A METHOD AND AN ARRANGEMENT FOR STARTING AN ELECTRICAL MACHINE HAVING VARYING RELUCTANCE

Abstract: The invention relates to a method and to an arrangement for starting an electric machine having three or more phase circuits (phase 1 - phase 3) with varying reluctance, in dependence on the position of the machine rotor in relation to its stator, so as to enable selection of the phase circuit or circuits which shall be activated to provide torque in a desired direction. The reluctance cycle of the machine is divided into a plurality of rotor position angle zones (zone 1 - zone 6) where for a magnitude (L) of clear relationship with the reluctance value of the machine in at least two of the phase circuits of the machine, there is found a clear relationship between these magnitudes within each zone. The mutual size relationship between the measured magnitudes is established by comparison, and therewith the zone which will indicate the position of the rotor in relation to the stator. The phase circuit or circuits which provides or provide driving in the desired direction is/are activated on the basis of the obtained zone.

Description: A Method and an Arrangement for Starting an Electrical Machine Having Varying Reluctance A method and an arrangement for starting an electrical machine having varying reluctance. The present invention relates to a method for starting an electrical machine having varying reluctance and being of the kind set forth in the preamble of Claim 1, and also to an arrangement for carrying out the method. Examples of machines of this kind are stepping motors and so-called switched reluctance motors (SR-motors). Background Art Machines having varying reluctance, hereinafter called reluctance machines, may be rotating or linear machines, meaning that when a movable part rotates or moves linearly, the reluctance in the magnetic circuit of the machine will vary. In the following, this mov- able part is called the rotor, irrespective of the type of machine concerned. The position of this movable part is thus called the rotor position. Normally, in order to utilize a reluctance machine optimally, the prevailing position of the movable part, i.e. the rotor position, is sensed and the machine is controlled on the basis of this sensed position. The position is often sensed with the aid of position sensors mounted on the machine, these sensors producing an output signal which denotes the absolute position. Sensors of a simpler kind which provide relative posi¬ tional information, for instance increment sensors, are also used. In recent times, methods have been proposed for con¬ trolling reluctance machines which do not include the use of physical sensors. Two such methods are described in U.S. 4,868,478 and S.E. 8604307-2. All of the embo- di ents described in these publications use different methods of sensing or measuring the inductance in a machine phase. However, one problem which is not resolved satisfac- torily by the methods described in these publications, is how the machine shall be started. Since the inductance is indirectly a measure of the reluctance, the drive circuits described in said publi- cations constitute examples of embodiments which can be used in practice as the basis of starting a reluctance machine. These drive circuits are not described in detail here. The invention, however, is not restricted to its use together with these drive circuits, but applies to all systems in which the reluctance or a magnitude connected thereto can be measured or sensed in the various magnetic circuits of the machine. Machines of the kind to which the invention relates include a stationary part, here referred to as the stator, irrespective of whether the machine concerned is a rotating machine or a linear machine, and a mov¬ able part, the rotor. Both the stator and the rotor are normally made from a soft magnetic material, such as soft magnetic iron, with embossed, tooth-like poles. Flux generating windings for the different phases and poles are normally applied to the stator, which affords favourable operation with respect to mechanical con¬ struction, operational reliabiity, working temperature, etc., although these windings may alternatively be mounted on the rotor instead, if so desired. In the case of specific machine designs, there is used a combination of varying reluctance and permanent magnets. An example of one such design is the so-called hybrid-stepping motor. A common feature of machines which employ varying reluctance is that these machines shall be controlled so that current will flow through the machine windings only when the reluctance for each individual winding circuit either increases with a change in rotor posi¬ tion, which applies to generator operation, or decrea¬ ses with a change in the rotor position, which applies to motor operation. The aforesaid rotor-position-depen- dency derives from this. For the sake of simplicity, the following description will be made solely with reference to rotating machines. It will be understood, however, that what is described applies also to linear machines. The object of the invention is to provide a method for engaging and starting a machine having varying reluc¬ tance, with the intention of establishing which phase circuit or phase circuits shall be activated in order to obtain torque in a