AC Theory

EE 101 Electrical Engineering

ac theory

3.0 Alternating Current Theory
The advantage of the alternating waveform for electric power is that it can be stepped up or stepped down in
potential easily for transmission and utilisation. Alternating waveforms can be of many shapes. The one that is
used with electric power is the sinusoidal waveform. This has an equation of the form
v(t)
v(t) = Vm sin(ω t + φ )
T
Vm
The peak value of the waveform is the maximum value of the
φ/ω
waveform and for the a.c. waveform it is Vm.
mean value = 1
T

t

To +T

∫ v( t ) ⋅ dt

T

To

The mean value of the sinusoidal a.c. waveform is 0 since positive and negative areas cancel. (It can also be
shown by integration). If To = φ/ω
vrect (t)
T +T
1 o
T
average value =

∫ v rect ( t ) ⋅ dt

T

To

The average value of the a.c. waveform is defined as the average
value of the rectified waveform and can be shown to be equal to 2 V
m
π

1
T

rms value =

To +T

∫v

2

t

T

( t ) ⋅ dt
T

v2(t)

To

The effective value or r.m.s. value of a waveform is defined as above
so that the power in a resistor is given correctly. That is
2
V effective

R

⋅T =

To +T

∫

To

v2 (t)
1
2
⋅ dt , or V effective =
R
T

To +T

∫v

2

( t ) ⋅ dt

t

T

To

The effective value or rms value of the waveform is thus the square root of the mean of the squared waveform
for the sinusoidal a.c. waveform.
and can be shown to be equal to 1
2

Vm

Unless otherwise specified, the rms value is the value that is always specified for ac waveforms, whether it be a
voltage or a current. For example, 230 V in the mains supply is an rms value of the voltage. Similarly when we
talk about a 5 A, 13 A or 15A socket outlet (plug point), we are again talking about the rms value of the rated
current of the socket outlet.
For a given waveform, such as the sinusoid, the peak value, average value and the rms value are dependant on
each other. The peak factor and the form factor are the two factors that are most commonly defined.
Form Factor =

rms value
average value

and for a sinusoidal waveform, Form Factor =

Vm
2

2Vm

=

π
2 2

= 1.1107 ≅ 1.111

π
The form factor is useful such as when the average value has been measured using a rectifier type moving coil
meter and the rms value is required to be found. [Note: You will be studying about these meters later]

Peak Factor = peak value
rms value
Peak Factor =
Vm

Vm
2

and for a sinusoidal waveform,

i

= 2 = 1.4142

t

The peak factor is useful when defining highly distorted waveforms
such as the current waveform of compact fluorescent lamps.
Some advantages of the sinusoidal waveform for electrical power applications
a. Sinusoidally varying voltages are easily generated by rotating machines
b. Differentiation or integration of a sinusoidal waveform produces a sinusoidal waveform of the same
frequency, differing only in magnitude and phase angle. Thus when a sinusoidal current is passed through
(or a sinusoidal voltage applied across) a resistor, inductor or a capacitor a sinusoidal voltage waveform (or
current waveform) of the same frequency, differing only in magnitude and phase angle, is obtained.

University of Moratuwa - JRL/Sep2008

21


ac theory

If i(t) = Im sin (ωt+φ),
for a resistor, v(t) = R.i(t) = R.Im sin (ωt+φ) = Vm sin (ωt+φ) − magnitude changed by R but no phase shift
for an inductor,

v( t ) = L.

di
d
= L . ( I m sin (ω t + φ )) = L . ω . I m cos (ω t + φ ) = L . ω . I m sin (ω t + φ + π / 2)
dt
dt

− magnitude changed by Lω and phase angle changed by π/2
for a capacitor,

v( t ) =

1
1
−1
1
. ∫ i ⋅ dt = ∫ I m sin (ω t + φ ) ⋅ dt =
. I m cos (ω t + φ ) =
. I m sin (ω t + φ − π / 2)
C
C
C ⋅ω
C ⋅ω

− magnitude changed by 1/Cω and phase angle changed by −π/2

c. Sinusoidal waveforms have the property of remaining unaltered in shape when other sinusoids having the
same frequency but different in magnitude and phase are added to them.
A sin (ωt+α) + B sin (ωt+β)
= A sin ωt. cos α + Α cos ωt. sin α + B sin ωt. cos β + B cos ωt. sin β
= (A. cos α + B. cos β) sin ωt + (A. sin α + B. sin β) cos ωt
= C sin (ωt + φ), where C and φ are constants obtained from trigonometry.
d. Periodic, but non-sinusoidal waveforms can be broken up to its fundamental and harmonics.
e. Sinusoidal waveforms can be represented by the projections of a rotating phasor.
a(t)

