energies

Article 3-D FEM Analysis, Prototyping and Tests of an Axial Flux Permanent- Wind Generator

Joya C. Kappatou 1,*, Georgios D. Zalokostas 2 and Dimitrios A. Spyratos 3 1 Department of Electrical and Computer Engineering, University of Patras, 26500 Patras, Greece 2 Electrical and Computer Engineer, Amazonon 3 Evrytania, 36100 Karpenissi, Greece; [email protected] 3 Electrical and Computer Engineering student, University of Patras, 26500 Patras, Greece; [email protected] * Correspondence: [email protected]; Tel.: +30-261-099-6413

Received: 20 June 2017; Accepted: 23 August 2017; Published: 26 August 2017

Abstract: This paper contributes to the research and development of Axial Flux Permanent Magnet Synchronous Machines (AFPMSM); and in particular the design, the construction stages and measurements of a double single internal non-ferromagnetic with a trapezoidal-concentrated winding machine for wind power generation applications. The initial dimensions of the machine were calculated using analytical formulas and a model was created and analyzed using the 3D Finite Element Method (FEM). The shape of the of the machine was optimized and presented in a previous paper and a prototype was constructed and tested in the laboratory. In addition, a temperature test of the stator was performed experimentally. Finally, the effect of the different axial widths of the two air gaps on the electrical magnitudes and the field of the machine were investigated using both FEM analysis and experiments.

Keywords: Axial flux machine; finite element analysis; permanent-magnet generator; wind power generator

1. Introduction Research on the design and construction of Axial Flux Permanent Magnet Synchronous Machines (AFPMSM) has been attracting increasing scientific interest over the past few years. These types of machines have been used in various industrial applications such as wind power generation, ship propulsion drives and electric vehicles as well as in numerous other related studies which can be found in the literature [1–6]. Despite the recent increase in published research on this topic, the investigation design and manufacturing processes for AFPM machines is still ongoing. Compared to the Radial Flux Synchronous Machines, the AFPMSMs offer advantages in certain applications. They have a compact and robust structure, the length of the air-gap is small, their volume is limited and their efficiency and power-to-weight ratio are high. They consist of several modules, which can be adjusted to power or torque requirements [7–11]. The number of permanent magnets can be large, making these machines suitable for high frequency or low speed applications such as electric vehicle traction or ship propulsion drives and wind power generation. They can also be directly coupled to low-speed turbines, wind, or hydro turbines, thus the use of gear-boxes, which is maintenance demanding is avoided and the system becomes lighter, less noisy, and more efficient [12]. Several AFPMSM topologies can be found in the literature concerning the number of modules: single sided machines, the one stator-one rotor and multiple air-gap machines, usually, double sided machines, one stator-two rotors or one rotor-two . They can be categorized also, according to the material of the stator and the winding topology. So, the stator can be constructed using ferromagnetic material, slotted or slot-less, or using non-magnetic materials.

Energies 2017, 10, 1269; doi:10.3390/en10091269 www.mdpi.com/journal/energies Energies 2017, 10, 1269 2 of 14

In the cases of the non-magnetic material stator core or the coreless stator, the hysteresis and eddy current stator losses, the cogging torque is eliminated and the rotor core losses and acoustic noise are reduced [13,14]. Furthermore, the weight of the machine is reduced and the axial magnetic attractive forces between stator and rotor, at no load are non-existent. The aforementioned forces can cause problems at the construction and when the air gap length is not equal between each rotor and the stator in the multiple air-gap machine topologies. On the other hand, with the use of a coreless stator, the winding inductances become smaller and stronger or larger permanent magnets are needed to produce the same magnetic field. For the study of different AFPMSM topologies and designs both analytical methods, verified through the Finite Element Method (FEM), or experiments were employed [15–19]. Other papers [20,21] examine this type of AFPMSG topology using analytical magnetic field computation techniques validated by comparison to Finite Element Analysis (FEA) results. The present work, while adopting the same topology of two rotors with one internal coreless stator, is distinctive in terms of having selected the shape of the magnets in relation to the winding shape (non-overlapping trapezoidal) and the machine electromagnetic and performance characteristics based on extensive research (a previously published paper [22]). The behavior of the machine was studied via a 3D FEA model and a prototype was constructed. The basic magnitudes of the machine were measured and compared to the simulation results. Furthermore, a stator temperature test was performed and the effect of the different axial width of the two air-gaps on the electrical and magnetic magnitudes of the machine was investigated. The synchronous generator is intended for wind power generation. The detailed characteristics of the machine are described in the section that follows.

2. AFPM Machine Characteristics The machine was chosen to have one coreless stator and two rotors. The stator consists of a trapezoidal-concentrated, non-overlapping winding that is buried in a composite material of high temperature epoxy resin. The permanent magnets consist of high energy material, neodymium–iron–boron (NeFeB N42); they are axially magnetized and placed so that a north pole is located after a south pole. The choice of coreless stator has the advantages of simple stator assembly and elimination of the cogging torque. Furthermore, rotor surface losses, magnetic saturation and acoustic noise are reduced compared to the iron stator core. The machine is designed to be directly coupled to the wind generation system and thus it must be designed with numerous poles to fit the frequency requirements. In cases of wind generation applications, such as the one under study, it is essential to use a Pulse Width Modulation (PWM) converter to maintain steady output voltages and frequency. However, this paper does not focus on the converter that will be the subject of future research. The initial dimensions of the machine were calculated by using known formulas [7,23,24]. The magnet dimensions, the number of poles, the coil shape, size and its number of turns, the air gap length and the overall diameter of the generator were the basic chosen parameters that were used in the theoretical equations to extract the remaining machine parameters. Several 3D Finite Element Models were designed and analyzed with the use of a commercial software package, opera 3D in order to study and optimize the performance of the generator. The machine is intended for small direct-drive wind-energy conversion systems. A realistic speed value for safe operation of the machine is 375 rpm, which corresponds almost to 5 bf wind for the chosen wind turbine. Firstly, the basic magnitudes of the 3-phase generator with two rotors and one inner stator were chosen according to our requirements and the good and safe operation of the machine [7], as presented in Table1. Energies 2017, 10, 1269 3 of 14

Table 1. Basic machine parameters.

