MXPA99007807A - Switched reluctance drive with high power factor - Google Patents

Abstract

Un circuito de reluctancia conmutada de polifase es suministrado desde un circuito de corrección del factor de energía pasivo. Las fases de la máquina son conmutadas de modo que existe siempre una fase queconsume o extrae corriente del enlace de CD y la corriente extraída del enlace de CD es siempre sustancialmente positiva.

Description

SWITCHED RELUCTANCE ACTUATOR WITH HIGH ENERGY FACTOR Field of the invention: This invention relates to a switched reluctance actuator system. In particular, it relates to a switched reluctance actuator system that is configured to draw current at a high power factor of an electrical supply. The characteristics and observation of switched reluctance machines are well known in the art and is described in, for example, "The Characteristics, Design and Application of Switched Reluctance Motors and Drives" by Stephenson and Blake, PCIM'93, Nürnberg, 21 -24 June 1993 incorporated here as a reference. Figure 1 shows a typical switched reluctance actuator in schematic form arranged to lay a load 19. The actuator comprises a switched reluctance motor 12 having a stator and a rotor, an energy converter 13 and an electronic control unit 14. The actuator is powered from a supply of. DC power (Direct Current) 11 that can be a battery or AC power grids (Alternating Current) rectified and filtered. The DC voltage provided by the power supply 11 is switched through phase coils 16 of the motor 12 REF .: 30898 by an energy converter 13 under the control of the electronic control unit 14. Figure 2 shows the circuit typical switching in the power converter 13 which controls the energization of the phase coil 16. In this circuit, a switch 21 is connected between the positive terminal of one power line and one end of the coil 16. Connected between the other end of the coil 16 and the negative terminal of the power supply is another switch 22. Between the switch 22 and the coil 16 is connected the anode of a diode 23, the cathode of which is connected to the positive line of the supply of Energy. Between the switch 21 and the coil 16 is connected the cathode of another diode 24, which is connected at its anode to the negative line to the power supply. The switches 21 and 22 act to couple and uncouple the phase coil 16 to the DC power source, so that the coil 16 can be energized or de-energized. Many other configurations of the switching circuit are known in the art, some of which are discussed in the Stephenson & Blake quoted above. For proper operation of the actuator, the commutation must be correctly synchronized to the rotation angle of the rotor. A rotor position detector 15 is typically employed to supply signals that correspond to the angular position of the rotor. The output of the rotor position detector 15 can also be used to generate a speed feedback signal. The rotor position detector 15 can take many forms. For example, it can take the form of the physical components that are shown schematically in the Figure 1, or of an algorithm of programs and programming systems that calculate the position of the other given parameters of the drive system, as described in EP-A-0573198 (Ray). In some systems, the rotor position detector 15 may comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices is required. in the power converter 13. The switched reluctance actuator is essentially a variable speed system characterized by voltages and currents in the phase coils 16 which are very different from those found in the additional machines. Figure 3 (a) shows a typical voltage waveform applied by the phase coil controller 16. At a predetermined rotation angle, the voltage is applied by switching over the switches 21 and 22 in the power converter 13 and applying a constant voltage for a given driving angle? c. The current rises from zero, typically reaches a peak and falls slightly as shown in Figure 3 (b). When? C has been traversed, the switches in the power converter 13 open and the action of the energy return diodes 23 and 24 places a negative voltage across the coil, causing the flow in the machine, and in consequence the current, fall to zero. There is then a period of zero current until the cycle is repeated. It should be clear that the phase consumes power from the supply during? C and returns a small amount to the supply later. So the supply, shown as 11 in Figure 1, needs to be a source of low impedance that is capable of receiving the returned energy for part of its operation cycle. Figure 3 (c) shows the current that is supplied to the phase coil 16 by the energy converter 13 during the power supply period and the current flowing back to the converter 13 during the energy return period period . Typically, the DC power supply 11 of Figure 1 is made by rectifying the supply of the AC electrical networks, as shown in Figure 4, where the power supply 30 is shown as an AC voltage source. 32 in series with an impedance of the source 34. In most cases, the impedance 34 is mainly inductive. This inductance can be increased by adding additional inductive components in series. A rectifier bridge 36 is provided having four terminals A, B, C and D, two of which, A and C, are connected to the power supply 30, the other two, B and D are connected through a capacitor 38. The rectifier bridge 36 rectifies the sinusoidal voltage of the source and the output voltage is modulated by the capacitor 38. Connected in parallel with the capacitor 38 and the rectifier bridge 36 is a switched reluctance actuator 39 (shown schematically) , which typically comprises blocks 12, 13 and 14 of Figure 1. Lines marked + V and -V in Figure 4 are generally known as the CD link, and capacitor 38 as the link capacitor. CD. In the absence of any load on the DC link, the capacitor 38 is charged up to successive voltage cycles at the peak voltage of the sine-wave supply 30. The capacitor 38 must therefore be rated for at least the peak supply voltage. When a resistive load is applied, and when the supply voltage is below the capacitor voltage, energy is drawn from the capacitor 38. When the rectified supply voltage rises above the capacitor voltage, the capacitor 38 is charged. The size of the capacitor 38 and the amount of current drawn by the load interact. Generally, the capacitor is sized so that it excites the relatively small amount of drop over the DC link voltage while the capacitor is supplying the load. Figure 5 shows the rectified voltage and DC link voltage for a capacitor of typical size, from which it can be seen that the DC link voltage remains approximately constant. The shape of the supply current is complex, since it depends not only on the size of the CD link capacitor but also on the size and nature of the impedance of the source. If capacitor 38 is very large (so that the voltage fluctuation is effectively zero) and the impedance of the supply is negligible, the graph of current versus time has a very large peak centered on the peak of a graph similar to that of the waveform of the rectified voltage. In practice, some impedance of the supply is always present and has the effect of increasing the width of the current pulses and consequently reducing their magnitude. However, the rectifier must be rotated to withstand the current of the upper peak.

