Power factor improvement in switched reluctance motor drive using pwm

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME
48
POWER FACTOR IMPROVEMENT IN SWITCHED RELUCTANCE
MOTOR DRIVE USING PWM CONVERTER
Mahavir Singh Naruka1
, D S Chauhan2
, S N Singh3
Uttarakhand Technical University, Uttrakhand, INDIA
ABSTRACT
This paper present the power factor improvement technique in the midpoint converter based
Switched Reluctance Motor (SRM) drive using a AC-DC three level Pulse Width Modulation
(PWM) converter. A conventional SRM drive produces very high level of harmonics content and
poor power factor at ac mains. The proposed converter with midpoint converter fed SRM drive
improves the power factor at ac mains with low current harmonics. The SRM drive with converter is
modeled and its performance is simulated in Matlab/Simulink environment.
Keywords: Midpoint converter, Power quality, THD, PFC, SRM , PWM converter
I. INTRODUCTION
Switched Reluctance Motor (SRM) have a simple and robust construction; they eliminate
permanent magnets, brushes, commutators and windings on the rotor side. As a result of its inherent
simplicity , SRM offers advantages of reliable and low cost variable speed drives [1-2]. This motor
drive needs power converters for its operation. Many type of power converters for SRM drive is
reported in literature [3-4]. These converters require stable DC supply as an input source. For this
purpose if a conventional rectifier unit is used as front end converter, the supply current drawn is in
pulse form. This supply current is of very low power quality.
The block diagram of conventional SRM drive is shown in fig-1. It can be divided into
supply utility, AC/DC converter, Capacitor network, DC/DC machine converter and SRM.
The attractive features of these converters are constant DC bus voltage, low harmonic
distortion of the utility current, bi-directional power flow & controllable power factor.
It is very difficult to maintain the balanced voltage across each capacitor at DC bus. Two
capacitor split the DC link voltage into two equal voltages. Phase voltage of SRM is provided by
each capacitor. By varying the conduction period of a phase, we can balance the capacitor voltage.
Source current drawn by the motor is in pulse form so it induces ripple voltage across the capacitors.
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ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 4, Issue 4, July-August (2013), pp. 48-55
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49
Torque ripples and THD (Total Harmonics Distortion) of supply current are increased due to the
unbalancing of voltages across capacitors. To improve the input power factor and DC capacitor
voltage balancing, a power factor controller is needed at input side of the converter. For this a three
level PWM converter fed SRM drive is designed, modeled and simulated in Matlab/Simulink.
Fig-1 Bridge rectifier as front end converter fed SRM drive
The proposed SRM drive system is capable of improving the power factor close to unity with
low THD of supply current. The power quality is also within IEEE standard [5]
II. PROPOSED SYSTEM CONFIGURATION
In this paper, a non linear control technique for a PWM three level voltage source AC-DC
converter associated with a mid point IGBT based machine converter is proposed.
To improve the power factor at the input ac mains, a PWM converter is used as front end
converter for SRM drive. Fig-2 shows the schematic diagram of the proposed converter. The
proposed system consists of a PWM converter which is an ac-dc boost converter and mid –point
converter based SRM drive. In this system the mid-point converter will be used as a machine side
converter & PWM will work as a front end converter.
Fig-2 proposed PWM converter fed SRM drive
The proposed converter has resistor-inductor series circuit on each input line & two capacitor
on the DC link. The converter has a three level bridge of selected forced-commutated power
electronics devices. The series RC snubber circuits are connected in parallel with each switch device.
The IGBT bridge converter has a combination of universal bridge and discrete IGBT switches. These
power electronic switches are controlled by gate pulses, which are given by PWM Gate pulse
generator system. The PWM gate pulse generator has a combination of voltage & current regulator.
The output of the controller is given to the discrete 3-phase PWM generator, which produces the
controlled pulse as per requirement.
Diode bridge
rectifier
Front-end
converter
Machine
converterAC
Source
Gate pulse
(PWM)
generator SRM

