2. WHAT IS AN ALTERNATING CURRENT (AC)?
• In alternating current, the electric
charges flow changes its direction
periodically.
• Alternating current can be identified in
waveform called a sine wave. In other
words, it can be referred to as the
curved line. These curved lines
represent electric cycles and are
measured per second.
• The measurement is read as Hertz or
Hz.
3. AC V/S DC
Alternating Current Direct Current
AC is safe to transfer longer distance even between two cities,
and maintain the electric power.
DC cannot travel for a very long distance. It loses electric
power.
The rotating magnets cause the change in direction of electric
flow.
The steady magnetism makes DC flow in a single direction.
The frequency of AC is dependent upon the country. But,
generally, the frequency is 50 Hz or 60 Hz.
DC has no frequency of zero frequency.
In AC the flow of current changes its direction backwards
periodically.
It flows in a single direction steadily.
Electrons in AC keep changing its directions – backward and
forward
Electrons only move in one direction – that is forward
4. ADVANTAGES OF AC OVER DC
• AC is less expensive and easy to generate than DC.
• The distance covered by AC is more than that of the DC.
• The power loss during transmission in AC is less when compared to the DC.
• The loss of energy during the transmission in AC voltage is less when
compared with the DC voltage and this makes its installations easy when the
transformers are at distance.
• AC voltage has the advantage of stepping up and stepping down as per the
requirement.
7. RMS VALUE OF AC
“RMS value of an alternating current is that steady state current (dc)
which when flowing through the given resistor for a given amount of
time produces the same amount of heat as produced by the alternative
current when flowing through the same resistance for the same time”
rms voltage is:
rms current is :
2
V
V m
2
I
I m
8. PHASE ANGLE
Phase Difference is used to describe the difference in degrees or radians
when two or more alternating quantities reach their maximum or zero
values.
The phase difference or phase shift as it is also called of a Sinusoidal
Waveform is the angle Φ (Greek letter Phi), in degrees or radians that the
waveform has shifted from a certain reference point along the horizontal
zero axis. In other words phase shift is the lateral difference between two or
more waveforms along a common axis and sinusoidal waveforms of the
same frequency can have a phase difference.
Phase Difference Equation:
•Where:
• Am – is the amplitude of the waveform.
• ωt – is the angular frequency of the waveform in radian/sec.
• Φ (phi) – is the phase angle in degrees or radians that the waveform has
shifted either left or right from the reference point
12. RESISTANCE IN AC CIRCUITS
ft
V
v m
2
sin
i
R
v
i
Instantaneous current
R
v
i
ft
2
sin
R
V
i m
ft
2
sin
I
i m
13. i
C
ft
V
v m
2
sin
i
v
dt
dv
C
i
ft
V
v m
2
sin
ft
fCV
i m
2
cos
2
f
2
t
CV
i m
cos
m
m CV
I
Phasor diagram and wave form
fCV
2
CV
I
CV
j
I
Current leads Voltage
by 90 degrees
C
fC
XC
1
2
1
C
C
C jX
V
jX
V
X
V
j
I
Capacitance Reactance
rms current
2
sin
t
CV
i m
i
Using complex numbers and the j operator
CAPACITANCE IN AC CIRCUITS
13
14. dt
di
L
v
ft
2
sin
V
v m
ft
2
cos
fL
2
V
i m
f
2
t
cos
L
V
i m
i – instantaneous current
Current lags Voltage
by 90 degree
0
t
m
m
L
V
I
fL
2
V
L
V
I
rms current
Using complex numbers and the j operator V
L
j
I
L
fL
2
XL
L
L jX
V
X
V
j
I
Inductive Reactance
2
t
sin
L
V
i m
i
L
ft
V
v m
2
sin
i
v
i
Phasor diagram and wave form
Inductance in an AC Circuits
14
15. IMPEDANCE
• A measure of the overall opposition of a circuit to current.
• It is like resistance, but it also takes into account the effects of
capacitance and inductance.
• Impedance is more complex than resistance because the
effects of capacitance and inductance vary with the frequency
of the current passing through the circuit and this
means impedance varies with frequency.
• The effect of resistance is constant regardless of frequency.
16.
17. RESONANCE (TUNED CIRCUITS)
• Resonance occurs whenever the phase angle of the circuit is zero, The only way that f = 0 is if XL = XC
• Generally resonance is achieved by varying the angular frequency the circuit until XL = X C.
18. SERIES RESONANCE
• In the RLC series circuit, when
the circuit current is in phase with
the applied voltage, the circuit is
said to be in Series Resonance.
• A series resonant circuit has the
capability to draw heavy current
and power from the mains; it is
also called acceptor circuit.
• At the resonance : XL – XC = 0 or
XL = XC
19. Effects of Series Resonance
• At resonance condition, XL = XC the impedance of the circuit is
minimum and is reduced to the resistance of the circuit. i.e. Zr = R
• At the resonance condition, as the impedance of the circuit is
minimum, the current in the circuit is maximum. i.e Ir = V/Zr = V/R
• As the value of resonant current Ir is maximum hence, the power
drawn by the circuit is also maximized. i.e. Pr = I2Rr
• At the resonant condition, the current drawn by the circuit is very
large or we can say that the maximum current is drawn. Therefore,
the voltage drop across the inductance L i.e. (VL = IXL = I x 2πfrL) and
the capacitance C i.e (VC = IXC = I x I/2πfrC) will also be very large.
20. Parallel Resonance
• Parallel Resonance means when
the circuit current is in phase
with the applied voltage of an
AC circuit containing an inductor
and a capacitor connected
together in parallel.
21. CHARACTERISTICS OF PARALLEL RESONANCE
• Below resonant frequency, circuit is inductive and impedance is small
since XL is small.
• Above the resonant frequency, circuit is capacitive and impedance is
small because of XC is low.
• At Resonance, Circuit is resistive and impedance is maximum since
XL = XC
• It is also called as Rejector Circuit.
