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EXPERIMENTAL INVESTIGATIONS ON A PERMANENT MAGNET BRUSHLESS
DC MOTOR FED BY PV ARRAY FOR WATER PUMPING SYSTEM
C.L. Putta Swamy, Bhim Singh, B.P. Singh and S.S. Murthy
Department of Electrical Engineering, IIT Delhi
Hauz Khas, New Delhi- 110 016, INDIA, Fax 11-6862037
Abstract: This paper describes the experimental study carried
out on a Permanent Magnet Brushless DC (PMBLDC) Motor
drive coupled to a pump load powered by photovoltaic (PV) array
I for water pumping system. A simple low cost prototype controller
has been designed and developed without current and position
sensors which reduces drastically the overall cost of the drive
system. This controller is used to test the dynamic behavior of
the PMBLDC motor drive system. The mathematical model of
the system is developed with a view to carry out a comparison
between experimental and simulated response of the drive
system. The necessary computer algorithm is developed to
analyze the performance under different conditions of varying
solar insolation for a pump load. The developed state space
equations are simulated to obtain the performance characteristics
which are also verified by conducting suitable experiment on the
developed system.
Key W o r d s : Permanent Magnet Brushless DC Motor,
Photovoltaic Array, Position Sensor, Sokr Insolation, Water
Pumping System
INTRODUCTION
New types of electric motors like Permanent Magnet(PM)
Motors, Switched Reluctance Motors (SRM) and Stepper
Motors(SM) have emerged
due to the development in
engineering material technology and tremendous improvement in
solid state devices and circuits[l]. Owing to the technical
improvements in motors, controllers and feedback techniques
electronically commutated motors (ECM) (also known as brushless
motors) are replacing brushed motors in many applications. The
main advantages of the ECM over the brushed motors are
reduced maintenance requirements, environmental effects and
electromagnetic radiation[2]. Within the last three decades,
several improved magnetic materials are developed for high
performance PM motors. Rare-earth magnets which are usually
Samarium Cobalt alloys, are still among the best performing
magnetic materials. The most recent developments are the
Neodymium-Iron-Boron alloys. The magnetic performance of
these alloys is about 30% better compared to Samarium Cobalt
magnets[l]. PMBLDC motor drives have received considerable
attention recently as their performance is superior to those of the
brushed dc motors and ac motors for servo and adjustable speed
iapplications. Specially designed low inertia motors fed by a
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MOSFBT based current controlled voltage source inverter(CCVSI), provide desirable features such as high power to weight
ratio, high torque to current ratio, fast response and above all
high operating efficiency. The features of high operating
efficiency, brushless construction, maintenance free onerationand
increasing awareness about energy conservation have given a
scope to employ PMBLDC motor in water pumping application
operated by PV array particularly in remote villages where
electric supply is either not available or reliable.
Needless to say that the global energy crisis experienced today
is the result of disproportionate and wasteful use of some of the
fossil fuels, such as petroleum, coal and natural gas, which took
several million years in to synthesis. The humankind today is
beset with the global environmental problems of the greenhouse
effect and acid rain. The key to resolving these problems lies in
the development of cleaner form of energy. Solar cells, which
convert the solar energy into electricity through the PV effects
of the semiconductors are very useful in tackling some of the
global environmental problems. One of the important
characteristics of solar energy systems is their 'invariant'
nature. They do not extract any material from earth and they do
not return any pollutant to the environment[4-6]. New processes
for manufacturing of PV cells towards enhanced efficiency are
currently being explored to at reduce cost per peak watt. Solar
energy is an ideal form of energy as it is environment friendly
and can substitute for dwindling energy resources. It is clean,
exhaustible and available all over the world with varying
intensity[7]. Further, silicon - the main material used in
manufacturing of solar cells is the second most plentiful element
available on the earth. Thus there is no problem of resource
availability.
In view of the above, countries like India have made it a
national mission to install large number of PV operated pump
sets to irrigate remote and rural areas. While adequate financial
input from global agencies is available, appropriate technological
input is found lacking. The PV panels energize the DC motors
which in turn drive the pumps. Since the system has to be
installed in technically unattended zones it must be robust,
economical and maintenance free. The PM DC motors using
mechanized commutators and brushes need regular maintenance
and are prone to failure. Electronically commutated PMBLDC
motors may be a natural alternative. Such motors used for other
applications such as aerospace are found to be prohibitively
expensive necessitating the development of a system which is
reliable and within reach of customer.