Parameter Value Number of poles in one rotor 16 Nominal frequency 50 Hz Rotor axial thickness 12 mm Magnet axial thickness 10 mm Air-gap axial thickness 3 mm Internal to external radius ratio (σ) 0.379

The rest of the basic magnitudes were calculated using proper sizing equations and assumptions [7] and they are listed in Table2. Equation (1) was used to compute the axial thickness of the stator, Equation (2) to compute the peak value of the phase voltage, and Equation (3) for the developing torque of the machine.   2·hm Br tw = − 1 − 2·g, (1) ksat·µrrec Bmg where, g is the air-gap axial thickness, hm is the magnet axial thickness, µrrec is the relative magnetic permeability of the magnets, Br is the remaining magnetization of the magnets, Bmg is the maximum density of magnetic flux that is just above the surface of the magnet.

q 4 E = ω Bρ N r l k k , (2) pc α p e p d

p 3p T = P = E I , (3) 2ω 4ω pc pc where, q is the number of coils per phase, p is the number of poles, α is the number of parallel circuits, l is the active length of the winding (ro − ri), N is the number of coil turns, Bρ is the maximum value of the magnetic flux density (1st harmonic) in the gap, re is the average radius of the stator winding ((ro + ri)/2), kd is the distribution factor of the winding, kp is the pitch factor of the winding, ω is the electrical speed.

Table 2. Calculated variables from the theoretical equations.

Parameter Value Number of coils 12 Stator axial thickness 18 mm Stator external radius 158 mm Stator internal radius 60 mm Number of turns per coil 210 Nominal number of revolutions 375 rpm

Each rotor disc consists of 16 magnets of alternating polarity. The permanent magnets were glued in the surface of the rotors. Their shape resulted from FEM analysis, published in a former paper [22], by comparing the machine electromagnetic and performance characteristics obtained for different magnet shapes [9,23–25]. The stator winding is concentrated and non-overlapping and plunged in a supporting structure made of high temperature epoxy resin (non-magnetic and non-conductive material). Multi-turns of trapezoidal coils of isolated conductors form the winding. The prototype machine that resulted from the optimized FEM analysis model was constructed in the university laboratory, apart from the metal parts which were made in the university machine shop. A test bench was also developed in the laboratory to measure the prototype performance. Accurate measurements were taken in order to compare and evaluate the constructed machine with the Energies 2017, 10, 1269 4 of 14 simulation 3D FEM model. In addition, the constructed prototype stator was submitted to temperature tests. Finally, an investigation of the effect of unequal width air-gaps on the electrical characteristics Energies 2017, 10, 269 4 of 14 and the field of the machine was conducted. 3. Design Optimization and Simulation Results 3. Design Optimization and Simulation Results The impact of magnet design in machine performance has been extensively investigated and The impact of magnet design in machine performance has been extensively investigated and presented in a previous paper [22]. The categorization of the magnets depends on their shape, hence presented in a previous paper [22]. The categorization of the magnets depends on their shape, hence their label categorization as radial, conventional skew, dual skew and finally triangular skew their label categorization as radial, conventional skew, dual skew and finally triangular skew magnets. magnets. Figure 1 presents the corresponding designs used in the simulations. In each model, the Figure1 presents the corresponding designs used in the simulations. In each model, the number and number and the radial length of the magnets is the same, but the volume of the magnetic material is the radial length of the magnets is the same, but the volume of the magnetic material is different, different, due to the different designs. In order for the simulation results to be comparable, all model due to the different designs. In order for the simulation results to be comparable, all model dimensions dimensions and test conditions apart from the magnets shape were kept unaltered. The magnets used and test conditions apart from the magnets shape were kept unaltered. The magnets used in the in the simulation are NdFeB, grade Ν42 and the magnetization curve (B-H) that was used was simulation are NdFeB, grade N42 and the magnetization curve (B-H) that was used was identical to identical to the magnetization curve provided by the manufacturer. the magnetization curve provided by the manufacturer.

FigureFigure 1.1. TheThe layoutlayout of thethe permanentpermanent magnets and the shape of the magnets as upper and lower, respectively.respectively. ( a ()a Radial) Radial magnets; magnets (b); conventional(b) conventional skew magnets;skew magnets (c) dual; ( skewc) dual magnets; skew ( magnetsd) triangular; (d) skewtriangular magnets. skew magnets.