3.1 Phasor Representation of Sinusoids
You may be aware that sin θ can be written in
terms of exponentials and complex numbers.
i.e. e jθ = cos θ + j sin θ

Amsinω t

P
ω
O

ωt

X

O

t

or e jω t = cos ω t + j sin ω t

T

Consider a line OP of length Am which is in the horizontal direction OX at time t=0.
If OP rotates at an angular velocity ω , then in time t its position would correspond to an angle of ω t.
The projection of this rotating phasor OP (a phasor is somewhat similar to a vector, except that it does not have
a physical direction in space but a phase angle) on the y-axis would correspond to OP sin ωt or Am sin ω t and
on the x-axis would correspond to Am cos ω t. Thus the sinusoidal waveform can be thought of being the
jωt
projection on a particular direction of the complex exponential e .
a(t)

P
ω
φ
0

Amsin (ω t+φ)

R
ωt

X

0

0

t

A
φ

P
R

T
Rotating Phasor diagram
If we consider more than one phasor, and each phasor rotates at the same angular frequency, then there is no
relative motion between the phasors. Thus if we fix the reference phasor OR in a particular reference direction
(without showing its rotation), then all others phasors moving at the same angular frequency would also be
fixed at a relative position. Usually this reference direction is chosen as horizontal on the diagram for
convenience.

Am

0

φ

A=

P

R

0

φ
Ax

Am
2
Ay
reference direction

Phasor diagram


22


ac theory

It is also usual to draw the Phasor diagram using the rms value A of the sinusoidal waveform, rather than with
the peak value Am. This is shown on an enlarged diagram. Thus unless otherwise specified it is the rms value
that is drawn on a phasor diagram.
It should be noted that the values on the phasor diagram are no longer time variables. The phasor A is
characterised by its magnitude A and its phase angle φ. These are also the polar co-ordinates of the phasor
and is commonly written as A φ . The phasor A can also be characterised by its cartesian co-ordinates Ax
and Ay and usually written using complex numbers as A = Ax + j Ay.
Note: In electrical engineering, the letter j is always used for the complex operator
is regularly used for electric current.
It is worth noting that

A=

Also, Ax = A cos φ,

− 1 because the letter i

2
2
Ax + Ay and that tan φ = Ay or φ = tan − 1  Ay 
 

Ax

 Ax 

jφ

Ay = A sin φ and A e = A cos φ + jA sin φ = Ax + j Ay

Note: If the period of a sinusoidal waveform is T, then the corresponding angle would be ω T. Also, the period
of a waveform corresponds to 1 complete cycle or 2 π radians or 3600. ∴ ω T = 2π
3.2 Phase difference
Consider the two waveforms Amsin (ω t+φ1) and Bmsin (ω t+φ2) as shown in the figure. It can be seen that they
have different amplitudes and different phase angles with respect to a common reference.
φ2− φ1
y(t)
Bmsin (ω t+φ2)
Amsin (ω t+φ1)

ωt

φ1 O
φ2− φ1

φ2

B=

Bm
2

ωT
j(ωt+φ )
A
1
These two waveforms can also be represented by either rotating phasors Am e
A= m
j (ωt+φ )
2 with peak amplitudes Am and Bm, or by a normal phasor diagram
and Bm e
2
φ2-φ1
with complex values A and B with polar co-ordinates A φ1 and B φ2 as
φ1
0
shown .
Any particular value (such as positive peak, or zero) of a(t) is seen to occur at a time T after the corresponding
value of b(t). i.e. the positive peak Am occurs after an angle (φ2 −φ1) after the positive peak Bm. Similarly the
zero of a(t) occurs after an angle (φ2 −φ1) after the corresponding zero of b(t). In such a case we say that the
waveform b(t) leads the waveform a(t) by a phase angle of (φ2 −φ1). Similarly we could say that the waveform
a(t) lags the waveform b(t) by a phase angle of (φ2 −φ1). [Note: Only the angle less than 180o is used to specify
whether a waveform leads or lags another waveform].
We could also define, lead and lag by simply referring to the phasor diagram. Since angles are always
measured anticlockwise (convention), we can see from the phasor diagram, that B leads A by an angle of
(φ2 −φ1) anticlockwise or that A lags B by an angle (φ2 −φ1).
Addition and subtraction of phasors can be done using the
C
same parallelogram and triangle laws as for vectors, generally
B
using complex numbers. Thus the addition of phasor A and
phasor B would be
A + B = (A cos φ1 + j A sin φ1) + (B cos φ2 + j B sin φ2)
= (A cos φ1 + B cos φ2) + j (A sin φ1 + B sin φ2)
= Cx + jCy = C φc = C

