2015 Tenth International Conference on Ecological Vehicles and Renewable Energies (EVER)
Photovoltaic Motors Review, Comparison andSwitched Reluctance Motor Prototype
Loıc Queval, Lionel VidoSATIE, University of Cergy-Pontoise
5 mail Gay Lussac95031 Cergy-Pontoise, FranceEmail: [email protected]
Alain CotyIndependent researcher
95031 Cergy-Pontoise, FranceEmail: [email protected]
Bernard MultonSATIE, ENS RennesCampus de Ker Lann35170 Bruz, France
Email: [email protected]
Abstract—The simplicity of photovoltaic motors makesthem ideal candidates for fully autonomous applicationsrequiring thousands of operating hours without main-tenance, like water pumping. Photovoltaic motors usephotovoltaic cells optically commutated by a shutter drivenby the motor rotor to convert light energy into mechanicalenergy, without the need of any brushes or other powerelectronics. With the decrease of photovoltaic cells price,photovoltaic motors could be more affordable and reliablethan conventional systems, and therefore particularly wellsuited for off-grid applications. The concept has beenpatented under various forms, but the scientific literatureis so far very scarce. In this article, we attempt toclassify photovoltaic motors, and to explain in details theirphysical working principle. Then we compare the differentarchitectures by defining two pre-design factors linked tothe system maximal output power. Finally, we report firstexperimental results on a photovoltaic switched reluctancemotor (PV SRM) prototype using a 6/4 switched reluctancemachine and 12 photovoltaic cells.
Keywords—photovoltaic motor; switched reluctance mo-tor; water pumping.
I. INTRODUCTION
Definition: A photovoltaic motor is an electric motorusing photovoltaic (PV) cells optically commutated by ashutter driven by the motor rotor to convert light energyinto mechanical energy, without the need of any brushesor other power electronics.
There have been developed electric motors driven bylight through photovoltaic cells. But, the prior art motorsrequired brushes or DC/AC converters. The purpose ofthis article is to introduce another family of conversionsystem : photovoltaic motors. We underline here thatsuch motors have been patented so far, but scientificliterature on the topic is limited [11], [18]. The attemptof this article is to fill the gap.
(a) (b)
Fig. 1. Photovoltaic motor applications [3]
A. PV cells + DC motor
Light energy can be converted into mechanical energyby first converting light energy to DC electric energyusing photovoltaic cells, and then converting this DCelectric energy to mechanical energy using a DC motor[1]. This system is simple, but the DC motor requiresbrushes which must be replaced periodically. This is animportant drawback for applications with reliability andmaintenance constraints, like water pumping in isolatedareas.
B. PV cells + power electronics + AC (brushless) motor
Light energy can be converted into mechanical energyby first converting light energy to DC electric energyusing photovoltaic cells, then converting this DC electricenergy to AC electric energy using DC/AC converter, andthen converting this AC electric energy to mechanicalenergy using an AC motor [2]. Thanks to its DC/ACconverter, the system can track the maximum power pointof the PV cells, and operate efficiently the AC motor.But converters tend to be expensive, bulky and can havereliability problems in harsh environment.
978-1-4673-6785-1/15/$31.00 c©2015 European Union
(a) (b)
(c)
Fig. 2. PV unipolar PMSM. (a) Overview of the motor proposed byHall in 1967 (Fig.1 from [4]). (b) Overview of the motor proposed byBraeutigam in 1967 (Fig.3 from [5]). (c) Working principle schematic,only one phase and one pole pair, simplified magnetic circuit.
C. PV cells + AC motor = Photovoltaic motor
Light energy can be converted into mechanical energyby first converting light energy to AC electric energyusing photovoltaic cells commutated by a shutter me-chanically connected to the rotor, and then converting thisAC electric energy to mechanical energy using an ACmotor. The system {commutated PV cells + AC motor}is called a photovoltaic motor. PV motors potentiallyentail low OPEX and CAPEX, because they requireno brush or DC/AC converter. In addition, the lowutilization factor of the PV cells (see section III) couldbe overcompensated by the rapid decrease of PV cellsprices. On the other hand, the absence of converter andthe use of a rotating shutter introduce new constraintsthat could limit the performances of the system.
