Performance of Six-Pulse Line-Commutated Converter in DC Motor Drive Application

Proceedings of the Pakistan Academy of Sciences 51 (3): 201–208 (2014) Pakistan Academy of Sciences
Copyright © Pakistan Academy of Sciences
ISSN: 0377 - 2969 (print), 2306 - 1448 (online)
Research Article
Performance of Six-Pulse Line-Commutated Converter in
DC Motor Drive Application
Syed Riaz ul Hassnain1, Haider Zaman1, Zunaib Ali1, Muqadsa Iftikhar1,
and Tariqullah Jan2*
1University of Engineering & Technology Peshawar, Abbottabad Campus, Pakistan
2University of Engineering & Technology, Peshawar, Pakistan
Abstract: This paper presents the speed control of DC motor using six pulse controlled rectifier. The
conventional Proportional Integral (PI) control is used for firing angle control. The armature current is fed
back and compared with reference current representing desired speed values. The proposed system is
simulated using SimPowerSystem and Control System Matlab toolbox. The time domain plot of reference
and actual armature current are shown in results section. The results are satisfactory with deleterious effect
on input current. The frequency plot of input current is provided to show the harmonic contents, generated
as a result of control operation.
Keywords: Silicon control rectifier (SCR), DC motor, proportional integral control, harmonics, displacement
factor and distortion factor, line-commutated converter (LCC)
1. INTRODUCTION
The continuous popularity of DC motors is due to
their efficient performance and control advantages
over ac motors. The scope of DC motor varies
from high speed automation to electric vehicles.
The dc motors are increasingly used in
applications where speed or torque needs to be
varied and controlled with high accuracy. The
most commonly used DC motors for variable
speed are series and separately excited [1], but
generally DC series motor is used for traction
purposes.
With the development of high power solid
state switches, the DC motor control applications
started to grow almost exponentially. Today,
broadly two methods are employed (based on
switches controllability) to set the applied voltage
as a variable parameter. In the first method applied
voltage can be made variable by using controlled
rectifiers which provide variable DC from a fixed
ac shown in Fig. 1(a) whereas in the second
method uncontrolled rectifiers are used to produce
a fixed DC from a fixed ac and at the output of
rectifier a DC chopper circuit is used which
provide variable DC just by changing the duty
cycle shown in Fig. 1(b) [2].
In this paper our major focus is to discuss the
use of SCR based converters for speed regulation
i.e. to get variable DC voltage from a fixed ac
voltage. The use of SCR allows simple voltage
variability just by varying the firing angle and
thereby the speed of the motor. For the speed
regulation of DC motor, the speed needs to be
monitored as the speed of DC motor changes with
load torque [3]. The control circuit corrects the
firing angle of SCR, which in-turn changes the
armature voltage, thereby regulating the speed.
The control circuit used here consists of PI current
controller, which takes current as feedback,
compares it to the reference and then generates
firing angle of SCRs to minimize the error and
regulate the speed [4-5]. The benefits of feedback
network (which is also called closed loop control
system) are of high level of accuracy,
compensation of load variations and system
nonlinearities, fast response and continuous
monitoring of process [6]. The block diagram of
DC motor fed with closed loop controlled rectifier
is shown on Fig. 2.
First, the mathematical model of DC motor is
developed in section 2. Working principle of six
————————————————
Received, July 2014; Accepted, August 2014
*Corresponding Author: Tariqullah Jan, Email: tariqullahjan@nwfpuet.edu.pk

202 Syed Riaz ul Hassnain et al
Fig. 1(a). Controlled rectifier fed motor.
Fig. 1(b). Uncontrolled rectifier followed by DC-DC converter.
Fig. 2. Block diagram of DC motor fed with closed loop controlled rectifier.
Fig. 3. Separetly excited DC motor.