TheThe under-investigationunder-investigation simulationsimulation modelsmodels werewere comparedcompared forfor thethe samesame loadload andand revolutionrevolution speed.speed. TheThe selectedselected modelmodel waswas thethe oneone withwith triangulartriangular skewskew magnets,magnets, FigureFigure1 1dd,, because because of of its its high high outputoutput power,power, powerpower factor,factor, lowlow costcost andand simplicitysimplicity ofof productionproduction [[2255].]. RegardingRegarding thethe FEAFEA models,models, thethe appropriateappropriate mesh mesh size size must must be be used used at at the the different different parts parts of of the the machine machine concerning concerning the the precision precision of theof the computation computation results. results. The The densest densest mesh mesh design design is needed is needed for for the the air-gap air-gap area, area, where where the the number number of theof the surface surface elements elements is ~75,000 is ~75,000 and and the the volume volume elements elements ~375,000. ~375,000. The The required required time time for one for time-stepone time- iterationstep iteration is ~7 minis ~7 with min Intelwith CoreIntel i5-3570KCore i5-3570K CPU @ CPU 3.5 GHz, @ 3.5 16 GHz, GB RAM16 GB and RAM 64-bit and operating 64-bit oper systemating (Microsoftsystem (Microsoft Windows, Windows, Washington, Washington, DC, USA). D.C,USA). TheThe finalfinal modelmodel waswas simulatedsimulated bothboth atat nono loadload (∞(∞[Ohm])[Ohm]) andand underunder loadload conditions.conditions. FigureFigure2 2 presentspresents thethe 3D3D simulatedsimulated modelmodel inin explodedexploded preview.preview. FigureFigure3 3 presents presents the the magnetic magnetic flux flux density density distribution in absolute values in a slice in the middle of the air gap where, as expected, the maximum values appear in the edges of the magnet. Similar distribution of the magnetic flux density appears throughout the axial length of the machine. The simulation results for the Root Mean Square (RMS) phase voltage at no-load (Back- Electromotive Force (EMF)) and output power under 70 Ω load versus the speed are presented in Figures 4 and 5 respectively. The simulation was non-linear taking

Energies 2017, 10, 1269 5 of 14

distribution in absolute values in a slice in the middle of the air gap where, as expected, the maximum values appear in the edges of the magnet. Similar distribution of the magnetic flux density appears throughout the axial length of the machine. The simulation results for the Root Mean Square (RMS) phase voltage at no-load (Back- Electromotive Force (EMF)) and output power under 70 Ω load Energies 2017, 10, 269 5 of 14 versusEnergies 2017 the,, speed 10,, 269 are presented in Figures4 and5 respectively. The simulation was non-linear taking5 of 14 into account the magnetization characteristics of the selected materials provided by our suppliers. intointo account account the the magnetization magnetization characteristi characteristics csof of the the selected selected materials materials provided provided by by our our suppliers suppliers. . Theinto waveformaccount the of magnetization the Back-EMF versuscharacteristi the speedcs of is the presented selected in materials Figure4 andprovided it is consistent by our suppliers with the. TheThe waveform waveform of of the the Back Back-EMF-EMF versus versus the the speed speed is ispresented presented in in Figure Figure 4 and4 and it isit isconsistent consistent with with the the linearThe waveform relationship of the between Back-EMF them. versus The the waveform speed is of presented the output in Figure power 4 versus and it theis consistent speed in Figurewith the5 linearlinear relationship relationship between between them them. The. The waveform waveform of of the the output output power power versus versus the the speed speed in in Figure Figure 5 5 presentslinear relationship the expected between form. them. The waveform of the output power versus the speed in Figure 5 presentspresents the the expected expected form form. .

FigureFigure 2. 2.23D.. 3D simulated simulated model model (exploded (exploded preview). preview).

Figure 3. Magnetic flux distribution in the middle of the air gap in absolute values. Figure 3.3.. Magnetic flux flux distribution in the middle of the air gap in absoluteabsolute values.values.

FigureFigure 4. 4 4.Plot.. Plot versus versus the the speed speed of ofthe the Back B Back-EMF,ack-EMF-EMF, RMS,,, RMS,RMS, with withwith 3 mm 33 mmmm air airair gaps, gaps,gaps, simulation simulationsimulation results. results.results.

Energies 2017, 10, 269 5 of 14 into account the magnetization characteristics of the selected materials provided by our suppliers. The waveform of the Back-EMF versus the speed is presented in Figure 4 and it is consistent with the linear relationship between them. The waveform of the output power versus the speed in Figure 5 presents the expected form.

Figure 2. 3D simulated model (exploded preview).

Figure 3. Magnetic flux distribution in the middle of the air gap in absolute values.

Energies 2017, 10, 1269 6 of 14 Figure 4. Plot versus the speed of the Back-EMF, RMS, with 3 mm air gaps, simulation results.

Figure 5. Plot versus the speed of the power output with 3 mm air gaps, simulation results.

4. Prototype Experimental Measurements

4.1. Construction Process In order to construct the machine with high precision compared to the simulation model, a specified process was followed; nevertheless, some necessary modifications of the constructed machine compared to the original design were made, as described here. The handmade winding is fabricated using copper of 0.8 mm diameter. The total length of the winding is 186.23 m per phase. Since the height of the winding was slightly larger that the predicted one, the outer radius of the constructed stator and rotor was increased. As widely documented, the ratio of internal to external radius, σ, of the machine affects significantly the performance characteristics, so it must be chosen appropiatelly [7,24]. Therefore, the height of the internal radius was equally increased, to keep the same ratio σ. The magnets, with the same shape and dimentions as per the FEM, were placed in the same relative position according to the winding, but the distance between two adjacent magnets was slightly increased too. The FEA model was modified accordignly to reflect the aforementioned modifications of the construcetd machine, in order to compare the simulation and experimental results in a robust manner. The constructed AFPM machine was tested under varius speeds under no-load (∞[Ohm]) and load conditions. Furtheremore, a stator temperature test was performed and finally, the machine performance with two unequal air gaps was investigated.

