππ2If ππ2 is real then ππππππ is real.So choose C to make ππ2 real.
Define ππ2 by omitting the 1-ohm resistor:
1 Ξ©1 Ξ©
C1 H
1 H
ππππππ
ππππππ 1 Ξ©πππ Ξ©
πππ Ξ©1 Ξ©
1ππππΆπΆ
ππ2πππ Ξ©
1ππππΆπΆ
1 Ξ©πππ Ξ©
4114
141441
12
jCj
jY
jZjZ
++
+=+=+=
Choose the value of C to make ππ2 real.
1)41(44142 ++
++=
jCjjjZ
CjCjjZ
4)161(4142 +β
++=
CjCjCjCjZ
4)161(41)4)161((4
2 +β+++β
=
Define ππ1 to be 1ππ4πΆπΆ
in parallel with 1 + πππ .
Since parallel admittances add,
ππ1 = ππππΆπΆ +1
1 + πππThe input impedance ππ2 is j4 in series with ππ1
ππ2πππ Ξ©
1ππππΆπΆ
1 Ξ©πππ Ξ©
ππ2πππ Ξ©
ππ1
CjCCjCZ
4)161()4644()161(
2 +β+β+β
=
ββββ
+ββ+β
=CjCCjC
CjCCjCZ
4)161(4)161(
4)161()648()161(
2
22
2
2 16)161())161(4)161)(648((4)648()161(
CCCCCCjCCCZ
+βββββ+β+β
=
To make ππ2 real, set the imaginary part to zero:
0))161(4)161)(648(( =ββββ CCCC
0)4648)(161( =βββ CCCThere is a common factor of (1 β 16C) so factor it out:
0)688)(161( =ββ CC
0)161( =β C 0)688( =β C1176.0
172
688
===C0625.0161==C F
and
F, and
Solution of AC Circuits Using Phasors
Change to phasors and impedance.
To solve an AC circuit:1.Find the impedance of each component in the circuit.2.Draw the circuit with phasors and impedances.3.Use mesh analysis or node analysis to solve the circuit, using complex arithmetic.4.The amplitude of the voltage is the magnitude of the phasor and the phase angle of the voltage is the angle of the phasor.
Find the amplitude and phase of the voltage across the resistor, π£π£π π . The frequency is 400 Hz.
Find the impedances:ππ = 2ππππ = 2πππππππ = 2,513 radians/secπππ π = π π = 5 ohmsπππΏπΏ = ππππππ = ππππ2ππ3πππ.ππππππ10β3 = ππ2 ohms
+π£π£π π β
π π = 5 Ξ©
ππ = 07958 mH
10 cosπππ‘π‘+πππ π β
5 Ξ©
ππ2 Ξ©
10
Solve the circuit using complex arithmetic:
Node analysis: 10 β πππ π ππ2 β
πππ π 5 = 0
Multiply by j10:50 β 5πππ π β ππ2πππ π = 05 + ππ2 πππ π = 50
πππ π =50
5 + ππ25 β ππ25 β ππ2 =
450 β πππππ29 = 8.621 β ππ3.448
Magnitude 8.6212 + (β3.448)2=9.284Angle tanβ1 β3.448
8.621=-21.8 degrees
Henceπππ π = 9.284β β 21.8Β°
Mesh analysis: 10 β ππ2πΌπΌ β ππΌπΌ = 0πΌπΌ = 1π₯
5+ππ25βππ25βππ2
= 5π₯βππ2π₯29
= 1.724 β πππ.6896 amps
πππ π = ππΌπΌ = 5 1.724 β j0.6896 = 8.621 β j3.448Same as by node analysis!πππ π = 9.284β β 21.8Β° volts
Amplitude = πππ π =9.284 volts
Phase = the angle of πππ π which is -21.8 degrees.
π£π£π π π‘π‘ = 9.284 cos πππ‘π‘ β 21.8Β°
+πππ π β
5 Ξ©
ππ2 Ξ©
10+πππ π β
5 Ξ©
ππ2 Ξ©
10
10 πππ π
πΌπΌ
Example: Solve a Circuit Using Phasors
β’ Solve this circuit to find the amplitude and phase of the load voltage ππ at ππ =2 GHz.β’ The generator in this circuit is 10 cosπππ‘π‘ volts at frequency ππ =2 GHz so the radian frequency is ππ =
2ππππ = 1.2πππππ101π₯ radians/second.
To solve an AC circuit:1.Find the impedance of each component in the circuit.2.Draw the circuit with phasors and impedances.3.Use mesh analysis or node analysis to solve the circuit, using complex arithmetic.4.The amplitude of the voltage is the magnitude of the phasor and the phase angle of the voltage is the angle of the phasor.
73 Ξ©
2.1 nH
11.8 pF
50 Ξ©
10 cosπππ‘π‘+ππβ
73 Ξ©
2.1 nH
11.8 pF
50 Ξ©
10 cosπππ‘π‘+ππβ 73 Ξ©
ππ2π.39 Ξ©
βπππ.744 Ξ©
50 Ξ©
10β πΒ°+ππβ
10β πΒ°+ππβ
50 + ππ2π.39 Ξ©
0.6178 β πππ.687 Ξ©
Homework: solve this circuit yourself without looking at the lecture notes.