φ2
0

φ1

A

where C = C 2 + C 2 = ( A cos φ + B cos φ ) 2 + ( A sin φ + B sin φ ) 2
1
2
1
2
x
y
and

C y

( A sin φ 1 + B sin φ 2 )

= tan −1 

Cx 
( A cos φ 1 + B cos φ 2 ) 





φ c = tan −1 


23


ac theory

Example
Find the addition and the subtraction of the two complex numbers given by 10 30o and 25 48o.
Addition = 10 30o + 25 48o = 10(0.8660 + j 0.5000) + 25(0.6691 + j 0.7431)
= (8.660 + 16.728) + j (5.000 + 18.577) = 25.388 + j 23.577 = 34.647 42.9o
Subtraction = 10 30o − 25 48o = (8.660 − 16.728) + j (5.000 − 18.577) = − 8.068 − j 13.577 = 15.793 239.3o
Multiplication and division of phasors is most easily done using the polar form of complex numbers.
Thus the multiplication of phasor A and phasor B would be
A * B = A φ1 * B φ2 = A e

jφ

1

∗ B e

jφ

2

= A*B e

j(φ + φ )
1

2

= A*B φ1+ φ2 =C φc

where C = A*B and φc = φ1+ φ2
In a similar way, it can be easily seen that for division

C = A/B and φc = φ1− φ2
Thus, whenever we need to do addition and subtraction, we use the cartesian form of complex numbers, whereas
for multiplication or division we use the polar form.
Example
Find the multiplication and the division of the two complex numbers given by 10 30o and 25 48o.
Multiplication = 10 30o * 25 48o = 250 78o
Division = 10 30o ÷ 25 48o = 0.4 −18o
3.3 Currents and voltages in simple circuit elements
3.3.1 Resistor
i (t)

R
v (t)
v(t) = R.i(t)
R

I

for a sinusoid, consider i(t) =Real part of [ Im e(jωt+θ) ] or Im cos (ωt+θ)

∴v(t) = Real [R. Im e(jωt+θ) ] = Real [Vm. e(jωt+θ)]

Imcos ω t

or v(t) = R.Im cos (ωt+θ) = Vm cos (ωt+θ )

Vmcos ω t

∴Vm = R.Im and Vm/√2 = R.Im/√2

O

ωt

i.e. V = R . I

ωT

Note: V and I are rms values of the voltage and
I
V
current and no additional phase angle change O
has occurred in the resistor .
Phasor diagram
Note also that the power dissipated in the resistor R is equal to R . I 2 = V . I
3.3.2 Inductor
V

i (t)

I

L

v (t)
d i( t )
v( t ) = L
dt
j ωL

for a sinusoid, consider i(t) =Real part of [ Im e(jωt+θ) ] or Im cos (ωt+θ)
∴v(t) = Real [L. d Im e(jωt+θ) ] = Real [L. jω. Im ej(ωt+θ) ] = Real [j Vm ej(ωt+θ) ]
dt
or v(t) = L. d Im cos (ω t+θ) = −L.ω.Im sin (ωt+θ) = L. ω.Im cos (ωt+θ+π/2)
dt
Imcos ω t
=Vm cos (ωt+θ+π/2)
Vmsin ω t
∴Vm = ωL.Im and Vm/√2 = ωL.Im/√2
π/2
O
ωt

V
It can be seen that the rms magnitude of voltage is related to the rms
magnitude of current by the multiplying factor ωL. It also seen that the
voltage waveform leads the current waveform by 90o or π/2 radians or that
the current waveform lags the voltage waveform by 90o for an inductor .
Thus it is usual to write the relationship as V = jωL.I or V = ω L.I 90ο

ωT
O

V

I

Phasor diagram
The impedance Z of the inductance may thus be defined as jω L , and V = Z . I corresponds to the generalised
form of Ohm’s Law.
Remember also that the power dissipation in a pure inductor is zero, as energy is only stored and as there is no
resistive part in it, but that the product V . I is not zero.


