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Abstract - The chief objective is to improve the power
quality by continuously monitoring the load power factor.
When the load power factor falls below a certain value it
results in the increase of line current, resulting in more
line loss and greater voltage drop. Thus, the aim is to inject
capacitances of required values when the power factor
falls below the specified level. Primarily, a signal of pulse
width proportional to the phase difference is generated.
From the ON time period of each pulse the power factor
can be determined. The exact value of the capacitance to
be injected is then found out using some mathematics.
Finally, the value of capacitance, so obtained, is to be
approximated with the standard values of capacitance.
Micro-controller will switch all the capacitors, which taken
together is very close to the exact value of the capacitance.
Keywords - power factor; capacitance; micro-controller.
Introduction
The cosine of the angle between the voltage and
current in an ac circuit is known as power factor. It may also
be defined as the ratio of KW and the KVA drawn by the
electrical load where KW is the actual load power and KVA is
the apparent power. It is a measure of how effectively the
current is being converted to useful work output and more
particularly is a good indicator of the effect of the load current
on the efficiency of the supply system.
Effects of low power factor :
a) Large copper losses:
The power factor plays an important role in ac
circuits since power consumed depends upon this
factor. P=√3VLILcosφ ( for 3 phase line)
=> IL=P/(√3VLcosφ)
It is clear from above that for fixed power and voltage, the
load current is inversely proportional to the power factor.
Lower the power factor, higher is the load current and vice-
versa. The large current at low power factor causes more I2
R
loss, resulting in poor efficiency.
b) Poor voltage regulation
The large current at low lagging power factor causes
greater voltage drops at the alternators, transformers,
transmission lines and distributors. This results in
decreased voltage available at the supply end, thus
impairing the performance utilization of devices.
Reasons of low power factor
a) Most of the ac motors are of induction type which have
low lagging power factor.
b) Arc lamps, electric discharge lamps and industrial
heating furnaces operate at low power factors.
Major steps for the power factor correction
Low power factor is undesirable from economic point of
view. Pic-microcontroller chip has been used, here for the
implementation of the power factor correction. The task is
divided into the following steps:
1. Measurement of power factor
2. Evaluating the value of capacitance to be injected for the
improvement of power factor.
3. Approximating the capacitance, obtained from step 2,
with some standard capacitances.
1. Measurement of power factor
The circuit diagram for the detection of power factor is
shown in Fig 1.
Fig 1: Circuit diagram for detection of power factor
Automatic Power Factor improvement using Microcontroller
Reetam Sen Biswas, 4th
year,
Electrical Enginering, Heritage Institute of
Technology, Kolkata.
Email id: reetsenbiswas@gmail.com
Dr. Satadal Mal, Professor and Head,
Department of Electrical Engineering,
Heritage Institute of Technology, Kolkata.
Email id: satadal14@gmail.com
IEMCON2015 Conference and Workshop on Computing & Communication

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The line current and the line voltage are first stepped down
using C.T and P.T. The objective is to produce a signal
having pulse width proportional to the phase difference
between voltage and current. Thus, to realize this, the
stepped down voltage and the line current are applied to the
zero-crossing detectors (ZCD1 and ZCD2 respectively). The
output voltage waveforms from ZCD1 and ZCD2 are shown
in Fig 2. The outputs from ZCD1 and ZCD2 are fed to the 2
input XOR gate. The XOR gate results high only when one
input is low and the other input is high. Hence, the output of
the XOR gate is a series of pulses whose width is
proportional to the phase difference between the two signals.
This is shown in Fig 2.
Fig 2: Waveforms at different points of the circuit diagram
It is clear that the ON time of the XOR gate is proportional to
the phase difference between 0o
and 360o
. This means that
the ON time(T) is indicative of the power factor.
Fig 3: Variation of TON with Ф
‘TON’ is measured by using micro-controller. PIC 18F458 of
20 MHz crystal frequency is utilized for the proposed
scheme.
Algorithm for ‘TON’ measurement
The output of the XOR gate, z(t) is fed as input at Port B, pin
0. COUNT is a register which stores a value indicative of
power factor.
1. Pin 0 of port B is set as an input pin. Initially,
COUNT is initialized to 0.
2. The status of the pin is continuously examined by the
micro-controller.
