GB2208456A - Reluctance motor drive circuit - Google Patents

Abstract

A multiphase converter for a reluctance motor has a dc supply (+V, O) and an array of switching elements (S41-S44) to supply unipolar pulses of current to the phase windings (21-24). Each switching element connects one end of at least one phase winding to one of the supply rails and each freewheel diode (D41-D44) provides a path for freewheeling motor current when the associated switching element is turned off. Each switch is rated at the voltage seen by the motor. Less than two switches are required per phase. Diodes (D21-24) in series with certain phases block unwanted current paths. A novel switching algorithm allows currents flowing in any phase windings which share the same switch to be simultaneously controlled. Motors with an odd number of phases cause no power imbalance in the dc supply. Such motors offer a multiple stepping mode and can be wound to reduce the motor hysteresis loss. <IMAGE>

Description

MULTIPHASE POWER CONVERTER CIRCUITS This invention relates to multiphase power converter circuits, suitable for switched reluctance motors.

The reluctance motor is a brushless stepping motor with a salient pole rotor. When powered from a suitable power converter circuit it is a fully controlled variable speed drive system, capable of producing high torque, especially at low speeds, making it suitable for use in traction and many other industrial applications. The reluctance of the flux path between two diametrically opposite stator poles varies as a pair of rotor poles moves in and out of alignment.

Since inductance is inversely proportional to reluctance, the inductance of the phase is a maximum when the rotor is in the aligned position, and minimum in the non-aligned position. A pulse of positive torque is produced if current flows in a phase winding as the inductance of that winding is increasing. A negative torque contribution is avoided if the current is reduced to zero before the inductance starts to decrease again. The rotor speed can be varied by changing the frequency of the phase current pulses while retaining synchronism with the rotor position.

A power converter is required which can increase and decrease the current in a phase winding as fast as possible. Unlike ac and permanent magnet motors the direction of the torque produced does not depend on the direction of the current flow. Prior art power converters for reluctance motors have used either one or two switching elements per phase as shown in Figure 1. Figure 1(a) shows a bifilar configuration requiring two closely coupled windings 1 and 2 on the same stator pole. The switch 3 and the diode 4 must be rated at more than twice the motor voltage. Figure l(b) shows a section of a power converter employing a split dc voltage rail. It also uses only one switch 6 per phase winding 7. This switch must be rated at twice the voltage seen by the motor.Energy is taken from one half of the supply when the switch 6 is on and returned to the other half via the freewheel diode 9 when the switch is turned off. Another phase would be connected in such a way as to reverse this power flow and thus maintain reasonably constant voltage levels on the capacitors, 5 and 8. The total number of phases in this power converter must therefore be even. The asymmetric half bridge (Figure l(c)) uses two switches 11 and 13 and two diodes 10 and 14 per phase winding 12.

These switches and diodes are rated at the rail voltage which is also the voltage seen by each motor winding. Of the three drive circuits the asymmetric half bridge gives the maximum motor voltage for a particular switch rating. It has the disadvantage that two switches are required per phase.

It is the object of this invention to provide power converter circuits for reluctance motors of any phase number, based on the asymmetric half bridge which can be configured in such a way so as the average number of switches per phase is less than two.

According to this invention there is provided a power converter circuit for a reluctance motor comprising supply means for connection to a dc supply, respective switching means for supplying current from the supply to each phase winding in only one direction, and frequency control means for switching the switching means so as to cause the current in the phase winding to increase and decrease during a specified time determined by the rotor position and load demand, wherein the switching means comprises at least one positive switching element for connection of one end of one or more phase windings to the positive supply rail of the supply means and at least one negative switching element for the connection of the second terminals of one or more of the phase windings to the less positive supply rail of the supply means, the connections arranged so that at least one switching element in the power converter has more than one phase winding connected to it, while also ensuring that no phase winding is connected to the same pair of positive and negative switching elements as any other phase winding, the maximum number of phase windings which may be connected to any positive switching element being equal to the total number of negative switching elements in the power converter and the maximum number of phase windings which may be connected to any negative switching element being equal to the total number of positive switching elements in the power converter.