24


3.3.3

ac theory

Capacitor
for a sinusoid, consider i(t) =Real part of [ Im ej(ωt+θ) ] or Im cos (ωt+θ)
i (t)
v(t) = Real [ 1 I e j (ω t +θ ) . dt ] = Real [ 1 . Im e(jωt+θ) ] = Real [ 1 Vm e(jωt+θ) ]
m
C∫
j
C. jω
v (t)
or v(t) = 1 I cos(ω t + θ ). dt = 1 .Im sin (ωt+θ) = 1 .Im cos (ωt+θ−π/2)
m
Cω
Cω
C∫
1
v ( t ) = ∫ i ( t ). dt
=Vm cos (ωt+θ−π/2)
Imcos ω t
C
1
∴Vm = 1 .Im and Vm/√2 = 1 .Im/√2
Vmsin ω t
I
jω C
ωC
ωC
π/2
O
ωt
C

V
It can be seen that the rms magnitude of voltage is related to the rms
magnitude of current by the multiplying factor 1 .
ωC
It also seen that the voltage waveform lags the current waveform by 90o
or π/2 radians or that the current waveform leads the voltage waveform by
90o for a capacitor.

ωT
I
V

O

Phasor diagram
Thus it is usual to write the relationship as V = 1 .I or V = 1 .I 90ο
jω C
ωC
The impedance Z of the inductance may thus be defined as 1 , and V = Z . I corresponds to the generalised
jω C
form of Ohm’s Law.
Remember also that the power dissipation in a pure capacitor is zero, as energy is only stored and as there is no
resistive part in it, but that the product V . I is not zero.
3.4 Impedance and Admittance in an a.c. circuit
The impedance Z of an a.c. circuit is a complex quantity. It defines the relation between the complex rms
voltage and the complex rms current. Admittance Y is the inverse of the impedance Z.
V = Z . I,

I = Y.V

where

Z = R + j X,

and

Y=G+jB

It is usual to express Z in cartesian form in terms of R and X, and Y in terms of G and B.
The real part of the impedance Z is resistive and is usually denoted by a resistance R, while the imaginary part of
the impedance Z is called a reactance and is usually denoted by a reactance X.
It can be seen that the pure inductor and the pure capacitor has a reactance only and not a resistive part, while a
pure resistor has only resistance and not a reactive part.
Thus Z = R + j 0 for a resistor, Z = 0 + jωL for an inductor, and Z = 1 = 0 − j 1 for a capacitor.
jω C
ωC
The real part of the admittance Y is a conductance and is usually denoted by G, while the imaginary part of the
admittance Y is called a susceptance and is denoted by B.
Relationships exist between the components of Z and the components of Y as follows.
−X
R
1
1
R − j X so that
G= 2
, and B = 2
G + jB =Y = =
= 2
2
2
Z R+ jX R + X
R +X
R +X2
The reverse process can also be similarly done if necessary.
However, it must be remembered that in a complex circuit, G does not correspond to the inverse of the
resistance R but its effective value is influenced by X as well as seen above.
3.5 Simple Series Circuits
In the case of single elements R, L and C we found that the angle difference between the voltage and the current
was either zero, or ± 90o. This situation changes when there are more than one component in a circuit.
3.5.1 R-L series circuit
V
R
VR

I

jωL
VL

In the series R-L circuit,
considering current I as reference
VR = R.I, VL = jωL.I, and V = VR + VL

∴ V = (R + jωL).I

so that

the total series impedance is Z = R + jωL

V

VL

VL

φ
VR
I
Phasor diagram

25


ac theory

The above phasor diagram has been drawn with I as reference. [i.e. I is drawn along the x-axis direction]. The
current was selected as reference in this example, because it is common to both the resistance and the inductance
and makes the drawing of the circuit diagram easier. In this diagram, the voltage across the resistor VR is in
phase with the current, where as the voltage across the inductor VL is 90o leading the current. The total voltage
V
V is then obtained by the phasor addition (similar to vector addition) of VR and VL.
φ
If the total voltage was taken as the reference, the diagram would just rotate as
VL
I
shown. In this diagram, the current is seen to be lagging the voltage by the
VR
same angle that in the earlier diagram the voltage was seen to be leading the
current. VL has been drawn from the end of VR rather than from the origin for
Phasor diagram
ease of obtaining the resultant V from the triangular law.
In an R-L circuit, the current lags the voltage by an angle less than 90o and the circuit is said to be inductive.
Note that the power dissipation can only occur in the resistance in the circuit and is equal to R . I 2 and that this
is not equal to product V . I for the circuit.
3.5.2 R-C series circuit