desired direction. The aforesaid object is achieved with a method having the characteristic features set forth in Claim 1. Further developments of the invention and other fea¬ tures thereof, and an arrangement for carrying out the method are set forth in the remaining Claims. According to the invention, the reluctance in the magnetic circuits of the machine is sensed, thereby obtaining information as to the absolute position of the rotor within a reluctance cycle. By sensing and processing this information in accordance with the inventive method, it is possible always to select the correct phase circuit, so as to obtain maximum torque in a desired direction. The method is based on the fact that the reluctance cycle can be divided into a given number of zones, where each zone has a unique size relationship between the respective reluctance values of the different phases. For the purpose of determining in which of these zones the rotor is positioned, the measured values are mutually compared so to establish the magnitude of these values. Although the inventive method is primarily suited for machines of the aforesaid kind which are not provided with physical rotor position sensors, the method can also be applied to machines in which rotor position sensors having limited function are used. The invention presumes that the reluctance or a mag¬ nitude coupled thereto, e.g. the inductance, in the various magnetic circuits of the machine is measured or sensed. This measuring or sensing of said reluctance or magnitude is effected in such a way that information relating to rotor position is obtained, such that activation of the machine can be effected in a reliable manner on the basis of this information. The desired operational state of the machine is achieved in this way, e.g. the maximum torque in the desired drive direction in those instances, when the machine is a drive motor. Brief Description of the Drawings 5 The invention will now be described in more detail with reference to the accompany drawings, in which Figure 1 illustrates schematically a diagram of induc¬ tance and static torque as a function of the rotor position of a reluctance motor having three separate phases, each with an individual winding circuit; Figure 2 illustrates schematically curves of the induc- tance and zone division for the three dif¬ ferent phases in the same machine as that shown in Figure 1, in one and the same dia¬ gram; Figure 3 illustrates diagrams of measured curves over the true static torque and the true inductance of a manufactured 3-phase SR-motor of a kind for which the invention is intended; and Figure 4 illustrates an embodiment of control means for a reluctance motor for carrying out the method according to the invention. Detailed Description of the Drawings Figure 1 illustrates schematically curve forms for static torque T in broken lines and for inductance L in full lines for a 3-phase reluctance machine which has a reluctance cycle that can be divided into six zones, zone 1 - zone 6. The curves for the three phases, Phase 1, Phase 2 and Phase 3 are shown separately in three separate diagrams, one beneath the other. It will be seen from the Figure that the zone boundaries or limits are defined by the positions in which two phases have equally large inductance values. The inductance is inversely proportional to the reluctance. Each zone has a unique combination of the inductance levels (i.e. also of the reluctance levels) in the different phases. This applies irrespective of the number of phases in the machine. For instance, a machine with four phases will have eight different zones within each reluctance cycle. This fact is uti¬ lized to sense or determine the absolute position in the reluctance cycle, so that the correct phase winding circuits can be activated for the desired operational state. That phase circuit which shall be activated within each zone is determined on the basis of curves over the measured values of the static torq ^ue Tme_a_s_,u,r_.e_ (see Figure 3). It can be read from the curves illus- trated in Figure 1 over the torque T that for the zones 1 and 2, activation of drive voltage for phase 3 will result in driving in the forward direction, whereas driving of Phase 2 will result in driving in the re¬ verse direction, while for zones 3 and 4, activation of drive voltage for Phase 1 results in driving in the forward direction and driving of Phase 3 results in driving in the reverse direction, and for zones 5 and 6, activation of drive voltage for Phase 2 will result in driving in the forward direction and driving of Phase 3 results in driving in the reverse direction. In the composite illustration of Figure 2, in which the inductance Phase 1 is shown in full lines, the induc¬ tance Phase 2 is shown in broken lines, and the induc- tance for Phase 3 is shown in dotted lines, it is shown that it is possible to establish in which rotor posi¬ tion zone the rotor is located by sensing which phase has the lowest, the intermediate and the highest induc¬ tance respectively, since each zone has a unique com- bination of these inductance-value relationships. Normally, it is necessary to treat the rotor positions around the zone limits, where the static torque T for a phase has its zero axis, in a special manner. The reason for this is because there is a risk of obtaining