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Your course, please
A. Science
B. Nanoscience
C. Theoretical Physics
Session ID: PY2P10EM Laptop: responseware.eu
AC Sources
โข Most present-day household and industrial power distribution systems operate with alternating current (ac).
โข Any appliance that you plug into a wall outlet uses ac.
โข An ac source is a device that supplies a sinusoidally varying voltage.
Sinusoids
A sinusoidal is a signal that has the form of the sine or cosine function, which is a time-varying excitation
It is a period function of time ๐ก with period ๐ =2๐
๐:
๐ฃ ๐ก + ๐๐ = ๐๐ cos ๐ ๐ก + ๐๐ + ๐ = ๐๐ cos ๐๐ก + ๐ + ๐๐๐
= ๐๐ cos ๐๐ก + ๐ + ๐๐2๐
๐= ๐๐cos ๐๐ก + ๐ + ๐2๐
= ๐๐cos(๐๐ก + ๐) = ๐ฃ(๐ก)
๐ฃ ๐ก = ๐๐ cos ๐๐ก + ๐Sinusoidal
the amplitude
angular frequency measured in radians/s and the cyclic frequency ๐ in Hz, ๐ = 2๐๐
the phase
Two sinusoids cos(๐ฅ + ๐/3) leads cos(๐ฅ) by ๐/3
cos(๐ฅ โ ๐/3) lags cos ๐ฅ by ๐/3
๐ฃ1 = cos ๐ฅ +๐
3, ๐1 =
๐
3
๐ฃ2 = cos ๐ฅ, ๐2 = 0
๐ฃ1 leads ๐ฃ2 by
ฮ๐ = ๐1 โ ๐2 =๐
3
Phase angleA sinusoid can be expressed in either sine or cosine form. When comparing two sinusoids, it is expedient to express both as either sine or cosine with positive amplitudes.
โข ฮ๐ > 0, ๐ฃ1 leads ๐ฃ2 by ฮ๐โข ฮ๐ = 0, ๐ฃ1 and ๐ฃ2 are in phase
โข ฮ๐ < 0, ๐ฃ1 lags ๐ฃ2 by ฮ๐
ฮ๐ = ๐1 โ ๐2
Phase angle
between two
signals:
๐ฃ1 = ๐๐1 cos(๐๐ก + ๐1)
๐ฃ2 = ๐๐2 cos(๐๐ก + ๐2)
Positive
Use cosine
โcos ๐ฅ = cos(๐ฅ ยฑ ๐)
sin(๐ฅ) = cos(๐ฅ โ๐
2)
โ๐ < ๐ < ๐
Q1: The phase angle between,
๐ฃ1 = โ10 cos(๐๐ก +๐
3) and
๐ฃ2 = 12 sin ๐๐ก +๐
6
is: ๐
3โ
๐
6=
๐
6. This means ๐ฃ1 leads ๐ฃ2 by
๐
6.
A. True
B. False
Answer to Q1:
Use the same form:
๐ฃ1 = โ10 cos ๐๐ก +๐
3= 10 cos ๐๐ก +
๐
3โ ๐ = 10 cos ๐๐ก โ
2๐
3
๐ฃ2 = 12 sin ๐๐ก +๐
6= 12 cos(๐๐ก +
๐
6โ๐
2) = 12 cos(๐๐ก โ
๐
3)
Compare:
ฮ๐ = โ2๐
3โ๐
3= โ
๐
3< 0
Therefore, ๐ฃ1 lags ๐ฃ2 by ๐
3.
Phasor
โข A phasor is a complex number that represents the amplitude (e.g. ๐๐) and phase (๐) of a sinusoid, ๐ฃ ๐ก = ๐๐ cos(๐๐ก + ๐).
โข The real part of a phasor represents the sinusoid signal ๐ฃ ๐ก .
โข Since we consider a single frequency, the phasor can be written as ๐ฝ = ๐๐๐
๐๐ , i.e. ๐๐๐๐ก is implicitly present.
Phasor : ๐๐๐๐ ๐๐ก+๐ = ๐๐ cos(๐๐ก + ๐) + ๐๐๐ sin(๐๐ก + ๐)
Exponential representation Rectangular representation
The sinusoid signal ๐ฃ(๐ก)
Q2: The phasor of ๐ฃ1 = โ10 cos(๐๐ก +๐
3) is
A. โ10๐๐๐
3
B. 10๐๐๐
3
C. 10๐โ๐๐
3
D. 10๐โ๐2๐
3
Q3: The ac sinusoid voltage ๐ฃ(๐ก) (๐ = 4 rads/s) that corresponds to a phasor ๐ฝ = 3V is
A. 3 V
B. cos(4๐ก + 3)V
C. 3 cos(4๐ก)V
D. 3 sin(4๐ก) V
E. 3 cos(4๐ก + ๐)V
Answer to Q2-Q3
Q2: To wirte the phasor for ๐ฃ1 = โ10 cos(๐๐ก +๐
3) , we first need to
convert it into the conventional form, i.e. a cosine with a positive amplitude,
๐ฃ1 = โ10 cos ๐๐ก +๐
3= 10 cos ๐๐ก +
๐
3โ ๐ = 10 cos(๐๐ก โ
2๐
3)
Therefore, ๐ฝ = 10๐โ2๐
3๐ = 10โ โ 120O
Q3: Note ๐ฝ = 3 = 3e๐โ 0, i.e. the amplitude is 3 V, the phase is 0, and ๐ = 4
Therefore๐ฃ ๐ก = 3 cos ๐๐ก + 0 = 3 cos(4๐ก) V
Phasor diagram
โข To represent sinusoidallyvarying voltages and currents, we define rotating vectors in the Argand plane called phasors.