4.2. Test Bench Construction and Measurement Instruments A test bench was constructed in the laboratory to perform measurements on the prototype machine. The AFPMSG was connected to a two pole 7.5 kW, 400 V, . The induction motor was driven by a voltage source inverter and during the load tests a variable 3-phase resistance was connected to the terminals of the generator. A NI-6211 usb card (LabVIEW-National Instruments, Austin, TX, USA) collected the current and torque measurements. The Datum M425-C torque meter (Datum electronics, East Cowes, UK) with a DYI interface measured the torque with 1000 samples/sec. An in-house current sensor was developed, utilizing LEM LAH25NP Hall Effect current transducers (LEM, Geneva, Switzerland) for the line current measurement. All the current measured signals were sampled with 30 kHz frequency. Also, the Zimmer LM6-500 power analyzer (extremely high measuring accuracy of 0.015% of reading + 0.01% of range at 45–65 Hz) (ZES ZIMMER Electronic Systems, Oberursel (Frankfurt), Germany) was used in order to measure precisely the power output of the axial flux generator. The 3D design of the test bench is illustrated in Figure6 and the constructed test bench in Figure7. Energies 2017, 10, 269 6 of 14

Figure 5. Plot versus the speed of the power output with 3 mm air gaps, simulation results.

4. Prototype Experimental Measurements

4.1. Construction Process In order to construct the machine with high precision compared to the simulation model, a specified process was followed; nevertheless, some necessary modifications of the constructed machine compared to the original design were made, as described here. The handmade winding is fabricated using copper of 0.8 mm diameter. The total length of the winding is 186.23 m per phase. Since the height of the winding was slightly larger that the predicted one, the outer radius of the constructed stator and rotor was increased. As widely documented, the ratio of internal to external radius, σ, of the machine affects significantly the performance characteristics, so it must be chosen appropiatelly [7,24]. Therefore, the height of the internal radius was equally increased, to keep the same ratio σ. The magnets, with the same shape and dimentions as per the FEM, were placed in the same relative position according to the winding, but the distance between two adjacent magnets was slightly increased too. The FEA model was modified accordignly to reflect the aforementioned modifications of the construcetd machine, in order to compare the simulation and experimental results in a robust manner. The constructed AFPM machine was tested under varius speeds under no-load (∞[Ohm]) and load conditions. Furtheremore, a stator temperature test was performed and finally, the machine performance with two unequal air gaps was investigated.

4.2. Test Bench Construction and Measurement Instruments A test bench was constructed in the laboratory to perform measurements on the prototype machine. The AFPMSG was connected to a two pole 7.5 kW, 400 V, induction motor. The induction motor was driven by a voltage source inverter and during the load tests a variable 3-phase resistance was connected to the terminals of the generator. A NI-6211 usb card (LabVIEW-National Instruments,Austin ,Texas,USA) collected the current and torque measurements. The Datum M425- C torque meter ( Datum electronics, East Cowes United Kingdom) with a DYI interface measured the torque with 1000 samples/sec. An in-house current sensor was developed, utilizing LEM LAH25NP Hall Effect current transducers (LEM, Geneva, Switzerland) for the line current measurement. All the current measured signals were sampled with 30 kHz frequency. Also, the Zimmer LM6-500 power analyzer (extremely high measuring accuracy of 0.015% of reading + 0.01% of range at 45–65 Hz) (ZES ZIMMER Electronic Systems, Oberursel (Frankfurt),Germany) was used in order to measure

Energiesprecisely2017 the, 10, power 1269 output of the axial flux generator. The 3D design of the test bench is illustrated7 of 14 in Figure 6 and the constructed test bench in Figure 7.

Figure 6. 3D design of the test bench. Energies 2017, 10, 269 Figure 6. 3D design of the test bench. 7 of 14

Figure 7.7. Constructed test bench.

4.3. Experimental Results 4.3. Experimental Results The constructed machine was tested at no-load condition as well as under different loads for the The constructed machine was tested at no-load condition as well as under different loads for the same speed range as in simulations. Figure 8 shows the line voltage waveform of the AFPMSG at no- same speed range as in simulations. Figure8 shows the line voltage waveform of the AFPMSG at load condition (Back-EMF) for a speed of 300 rpm (as it is shown in oscilloscope, it has a sinusoidal no-load condition (Back-EMF) for a speed of 300 rpm (as it is shown in oscilloscope, it has a sinusoidal form and the RMS value of the voltage is 118.6 V). Figure 9 presents the variation of the phase voltage form and the RMS value of the voltage is 118.6 V). Figure9 presents the variation of the phase voltage at no-load (Back-EMF) for different speed values, and Figure 10 shows the output power versus the at no-load (Back-EMF) for different speed values, and Figure 10 shows the output power versus the speed for a given load of 70 Ω. Finally, Figure 11 presents simulation and experimental results for a speed for a given load of 70 Ω. Finally, Figure 11 presents simulation and experimental results for speed of 375 rpm of the output power versus the load per phase. A comparison to the simulated a speed of 375 rpm of the output power versus the load per phase. A comparison to the simulated results is presented in the comparable figures. As is shown, the difference between the simulation results is presented in the comparable figures. As is shown, the difference between the simulation and experimental results is insignificant, with experimental results showing slightly lower values and experimental results is insignificant, with experimental results showing slightly lower values than simulation. than simulation. FEM and experimental results are close to each other, especially in the low speed region. At the FEM and experimental results are close to each other, especially in the low speed region. At the nominal speed of 375 r/min and a load equal to 70 Ω/phase, an electrical power output of 274.89 W nominal speed of 375 r/min and a load equal to 70 Ω/phase, an electrical power output of 274.89 W was measured, while the mechanical power input equals 318.47 W. The power losses were esimated was measured, while the mechanical power input equals 318.47 W. The power losses were esimated by by subtraction of the mechanical input and were measured via the torque sensor mounted on the axis subtraction of the mechanical input and were measured via the torque sensor mounted on the axis and and the electrical power output. Eddy current losses in the permanent magnets, mechanical losses in the electrical power output. Eddy current losses in the permanent magnets, mechanical losses in the the bearings, and windage losses are present alongside the stator copper losses and the rotor iron bearings, and windage losses are present alongside the stator copper losses and the rotor iron losses. losses. At this load condition of 70 Ω/phase, a power loss of 43.58 W is calculated. The RMS phase At this load condition of 70 Ω/phase, a power loss of 43.58 W is calculated. The RMS phase current current was mesured at 1.1 A; taking into consideration the resistance of the winding per phase, the was mesured at 1.1 A; taking into consideration the resistance of the winding per phase, the copper copper losses are calculated at 22.506 W. Figure 12 shows the output power versus the speed under losses are calculated at 22.506 W. Figure 12 shows the output power versus the speed under 70, 26 and 70, 26 and 130 Ω/phase load with 3mm air gaps. 130 Ω/phase load with 3 mm air gaps.

