CHAPTER 4 HARDWARE DEVELOPMENT OF DUAL ROTOR RADIAL FLUX PERMANENT MAGNET GENERATOR FOR STAND-ALONE WIND ENERGY SYSTEMS

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1 66 CHAPTER 4 HARDWARE DEVELOPMENT OF DUAL ROTOR RADIAL FLUX PERMANENT MAGNET GENERATOR FOR STAND-ALONE WIND ENERGY SYSTEMS 4.1 INTRODUCTION In this chapter, the prototype hardware development of proposed 1kW, 78V, 120 rpm, DRFPMG is described in detail. The fabrication of rotor and stator, details of winding and final mechanical assembly are explained. The testing of the prototype is carried out using an experimental set up. The experimental set up, operation of proposed DRFPMG and test results are discussed. The important experimental results are compared with the results of theoretical design and FEA. The merits of the DRPFMG are discussed in detail. 4.2 PROTOTYPE FABRICATION OF DRFPMG In order to validate the theoretical predictions through experimental tests a prototype of the proposed 1 kw, 78 V, 120 rpm DRFPMG is fabricated and tested. The important specifications of the prototype are given in Table 4.1. From Table 3.3, the cogging torque reduction technique, shifting of slot openings reduces the cogging torque below 1 Nm, as needed by wind turbine systems. From Table 3.5, the percentage reduction in induced emf is less when compared to other cogging torque reduction methods. Hence, in this prototype the cogging torque reduction technique adopted is shifting of slot openings.

2 67 Table 4.1 Specifications of prototype Symbol Values Unit P R 1048 kw N 120 rpm V 78.2 volts D roo m D roi m D rii m D rio m D m D m L 0.04 m T m m N si 45 - N so 45 - N pri 46 - N pro Fabrication of Dual Rotor The rotor cores are made up of silicon steel laminations of 0.45 mm thickness. The number of poles present in inner and outer rotor is 46 each. The high energy NdFeB permanent magnets are used. The grade is N 35. They are uniformly magnetized along inward/outward radial direction. The adjacent magnets of both outer rotor and inner rotor are magnetized in the opposite direction. The important characteristics of the magnets are: Residual magnetic flux density = 11.7 to 12.1 T ; Tensile strength = 7.5kg/mm 2 ; Electrical Resistance = 160 µ-ohm-cm and Maximum operating temperature = 80ºC. The size of the inner rotor magnets is 40 x 10 x 5 mm and the size of the outer rotor magnets is 40 x 20 x 5 mm. The magnets are

68 surface mounted on the outer periphery of the inner rotor core and inner periphery of the outer rotor core as shown in Figure 4.1(a) and 4.1(b) respectively.

(a) (b) (c) Figure 4.1 Dual rotor of DRFPMG 4.2.2 Fabrication of Stator The stator core is made up of silicon steel laminations of 0.45 mm thickness.

Both the working surfaces of the stator core are used with common back iron, unlike the conventional machines. The common back iron serves as return path for the flux lines.

3 68 surface mounted on the outer periphery of the inner rotor core and inner periphery of the outer rotor core as shown in Figure 4.1(a) and 4.1(b) respectively. In order to rotate the inner rotor and outer rotor at the same speed they are connected together using an end disc. The Figure 4.1(c) shows the dual rotor assembly with the shaft. (a) (b) (c) Figure 4.1 Dual rotor of DRFPMG Fabrication of Stator The stator core is made up of silicon steel laminations of 0.45 mm thickness. It consists of slots on both outer and inner periphery. The number of slots present in the outer periphery and inner periphery of the stator is 45 each. Both the working surfaces of the stator core are used with common back iron, unlike the conventional machines. The common back iron serves as return path for the flux lines. The number of coils per phase is 15. The grouping of coils is done such that the electrical angle between any two phases is -120 o as given in Table 4.2. The electrical angle between any two phases is the product of the number of slots per phase and slot angle in electrical degree. A section of the winding is shown in Figure 4.2.

& R Single group of 15 each 2760 = [(8x360) -120] = -120 Figure 4.

4 69 Table 4.2 Grouping of coils N m N S m e N sph Grouping of Coils Electrical Angle between R&Y, Y & B and B & R Single group of 15 each 2760 = [(8x360) -120] = -120 Figure 4.2 Section of the winding diagram The winding pattern is common for slots present in inner and outer periphery of the stator. The three phase windings present in the inner slots are connected in series with the three phase windings present in the outer slots in order to add up the induced emf. The wound stator of proposed DRFPMG is shown in Figure 4.3(a). The wound stator fixed to a metal frame is shown in Figure 4.3(b). (a) (b) (c) Figure 4.3 Wound stator and mechanical assembly of DRFPMG

5 Mechanical Assembly of DRFPMG The final mechanical assembly of the proposed DRFPMG is shown in Figure 4.3(c). The stator is mounted at one end to a metal frame. The dual rotor assembly is fixed such that the stator is nestled between the inner rotor and outer rotor. One bearing set is provided at each end between the shaft and two frames. An air-gap is formed between inner periphery of the outer rotor and outer periphery of the stator. Another air-gap is formed between outer periphery of the inner rotor and inner periphery of the stator. Thus dual air-gap is formed. The radial forces are balanced by an equal and opposite attractive forces due to symmetry. As the DRFPMG is run at low rpm the cantilevered mechanical structure of the outside rotor and the stator is preferred. 4.3 EXPERIMENTAL SETUP OF DRFPMG The experimental set up consists of a three phase induction motor mechanically coupled with the prototype of DRFPMG, as shown in Figure 4.4. The three phase induction motor acts as a prime mover and is controlled by an autotransformer. The entire set up is a laboratory model of direct coupled standalone wind energy system employing a dual rotor radial flux permanent magnet generator, for power generation.