R

∴

I

In the series R-C circuit,

V

VR = R.I, VC =

1

I

jω C

φ

.I=−j 1 .Ι
jω C
ωC
1

VC

V

and V = VR + VC

VR
V = (R + 1 ).I
jω C

VR

VC
so that the total series impedance is Z = R +

Phasor diagram

1

VR

jω C

I

VL

φ

The phasor diagrams has been drawn first with current I as reference and then
with voltage V as reference.

V

o

In an R-C circuit, the current leads the voltage by an angle less than 90 and the circuit is said to be capacitive.
3.5.3 L-C series circuit
V
1
jω C

jωL

I

VL = jωL.I, VC =

VL

VC

.I = − j 1
jω C
ωC
1

V

VC
or V

I
VC

and V = VL + VC

∴ V = (jωL + 1 ).I

VL

VL

In the series L-C circuit,

I

Phasor

= jωL − j 1
jω C
jω C
ωC
It is seen that the total impedance is purely reactive, and that all the voltages in the circuit are inphase but
perpendicular to the current. The resultant voltage corresponds to the algebraic difference of the two voltages
VL and VC and the direction could be either up or down depending on which voltage is more.
so that the total series impedance is Z = jωL +

1

When ωL = 1 the total impedance of the circuit becomes zero, so that the circuit current for a given supply
ωC
voltage would become very large (only limited by the internal impedance of the source). This condition is
known as series resonance. In an L-C circuit, the current either lags or leads the voltage by an angle equal to 90o
and the resultant circuit is either purely inductive or capacitive.
Note that no power dissipation can occur in the circuit and but that the product V . I for the circuit is non zero.
3.5.4 R-L-C series circuit
R

V
jωL

I

jω C

VL

VR

∴ V = (R + jωL + 1 ).I
jω C

In the series R-L-C circuit,

1

VC

VR = R.I, VL = jωL.I, VC =

.I=−j 1 .Ι
jω C
ωC
1

and V = VR +VL + VC

so that the total series impedance is Z = R +jωL + 1 = R + j(ωL − 1 )
jω C
ωC


26



1 
Z = R + ω L −


ωC

2

2

ac theory

has a minimum value at ωL = 1 . This is the series resonance condition.
ωC

In an R-L-C circuit, the current can either lag or lead the voltage, and the phase angle difference between the
current and the voltage can vary between −90o and 90o and the resultant circuit is either inductive or capacitive.
3.6 Simple Parallel Circuits
3.6.1 R-L parallel Circuit

IR
I
IL

R

V

IR

In the parallel R-L circuit,

V

φ

considering V as reference
V = R.IR, V = jωL.IL, and I = IR + IL

IL

∴ I =V + V

jωL

Phasor diagram

R

jω L

I

IL

∴ total shunt admittance = 1 + 1
R

jω L

3.6.2 R-C parallel Circuit
V
IR

In the parallel R-C circuit,

R

I

V = R.IR, IC = jωC.V, and I = IR + IL

1

IC

jω C

IL

φ

∴ I = V + V . jω C

IR

R

∴ total shunt admittance = 1 + jω C

I

IL


V

Phasor diagram

R

3.6.3 R-L-C parallel Circuit
V
IR
I
IL

In the parallel R-L-C circuit ,

R

and I = IR + IL + IC
1

R

jω L

IL+IC

V

Phasor diagram

∴ total shunt admittance = 1 + 1 + jω C
R

I
φ

∴ I = V + V + V . jω C

jω C

IL

IR

V = R.IR, V = jωL.IL, IC = jωC.V

jωL

IC

IC


jω L

As in the case of the series circuit, shunt resonance will occur when
shunt admittance.