Performance of Six-Pulse Line-Commutated Converter in DC Motor Drive Application 203
pulse line-commutated converter is described in
section 3. The section 4 presents the complete
system responses under a transient condition,
followed by a conclusion.
2. DC MOTOR MODELING
The equivalent circuit of commonly used DC
motor i.e. separately excited DC motor is shown in
Fig. 3. In case of separately excited DC motor the
armature circuit and field circuit is independent of
each other. When the armature current ݅஺ is passed
through the armature circuit placed in a magnetic
field produced by field current ݅௙passing through
the field circuit, the motor produces a back
electromotive force ܧ஺ and induced torque ߬௜௡ௗ ,
which balance the load torque at a particular
speed. For separately excited DC motor, the
current passing through the field windings is
independent of the current passing through
armature windings. The transient and steady state
analysis of separately excited DC motor is derived
using Fig. 3.
Transient Analysis
The instantaneous field current is given by
differential equation:
ܸ௙ ൌ ݅௙ܴ௙ ൅ ܮ௙
ௗ௜೑
ௗ௧ (1)
The instantaneous armature current can be
calculated from:
ܸ஺ ൌ ݅஺ܴ஺ ൅ ܮ஺
ௗ௜ಲ
ௗ௧ ൅ ܧ஺ (2)
Where ܧ஺ is back emf of the motor and is given by
ܧ஺ܭ߱݅௙ (3)
The torque induced by the motor ߬௜௡ௗ is:
߬௜௡ௗ ൌ ܭ݅௙݅஺ (4)
The induced torque in terms of load torque is:
߬௜௡ௗ ൌ ܬ ௗఠ
ௗ௧ ൅ ܤɘ ൅ ߬௟௢௔ௗ (5)
Where,
߱ ൌ a‰—Žar s’‡‡d ሺrad⁄sሻ
ܤ ൌ …‘s–a– ‘ˆ ˜‹s…‘—s ˆr‹…–‹‘ ൬N.
m
rad⁄s
൰
ܭ ൌ ˜‘Ž–a‰‡ …‘s–a– ሺ⁄ െ radȀs)
ܮ஺ ൌ arma–—r‡ …‹r…—‹– ‹d—…–a…‡ ሺሻ
ܮ௙ ൌ ˆ‹‡Žd …‹r…—‹– ‹d—…–a…‡ ሺሻ
ܴ஺ ൌ arma–—r‡ …‹r…—‹– r‡s‹s–a…‡ ሺπሻ
ܴ௙ ൌ ˆ‹‡Žd …‹r…—‹– r‡s‹s–a…‡ ሺπሻ
߬௟௢௔ௗ ൌ Ž‘ad –‘r“—‡ ሺN. mሻ
ܬ ൌ ‘m‡– ‘ˆ ‹‡r–‹a
Steady-state Analysis
For the steady state analysis all the derivatives
with respect to time are put to zero. The resulting
steady state average quantities are:
ܸ௙ ൌ ܫ௙ܴ௙ (6)
ܸ஺ ൌ ܫ஺ܴ஺ ൅ ܧ஺
ൌ ܫ஺ܴ஺ ൅ ܭ߱ܫ௙ (7)
߬௜௡ௗ ൌ ܭܫ௙ܫ஺
ൌ ɘ ൅ ߬௟௢௔ௗ (8)
The power induced or developed is:
ܲ௜௡ௗ ൌ ߬௜௡ௗ߱ (9)
In modeling of DC motor, the main concern is to
find the relationship between speed of DC motor
and armature voltage. The speed of the separately
excited DC motor in terms of armature voltage can
be derived using ܧݍ. ሺ͹ሻ i.e.
߱ ൌ ௏ಲିூಲோಲ
௄ூ೑
(10)
Using ܧݍ. ሺ͸ሻ,ܫ௙ ൌ ܸ௙⁄ܴ௙
߱ ൌ ௏ಲିூಲோಲ
௄௏೑⁄ோ೑ (11)
From ܧݍ. ሺͳͳሻ it is cleared that the speed of motor
is directly related to armature voltage ܸ஺, larger
the armature voltage greater will be the speed and
vice versa[6]. The type of control in which the
controlling parameter is voltage, is called voltage
control. In practice, the armature and filed currents
are kept constant to fulfill the demand of torque
while armature voltage is made variable to control
the speed. So, in this study a DC motor is fed with
the output of a three phase SCR bridge.
3. WORKING PRINCIPLE OF SIX PULSE
CONVERTER
The circuit diagram for 3-phase six pulse
controlled rectifier is shown in the Fig. 4.The
circuit consist of six thyristors, i.e.,

Fig. 4. Six pulse controlled rectifier.
Fig. 5. The૜׎ line to line voltage waveform ࢂሺ࣓࢚ሻ, converter output voltage waveform ࢂ࢕ሺ࣓࢚ሻ, phase ǲࢇǳ current
waveform ࢏ࢇሺ࣓࢚ሻ.

