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Switched Reluctance Generators and Their Control

By Tom Kennedy,2014-04-08 15:19
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Switched Reluctance Generators and Their Control

    Switched Reluctance Generators and Their

    Control

    Arthur Radun

    To obtain generating action with the SRM, the phase ;

    current must be timed relative to the rotor position as shown Abstract--This paper reviews the characteristics of the

    in Fig. 1 for a 6/4 SRM. In the figure it is assumed that the switched reluctance machine (SRM) operating as a generator. Many of these characteristics are unique compared to those of machine is turning in the positive angular direction so that other machine types because the SRM does not employ generating occurs when the phase torque is negative. Thus permanent magnets or a field winding on its rotor. Not the phase current is timed to flow for those rotor positions employing permanent magnets or a field winding on its rotor where the torque is negative and to be zero for those rotor allows the SRM's rotor to be operated at high temperatures positions where the torque is positive. Because the sign of and speeds. Further it means the SRM, operating as a the SRMs phase torque is independent of the sign of the generator, does not possess the inherent problem of generating SRM's phase current, motoring or generating action is into a shorted winding like a permanent magnet machine. totally controlled by the rotor positions for which the phase Though not having permanent magnets or a field winding on

    current is nonzero [5]. the SRM's rotor gives it certain advantages, the lack of a separate excitation source or winding requires special consideration during the design of an SRM generating system Torque(SRG). This paper describes the SRG's excitation and the effect this excitation has on the SRG's operation. The issue of excitation is especially important during load faults. Also this paper describes the duality of SRM generator and motor 2040operation. The implications this duality has for the SRG's ;control are described. The paper concludes with results for 9060

    controlling the SRG.

Index Terms--Switched Reluctance Motor, Switched

    Reluctance Machine, Switched Reluctance Generator, Phase CurrentGenerators, Motor Drive, Power Electronics

    I. INTRODUCTION Ideal

    There has been significant interest in developing SRMs

    for numerous variable speed applications. These range from 090602040low cost consumer applications to high performance aerospace applications [1-6]. This interest in SRMs is due Fig. 1 Static torque and phase current for one phase during generation. to the machine's potential for low cost and/or fault tolerance Zero and 90 degrees are the aligned position and 45 degrees is the [1-7]. Most SRM applications addressed in the literature unaligned position. have utilized the SRM as a motor and have addressed its motoring performance [1-6]. The SRM can also be applied A typical drive system for controlling the SRM's phase as a generator [8-11]. currents relative to its rotor position is shown in Fig. 2. This The SRM requires power electronics to operate as a drive system is basically the same whether or not the SRM generator just as it does to operate as a motor. This means is being used as a motor or as a generator. The drive system that it is best suited for applications that require variable consists of the SRM, a rotor shaft position sensor, a speed. This makes the SRM a candidate for applications controller, and a power electronic converter. Figure 3 shows such as aircraft engine starter/generators, automotive the three states any one of the phases of the power starter/generators, and windmill generators. In addition, electronic converter can be in. In the first state, the two generating issues arise in applications that regenerate. IGBT switches are on and the SRM's phase voltage is Examples include washing machines, flywheels, and hybrid positive, tending to increase the phase current. In the and electric cars. second state, one IGBT switch remains on while the other

    one is turned off. The current freewheels through the on

    IGBT switch and one diode making the phase voltage zero. The phase current may either increase or decrease. In the ;University of Kentucky third state both IGBT switches are turned off, turning on

    Vbusi()IMotoringGeneratinggen

    ILoadISSRMCPh1Ph2Ph3

    Aligned+2+--112Starting andPositionConverter Control Fault SourceShaftFig. 4 Phase current during generating is the phase current during Positionmotoring mirrored about the aligned position. Microprocessor Control Information busmVFig. 2 Typical SRM drive system for motoring or generating. (2) (1)(2)(1(2)) m~VbusFor the generating case, integrate (1) over a generating

    interval where the angles are greater than zero to obtain C+busgVV (3) (2)(1)(21)-m~

    Now require that (-) be equal to () and eliminate 22these variables from (2) and (3) by adding the two VbusVbusVbusequations together

    +busmbusg()VVCCC++ (4) (1)(1)(21)VVV-m~--For the flux linkage at - to equal the flux linkage at +, 11V must be equal to the negative of V. Because the busmbusgState 2. OneState 3. BothState 1. BothSRMs flux linkage curves are symmetrical around the Switch On, OneSwitches OffSwitches Onaligned position, the phase currents will also be equal at - 1Switch Off and +. The equality of the phase flux and current at +/- 11Fig. 3 States of one phase of the power electronic converter. and +/- means that the SRM's phase current is mirrored 2 around the aligned position from motoring to generating. both diodes. This reverses the phase voltage decreasing the This mirroring of the phase current around the aligned phase current [12,13]. rotor position is shown in Fig. 5. Here the simulated phase

    currents for low speed motoring and generating are shown II. DUALITY BETWEEN MOTORING AND GENERATING with the converter states labeled. The SRG simulated has a The SRM as generator is the dual of the machine as a rated speed of 25,000rpm, rated voltage of 270Vdc, and motor [12,13]. In fact the machine phase current waveforms rated power of 33kW [12]. In the figure the simulated during generating are simply the mirror images, around the motoring and generating phase currents are shown at a aligned rotor position, of the phase currents during machine speed of 5,000rpm and a bus voltage of motoring as illustrated in Fig. 4. This statement can be 270Vdc.During these simulations all of the system losses, proven precisely if the machine's winding resistance is zero phase resistance etc., were set to zero. At the speed and it is essentially true for actual machines with reasonable simulated, the converter chopped using a strategy called efficiencies. The flux linked by an energized phase winding is 500 State 2State 2dtd()()V400 (1) bus~mdtd 300where the bus voltage V is approximately a constant. In busState 3State 1iphwhat follows the rotor position angles, defined in Fig 4, are 200less than zero if they occur before the aligned rotor position State 3State 1and greater than zero for angles greater than the aligned 100rotor position. It will be assumed that the machine is always

    turning in the same direction so that the rotor angle is

    always increasing for both motoring and generating. Thus 0-50-40-10-30-2001020304050the generating angles are greater in value than the motoring (t) angles. For the motoring case, integrate (1) over a motoring Fig. 5 Simulated phase current for low speed motoring and generating with converter states labeled. interval where the angles are less than zero to obtain

freewheel chopping. In motoring, both IGBT switches are Motoring Generating

    Both switches on - Current Both switches off - Current first turned on (state 1) until the current command is increases decreases reached. At this point one IGBT switch is turned off so the One switch on, one switch off One switch on, one switch off - current freewheels through the remaining IGBT and one - Current decreases Current increases diode (state 2). This shorts the phase and the back EMF of Both switches off - Current