D. Potential applications
PV motors are envisioned to be used in fully au-tonomous applications where reliability is a major con-straint. One of such application is water pumping inisolated area. PV motors can be used as autonomousmotor-pumps to pump water from a well, to irrigatecrops, or to store water in a tank (Fig.1). For water
(a)
(b)
(c) (d)
(e)
Fig. 3. PV bipolar PMSM. (a) Overview of the motor proposedby Marrison in 1959 (Fig.1 from [6]). (b) Overview of the motorproposed by Nakamats in 1987 (Fig.1-2 from [7]). (c) Overview ofthe motor proposed by Izawa in 1988 (Fig.2 from [8]). (d) Overviewof the motor proposed by Shea in 1995 (Fig.2 from [9]). (e) Workingprinciple schematic, only one phase and one pole pair, simplifiedmagnetic circuit.
(a)
(b)
Fig. 4. PV SRM. (a) Overview of the motor proposed by Coty in2012 (Fig.4-5 from [10]). (b) Working principle schematic, only onephase and one tooth pair, simplified magnetic circuit.
pumping application, an output power of 100 to 200 Wis required. Coupled with blades, PV motors can be usedas fans. It can be fixed to the roof of a greenhouse orhousing, and drive the fan inside (Fig.1) when sun shines.For ventilation application, an output power of about50 W is required. Other possible applications includefan-cooled parasols, solar water-heaters, mills, toys andadvertising items.
II. PHOTOVOLTAIC MOTORS REVIEW
For simplicity’s sake, we describe here elementaryPV motors having 1 phase and 2 poles (or 2 statorteeth and 2 rotor teeth for switched reluctance motors).The aim is not to introduce optimal designs, but todemonstrate the physical working principles behind PVmotors. It is clear that motors with 3 or more phases,and more poles (or rotor teeth) would be preferred forreal world applications. This would allow for highertorque and facilitate self-starting. In the following, theshutter is mechanically connected to the rotor, and rotatessynchronously with it. In Fig.2 and following, CCWmeans counterclockwise direction.
A. PV unipolar PMSM
This PV motor was proposed the same year by Hall[4] and Braeutigam [5] (Fig.2a-b). In its elementary form,it comprises: a permanent magnet synchronous motor(PMSM), one photovoltaic cell (PV1), and a shutter(Fig.2c). The PMSM is a single-phase 2-pole machinewith non-magnetic teeth. The permanent magnet is onthe rotor. The single-phase armature coil is on the stator.The PV cell is connected to the armature coil. The term”unipolar”, selected by analogy with stepper motors,means that the sign of the current in the armature coil isconstant.
A step-by-step operation of this motor is shown inFig.5. Assuming the rotor initial angle is between 0 and180 degrees, the shutter lets the light hit PV1. PV1 pro-duces a current i, that magnetizes the armature coil. Thiscauses the PM rotor to rotate under repulsion/attractionforce. After a 180 degrees rotation, the shutter blocksthe light on PV1. The current i goes to zero, as wellas the armature coil magnetization. Neglecting possibledetent torque, the PM rotor continues to rotate by inertiauntil 360 degrees. Thus, a positive torque is generatedintermittently.
B. PV bipolar PMSM
This PV motor was proposed under different formsin [6]–[9] (Fig.3a-d). Experimental results on a pro-totype with two 10 mm2 PV cells are reported in[Bobitski2004]. In its elementary form, it comprises:a permanent magnet synchronous motor (PMSM), twophotovoltaic cells (PV1 and PV2), and a shutter (Fig.3e).The PMSM is a single-phase 2-pole machine with mag-netic teeth. The permanent magnet is on the rotor. Thesingle-phase armature coil is on the stator. The two PVscells are connected in anti-parallel relationship to eachother, and are connected to the armature coil. The term”bipolar” means here that the sign of the current in thearmature coil varies.
A step-by-step operation of this motor is shown inFig.6. Assuming the rotor initial angle is between 0and 180 degrees, the shutter lets the light hit PV1, butblocks the light on PV2. PV1 produces a current i, thatmagnetizes the armature coil. This causes the PM rotorto rotate under repulsion/attraction force. After a 180degrees rotation, the shutter blocks the light on PV1, butlets the light hit PV2. The current i changes in direction,as well as the armature coil magnetization. The PM rotorcontinues to rotate under repulsion/attraction force. Thus,a positive torque is generated continuously.
