CHAPTER 3 FABRICATION OF SOFT MAGNETIC COMPOSITE SWITCHED RELUCTANCE GENERATOR PROTOTYPE

Transcription

1 103 CHAPTER 3 FABRICATION OF SOFT MAGNETIC COMPOSITE SWITCHED RELUCTANCE GENERATOR PROTOTYPE 3.1 INTRODUCTION This chapter describes elaborately the developmental aspects of SMC-SRG prototype, covering the fabrication details of the machine, description of power converter, position sensor, logic circuit and the choice of electric motor that emulates the prime mover that has been coupled to the soft magnetic composite switched reluctance machine. The cost involved in the fabrication of single SRG prototype from SMC powder through powder metallurgical process is quite prohibitive. The cost effective method adopted to fabricate the SRG prototype is to utilize the pre-form blanks or the prototyping blanks available in standard dimensions from Höganäs AB-Sweden. The machining of these pre-form blanks necessitates the deployment of high speed and high precision machinery as recommended by the SMC manufacturer, Engstrom et al. (1994). As this kind of precision machinery is yet to be available where this research work has been carried out, it has been decided to fabricate the SRG indigenously making use of the available machinery, at the same time by exercising extreme caution taking cognizance of the brittle nature of the SMC material. The machining process of stator and rotor from pre-form SMC SOMALOY10003P blanks has been explained. Next, the process of assembling of constituent parts of soft

2 104 magnetic composite switched reluctance machine which include windings, slotted angle disk, position sensors, shaft, casing and end caps has been explained. The development of power converter and the logic circuit for generator operation of the fabricated soft magnetic composite switched reluctance machine has been elaborated. Finally the selection of motor that emulates prime mover for SRG has been explained along with the description its coupling to fabricated soft magnetic composite switched reluctance machine to result a SMC-SRG prototype test platform. 3.2 STATOR AND ROTOR CORE MACHINING AND ASSEMBLY The use of Soft Magnetic Composites (SMC) and compaction technology allows creating complex 3D ferrous structures. In these structures the core is subject to a 3D variation of the magnetic flux over an excitation cycle, and the trajectory of the flux vector forms different shapes in different parts of the core. Pre-form or prototyping blanks of SOMALOY10003P have been imported from Höganäs AB-Sweden to fabricate the stator and rotor cores of SMC-SRG. The choice of lubricating binder in SMC plays a significant role in the machinability of the prototyping material blanks. The lubricating binder 0.3%LB1 facilitates better machinability of the SMC pre-form blanks owing to higher transverse rupture strength(140mpa) and higher resistivity, Lefebvre et al. (2002). The circular SMC prototyping blank has the following dimensions: outer diameter of 120mm and height of 20mm.Six such pre-form blanks have been employed in the fabrication of stator and rotor core, three for stator and three for rotor. A typical prototyping SOMALOY10003P blank for stator and rotor core fabrication is shown in Figure 3.1.

3 105 Figure 3.1 SOMALOY10003P prototyping material blank (OD120mm and H20mm) Stator Core Machining Three circular shaped blanks (OD120mm, Height20mm) were glued together by using epoxy glue LOCTITE -638 before machining to fabricate stator core of the prototype machine. The layer of the glue between SOMALOY10003P prototyping material blanks is 0.02mm thick, Guo et al. (2003). The stator core material after glued but before machining is shown in Figure 3.2. Figure 3.2 Glued prototype material blanks for stator core before machining SOMALOY10003P prototyping material blanks can be machined by drilling, turning and milling into prototype components for soft magnetic applications. In order to machine larger components, the outer radius and

4 106 radial edges are milled first by clamping the blank of SMC axially. A jig must be built to house the partially machined SMC sections at the correct diameter, with an adjustable clamp pressing each section in place from the inner radius. An aluminum plate is then bolted over the partially machined sections to clamp them in place axially and the radial clamps are then removed. The inner radius of the arcs is then milled out as a complete circle, guaranteeing concentricity. The key feature in this method is that the edges of the SMC are supported, reducing corner damage because the SMC is sandwiched axially between two sacrificial aluminum plates. The outer diameter of the prototyping material is milled to the required dimension to result in a good tolerance in the stator core outer profile allowing a close fit with the outer casing. After the outer diameter is machined to size, the remainder of the former jig is unbolted and removed, leaving the finished stator core prototyping section. Any subsequent machining must be by grinding but the abrasive used in the grinding wheel must be very strong. Abrasives are considerably harder than cutting-tool materials, Bayramli et al. (2005). They allow removal of very small quantities of material from the work-piece surface. Consequently, very fine surface finish and dimensional accuracy can be obtained. The stator core after machining is shown in Figure 3.3 Figure 3.3 Machined SMC stator core

