MOTOR DRIVE SYSTEM

Abstract: A switched reluctance motor (10) has a rotor position sensing system (22) to provide for each rotor revolution one set of logic state transitions having time intervals and time durations determined by the angular rotor position with respect to a datum. The output from system (22) is delivered to a logic circuit device (24) with several channels and arranged to derive further sets of logic transitions by combinatorial logic. One set of logic transitions is selected by a selector (25) and is applied to the commutation switches of the motor (10).

Description: MOTOR DRIVE SYSTEM The present invention relates to a motor drive system for a switched reluctance motor. A switched reluctance motor is a form of d.c. brushless motor wherein the controllable commutation switches of the motor are switched on and off at instants corresponding to particular rotor angular positions. This is achieved by using a rotor position sensing system, usually comprising a slotted disc or magnetised ring placed on the motor rotor and a number of sensors adjacent to said disc or ring. The sensing system is arranged to produce pulses or digital logic state transitions at the output of the sensors at particular rotor angles. The pulses are fed directly to the controllable commutation switches of the motor and control the switching operation and consequently the speed/torque characteristics of the motor. It is an object of the present invention to provide a new and improved drive system for a switched reluctance motor. According to the present invention there is provided a motor drive system for a switched reluctance motor having a rotor and a stator with a plurality of stator phase windings each series connected with a controllable commutation switch, said drive system comprising a rotor position sensing system arranged to provide for each rotor revolution one set of logic state transitions having time intervals and time durations determined by the angular rotor position with respect to a datum, and commutation-signal delivery means connected between the output of the sensing system and the controllable commutation switches of the motor, and wherein said delivery means comprises a plurality of signal conditioning channels with respective logic circuits arranged to derive respective sets of logic state transitions from said one set, and selector means for connecting a selected one of said channels between the input and output of the delivery means whereby the controllable commutation switches of the motor rotor are commutated at time intervals and for time durations determined by the logic circuit of the selected channel. By virtue of the present invention the speed/torque characteristics of the motor can be substantially widened according to the logic circuit selected to control the commutation switches without any variation of the rotor position sensing system. Typically at constant torque the motor speed range can be doubled or at constant speed the torque can be doubled. The sensing system typically utilises inexpensive Hall effect or optical shaft position sensors. Both 3-phase and 4-phase motors may be controlled in accordance with the present invention utilising only two sensors which is an important economy. Forward or reverse rotation of such motors is also possible in both motoring and braking modes. Plural motoring modes and plural braking modes are also practical. All forms of 3-phase switched reluctance motors may be operated with a single drive system regardless of the number of rotor and stator poles, the only differences arising with different motor forms being restricted to the position sensor system. Selection of a particular channel may be made manually or automatically during running of a motor without protective circuitry or logic effective during the changeover so that flexible control is enabled with only the simplest forms of circuitry. Variant embodiments are very numerous and provide a wide range of characteristics, ranging from the simple systems which are economical in sensors to those which provide almost as much control flexibility as continuously controllable switching angles. Conveniently said datum with respect to the angular - _- position of motor rotor has a value of zero degrees and in use said rotor position sensing system and a first rotor pole of the rotor are initially aligned with the datum whereby said drive system will control a motor having forward and reverse symmetrical operation. Alternatively where said sensing system is not aligned with said datum and is angularly disposed from said datum for values other than multiples of the angular displacement between the sensors of said position sensing system, said drive system will control a motor in one direction only. Embodiments of the present invention will now be described by way of example and with reference to the following drawings in which:- Fig. 1 illustrates the mechanical form of a typical switched reluctance motor; Fig. 2 illustrates the electrical circuit of Fig. 1 motor with a motor drive system in accordance with the present invention; Fig 3 illustrates one example of a particular detail of Fig. 2; Fig. 4 illustrates variation of inductance arising in the motor from use of the Fig. 3 detail; Fig. 5 illustrates four different forward rotation commutation control modes provided in accordance with the present invention by the Fig. 2 circuit utilising the Fig. 3 detail; Fig. 6 illustrates reverse rotation commutation control modes provided in accordance with the present invention by the Fig. 2 circuit utilising the Fig. 3 detail; Fig. 7 illustrates a modified form of the Fig. 3 detail together with a graph of inductance variation and commutation control modes arising therefrom; Fig. 8 illustrates a further form of the Fig. 3 detail together with a graph of inductance variation and commutation control modes arising therefrom; and Fig. 9 illustrates a modified form of Fig. 8. Fig. 1 shows a switched reluctance motor 10 having three phases, each phase winding comprising