ܶଵ,ܶଶ,ܶଷ,ܶସ,ܶହand ܶ଺ for rectification purpose.
Three phase system consist of three sinusoidal
input voltages (ܸ௔ǡ ܸ௕ܸ௖) with same frequency
and magnitudes i.e. (݂௔ ൌ ݂௕ ൌ ݂௖ ൌ ݂)
(ܸ௠௔ ൌ ܸ௠௕ ൌ ܸ௠௖ ൌ ܸ௠) but shifted 120° from
each other. The operating principle of the circuit is
that, the pair of SCR connected between the lines
havinghighest amount of line-to-line voltage will
conduct provided that the gate signal is applied to
SCRs at that instant. i.e. SCR needs gate signal in
addition to ܸ஺௄ ൐ 0. The input and output voltage
wavefroms and input current waveform for phase
“ܽ” are shown in Fig. 5.
Between 0 ൑ ߱ݐ ൑ ߨ
ൗ3 the highest line-to-line
voltage is ܸ௖௕, with ܶସ ܶହ intially
conducting. By firing ܶ଺ at delay angle of ߙ,results
ܸ௖௕at load. It should be noted is that ܸ௖௕apperas
across load till ߨ
ൗ3 ൅ ߙ. Although beyond the
point ߨ
ൗ3 the line-to-line voltage ܸ௔௕ has the
highest value but until ܶଵ is not fired, it will not
appear across the load and the maximum voltage
across load is still ܸ௖௕.As ܶଵ is fired, ܶହ turns off
and now current passes through ܶଵ ܶ଺ i.e. ܸ௔௕
appears across the load. Another impotant
conclusion is that, only one SCR needed to be
fired at a time except for the first cycle. Because
first SCR of the pair already conducts due to phase
sequence.
From Fig. 5 it can be seen that frequency
of output pulsating voltage is ͸݂ and in this way
the harmonic components are shifted to higher
frequencyAs compare to three pulse converter, it
reduces the need of filtering at the output[7-8].The
output DC voltage of a controlled rectifier is a
function of the firing angle ߙ and it can be
calculated from Fig. 5. The relation is:
ܸௗ௖ ൌ
1
ߨ
ൗ3
ఈାଶగ
ൗଷ
න √3ܸ௠ sinሺ߱ݐሻ ݀ሺ߱ݐሻ
ఈାగ
ൗଷ
ሺ12ሻ
ܸௗ௖ ൌ
3√3ܸ௠
ߨ
ఈାଶగ
ൗଷ
න sinሺ߱ݐሻ ݀ሺ߱ݐሻ
ఈାగ
ൗଷ
ܸௗ௖ ൌ
3√3ܸ௠
ߨ
െcosሺ߱ݐሻ
ߙ ൅ 2ߨ
ێێێێ
ۏ
ൗ3
ߙ ൅ ߨ
ൗ3
ܸௗ௖ ൌ
െ3√3ܸ௠
ߨ
ൗ3൯ െ cos൫ߙ ൅ ߨ
ൣcos൫ߙ ൅ 2ߨ
ൗ3൯൧
ܸௗ௖ ൌ
3√3ܸ௠
ߨ
cosሺߙሻ ሺ13ሻ
ܸௗ௖ ൌ ܸௗ௠ cosሺߙሻ ሺ14ሻ
ܸ௡ ൌ
ܸௗ௖
ܸௗ௠
cosሺߙሻ ሺ1ͷሻ
Where ܸௗ௠ is the average output voltage atߙ ൌ
0and ܸ௡ is normanilzed average voltage.The plot
in Fig. 6 shows the normalized output DC voltage
verses the firing angle ߙ.
It can be seen from Fig. 5 that for ߙ ൌ ߨ
ൗ3
the rectified output voltage reaches zero crossing.
If ߙ is increased beyond ߨ
ൗ3 i.e. ߙ ൐ ߨ
ൗ3, the load
voltage becomses discontinous for resistive load
where as for inductive load the negative voltage
appears across load. Fig. 6 shows that by varying
ߙ between 0 ݐ݋ ߨ
ൗ2 output varies between 1 0
i.e. rectification region and by varying ߙ between
ߨ
ൗ2 ݐ݋ ߨ output varies between 0 െ 1 i.e.
inversion region. Rectification region is
represented by 1ୱ୲Quardant and inversion region
by 4୲୦Quardant resulting in 2 Quardant
operation.
There is also an issue in using six pulse
converter i.e. the effect of converter on input
power factor. The power factor for highly
inductive load (motor) can be calculated as:
ܲ௔௩
ܲ. ܨ ൌ
3 ܸ௜௥௠௦ ܫ௜௥௠௦
ܲ. ܨ ൌ
͸ ܸ௠ ܫ௢
3 ܫ௢ߨ ή ܸ௠⁄√2
cosሺߙሻ
ܲ. ܨ ൌ 0.9 cosሺߙሻ ሺ1͸ሻ
From ܧݍ. ሺ1͹ሻ it can be seen that by varying ߙ
between 0 ݐ݋ߨ , the power factor also vary
between 0.9 0for rectification region and
0 െ 0.9 for inversion region.
4. RESULTS AND DISCUSSION
The circuit consist of sixpulse line-commutated
converter (LCC), DC motor and the control
system; consisting of PI controller is simulated
using Simulink. The simulated system is shown in
the Fig. 7. The Synchronized 6-Pulse Generator