In a preferred embodiment of the invention the switching means further incorporates a freewheel diode for each positive switching element, its cathode connected to the load connection node between the positive switching element and the first terminals of the phase windings connected to it, its anode connected to the less positive supply rail of the supply means, and a freewheel diode for each negative switching element, its anode connected to the load connection node between the negative switching element and the second terminals of the phase windings connected to it, its cathode connected to the positive supply rail of the supply means, whereby each freewheel diode provides a path for freewheeling motor current when the associated switching element is switched off.

It is also advantageous to add a diode in series with any phase winding which shares the switching elements at both its ends with other phase windings, wherein the diode is connected at any point in the phase winding circuit, its cathode at a lower potential than its anode when both the switching elements associated with that phase are turned on. There are normally two coils associated with each phase of a reluctance motor, connected in series or parallel to form the phase winding. Each coil is placed on diametrically opposite stator poles and wound so as, when current flows in the winding, one pole acts as a north magnetic pole, the other as a south magnetic pole. When the coils are connected in parallel the switches would be rated to carry twice the current in order to produce the same ampere turns in the phase winding.The rate of rise of the current for a given voltage would be increased by a parallel connection. Alternatively there may only be one coil associated with each phase winding.

It is also possible to have a motor construction such that there are more than one pair of diagonally opposite stator poles associated with each phase of the motor. The phase winding in the power converter may also include further coils from these poles either connected in series or parallel. Some reluctance motor designs have two or more teeth per stator pole. The phase winding however is still wound around the whole stator pole.

In order to fully understand the invention reference will now be made to the accompanying drawings in which; Figure 1 has already been referred to and shows three prior art power converters for the supply of unipolar current to a phase winding; Figure 2 is a diagram of a four phase reluctance motor drive system according to the invention; Figure 3 shows simplified current/rotor angle waveforms for a four phase reluctance motor operating in chopping mode; (a) low speed motoring; (b) high speed motoring; Figure 4 is a diagram of a seven phase power converter according to the invention; Figure 5 is a section from a power converter according to the invention to illustrate the general switching logical equation; Figure 6 is a schematic diagram of the laminations of a seven phase motor to illustrate the flux paths in the iron.

The asymmetric half bridge (AHB) shown in Figure l(c) uses two switches and two freewheel diodes to control a unipolar current in the phase winding. It has three modes of operation for current control. The first, a positive volt loop (PVL), occurs when both switches are closed. The current increases in the winding. The second is a zero volt loop (ZVL) occurring if either one of the two switches is open, and current freewheels through one of the diodes. The final mode of operation is a negative volt loop (NVL). Both switches are open and the current falls quickly as energy is returned from the winding to the supply via both diodes.Despite the advantages offered by the AHB in current control, the extra cost of using two switches per phase winding, as compared to the circuits in Figure 1(a) and l(b) which use only oner has hampered the use of the AHB in commercial reluctance motor power converters, even though, unlike Figure 1(a) and l(b), the switches are rated at the same voltage as seen by the motor. This invention allows the advantages of the AHB to be exploited while the average number of switches in the power converter will be less than two per phase winding.

There is a mathematical limit to the number of phase windings which may be connected to the multiphase power converter according to the invention. If, for example, a power converter has p positive switching elements and associated diodes and n negative switching elements and associated diodes, then the maximum number of phases which can be driven is given by; Nphase = p x n.