two coils wound on opposite poles a, a'; b, b 1 ; c, c' of the motor stator and connected usually in series. The stator has six poles and the rotor 12 has four poles, this being a common form of the SR motor. Other combinations of phases and pole numbers are possible but the 6/4 3-phase motor is used by way of example. The motor drive system to be explained is essentially the same for all 3-phase motors, regardless of the number of poles, which is important in that it permits a unified design for all 3-phase motors. Motors having other phase numbers conform to similar principles but the resulting logic circuitry is different in detail. Fig. 2 schematically illustrates the motor 10 with its motor drive system 20. Each phase winding A, B, c is controlled by two switches in the form of transistors 16, 17 and the description following is based on the case where the lower transistor 16 is 'commutated' (switched) on and off by the system 20, the upper transistor 17 being 'chopped' or pulse-width modulated to control the average voltage applied to the winding during the conduction interval. While this particularises the description, the invention applies equally wherein the upper transistor 17 is the commutating one and the lower transistor 16 is chopped; or where both transistors 16, 17 are commutated and chopped simultaneously; or where neither transistor 16, 17 is chopped but both are commutated simultaneously. The system 20 comprises a rotor position sensing system 22 arranged to provide for one revolution of the rotor 12 one set of digital logic state transitions having time intervals and durations determined by the angular position of the rotor 12 with respect to a datum, the sensing system 22 is connected to commutation signal delivery means 24 the output of which is connected to operate the controllable commutation switches 16. The commutation signal delivery means 24 has a plurality of signal conditioning channels with respective logic circuits arranged to derive respective sets of logic state transitions from the set provided by system 22. One of the channels may retain the same logic state transitions at the output as seen at the input of means 24. Selector means 25 are provided for connecting a particular channel between the input and output of the delivery means 24 so that the switches 16 are commutated at time intervals and for time durations which are determined by the logic state transitions at the output of the signal delivery means 24 and therefore by the selected logic circuit channel of said means 24. The rotor position sensing system 22 is a combination of a slotted disc 22A and a number of optical sensors 22B adjacent to the disc 22A, a particular example being illustrated in Fig. 3 where the axes labelled 1, 2, 3 are the physical axes of the three phases in the motor 10 shown equi-spaced at 120-degree intervals for clarity and symmetry only. In practice the phase current may be poled positively or negatively so that positive m.m.f. may lie along one of the axes shown, or in the opposite direction. The direction of the current does not affect the torque productions or the direction of the torque. In forward rotation any rotor pole passes the phases in the order 1 " 2 3 . The shaft position sensors 22B are labelled A, B and C and their angular locations are shown. The four poles of the rotor 12 are also shown for a particular instant, namely when phase 1 has maximum inductance, ie. phase 1 is in the 'aligned' condition. When the rotor is 45-degrees away from this position, in either direction, then phase 1 is in the unaligned' position and its inductance is minimum and the two stator poles of phase 1 are then each midway between two rotor poles. The variation of the inductance of phase 1 is shown by way of example in Fig. 4 in an idealised form, with the aligned condition occurring at 0 and 90m degrees, where m is an integer, m=l, 2, ; and the unaligned condition is shown at 45 degrees and thereafter at 45 + 90m degrees. In practice the inductance diagram has rounded corners and may depend on the phase current, but such effects are secondary. The corner at 60 degrees corresponds to the condition wherein the leading corner of the rotor pole has just reached alignment with the first corner of the stator pole which it is approaching. As is well known, to produce motoring torque in the motor, phase current for each phase must be switched on to flow substantially during the period of rising inductance, ie. while the rotor poles are approaching the aligned position, and to produce braking torque, phase current must be switched on to flow substantially during the period of falling inductance, ie. while the rotor poles are approaching the unaligned position. In Fig. 4, motoring torque is produced if current flows between 45 and 90 degrees, whereas braking torque is provided if current flows between 90 and 135 degrees, it being understood that the rotor is rotating in the positive direction so that the rotor angle is continuously increasing. In prior-art systems, the slotted disk 22A and its sensors 22B are arranged to provide one set of fixed logic state transitions that are used to switch the transistors 16, 17 in series with the phase windings on and off at angles which are fixed in relation to rotor angular position, and there is no variation of these angles. The angular width of the disc slot must be related to the conduction angle, and if one phase is controlled exclusively by one sensor, then the slot width must equal the conduction angle. For motors intended to have bidirectional rotation the forward conduction period must be symmetrical with the reverse conduction period, such that in general neither can be optimum for torque production or efficiency. It will of course be understood that when a slot of the disc 22A is opposite to the sensor, the sensor signal is high (logic 1), and when a 'fence' is opposite to the sensor, the signal is 'low' (logic 0), the transitions between 0 and 1 containing the important timing information for the drive system. In