Rectification Region Inversion Region
0 0.5 1 1.5 2 2.5 3
1
0.5
0
-0.5
-1
α (rad)
Normalized Average Voltage
Fig. 6. Normalized average voltage as function of ࢻǤ
Source Impedence DC motor equivalent circuit
Iref
IA
Fig. 7. Model for speed control of DC motor.
208 V rms L-L
3-phase Source
DIstortion Factor
30
25
20
15
10
5
Fig. 8. Variation of Motor Current Corresponding to Reference Current.
Continuous
[A]
phase a Current
+ i-ia
alpha plot
Vo IA+Iref
v +-
Vo
v +-
Vca
Vc
v +-
Vbc
Vb
v +-
Vab
Va
g
A
B
C
+
-
Thyristor Converter
alpha_deg
AB
BC
CA
Block
pulses
Synchronized
6-Pulse Generator
Input Current
DSPF
Power Factor
Power Factor Calculation
0.5043
Power Factor
alpha_deg Dif f
PI Controller
Mux
Iref
+ i-
Id
0.9589
Distortion Factor
0.526
Displacement Factor
0
[A]
Vo
0 0.02 0.04 0.06 0.08 0.1
0
Time (sec)
Motor and Reference Current
Motor Current Id
Reference Current Idref

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
120
100
80
60
40
Fig. 9. Variation in ࢻ to achieve desired Reference Current.
300
250
200
150
100
50
0
-50
Fig. 10. Variation inconverter output to achieve desired Reference Current.
100
80
60
40
20
Fig. 11. Frequency Specturm for ࢻ ൌ ࣊
ൗ૟
20
Time (sec)
α (degree)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
-100
Time (sec)
LCC Output
0 60 120 180 240 300 360 420 480 540 600 660
0
α=π/6
Frequency Hz
Normalized Harmonic Magnitude