Several authors have studied different aspects of steady state
performance of various types of electric motors operated from
a PV array [3-8]. Appelbum et al [7] have examined the starting
characteristics of PM brushed DC and series motors powered by
solar cell. Rehman [3] have discussed on the diversity in the
application of the photovoltaic systems. Alhuwainem[5] has
analyzed steady state performance of DC motors supplied from
PV generators with step up conversion. Bhat et al[6] have
discussed the performance optimization of an induction motor
pump system using PV energy source.
The PMBLDC motor is considered as one of the best motor
which exhibits the highest efficiency among all conventional
motors[l]. Due to its performance superiority, this motor is more
suitable for solar energy powered water pumping application.
Water pumping system does not require accurate speed control
for its satisfactory performance but cost reduction is the prime
consideration in such systems. Hence, in this paper a new low
cost scheme has been proposed for PV array powered PMBLDC
motor coupled to water pump. A prototype laboratory model is
developed with a view to extend experimental verification for the
simulated results.
can be obtained by estimating the position by sensing the back
emf. In this system the current is restricted without current
controller, because the PV array itself controls the current up to
its maximum value.
ANALYSIS OF THE DRIVE SYSTEM
All the acessories of the drive system are modeled independently
and integrated together
for the purpose of performance
simulation.
PV Array The solar cell are connected in series and parallel
combinations in order to get the desired level of voltage and
current. The equivalent circuit of PV array is shown in Fig.2.
The I-V equations of a solar cell is given by
V = - I R, + (1/D) In (1 + (Iph -I)/Io))
Where 1^ = Photon current proportional to the insolation
Rj = Series resistance of the cell
Io = Cell reverse saturation current
D = q/AKT, q = Electric charge of an electron
CONTROL STRATEGY
K= Boltzman constant T = Absolute temperature
Fig.l shows a schematic diagram of the PMBLDC motor drive
coupled to water pump powered by PV array through the filter
and an inverter. The PV-array directly converts solar insolation
into DC electrical power. The magnitude of PV-array current
depends on the intensity of sunlight. This current is fed to the
MOSFET based VSI which supplies the necessary power to the
PMBLDC motor to drive the water pump. The switching pattern
SOLAR
PV
ARRAY
Vdc
"T
A = Compilation Factor
Inverter and Motor
The detail of the modeling is already reported in [8]. The voltampere equations in state space current derivative form:
M08FET
BASED
VSI
SWITCHING
SIGNALS
PMBLDC
MOTOR
6
CONTROL
CIRCUIT
Fig.l
(1)
BACK EMFS
Schematic diagram of PV-array fed PMBLDC motor drive
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•JNAA/NA-Tjc
Ph
t
Rah
Varray
JX.
Fig.2
Equivalent
Circuit
of
PV-array
pib = [1/(L8 + M)] [vta - ibR - ej
(3)
pic = [V(L, + M)] [vm - i,R - e j
(4)
Where i^, ib and ic are winding currents of the phases a, b and
c. L5 is the self inductance and M is the mutual inductance. v m ,
v^ and \m are phase voltages, e,,,,, e^ and e^ are phase to
neutral back emfs and R is the resistance per phase of stator
winding.
The torque expression becomes
Te = Kb [ U6,)k + fb(0r)ib + WA 1
(5)
Where fa(0t), fb(0r) and fc(0j.) are functions of rotor position.
(B) Lin,
The mechanical equation of the motion in speed derivative form
can be expressed as:
pwt = (P/2) (Te - T, - Bwr)/J
D«t«ction o£ Position Signal U.infl
B*ck Eafa.
(6)
Where T[ is the load torque, B is the frictional coefficient,? is
the number of poles and J is the moment of inertia.
Position Detection by Using Back emf
The induced emf in the phase windings E a , Eb and Ec are
generally in trapezoidal shape as shown in Fig.3. The induced
voltages across the line, Eab, E^ and E ra are derived from the
phase induced voltages as Eab = Ea-Eb, E^ = Eb-Ec and E r a =
Ec- Ea. From three line voltages Eab, E^ and EM six pulse
signals of 180 degree duration are obtained with the help of zero
crossing detectors. These signals can be represented as Sa, Sb,
Sc, Sd, Se and Sf as shown in Fig.3. The necessary 6 pulse
signals namely, F l , F2, F3, F4, F5 and F6 for conduction
period and six pulse signals namely, F l l , F12, F13, F14, F15
and F16 for commutation period are obtained by logicaEy
ANDing the 180 degree pulse signals. The signals so obtained
have 120 degree conduction period and 60 degree commutation
period as are shown in Fig.3.