Figure 8. Line voltage waveform of the AFPMSG at no-load (Back-EMF) for a speed of 300 rpm, as shown in oscilloscope.

Energies 2017, 10, 269 7 of 14

Figure 7. Constructed test bench.

4.3. Experimental Results The constructed machine was tested at no-load condition as well as under different loads for the same speed range as in simulations. Figure 8 shows the line voltage waveform of the AFPMSG at no- load condition (Back-EMF) for a speed of 300 rpm (as it is shown in oscilloscope, it has a sinusoidal form and the RMS value of the voltage is 118.6 V). Figure 9 presents the variation of the phase voltage at no-load (Back-EMF) for different speed values, and Figure 10 shows the output power versus the speed for a given load of 70 Ω. Finally, Figure 11 presents simulation and experimental results for a speed of 375 rpm of the output power versus the load per phase. A comparison to the simulated results is presented in the comparable figures. As is shown, the difference between the simulation and experimental results is insignificant, with experimental results showing slightly lower values than simulation. FEM and experimental results are close to each other, especially in the low speed region. At the nominal speed of 375 r/min and a load equal to 70 Ω/phase, an electrical power output of 274.89 W was measured, while the mechanical power input equals 318.47 W. The power losses were esimated by subtraction of the mechanical input and were measured via the torque sensor mounted on the axis and the electrical power output. Eddy current losses in the permanent magnets, mechanical losses in the bearings, and windage losses are present alongside the stator copper losses and the rotor iron losses. At this load condition of 70 Ω/phase, a power loss of 43.58 W is calculated. The RMS phase current was mesured at 1.1 A; taking into consideration the resistance of the winding per phase, the

Energiescopper2017, losses10, 1269 are calculated at 22.506 W. Figure 12 shows the output power versus the speed under8 of 14 70, 26 and 130 Ω/phase load with 3mm air gaps.

FigureFigure 8. Line8. Line voltage voltage waveform waveform of the the AFPMSG AFPMSG at atno no-load-load (Back (Back-EMF)-EMF) for a forspeed a speed of 300 ofrpm 300, as rpm, Energiesas 2017 shownshown, 10, in269 in oscilloscope.oscilloscope. 8 of 14 EnergiesEnergies 2017 ,2017 10, ,269 10, 269 8 of 148 of 14

Figure 9. Plot versus the speed of the output phase voltage at no-load, simulation and experimental Figure 9 9.. PlotPlot versus versus the speed the speedof the output of the phase output voltage phase at voltageno-load, atsimulation no-load, and simulation experimental and Figureresults. 9. Plot versus the speed of the output phase voltage at no-load, simulation and experimental results.experimental results. results.

Figure 10. Plot versus the speed of the output power simulation and experimental results at 70 Ω/phase load. Figure 10. Plot versus the speed of the output power simulation and experimental results at 70 Figure 10 10.. PlotPlot versus versus the the speed speed of of the the output output power power simulation simulation and and experimental experimental results results at 70 at Ω/phase load. 70Ω/phaseΩ/phase load load..

Figure 11. Plot versus the load per phase of the output power simulation and experimental results for a speed of 375 rpm.

Figure 11. Plot versus the load per phase of the output power simulation and experimental results for Figure 11. Plot versus the load per phase of the output power simulation and experimental results for aFigure speed 11. of 375Plot rpm. versus the load per phase of the output power simulation and experimental results for a speed of 375 rpm. a speed of 375 rpm.

Figure 12. Plot versus the speed of the output power under 70, 26 and 130 Ω/phase load with 3 mm air gaps, experimental results.

4.4. Temperature Test Figure 12. Plot versus the speed of the output power under 70, 26 and 130 Ω/phase load with 3 mm FigureA temperature 12. Plot versus test the was speed performed of the output before thepower construction under 70, of26 the and rotors 130 Ω in/phase order load to check with the 3 mm safe air gaps, experimental results. operationair gaps, experimental of the machine. results. The crucial magnitude is the temperature of the stator, which consists of

4.4. Temperature Test 4.4. Temperature Test A temperature test was performed before the construction of the rotors in order to check the safe A temperature test was performed before the construction of the rotors in order to check the safe operation of the machine. The crucial magnitude is the temperature of the stator, which consists of operation of the machine. The crucial magnitude is the temperature of the stator, which consists of

Energies 2017, 10, 269 8 of 14

Figure 9. Plot versus the speed of the output phase voltage at no-load, simulation and experimental results.

