HSRM paper单相永磁SRM.doc

Design and Driving Characteristics of Single-phase 4/4 Hybrid SRM with Non-uniform Air-gapGuang-Il Jeong, Ying Tang, Jin-Woo AhnDept. of Mechatronics EngineeringKyungsung UniversityBusan, KAbstractThis paper presents a novel single-phase 4/4 poles hybrid Switched Reluctance Motor(SRM) intended for high speed operation used in hammer drill. This single phase hybrid SRM with special non-uniform air-gap design is driven by the simplest converter, unipolar converter, which allows the motor to operate at high speed with low switching loss. Compare with the conventional single-phase SRM, it has an increased torque density and relatively low torque ripple by properly making use of the cogging torque generated by permanent magnets. The proposed hybrid SRM design has the self-start capability due to the parking function of permanent magnets. To verify the effectiveness of the design, finite element analysis software (ANSOFT) is employed to get the electrical characteristics of the proposed motor. Based on the analysis, a prototype motor is manufactured and operated to catch the experimental results.Keywordshybrid SRM; non-uniform air-gap; low-cost; torque ripple; hammer drill,ANSOFTI. Introduction Switched Reluctance Motor drives make their entrance into a wide range of industrial and domestic applications. Their unique capability to operate under harsh and high speed ambient conditions, in particular, makes them a powerful candidate for many automotive and aerospace products. Switched Reluctance motors are relatively cheap and easy to be manufactured. This, however, does not entirely cover the high cost of power and control electronics.For various industrial applications, cost is an important issue, especially for pump, hammer drill or fan-type electrical drives due to their mass production. The multi-phase switched reluctance machine has dominated the electrical drives for long decades. But the multi-phase SRM suffers from the poor efficiency and high manufacture cost due to the switches and switching loss, especially operated in high speed range. So, there is a strong demand in the industrial application for a new kind of low-cost SRM drives, to replace the traditional multi-phase SRM drives. To cut down the cost when needed to operate at a high speed, an effective way is to use unipolar drives instead of bipolar drives by reducing the number of switches in converter. Single phase switched reluctance (SR) motors suit naturally unipolar excitations and have been involved in many studies as an effective low cost drive system 12. But the focus of the current research work on low-cost SR drive system is mainly on the converter side. For example, a single-controllable-switch converter 3 was proposed for controlling an asymmetric two-phase SR motor, and a modified topology was proposed in 4 with improved controllability of the current. However, there is no doubt that in order to achieve a good low-cost solution, it is important that the converter, the motor control, and the motor design should be researched together. A single-phase version of SR motor, and its corresponding converter, will be both the simplest structure and the lowest cost to produce among different SR motor designs. But for conventional single phase SR machine, there is a dead torque zone occupying half of one electrical period, which results in high torque ripple. And the initial positioning of a single phase SR machine to enable self-starting is also a problem. Often, parking magnets or parking windings are used in single-phase SR machines to suspend the rotor at a proper position which allows self-starting 5. In 6 ,a hybrid single phase SRM using cheap ferrite magnets which is formed in a V-shape flux concentration manner is proposed, offering more flexibility in controlling and shaping the magnet field. Similar to other machines with embedded stator magnets, the stator of the design presented in 6 is made of several segments, resulting in a weaker stator mechanical structure compared to a solid stator construction. Also, the challenge faced by segmented stator design is that it is not easy to locate every stator segment on the same stator inner radius when piecing them together, so the mismatched stator segments may cause a degraded machine performance.In this paper, a new single-phase hybrid switched reluctance (HSR) motor which uses both the reluctance torque and permanent magnet interaction torque together to generate high torque density is presented. The cogging torque produced from permanent magnets is properly utilized to reduce the torque ripple. And self-starting capability is captured by the non-uniform air gap design. Some experimental results are done to verify the performance of proposed hybrid switched reluctance motor.II. The proposed single phase hybrid srmThis section presents the basic design details and working principles of the hybrid SRM with non-uniform air gap. Furthermore, several simulation results from finite element analysis software(ANSOFT) are also presented. A. The design of proposed hybrid SRMThe structure of proposed HSR machine is shown in Fig.1. It has four rotor poles, four stator reluctance poles, and two permanent magnets are mounted on the surface of the stator in between two adjacent reluctance poles as indicated in Fig. 1. Each coil consists of a winding that embrace the two neighborhood stator poles. These two windings may be connected in parallel or in series to form a phase. The non-uniform air gap on one side of each rotor pole is adopted for parking rotor to realize self-starting ability of the machine. Fig. 1. Three-dimensional model of proposed motor design with non-uniform air gapAlso, motor is predeterminated to rotate in only the anticlockwise direction as viewed in Fig. 2.Fig. 2. Section view of proposed motor B. The working principle of proposed hybrid SRMStage 0(start positon): When there is no current flowing in the motor, the rotor will park at the position shown in Fig. 3 (defined as the zero degree of rotor position), due to the asymmetrical structure of the rotor poles under the effect of the two PM embedded in stator.Stage 1(wingding working period): When voltage is added to winding terminals, the generated current will flow in the coils and produce flux to circulate in the machine, thus, will produce a positive reluctance torque to pull the rotor to rotate forward, until the rotor poles are aligned with the stator reluctance poles shown in Fig.4, which is around 45 mechanical degrees for this machine. Attention should be paid to that the total flux is generated by both PM and winding current.Stage2(only PM working period):After about 