1

ωL

= ω C giving a minimum value of

Note that even in the case of a parallel circuit, power loss can only occur in the resistive elements and that the
product V. I is not usually equal to the power loss.
3.7 Power and Power Factor
It was noted that in an a.c. circuit, power loss occurs only in resistive parts of the circuit and in general the
power loss is not equal to the product V . I and that purely inductive parts and purely capacitive parts of a
circuit did not have any power loss. To account for this apparent discrepancy, we define the product V . I as the
apparent power S of the circuit. Apparent power has the unit volt-ampere (VA) and not the watt (W), and watt
(W) is used only for the active power P of the circuit (which we earlier called the power)
apparent power S = V . I
Since a difference exists between the apparent power and the active power we define a new term called the
reactive power Q for the reactance X.

27


ac theory

The instantaneous value of power p(t) is the product of the instantaneous value of voltage v(t) and the
instantaneous value of current i(t).
i.e.

p(t) = v(t) . i(t)

If v(t) = Vm cos ω t and i(t) = Im cos (ω t − φ),
where the voltage has been taken as reference and the current lags the voltage by a phase angle φ
then

p(t) = Vm cos ω t . Im cos (ω t − φ) = Vm Im .
=

. 2 cos ω t . cos (ω t − φ)

Vm Im [cos (2ω t − φ) + cos φ]
p(t)

p(t)

v(t)

p(t)
Vm Im cos φ

T

t

t
i(t)

current lagging voltage by angle φ

inphase

quadrature

It can be seen that the waveform of power p(t) has a sinusoidally varying component and a constant component.
Thus the average value of power (active power) P would be given by the constant value ½ Vm Im cos φ.
active power P = ½ Vm Im cos φ = Vm . I m .cos φ = V . I cos φ
2 2
The term cos φ is defined as the power factor, and is the ratio of the active power to the apparent power.
Note that for a resistor, φ = 0o so that P = V . I
and that for an inductor, φ = 90o lagging (i.e. current is lagging the voltage by 90o) so that P = 0
and that for an capacitor, φ = 90o leading (i.e. current is leading the voltage by 90o) so that P = 0
For combinations of resistor, inductor and capacitor, P lies between V. I and 0
For an inductor or capacitor, V. I exists although P = 0. For these elements the product V. I is defined as the
reactive power Q. This occurs when the voltage and the current are quadrature (90o out of phase). Thus
reactive power is defined as the product of voltage and current components which are quadrature. This gives
reactive power Q = V. I sin φ
Unlike in the case of inphase, where the same direction means positive, when quantities are in quadrature there
is no natural positive direction. It is usual to define inductive reactive power when the current is lagging the
voltage and capacitive reactive power when the current is leading the voltage. It is worth noting that inductive
reactive power and capacitive reactive power have opposite signs.
Although reactive power does not consume any energy, it reduces the power factor below unity. When the
power factor is below unity, for the same power transfer P the current required becomes larger and the power
losses in the circuit becomes still larger (power loss ∝ I2). This is why supply authorities encourage the
industries to improve their power factors to be close to unity.
3.8 Three Phase Power
As you already know, to transmit power using single phase ac, we need two wires. Of course you may have an
earth wire for protection, but this wire does not usually carry any current. You may also have seen that
distribution lines usually have 4 wires. What are these 4 wires ? It is the three phase wires and the neutral wire.
3.8.1 Balanced Three Phase
Now why do you need three phase wires ? This is in order to make our transmission efficient. You are already
aware that if we have three equal forces at angles of 120o to each other, if we use the triangular law we get an
equilateral triangle in which the resultant force is zero. A similar thing happens in three phase except that we
have phasors instead of vectors. That is, we have 3 voltages (or currents) which are equal in magnitude but
differing in phase angle by 120o from each other.


28


ac theory

R phase

current

Y phase

B phase

100
50

angle (rad)

0

π/ 2

0
-50

π

3π/ 2

2π

-100
Three phase waveforms

R

120o
B

120o

120o

Y

If the R phase is taken as reference and drawn vertical (reference need not always be
drawn horizontally), then the three phases would have the phasor diagram shown.
If vR(t) = Vm cos ω t, then vY(t) = Vm cos (ω t −2π/3) and vB(t) = Vm cos (ω t +2π/3)
It can easily be seen that the addition of these three waveforms at any instant is zero.
This is not only true for balanced three phase voltages, but for balanced three phase
currents. Thus if we have a balanced three phase system of currents, then their addition
would become zero, and no neutral wire would be required. However in practice, three
phase currents are never perfectly balanced and the neutral wire would carry the
unbalance.
If VR, VY, and VB are the r.m.s.voltages of the three phases with respect to the
neutral, then they are normally called the phase voltages. The voltage between
any two phase wires (or lines) is called the line voltage.
As can be seen from the diagram, the magnitude of the line voltage is √3 times
the phase voltage for a balanced system. In a three phase system, it is always
the r.m.s. line voltage that is specified, unless otherwise specified. For example
in the domestic supply, in single phase we say that the voltage is 230 V and in
three phase we say that the voltage is 400 V (~ 230 × √3).

