AC Fundamentals.pdf

Alternating Current
(ac)
Fundamentals

Alternating Current (ac) Fundamentals
• Reasons for concentrating on sinusoidal voltage
• voltage generated by utilities throughout the world is ac and
• its application in electrical, electronic, communication, and
industrial systems
• Sinusoidal ac voltages are available from a variety of sources.
• Common source is the typical home outlet, which provides an
ac voltage that originates at a power plant; such a power plant
is most commonly fueled by water power, oil, gas, or nuclear
fission.
• Different forms of ac

Alternating Current (ac) Fundamentals: Definitions
• Waveform: The path traced by a
quantity, such as the voltage in Fig.
plotted as a function of some
Variable such as time, position,
degrees, radians, and so on
• Instantaneous value (e1): The
magnitude of a waveform at any
instant of time. Sine wave
•Peak amplitude (Em): The maximum value
of a waveform as measured from its average
or mean value. It is the maximum value,
positive or negative, of an alternating quantity.
Cosine wave

Alternating Current (ac) Fundamentals: Definitions
• Peak-to-peak value (EP-P): The
maximum value of a waveform
from positive to negative peaks.
• Periodic waveform: A waveform
that continually repeats itself after
the same time interval. Waveform
of Fig. is a periodic waveform.
• Cycle: One complete set of positive and negative values of
alternating quantity is known as a cycle.
• Period (T1 or T2): The time taken by an alternating quantity to
complete one cycle is called its time period T. For example, a 50
Hz alternating current has a time period of 1/50 seconds.
• Frequency: The number of cycles that occur in 1 s. The unit of
frequency is hertz (Hz), where 1 Hz = 1 cycle per second.

General form of ac current or voltage
The basic mathematical form for
sinusoidal waveform is
y = A sin α = A sin ωt … (5.1)
Here , Am = amplitude
ω = angular frequency
t = time
α= angular distance
y = instantaneous value
• Valid when the waveform passes
through origin.
• If the wave form shifted to the right or
left of 0°, the expression becomes
y = A sin = A sin (ωt ± θ)

y = A sin α = A sin ωt … (5.1)
• Valid when the waveform passes
through origin.
• If the wave form shifted to the right or
left of 0°, the expression becomes
y = A sin = A sin (ωt ± θ)
• If the waveform passes through the
horizontal axis with a positive slope
before 0°, expression is
y = A sin = A sin (ωt + θ) … …. (5.3)
• If passes after 0° expression is
y = A sin = A sin (ωt - θ)

y = A sin α = A sin ωt
• If the waveform crosses the horizontal axis
with a positive-going slope 90° (π/2) sooner,
it is called a cosine wave; that is
sin (ωt + 90°) = sin (ωt + π/2) = cos ωt
Or sin ωt = cos (ωt - 90°) = cos (ωt - π/2)
• The term lead and lag are used to indicate
the relationship between two sinusoidal
waveforms of the same frequency plotted
on the same set of axes. In Fig. 5.3 (d),
the cosine wave is said to lead the sine
curve by 90º, and the sine curve is said to
lag the cosine curve by 90º. The 90º is
referred to as the phase angle between the
two waveforms.

Phase and phase difference
• By phase of an ac is meant the fraction of
the time period of that alternating current which
has elapsed since the current last passed through
the zero position of reference.
• if the two ac or emf reach their maximum and
zero at the same time such ac or voltages are
said to be in phase with each other.
The two voltages will have the equations,
e1 = Em1 sin ωt and e2 = Em2 sin ωt

Root-Mean-Square (RMS or rms) value of ac
• The rms value of an alternating current is given by that steady (dc) current
which when flowing through a given circuit for a given time produces the same
heat as produced by ac current when flowing through the same circuit for the
same time.
• Also known as the effective value of
alternating current or voltage
• RMS value can be computed using either
mid-ordinate method or analytical method.
• In home outlets what we measure (220 V) is rms value of alternating voltage
i.e. v(t) = (√2) 220 Sin 314 t
In general alternating voltage v(t) = Vm Sin ωt = Vm Sin (2 π f t)
And alternating voltage i(t) = Im Sin ωt = Im Sin (2 π f t)

Root-Mean-Square (R.M.S.)
Computing rms value using analytical method:
Standard form of ac is i =Im Sin ωt = Im Sin θ.
Sum of the squares of instantaneous values of currents in = ∫ i2
dθ
Mean of the squares of the instantaneous values of current over one complete
cycle is
= ∫ [ (i2
dθ)/2π]
Square root of this value is, = √ [(i2
dθ)/2π]
Hence, the rms value of ac is, = √ [(i2
dθ)/2π]
which yields, Irms = Ieff = Im/√2 = 0.707 Im
• Hence, rms value of ac = 0.707 X maximum value of current.
• In general rms value or effective value of
any alternating quantity (v, I, etc) is

Computing rms value using analytical method

Average value of alternating current or voltage
• The average value Iav of an ac is expressed by that steady current
which transfers across any circuit the same charge as is transferred by
that ac during the same time.
• In case of a symmetrical ac (i.e. one whose two half cycles are exactly
similar, whether sinusoidal or non-sinusoidal), the average value over a
complete cycle is zero.
• Hence, in their case, the average value is obtained by adding or
integrating the instantaneous values of current over one half cycle only.
• Iav = ∫ [(i dθ)/2π ] here integration limit is 0 to π.
• Therefore, Iav = Im/(π/2) = twice the maximum current / π
Average value of current or voltage = 0.637 X Maximum value.
Form factor, Kf = (rms value)/(average vakue) = 0.707 Im/ 0.637 Im = 1.11
Peak or Amplitude factor, K = (Maximum value)/(rms value) = I /(I /√2) = √2=1.414

Alternating current or voltage
Ex.5.1: Determine the angular velocity of a sine wave having a frequency of 60 Hz.
Ex. 5.2: Determine the frequency and Time period of a sine wave having angular velocity
of 500 rad/s.
Ex.5.3: Determine the average value and rms value of the sine wave shown in Fig. 5.5.
Vav = (0.637) (Vm) = (0.637) (169.7 V) = 108 V
Vrms = (0.707) (Vm ) = (0.707) (169.7 V) = 120 V

AC Fundamentals.pdf

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AC Fundamentals.pdf