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Power Factor Correction with Synchronous Motor for
Rice Mill
May Zin Tun, Pyone Lai Swe
Abstract— Interconnection of electric power systems is
becoming increasingly widespread as part of the power exchange
between countries as well as regions within countries in many
parts of the world. There are few solutions, that allow handle the
problem of power factor correction. Synchronous motors are
used principle in large power applications because of their high
operating efficiency, reliability, controllable power factor, and
relatively low sensitivity to voltage dips. The synchronous motor
can be performed both generating kVAR as a compensator and
moveable machine as a motoring function in the power system.
The kVAR generated from synchronous motor are compensated
for any other related induction machine in Rice Mill. The result
data of overall power factor improvement and the best condition
of power factor correction are presented in this paper.
Keywords – Power factor, Power factor correction,
Synchronous motor, Rice mill
I. INTRODUCTION
Manuscript received Oct 15, 2011.
May Zin Tun, Department of Electrical Power Engineering, Mandalay
Technological University (e-mail:[email protected]).Mandalay,
Myanmar,09-422482029
Pyone Lai Swe, Department of Electrical Power Engineering, Mandalay
Technological University (e-mail:[email protected]) Mandalay,
Myanmar
II. POWER FACTOR
A power factor of an AC circuit is defined as the cosine
of the angle between the current and voltage vectors and the
terms leading or lagging indicate that the current leads or lags
on the voltage. A voltage applied to a capacitor produces a
current which leads the voltage by 90 electrical degrees, a
voltage applied to a pure resistance produces a current in
phase with it, and a voltage applied to an inductance produces
a current lagging it by 90 electrical degree. In practice all
loads posses at least a small amount of resistance, inductance
and capacitance but usually one predominates. All loads can
be represented by a combination of a resistance and a
reactance, either inductive or capacitive.
The power factor of a system is composed of two
elements, active power and apparent power. Active power is
the useful power. Apparent power is the aggregate of active
power and reactive power. The power factor ratio is given in
Equation (1). [1]
Active Power
Power Factor =
(1)
Apparent Power
This ratio is also equal to the cosine of the angle between the
voltage and the current of the system. The power factor ratio is
given in Equation (2).
Power Factor=cos (Φ)
(2)
The power factor is the ratio of kW component to the kVA
component, consider Figure 1.
Active Power P
Φ
Ap
par
ent
Po
we
rS
Figure1. Power Vector Diagram [1]
Reactive Power Q
The large industries have many loads such as induction
motors, and others inductive loads. They are of power factor
lagging loads. Low power factor can increase electricity
generating costs, reduce electrical distribution capacity and
increase voltage drop. Power factor is the factor by which
apparent power, or kVA, is multiplied to obtain actual power, or
kW, in an ac system. It is the ratio of the in-phase component of
current to total current. It is also the cosine of the angle by which
the current lags (or leads) the voltage. In practice, the static
capacitors, synchronous motors and synchronous condensers are
used as the load compensator. Overall power factor of the plant
is improved because of unity or leading power factor of
synchronous motor. The power factor of a synchronous motor is
controllable within its design and load limits. It may operate at
unity, leading, or in rare cases, lagging power factor and may be
used to modify the power factor of the system to which it is
connected. A simplified explanation of phasor relationships will
describe what takes place under various load and excitation
conditions. Phasors are vectors that represent the relationship
between voltages or currents.
The phasor length represents the magnitude and, as it is
allowed to rotate counterclockwise about the origin, the
projection on the x axis is the instantaneous value. The power for
unity power factor is always positive while both leading and
lagging power factors have some negative power values, and the
peak is less than for unity. Thus, the average value of power is
less for either leading or lagging power factor for the same
current or voltage. Synchronous motor can also be used for
power-factor correction in an industrial plant consisting of a
large number of induction motors. [4]
An induction motor takes lagging current from the supply to
produce flux in it magnetizing current even when supplying no
shaft torque. When the shaft torque is increased, both the
amplitude of the current and its power factor change. If the
excitation is kept constant, variation of the shaft torque causes
the power factor to vary. The variation in reactive kVAR with
load depends on the shape of the characteristics, that is, it
depends on the synchronous reactance. In general, the power
factor of industrial load is lagging and the effects of low
lagging power factors are considerable. The various items
such as transformer, transmission lines and machines posses
resistance and reactance and when the power factor of the load
is unity, the power factor of the generation will be lagging. To
obtain unity power factor on the generators, the load would
have to have a leading power factor.
To supply a fixed amount of kW to a load, it is necessary to
provide a greater amount of kVA. The lower power factor
becomes in kVA implies increased losses in generation,
transmission and distribution and also requires greater
generation and distribution capacity. The various items in a
transmission and distribution system have some resistance and
this produces a loss when current flows. The amount of loss is
proportional to the square of current and is associated with the
kVA transmitted and not the kW power. Maximum efficiency
and utilization of generation and distribution capacity would
be obtained if the power factor were increased to unity.
However, it is not generally possible to operate all the plant in
an electrical supply system at unity power factor. The various
items such as transformer, transmission lines and machines
posses resistance and reactance and when the power factor of
the load is unity, the power factor of the generation will be
lagging. To obtain unity power factor on the generators, the
load would have to have a leading power factor. [6]
III. POWER FACTOR CORRECTION
Power factor correction usually means the practice of
generating reactive power as close as possible to the load
which requires it, rather than supplying it from a remote power
station. The objective of power factor correction is to reduce
the current flowing in the circuit connecting the consumer’s
load with the source of supply. When the power factor is high,
the reactive component is a small percentage of the total and
so a given change in the reactive component produces only a
small change in the total. Accordingly, it is unnecessary to
correct the power factor to exactly unity. The phasor diagram
of single-phase load of admittance is shown in Figure 2.
IR = VGL= ILcosϕ
V