The general form of the supply current as a function of time is shown in Figure 5, where? it should be noted that the current is zero for a significant portion of the total cycle. This has an effect undesirable about the energy factor of the total circuit. The energy factor is defined as the ratio of the actual energy supplied to the load to the apparent energy (ie volt-amperes) supplied to the circuit. With a low supply impedance, the power factor is i 10 typically around 0.5: with the inductance added to the supply it is possible to increase the current pulse width and consequently increase the power factor, but a value of about 0.65 is generally considered the practical limit and the effective cost. 15 Those low energy factors can cause problems for electrical equipment designers, for two reasons. First, the supply may have a minimum limit on the energy factor that can be deferred or consumed, in which case the energy factor has to be corrected by some other means. Second, for home appliances that operate with domestic power supply outlets there is a fixed current limit: for example, US home appliances are often limited to 15A to 120V. This allows a rated energy of 1800 to be consumed at a unit energy factor, but proportionally less at reduced energy factors (typically 1000W using the circuit? Of Figure 4). For these reasons, power factor correction (PFC) circuits have been developed to raise the power factor of a given load. EP-A-0805548 (Sudgen) describes several active energy factor correction circuits. These are known as "active" circuits because they typically use a switch placed through the rectifier output to I 10 demodulate the current attached or extracted from the supply and forces it to follow the phase and waveform of the supply voltage. However, although those circuits can greatly improve the energy factor, they are expensive and bulky. A cheaper and smaller circuit is required, particularly for home appliances. In addition to active PFC circuits, passive PFC circuits are known. They do not use active switches but use combinations of passive components to improve the power factor. One such circuit is describes in "Improved Valley-Fill Passive Power Factor Correction Current Shaper Approaches IEC Specification Limits". PCIM Journal, February 1998, pp 42-52. Sum, KK. This circuit is shown in Figure 6, and includes the supply 30 and rectifier bridge 36 described with Referring to Figure 4, however, in this case, connected through terminals B and D of bridge rectifier 36, there is a series combination comprising a capacitor Cl connected to the anode of a diode D3 which is connected via its cathode to another capacitor C2. Connected between the capacitor Cl and the diode D3 is the cathode of another diode Dl, the anode of which is connected to the line -V of the CD link. Connected between capacitor C2 and diode D3 is the anode of another diode D2, the cathode of which is connected to the + V line of the DC link. When the supply voltage of the circuit of Figure 6 reaches its peak value, the charging current is able to flow through the rectifier 36 to the series connection of Cl, D3 and C2. The capacitors are each graduated at half the peak of the rectified voltage. When a resistive load RL is applied, the action of Dl and D2 is to connect Cl and C2 in parallel, so that when the rectified voltage drops to half its peak value, the load is supplied from the two capacitors. When the rectified voltage is above half its peak value, the load is supplied directly from the rectifier. Figure 7 shows the voltage waveforms. The circuit in this way partially fills the depression or valley between the voltage circuits, which is known as "filling the valley".