10β πΒ°+ππβ
50 + ππ2π.39 Ξ©
0.6178 β πππ.687 Ξ©
πΌπΌ πΌπΌ
Power into a Resistor in AC Circuits
If a circuit is driven by an A.C. voltage, then how much power flows?π£π£ π‘π‘ = ππππ cos πππ‘π‘ + ππππ π‘π‘ = πΌπΌππ cos πππ‘π‘ + ππ
Instantaneous power: ππ π‘π‘ = π£π£ π‘π‘ ππ(π‘π‘)
Average power: ππππππ = 1ππ β«π₯
ππ ππ π‘π‘ πππ‘π‘
Is there a convenient way to calculate the power directly from the phasors V and I?
π£π£ π‘π‘ = ππππ cos πππ‘π‘ + ππ
ππ(π‘π‘) = πΌπΌππ cos πππ‘π‘ + ππ
ππ(π‘π‘)+π£π£(π‘π‘)β
Phasor ππ = ππππβ ππ
Phasor πΌπΌ = πΌπΌππβ ππ
πΌπΌ+ππβ
Instantaneous and Average Power Delivered to a Resistor
The AC voltage π£π£ π‘π‘ = ππππ cos πππ‘π‘ is connected across a resistor R.Find the instantaneous power and the average power delivered to the resistor.
The instantaneous power is defined asππ π‘π‘ = π£π£ π‘π‘ ππ π‘π‘
π£π£ π‘π‘ = ππππ cos πππ‘π‘
ππ π‘π‘ =πππππ π cos πππ‘π‘
The instantaneous power is:
ππ π‘π‘ = ππππ cos πππ‘π‘πππππ π cos πππ‘π‘ =
ππππ2
π π cos2 πππ‘π‘
Trig identity: cos2ππ = 12
1 + cos(2ππ)
ππ(π‘π‘) =ππππ2
2π π 1 + cos2πππ‘π‘
π£π£ π‘π‘ = ππππ cos πππ‘π‘ ππ(π‘π‘) = πΌπΌππ cos πππ‘π‘π π
Time t
Instantaneous Power ππ(π‘π‘)ππππ2
π π
ππππ2
2π π
ππ2
ππ2
Average Power Delivered to a Resistor
The instantaneous power is: ππ π‘π‘ = π£π£ π‘π‘ ππ π‘π‘ = ππππ cos πππ‘π‘ πππππ π
cos πππ‘π‘ = ππππ2
π π cos2 πππ‘π‘ = 1
2ππππ2
π π 1 + cos2πππ‘π‘
The average power delivered to the resistor is defined as the average of p(t) over one AC cycle of length T:
ππππππ =1πποΏ½π₯
ππππ π‘π‘ πππ‘π‘
so
ππππππ =1πποΏ½π₯
ππ ππππ2
2π π 1 + cos2πππ‘π‘ πππ‘π‘ =1πποΏ½π₯
ππ ππππ2
2π π πππ‘π‘ +1πποΏ½π₯
ππ ππππ2
2π π cos2πππ‘π‘ πππ‘π‘ =ππππ2
2π π + 0 =ππππ2
2π π
ππππππ = ππππ2
2π π where ππππ is the amplitude of the AC voltage.
This is often written as
ππππππ =ππππ2
2π π
=ππππ2
2
π π = πππ π π π π π
2
π π where πππ π π π π π = ππππ
2is the βRMS valueβ of the AC voltage.
π£π£ π‘π‘ = ππππ cos πππ‘π‘
ππ π‘π‘ =πππππ π
cos πππ‘π‘
What is meant by βRMS valueβ?
π£π£ π‘π‘ = ππππ cos πππ‘π‘ ππ(π‘π‘) = πΌπΌππ cos πππ‘π‘π π
Amplitude and RMS ValueSuppose an AC voltage is given byπ£π£ π‘π‘ = ππππ cos πππ‘π‘Then the amplitude of the voltage is ππππ.
The βroot mean squareβ value or βRMSβ value of a periodic function π£π£(π‘π‘) with period ππ is defined as
ππππππππ =1πποΏ½π₯
πππ£π£2 π‘π‘ πππ‘π‘
This is the average value or βmean valueβ of the square of the voltage function.
For a sinusoid π£π£ π‘π‘ = ππππ cos πππ‘π‘ we can evaluate the RMS value of π£π£(π‘π‘) as
ππππππππ2 = 1
ππ β«π₯ππ ππππ cos πππ‘π‘ 2πππ‘π‘ = ππππ2
ππ β«π₯ππ 12
1 + cos 2πππ‘π‘ πππ‘π‘=ππππ2
ππ β«π₯ππ 12πππ‘π‘ + ππππ2
ππ β«π₯ππ 12
cos 2πππ‘π‘ πππ‘π‘ = ππππ2
ππππ2
= ππππ2
2
ππππππππ =ππππ
2
Thus the average power delivered to a resistor is
ππππππ = ππππ2
2π π =
ππππ2
2
π π = πππ π π π π π
2
π π
Phasors Relative to RMS ValueWe can write phasors βrelative to amplitudeβ or we can write phasors βrelative to RMS valueβ.π£π£ π‘π‘ = ππππ cos πππ‘π‘
Phasors relative to amplitude: ππ = ππππππππππThe magnitude of the phasor ππ = ππππ is the amplitude of the cosine voltage ππππ cos πππ‘π‘
Phasors relative to RMS value:ππ = ππππππππππππππ
The magnitude of the phasor ππ = ππππππππ is the RMS value of the cosine voltage, so ππππ = 2ππππππππ and the cosine is π£π£ π‘π‘ = 2ππππππππ cos πππ‘π‘
Which should you use?