50
III. DESIGN & ANALYSIS OF CONVERTER
Traditionally Diode Bridge Rectifier (DBR) are used for rectification. This rectifier can only
produce a constant DC voltage, which is a function of system voltage. A control rectification system
can be used to produce variable dc output voltage. But both these rectifier behave as a non-linear
load. A pulse width modulated (PWM) rectifier, shown in Fig-3, draws near sinusoidal current from
the supply mains. Also the DC link voltage can be regulated & the supply power factor is adjustable.
By applying KVL on supply side and KCL on load side gives
os
s
SVv
dt
di
L −= ----------------(1)
os
o
V
R
Si
dt
dv
C
1
−= ----------------(2)
where S is the switching function of the PWM converter & Vs=Vm sinωt is the input ac
source voltage with the amplitude of Vm and angular frequency of ω. Resistance on the ac side is
neglected.
The main control objectives for such PWM converter are to produce an input ac current is
with low harmonics content at a high power factor and to control the average DC voltage Vo. It is
clear that unity pf can be achieved , if the ac input current tracks the following reference current.
Is*=Im sinωt -------------------------(3)
By the combination of eq (1), (2) & (3) , we can write
)(
1
)(
*
dt
di
Lv
V
tS s
s
sso
o −=
dt
dv
C
R
V
i
tS ossso
s
+= (
1
)( *0 ------------------(4)
where Vsso=Vo+Vhr is the steady state output voltage with a DC reference voltage Vo and a
harmonic ripple content Vhr.
Now let
X1=is-is* , X2=Vo-Vsso, S=So+ S -----------------(5)
Where x1 & x2 are the state variables and S is the perturbation of the switching function S.
By using the above equations the following resulting equations are obtained
)( 22
.
1 xVSxSxL ssoo +∆−−=
−
------------------(6)
R
x
xiSxSxC so
2
11
.
2 )*( −+∆−=
−
---------------(7)
With the switching function variable (6) & (7) are non-linear & time varying.

51
A. Control strategy
According to the Lyapunov stability theory, any linear or non-linear system must eventually
settle down to an equilibrium point means every system is stable if there exists a positive definite
Lyapunov function V(x), whose time derivative is negative definite[5].
Then the equilibrium point at the origin (X1=0,X2=0) is globally asymptotically stable. Now consider
the following positive definite Lyapunov function for the converter
.
2
2
2
1
2
1
2
1
)( CxLxxV +=
--------------------- (8)
Taking the derivative of eq(8) with respect to the time gives
-----------------------(9)
If we put the value of Lx1 & Cx2 from eq(6),(7) to eq (9)
-------------(10)
By the equation (10), we can conclude that derivative of V(x) along with any system
trajectory becomes negative definite , if S is chosen to be
, β<0 ----------------------(11)
where β is an arbitrary real constant number. In order to generate the switching function S (or
compute in a digital implementation), it is necessary to predict the time varying steady state output
voltage Vsso=Vo+ Vhr(t) for the present operating point of the converter. In this method there is a
problem for calculation of Vhr(t) because the methods for finding the ripple component Vhr is
complex and less accurate so if we neglect this component, the calculation will be simple.
, β<0 -----------------------(12)
There are three criterion for selection of arbitrary real constant (β)
(i) It gives a stability region as large as possible.
(ii) Satisfactory dynamic response is obtained over the operating range of the converter.
(iii)Ripple content should be minimum.
A typical range of the β for the converter in this study is found to be -0.002≤β≤-0.00015.
B. PWM gate pulse generation
For the purpose of triggering pulses the input supply voltage Vabc , supply current Iabc &
generated DC link voltage Vdc are filtered through low-pass filters. The normalized supply voltage
are fed to the Phase Locked Loop (PLL) system. This PLL can be used to synchronize on a set of
variable frequency , three phase sinusoidal signals. The output of this PLL system are fed to current
& voltage regulator incorporate with normalized supply currents & voltages. The signal generated by
the combination of these regulators is fed to the discrete 3-phase PWM generator in series with delay
function. The block diagram are shown in Fig-3 & Fig-4
.
2211)(
ooo
xCxxLxxV +=
.
2
2
12 )*()(
R
x
SVxixxV ssos
o
−∆−=
.
12 )*( ssos VxixS −=∆ β
.
12 )*( os VxixS −=∆ β

52
Vdc ref
Idref
Vdc
Fig-3 Block diagram of DC voltage regulator
Vabc
Pulse
Iabc
ddc Unit Delay
Fig-4 Block diagram of regulator
A discrete 3-phase PWM generator block generates the pulse for carrier based pulse-width
modulation converter. The block can be used to fire the forced-commutated devices (FET’s, GTO’s
or IGBT’s) of 2-level or 3-level converters. By using a single bridge or twin bridges connected in
twin configuration vectorized outputs P1 & P2 which contains either 6-pulses( 2-level) or 12-
pulses(3-level) are used for triggering.
C. Control of SRM
The model used in this scheme is a current-controlled 60-kW 6/4 SRM drive using the SRM
specific model based on measured magnetization curves. The SRM is fed by a three-phase
asymmetrical power converter having three legs, each of which consists of two IGBTs and two free-
wheeling diodes. During conduction periods, the active IGBTs apply positive source voltage to the
stator windings to drive positive currents into the phase windings. During free-wheeling periods,
negative voltage is applied to the windings and the stored energy is returned to the power DC source
through the diodes. The fall time of the currents in motor windings can be thus reduced. By using a
position sensor attached to the rotor, the turn-on and turn-off angles of the motor phases can be
accurately imposed. This switching angle can be used to control the developed torque waveforms.
The phase currents are independently controlled by three hysteresis controllers which generate the
IGBTs drive signals by comparing the measured currents with the references. The IGBTs switching
frequency is mainly determined by the hysteresis band.
IV. MATLAB SIMULATION
The PWM converter along with midpoint converter based SRM drive is modelled and it is
simulated in Matlab/Simulink environment shown in fig-5. Three phase 600V, 50Hz ac supply is
given to the PFC converter & the DC link voltage is controlled to 260V. The IGBT based midpoint
machine converter is considered for 60kW, 6/4 SRM in this simulation. Object of this paper to
reduce the input current harmonics and give the almost unity power factor on supply side. The
concerned waveforms of simulation are shown in fig-6 & table-I.
+_
-
PI
Saturation
Controller
ZeroOrderHold
Controller
Anti-Aliasing
Filters
G1/Z