Fig. 5. PV unipolar PMSM, details of the operating principle.
Fig. 6. PV bipolar PMSM, details of the operating principle.
Fig. 7. PV SRM, details of the operating principle.
C. PV SRM
This motor was first proposed in [10] (Fig.4a). Inits elementary form, it comprises: a switched reluctancemotor (SRM), one photovoltaic cell (PV1), and a shutter(Fig.4b). The SRM is a single-phase machine with twostator teeth and two rotor teeth (SRM 2/2). The single-phase armature coil is on the stator. The PV cell isconnected to the armature coil.
A step-by-step operation of this motor is shown inFig.7. Assuming the rotor initial angle is between 0and 90 degrees, the shutter lets the light hit PV1. PV1produces a current i, that magnetizes the armature coil.This causes the rotor to rotate to reduce reluctance. Aftera 90 degrees rotation, the shutter blocks the light on PV1.The current i goes to zero, as well as the armature coilmagnetization. The rotor continues to rotate by inertiauntil 180 degrees. Thus, a positive torque is generated
intermittently.
D. Discussion
Although we showed PV motors with stationary PVcell, stationary coil and rotating shutter, a simple rear-rangement of the components would lead to a rotatingcoil, rotating PV cell and stationary shutter. Anotherrearrangement would allow having the light coming froma direction orthogonal to the shaft. A combination of bothrearrangements even opens the possibility of removingthe shutter (Fig.8): such configuration is known in thehobby community as a ”Mendocino motor” [12], [13].
Finally, we underline that other kinds of PV motorshave been proposed. Izawa introduced a PV Faradaymotor [16]. Morikawa proposed a PV electrostatic motor[17]. Petru and Ungureanu [18] built a PV motor withrolling rotor. These motors are not described in this
Fig. 8. ”Mendocino motor”, adapted from [15]. This is a PV bipolarPMSM with typically 4 PV cells (24mm × 33mm), 2 air-cored coils(100 turns), and one permanent magnet [14]. It uses magnetic bearingto reduce friction.
article, because of their probable very low performancesor high mechanical complexity.
III. PRE-DESIGN FACTORS
To help the designer in comparing the various kindsof PV motors, we define two pre-design factors. Bothcharacterize the input power of the system, and thereforegive information on its maximal output power. Theshutter sweep area filling factor ks is defined as,
ks =∑
k
θPV,k
2π(1)
where θPV,k is defined in Fig.9. A high ks means thatmost of the surface under the shutter sweep area has PVcells, ie. this surface is well used. The PV utilizationfactor kPV is defined as,
kPV =∑
k
θs,k2π
(2)
where θs,k is defined in Fig.9. A high kPV means thatmost of the time the shutter lets the light hit the PV cells,ie. the PV cells are well used. The pre-design factors ofvarious PV motors are summarized in Table I (calculationdetails are not included due to lack of space).
The factor kskPV can then be used to comparedifferent PV motors assuming they have the same shutterradius. With kskPV = 0.33, the PV bipolar PMSM with3 phases and 2 poles is the best configuration. Assumingan illumination of 1000 W/m2, a PV cell efficiency of18.8 %, and a motor efficiency of 80 %, the output powerwould be 200 W for a shutter radius of 113 cm. With
Fig. 9. Definition of θPV,k and θs,k.
kskPV = 0.3, the PV SRM 10/2 is the second bestconfiguration. With the same assumptions, the outputpower would be 200 W for a shutter radius of 119 cm.
IV. PV SWITCHED RELUCTANCE MOTOR PROTOTYPE
As seen above, the PV SRM 10/2 has one of thebest factor kskPV . Besides, it has the ability to self-starteven for low illumination thanks to its 5 phases and theabsence of cogging torque (no magnet). Therefore, thePV SRM 10/2 is considered to be a good candidate fora PV motor for water pumping application.