In the lecture notes I use phasors relative to amplitude.
However, sometimes phasors relative to RMS value is preferred.
Everybody knows that the AC line voltage is 110 volts.
Is this amplitude or RMS value?
Answer: in the power industry the custom is to use phasors relative to RMS value so 110 volts is the RMS value.
The amplitude is ππππ = 2ππππππππ = 1.ππππππππ = 155.5 volts.
Most βDVMsβ or βdigital voltmetersβ read RMS values on the AC voltage setting.
Power to an Impedance?
We will return to the topic of power in AC circuits later in the course.
The course web site is: http://users.encs.concordia.ca/~trueman/web_page_273.htm
ELEC273 Lecture Notes Set 10 Phasors and Impedance
Final Exam: Friday December 15, 2017 from 9:00 to 12:00 (confirmed)
Mesh Equations in the βPhasor Domainβ1. Convert all the components to impedances at the operating frequency. Convert the sources to phasors. 2. Assign mesh currents πΌπΌ1, πΌπΌ2, πΌπΌ3, β¦3. Write a KVL equation for each mesh path and an equation for each current source.
Example #1: really easy!!!!!
The operating frequency is 60 Hz.The sources areπ£π£1 π‘π‘ = 10 cosπππ‘π‘π£π£2 π‘π‘ = 16 sinπππ‘π‘Write mesh equations for this circuit.Solve the equations to find the values of the mesh currents.
+π£π£1(π‘π‘)β
+π£π£2(π‘π‘)β
2 Ξ©5 Ξ©
1 Ξ©
663.1 Β΅F
5.3 mH 10.6 mH
Step 1: Convert all the components to impedances at the operating frequency. Convert the sources to phasors.
The operating frequency is ππ =60 Hz. The radian frequency is ππ = 2ππππ = 2ππππππ = π2πππ = 376.99 β 377 radians/second. For the 5.3 mH inductance, πππΏπΏ = ππππππ = ππππ3πππππ.3ππ10β3 = πππ.9981 β ππ2 ohmsThe 10.6 mH inductance has an impedance of πππ ohms.The 663.1 microfarad capacitance has an impedance of πππΆπΆ = 1
πππππΆπΆ= 1
πππππΆπΆβππβππ
= βπππππΆπΆ
= βππ377π₯π₯663.1π₯π₯1π₯β6
= βπππ ohms.
Write the sources as phasors:π£π£1 π‘π‘ = 10 cosπππ‘π‘ becomes phasor ππ1 = 10β πΒ°π£π£2 π‘π‘ = 16 sinπππ‘π‘Change sine to cosine using the trig identity sinπππ‘π‘ = cos(πππ‘π‘ β 90Β°) so π£π£2 π‘π‘ = 16 sinπππ‘π‘ = 16cos(πππ‘π‘ β 90Β°) which becomes phasor ππ2 = 16β β 90Β°Since 1β β 90Β° = βππ we can write ππ2 = 16β β 90Β° = βππππ
+π£π£1(π‘π‘)β
+π£π£2(π‘π‘)β
2 Ξ©5 Ξ©
1 Ξ©
663.1 Β΅F
5.3 mH 10.6 mH ππ1 =10
ππ2 =16β β 90Β°= βππππ
2 Ξ©5 Ξ©
1 Ξ©
βj4 Ξ©
ππ2 Ξ© πππ Ξ©
ππ2=βππππ
ππ1 = 10
2 + j2 Ξ© 1 + j4 Ξ©
5 β j4 Ξ©
πΌπΌ1 πΌπΌ2
Mesh path FABEF:+10 β 2 + ππ2 πΌπΌ1 β 5 β πππ πΌπΌ1 β πΌπΌ2 = 0
Mesh path EBCDE:+ 5 β πππ πΌπΌ1 β πΌπΌ2 β 1 + πππ πΌπΌ2 β βππππ = 0
Collect terms:β2 β ππ2 β 5 + πππ πΌπΌ1 + 5 β πππ πΌπΌ2 = β10β7 + ππ2 πΌπΌ1 + 5 β πππ πΌπΌ2 = β10
5 β πππ πΌπΌ1 + β5 + πππ β 1 β πππ πΌπΌ2 = βπππ65 β πππ πΌπΌ1 + β6 πΌπΌ2 = βπππ6
Step 2: Assign mesh currents πΌπΌ1 and πΌπΌ2:
Step 3:Write a KVL equation for each mesh path and an equation for each current source.
