Lecture 1: AC Fundamental

Electrical Circuits
Part (2) : AC Circuits
Lecture 1
‫ باسم ممدوح الحلوانى‬.‫د‬
Circuit Analysis – Theories and Practice
Robinson &amp; Miller
Chapter (15): AC Fundamentals
Chapter (16): R, L, and C Elements and the Impedance Concept
Chapter (17): Power in AC Circuits
Chapter (18): AC Series-Parallel Circuits
Chapter (19): Methods of AC Analysis
Chapter (20): AC Network Theorems
Chapter (21): Resonance
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Course Web Page
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Chapter (15): AC Fundamentals
 Previously you learned that DC sources have ﬁxed polarities and constant
magnitudes and thus produce currents with constant value and unchanging
 In contrast, the voltages of ac sources alternate in polarity and vary in
magnitude and thus produce currents that vary in magnitude and alternate in
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Chapter (15): AC Fundamentals
 Sinusoidal ac Voltage
One complete variation is referred to as a cycle.
Starting at zero,
the voltage increases to a positive peak amplitude,
decreases to zero,
changes polarity,
increases to a negative peak amplitude,
then returns again to zero.
 Since the waveform repeats itself at regular intervals, it is called a periodic signal.
 Symbol for an ac Voltage Source
Lowercase letter e is used
to indicate that the voltage varies with time.
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Sinusoidal ac Current
 During the ﬁrst half-cycle, the
source voltage is positive
 Therefore, the current is in the
clockwise direction.
 During the second half-cycle, the
voltage polarity reverses
 Therefore, the current is in the
counterclockwise direction.
 Since current is proportional to voltage, its
shape is also sinusoidal
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Generating ac Voltages (Method A)
 One way to generate an ac voltage is to rotate a coil of wire at constant
angular velocity in a ﬁxed magnetic ﬁeld
 The magnitude of the resulting voltage is proportional to the rate at which ﬂux
lines are cut
 its polarity is dependent on the direction the coil sides move through the ﬁeld.
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Generating ac Voltages
 Since the coil rotates continuously, the voltage produced will be a repetitive,
Time Scales
 Often we need to scale the output voltage in time.
 The length of time required to generate one cycle depends on the
velocity of rotation.
600 revolutions in 1 minute = 600 rev / 60 s
= 10 revolutions in 1 second.
The time for 1 revolution = one-tenth of a second
= 100 ms
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Generating ac Voltages (Method B)
 AC waveforms may also be created electronically using function (or signal)
 With function generators, you are not limited to sinusoidal ac. gear.
 The unit of Figure can produce a variety of variable-frequency waveforms,
including sinusoidal, square wave, triangular, and so on.
 Waveforms such as these are commonly used to test electronic
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Instantaneous Value
 As the coil voltage changes from instant to instant. The value of voltage at any
point on the waveform is referred to as its instantaneous value.
 The voltage has a peak value of 40 volts
 The cycle time of 6 ms.
 at t = 0 ms, the voltage is zero.
 at t=0.5 ms, the voltage is 20V.
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Voltage and Current Conventions for ac
 First, we assign reference polarities for the source and a reference direction for
the current.
 We then use the convention that, when e has a positive value, its actual polarity is the
same as the reference polarity, and when e has a negative value, its actual polarity is
opposite to that of the reference.
 For current, we use the convention that
when i has a positive value, its actual
direction is the same as the reference
 and when i has a negative value, its actual
direction is opposite to that of the
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Voltage and Current Conventions for ac
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Attributes of Periodic Waveforms
 Periodic waveforms (i.e., waveforms that repeat at regular intervals), regardless
of their wave shape, may be described by a group of attributes such as:
 Frequency, Period, Amplitude, Peak value.
Frequency: The number of cycles per second of a waveform is defined
 Frequency is denoted by the lower-case letter f.
 In the SI system, its unit is the hertz (Hz, named in honor of pioneer researcher Heinrich
Hertz, 1857–1894).
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Attributes of Periodic Waveforms
Frequency Ranges:
 The range of frequencies is huge.
 Power line frequencies, for example, are 60 Hz in North America and 50 Hz
in many other parts of the world.
 Audible sound frequencies range from about 20 Hz to about 20 kHz.
 The standard AM radio band occupies from 550 kHz to 1.6 MHz
 The FM band extends from 88 MHz to 108 MHz.
 TV transmissions occupy several bands in the 54-MHz to 890-MHz range.
 Above 300 GHz are optical and X-ray frequencies.
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Attributes of Periodic Waveforms
 Period:
 The period, T, of a waveform, is the duration of one cycle.
 It is the inverse of frequency.
 The period of a waveform can be measured between any two corresponding
points ( Often it is measured between zero points because they are easy to
establish on an oscilloscope trace).
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Attributes of Periodic Waveforms
Amplitude , Peak-Value, and Peak-to-Peak Value
Amplitude (Em):
The amplitude of a sine wave is the distance
from its average to its peak.
Peak-to-Peak Value (Ep-p):
It is measured between minimum and maximum peaks.
Peak Value
The peak value of a voltage or current is its maximum
value with respect to zero.
In this figure : Peak voltage = E + Em
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The Basic Sine Wave Equation
The voltage produced by the previously described generator is:
• Em: the maximum coil voltage and
• α : the instantaneous angular position of the coil.
 For a given generator and rotational velocity, Em is constant.)
 Note that a 0&deg; represents the horizontal position of the coil and that one
complete cycle corresponds to 360&deg;.
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Angular Velocity (ω)
If the coil rotates through an angle of 30&deg; in one second, its
angular velocity is 30&deg; per second.
rotated, you can compute the angle through which it has turned using:
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Radian Measure
 In practice, q is usually expressed in radians per second,
 Radians and degrees are related by :
For Conversion:
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Relationship between ω, T, and f
 Earlier you learned that one cycle of sine wave may be represented as either:
 Substituting these into:
Sinusoidal Voltages and Currents as Functions of Time:
 We could replace the angle α as:
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Voltages and Currents with Phase Shifts
 If a sine wave does not pass through zero at t =0 s, it has a phase shift.
 Waveforms may be shifted to the left or to the right
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Introduction to Phasors
 A phasor is a rotating line whose projection on a vertical axis can be used
to represent sinusoidally varying quantities.
 To get at the idea, consider the red line of length Vm shown in Figure :
The vertical projection of this line (indicated in dotted red) is :
 By assuming that the phasor rotates at angular velocity of ω rad/s in the
counterclockwise direction
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Introduction to Phasors
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Introduction to Phasors
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Shifted Sine Waves Phasor Representation
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Phasor Difference
 Phase difference refers to the angular displacement between different
waveforms of the same frequency.
 The terms lead and lag can be understood in terms of phasors. If you observe
phasors rotating as in Figure, the one that you see passing first is leading and
the other is lagging.
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AC Waveforms and Average Value
 Since ac quantities constantly change its value, we need one single numerical
value that truly represents a waveform over its complete cycle.
Average Values:
 To find the average of a set of marks for example, you add
them, then divide by the number of items summed.
 For waveforms, the process is conceptually the same. You
can sum the instantaneous values over a full cycle, then
divide by the number of points used.
 The trouble with this approach is that waveforms do not
consist of discrete values.
Average in Terms of the Area Under a Curve:
Or use area
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AC Waveforms and Average Value
 To find the average value of a waveform, divide the area under the waveform by
the length of its base.
 Areas above the axis are counted as positive, while areas below the axis are
counted as negative.
 This approach is valid regardless of waveshape.
 Average values are also called dc values, because dc meters indicate average
values rather than instantaneous values.
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AC Waveforms and Average Value
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