At the time of starting of the motor the induced emfs are zero
and hence the information about the rotor position cannot be
extracted. To solve this problem, low frequency currents are
injected through the motor phase. This results in a production of
an average positive torque which provides rotation of the motor
shaft. When rotor shaft picks up speed the induced emfs are
detectable and can be used to obtain the switching signals for
controlling the CC-VSI.
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+5V
• J
wmm.
S-8TACE
q
CLKI
RING COUNTBR —
- •••
STARTING
OSCILLATOR
»
J
SNABLS
Tr
S-ST ACS
Q
+
1
. . .'(
K
1.
-11 in n TT.rri»
R1NC COUNTBR
CLKZ
,?
4
e
1
Vcc
\ I
Fig. 4
GND
SIGiNflL
S t a r t i n g scheme of PMBLDC motor d r i v e
JT_
Fig.5
Normal running scheme of PMBLDC motor d r i v e .
HARDWARE DESCRIPTION
Scheme for Normal Running
Figures 4 and 5 show the block diagram of starting scheme and
normal running scheme. In order to obtain square wave pulses
of low frequencies, an astable multivibartor using the 555 IC is
used. The output of the astable multivibartor is not a
symmetrical square wave. Therefore a divide by two circuit is
used to generate a symmetrical square wave and which reduces
the frequency of the signal obtained from 555 to the half. Two
three stage ring counters are used for generating signals for six
inverter devices. The IC 74121 is used to provide the necessary
delay signal. The two output stages of ring counters are used to
determine, whether the gating signals to the MOSFET are
obtained from the starting circuit or from motor terminals. The
output of the ring counter and the Q of the 74121 are connected
to input of the AND gate or OR gate. These two operations
(AND and OR) ensure that the gate driver signals obtained from
starting circuit when output Q of 74121 is high.
The PMBLDC Motor has star connected windings without
neutral, it is not possible to directly sense the back emfs across
the phases. Therefore, the line voltages are sensed and their zero
crossing signals are obtained. These zero crossing signals(a, b
and c) and their inverted signals (a", b and "c) are used to derive
the gating signals as below;
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Gl = a.c, G2 = b.c, G3 = b.a, G4 = c.a, G5 = c.b, G6 =
a.b,
Where Gl to G6 are gating signals of the MOSFETs 1 to 6
respectively. While a, b and c are the positive zero crossing
signals of line voltages Eab, Ebc and Eca respectively.
These signals (Gl to G6) are fed to one of the inputs of two input
AND gate, the other input being the signal Q of one shot. The
output of these AND gates are connected to the one input
terminal of OR gates, while other input for OR gate is fed from
the starting circuit.
array
60.ee160.08
0)
01
-p
B. 00
8.10
XTV | I'l I IT I II IJ II IT T I IT I f-T-TTI-TI-I T I M i n i T . > r p ,
e.ee a.ie e.ie a. be e.'
e.ee o . i e a.20 0.30
Time(s)
(a) Array voltage
(b)
B.C
T i m e (S)
Array current
e.20
eTba
e.
Time(S)
(d)
Rotor Speed
1.UO
5
53
0
EH
-2.ee
" ' 0' '10'
e.fee
e.Sa
eT-«e
T i m e (S)
0.10 e.ae e.St 0.40
Time (S)
(c)
Winding Current
Fig.6
(e) Electromagnetic torque
Dynamic response of PV-array fed PMBLDC motor drive
RESULTS AND DISCUSSIONS
The numerical technique .fourth order Runge-Kutta method is
used to solve the nonlinear equations (1) - (6) in order to obtain
the dynamic performance of the PMBLDC motor fed by
PV-Array
The dynamic response of a 4-pole, 1.0 A PMBLDC motor
coupled to a water pumping system is shown in Fig.6. To reveal
the effectiveness of the drive system, the following observations
are made from the results obtained. Figures (a), (b), (c) ,(d) and
(e) respectively the variations of array voltage, array current,
motor winding current, rotor speed and electromagnetic torque
with time at a solar insolation level of 800 watts/m2. From Fig.