The apparent power supplied to the loads is
SL = V2GL- jV2BL
= PL+jQL
The power factor is expressed by the equation
cos  L =
(3)
PL
SL
(4)
where,
PL=active power supplied to the load (kW)
QL=reactive power supplied to the load (kVAR)
SL=apparent power supplied to the load (kVA)
Iγ = -VBL
IS = IR
V

IX = +VBL
Figure 3. Phasor Diagram of Single-phase Load (Compensated) [2]
Figure 3 shows the phasor relationships in which supply
current is in phase with V, making the overall power factor
unity. The supply current I S now has the smallest value
capable of supplying full power PL and the voltage V, and all
the reactive power required by the load is thus totally
compensated. Relieved of the reactive requirements of the
load, the supply now has excess capacity which is available
for supplying other loads.
The apparent power exchanged with the supply system is
Sγ=Pγ+ Qγ = VIγ*
=jV2BL
(5)
Most loads are inductive, requires capacitive compensation
for total compensation of reactive power, the reactive power
rating of the compensator is related to the rated power P L of
the load by
QL= PL tan  L
(6)
The rated apparent power SL of the load is described by the
equation
QL = SL sin  L
 SL 1  cos2L
(7)
where,
Sγ = apparent power of the compensator (kVA)
Qγ= reactive power of the compensator (kVAR)
Pγ= active power of the compensator (kW)
IX = VBL= ILsinϕ
IL=VYL
Figure 2. Phasor Diagram of Single-phase Load (Uncompensated) [2]
Power factor improvement can be obtained by adding
equipment designed solely to take leading kVAR, by replacing
equipment taking lagging kVAR and by equipment operating
at unity power factor or at a leading power factor or
installation new plant which would operate at leading power
factor. Power factor correcting equipment may consist of static
capacitor, synchronous condensers or synchronous motor. All
have advantages and disadvantages and the choice of which to
use in particular case may only be described after considering:
[6]
 The amount of leading kVAR required
 Electrical power changes
 The variation of kVAR required
 Method of control required
 Type of load where power factor is improved
 Any other power factor correcting plant installed
Future development of load
action; negative Q means delivering capacitive VARs for a
generator action, or receiving capacitive VARs for a motor
action. It can be observed from Figure 2 that the power factor
is lagging when P and Q have the same sign, and leading when
P and Q have opposite signs.
B. Synchronous Motor with Different Excitations
A synchronous motor is said to have normal excitation
when Ef = Vt. If field excitation is such that Ef < Vt, the motor
is said to be under-excited. In both these conditions, it has a
lagging power factor as shown in Figure 5.
Ef
Ia
j I a Xs

j I a Xs
Ef
Vt
 δ
Ia
Vt
(i)
- Ia
(ii)
Ia

t
δ
Ia

j IaXs
Vt
δ
Ef
Ef
(iii)