Figure 10. Plot versus the speed of the output power simulation and experimental results at 70 Ω/phase load.

EnergiesFigure2017, 1011,. 1269 Plot versus the load per phase of the output power simulation and experimental results for9 of 14 a speed of 375 rpm.

FigureFigure 12.12. PlotPlot versusversus thethe speedspeed ofof thethe outputoutput powerpower underunder 70,70, 2626 andand 130130Ω Ω/phase/phase load with 33 mmmm airair gaps,gaps, experimentalexperimental results.results.

4.4.4.4. Temperature TestTest AA temperature testtest waswas performedperformed beforebefore thethe constructionconstruction ofof thethe rotorsrotors inin orderorder toto checkcheck thethe safe operationoperation of thethe machine.machine. The crucial magnitude is the temperature of the stator,stator, whichwhich consistsconsists ofof windings plunged in the supporting structure made of high temperature epoxy resin with a highest permitted temperature of 120 ◦C. Several thermocouple sensors were plunged in the supporting structure,Energies 2017 the, 10, 269 epoxy resin. Their exact position is presented in Figure 13. Point 1 is located on9 of the 14 surface of the winding, while point 4 is located 28 mm away from the end of the winding. Point 3 is placedwindings in theplunged center in of the a coil supporting area. structure made of high temperature epoxy resin with a highest permittedWith only temperature the stator of mounted 120 °C. Several on the shaft thermocouple and fed by sensors a 2 A dc were current, plunged Figure in 14 the presents supporting the variationstructure,of the the epoxy temperature resin. T overheir timeexact inposition different is sectorspresented of the in stator.FigureAs 13 observed,. Point 1 is the located critical on parts the aresurface those of between the winding the coils,, while point point 2 in4 is Figure located 13 .28 mm away from the end of the winding. Point 3 is placedWe in note the center that in of the a coil same area. test during normal operation, with the whole machine assembled, the highestWith only temperature the stator in mounted the stator on did the not shaft exceed and 22 fed◦C by because a 2 A of dc the current, air-flow Figure due to 14 the presents rotation the of thevariation rotors. of These the temperature results show over the time safe in operation different of sectors the machine of the stator. under As nominal observed, conditions the critical without parts statorare those deformation between the problems. coils, point 2 in Figure 13.

Figure 1 13.3. Constructed stator with the temperature measuring points indicated.

Figure 14. Temperature test, only stator mounted in the shaft, 2 A dc current to the winding.

We note that in the same test during normal operation, with the whole machine assembled, the highest temperature in the stator did not exceed 22 °C because of the air-flow due to the rotation of the rotors. These results show the safe operation of the machine under nominal conditions without stator deformation problems.

5. Variation of the Axial Width of the Two Air-Gaps Due to the manufacturing process, these machines may present imperfections or asymmetries compared to the original theoretical design (manufacturing imperfections). We have considered the case of unequal air-gaps, e.g., due to anomalies on one surface of the epoxy resin in the stator. This section discusses the effect of the different axial width of the two air gaps on the electrical magnitudes and the field of the machine. We have considered two different cases. In the first case, the axial width of the two air gaps is the same, 3 mm, while in the second case, the widths of the two air gaps are unequal, 2 mm and 4 mm, respectively. We note that in both cases, the total width of the

Energies 2017, 10, 269 9 of 14

windings plunged in the supporting structure made of high temperature epoxy resin with a highest permitted temperature of 120 °C. Several thermocouple sensors were plunged in the supporting structure, the epoxy resin. Their exact position is presented in Figure 13. Point 1 is located on the surface of the winding, while point 4 is located 28 mm away from the end of the winding. Point 3 is placed in the center of a coil area. With only the stator mounted on the shaft and fed by a 2 A dc current, Figure 14 presents the variation of the temperature over time in different sectors of the stator. As observed, the critical parts are those between the coils, point 2 in Figure 13.

Energies 2017, 10, 1269 10 of 14 Figure 13. Constructed stator with the temperature measuring points indicated.

FigureFigure 1 14.4. TemperatureTemperature test, only stator mounted in the shaft, 2 A dc current to the winding.