Assuming that the capacitors are fully charged by the peak portion of the voltage, they begin to supply current when the supply voltage falls to the half of its peak, that is 150 °. Neglecting any drop in capacitor voltage, it supplies current to the next value of half the peak voltage, ie at 30 °, when Dl and D2 are inversely deflected. Between those angles, the current for the load is supplied entirely from the rectifier 36. If the capacitors have little voltage drop, their load current is centered around the peak voltage, giving the composite current waveform shown in Figure 7. In practice, however, some drop is accepted to gain economy in a capacitor size, so that the load peak is dispersed and the rectifier 36 conducts before 30 °. Two things should be noted about the circuit of Figure 6. First, the supply current diffuses better than with the traditional circuit of Figure 4, and consequently has a lower harmonic content. This leads to an improved energy factor. Second, capacitors Cl and C2 rotate only at half the supply voltage and supply charge current only during the "valley". This allows smaller capacitors to be used and leads to an economic circuit.

In the Sum article cited above, it is explained that the circuit of Figure 6 is good for improving the energy factor, but that it is only suitable for small resistive loads that do not return power to the supply (ibid, p 44). As an example, Sum describes how the basic circuit can be adapted for use with fluorescent lighting loads by adding a voltage doubler. This improves the energy factor even more but at the expense of efficiency. This is inappropriate for a switched reluctance actuator, where high efficiency of the power conditioning circuits is essential. For switched loads (resistive or inductive) where the switching frequency is different from the supply frequency of the electrical networks, the valley filled circuit of Figure 6 is considered to be of little value. This is due to the presence of inductance in the supply impedance, which forces the capacitor voltage to rise when the current to the load is interrupted. There is no guarantee that sufficient capacitor charge will be taken during the "valley filling" period, this mechanism can lead to excessive capacitor voltage and eventual failure of the capacitor. Although the increase in capacitor voltage can be accommodated in a very small actuator, increasing the size of the capacitor in a larger actuator to overcome the problem cancels the object of achieving an efficient, low cost circuit. As mentioned above, a switched reluctance actuator is an inductive switched load that returns power to the supply circuit during part of each operation cycle. If operated using traditional control methods and coupled with the circuit of Figure 6, this returned energy adds to the problem described above with the supply inductance to further strain the capacitors. With the traditional supply circuit of Figure 4, this is not a problem because, although the rectifier is not receptive to the returned power, the capacitor of the DC link is typically large enough to absorb the energy without problem. Although the circuit to fill the valley is attractive in the sense that the power factor potentially improves, the small capacitors associated with it can not cooperate with the returned power of the machine as well as the energy of the supply inductance, as described previously. There is, therefore, a need for a PFC circuit that can operate successfully with a switched reluctance actuator. An object of this invention is to provide a switched reluctance actuator, high energy, cheap, which consumes energy at a high energy factor of supply. According to one aspect of the invention there is provided a switched reluctance actuator as specified in claim 1. Some features of the invention are specified in the claims depending on claim 1. According to another aspect of the invention, provides a method for operating a switched reluctance actuator as specified in the claim 9. Some preferred features of the method are specified in the dependent claims. The invention can be practiced in numerous ways, some of which will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a prior art switched reluctance actuator. typical Figure 2 is the diagram of a circuit of a standard switching arrangement; Figure 3 (a) is a graph of the voltage applied to a phase coil of the actuator of Figure 1, as a function of the angle of the rotor; Figure 3 (b) is a graph of the resulting phase current as a function of the rotor angle; Figure 3 (c) is a graph of the waveform of the current in the CD link as a function of the rotor angle; Figure 4 is an equivalent circuit for the power supply side of a switched reluctance actuator of the prior art; Figure 5 is a graph of the voltage and current of the circuit of Figure 4 as a function of time; Figure 6 is a diagram of a power factor correction circuit of the prior art; Figure 7 shows the voltages and current of the circuit of Figure 6; Figure 8 is an equivalent circuit for a switched reluctance actuator in which the invention is incorporated; Figure 9 is a graph of the voltages and supply current of the circuit of Figure 8 as a function of time in one mode of operation: Figure 10 (a) is a graph of the current in the CD link as a function of the angle when the machine is operated according to the traditional control means; Figure 10 (b) is a graph of the current in the CD link as a function of the angle when the machine is operated according to the invention in the single-pulse mode; Figure 10 (c) is a graph of the current in the CD link as a function of the angle when the machine is operated according to the invention in the suppression or modulation mode; and Figure 11 shows the DC link voltage and the supply current for the actuator of Figure 8 when operating according to the invention. Figure 8 shows the circuit to fill the .valle 67 of Figure 6 applied to the converter circuit 69 of a polyphase switched reluctance actuator. In this case, the machine has two phases and the appropriate power switches 71, 72, 76 and 77 are connected to the coils 16 to switch them through the DC link at the appropriate times. The usual method for operating a switched reluctance machine, as taught, for example in the Stephenson article cited above, is to adjust the conduction angle, Δc, in Figure 3 (a), as a function of speed and load . In this way a, say, half the velocity of the value of? C can be about 20% of the angular period of the excitation cycle of the phase. When the speed is raised to its maximum value for the actuator, the driving angle would be increased towards a maximum value, with frequency chosen around 45% of the angular period.