6 71 Figure 4.4 Experimental setup of DRFPMG 4.4 OPERATION OF DRFPMG The experimental measurement of no load induced emf and power capacity of proposed DRFPMG prototype is done, using the experimental set up. The prototype of proposed DRFPMG was run at a speed of 120 rpm using the experimental set up. This speed is suitable for direct coupling of wind turbines with proposed DRFPMG and the measurements are taken at this speed. The fluxes starts from one pole, travel along the circumference of the stator core, link the single layer three phase windings present in the slots and reaches the adjacent opposite pole. As the dual rotor rotates the flux produced by the inner rotor magnets are cut by three phase windings of the inner slots of the stator core. Simultaneously, the flux produced by the outer rotor magnets is cut by the three phase windings of the outer slots of the stator core. The emf is induced in the windings present in the inner and outer slots of the stator simultaneously. Since both the three phase windings are connected in series the emf produced gets added up and available at the terminals of DRFPMG. Both the working surfaces of the stator core are used with common back iron, unlike the conventional machines. Thus, the proposed DRFPMG, work as two conventional radial flux permanent magnet generators connected in series.

72 4.5 EXPERIMENTAL RESULTS The trapezoidal no load induced emf waveform observed using an oscilloscope in one of the three phases of proposed DRFPMG is shown in Figure 4.5. At 120 rpm, the no load peak voltage is 78.

7 EXPERIMENTAL RESULTS The trapezoidal no load induced emf waveform observed using an oscilloscope in one of the three phases of proposed DRFPMG is shown in Figure 4.5. At 120 rpm, the no load peak voltage is 78.2 V. The cogging torque of DRFPMG is measured with piezo electric reaction torque sensors. The cogging torque observed is Nm. The power capacity of the DRFPMG prototype is tested under different loading conditions at four different rotational speeds. The output power at each of these speeds is shown in Figure 4.6. At 120 rpm, under full load condition, the output power of the DRFPMG prototype is 1048 W. The no load induced emf, cogging torque and full load output power of proposed DRFPMG, at 120 rpm, obtained from FEA and experiment are compared in Table 4.3. The experimental results have shown a good agreement with FEA results. Figure 4.5 No load induced emf of DRFPMG prototype

8 73 Figure 4.6 Output power of DRFPMG prototype at different speeds Table 4.3 Comparison of FEA and experimental results Parameter FEA Experiment Unit V volts T cog Nm P R kw 4.6 MERITS OF DRFPMG In DRFPMG having surface mounted magnets, high-energy Neodymium Iron Boron (NdFeB) magnets are necessary, to provide required amount of flux density in the air gap. This will improve the lower power density achieved by ferrite magnets. But the use of NdFeB raises the cost of machines. The doubled air gap associated with dual rotors can produce more power in a slightly enlarged volume. The material cost is therefore sharply reduced. The DRFPMG structure is suitable for both short and long machines as it has non-slotted rotor core. The design of hybrid DRFPMG (surface mounted magnets in outer rotor and buried magnets in inner rotor) reduces the difference between the d-axis and q-axis reactance.

9 74 Both the working surfaces of the stator core are used. This allows the DRFPMG to exploit a higher percentage of the stator winding, for the production of the output power in comparison with conventional machines. This leads to high efficiency, high ratio of diameter to length, and very short end winding. The large armature reaction and low overload capability caused by slotting in the stator can be compensated using surface mounted permanent magnets. Non-slotted toroidal winding is possible in the stator. Smooth torque and low noise are advantages of the non-slotted configurations. Absence of slots and teeth lead to a constant back emf even when the load connected to the DRFPMG varies. In DRFPMG the cogging torque is small. The cogging torque produced in the DRFPMG can be reduced, as needed by wind turbines, using three simple techniques namely, shifting of slot openings, rearrangement of magnet pole arc width and slot opening width. In DRFPMG, the radial forces, in addition to the alignment forces, are balanced. The radial forces are generated that attempt to close the air gap and bring the rotor and stator into contact with each other. These forces are balanced by an equal and opposite attractive forces due to symmetry. Hence the mechanical stress is greatly reduced and it enables a robust mechanical assembly. 4.7 CONCLUSION In this chapter, the hardware development of a 1 kw, 78 V, 120 rpm, DRFPMG is described in detail. In the hardware development of proposed DRFPMG, shifting of slot openings method is used to reduce the cogging torque. The testing of the prototype is carried out using an experimental set up. The experimental set up and results are discussed. At 120 rpm, under full load condition, the DRFPMG prototype produced an output power of kw. The experimental results have shown a good agreement with the theoretical design and FEA results. The merits of the DRPFMG are discussed in detail. Thus, the proposed DRFPMG has a potential application in building robust direct coupled stand-alone wind energy systems.

CHAPTER 3 DESIGN OF THE LIMITED ANGLE BRUSHLESS TORQUE MOTOR

33 CHAPTER 3 DESIGN OF THE LIMITED ANGLE BRUSHLESS TORQUE MOTOR 3.1 INTRODUCTION This chapter presents the design of frameless Limited Angle Brushless Torque motor. The armature is wound with toroidal