an erroneously directed torque from that phase whose static torque T changes sign with the zone limit around these positions, due to incompleteness of the measure¬ ments. Consequently, margins or tolerances are intro- duced for the measurements around these positions, so as to eliminate this risk. In practice, this is done so that when the introductory comparisons indicate a given zone, further comparisons are made in order to verify the zone position. A constant value is first added to or subtracted from the measured inductance value of one of the phases which constitutes the one zone boundary or limit, whereafter the introductory comparisons are repeated. If the comparisons indicate another zone, the rotor is located close to a zone limit. There is then selected the zone whose phase combination will posi¬ tively provide a drive torque in the desired direction. If the comparisons indicate the same zone as earlier, similar comparisons are made for the phases of the other zone, in order to ascertain whether or not the rotor stands close to this zone limit. In machines which have more than three phase circuits, it can suffice to measure the inductance in solely three of the phase circuits, in order to achieve a positive start, when requirements are not especially high. In this case, however, the maximum torque is not obtained in all positions, due to the inferior resolu¬ tion resulting from such simplification. In some cases, when requirements are sufficiently low, it may even suffice to measure the inductance in only two phases. The diagram in Figures 1 and 2 is a principle diagram. The diagram in Figure 3 is a true measurement diagram. It will be seen here that __ IU,6_O_.S_„U.,JT.6_ has a shape which deviates from the ideal shape, although activation of the aforedescribed phases on the basis of obtained zones results in the intended driving direction. In the aforegoing, solely activation of driving only one of the phases has been described, although it will be obvious that all phases which result in driving in the desired direction can be activated simultaneously, e.g. simultaneous activation of both Phases 1 and 3 when zone 2 is indicated. Since the relucatance is inversely proportional to the inductance, the measurement values of the reluctance can, of course, be used instead of the measurement values of the inductance. Other magnitudes which have a clear relationship with the reluctance may, of course, also be used. Figure 4 illustrates an embodiment of a circuit which can be used for carrying out the inventive method. This circuit is the same as that illustrated in U.S. 4,868,478 (corressponding to SE 8604308-0) with the same applicant as that of the present invention. The difference between the circuit used in the present invention and the circuit described in the aforesaid U.S. Patent Specification lies in the software in the microprocessor incorporated in the circuit, this micr¬ oprocessor carrying out the aforesaid comparisons. Since the invention is used at the start-moment for the purpose of establishing the rotational angular position of the motor, no winding has voltage supplied thereto, but that the microprocessor will pulse-feed successively each winding whose inductance is to be established in the manner described in US 4,868,478 and makes the comparison on the inductance values obtained with the successive measuring processes. Figure 4 illustrates three phase windings Fl, F2 and F3 of the stator of a three-phase reluctance motor. The invention is not limited to the phases of the motor. The motor is driven by a d.c. voltage +V. The phase winding Fl lies between the zero point and the plus pole of the voltage source, in a circuit which includes a current measuring resistor Rl, the emitter-collector- path of a power transistor Tal, the phase winding Fl and a power transistor Tbl. The lower transistor Tal in the circuit is controlled to saturation during the whole of the period in which the phase winding Fl shall have driving, while the upper transistor Tbl is pulsed during the drive inter- val of the phase in the manner conventional with swit¬ ched reluctance motors, where each drive pulse for a phase is divided into part pulses. Since driving of the motor does not form any part of the actual invention, it will not be described in detail here. The anode of a diode Dal is connected to the zero point and the cathode of the diode is connected to that part of the phase winding Fl remote from the transistor Tal, so as to maintain the current through the phase winding Fl over the whole of the drive pulse interval. The anode of a diode Dbl is connected to that part of the phase winding Fl remote from the transistor Tbl and the cathode of said diode is connected to the plus pole in order to provide a current circuit in which current is 10 rapidly led away in the phase winding Fl as soon as the transistor Tal is disconnected, i.e. blocking. Each of the remaining phase windings F2 and F3 is connected to a respective circuit of the same kind. Thus, the phase winding F2 is connected to a circuit comprising a current measuring resistor R2, two power transistors Ta2 and Tb3 and two diodes Da2 and Db2. The phase winding F3 is connected in a circuit comprising a current measuring resistor R3, two power transistors Ta3 and Tb3 and two diodes Da3 and Db3. A control