The exact relationship is often stored in a look-up table as a function of speed. This has the effect of producing the so-called "single pulse" waveforms in the phase coil as shown previously in Figure 3 (b) and the reversion of the waveform as shown in Figure 3 (c). ). The inverted current must be absorbed by the capacitor in the DC link. The circuit of Figure 8 provides two phases, which are switched to 180 ° apart from each other. Each phase, when switched in the mode of a single normal pulse, has a current of the general shape of Figure 3 (b). When those are combined, the current in the CD link has the form shown in Figure 10 (a). This would lead to an unstable operation of the circuit of Figure 8, For two reasons. First, the abrupt switching of the current from a finite value to zero at point A in the cycle would cause difficulties associated with the supply inductance. The supply current, which flows through the inductance of the supply, would cause an increase in voltage over the DC link. This is shown by the large voltage peaks in Figure 9, which shows the approximate voltage that appears on the CD link. The voltage would rise until the energy stored in the supply inductor was transferred to capacitors Cl and C2. Second, the reversal of the current on the DC link between points A and B would cause a significant additional increase in the voltage of the capacitors so they are forced to absorb all the energy that is being returned from the machine. High voltage peaks (up to twice the peak supply voltage) make the capacitor design difficult. The supply current is of the approximate shape shown in Figure 9, from which it can be seen that it has a poor energy factor, which has some periods at zero and mainly discontinuities. Note that, for clarity, Figure 9 shows only one cycle of the supply current and the corresponding variation of the DC link voltage. Therefore, it is clear that the circuit filling the valley is unstable for use with the switched reluctance actuator operated in the traditional manner. However, if the machine is operated so that at any one time there is at least one switched phase, that phase can help absorb any energy associated with the inductance of the supply that is returned by other phases. This is shown in Figure 10 (b) for a single-impulse waveform, where each phase is switched for exactly half the cycle. Although there is still some net energy returned to the converter current, it is much smaller than in Figure 10 (a) and can usually be accommodated by capacitors Cl and C2 without a large voltage shift. The corresponding DC link voltage and supply current are shown in Figure 11, where the significant improvement is in the shape of the supply current is clear when compared to Figure 9. The power factor of the actuator substantially improves . In summary, if the phases of the machine are switched so that there is always at least one phase consuming current the DC link and the current consumed from the DC link is always substantially positive, then the power factor of the actuator can be improved . This method of operation of the switched reluctance machine goes against conventional teachings. In general, for optimal operation and efficiency, it is considered that the machine should be operated with driving angles that depend on the speed of the load, rather than with a "completely open" angle, constant, regardless of speed. The detailed operation of the circuit is as follows. When the switches in the converter circuit 69 are activated to de-energize a phase, and another phase is activated simultaneously, the capacitors Cl and C2 are connected in series by the action of D3 which is deflected forward, and are charged from the motor phase. in progress and in supply as described above. This ensures that the power return of the machine is at a high DC link voltage. In addition, because the other phase is activated simultaneously, the above action ensures that the initial energization of the machine in this next phase is also from a high voltage. It is known that both factors are beneficial for the operation of the switched reluctance machine, particularly when operating at high speed. This installation, however, is not available when the machine is connected to a standard circuit such as that shown in Figure 4 because the CD link of Figure 4 is substantially constant so that both the power supply and the the return of energy are associated with the same fixed voltage. It should be appreciated that the invention is more beneficial when there are many circuits of operation of the switched reluctance actuator that occur within a supply cycle of the electric-networks (e.g., as shown in Figure 11). This is achieved either by operating a machine with a low number of poles at high speed or by operating a machine with a high number of poles at a lower speed. The single-impulse waveform of Figure 10 (b) corresponds to the maximum energy that is consumed by the actuator. The prior art method for reducing the power output of the actuator would be to reduce the driving angle. As explained above, this would cause an excess voltage of capacitors Cl and C2 and could not be tolerated. However, the requirement to maintain the conduction in at least one phase at all times can be satisfied at lower output energies by using the control of suppression or modulation of the current while still maintaining a completely open, ie electric, conduction angle. 180 °. As explained in the article by Stephenson quoted I 10 above, there are two main methods of suppression or modulation. The simplest method is to simultaneously open the two switches associated with a phase coil, for example switches 71 and 72 in Figure 8. This causes the power to be returned from the machine to the DC link in each "suppression or modulation". This is sometimes referred to as "suppression or hard modulation". The alternative method is to open only one of the switches, for example 71, allowing the current to circulate or roll freely around the loop formed by the coil 16, the other switch 72 and diode 74. This is known as "freewheel suppression or modulation" or "soft suppression or modulation". In this control mode, no energy is returned to the CD link until the end of each period of the phase. Figure 10 (c) shows a current of typical converter in suppression or soft modulation control mode, where five suppression or modulation cycles were used in a single driving cycle. The returned current is very small and is easily handled by location capacitors Cl and C2. Although this suppression or modulation control method damages the total power factor of the actuator (because the supply current is significantly discontinuous), the mode is only used at low energies, where the limit on the level of the absolute current is generally It is not a concern. From the above description, it has been shown that a control scheme for a switched reluctance actuator can be implemented which allows the circuit shown in Figure 8 to be operated so that its power factor is maximum at the maximum power output of the actuator. In practice, an energy factor greater than 0.9 is storable, which allows a 1650 household appliance to be operated from a 15A supply of 120V. Operation at lower energy (also with a reduced energy factor) can also be achieved within the limit of the supply current. The illustrative embodiment described above uses a two-phase switched reluctance actuator, but any larger number of phases can also be used, since the larger number of phases makes it easier to ensure that there is always a net current consumption of the DC link. . Those skilled in the art will appreciate that variations of the described arrangements are possible without departing from the invention. Consequently, the above description of the different modalities are made by way of example and not for the purpose of limiting. It should be clear to one skilled in the art that minor modifications can be made to the converter circuit without significant changes to the operation described above.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (11)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A switched reluctance actuator, characterized in that it comprises a polyphase switched reluctance machine, a power factor correction circuit for improving the power factor of a DC link fed by AC, and an energy converter connected to the correction circuit of the energy factor for supplying power from the DC link to the switched reluctance machine, the power converter circuit is adapted to substantially maintain the conduction in at least one phase of the polyphase switched reluctance machine.