3. If B0 pin is high, the COUNT is incremented and a
fixed time delay subroutine is called.
4. Again we go back to step 2.
5. Once the status of B5 pin is low the control comes
out of the loop.
6. The value stored in the COUNT is indicative of the
power factor.
Algorithm for Delay subroutine
Timer clock frequency is one-fourth of the crystal
frequency=5 MHz. One clock period is 1/5 µs =0.2 µs. Here, a
fixed time delay of 100 µs or 500 clock pulses is being
generated.
TIMER0 register is selected for the purpose. When the timer
reaches its maximum value of FFFF it rolls over to 0000 and
TMR0IF is set to 1.Thus, for a count of 500 clock pulses the
initial value of TIMER0 is set to FE0BH because FFFFH-
FE0BH=1F4H=500D .
1. The TOCON( control register) is first set with the
control word(which is 08H in this case).
2. TMR0H(the higher byte) is set to FEH.
3. TMROL(the lower byte) is set to 0BH.
4. Counting of TIMER0 is started by setting the
TMR0ON bit of the T0CON.
5. The status of TMR0IF is continuously checked.
When it is set(1) control returns to the main
program.
Fig 4: Flowchart for evaluating power factor

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Relationship between COUNT and φ
Now, the number stored in COUNT has to be related
to the power factor. Time period of each clock pulse(T) is 0.2
µs. Total time for which pulse is high(Ttot) =COUNT×T×500.
Thus, Phase angle(φ in rads) corresponding to ON period of
the pulse =2πf× Ttot =2×3.14×50×COUNT×T×500
= 0.03146×COUNT.
Since we get φ, power factor can also be evaluated by cos
(φ). However, since it is difficult to do trigonometric operation
in micro-controller a look up table approach is used to relate
the power factor with COUNT. tan φ is also evaluated, as it
shall be required later on. The look up table is given in Table
1.
Table 1: Look up table
COUNT φ(in rad) φ(in deg) cos(φ) tan(φ)
1 0.03146 1.803439 0.999505 0.03147
2 0.06292 3.606879 0.998021 0.063003
3 0.09438 5.410318 0.99555 0.094661
4 0.12584 7.213758 0.992093 0.126508
5 0.1573 9.017197 0.987654 0.15861
6 0.18876 10.82064 0.982238 0.191034
7 0.22022 12.62408 0.975849 0.22385
8 0.25168 14.42752 0.968495 0.257132
9 0.28314 16.23096 0.960183 0.290957
10 0.3146 18.03439 0.95092 0.325407
11 0.34606 19.83783 0.940716 0.36057
12 0.37752 21.64127 0.929582 0.39654
13 0.40898 23.44471 0.917527 0.433419
14 0.44044 25.24815 0.904564 0.471318
15 0.4719 27.05159 0.890706 0.510358
16 0.50336 28.85503 0.875967 0.550673
17 0.53482 30.65847 0.86036 0.59241
18 0.56628 32.46191 0.843903 0.635733
19 0.59774 34.26535 0.82661 0.680824
20 0.6292 36.06879 0.808499 0.72789
21 0.66066 37.87223 0.789587 0.777163
22 0.69212 39.67567 0.769895 0.828906
23 0.72358 41.47911 0.74944 0.883422
24 0.75504 43.28255 0.728244 0.941055
25 0.7865 45.08599 0.706327 1.002206
26 0.81796 46.88943 0.683711 1.06734
27 0.84942 48.69287 0.660419 1.137002
28 0.88088 50.49631 0.636473 1.211834
29 0.91234 52.29975 0.611897 1.2926
30 0.9438 54.10318 0.586715 1.380216
31 0.97526 55.90662 0.560953 1.475789
32 1.00672 57.71006 0.534635 1.580671
33 1.03818 59.5135 0.507789 1.696534
34 1.06964 61.31694 0.48044 1.825468
35 1.1011 63.12038 0.452616 1.970118
36 1.13256 64.92382 0.424343 2.13389
37 1.16402 66.72726 0.395651 2.321242
38 1.19548 68.5307 0.366567 2.538123
39 1.22694 70.33414 0.33712 2.792659
40 1.2584 72.13758 0.30734 3.096246
41 1.28986 73.94102 0.277255 3.465384
42 1.32132 75.74446 0.246897 3.92489
43 1.35278 77.5479 0.216293 4.513909
44 1.38424 79.35134 0.185476 5.297981
45 1.4157 81.15478 0.154475 6.395824
46 1.44716 82.95822 0.123322 8.046984
47 1.47862 84.76166 0.092046 10.81803
48 1.51008 86.5651 0.060679 16.44979
49 1.54154 88.36854 0.029252 34.17089
2. Evaluating the value of capacitance to be
injected for the power factor correction
Let us consider the circuit below. The associated phasor
diagram is also being shown.