For a given total number of switching elements, the number of phase windings will be a maximum if I (p-n) I = 1 or 0. To obtain maximum use of the total number of switching elements it is recommended that the number of positive switching elements, p, differs by no more than one from the number of negative switching elements, n. Other power converter circuits are possible with less than maximum switch utilization, having simpler switching algorithms, and still less than two switches per phase. In some configurations according to the invention certain phase windings will share the switching elements at both its ends with other phase windings in the circuit. In such power converters it is possible for current to flow in the phase winding in the opposite direction to the normal unipolar current, when other switches are turned on.It is advantageous to block such currents with a diode in series with each phase winding in which this current could occur. The diode can be connected at any point in the phase winding circuit providing its cathode is at a lower potential than its anode when both the switching elements associated with that phase winding are turned on. When added this diode also blocks the only possible path of reverse switch current.

In one possible embodiment of the invention, illustrated in Figure 2 two positive switching elements,S42 and S44, two negative switching elements,S41 and S43, and their freewheel diodes, D41-D44, are used in conjunction with four further diodes, D21-D24, one connected in series with each phase winding, in a power converter for a four phase motor. Each switching element has two phase windings connected to it Although there are eight winding terminals, common connection points within the motor ensure that only six wires run from the power converter to the motor. The diodes D21-D24 could be mounted on the motor frame, reducing the connection wires to only four. The diagram also shows how each phase winding is split into two coils connected in series, the second wound to give opposite magnetic polarity on the stator pole diametrically opposite the first.The second coil is not essential for the operation of the motor but increases the flux linking the two poles, and thus improves the motor's performance.

To ensure starting torque from any position the reluctance motor is designed so that, during rotation of the rotor, the regions of increasing inductance of adjacent phases overlap and so if full torque is to be produced the common switch between two phases will have to carry both currents simultaneously and must be rated accordingly. To maintain control over both these phase currents a novel switching algorithm must be adopted. In the description of this algorithm reference will be made to the four phase reluctance motor drive system in Figure 2 and the current waveforms in Figure 3. The current waveforms in Figure 3(a) are representative of the actual phase currents during low speed anti-clockwise rotation of the rotor of the four phase motor in Figure 2. The current waveforms in Figure 3(b) are for higher speed operation. Consider point 31 in Figure 3(a).The current in phase winding 21 is being chopped to maintain a constant magnitude. This is achieved by keeping switch S42 permanently on while using switch S41 to alter the current path. The current in phase winding 21 can be increased in a PVL when switch S41 is closed, and will decay slowly in a ZVL via diode D41 and switch S42 when switch S41 is opened. The switching signals to switch S41 could originate from either an open or closed loop current control system. Meanwhile, switch S43 can be used to increase and chop the current in phase winding 22, without effecting the current in phase winding 21. To ensure that no negative torque component is produced the current in phase winding 21 must be reduced to zero by point 33.

To achieve this a commutation signal for 21 would have been issued by the controller at point 32. At this time phase current 22 is in chopping mode and control of this is passed to switch S42 while switch S43 is now left permanently on. Switch S41 is simultaneously opened and kept open. when the current control circuit for phase winding 22 requests a ZVL switch S42 is opened forcing the current to flow through diode D42. The current in phase winding 21 is thus forced to flow in a NVL through diodes D41 and D42 and will decay rapidly. These NVL's will be interspersed with ZVL's during periods when phase winding 22 is in a PVL.

The overall effect of this switching algorithm is that the time average negative voltage available for commutation has been reduced and current fall time will therefore be longer. At low speeds, where the reluctance motor drive system realises its full potential, the effect on the torque output of the motor will be minimal. At higher speeds earlier commutation may be necessary to ensure that the current reaches zero before excessive negative torque is produced.

However, for a given switching element the positive and negative voltages across the phase winding are twice those from any other power converter using one switch per phase. This will help to offset any increase in the current fall time caused by the move to this power converter.

Commutation of the current in the phase winding occurs at a point such that there is enough time to reduce the current in the phase winding to zero before the rotor moves beyond alignment. This is equivalent to ensuring that the flux linking the phase winding is reduced to zero. The rate of change of flux linkage associated with a phase winding is related to the applied voltage by, v = iR + dtv dt (v) dt where v instantaneous applied voltage; i instantaneous phase current; R phase winding resistance; v flux linkage associated with the phase winding.