If Vp is the phase voltage magnitude, VR = Vp ∠0, VY = Vp ∠-2π/3, VB = Vp ∠2π/3 .
The Line voltage magnitude will be √3×Vp = VL and the line voltages will be VRY = VL ∠π/6,
VYB = VL ∠(-2π/3+π/6) = VL ∠-π/2 , VBR = VL ∠(2π/3+π/6)= VL ∠5π/6.
Balanced loads may be connected either in
star or delta as shown.
A balanced star connected load can be
converted to an equivalent delta and the other
way round as well. In such a case it can be
shown that

ZD

ZS
ZS

ZD

ZS
ZD

ZD = 3 ZS
It can also be shown that the total three phase power in a star connected load, or delta connected load can be
expressed in terms of the line quantities as
P = √3 VL IL cos φ
where cos φ is the power factor of the load, VL is the line voltage and IL is the line current.
The reactive power in a three phase circuit may be similarly defined as

P

Q = √3 VL IL sin φ

φ

and the apparent power in a three phase circuit may be defined as
S = √3 VL IL
Note: In both the single phase circuit as well as the three phase circuit,
S2 = P2 + Q2

and

Q

S

tan φ = Q/P

Since the magnitudes of the three phases are equal (for both voltage and current) in a balanced three phase
system, it is sufficient to calculate the quantities for one of the phases and obtain the others by symmetry.

29


ac theory

Example
A 3 phase, 12 kW balanced load at a lagging power factor of 0.8 is supplied from a 3 phase, 400V, 50 Hz
supply. Determine the line current in magnitude and phase relative to the supply voltage, the apparent power
and the reactive power drawn.
P = 12 kW, VL = 400V, cos φ = 0.8 lag, P = √3 VL IL cos φ, i.e. 12000 = √3 × 400 × IL × 0.8

∴ IL = 21.65 A, φ = cos-1(0.8) = 36.87o.

i.e. IL = 21.65 ∠-36.87o A.

Apparent power = √3VL IL = √3 × 400 × 21.65 = 15000 = 15 kVA.
Reactive power = √3 VL IL sin φ = √3 × 400 × 21.65 × sin (36.87o) = 9000 = 9 k var
3.8.1 Unbalanced three phase systems
Unbalanced three phase systems would consist of either star connected or delta connected sources which need
not be balanced and an unbalanced star connected or delta connected load. Such systems cannot be solved
using one phase of the system, but could be solved using the normal network theorems.
3.9 Power factor correction
Since most practical loads are inductive in nature (as they have coils rather than capacitors), the power factor
can be improved by using capacitors across the load. They may be either connected in star or delta.
A pure capacitor affects only the reactive power and not the active power.
Example
In the above example a delta connected bank of capacitors are used to improve the power factor to 0.95 lag.
What should be the value of the required capacitors.
New power factor = 0.95 lag.

∴ new power factor angle = cos-1 (0.95) = 18.19o

The active power of the load is unchanged by the capacitor bank, ∴ P = 12000 W

∴ new reactive power = P tan φ = 12000 × tan (18.19o) = 3944 = 3.944 k var
∴ reactive power supplied from capacitor bank = 9000 - 3944 = 5056 = 5.056 k var.
Each of the 3 capacitors in the delta connected bank must supply = 5.056/3 = 1.685 k var.
Since they are connected in delta, each would get a voltage of 400 V.

∴ VL2 C ω = 4002 × C × 2 π × 50 = 1685
∴ C = 33.5 × 10-6 = 33.5 µF.
If the capacitors had been connected in star, then the reactive power would still have been the same, but the
voltage across each would have been 400/√3 and the value of each capacitor would be
Vp2 C' ω = (400/√3)2 × C' × 2 π × 50 = 1685
∴ C' = 100.6 × 10-6 = 100.6 µF.


30

AC Theory

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