accordance with the present invention each phase can be selectively commutated by a logical combination of logic state transitions derived from the position sensing system 22 which may have as few as two sensors. The specific combinatorial/logic together with the slot width, the orientation of the disk relative to the rotor, and orientation of the sensors relative to the disc may be particularly selected to achieve different results as will now be explained. Fig. 5 shows how three alternative forward motoring (FM) and one forward braking mode (FB) are generated from the three output signals conveniently denoted A, B, C of the sensors A, B, c of Fig. 3 by four different sets of logical combinations of the sensor signals A, B, C and their complements. The logical expressions on the right of Fig. 5 uniquely determine the logical combination of these signals, and it will be understood that various different circuit arrangements could be used to implement them in the respective channels of the delivery means 24. The FM-NORMAL mode is defined by switching angles of 52.5 degrees, and 82.5 degrees and the torque produced in this mode is represented as 100%. In the FM-BOOST mode the angles are 37.5 degrees and 67.5 degrees, (ie, the same conduction angle of 30 degrees but advanced by 15 degrees relative to the normal mode) and the torque is 140%. Note that the slot width on the disk is 45 degrees and that these two modes use simple logical combinations of only two sensor signals (or their complements) to generate the phase switching signals. The FM-LONG DWELL mode is defined by switching angles of 37.5 degrees and 82.5 degrees ie, a conduction angle or dwell of 45 degrees, which is 50% longer than in the other two modes which provides a torque of 280%. The fourth mode FB provides braking torque, with conduction angles of 82.5 degrees and 112.5 degrees and produces - 140% torque. Fig. 6 shows similar modes to those of Fig. 5 but for reverse rotation being derived from the same sensor signals A, B, C because, of course, the state transitions of the sensors 22B are uniquely associated with rotor position and are not affected by the direction of rotation, the main difference between forward and reverse is the exchange of an On* angle for an 'off angle, and the logical expressions for deriving the Fig. 6 waveforms from the sensor signals A, B, C are shown together with the Fig. 4 inductance diagram (in the interests of clarity) . In Fig. 7 is shown a sensing system arrangement for a 3-phase motor with three sensors and a disc configured so that the conduction angle is equal to the slot angle of 34 degrees. Optimized angles for turn-on and turn-off can be arranged with this configuration, by adjusting the 'user-definable* phase shift which is the orientation of the disc relative to the rotor. However, symmetry between forward and reverse rotation is absent for all values of this phase shift other than zero and integral multiples of 15 degrees. For conduction angles of 30 degrees only, the third sensor signal may be logically, derived from the other two and is therefore redundant. Fig. 8 shows such an arrangement for operating either a 3-phase or a 4-phase motor and which also has a similar user-definable phase-shift to that in Fig. 7 but for all values of phase shift not equal to zero or an integral multiple of 15 degrees, symmetry between forward and reverse operation is lost. In Fig. 9 is shown a two-sensor version for operating either a 3-phase or a 4-phase motor and which has symmetry between forward and reverse operation; the user-definable phase-shift is set to zero, giving switching angles of 45 and 75 degrees and forward and reverse motoring and braking modes.

Claims: _ 1 Q _ CLAIMS 1. A motor drive system for a switched reluctance motor 10 having a rotor 12 and a stator with a plurality of stator phase windings A, B, C each series connected with a controllable commutation switch 16, said drive system 20 comprising a rotor position sensing system 22 arranged to provide for each rotor revolution one set of logic state transitions having time intervals and time durations determined by the angular rotor position with respect to a datum, and commutation-signal delivery means 24 connected between the output of the sensing system 22 and the controllable commutation switches 16 of the motor, and wherein said delivery means 24 comprises a plurality of signal conditioning channels with respective logic circuits arranged to derive respective sets of logic state transitions from said one set, and selector means 25 for connecting a selected one of said channels between the input and output of the delivery means 24 whereby the controllable commutation switches 16 of the motor rotor 12 are commutated at time intervals and for time durations determined by the logic circuit of the selected channel. 2. A motor drive system as claimed in claim 1, wherein the rotor position sensing system 22 comprises only two sensors. 3. A motor drive system as claimed in either preceding claim, wherein said datum with respect to the angular position of motor rotor 12 has a value of zero degrees and in use said rotor position sensing system 22 and a first rotor pole of the rotor 12 are initially aligned with the datum whereby said drive system 20 will control a motor having forward and reverse symmetrical operation. 4. A motor drive system as claimed in any preceding claim, wherein said selector means 25 is manually operable, 5. A method of operating a switched reluctance motor comprising generating one set of logic state transitions having time intervals and time durations representative of the angular position of the rotor with respect to a datum, logically deriving further sets of logic state transitions from said one set, and applying a selected set of said logic state transitions to control the commutation of the phase windings of the motor.

Inventors: BECERRA ROGER C, MILLER TIMOTHY JOHN EASTHAM
Applicants: UNIV GLASGOW
Application year: 1989
Main classification: H02P7
IPC codes: H02P6, H02P25