The flux linkage associated with a phase winding can therefore be calculated at any time by integrating the net applied voltage with respect to time. To ensure that the current, and hence the flux linkage, reaches zero before the rotor moves beyond alignment the integrals of the applied voltage before and after commutation must be equal and opposite. As the speed increases, for a given phase current, the back-EMF generated in the motor phase winding increases.

The average chopping voltage which is required to maintain a constant current in the phase winding is therefore larger and the mark:space ratio of the chopping voltage must therefore increase. The negative volts available to an adjacent phase during a region of alternate zero and negative volt loops is reduced accordingly. The commutation must therefore occur earlier to ensure that the flux linkage is still reduced to zero at the correct time. In the 4 phase power converter of Figure 2, commutation of the current in phase winding 21 would mean turning off or chopping switch S42. As switch S42 is the common switch between phase windings 21 and 22 it cannot be turned off or chopped until the current in phase winding 22 reaches its chopping value, (point 31 in Figure 3(a)).The current in phase winding 21 cannot flow in a negative volt loop until after the current in phase winding 22 has reached its chopping level. However, although switch S42 cannot be turned off the other switch associated with phase winding 21, i.e. switch S41, can be turned off prior to point 31.

This will have two effects. Firstly, the current in phase winding 21 will decay slowly in a zero volt loop flowing through diode D41 and switch S42. The flux linkage associated with the phase winding will remain relatively constant during this time as there are no reverse volts across the phase winding. The second effect of turning off switch S41 at some point prior to final commutation is that any remaining current in phase winding 24 will be forced to flow in a negative volt loop through diodes D41 and D44.The current waveforms of Figure 3(b) show how the zero volt loop is inserted prior to point 31. Between point 31 and point 34, switch S42 is modulating the current in phase winding 22 and so there is a period of alternate negative and zero volt loops in phase winding 21.At point 34 (Figure 3(b)) one step-angle after phase winding 21 started a zero volt loop, phase winding 22 will do likewise. Therefore, after point 34, any remaining current in phase winding 21 decays rapidly as energy is returned to the supply via diodes D41 and D42. This negative volt loop will be advantageous in assisting the fall of the flux linkage if it occurs before the start of the decreasing inductance region.

As the speed increases, the mark:space ratio of the chopping voltage must increase to overcome the increased back-EMF, if constant current is to be maintained in the winding. For a given current magnitude there is a speed when the generated back-EMF is equal to the full supply voltage. At this speed, known as the base speed, a constant current is maintained in the phase winding without chopping. At and above base speed, the current in an adjacent phase can not therefore flow in a negative volt loop between points 31 and 34. The zero volt loop will occur during a complete step-angle and will be followed by a negative volt loop. This negative volt loop is most important at higher speeds, when it can assist the fall of current in the phase winding and its associated flux linkage, offsetting the loss of negative volts during the alternate zero and negative volt loop period.The starting point of the zero volt loop can still be evaluated by ensuring that the integrals of the voltage with respect to time are equal before and after commutation.

The switching algorithm is repeated at each switch in turn for the control of the current in each phase winding. The design requirements for each switch in the converter are identical, and any converter power loss will be evenly distributed among the switches. A similar algorithm applies when the rotation of the motor is reversed.

These algorithms are summarised by the tables below. The left side of the tables show the various operational states of the motor phases (positive, negative or zero volt loops) as the rotor turns through .4 step-angles. The sequence then repeats itself. The right side of the table lists the actual switch positions in the power converter. A logic "H" indicates that the switching element concerned should be turned on and kept on until the controller changes the phase request.