unit 1 in the form of a microprocessor con¬ trols the switching on and off of the transistors. The control unit 1 has six outputs, each of which is con¬ nected via a respective amplifier 2 to the control on a respective one of the transistors Tal, Ta2, Ta3, Tbl, Tb2, Tb3, for the purpose of controlling each transis¬ tor individually. The inductance level is sensed successively in each phase to be investigated, i.e. all the phases in a three phase arrangement. This is carried out in the aforesaid manner in order to detect which phase has the highest inductance value, which phase has the inter¬ mediate inductance value and which phase has the lowest inductance value, which provides an indication of the zone in which the rotor position angle θ is located (see Figure 2) . Since the rotor is stationary and no phase has drive current, pulses of short duration are delivered from the control unit to the controls on both power tran¬ sistors Tai and Tbi in the drive circuit for the wind- ing Fi in the phase Phase i to be investigated. Each 11 pulse has a determined duration which is small in relation to the period time, so that the current in the monitored winding is able to go down to zero between each pulse. At the end of each pulse, the current is measured by an analog/digital converter 11 connected to an input on the control unit 1. The current rise during the duration T of each pulse provides a measurement of the instantaneous inductance level in the monitored winding. Accordingly, a multiplex unit 5 has three inputs, each connected to a respective resistor Rl, R2 and R3, all of which have mutually the same resistance. The multiplex unit 5 also has a fourth input, which is connected to a voltage divider with the resistors Rx and Ry connected between the supply voltge +V and earth. The voltage U' across the resistor Ry is propor¬ tional to the supply voltage +V. As disclosed in U.S. 4,520,302 and US 4,868,478, the supply voltage can vary, partly due to normal voltage variations and partly due to the voltage ripple which occurs in those instances when +V is a recitified alternating voltage, and consequently it may be suitable to investigate the value of the drive voltage after each value of the voltage across one of the resistors Rl, R2 or R3, in order to obtain the correct result of the inductance, since the inductance is calculated by approximating the equation (U-Ri) = d(L*i)/dt which describes the voltage-current ratio for a coil having the resistance R, the inductance L, the current throughflow i and the applied voltage U with U - L*di/dt 12 which in the case of a motor winding is a good aproxi- mation for small currents i and low motor speeds, i.e. small dL/dt. It is also assumed that di/dt = (i--i 1 )/(t 2 -t 1 ) (1) where i designates the current at t 2 and i. designates the current at t_ for short periods T = (t.-t-). In this case, i. = 0 and i 2 is the current level at the end of the pulse delivered to the monitored phase winding. The voltage U is corresponded by the drive voltage +V. The inductance is thus calculated as: L = V/(i 2 /T) = V*T/i 2 The risk of voltage flucuations at precisely the time of making the investigation at the start moment is relatively small, and consequently it may be unneces- sary for the invention to investigate the voltage above Ry for each phase investigated. There is no disadvan¬ tage, however, in carrying out the additional inves¬ tigation of ϋ', which after all is proportional to +V, and when using, at the same time, the control facility described in U.S. Patent Specification No. 4,868,478, it may result in simpler software, since this additional investigation shall nevertheless be carried out when the machine has started, and conse¬ quently both of the cases with and without an addi- tional investigation of ϋ' can be used. The multiplex unit 5 is controlled by the control unit 1 with a digital signal, so as to indicate which of the inputs of said multiplex unit 5 shall be connected to its output. The signal from the unit 5 is supplied to 13 the A/D-converter 11. The control unit 1 pulse-controls the phases in sequence and, for each phase, controls the multiplex unit 5 and the analog/digital unit 11 so that the voltage across the phase resistor Ri in the phase i just investigated via the multiplex unit 5 is delivered to the analog/digital converter 11 at the time T after pulse start in the phase in question. The unit 1 reads the digital signal on the output of the converter 11 and divides the signal with the resistance of the resistor Ri, so as to obtain the value of the phase current i and calculates the inductance according to the aforegiven equation (1) . Subsequent to comple¬ tion of a pulse control cycle including calculation of the inductance in all phases investigated, the unit 1 investigates the mutual size or value of the induc¬ tances and delivers drive voltage to the phase which provides torque in the desired direction in accordance with a stored preference program and also carries out those calculations described above with reference to Figures 1-3. No flow diagram is given for these cal¬ culations, since it is extremely simple for an ordinary programmer to compile a program with the aid of the information given above. It will be understood that many modifications are possible within the scope of the invention.