Fig 5(a)Circuit diagram for power factor improvement
(b)Corresponding Phasor diagram
V= The load voltage
I = The line current
IC= The current flowing through the capacitor
IL=The current drawn by the load
φL=The load angle before connecting capacitance
φ= Load angle after connecting capacitance
Power consumed by load (P1) before correction =V× IL×cosφL
Power consumed by load (P2) after correction = V×I×cosφ
P1 should be equal to P2.
=> P1 = P2 => V× IL×cosφL = V×I×cosφ
=> I = ……..(i)

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From the phasor, it is evident that AB = OA – OB.
But, AB = |IC|
=> |IC| = ILsin φL – I sinφ……….(ii)
Substituting I from eq. (i) into eq.(ii) we get:
|IC|= ILsin φL - ILcos φLtanφ……….(iii)
Since, tanφ is already known from the look up table,
it may be treated as a constant.
Let tanφ=k, => |IC|= ILsin φL - kILcos φL
Again, IL= …………......(iv)
Substituting IL from eq. (iv) to eq. (iii) we get –
IC = tanφL-
XC= => XC=
=> C = = ( tanφL - )
=> C= tanφL – …………(v)
Now, V is the load end voltage kept at a fixed value and P is
the active power consumed by load. If load varies with time,
P also changes with time. However, as because the power
factor is to be improved at any particular point of time, we
can consider P to be fixed for that time.
Thus, C = a tanφL-b…………………………(vi) where, a=
b=
Table 2: Variation of C versus φL
The value of C obtained, when φL is 90
0
is
abnormal. But, it implies that the active power delivered is
zero. However, such a case shall never arise in reality. Thus,
we may ignore the last case. The graph of C versus φL is
shown in Fig 6.
Fig 6: Graph of capacitance versus φL
3. Approximating the actual capacitances with
standard capacitances
Interfacing of static capacitors with the micro-controller-
Fig 7 shows the connection of capacitors to the different port
pins of the micro-controller with the help of npn transistor and
relay. When the micro-controller output signal is high, the
transistor is switched on, establishing sufficient current
through the coil of the electromagnet to close the relay and the
capacitor will be connected in parallel to the load. But,
problem can arise when the microcontroller signal is removed
from the base to turn off the transistor and de-energize the
relay. Trying to change the current through an inductive
element too quickly may result in an inductive kick that could
damage surrounding elements or the system itself. This
destructive action can be subdued by placing a diode across
the coils shown in Fig.7.During the on state of transistor, the
diode is reverse-biased; it sits like an open circuit and doesn’t
affect any thing. However, when the transistor turns off, the
voltage across the coil will reverse and will forward-bias the
diode, placing the diode in it’s on state. In such a situation the
relay is de-energized and the capacitor is removed from the
load circuit.
Fig 7: Circuit diagram to switch the capacitors using micro-controller

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Table 3: Capacitors connected to different pins of the
microcontroller
PORT C PORT C
PA0 10 µf PC0 700 µF
PA1 20 µF PC1 800 µF
PA2 30 µf PC2 900 µF
PA3 40 µf PC3 1 mF
PA4 50 µf PC4 2 mF
PA5 60 µf PC5 3 mF
PA6 70 µf PC6 4 mF
PA7 80 µf PC7 5 mF
PB0 **** PD0 6 mF
PB1 90 µF PD1 7 mF
PB2 100 µF PD2 8 mF
PB3 200 µF PD3 9 mF
PB4 300 µF PD4 10 mF
PB5 400 µF PD5 11 mF
PB6 500 µF PD6 12 mF
PB7 600 µF PD7 13 mF
Table 3 shows which capacitances are interfaced to the
different pins of the micro-controller. The exact value of the
capacitance(C) was calculated by the formula C=atanφL-b.