Similarly a "L" indicates that the switching element would be kept turned off (open circuit). In some cases the switching signals to a particular switch are derived directly from the current control circuit of a particular phase winding, either operating open or closed loop, indicated in the table by C21...C24 respectively. If the current in the phase winding "xx" was to be increased "Cxx" would be logic "H", and if the current in the phase winding was to decrease then "Cxx1' would be a logic "L". The first two tables show the switching algorithm for low speed operation of the 4 phase power converter. The extended zero volt loop is not required. As the speed increases the states marked *, representative of the times when two phases are simultaneously carrying current, will gradually take up a smaller proportion of each step angle LOW SPEED ANTI-CLOCKWISE ROTATION MODES OF EACH PHASE WINDING SWITCH POSITIONS 21 22 23 24 S41 S42 S43 S44 *pos & ero neg neg & ero pos & ero H C21 L C24 pos & ero neg neg neg & ero C21 H L L *pos & ero pos & ero neg neg & ero C21 H C22 L neg & ero pos & ero neg neg L C22 H L *neg & ero pos & ero pos & ero neg L C22 H C23 neg neg & ero pos & ero neg L L C23 H * neg neg & ero pos & ero pos & ero C24 L C23 H neg neg neg & ero pos & ero H L L C24 LOW SPEED CLOCKWISE ROTATION MODES OF EACH PHASE WINDING SWITCH POSITIONS 21 22 23 24 S41 S42 S43 S44 neg & ero neg neg pos & ero C24 L L H *neg & ero neg pos & ero pos & ero C24 L C23 H neg neg pos & ero neg & ero L L H C23 * neg pos & ero pos & ero neg & ero L C22 H C23 neg pos & ero neg & ero neg L H C22 L *pos & ero pos & ero neg & ero neg C21 H C22 L pos & ero neg & ero neg neg H C21 L L *pos & ero neg & ero neg pos & ero H C21 L C24 When the speed is such that commutation occurs so early that the states marked "*" do not exist, then the extended zero volt loop is introduced to allow a further increase in speed. The following two tables describe the algorithm for the high speed operation of the motor.

HIGH SPEED ANTI-CLOCKWISE ROTATION MODES OF EACH PHASE WINDING SWITCH POSITIONS 21 22 23 24 S41 S42 S43 S44 pos & ero neg neg neg & ero C21 H L L zero neg neg neg L H L L neg & ero pos & ero neg neg L C22 H L neg zero neg neg L L H L neg neg & ero pos & ero neg L L C23 H neg neg zero neg L L L H neg neg neg & ero pos & ero H L L C24 neg neg neg zero H L L L HIGH SPEED CLOCKWISE ROTATION MODES OF EACH PHASE WINDING SWITCH POSITIONS 21 22 23 24 S41 S42 S43 S44 neg & ero neg neg pos & ero C24 L L H neg neg neg zero L L L H neg neg pos & ero neg & ero L L H C23 neg neg zero neg L L H L neg pos & ero neg & ero neg L H C22 L neg zero neg neg L H L L pos & ero neg & ero neg neg H C2l L L zero neg neg neg H L L L As the speed increases the zero volt loop occurs over a larger rotor angle. At and above the base speed chopping is no longer required and so all the signals "Cxx" become "H". The regions of alternate negative and zero volt loops will then disappear. The zero volt loop then lasts for a complete rotor step-angle and is immediately followed by a negative volt loop. This is an example of a suitable algorithm for the operation of the 4 phase converter according to the invention. Other algorithms are possible depending on the exact performance requirements of the drive.

The 4 phase power converter just described is one example of the invention. It is also a member of a set of power converters according to the invention in which each switching element is connected to no more than two phase windings. These two phase windings must be adjacent to each other in the normal phase switching sequence for motor rotation in both directions. As before, each phase winding which shares a switching element with another phase at both its ends will have a diode connected in series with it. Such an arrangement allows more than two switches to be on at one time since any currents which do flow will be in phases which will produce torque in the correct sense. The maximum switch utilization of this family is obtained when there is equal numbers of positive and negative switching elements, each one connected to two different phase winding terminations.There is then an average of one switch per phase. The four phase power converter in Figure 2 is an example of this.

However, if there is an odd number of phases, Nphase, then Nphase+l switches will be needed.