5 Rotor Core Machining and the Shaft The most challenging aspect in the machining of rotor part is in ensuring that the angles between the projected rotor poles are in outside radial direction, accurate enough that the rotor is cylindrical with the correct diameter and ensures very small air gap (0.5mm) between stator and rotor components. The inner radius of the arcs is then milled out as a complete circle with a shaft key hole, guaranteeing concentricity. The carbon steel shaft is stepped down to 75mm after the bearing seat. This facilitates a light coupling like a rotating slotted disk which is used for sensing the change of states in rotor position sensor signals. The rotor core is shown in Figure Windings Switched reluctance machines invariably use a concentrated copper coil winding on each stator pole. The coils may be interconnected in a variety of ways and a multi-layer winding and/or a multi-strand of wires connected in parallel may be used. For convenience of interconnection, stator pole coils for each phase are connected in series. However, for the number of winding layers per coil there is no wide range of possibilities and the choice is largely dependent upon the available slot area. The coil must be sandwiched between the two stator pole sides to give a rectangular slot profile for the winding. The objective is to fill this slot with as much copper as possible and limit the air gaps between the coil and the surrounding stator SMC core to improve the heat transfer from the coil. The coils were individually wound in situ on each stator pole. They were then connected in series on alternative stator poles in such a direction as to provide two phases each phase having alternative northsouth stator magnetic poles upon excitation. The coil has been constructed from round enamel coated copper conductor of AWG20. The number of coil

6 108 turns per stator pole has been fixed as 70. Before coils were interconnected, an insulating layer was used to contain individual coils and to help absorbing the resin that was later used. Sixteen small strips of mica, each 25 mm long were pressed on the coil sides in each slot in order to help the resin to firmly hold the coils in place. Coils of each phase were then interconnected by brazing Outer Casing and End Caps The outer casing is manufactured from alloy steel. It is a cylinder with a flange on the inside diameter for housing of SMC stator. Around the edges the outer casing is tapped to take bolts for holding the end caps. Both end caps are of the same design and as in the case of outer casing, are manufactured from alloy steel. They comprise a circular disc with a projecting bearing housing and a central hole for the shaft to protrude. On the other side of the disc is a flange located on the outer casing to maintain concentricity. An end cap with bearing is shown in figure 3.4. On the dorsal side of the end plate holes have been drilled to insert the thermocouple sensors to measure temperatures of winding and the stator core. Figure 3.4 Rotor core and End cap with bearing fixed

7 Assembly of Stator and Rotor and the Rotor Position Sensor- Mechanical Details The major requirement is that the rotor and all other rotating parts of the machine should be dynamically balanced, having their centre of mass on the axis of rotation to avoid undue vibration and possible bending of the shaft. The rotor length should be such that the ratio of the maximum length to the maximum rotor radius in practice should not exceed 5.The stator being fitted into casing is illustrated in Figure 3.5 while the entire assembly with one end cap open is shown in Figure 3.6. Figure 3.5 Stator being assembled into the casing Figure 3.6 Assembled stator and rotor

8 110 Rotor position sensors that have been employed in this work are optical sensors with a light emitter and a photo detector, Bilgin and Emadi (2011). The requirements of the mechanical work are as follows: 1) To prepare a disk of appropriate size made of sheet metal plate. 2) To wire cut as many slots in the disk as there are rotor poles. 3) An arrangement for adjusting the position of the opto-device radially and tangentially with regard to a fixed point on the stator. The rotor position detector has been installed on the shaft extension part of SMC-SRG. The rotor position detector system is comprised of three photoelectric transducers and a slotted disk. The slotted disk has four teeth with 45 o width per tooth and four slots with 45 o width per slot and is fixed on the same shaft of the rotor. The three photoelectric transducers are fixed on the extension of the end shield of the SRG with a 30 o interval. The disk has been connected to the shaft using an inside threaded spindle. Screws were used to join the disk to the spindle head. The length of the spindle allows the disk to rotate in a plane which can be easily reached by the slotted optodevice. The wire cut slotted steel disk and the arrangement of slotted disk and position sensors is depicted in Figure 3.7.The three photoelectric transducers are pairs of light emitting diode(mled 930) and the photo transistor(mrd 5009) which has been fixed on opposite sides of the metal sleeves in a collinear fashion using washers and adhesives. Care has been exercised while fixing the photoelectric transducers to preserve the angular difference between the pairs of light emitting diode and the phototransistor.