Label the circuit diagram with the voltage across each impedance. Then it is easy to get the signs correct in the mesh equations:
ππ2= βππππππ1 = 10
2 + j2 Ξ© 1 + j4 Ξ©
5 β j4 Ξ©
πΌπΌ1 πΌπΌ2
10 βππππ
πΌπΌ1 πΌπΌ2
+ 2 + j2 πΌπΌ1 β + 1 + j4 πΌπΌ2 β
+5 β j4 (πΌπΌ1 β πΌπΌ2)β
A B C
DEF
Solve the equations: Solve the equations by hand:β7 + ππ2 πΌπΌ1 + 5 β πππ πΌπΌ2 = β105 β πππ πΌπΌ1 + β6 πΌπΌ2 = βπππ6
Eliminate πΌπΌ2:β7 + ππ25 β πππ
πΌπΌ1 + πΌπΌ2 =β10
5 β πππ5 β πππβ6 πΌπΌ1 + πΌπΌ2 =
βππππβ6
Evaluate the coefficients:(β1.049 β πππ.4390)πΌπΌ1 + πΌπΌ2 = (β1.220 β πππ.9756)(β0.8333 + πππ.6667)πΌπΌ1 + πΌπΌ2 = ππ2.667Subtract:(β0.2157 β ππ1.106)πΌπΌ1 = (β1.220 β ππ3.643)πΌπΌ1 = 3.380 β πππ.4438 = 3.πππβ β 7.5Β°
Evaluate πΌπΌ2: πΌπΌ2 = ππ2.667 β (β0.8333 + πππ.6667)πΌπΌ1πΌπΌ2 = 2.521+j0.0435=2.521 β π.0Β°
Homework: solve the equations by hand yourself!Use determinants and show that you get the same answer.
Example 2: Mesh Equations with a current source. 1. Convert all the components to impedances at the operating frequency. Convert the sources to phasors. 2. Assign mesh currents πΌπΌ1, πΌπΌ2, πΌπΌ3, β¦3. Write a KVL equation for each mesh path and an equation for each current source.
The operating frequency is 60 Hz.The sources areπ£π£1 π‘π‘ = 10 cosπππ‘π‘π£π£2 π‘π‘ = 2 sinπππ‘π‘Write mesh equations for this circuit.
Step 1: Convert the sources.π£π£ππ π‘π‘ = 10 cosπππ‘π‘ becomes phasor ππ1 = 10β πΒ°ππππ π‘π‘ = 2 sinπππ‘π‘ = 2 cos(πππ‘π‘ β 90Β°) becomes ππ2 = 2β β 90Β° = βππ2
10 cosπππ‘π‘
2 Ξ© 1 Ξ©
331.6 Β΅F
5.3 mH 10.6 mH
2 sinπππ‘π‘
Step 1, continued: Convert the components to impedances.
The operating frequency is ππ =60 Hz. The radian frequency is ππ = 2ππππ = 2ππππππ = π2πππ = 376.99 β 377radians/second. For the 5.3 mH inductance, πππΏπΏ = ππππππ = ππππ3πππππ.3ππ10β3 = πππ.9981 β ππ2 ohms
The 10.6 mH inductance has an impedance of πππ ohms.
The 331.6 microfarad capacitance has an impedance of πππΆπΆ = 1
πππππΆπΆ= 1
πππππΆπΆβππβππ
= βπππππΆπΆ
= βππ377π₯π₯331.6π₯π₯1π₯β6
= βπππ ohms.
Step 2: Assign mesh currents πΌπΌ1 and πΌπΌ2.
10 cosπππ‘π‘
2 Ξ© 1 Ξ©
331.6 Β΅F
5.3 mH 10.6 mH
2 sinπππ‘π‘ 10
2 Ξ© 1 Ξ© j4 Ξ©
-j8 Ξ©βππ2j2 Ξ©
2+j2 Ξ©
10 βππ2
1-j4 Ξ©
πΌπΌ1 πΌπΌ2
Step 3: Write a KVL equation for each mesh.Write a βconstraint equationβ for each current source.
Constraint equation for the current source: πΌπΌ2 β πΌπΌ1 = β2ππ
Supermesh Path ABCDEFA: +10 β 2 + ππ2 πΌπΌ1 β 1 β πππ πΌπΌ2 = 0
2 + ππ2 πΌπΌ1 + 1 β πππ πΌπΌ2 = 10
Hence the equations are:βπΌπΌ1 + πΌπΌ2 = β2ππ2 + ππ2 πΌπΌ1 + 1 β πππ πΌπΌ2 = 10
Homework:1.Solve the equations for πΌπΌ1 and πΌπΌ2πΌπΌ1 =5.02 amps, angle 40.0 degreesπΌπΌ2 =4.04 amps, angle 17.7 degrees
2.Verify the solution with LTSpice.
2+j2 Ξ©
10 βππ2
1-j4 Ξ©
πΌπΌ1 πΌπΌ2
A
B C D
EF
Mesh Equations in the βPhasor Domainβ1. Convert all the components to impedances at the operating frequency. Convert the sources to phasors. 2. Assign mesh currents πΌπΌ1, πΌπΌ2, πΌπΌ3, β¦3. Write a KVL equation for each mesh path. Write an equation for each current source.