โข Shown is a phasor diagramfor sinusoidal voltage and current with their initial phases ๐ and โ๐.
Time domain and phasor (frequency) domain
Time-domain representation is time dependent and always real, and its phasor (or frequency) domain counterpart is time-independent, generally complex. The phasor domain is for a constant ๐, i.e. we consider signals which have the same frequency. Circuit response depends on ๐. If we switch from one frequency to another, the circuit responses changes.
๐๐๐๐๐
Time-independent and complex
Phasor Time
Time-dependent and real
๐๐ cos(๐๐ก + ๐)
Derivative and integral in phasor domain
In phasor representation, the time derivative of a sinusoid becomes just multiplication by the constant ๐๐; integrating a
phasor corresponds to multiplication by 1
๐๐.
PhasorTime
๐ฃ ๐ก = ๐๐ cos(๐๐ก + ๐)๐๐ฃ ๐ก
๐๐ก= โ๐๐๐ sin ๐๐ก + ๐ = ๐๐๐ cos(๐๐ก + ๐ +
๐
2)
เถฑ๐ฃ ๐ก ๐๐ก =๐๐๐sin(๐๐ก + ๐) =
๐๐๐cos(๐๐ก + ๐ โ
๐
2)
๐ฝ = ๐๐๐๐๐
๐๐๐๐๐ ๐+
๐2 = (๐๐๐
๐๐)๐๐๐๐2 = ๐๐๐ฝ
๐๐๐๐๐ ๐โ
๐2 = ๐๐๐
๐๐๐โ๐
๐2
๐=
๐ฝ
๐๐
Try this: ๐ ๐๐๐๐ ๐๐ก+๐
๐๐ก
Q4: In an ac circuit, the voltage across a 4ฮฉresistor is ๐ฃ ๐ก = 4 cos(10๐ก + ๐/3), the phase of the current through the resistor is
A. 0
B. โ๐
3
C.๐
3
D. None of the above
Q5: For a resistor, its voltage and current are always in phase.
A. True
B. False
Resistor in an ac circuit
โข The resistance does not depend on the frequency of the ac source.
โข The voltage and current are related by Ohmโs law: ๐ฃ๐ (๐ก) = ๐๐ ๐ก ๐ and Ohmโs law holds in phasor domain.
โข Current and voltage are in phase.
PhasorTime
๐๐ ๐ก = ๐ผ๐ cos(๐๐ก + ๐)๐ฃ๐ ๐ก = ๐๐ ๐ก ๐ = ๐ผ๐๐ cos(๐๐ก + ๐)
๐ฐ๐น = ๐ผ๐๐๐๐
๐ฝ๐น = ๐ผ๐๐ ๐๐๐ = ๐ ๐ฐ๐น
Q6: For an inductor, its voltage and current are always in phase.
A. True
B. False
Inductor in an ac circuit
โข The inductance does not depend on the frequency of the ac source.
โข The voltage and current are related by :
๐ฃ๐ฟ(๐ก) = ๐ฟ๐๐๐ฟ ๐ก
๐๐ก.
โข Voltage leads current by ๐/2
PhasorTime
๐๐ฟ ๐ก = ๐ผ๐ cos(๐๐ก + ๐)
๐ฃ๐ฟ ๐ก = ๐ฟ๐๐๐ฟ ๐ก
๐๐ก= โ๐ฟ๐๐ผ๐ sin(๐๐ก + ๐)
๐ฐ๐ณ = ๐ผ๐๐๐๐
๐ฝ๐ณ = (๐๐๐ฟ)๐ฐ๐ณ
Q7: For a capacitor, its voltage and current are always in phase.
A. True
B. False
Capacitor in an ac circuit
โข The capacitance does not depend on the frequency of the ac source.
โข The voltage and current are related by :
๐๐ถ(๐ก) = ๐ถ๐๐ฃ๐ถ ๐ก
๐๐ก.
โข Current leads voltage by ๐/2.
PhasorTime
๐ฃ๐ ๐ก = ๐๐ cos(๐๐ก + ๐)
๐๐ถ ๐ก = ๐ถ๐๐ฃ๐ ๐ก
๐๐ก= โ๐ถ๐๐๐ sin(๐๐ก + ๐)
๐ฐ๐ช = ๐ผ๐๐๐๐
๐ฝ๐ช =๐ฐ๐ช๐๐๐ถ
Impedance and admittance
โข Impedance represents the opposition to the flow of sinusoidal current.
โข ๐ is generally a complex number (ฮฉ).
โข Admittance, ๐ = ๐/๐ is the inverse of impedance (S).
๐ฝ = ๐๐ฐ
๐ฝ = ๐ ๐ฐ๐ฝ = ๐๐๐ฟ ๐ฐ
๐ฝ =1
๐๐๐ถ๐ฐ
Phasor
Impedance
Continued on next page
โข ๐ < 0, capacitive/leading reactance, e.g. ๐ = โ1
๐๐ถ๐
โข ๐ > 0, inductive/lagging reactance, e.g. ๐ = ๐๐ฟ ๐
Impedance:
Admittance:
๐ = ๐ + ๐๐
๐ =๐
๐= ๐บ + ๐๐ต
Resistance Reactance
Conductance Susceptance
Capacitor Inductor
๐ โ 0 ๐ = โ1
๐๐ถ๐ โ โ
open circuit
๐ = ๐๐ฟ ๐ โ 0short circuit
๐ โ โ ๐ = โ1
๐๐ถ๐ โ 0
short circuit
๐ = ๐๐ฟ ๐ โ โopen circuit
Circuit response depends on the frequency