When a power converter is required for a motor with an odd number of phases, Nphase, it is recommended that Nphase+l switches are used. An example of such a power converter is shown in Figure 4. Eight switching elements, S81-S88, and their associated freewheel diodes, D81-D88, are connected to a seven phase reluctance motor. Five series diodes, D72-D76, have been added. No more than two phases are associated with each switching element, and in every case these two phases are consecutive in the phase switching sequence for the rotation of the motor in both directions. A switching algorithm of the form described earlier will have to be adopted in this power converter. As the number of phase windings in a reluctance motor increases, the ratio of rotor pole arc to rotor step-angle increases rapidly.The time delay between consecutive phase currents is a much smaller proportion of the total conduction time of a phase winding.

It is no longer necessary to insert the zero volt loop to extend the speed range of the drive. After commutation there are alternate zero and negative volt loops for the subsequent step-angle, after which the remaining current in the phase winding decays rapidly in a negative volt loop. As the zero volt loop is not required, the switching algorithm for each switch is simplified and can be expressed in a general logic equation, covering the entire speed range.

Figure 5 shows a generalized section of a drive circuit. This can be used to derive a general logic equation for a switching element. The switch S(x,x+l) is defined as the switching element which is connected to phases P(x) and P(x+l). If the forward direction (FWD=logic H) is representative of a phase excitation sequence P(x 2),P(x-l),P(x),P(x+l),P(x+2)..., then;

where logical AND; + logical OR; P(y) NOT P(y).

The signals "P(y)" are derived from the position controller and "C(y)" is a higher frequency chopping signal, to control the current level in phase y by either closed or open loop methods. If the value y in any term P(y) or C(y) is not in the range l < y < Nphase, then modular arithmetic is applied to give a new value y' which is the integer remainder after the division of y by Nphase. Positive logic has been assumed and so if P(y) is logic "H" when the current in phase y is to be increased or maintained, then the output S(x,x+l) will be "H" when the switch should be on. Taking care to satisfy any timing requirements of the particular semiconductor switch being used, the logic signal S(x,x+l) can be applied to the power converter to directly control the switch.

The general logic equation is ideally suited to controlling the switches in the 7 phase reluctance motor over its entire sped range.

Although there is a break in the phase connection chain, the phase sequence is still modular and so modular arithmetic can still be used to derive the switching equations. There are however two switches which have only one phase winding associated with them. The switching equation for these is thus simplified and is given by;

Figure 6 shows a schematic diagram of the laminations of a seven phase motor. If the phase windings in Figure 6 are connected to the power converter as shown in Figure 4 then when FWD="H" in the above equations, and the position controller excites the phases in numerically ascending order, the rotor, 101, will rotate in an anticlockwise direction.

The general logic equation can be applied to any switch in a power converter circuit according to the invention providing no more than two phase windings are connected to any switch, and these phase windings are consecutive in the phase excitation sequence. The general logic equation used as an example does not implement extended zero volt loops. It- can however still be used to predict the low speed switching algorithm of the four phase power converter, prior to the insertion of the extended zero volt loops.

The seven phase power converter according to the invention can be used to exploit two additional features of a motor with an odd number of phases. Firstly there is a multiple stepping action. Rather than switching the phases in their usual consecutive order, every alternate phase is energized. When the total number of phases is odd, they will all still carry current but in a different order. The result is that the rotor moves twice as far with each phase current pulse. For example, consider the seven phase power converter and motor shown in Figures 4 and 6. The phase sequence for rotation in the anti-clockwise direction would normally be 71-72-73-74-75-76-7771. However if double stepping were introduced the sequence would become 71-73-75-77-72-74-76-71. This multiple stepping reduces the number of steps per revolution and so the torque output of the motor is reduced accordingly.The method could be used in an application where a high inertia load required full motor torque at starting but required less torque when running at constant speed. In such a case the switching losses would be reduced and the system efficiency improved. When the phase controller moves into double stepping mode the switching algorithm is simplified because only one of the phase windings associated with each switching element will have to carry current at any time. However the general logic equation still applies though in a greatly simplified form. This means that the switching control logic does not need to be altered for a move to multiple stepping. The only change is in the phase excitation sequence. There will be a small uncontrolled build up of current in the common phase linking the switching elements of two double stepped phases.This current will not be detrimental to torque production in the direction of rotation and will decay to zero after the phases are commutated.