45 mechanical degrees later, the current in winding coils should then be maintained zero for the remaining about 45 mechanical degrees to allow the rotor align with stator poles again under the function of PM as shown in Fig.3.By repeating this procedure, a steady state operation can be achieved.Fig. 3. Start position(unaligned position) of proposed motor Fig.4. Aligned position of proposed motorC. The electronical characteristic of the proposed hybridSRMIn order to get familiar with the electronica characteristic of proposed motor, the inductance profile under different energizing currents is simulated with no PM function in motor(Fig.5.). From the profile it can be see that the greater the current flowing in winding, the more saturation is reached. A family of curves over the electromagnetic toque as a function of the rotor position for different values of the motor current is shown in Fig.6. The torque curve at zero motor current is the torque exerted by the permanent magnets alone. At point of 0 degree, there is a stable equilibrium s the parking position due to the non-uniform air gap on the rotor side(see Fig.2.). At that position energizing the motor winding with the rated current amplitude gives the momentary starting torque. Thanks to the fact that the current flowing in winding weakens the field of the permanent magnets, a fraction of the rated current, the deparking current is sufficient to counterbalance the holding torque of the PM.The current should be zero when the rotor has reached the position aligned with the reluctance poles. Then by properly making use of PM flux linkage, a positive cogging torque generated by PM flux linkage will then continue to pull the rotor poles to the position aligned with the PM-poles. During this period the torque density has been improve and the great Fig.5.Inductance profile under different energizing currents is simulated with no PM function in motorFig.6. Family of shaft torque curves at different current levelstorque ripple that always happen in conventional single phase SRM has been drastically reduced by effect of PM. The reason is demonstrated in Fig.7. For a normal SR motor, the average torque for a DC current is determined by the area enclosed by curve 1, curve 2, and line AB. If curve 2 could be moved along the negative flux linkage axis, like curve 3, the total enclosed area for the same current will be increased. The flux produced by the permanent magnets placed in the middle ofFig.7. Demonstration of how the PM flux linkage could be used to increasethe total torquereluctance poles can be used to move the flux-linkage curve in such a manner as to increase the motors torque density. This increase is due to the additional PM interaction torque component. It should be pointed out that the cogging torque has no contribution to the average torque. The cogging torque is negative in the region where the reluctance torque and the PM interaction torque are positive. The cogging torque is used to move some of the positive torque produced by the winding current to the region where the current is zero. Compared to aconventional single-phase SR motor, whose output torque is zero when the phase current is off, the torque ripple of this HSR motor is thus greatly reduced. Fig. 8 shows the waveform of the instantaneous cogging torque and continuous torque at a fixed DC current for one electrical period is shown in Fig.9. The green line used to do comparison with proposed non-uniform design is the uniform air gap structure model. It can be found out that by the non-uniform air gap design, the starting torque can be increased at both parking position and align position, from these three figures.Fig.8. Cogging torque of proposed motor(mechanical degree)Fig.9. Continuous torque of proposed motor under constant current of 16A(mechanical degree)D. The optimization of proposed hybrid SRMThe self-start capability is acquired by increasing the air gap on a fractional part of the rotor pole arc. Thus, in order to get the better starting performance of this structure, some simulations with different air gap shape are built to get the best design specification. Three different air gaps are modeled as presented in Fig.10. The only difference between these four (a) HKY(uniform air gap) (b)KY1.0 (c) KY1.1 (d) KY1.2Fig.10.Proposed motor design with different air gapFig.11. Continuous torque of proposed motor design with different air gapmodels is air gap size(Table 2). From Fig.11, it can be seen that the best properties belong to KY1.2 owing to its bigger start ability and less torque dead zone. Because of that, the dimension of KY1.2 is devoted to motor manufacture.Table 1 Air gap size in four modelsParametersHKYKY1.0KY1.1KY1.2Air-gap (mm)0.50.5 to 1.00.5 to 1.10.5to1.2Outer radius of rotor (mm)1716.5to15.816.5to15.816.5to15.8I. experiment relutsTo verify the proposed design, a prototype hybrid SRM has been manufactured, which is shown in Fig. 12. Some of the main dimensions and specifications of the motor are given in Table 2. (a) Rotor of proposed design (b) Stator of proposed design (c) Assembly Fig.12. Prototype hybrid SRMTable 2 Main dimensions and specifications of prototype motorParametersHKYNumber of stator pole4Number of PM2Number of rotor pole4Length of stator (mm)78Width of stator (mm)58Stack length (mm)30Yoke thickness of stator (mm)8.2Inner radius of stator (mm)17.5Air-gap (mm)from0.5 to 1.2Outer radius of rotor (mm)17Inner radius of rotor (mm)14Radius of shaft (mm)6.55Stator pole arc (deg.)30Rotor pole arc (deg.)42/30Turn number per phase184Resistance per phase (Ohm)0.577PM Amount (mm3)12112A. Experiment waveformsFig.13. presents the experimental setup of the single phase hybrid SRM. The load is supplied by the dynamometer and the output power is measured by high-speed dynamometer.The input power is measure by the power analyzer PPA2530. Since this motor is developed for hammer drill whose target is running at a rated speed of 18000r/min with different load, theloadcapability is tested in experiment and indicated in Fig.14.Fig.13. Workbench setup(a) Torque = 0.1Nm and speed = 18000RPM(b) Torque = 0.3Nm and speed = 18000RPMThough attempts have been made to reduce the torque ripple, this prototype motor still appears to be noisy. To perfect this proposed design, idea on noise reduction should be further considered.CONCLUSIONSThis hybrid SRM is cost efficient both on the electronics and on the motor side. The number of power switch is reduced since the single phase converter is applied. The motor has high torque density and low torque ripple owing to a combination of PM-bias and multi-pole excitation and its design permits massive produ