To verify the concept, we built a prototype using aSRM 6/4. Each stator teeth has 2 windings (Fig.11).The average winding resistance is 0.45 Ω. The averageunaligned and aligned linear inductances are 2 mH and35 mH. Each winding is connected to one PV cell(Fig. 10). The PV cells are MPO Solo (Fig.12), 18764mm2 after cutting, efficiency 18.8 %, Isc = 8.897 A,Voc = 0.640 V at standard testing conditions. The PVassembly is shown in Fig.13. The PV cells are protectedfrom overvoltage with transil diodes (Fig.10). The shutterradius is 33 cm. The PV SRM prototype mounted onhis test bench is shown in Figs.14 and 15. With theselected aperture angle of PV cell and of shutter (TableI), we obtained kskPV = 0.333, but the motor has somegenerating (breaking) torque. Assuming an illuminationof 1000 W/m2, a PV cell efficiency of 18.8 %, and amotor efficiency of 80 %, the maximum output power ofour prototype is 11.3 W.
We report our first test results now. We successfullyoperated the PV motor at no-load, under a mean il-lumination of 1090 W/m2. It was able to self-start,and the steady-state rotor speed was 98 rpm. The no-load voltage and current of one stator coil are shownin Fig.16. We observe that the peak current is far fromthe maximum current that the PV cell could generate atthe test illumination. This is linked to a bad impedancematching between the PV cells and the motor windings
TABLE I. PRE-DESIGN FACTORS OF SEVERAL PV MOTORS
Motor m Ns Nr nPV ns θPV [deg] θs [deg] kPV ks kskPV
PV unipolar PMSM 1 2 2 1 1 90 90 1/4 1/4 0.0625PV unipolar PMSM 3 6 2 3 1 90 90 1/4 3/4 0.1875PV bipolar PMSM 1 2 2 2 1 90 90 1/4 1/2 0.125PV bipolar PMSM 3 6 2 6 1 60 120 1/3 1 0.333PV SRM 1 2 2 2 2 45 45 1/4 1/4 0.0625PV SRM 3 6 4 12 4 22.5 22.5 1/4 3/4 0.1875PV SRM 5 10 2 10 2 36 54 3/10 1 0.3PV SRM (prototype) 6 6 4 12 4 30 30 1/3 1 0.333
m = No. of phases ; Ns = stator poles ; Nr= rotor poles ; nPV = No. of PV cells ; ns = No. of shutter apertures ; θPV =aperture angle of PV cell ; θs = aperture angle of shutter ; kPV = PV utilization factor ; ks = shutter sweep area filling factor.
Fig. 10. Electrical circuit of PV SRM prototype (all phases are shown).
Fig. 11. Overview of PV SRM prototype (only one phase is shown).
Fig. 12. Characteristics of photovoltaic cells MPO Solo (Monocrys-talline silicon, STC AM1.5, 25◦C) [19].
Fig. 13. PV cells assembly of the PV SRM prototype.
in this first prototype. The dynamic interaction betweenthe PV cells and the SRM is complex, and will needdetailed modeling to be explained. Similar results wereobtained for various illuminations, and with a light load(fan). The output torque was too low to be measuredaccurately. Nevertheless, these results show that the PVswitched reluctance motor is operating as expected. Weare now focusing our efforts on increasing the outputmechanical power of our prototype.
Fig. 14. Bottom view PV SRM prototype.
Fig. 15. Top view PV SRM prototype and testbench.
Fig. 16. PV SRM voltage and current of one stator coil at no-load.
Fig. 17. PV SRM voltage and current of one stator coil under lightload.
V. CONCLUSION
Photovoltaic motors are little known conversion sys-tems, with potentially low price and low maintenanceneed. They are particularly well suited for fully au-tonomous applications in isolated areas where reliabilityis a major concern, like water pumping. We reviewedhere different kinds PV motors, classified them andillustrated their operation. We introduced two pre-designfactors for the {PV cell-shutter} system, that helpedus identifying the PV switched reluctance motor as apromising architecture. To verify the concept, we as-sembled and tested successfully the first PV switchedreluctance motor. Further investigations are now neededto elaborate PV motor models, which could help us tounderstand the interaction between the PV cells and themotor, and to increase the output power of such systems.
ACKNOWLEDGMENT
We would like to thank the Laboratory for ElectricalMachines of Fachhochschule Dusseldorf for its technicalsupport.
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