9 111 Figure 3.7 Slotted disk and the arrangement of slotted disk and rotor position sensors 3.3 POWER CONVERTER AND LOGIC CIRCUIT DEVELOPMENT The objective of this section is to introduce the basic choices for the power converter of SMC-SRG. Since the machine is conceived and built as a prototype for research purpose rather than for optimum power conversion, a simple and versatile converter is required. The basic function of the converter is to energize the phase windings with excitation pulses at predetermined firing angles Selection of Power Converter Topology The criterion for the selection of power converter topology is to achieve simplicity of the power electronics and drive circuits and flexibility of the power supply. Despite simplicity, converters such as the C-dump were not considered suitable for this application. Bifilar winding converter is inappropriate due to extra copper and poor thermal management. The stress

10 112 on the switching device at turn-off is also a negative attribute of the bifilar converter. It is not strictly one-switch-per phase arrangement because of the need for the addition of a chopper transistor. The split supply converter is feasible with even number of phases because for the capacitor voltage to remain stable at half the supply voltage, the mean current of the phases should be equal. Although phase windings can be rated for the capacitor voltage, each power switch must be rated for full supply voltage Vs. Based on the above delibrations and to ensure control flexibility, an asymmetric halfbridge converter has been chosen for this research work. It is robust, allows rapid changes in control strategy and permits independent control of the phases with fault tolerant operation. The asymmetric half bridge converter with three phase legs, developed for this research work, is shown in Figure 3.8 Bridge Rectifier A Q1 C Q3 E Q5 230V,50Hz AC Supply C1 470uF C2 470uF C3 470uF From Gate driver I IRF450 B C4 470uF D1 MUR3060 From Gate driver II D D2 1 MUR3060 Phase1 Phase2 D3 2 MUR3060 IRF450 IRF450 From Gate driver III F D4 1 1 MUR3060 Phase3 D5 2 2 MUR3060 D6 MUR3060 X Q6 IRF450 Y Q4 IRF450 Z Q2 IRF450 0 Figure 3.8 Asymmetric Half Bridge Converter for SMC-SRG To control the upper phase leg transistors independently separate ground is necessary, which is normally achieved through level shifting. Though not shown explicitly in the circuit for power converter, this has been incorporated in the actual circuit so that control strategies such as hysteresis current control can easily be applied.

11 Selection of Switching Components For the SMC-SRG, the excitation voltage is derived from a bridge rectifier connected to ac mains through an auto transformer. The capacitors C1 through C4 are the filters as well as DC link capacitors of 450 µf with the voltage rating of 450V. MOSFET is considered as the optimum choice as power electronic switching device for low power application up to 2-3kW. It is characterized by simple gate driving and snubberless operation and can therefore also be considered as the most economic choice for this application. A power MOSFET with 50A peak and 18A continuous rating has been selected. The conducting losses of power MOSFETS tend to be high. However, the selected transistor, IRF 450, features a low, 0.18 ON state resistance which ensures reasonable transistor losses of less than 10W per phase at the maximum expected transistor current. To protect the transistors and to ensure low conduction losses, the transistor of each phase is mounted on a heat sink along with the power diode of the relevant phase. Power diode losses are not expected to have an impact on the thermal capability of the heat sink to dissipate the heat from the transistor and hence a matt black heat sink of 1.1ºk/w has been chosen for this power converter. A fast recovery MUR3060 diode has been chosen as the freewheeling diode whose continuous current rating should be slightly higher than that of the power transistor, as the peak current will exceed that of the transistor in generator applications. The upper MOSFET is floating and it can be either at 0 V or 160 V. This floating ground would generate a lot of common mode noise. This would interfere with the normal operation of the circuit and it might malfunction. To avoid this situation, the ground of each gate driver output is isolated from the grounds of the other gate drivers and also from the input stage ground.