53
Fig-5: Matlab simulation model
Fig-6 Steady State Response Of Input Supplied Current, Voltage Waveform, Dc Link Output & Thd
Of Supplied Current

54
TABLE I
PERFORMANCE PARAMETERS OF PROPOSED PFC FOR SRM DRIVE
Speed
(rpm)
Torque
(N-m)
Vdc
(Volts)
THD
(%)
Power
factor
2000 81.78 263.3 1.5 0.9988
2500 88.65 262.7 1.2 0.9989
3000 79.01 269.7 1.01 0.9989
3500 76.1 264.1 0.99 0.9990
4000 68.28 262.5 0.70 0.9990
4500 49.64 262.7 0.46 0.9992
5000 18.18 262.7 0.36 0.9992
V. RESULT & DISCUSSION
The performance of the proposed PWM converter fed SRM drive has been simulated in
Matlab/Simulink environment.
The results are compared with the conventional converter fed SRM from the bridge rectifier.
The current drawn by the conventional converter is non-sinusoidal, distorted & containing high level
of harmonic distortions. The percentage THD of supply current is 61.65% with fundamental current
3.614(r m s). AT 20% of rated load, THD of supply current is 92.39% with fundamental current of
1.149 %(r m s). As the load torque reduced the THD of supply current is increased. This shows that
the conventional bridge rectifier fed SRM has a very high THD & low power factor of supply current
[6].
Fig-6,Show the results of Steady state & dynamic performance of supply current, THD & dc
link voltage for the proposed PWM converter based SRM.
VI. CONCLUSION
The PWM converter along with midpoint converter based SRM drive is designed &
simulated in Matlab/Simulink environment. Three phase 600V, 50Hz ac supply is given to the PFC
converter & the DC link voltage is controlled to 260V. The IGBT based midpoint machine converter
is considered for 60kW, 6/4 SRM in this simulation. The obtained results have been compared with
conventional rectifier. Using this technique, the power quality has been improved as compared to the
conventional bridge converter. It has been seen that this scheme can withstand under wide range of
speed with almost unity power factor. The THD of supply current and power factor are well within
IEEE 519 standard limits [7].
REFERENCES
[1] J. W. Ahn, “Switched Reluctance Motor” (Korean), Osung Media, 2004.
[2] R. Krishnan, “Switched Reluctance Motor Drives: Modeling, Simulation, Analysis, Design
and Applications” CRC Press, 2001.
[3] S. Vukosavic and V. R. Stefanovic, “SRM Inverter Topologies: A Comparative evaluation”’
IEEE Trans. On industry Applications, vol. 27, no. 6,pp 1034-1047, Nov/Dec.1991
[4] Pollock and B. W. Williams, “Power converter circuits for Switched Reluctance Motors with
the minimum number of switches” in Proc. Inst. Elect. Eng., vol 137, pt. B, no-6, 1990,
pp 373-384.

55
[5] Hasan K. C. and Osman K., “Control strategy for single phase PWM ac/dc voltage source
converters based on Lyapunov’s direct method”, International Journal of electronics, 2000,
Vol.-87, No.-12, pp 1485-1498.
[6] M Rajesh, Bhim singh “Power quality improvement in Switched Reluctance motor using
Vienna Rectifier” (2012) The IEEE website. [Online]. Available: http://www.ieee.org/
[7] M. Shell. (2002) IEEEtran homepage on CTAN. [Online]. Available:
http://www.ctan.org/tex/archive/macros/latex/contrib/supported/IEEEtran.
[8] Dr. Hina Chandwani, Himanshu N Chaudhari and Dhaval Patel, “Analysis and Simulation of
Multilevel Inverter using Multi Carrier Based PWM Control Technique”, International
Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 3, 2013,
pp. 200 - 208, ISSN Print : 0976-6545, ISSN Online: 0976-6553.
[9] Pradeep B Jyoti, J.Amarnath and D.Subbarayudu, “The Scheme of Three-Level Inverters
Based on Svpwm Overmodulation Technique for Vector Controlled Induction Motor Drives”,
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[10] K.Vijaya Bhaskar Reddy and G.V. Siva Krishna Rao, “Modeling and Simulation of Modified
Sine PWM Vsi Fed Induction Motor Drive”, International Journal of Electrical Engineering
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ISSN Online: 0976-6553.

Power factor improvement in switched reluctance motor drive using pwm

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