We note that in the same test during normal operation, with the whole machine assembled, the 5. Variation of the Axial Width of the Two Air-Gaps highest temperature in the stator did not exceed 22 °C because of the air-flow due to the rotation of the rotors.Due to These the manufacturing results show the process, safe operation these machines of the machine may present under imperfections nominal condition or asymmetriess without statorcompared deformation to the original problems. theoretical design (manufacturing imperfections). We have considered the case of unequal air-gaps, e.g., due to anomalies on one surface of the epoxy resin in the stator. 5. VariationThis section of the discusses Axial Width the effect of the of Two the differentAir-Gaps axial width of the two air gaps on the electrical magnitudes and the field of the machine. We have considered two different cases. In the first case, EnergiesDue 2017 to, 10 the, 269 manufacturing process, these machines may present imperfections or asymmetries10 of 14 the axial width of the two air gaps is the same, 3 mm, while in the second case, the widths of the two compared to the original theoretical design (manufacturing imperfections). We have considered the air gaps are unequal, 2 mm and 4 mm, respectively. We note that in both cases, the total width of the twocase airof -unequalgaps is theair -same,gaps, e.g.6 mm., due Several to anomalies 3D FEM on analyses one surface and of laboratory the epoxy tests resin were in the performed stator. for two air-gaps is the same, 6 mm. Several 3D FEM analyses and laboratory tests were performed for each Thiscase andsection the discussesresults are the presented effect of and the differentcompared axial below. width of the two air gaps on the electrical each case and the results are presented and compared below. magnitudes and the field of the machine. We have considered two different cases. In the first case, 5.1. Simulation Results the5.1. axial Simulation width Resultsof the two air gaps is the same, 3 mm, while in the second case, the widths of the two air gapsFigure are 15unequal, presents 2 mm the anddistribution 4 mm, respectively. of the normal We componentnote that in ofboth the cases magnetic, the total flux width density of forthe Figure 15 presents the distribution of the normal component of the magnetic flux density for both both scenarios, equal and unequal air-gaps. Owing to the large air gap, the maximum flux density scenarios, equal and unequal air-gaps. Owing to the large air gap, the maximum flux density does not does not exceed 0.44 T in both sides; therefore, the magnetic circuit is unsaturated. Also, it can be exceed 0.44 T in both sides; therefore, the magnetic circuit is unsaturated. Also, it can be noticed that noticed that the unequal air-gaps do not result in the increase of the field in the side of the smaller the unequal air-gaps do not result in the increase of the field in the side of the smaller air-gap, but only air-gap, but only in a slight shift of the waveform. in a slight shift of the waveform.

FigureFigure 15 15.. NormalNormal component component of of the the magnetic magnetic density density across across the the axial axial length length of ofthe the machine machine with with 3 mm3 mm air air gaps gaps compared compared with with the the 2– 2–44 mm mm air air gaps gaps machine. machine.

Nonetheless,Nonetheless, the the shift shift of of the the magnetic magnetic field field does does not not materially materially affect affect the the results results shown shown in in FigureFiguress 16 andand 17 17,, presenting presenting the the variation variation of theof the phase phase voltage voltage at no at load no andload the and output the output power power versus versusthe speed, the speed respectively., respectively.

Figure 16. Plot versus the speed of the output phase voltage at no-load (Back-EMF) in simulation model with 3 mm air gaps compared to that of the simulation model with 2–4 mm air gaps.

Energies 2017, 10, 269 10 of 14 two air-gaps is the same, 6 mm. Several 3D FEM analyses and laboratory tests were performed for each case and the results are presented and compared below.

5.1. Simulation Results Figure 15 presents the distribution of the normal component of the magnetic flux density for both scenarios, equal and unequal air-gaps. Owing to the large air gap, the maximum flux density does not exceed 0.44 T in both sides; therefore, the magnetic circuit is unsaturated. Also, it can be noticed that the unequal air-gaps do not result in the increase of the field in the side of the smaller air-gap, but only in a slight shift of the waveform.

Figure 15. Normal component of the magnetic density across the axial length of the machine with 3 mm air gaps compared with the 2–4 mm air gaps machine.

Nonetheless, the shift of the magnetic field does not materially affect the results shown in Figures 16 and 17, presenting the variation of the phase voltage at no load and the output power Energies 2017, 10, 1269 11 of 14 versus the speed, respectively.

Figure 16.16. Plot versus the speed of thethe outputoutput phasephase voltagevoltage atat no-loadno-load (Back-EMF)(Back-EMF) inin simulationsimulation model withwith 33 mmmm airair gapsgaps comparedcompared toto thatthat ofof thethe simulationsimulation modelmodel withwith 2–42–4 mmmm airair gaps.gaps.

Energies 2017, 10, 269 11 of 14

Figure 17.17. Plot versus the speed of the output p powerower in the simulation model with 3 mm air gaps compared to that of the simulation model with 2–42–4 mm air gaps.gaps.

5.2. Experimental Results The comparison betweenbetween thethe constructed constructed model model with with 3 mm3 mm and and 2–4 2– mm4 mm air air gaps gaps reveals reveals that that the theoutput output phase phase voltage voltage at no-load at no-load (Back-EMF) (Back-EMF) remains remains unaffected, unaffected, as it isas producedit is produced by the by magnets the magnets field whichfield which remains remains constant. constant. This outcomeThis outcome can be can easily be easily deduced deduced from from Figure Fi 18gure. 18.

Figure 18 18.. Plot versus the speed of the output phase voltage at no no-load-load in the constructed model with 3 mm air gaps compared to that of the constructed model with 2–42–4 mmmm airair gaps.gaps.

FigureFiguress 19 and 20 demonstrate the variation of the phase current current and and phase phase voltage voltage under under load load,, respectivelyrespectively,, versusversus thethe speed speed for for the the two two air air gap gap cases. cases. The The constructed constructed machine machine with with two two unequal unequal air airgaps gaps presents presents slightly slightly less less current current and and voltage voltage than than the the machine machine with with equal equal air air gaps.gaps. ThisThis explains the similar reduction of the output power in the case of two unequal air gaps, compared to the case of equal air gaps, as presented in Figure 21. Potential causes of the discrepancy between FEM and experimental results are small asymmetries of the constructed machines and the variation of the ohmic resistances with the temperature.

Figure 19. Plot versus the speed of the output phase current in constructed model with 3 mm air gaps compared to that of the constructed model with 2–4 mm air gaps.

Energies 2017, 10, 269 11 of 14

Figure 17. Plot versus the speed of the output power in the simulation model with 3 mm air gaps compared to that of the simulation model with 2–4 mm air gaps.

