1. Field of the Invention
This invention relates to the operation of a polyphase electrical machine, particularly, but not exclusively, a polyphase switched reluctance machine.
2. Description of Related Art
The characteristics and operation of switched reluctance systems are well known in the art and are described in, for example, “The characteristics, design and application of switched reluctance motors and drives” by Stephenson and Blake, PCIM'93, Nüurnberg, 21–24 Jun. 1993, incorporated herein by reference. FIG. 1 shows a typical switched reluctance drive in schematic form, where the switched reluctance motor 12 drives a load 19. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive, and a rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. One of the most common configurations is shown for a single phase of a polyphase system in FIG. 2, in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the “DC link” of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A resistor 28 is connected in series with the lower switch 22 to provide a current feedback signal. A capacitor 25, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”) which cannot be drawn from or returned to the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter.
A polyphase system typically uses several “phase legs” of FIG. 2 connected in parallel to energize the phases of the electrical machine. Because switched reluctance machines typically have very low mutual inductances between phases, it is the standard practice in the art to consider firstly the operation of one phase acting alone and simply add contributions corresponding to the other phases, but each time-shifted by an appropriate amount.
FIGS. 3(a)–(c) show typical waveforms for an operating cycle of the circuit shown in FIG. 2. FIG. 3(a) shows the voltage being applied for the duration of the conduction angle θc when the switches 21 and 22 are closed. FIG. 3(b) shows the current in the phase winding 16 rising to a peak and then falling slightly. At the end of the conduction period, the switches are opened and the current transfers to the diodes, placing the inverted link voltage across the winding and hence forcing down the flux and the current to zero. At zero current, the diodes cease to conduct and the circuit is inactive until the start of a subsequent conduction period. The current on the DC link reverses when the switches are opened, as shown in FIG. 3(c), and the returned current represents energy being returned to the supply. The shape of the current waveform varies depending on the operating point of the machine and on the switching strategy adopted. As is well-known and described in, for example, the Stephenson paper cited above, low-speed operation generally involves the use of current chopping to contain the peak currents, and switching off the switches non-simultaneously gives an operating mode generally known as “freewheeling”.
The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. Detection of rotor position is conventionally achieved by using a transducer 15, shown schematically in FIG. 1, such as a rotating toothed disk mounted on the machine rotor, which co-operates with an optical or magnetic sensor mounted on the stator. A pulse train indicative of rotor position relative to the stator is generated and supplied to control circuitry, allowing accurate phase energization. This system is simple and works well in many applications. However, the rotor position transducer increases the overall cost of assembly, adds extra electrical connections to the machine and is, therefore, a potential source of unreliability.
Various methods for dispensing with the rotor position transducer have been proposed. Several of these are reviewed in “Sensorless Methods for Determining the Rotor Position of Switched Reluctance Motors” by W. F. Ray and I. H. Al-Bahadly, published in the Proceedings of The European Power Electronics Conference, Brighton, UK, 13–16 Sep. 1993, Vol. 6, pp 7–13, incorporated herein by reference.
Some of these methods proposed for rotor position estimation in an electrically driven machine use the measurement of one or more machine parameters from which other values can be derived. For example, phase flux-linkage (i.e. the integral of applied voltage with respect to time) and current in one or more phases can be monitored. Position is calculated using knowledge of the variation in inductance of the machine as a function of angle and current. This characteristic can be stored as a flux-linkage/angle/current table and is depicted graphically in FIG. 4. The storage of this data involves the use of a large memory array and/or additional system overheads for interpolation of data between stored points.
Some methods make use of this data at low speeds where “chopping” current control is the dominant control strategy for varying the developed torque. Chopping control is illustrated graphically in FIG. 5(a) in which the current and inductance waveforms are shown over a phase inductance period. (Note that the variation of inductance is depicted in idealized form.) These methods usually employ diagnostic energization pulses in non-torque-productive phases (i.e. those phases which are not energized directly from the power supply at a particular moment). A method suited to low-speed operation is that proposed by N. M. Mvungi and J. M. Stephenson in “Accurate Sensorless Rotor Position Detection in an S R Motor”, published in Proceedings of the European Power Electronics Conference, Firenze, Italy, 1991, Vol.1, pp 390–393, incorporated herein by reference. These methods work best at relatively low speeds, where the length of time taken up by a diagnostic pulse is small compared to the overall cycle time of an inductance period. As speed rises, the pulse occupies a longer part of the cycle and soon the point is reached where reliable position information is not available.
Other methods operate in the “single-pulse” mode of energization at higher speeds. This mode is illustrated in FIG. 5(b) in which the current and inductance waveforms are shown over a phase inductance period. These methods monitor the operating voltages and currents of an active phase without interfering with normal operation. A typical higher speed method is described in International Patent Application WO 91/02401, incorporated herein by reference.
Instead of opening both switches simultaneously, there are circumstances in which it is advantageous to open the second switch an angle θf later than θon, allowing the current to circulate around the loop formed by the closed switch, the phase winding and a diode. A typical waveform is illustrated in FIG. 5(c). This technique is known as “freewheeling” and is used for various reasons, including peak current limitation and acoustic noise reduction.
The phase inductance cycle of a switched reluctance machine is the period of the variation of inductance for the, or each, phase, for example between maxima when the rotor poles and the relevant respective stator poles are fully aligned. FIG. 5(a) shows the inductance profile in idealized form, whereas in practice the corners of the profile are rounded due to flux fringing in the air and to saturation of the ferromagnetic paths.
It is known that the shape of the phase current waveform of a switched reluctance machine in single-pulse mode is related to the inductance profile of the phase winding. In particular, the start of the rising portion of the inductance profile, which is due to the onset of overlap between the stator and rotor poles, corresponds to the rollover when the phase current changes from rising to falling in the phase inductance cycle. European Patent Application EP 1109309A, incorporated herein by reference, discusses this phenomenon and uses the natural peak in current, in single-pulse operation, as the basis of a rotor position detection method.
In all of these methods, the magnitude of the waveform of the parameter being employed for rotor position detection must be large enough to allow the chosen algorithm to work reliably. While this is not a problem when the machine is on full load or a significant fraction of it, if the machine is called on to operate at a particular speed at no-load or a very small output, the flux or current (for example) is often so small that the position detection algorithms are at best error prone and often completely fail, and the control system consequently loses control of the machine. In many applications this is unacceptable.