Double stepping can also be applied to a motor with an even number of phases, but some of the windings would not carry current at all and uneven distribution of the heating losses in the motor would result.

This happens if the total number of phase windings is a multiple of the number of steps taken with each phase current pulse. Depending on the number of phase windings in the motor other multiple stepping modes are possible which still use all the phase windings. A third stepping mode in the seven phase motor would mean that the rotor would move through three normal step-angles with each phase current pulse. However the torque associated with this and higher stepping modes may be too low for practical purposes.

The second advantage of an odd number of phase windings arises if the phase coils are wound on each stator pole so as when the unipolar current flows in each it produces a stator pole magnetic field of opposite magnetic polarity to that of the two neighbouring poles.

This will reduce the hysteresis loss in the lamination and increase the efficiency of the drive system. Consider again the seven phase motor laminations shown in Figure 6. The arrows on each stator pole represents the direction of the flux when the pole is energized. Due to phase overlap, at least two phases will be on at one time. If phases 71 and 72 are on then the direction of flux in the back iron is such that it will be minimized everywhere except between the two phases. This means that a beneficial shorter flux path of minimum reluctance is formed linking the two phases via the rotor teeth.

Similar paths are created by each phase in turn ensuring that the rotor and stator hysteresis losses are kept to a minimum, despite the increase in the fundamental switching frequency caused by the extra phases.

The above mentioned advantages are only obtained with an odd number of phases thus emphasizing the importance of a multiphase power converter offering a free choice in the number of phase windings, without the penalty of too many switches. The power converter according to this invention is not restricted to its use in reluctance motor drives. The phase windings could be replaced by any two terminal electric load requiring unipolar current from a dc supply.

In summary a family of multiphase power converters for controlling the unipolar phase current in the phase windings of a reluctance motor or any unipolar load have been disclosed. Examples have been given of four phase and seven phase reluctance motor power converters. The switching elements used in these or any other power converter according to the invention could be any suitable semiconductor devices. Switching algorithms have been described for the control of the switches in these power converters to achieve independent current control for the phase windings in the motor.

Claims (16)

1. A multiphase power converter for a reluctance motor comprising a supply means for connection to a dc supply, switching means for supplying unipolar current from the supply to each phase winding of the motor, and frequency control means for switching the switching means so as to cause the current in the phase winding to increase and decrease during a specified time determined by the rotor position and load demand, wherein the switching means comprises at least one positive switching element for connection of one end of one or more phase windings to the positive supply rail of the supply means and at least one negative switching element for the connection of the second terminals of one or more of the phase windings to the less positive supply rail of the supply means, the connections arranged so that at least one switching element in the power converter has more than one phase winding connected to it, while also ensuring that no phase winding is connected to the same pair of positive and negative switching elements as any other phase winding, the maximum number of phase windings which may be connected to any positive switching element being equal to the total number of negative switching elements in the power converter and the maximum number of phase windings which may be connected to any negative switching element being equal to the total number of positive switching elements in the power converter.

2. A multiphase power converter according to claim 1 wherein the switching means further incorporates a freewheel diode for each positive switching element, its cathode connected to the load connection node between the positive switching element and the terminals of the phase windings connected to it, its anode connected to the less positive supply rail of the supply means, and a freewheel diode for each negative switching element, its anode connected to the load connection node between the negative switching element and the terminals of the phase windings connected to it, its cathode connected to the positive supply rail of the supply means, whereby each freewheel diode provides a path for freewheeling motor current when the associated switching element is switched off.