5.2. Experimental Results The comparison between the constructed model with 3 mm and 2–4 mm air gaps reveals that the output phase voltage at no-load (Back-EMF) remains unaffected, as it is produced by the magnets field which remains constant. This outcome can be easily deduced from Figure 18.

Figure 18. Plot versus the speed of the output phase voltage at no-load in the constructed model with 3 mm air gaps compared to that of the constructed model with 2–4 mm air gaps.

Figures 19 and 20 demonstrate the variation of the phase current and phase voltage under load, Energiesrespectively2017, 10, ,versus 1269 the speed for the two air gap cases. The constructed machine with two unequal12 of 14 air gaps presents slightly less current and voltage than the machine with equal air gaps. This explains the similar reduction of the output power in the case of two unequal air gaps, compared to the case of equal airair gaps,gaps, asas presentedpresented inin FigureFigure 2121.. Potential causes of the discrepancydiscrepancy between FEM and experimental results results are are small small asymmetries asymmetries of the of the constructed constructed machines machines and the and variation the variation of the ohmicof the resistancesohmic resistance with thes with temperature. the temperature.

Figure 19.19. Plot versus the speed of the output phase current in constructed model with 3 mm air gaps Energies 2017, 10, 269 12 of 14 Energiescompared 2017, 10, 269 toto thatthat ofof thethe constructedconstructed modelmodel withwith 2–42–4 mmmm airair gaps.gaps. 12 of 14

FigureFigure 20.20. PlotPlott versusversus the speedspeed of thethe terminalterminal phasephase voltagevoltage in constructedconstructed model with 3 mmmm airair gapsgaps comparedcompared toto thatthat ofof thethe constructedconstructed modelmodel withwith 2–42–4 mm mm airair gaps.gaps.

Figure 21. Plot versus the speed of the output power in constructed model with 3 mm air gaps Figure 21. Plot versus the speed of the output power in constructed model with 3 mm air gaps compared to that of the constructed model with 2–4 mm air gaps. compared to that of the constructed model with 2–4 mm air gaps.

Finally, the ratio (Pout_symmetric – Pout_asymmetric)/Pout_symmetric, describing the influence of the air gap Finally,Finally, thethe ratio ratio (P (Pout_symmetricout_symmetric –− PPout_asymmetricout_asymmetric)/Pout_symmetric)/Pout_symmetric, describing, describing the influence the influence of the of air the gap air gapvariation variation,,, was was computed computed for the for same the same load load and anda variety a variety of speed of speed values. values. This This ratio ratio isis negligiblenegligible is negligible forfor forthethe thesimulationsimulation simulation resultsresults results ((0.6%, (0.6%, Figure Figure 17),), 17 while), while forfor forthethe the experimentalexperimental experimental resultsresults results,,, Figure Figure 21, , 21 itit ,isis it 4.9 is%. 4.9%.

6.. Conclusions The objective of this paper was to examine the performance of a low speed,, axial flux permanent magnet synchronous generator for small power wind applications. Firstly, the chosen parameters of thethe AFPMAFPM machinemachine werewere indicated,indicated, asas wellwell asas thethe calculatedcalculated variablesvariables fromfrom tthe analytical equations. Using nonlinear 3D FEM electromagnetic analysis, the proper magnet topology was selected,, as as presented in a former published paper,, and the model was simulated for a variety of speeds under no--loadload and and load load conditions. The prototype was constructed in the university laboratory, investigatedinvestigated and and lastly lastly the the experimental experimental results results were compared toto thethe simulation simulation ones. ones. The The comparison indicated that the experimental results confirm thethe simulation. simulation. An experimental temperaturetemperature testtest forfor thethe pprototype stator was performed showing the safe operation of the machine and finally the machine was testedtested forfor thethe twotwo unequal air gaps operating condition. FEM results show that unequal air gaps, maintaining the same total axial width of the machine as the machine with two equal air gaps, do not affect machine Back EMF, the stator current, the terminal voltage and thethe outputoutput powerpower ofof thethe machinemachine.. Experimental results indicate that unequal air gaps, maintaining thethe same total axial width of the machine as the machine with two equal air gaps, do not affect machine Back EMF, while the stator current,, thethe terminalterminal voltagevoltage and the output power are slightly decreased..

Author Contributions: All authors contributed in every part of the design, experiment, measurements, data analysis and the authorship of this paper.

Energies 2017, 10, 1269 13 of 14

6. Conclusions The objective of this paper was to examine the performance of a low speed, axial flux permanent magnet synchronous generator for small power wind applications. Firstly, the chosen parameters of the AFPM machine were indicated, as well as the calculated variables from the analytical equations. Using nonlinear 3D FEM electromagnetic analysis, the proper magnet topology was selected, as presented in a former published paper, and the model was simulated for a variety of speeds under no-load and load conditions. The prototype was constructed in the university laboratory, investigated and lastly the experimental results were compared to the simulation ones. The comparison indicated that the experimental results confirm the simulation. An experimental temperature test for the prototype stator was performed showing the safe operation of the machine and finally the machine was tested for the two unequal air gaps operating condition. FEM results show that unequal air gaps, maintaining the same total axial width of the machine as the machine with two equal air gaps, do not affect machine Back EMF, the stator current, the terminal voltage and the output power of the machine. Experimental results indicate that unequal air gaps, maintaining the same total axial width of the machine as the machine with two equal air gaps, do not affect machine Back EMF, while the stator current, the terminal voltage and the output power are slightly decreased.

Author Contributions: All authors contributed in every part of the design, experiment, measurements, data analysis and the authorship of this paper. Conflicts of Interest: There are no conflicts of interests.

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