Now, the exact capacitance C needs to be approximated with
the standard capacitance values, which are connected to the
micro-controller as shown in the table above. For example,
let us assume the C=1256 µF. The higher byte and the lower
byte of C are separated and stored in separate registers say,
CH and CL respectively. Thus, in this case CH=12 and CL=56.
To approximate CL=56, PA5 pin of the micro-controller should
be switched on;- i.e 56 µF is to be approximated by 60 µF.
Then, to approximate CH=12, PC3 and PB3 needs to be
switched on as 1mF and 200 µF are connected in those pins
respectively.
Algorithm for approximating CL
All the ports A,B,C and D are set as output ports. C1 is a
counter, which finds out the exact pin of port A which should
be set high for some value of CL. This is achieved by
calculating the number of times 10 may be subtracted from
CL. However, there can arise two special cases:
1. If CL=00, C1 remains 0 and thus the algorithm is stopped
immediately.
2. Again if C1 becomes greater than 8, PB1 needs to be
switched on as 90 µF is not interfaced to port A.
The flowchart in Fig 8 explains the algorithm clearly.
Fig 8: Flowchart for approximating CL
Algorithm for approximating CH
In this case CH is unpacked and the ten’s digit is stored in H
and the unit’s digit is stored in L. The value of H and L are
then used to switch on the appropriate capacitors. The
flowchart given in Fig 8 explains the algorithm.
Fig 9: Flowchart for approximating CH

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Approximation of L involves 3 steps :-
1. If L=0 , no capacitance value should be injected.
2. If L<7, the corresponding pin of port B, should set
high.
3. If L>7, the corresponding pin of port C should be set
high, since capacitors from 700 µF onwards are
interfaced to port C pins.
Similarly, approximation of H involves 3 steps again :-
1. If H=0, no capacitance is injected.
2. If H<6, the corresponding capacitance of port C
should be injected.
3. And if H>6, the respective capacitance of port D,
shall be injected, as from 6 mF onwards capacitors
are connected to port D pins.
This is absolutely clear from the flowchart in Fig 9.
Conclusion
There is some significant difference between this work and
earlier research on automatic power factor improvement that
I have referred. In the papers [1], [2], [3] and [4] a trial and
error approach was adopted. After detecting poor power
factor, their controller is so designed that the system
switches one capacitor at a time out of a group of many
capacitors. If goal to achieve the specified power factor is
met, the control comes out of the loop, else switching of
capacitor continues till compensation is under control.
On the other hand in this work, the exact value of
capacitance to be injected is calculated mathematically and
not by injecting one capacitance at a time and checking the
new power factor. In earlier methods more number of
switching had to be done, resulting in greater switching loss.
Such a disadvantage is overcome in this technique.
In this work, observing Table 3 we can say that the
resolution of this correction technique is 10 µF (as the
smallest increment that can be taken by the capacitance bank
is 10 µF). Now, if the earlier works want to maintain this same
resolution, their maximum range of operation is only 320
µF(as micro-controller has 32 pins and 10 µF is connected to
each pin), where as this technique permits a far greater range
of 10-12 mFs. Thus for dynamic load on a power system,
which varies through great limits, this work will be more
efficient.
In the reverse way it may be said that if the earlier
works are to maintain the same range of operation, their
resolution will become poor and thus accuracy will fall.
References
1. Nagaranjan M, Kandasamy K.V(2012) “Automatic
power factor correction for inductive load using
PIC.”International Conference on Modeling,
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2. Anant Kumar Tiwari, Durga Sharma, Vijay Kumar
Sharma (2014) “ Automatic power factor correction
using capacitive bank.”Int Journal of Engineering
Research and Applications, Volume-4, Issue-2,Feb-
2014,pp.393-395.
3. Prasad Phad(2014) “Minimizing the penalty in
industrial sector by engaging automatic power factor
correction panel using micro-
controller.”Multidisciplinary Journal of Engineering
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73-79.
4. Abhinav Sharma, Vishal Nayar, S. Chatterjee, Ritula
Thakur, P.K Lehana (2013)“PIC micro-controller
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