12 Power Transistor Gate Driver Circuit MOSFET power transistors are generally easier to drive than other switching devices. They are considered as voltage controlled devices. A sufficient voltage exceeding the threshold of the gate-source requirement and an initial small charging current for the gate-source capacitor are basically required to drive the transistor. Good isolation between power circuit connection and electronic circuit is achieved through driver circuit stage. The circuit that achieves the drive requirements for the power transistor is shown in Figure 3.9. Normally if hysteresis type controller is deployed, it demands frequent turn on and turn off of upper MOSFET switches Q1, Q3 and Q5.Three separate gate driver circuits are required with separate/ isolated power supply. The switching signals are fed to the gates of MOSFET switches through an optocoupler- zener diode combination. D7 D1N D9 D1N4007 U Ohm R1 Optocoupler1 TLP250 Q7 2N uF C8 C9 0.1uF From step down Transformer D8 D1N4007 D10 D1N4007 C1 470uF C2 220uF 0.1uF C2 0 R1 680Ohm Q8 2N2907 R1 47Ohm R2 Z D1 A 4.7k BZ18V B Figure 3.9 Gate driver circuit for switch Q1

13 115 A high-speed opto coupler is used to transfer the control signal from the input stage to the gate driver stage. An isolated DC/DC converter ensures that grounds of the gate driver circuits are isolated from each other. A common supply and a common ground is sufficient for the gate driver circuit of lower rung MOSFET switches(q2,q4and Q6) as shown in Figure From step down Transformer D20 D19 D21 D22 1 C20 470uF U C21 220uF 3 0.1uF C22 R19 680Ohm 220uF C27 0.1uF Q7 0.1uF 0.1uF Q17 2N2222 Q7 2N N R1 R1 Y R20 R1 R1 47Ohm Q18 220uF 220uF C8 C28 C9 C8 C9 X 2N2907 R21 ZD1 4.7k 680Ohm 47Ohm Q8 2N2907 R2 4.7k 680Ohm 47Ohm Q8 2N2907 R2 ZD1 4.7k Z ZD1 0 Figure 3.10 Gate driver circuit for lower rung switches Logic Circuit for SMC-SRG Operation A simple digital logic circuit for the operation of SMC-SRG has been developed. When the rotor of the generator is rotated, the photoelectric transducers provide the square wave signals, which represent the information of the rotor position as shown in Figure 3.11.

14 116 Figure 3.11 Output signals of the photoelectric transducers The position sensor is a light emitter photodiode receiver package as shown in Figure Figure 3.12 Photo sensor configuration The circuit is designed such that based on position sensors output, power converter switches must be turned on and off during the declining region of the phase winding inductance. The logic circuit developed for this research work is shown in Figure 3.13.

15 117 B U4A U1A A U5A U2A C U6A U3A Figure 3.13 Logic circuit to drive SMC-SRG The logic circuit, with terminations for rotor position signals and the printed circuit board layout of logic circuit with assembled components on board are shown in Figure Figure 3.14 SMC-SRG logic circuit Power Circuit Layout and Assembly The main concern for the layout of power converter is to reduce the effects of stray inductances, thereby reducing voltage spikes and stresses on

16 118 the power switches. This requires connection of the power capacitors and the power switches to phase winding to be as short as possible. The power circuit has been kept very close to the machine to avoid using long leads for connections. To protect the transistors and to ensure low conduction losses, the transistor of each phase is mounted on a separate heat sink along with the power diode of the relevant phase. Power diode losses are not expected to have an impact on the thermal capability of the heat sink to dissipate heat from transistor. A matt black heat sink operated by natural convection of ambient air to dissipate heat out of power MOSFET is considered here. The maximum junction temperature of the device, T jmax is 125 C and ambient air temperature, T a is considered 25 C. Maximum power dissipation in the transistor, P diss will approximately be 10 W. Using the thermal equation, R j a T jmax P diss T a (3.1) the thermal resistance between the device junction and air is estimated to be 10 o C/W, and with heat sink this will be, R j a R j c R c s R s a (3.2) The first two terms relate to the thermal resistance of junction to case, given as 1 C/W, and case to sink, given as 1.2 C/W.The sink to air thermal resistance will then be 7.8 C/W. The matt black heat sink of 1.1 C/W is very sufficient for cooling the power transistor as well as the accompanying power diode even if a 20% derating is applied. Hence individual heat sinks of the MOSFETS and power diodes have been connected to matt black finish main heat sink. The other side of the main heat sink houses fins for effective heat transfer. The power converter board with the assembled components is shown in Figure 3.15.