Write mesh equations at frequency ππ = 100 rad/sec
The sources areπ£π£ππ π‘π‘ = 14.14cos(πππ‘π‘ + 45Β°)ππππ π‘π‘ = 2sin(πππ‘π‘)
Solve the mesh equations.
Verify your solution with LTSpice.
Example
SolutionConvert the components to impedances: ππ = 100 rad/sec.
ππ = 20 mH πππΏπΏ = ππππππ = πππππππ2πππ10β3 = ππ2πΆπΆ = 0.5 mF πππΆπΆ = 1
πππππΆπΆ= βππ
1π₯π₯π₯π₯π₯.5π₯π₯1π₯β3= βππ2π
8 Ξ© 20 mH 0.5 mF 8 j2 -j20 8-j18
30 mH
8 Ξ©
j3
33+j3
9 Ξ©50 mH
9j5
9+j5
Draw the circuit with phasors and impedances:8 Ξ© 20 mH 0.5 mF 8 j2 -j20 8-j18
30 mH
8 Ξ©
j3
33+j3
9 Ξ©50 mH
9j5
9+j5 Convert the sources to phasors: π£π£ππ π‘π‘ = 14.14cos(πππ‘π‘ + 45Β°) becomes phasor ππππ = 14.14β ππΒ°ππππ π‘π‘ = 2 sin πππ‘π‘ = 2cos(πππ‘π‘ β 90Β°) becomes phasor πΌπΌππ = 2β β 90Β° = βππ2
πΌπΌπ π = βππ2
ππππ =10 + ππ10
πΌπΌ3
9+j5
8-j18 -j9
3+j312
πΌπΌ1 πΌπΌ2
πΌπΌπ π = βππ2
ππππ =10 + ππ10
πΌπΌ3
9+j5
8-j18 -j9
3+j312
πΌπΌ1 πΌπΌ2
Write the mesh equations:
Constraint equation: πΌπΌ2 β πΌπΌ1 = βππ2
Mesh path ABCDEA: β12πΌπΌ1 β 8 β ππππ πΌπΌ1 β πΌπΌ3 β βπππ πΌπΌ2 β πΌπΌ3 β 3 + ππ3 πΌπΌ2 β 10 + ππππ = 0
Mesh path BFGDCB: β 9 + πππ πΌπΌ3 + βπππ πΌπΌ2 β πΌπΌ3 + 8 β ππππ πΌπΌ1 β πΌπΌ3 = 0
A
B DC
E
FG
= 10 + ππ10
Collect terms:Mesh path ABCDEA: β12πΌπΌ1 β 8 β ππππ πΌπΌ1 β πΌπΌ3 β βπππ πΌπΌ2 β πΌπΌ3 β 3 + ππ3 πΌπΌ2 β (10 + ππππ) = 0
β12 β 8 + ππππ πΌπΌ1 + πππ β 3 β ππ3 πΌπΌ2 + 8 β ππππ β πππ πΌπΌ3 = 10 + ππππ
β20 + ππππ πΌπΌ1 + β3 + ππ6 πΌπΌ2 + 8 β ππ27 πΌπΌ3 = 10 + ππππ
Mesh path BFGDCB: β 9 + πππ πΌπΌ3 β βπππ πΌπΌ3 β πΌπΌ2 β 8 β ππππ πΌπΌ3 β πΌπΌ1 = 0
8 β ππππ πΌπΌ1 β ππππΌπΌ2 + β9 β πππ + πππ β 8 + ππππ πΌπΌ3 = 0
8 β ππππ πΌπΌ1 β ππππΌπΌ2 + β17 + ππ22 πΌπΌ3 = 0
Current generator: πΌπΌ2 β πΌπΌ1 = βππ2
The mesh equations:
β20 + ππππ πΌπΌ1 + β3 + πππ πΌπΌ2 + 8 β ππ2π πΌπΌ3 = 10 + ππππ8 β ππππ πΌπΌ1 β ππππΌπΌ2 + β17 + ππ22 πΌπΌ3 = 0πΌπΌ2 β πΌπΌ1 = βππ2
Solve the mesh equations:
πΌπΌ1 = 0.5919β πππ.2Β°πΌπΌ2 = 1.884β β 107.2Β°πΌπΌ3 = 0.8574β β 172.0Β°
Node Equations in the βPhasor Domainβ1. Convert all the components to impedances at the operating frequency. Convert the sources to phasors. 2. Assign a datum node and node voltages ππ1, ππ2, ππ3, β¦3. Write a KCL equation for each node.
Example - really easy!
The operating frequency is 60 Hz.
π£π£1 π‘π‘ = 10 cosπππ‘π‘π£π£2 π‘π‘ = 16 sinπππ‘π‘
Write node equations for this circuit. Find the voltage across 5 ohms in series with 663.1 microfarads.