6(a) it may be seen that the array voltage starts from its initial
low value and increase slowly till the motor reaches the steady
state condition. Under the steady state condition the array voltage
is maintained at a constant value. The voltage suffers from small
: fluctuations at every 60 degrees due to commutation. The array
current rises very rapidly to the level of photon current at
: standstill condition, and current reduces to a nominal value as
the motor accelerates thereby attaining a constant value with
small fluctuation. The winding current also fluctuates due to
regular commutation of inverter devices. The speed steadily
increases till the load torque equals the developed electromagnetic
torque and remains at constant value under steady state condition.
The measured response is shown in Fig.7. Part (a) of this figure
shows the variation of the speed and array current. The winding
current and the dc link current are shown in part (b). Part (c)
shows the winding current and dc link voltage. Both dc link
current and dc link voltage are shown in part (d). In the
experimental results also, small oscillations are found due to the
commutation at every 60 degrees.
CONCLUSION
The proposed model of a low cost PMBLDC motor has been
found to be effective in analyzing the dynamic performance of
the drive system. The prototype developed controller works
satisfactorily with different solar insolation. The elimination of
use of rotor position and current sensors have made the system
simpler which also helps in overall cost reduction of the drive
system. The drive system has been found suitable to pump
water even during the condition of low level of solar
insolation. An L-C filter introduced between the PV array and
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m
e
•a
• e
•a
U
o
o
O
o
o
tl
in
o
o
II
u
II
a
II
u
X
!X
(a) Speed and array current
!c) Winding current and dc link voltage
n
<
Q
o
*
II
si
m
<
u
"0
i—i
o
•
II
n!
•H
(b) Winding current and dc link current
Fig.7
Experimental results of
"0
UD
o
axis
m
•0
"O
(d) DC link current and voltage
PV-fed PMBLDC motor drive system
the inverter has been found instrumental in reducing current
and voltage fluctuations thereby smoothing the input supply to
the inverter. It also works as a protective valve for PV-array.
The position detection through the sensing of back emf has been
found to be easier to implement. It also works satisfactorily
with different solar insolation. It is hoped that owing to its
improved performance, high efficiency and low cost, the
PMBLDC motor drive would become more suitable for water
pumping application specially in isolated places.
REFERENCES
1. R. Krishnan and R.Ghosh, "Starting algorithm and
performance of a permanent magnet brushless motor drive
with no position sensor", IEEE PESC- 1987, pp 596-606.
2. Henichi Hzuka, Hideo Uzuhashi, Minoru Kano, Isanehuro
Endo and Katsuo Mohri, "Microcomputer control for
sensorless brushless motor:, IEEE Trans, on Industry
Applications, Vol. IA-21, No.4, May/June 1985, pp
595-601.
3. Saifur Rahman, M.A. khallat and B.H. Chowdury, "A
discussion on the diversity in the applications of photovoltaic
systems", IEEE Trans, on Energy Conversion, Vol. 3,
No.4, Dec. 1988, pp 738-746.
4. R. Ramkumar, H, J, Allison and W.L. Huges, "Solar
energy conversion and storage systems for the future", IEEE
Trans, on PAS, Vol. PAS-94, No.6, Nov/Dec. 1975, pp
1926-1934.
5. S.M. Alghuwainem, "Steady state performance of dc
motors supplied from photovoltaic generators with step up
converters", IEEE Trans, on Energy Conversion, Vol. 7,
No.2, June 1992, pp 267-271.
6. S.R. Bhat, Andre Pittet and B.S. Sonde, "Performance
optimization of induction motors- pump system using
photovoltaic energy sources", IEEE Trans, on Industry
Applications, Vol. IA- 23, No.6, Nov./Dec, 1987, pp
995-1000.
7. J.Appelbaum and S.Singer, "Starting characteristics of
permanent magnet and series excited motors powered by
solar cells: Variations with solar radiation and temperature",
Electric Machines and Power Systems, Vol.20, 1992, pp
173-181.
8. Putta Swamy C.L., Singh Bhim and Singh B.P., Dynamic
performance of permanent magnet brushless dc motor
powered by a PV array for water pumping", Journal of Solar
Energy Materials and Solar Cells, Netherland, Vol.36,
No.2, pp 187-200. Feb.-1995.
APPENDIX
Parameters of the Drive System
R, = 0.03 Ohms, R = 5 Ohms Ls = 0.006H/phase, Kb = 0.13
V/rpm, P= 4, J=0.0022 Kg/m2, I o = 0.000025 A, A= 1.66,
C=0.001 F, L = .OO25 H
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