17 119 Figure 3.15 Power converter circuit board after assembly The entire layout of the power electronic converter circuit along with logic circuit is shown in Figure Figure 3.16 Power converter circuit in its entirety

18 CONSTRUCTION OF SMC-SRG PROTOTYPE TEST PLATFORM A prime mover that provides mechanical energy input, to the fabricated SMC-SRG is required. This can be achieved using an electric system comprising a motor, with a coupling and bearings for the SR machine. A suitable prime mover has to be selected in order to couple with the fabricated SMC switched reluctance machine. The power capability of the prime mover motor must be higher than the switched reluctance machine that it drives. In this research a simple permanent magnet DC motor has been selected as the prime mover. The motor specifications are, 180V DC, 1500 rpm and 5.2 A. A base frame has been fabricated and the two machines are laid on the base and fitted to the base frame through suitable U shaped clamps. The shafts of the two machines are then level coupled through LOVE-JOY coupling as shown in Figure 3.17 Figure 3.17 SMC-SR machine coupled to DC motor prime mover

19 121 In order to control the operation of the prime mover DC motor an autotransformer and rectifier unit has been employed as shown in Figure Figure 3.18 Rectifier unit connected to DC motor prime mover The entire test plat form with its various units is shown in Figure Figure 3.19 SMC-SRG prototype test platform

20 SUMMARY The fabrication details of the soft magnetic composite switched reluctance generator have been explained in detail in this chapter. The initial cost of establishment to manufacture SMC components through powder metallurgical process is exorbitant even though the cost of SMC powder is cheap and this has proscribed to adopt the powder metallurgical process to develop the prototype. Prototyping through prototyping blanks popularly known as blanks approach can be cheap and fast, but it also has the drawback that the properties will in most cases be inferior to those obtained from compaction. Hence it has been decided to employ pre fabricated prototyping blanks of SOMALOY10003P to fabricate the small prototype of soft magnetic composite switched reluctance generator developed for this research. Owing to absence of windings and permanent magnets on the rotor, an auxiliary excitation voltage source is required for the SMC-SRG. Field excitation can be either from the output DC bus, or a separate bus. The latter kind of excitation is the separate excitation, wherein the excitation behavior is independent of generation. The winding excitation current is supplied by an external source during the whole running process. There is no coupling between excitation voltage and output voltage at that time and the two voltages can be adjusted independently. Hence the control of separate excitation is more straight forward and easier. Phase windings of the SRG are connected to the power converter, which provides field excitation for the phases and rectifies the generated current to the output DC bus. SMC -SRG drive converter for every excitation cycle of each phase should be able to magnetize the phase coil, retrieve the converted energy, and demagnetize the phase coil. The power produced by each phase can be controlled by varying the amplitude and the timing of current pulses in synchronism with the rotor position. It should be excited near the aligned position and then turned off before the unaligned position. Because there is no ideal current source, it has

21 123 been implemented with an asymmetric half-bridge voltage source, current controlled converter whose developmental details are explained in detail. The selection of switching components in the power converter has been explained. The necessary gate driver circuits to turn ON and OFF of transistor switches have also been explained, followed by the logic circuit development for SMC-SRG operation.the magnetic properties of the lamination stack are very good in the plane of the laminates but at the same time, poor in the normal direction. As SMC materials cannot be sintered, the strength is lower than is possible in laminates or sintered component. Typically the rotor teeth are exposed to both forces due to rotation and forces due to the magnetic pull of the stator. The stator will be exposed to the same magnetic pull but no rotational forces. In relatively small machines like the prototype SMC-SRG, when running at low speed, none of these forces will be very large. Finally the selection of appropriate prime mover emulator has been described along with its coupling process to the fabricated SMC switched reluctance machine to result a SMC-SRG test platform that enabled operational testing of developed SMC-SRG system.