+π£π£1(π‘π‘)β
+π£π£2(π‘π‘)β
2 Ξ©5 Ξ©
1 Ξ©
663.1 Β΅F
5.3 mH 10.6 mH
Convert the circuit into the βphasor domainβ:
We converted this circuit to the phasor domain on a previous slide!β’ Sources:π£π£1 π‘π‘ = 10 cosπππ‘π‘ becomes phasor ππ1 = 10β πΒ°π£π£2 π‘π‘ = 16 sinπππ‘π‘ = 16cos(πππ‘π‘ β 90Β°) becomes phasor ππ2 = 16β β 90Β° = βππππβ’ The operating frequency is ππ =60 Hz. The radian frequency is
ππ = 2ππππ = 2ππππππ = π2πππ = 376.99 β 377 radians/second. β’ For the 5.3 mH inductance, πππΏπΏ = ππππππ = ππππ3πππππ.3ππ10β3 =
πππ.9981 β ππ2 ohmsβ’ The 10.6 mH inductance has an impedance of πππ ohms.β’ The 663.1 microfarad capacitance has an impedance of πππΆπΆ =
1πππππΆπΆ
= 1πππππΆπΆ
βππβππ
= βπππππΆπΆ
= βππ377π₯π₯663.1π₯π₯1π₯β6
= βπππ ohms.
+π£π£1(π‘π‘)β
+π£π£2(π‘π‘)β
2 Ξ©5 Ξ©
1 Ξ©
663.1 Β΅F
5.3 mH 10.6 mHππππ
2 Ξ©5 Ξ©
1 Ξ©
βj4 Ξ©
ππ2 Ξ© πππ Ξ©10
ππππ10
2 + j2 Ξ© 1 + j4 Ξ©
5 β j4 Ξ©
Choose a datum and a node voltage:
Write a KCL equation at the node:
10 β ππ12 + ππ2 β
ππ15 β πππ β
ππ1 β βππππ1 + πππ = 0
Collect terms:βππ1
2 + ππ2 βππ1
5 β πππ βππ1
1 + πππ =β10
2 + ππ2 +ππππ
1 + πππβ1
2 + ππ2 β1
5 β πππ β1
1 + πππ ππ1 =β10
2 + ππ2 +ππππ
1 + πππ
One way to solve this quation is to use a common denominator for all the terms:β 5 β πππ 1 + πππ β 2 + ππ2 1 + πππ β (2 + ππ2)(5 β πππ)
2 + ππ2 (5 β πππ)(1 + πππ) ππ1
=β10 1 + πππ + ππππ(2 + ππ2)
(2 + ππ2)(1 + πππ)A better way is to do the division term by term; see next page.
ππππ10
2 + j2 Ξ© 1 + j4 Ξ©
5 β j4 Ξ©
ππ1
Solve:β1
2 + ππ2β
15 β πππ
β1
1 + πππππ1 =
β102 + ππ2
+ππππ
1 + πππ
Change the coefficients from ratios to simple rectangular form using your calculator:
β0.25 + πππ.25 β 0.1220 β πππ.09756 β 0.05882 + πππ.2353 ππ1 = β2.5 + j2.5 + 3.765 + j0.9412
Do the addition of real parts and of imaginary parts:β0.4302 + πππ.3877 ππ1 = 1.265 + ππ3.441
Do the division:
ππ1 =1.265 + ππ3.441
β0.4302 + πππ.3877 = 2.355 β πππ.876 = 6.33πβ β 68.1Β°
As a working engineer, would I do this calculation by hand?Probably not! I could easily make a mistake, and hand calculation takes a lot of time. Iβd use a short computer program to solve the equation.
Node Equations with a current generator
Example #2
Procedure for writing node equations in the βfrequency domainβ:1. Convert all the components to impedances at the operating frequency. Convert the sources to phasors. 2. Assign a datum node and node voltages ππ1, ππ2, ππ3, β¦3. Write a KCL equation for each node.
The operating frequency is 60 Hz.The sources areπ£π£1 π‘π‘ = 10 cosπππ‘π‘π£π£2 π‘π‘ = 2 sinπππ‘π‘Write a node equation for the voltage across the current generator.
10 cosπππ‘π‘
2 Ξ© 1 Ξ©
331.6 Β΅F
5.3 mH 10.6 mH
2 sinπππ‘π‘
Step 1: Convert to the phasor domain.
We did this conversion for mesh analysis, see above!
Convert the sources.π£π£ππ π‘π‘ = 10 cosπππ‘π‘ becomes phasor ππ1 = 10β πΒ°ππππ π‘π‘ = 2 sinπππ‘π‘ = 2 cos(πππ‘π‘ β 90Β°) becomes ππ2 = 2β β 90Β° = β2ππ
The operating frequency is ππ =60 Hz. The radian frequency is ππ = 2ππππ = 2ππππππ = π2πππ = 376.99 β 377 radians/second.
For the 5.3 mH inductance, πππΏπΏ = ππππππ = ππππ3πππππ.3ππ10β3 = πππ.9981 β ππ2 ohms
The 10.6 mH inductance has an impedance of πππ ohms.
The 331.6 microfarad capacitance has an impedance of πππΆπΆ = 1
πππππΆπΆ= 1
πππππΆπΆβππβππ
= βπππππΆπΆ
= βππ377π₯π₯331.6π₯π₯1π₯β6
= βπππ ohms.