Ia
Ia
ER
θ

ER
Ef
θ
α
α
Vt
Ef = V
Ef < V
Lagging pf
Lagging pf
(iii)
Vt
Lagging pf
(ii)
(i)
α
α

Ef < V Ia
Vt
Lagging pf
Ef
θ
α
α
Ef = V
ER
α
 =0
Ia Vt
(iv)
Figure 5.Vector Diagrams for Various Power Factors
Ef
δ
θ
α
IV. PRINCIPLE OF SYNCHRONOUS MOTOR
A. Power Angle and Other Performance characteristic on
Synchronous Motor
The real and reactive power delivered by a synchronous
generator, or received by a synchronous motor, can be
expressed in terms of the terminal voltage Vt, the generated
voltage Ef, the synchronous impedance Zs, and the power
angle or torque angle δ. Referring to Figure 4, it is convenient
to adopt a convention that makes positive the real power P and
the reactive power Q delivered by an overexcited generator.
Accordingly, the generator action corresponds to positive
values of δ, whereas the motor action corresponds to negative
values of δ. With the adopted notation it follows that P > 0 for
generator operation, whereas P < 0 for motor operation.
Ef
ER
j I aXs
-I a
(iv)
Figure4. Four Possible Cases of operation of a Round-rotor
Synchronous Machine with Negligible Armature
Resistance [3]
(i)Overexcited generator (pf lagging),P>0,Q>0,δ>0
(ii)Under excited generator (pf leading),p>0,q<0,δ>0
(iii)Overexcited generator (pf leading),p<0,q>0,δ<0
(iv)Under excited generator (pf lagging),p<0,q<0,δ<0
Further, positive Q means delivering inductive VARs for
a generator action, or receiving inductive VARs for a motor
On the other hand, if DC field excitation is such that Ef >
Vt, then motor is said to be over-excited and draws a leading
current, as shown in Figure 5(a). There will be some value of
excitation for which armature current will be in phase with Vt,
so that power factor will become unity, as shown in Figure
5(b). The value of α and hack e.m.f Ef can be found with the
help of vector diagrams for various power factors, shown in
Figures 5 (c) and (d).
V. DESIGN AND CALCULATION OF POWER FACTOR
CORRECTION BY USING SYNCHRONOUS MOTOR
The designed rice mill consists of twelve numbers of
induction motors is shown in figure 6. The twelve numbers of
induction motors are two numbers of induction motors used
for Huller, two numbers of induction motors for Fan Motor,
two numbers of induction motors for Chaff Separator, two
numbers of induction motors for Rice Huller, two numbers of
induction motors for Work Polisher, two numbers of induction
motors or White Rice Grader. The ratings of induction motors
are 24.45kW, 52kW, 30kW, 47.76kW, 60kW and 110kW. The
existing real and reactive power loadings of Rice Mill at full
load are shown in figure 7.
So, the total real power loadings
P T = 648.42 kW
Total reactive power loadings,
Q T =571.86kVAr
Total apparent power loadings,
S T = 864.56 kVA
And the overall power factor of Rice Mill = P T/ST
= 0.75 lagging
(2) Designed condition two is established by using 0.8 leading
power factor synchronous motor and induction motors.
Table I
Percentage of Induction Motors Loading
% of Load
VCB
Husker
IM
Fan
Motor
Chaff
Separator
IM
Rice
Huller
IM
24.45kW, 52kW,
0.75(lag) 0.75(lag)
IM
30kW, 47.76kW,
0.75(lag) 0.75(lag)
Work
Polisher
White Rice
Grader
25
Real Power
(kW)
162.1
Reactive
Power(kVAR)
142.9
50
324.2
285.9
75
486.3
428.9
100
648.4
571.8
IM
IM
60kW,
0.75(lag)
Table II
Result of Condition One (600kW, pf unity) of Synchronous
Motor
110kW,
0.75(lag)
Figure 6. Induction Motors Loading in Rice Mill
Induction
Motor
Synchronous Motor
Overall Plant
% of Load
Reactive
Power
(kVAR)
0
Real
Power
(kW)
912.1
Reactive
Power
(kVAR)
142.9
Pf
lag
220
Real Power
Reactive Power
160
25
Real
Power
(kW)
600
140
50
600
0
1074.2
285.9
0.97
120
75
600
0
1236.3
428.9
0.94
100
600
0
1398.4
571.8
0.93
125
600
0
1560.5
714.8
0.91
Real Power(kW)& Reactive Power(kVAR)
200
180
100
80
0.99
60
40
1
2
3
4
5
1
6
pf
Plants