3. A multiphase power converter for a reluctance motor according to either of the preceding claims wherein a diode is placed in series with each phase winding which shares a switching element with other phase windings at both its ends, the diode connected at any point in the phase winding circuit providing the cathode is at a lower potential than the anode when both switching elements associated with the particular phase are switched on.

4. A multiphase power converter for a reluctance motor according to any of the preceding claims wherein a pair of coils is associated with each phase of the motor, which are connected in series or parallel to form the phase winding, with each coil placed on diametrically opposite stator poles and wound so as when current flows in the winding, one stator pole acts as a north magnetic pole, the other stator pole, diametrically opposite the first, acts as a south magnetic pole.

5. A multiphase power converter according to any of claims 1,2 or 3 wherein there is more than one pair of diagonally opposite stator poles associated with each phase of the motor, with a coil on each stator pole wound as in claim 4, each coil connected in any series or parallel combination with the other coils associated with that phase to form the phase winding.

6. A multiphase power converter according to any of the preceding claims wherein the frequency means can control the current level in any phase winding of the motor using only the two switching elements associated with that phase winding, whereby a positive volt loop can be created by switching on both the positive switching element and the negative switching element associated with that phase winding, a zero volt loop can be created by switching on either one, but only one, of the switching elements associated with the phase winding, and a negative volt loop can be created by switching off both the switching elements associated with the particular phase winding.

7. A multiphase power converter according to any of the preceding claims wherein the frequency means can control the switching means in such a way that independent current control is maintained in all phase windings which share a common switching element, whereby if the common switching element is switched on then the individual phase currents of the associated phase windings can flow in either a positive volt loop or a zero volt loop determined by the state of the other switching element associated with the respective phase winding and if the common switching element is switched off then the individual phase currents of the associated phase windings can flow in either a negative volt loop or a zero volt loop determined by the state of the other switching element associated with the respective phase winding.

8. A multiphase power converter for a reluctance motor according to any of the preceding claims wherein each switching element of the switching means has no more than two phase windings connected to it, and when there are two, these phase windings must be sequential in the phase excitation sequence during normal rotation of the motor in either direction, resulting in a power converter which has the same number of switches as phase windings when there is an even number of phase windings or if there is an odd number of phase windings there will be one extra switching element over the total number of phase windings.

9. A multiphase power converter according to any of the preceding claims wherein the switching elements in the switching means are any suitable semiconductor device.

10. A reluctance motor drive system including a power converter according to any of the preceding claims and a reluctance motor with an odd number of phases wherein the coils comprising each phase winding of the reluctance motor are wound so as every stator pole has opposite magnetic polarity to both the adjacent stator poles, of the neighbouring phases so that when current flows in two neighbouring phases during an overlap period, magnetic flux will link the two adjacent phase coils via two adjacent rotor teeth without passing through the main rotor body or the bulk of the back iron.

11. A reluctance motor drive system including a power converter according to any of the claims 1-9 wherein two or more phase windings which are connected to the same switching element are connected together inside the motor frame and only one wire runs from the motor to the switching element in the power converter.

12. A reluctance motor drive system including a power converter according to any of the claims 1-9 wherein the frequency means can alter the normal phase switching sequence so that every alternate phase winding is energized in turn, thus forcing the rotor to turn twice as far for each phase current pulse.

13. A reluctance motor drive system including a power converter according to any of the claims 1-9 wherein the frequency means can alter the normal phase switching sequence so that every third phase winding is energized around the periphery causing the rotor to move three times as far with each phase current pulse.

14. A reluctance motor drive system including a power converter according to any of the claims 1-9 and a reluctance motor with two or more stator teeth per pole.

15. A multiphase electric power converter according to any of the claims 1-9 where each phase winding is replaced by any two terminal electrical load requiring unipolar current.

16. A reluctance motor drive system substantially as hereinbefore described with reference to Figures 2 to 6 of the accompanying drawings.