2. The switched reluctance actuator according to claim 1, characterized in that the correction circuit of the energy factor is passive.

3. The switched reluctance actuator according to claim 2, characterized in that the power factor correction circuit comprises a combination of capacitances and diodes.

4. The switched reluctance actuator according to claim 3, characterized in that the power factor correction circuit comprises a first capacitor connected between a positive supply line of the DC link and an anode of a first diode and a second capacitor which is connected between a negative supply line of the DC link and a cathode of the first diode, where a cathode of a second diode is connected between the first capacitor and the first diode, the anode of said second diode is connected to the negative line of the link of CD and between the second capacitor and the first diode of the anode of a third diode, the cathode of said third diode is connected to the positive line of the DC link. The switched reluctance actuator according to any of the preceding claims, characterized in that the power converter is adapted to suppress or modulate the current supplied to the polyphase switched reluctance machine. The switched reluctance actuator according to any of the preceding claims, characterized in that the polyphase switched reluctance machine is a linear machine. The switched reluctance actuator according to any of claims 1 to 5, characterized in that the polyphase switched reluctance machine is a rotary machine. The switched reluctance actuator according to any of the preceding claims, characterized in that the machine is a two-phase machine. 9. A method for operating a switched reluctance actuator having a polyphase switched reluctance machine and a power factor correction circuit to improve the power factor of a DC link supplied by the AC connected to the front end of the actuator. of switched reluctance, the method is characterized in that it comprises substantially maintaining the conduction of at least one phase of the polyphase switched reluctance machine. The method according to claim 9, characterized in that it comprises suppressing or modulating the current applied to the polyphase switched reluctance machine. 11. The method according to the claim 9 or claim 10, characterized in that the correction circuit of the energy factor is passive.