0.99
In this paper, the rating of synchronous motor to compensate
the reactive power of Rice Mill Factory is chosen as 804hp. It
is more cost and so that only chosen 600kW (804hp)
synchronous motor. This rating is to improve the power factor
0.75 to 0.98lagging.Table I, II and III are detail result data.
(10)
S=
0.97
0.96
0.95
0.94
0.93
0.92
0.91
Using the following Equations,
P = VIcosΦ
Q = P×tanΦ
p.f
0.98
Overall PowerFactor
Figure7. Existing Real and Reactive Power Loadings of Rice
Mill
(8)
(9)
=
P
p.f
40
(11)
60
80
100
120
140
Percent Load ofInductionMotors
kW
kVA
The power factor of plant has in the designed power factor
limit. There are two methods to improve the design power
factor of plant .The two methods are
(i)Addition of synchronous motors and
0.9
20
Figure 8. Relation of Percent Loads of Induction Motors and
Overall Power Factor by using unity Power Factor
of Synchronous Motor
Table III
Result of Condition Two (600kW, pf0.8) of Synchronous
Motor
Induction
Motor
% of Load
Synchronous Motor
Overall Plant
Reactive
Power
(kVAR)
562.5
Real
Power
(kW)
912.1
Reactive
Power
(kVAR)
419.6
Pf
lag
The characteristic features of the both solution are:
25
Real
Power
(kW)
600
(1) Designed condition one is established by using unity
power factor synchronous motor and induction motors.
50
600
562.2
1074.2
276.6
0.97
75
600
562.5
1236.3
133.6
0.99
(ii)Reduction of induction motors
0.91
100
600
562.5
1398.4
9.3
1
125
600
562.2
1560.5
152.6
1
[1]
[2]
When the loads of induction motor vary from 25 percent to
125 percent; the power factor of overall plant also changes from
0.99 to 0.91. This result is shown in Figure 8. From Table II,
when the loads of induction motor vary from 25% to 125%, the
power factor of overall plant also change from unity to 0.91
lagging power factor. This result is shown in Figure 9. In
comparison of Table I and II, the power factor of overall plant
from Table II is more improve than from Table I because of 0.8
leading power factor of synchronous motor. Therefore, the using
of 0.8 leading power factor of synchronous motor is best
condition to improve the power factor of the plant.
1
pf
Overall Power Factor
0.98
0.96
0.94
0.92
0.9
20
30
40
50
60
70
80
90
100
110
120
130
Percent Loads of Induction Motor
Figure 9. Relation of Percent Loads of Induction Motors and
Overall Power Factor by Using 0.8 Leading Power
Factor of Synchronous Motor
VI. CONCLUSION
The feature of power factor correction makes the
synchronous motor as a most useful industrial machine. In this
paper, according to compare test of load changes of induction
motors, the condition two is better than condition one. In
condition one, the power factor of plant is improved by using 600
kW, unity power factor of synchronous motor. In condition two,
the power factor of design plant is improved by using 600 kW,
0.8 leading power factor of synchronous motor. Therefore, the
synchronous motor at 0.8 leading power factor more improve
than at unity power factor. By increasing field excitation of
synchronous motor, the power factor is varied from unity to
leading power factor. Therefore, the power factor limits between
0.95 and unity by increasing field excitation of synchronous
motor.
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[3]
[4]
[5]
[6]
Jakub Kepka, Faculty of Electrical Engineering. “ Reactive Power
Compensation ”, Wroclaw University of Technology.
Jihn, Reactive Power Control in Electric System, Wiley & Sons Inc.,
T.J.E.MILLER,(1982).
Kumar, K. Murugesh. “Introduction and Synchronous Machines. ” Vikas
Publishing Company Ltd, (2000).
Brosan, G.S., and Hayden, J. T. 1996.Advance Electrical Power and
Machine.
Stevenson, JR.W.D. 1969. Element of Power System.
Anonymous: Introduction to Compensation System, Nokian Capacitor
Ltd., Finland,(2005).