10 cosπππ‘π‘
2 Ξ© 1 Ξ©
331.6 Β΅F
5.3 mH 10.6 mH
2 sinπππ‘π‘ 10
2 Ξ© 1 Ξ© j4 Ξ©
-j8 Ξ©βππ2j2 Ξ©
2+j2 Ξ©
10 βππ2
1-j4 Ξ©ππ1
Write the node equation at ππ1:
10 β ππ12 + ππ2 + β2ππ β
ππ11 β πππ = 0
βππ12 + ππ2 β
ππ11 β πππ = 2ππ β
102 + ππ2
β12 + ππ2 β
11 β πππ ππ1 = 2ππ β
102 + ππ2
Homework: Solve the equation.Verify the solution with Spice.
Step 2: Assign a datum node and node voltages ππ1, ππ2, ππ3, β¦Step 3: Write a KCL equation for each node.
2+j2 Ξ©
10 βππ2
1-j4 Ξ©ππ1
Example: Node equations with a supernode.
The frequency is 1950 MHz. Write node equations for this circuit.Find the amplitude and phase of ππππ.
Remark: β’ A voltage generator embedded inside a node is called a βsupernodeβ. β’ Only one unknown node voltage is needed for a supernode.β’ We write a KCL equation for the closed surface that encloses the voltage generator.
2 cos πππ‘π‘5 cos πππ‘π‘ + 45Β°
81.63 pH 2 Ξ©
5 Ξ©16.32 pF
10 Ξ©
+ππππβ
2 cos πππ‘π‘5 cos πππ‘π‘ + 45Β°
81.63 pH 2 Ξ©
5 Ξ© 16.32 pF10 Ξ©
Convert to phasors and impedances:
Sources:ππππ π‘π‘ = 2 cosπππ‘π‘ is represented by phasor πΌπΌππ = 2π£π£ππ π‘π‘ = 5 cos(πππ‘π‘ + 45Β°) is represented by phasor ππππ = 5β ππΒ°
ππ = 1950 MHz so ππ = 2ππππ =1.22πππ101π₯ rad/sec
ππππππ = πππππ.22πππ101π₯πππ.ππ3ππ10β12 = πππohm1
πππππΆπΆ= βππ
1.225π₯π₯1π₯10π₯π₯16.32π₯π₯1π₯β12= βπππ ohms
25β 45Β°
πππ Ξ© 2 Ξ©
5 Ξ©
βπππ Ξ©10 Ξ©
+ππππβ
Combine impedances:
25β 45Β°
πππ Ξ© 2 Ξ©
5 Ξ©βπππ Ξ©10 Ξ©
2 10 Ξ©5β 45Β°
Series:πππ in series with 2 is equivalent to (2 + πππ) ohms
Parallel:5 in parallel with βπππ is equivalent toπππ βπππ
5 β πππ=βππ2π5 β πππ
= 2.5 β ππ2.5
2 + πππ Ξ©
2.5 β ππ2.5 Ξ©+ππππβ
Node equations:
2 10 Ξ©5β 45Β° 2 + πππ Ξ©
2.5 β ππ2.5 Ξ©
ππ1 ππ2 πππ₯
Constraint equation: ππ2 β ππ1 = 5β ππΒ°
At the supernode:
2 βππ110 β
ππ2 β πππ₯2 + πππ = 0
At the πππ₯ node: ππ2 β πππ₯2 + πππ β
πππ₯2.5 β ππ2.5 = 0
+ππππβ
Use three nodes:
For the supernode: ππ1 and ππ2.
For the output, use πππ₯.
There are three nodes so three equations to solve, in three unknowns.
Solve to find:ππππ = 5.772 β β 30.5Β°
Amplitude of πππ₯ is 5.772 voltsPhase of πππ₯ is -30.5 degrees.
Can we solve this circuit βsmarterβ?
2 10 Ξ©5β 45Β° 2 + πππ Ξ©
2.5 β ππ2.5 Ξ©
+ππππβ
ππ1Use only one unknown node voltage, ππ1.
Build the constraint equation into the supernode directly.
Once the value of ππ1 has been found, use the voltage-divider relationship to find πππ₯:
πππ₯ = 2.5 β ππ2.5ππ1 + 5β ππΒ°
2 + πππ + 2.5 β ππ2.5
We can solve the node equation to findππ1 = 5.884β β 44.2Β°And then the voltage divider to findπππ₯ = 5.772β β 44.2Β°
This agrees with the βdumberβ solution!
ππ1 + 5β ππΒ°
We only need one node equation to find node voltage, ππ1.At the supernode:
2 βππ110 β
ππ1 + 5β ππΒ°2 + πππ + 2.5 β ππ2.5 = 0
So
βππ110 β
ππ14.5 β πππ.5 = β2 +
5β ππΒ°4.5 β πππ.5
Node Equations in the βPhasor DomainβWe did this example using mesh analysis. Letβs do it by node analysis.
Convert the components to impedances: ππ = 100 rad/sec.
The frequency is ππ = 100 rad/sec.π£π£ππ π‘π‘ = 14.14cos(πππ‘π‘ + 45Β°)ππππ π‘π‘ = 2 sin πππ‘π‘ = 2cos(πππ‘π‘ β 90Β°)
Write a set of node equations in matrix form.