Claims: 14 Claims 1. A method for starting an electric machine having three or more phase circuits (Phase 1 - Phase 3) with varying reluctance in dependence on the position of a movable part in relation to a stationary part of the machine, in order to select which of the phase circuits shall be activated in order to provide torque in a desired direction, c h a r a c t e r i z e d by dividing the reluctance cycle of the machine into a number of rotor position angular zones (zone 1 - zone 6) , where for a magnitude having clear relation¬ ship with the reluctance value of the machine in at least two of the phase circuits of said machine, there is found a clear relationship between these magnitudes within each zone; measuring the magnitude in those phase circuits which are relevant; establishing by comparison the mutual size relationships of the mea¬ sured magnitudes, and therewith the zone which dis- closes the position of the movable part in relation to the stationary part; and activating on the basis of the obtained zone of the phase circuit or circuits which produces or produce driving in the desired direction. 2. A method according to Claim 1, c h a r a c ¬ t e r i z e d by measuring said magnitude in all phase circuits. 3. A method according to Claim l or 2, c h a r a c - t e r i z e d by making a further comparison between said magnitudes, subsequent to establishing said zone, in order to verify the the zone obtained and to detect whether or not the zone position is located close to the one or the other of the limits of said zone, by adding or subtracting the obtained relevant measured 15 values with a constant value prior to said comparison, for those phase circuits which determine the zone limits. 4. An arrangement for starting an electrical machine having three or more phase circuits with varying reluc¬ tance in dependence on the position of a movable part in relation to a stationary part of the machine, so as to enable selection of the phase circuit or phase circuits which shall be activated in order to provide torque in a desired direction, said arrangement includ¬ ing a calculating and control unit (1) which is con¬ nected to control means (Tal, Tbl; Ta2, Tb2; Ta3, Tb3) for the different phase circuits and which is connected to detect a magnitude of clear relationship with the prevailing reluctance in each of the phase circuits, c h a r a c t e r i z e d in that the calculating and control unit (1) functions to sense said magnitude for relevant phase circuits of said phase circuits in sequence and thereafter establishes the size relation¬ ships between the sensed magnitudes and establishes the rotor position zone within which the movable part of the machine is located in relation to the stationary part; in that the reluctance cycle of the machine is divided into a number of rotor position angle zones where a clear relationship between the magnitudes is found within each zone; and in that the calculating and control unit (1), on the basis of the obtained zone and the position between the movable part of the machine in relation to the stationary part thereof indicated by said obtained zone, activates that or those phase circuit or phase circuits which provides or provide driving in the desired direction. 16 5. An arrangement according to Claim 4, in which the calculating and control unit (1) is connected to de¬ tect, via a controllable multiplex unit (5), a mag¬ nitude which is proportional to the current in each of the phase circuits, c h a r a c t e r i z e d in that the calculating and control unit (1) is intended to investigate in sequence the inductance in each phase circuit, by steering the relevant phase circuit to activation over a time period of predetermined duration and to detect the current of the phase circuit at the end of the activation time. 6. An arrangement according to Claim 5, c h a r a c ¬ t e r i z e d in that the calculating and control unit (1) is intended to calculate the inductance L in each phase circuit with the aid of the equation L = V*T/i 2 , where V is the phase drive voltage, T is the predeter¬ mined duration and i 2 is the current detected at the end of the activation time. 7. An arrangement according to Claim 5 or 6, c h a r a c t e r i z e d in that the calculating and control unit (1), subsequent to establishing the zone, is intended to carry out a further comparison between the magnitudes, in order to verify the obtained zone and with an incorporated logic operation determine whether the zone position is located to the one or the other of the limits of said zone, by adding or sub¬ tracting for those phase circuits which determine the zone limits, the relevant obtained measured values with a constant value prior to said comparison.

Inventors: ZELLMAN PER, ZELLMAN, PER
Applicants: EMS ELECTRONIC MOTOR SYSTEMS, EMS ELECTRONIC MOTOR SYSTEMS AB
Application year: 1990
Main classification: H02P6
IPC codes: H02P1, H02P6, H02P25