block is used here to fire the six thyristors
(Thyristor Bridge) of a six-pulse converter. The
output of the block is a vector of six pulses
individually synchronized on the six thyristor
voltages. The DC motor is modelled as series
connected RL and a DC source, where the DC
source representing the back emf.
In this paper the speed is controlled by
conrolling the current through the motor. For a
desired value of speed, the reference current is
provided and using PI controlled the reference
current is achieved in a little time. Thetime
domain plot for variation of motor armature
current ܫ஺ for corresponding reference current ܫ௥௘௙
is shown in Fig. 8. The pattern of firing angle ߙ
and LCC output voltage for desired ܫ௥௘௙ is shown
in Fig. 9 and Fig. 10, respectively. Fig. 8 shows
that intially, the current through the motor was
zero but when the value of reference current is set
as 10, the motor current started increasing from
zero level and reached the same level as reference
current within microseconds. As the reference
current is achieved, current through the motor
remained constant for the rest of time. Again,
when after sometime the reference current is
varied from 10 ݐ݋ ʹͷ, the motor achieved and
maintained the same current, which implies that
the current is regulated. The current regulation
directly implies that the speed is also regulated.
One of the important issue needed to be
analyzed, is the effect of converter on input power
factor because according to ܧݍǤ ሺ1͹ሻ the power
factor is the function of firing angle ߙ. The power
factor block in Fig. 7 is used to compute the power
factor . Power factor is product of displacement
factor(due to phase difference between voltage and
fundamental component of input current) and
distortion factor(due to distortion in input current).
For different values of ߙ, the value of distortion
and displacement factor is listed in Table.1. For
ߙ ൌ ߨ
ൗ6 specturm of input current is shown in
Fig. 11
Table 1. Six pulse converter power factor,
distortion factor and displacement factor for
different α’s.
ࢻ Displacement
Factor
Distortion
Factor
Power
Factor
ߨ
ൗ6 0.758 0.975 0.739
ߨ
ൗ4 0.639 0.967 0.618
ߨ
ൗ3 0.487 0.955 0.465
5. CONCLUSIONS
The line-commutated converter with conventional
PI control was used for speed regulation of DC
motor. The system was tested for the step change
in reference current and it provided satisfactory
results. When the load torque increases due to
extra load i.e.߬௟௢௔ௗ ൐ ߬௜௡ௗ, the motor speed
ultimately decreases, this will increase the
error ݁. To minimize the error the PI controller
changes the firing angle of SCR to increase the
voltage ܸ஺ applied to the armature of DC motor,
which increases the armature current ܫ஺, thereby
increasing the speed of DC motor. The problem
arises at low speed values (i.e. large values ofߙ) as
displacement factor reduces by keeping ߙ large.
6. REFRENCES
1. Kushwah, R. S. Wadhwani. Speed control of
separately excited DC motor using fuzzy logic
controller. International Journal of Engineering
Trends and Technology 4(6): 2518-2523 (2013).
2. Afrasiabi, N. M.H. Yazdi. DC motor control using
chopper. Global Journal of Science Engineering and
Technology 8: 67-73 (2013).
3. Shastri, S. P. Pandey. A comparative analysis of
firing angle based speed control scheme of DC motor.
International Journal of Engineering Research and
Applications 3(4): 232-235 (2013).
4. Jaiswal, M. M. Phadnis. Speed control of DC
motor using genetic algorithm based PID controller.
International Journal of Advanced Research in
Computer Science and Software Engineering 3(7):
247-253 (2013).
5. Pal, A.K. R.K. Mudi. Speed control of DC motor
using relay feedback tuned PI, fuzzy PI and self-tuned
fuzzy pi controller. Control Theory and Informatics
2(1): 24-32 (2012).
6. Rashid, M.H. Multilevel inverters. In: Alice Dworkin,
and Dona King. Power Electronics Circuits, Devices
and Applications, 3rd ed. Pearson Education, p. 409-
419 (2003).
7. Pyakuryal, S. M.A. Matin, Feedback controller for
a 3-phase 6-pulse rectifier. The International Journal
of Engineering and Science 2(8): 23-27 (2013).
8. Pyakuryal, S. M. Matin. Harmonic analysis for a 6-
pulse rectifier. International Organization of
Scientific Research Journal of Engineering 3 (3): 57-
60 (2013).
9. Nedeljkovic, M. Z. Stojiljkovic. Fast current
control for thyristor rectifiers. IEE Proceedings-
Electric Power Applications 15(6): 636- 638 (2003).
10. Gupta, R., R. Lamba S. Padhee. Thyristor based
speed control techniques of DC motor: A
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Scientific and Research Publications 2(6): 1-6 (2012).

Performance of Six-Pulse Line-Commutated Converter in DC Motor Drive Application

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