ππ = 20 mH πππΏπΏ = ππππππ = πππππππ2πππ10β3 = ππ2πΆπΆ = 0.5 mF πππΆπΆ = 1
πππππΆπΆ= βππ
1π₯π₯π₯π₯π₯.5π₯π₯1π₯β3= βππ2π
8 Ξ© 20 mH 0.5 mF 8 j2 -j20 8-j18
30 mH
8 Ξ©
j3
33+j3
9 Ξ©50 mH
9j5
9+j5
Draw the circuit with phasors and impedances:Convert the sources to phasors: π£π£ππ π‘π‘ = 14.14cos(πππ‘π‘ + 45Β°) becomes phasor ππππ = 14.14β ππΒ°ππππ π‘π‘ = 2 sin πππ‘π‘ = 2cos(πππ‘π‘ β 90Β°) becomes phasor πΌπΌππ = 2β β 90Β° = βππ2
Write Node EquationsNode 1:
βππ112
βππ1 β ππ28 β ππππ
βππ1 β ππ39 + πππ
= 0
Node 2:ππ1 β ππ28 β ππππ
+ (βππ2) βππ2 β ππ3βππ9
= 0
Node 3:ππ1 β ππ39 + πππ +
ππ2 β ππ3βπππ β
ππ3 β 14.14β 45Β°3 + ππ3 = 0
Collect terms in the node equations:Node 1: Multiply by 12(8 β ππππ)(9 + πππ)
βππ112
βππ1 β ππ28 β ππππ
βππ1 β ππ39 + πππ
= 0
β(8 β ππππ)(9 + πππ)ππ1 β 12(9 + πππ)(ππ1βππ2) β 12(8 β ππππ)(ππ1 β ππ3) = 0β(72 β ππππ2 + ππππ + 90)ππ1 β (108 + ππ60)(ππ1βππ2) β (96 β ππ216)(ππ1 β ππ3) = 0β(162 β πππ22)ππ1 β (108 + ππππ)(ππ1βππ2) β (96 β ππ2ππ)(ππ1 β ππ3) = 0β162 + πππ22 β 108 β ππππ β 96 + ππ2ππ ππ1 + 108 + ππππ ππ2 + (96 β ππ2ππ)ππ3 = 0β366 + ππ2ππ ππ1 + 108 + ππππ ππ2 + (96 β ππ2ππ)ππ3 = 0
Node 2:ππ1 β ππ28 β ππππ + (βππ2) β
ππ2 β ππ3βππ9 = 0
(βπππ)(ππ1 β ππ2) + (8 β ππππ)(βπππ)(βππ2) β (8 β ππππ)(ππ2 β ππ3) = 0(βπππ)(ππ1 β ππ2) + (βπππ2 β 162)(βππ2) β (8 β ππππ)(ππ2 β ππ3) = 0(βπππ)(ππ1 β ππ2) + (β144 + ππ32π) β (8 β ππππ)(ππ2 β ππ3) = 0βπππ ππ1 + (πππ β 8 + ππππ)ππ2+(8 β ππππ)ππ3 = β β144 + ππ32πβπππ ππ1 + (β8 + ππ27)ππ2+(8 β ππππ)ππ3 = β β144 + ππ32π
Arrange the equations into matrix form:Node 3:ππ1 β ππ39 + πππ
+ππ2 β ππ3βπππ
βππ3 β 14.1β 45Β°
3 + ππ3= 0
(βπππ)(3 + ππ3)(ππ1 β ππ3) + (9 + πππ)(3 + ππ3)(ππ2 β ππ3) β (9 + πππ)(βπππ)(ππ3 β 14.1β 45Β°) = 0(βππ27 + 27)(ππ1 β ππ3) + (27 + ππ15 + ππ2π β 15)(ππ2 β ππ3) β (βππππ + 45)(ππ3 β 14.1β 45Β°) = 0(βππ2π + 27)(ππ1 β ππ3) + (12 + ππ42)(ππ2 β ππ3) β (βππππ + 45)ππ3 = (βππππ + 45)(β14.πβ ππΒ°)(βππ2π + 27)ππ1 + (12 + πππ2)ππ2 + (ππ2π β 27 β 12 β πππ2 + ππππ β 45)ππ3 = (βππππ + 45)(β14.πβ ππΒ°)βππ2π + 27 ππ1 + 12 + πππ2 ππ2 + β84 + ππ66 ππ3 = β1256 + ππ3ππ.9
Hence the node equations are:β366 + ππ2ππ ππ1 + 108 + ππππ ππ2 + (96 β ππ2ππ)ππ3 = 0βπππ ππ1 + (β8 + ππ2π)ππ2+(8 β ππππ)ππ3 = β β144 + ππ32πβππ2π + 27 ππ1 + 12 + πππ2 ππ2 + β84 + ππππ ππ3 = β1256 + ππ3ππ.9
Arrange the node equations into a matrix equation:β366 + ππ2ππ 108 + ππππ (96 β ππ2ππ)
βπππ (β8 + ππ2π) (8 β ππππ)βππ2π + 27 12 + πππ2 β84 + ππππ
ππ1ππ2ππ3
=0
144 β ππ32πβ1256 + ππ3ππ.9
ππ1 = 7.091β β 19.8Β°ππ2 = 1.448β 168.9Β°ππ3 = 14.00β π2.0Β°