Method of powering an electrical circuit

Abstract: A method of operating an electrical circuit which requires an electrical power supply, such as a rotor position sensor (20) of an electrical motor, includes the step of deriving the power supply from an accessible magnetic field associated with the motor.

Description: This invention relates to an electrical machine system and to a method of powering an electrical circuit from an electrical machine system. The invention particularly, but not exclusively, relates to a method of operating a circuit including sensing means for sensing the position of a rotor relative to a stator of an electric machine of the kind in which data concerning the position of the rotor relative to the stator is utilised by a motor controller to energise appropriately windings associated with the stator. Particularly in switched reluctance and brushless types of machine, it is a requirement for the position of the rotor relative to the stator to be known so that the various windings of the stator can be energised at appropriate times, i.e. when respective poles of the stator and rotor are appropriately positioned. A more detailed description of the switched reluctance machine can be found in the article "The Characteristics, Design and Applications of Switched Reluctance Motors and Drives" by Stephenson et al., PCIM '93, June 21 - 24, 1993. It will be appreciated that the switched reluctance machine, for example, can be run equally well as a motor or a generator. Conventionally, the rotor carries a timing disc. A sensor is able to sense the rotational position of the disc and provide an input to an electronic circuit which generates data for transmission to the motor controller where the data is processed. The motor controller energises a respective winding of the stator in accordance with the position of the stator. The electronic circuit is conventionally powered from a power supply in the motor controller. In some applications, e.g. where the motor is a compressor motor in an air ventilator and/or air conditioning/circulating system of a vehicle, such as a carriage of a train, it is a requirement for the motor controller to be positioned remote from the motor. Thus, long lengths of cabling are required from the controller to the motor, including cabling necessary to provide a power supply to the electronic circuit of the sensing means. This introduces the risk of voltage transients and noise being induced in the power supply to the electronic circuit, particularly where the motor controller is located in a hostile electromagnetic environment, where transients may need to be suppressed at either or both ends of the power supply cabling. Both the cabling for the power supply to the electronic circuit and the apparatus required to achieve transient suppression impose a cost penalty. Systems which transmit power to rotating parts of electrical machines are well known. For example JP-A-57088856 discloses a method of transmitting power across the airgap of a generator which is then rectified to provide power for an exciter winding or a telemetry system. All such systems, however, require one or more rectifiers to be mounted on the rotating member. The invention is defined in the accompanying independent claims. Preferred features are recited in the dependent claims. In the case where sensing means are provided to sense the position of a rotor relative to a stator of an electrical machine, the requirements or cabling to supply power to the electrical circuit where no appropriate local power supply is available, and for the suppression cf transients in that power supply, are avoided. Furthermore, the sensing means and, in particular, the electrical circuit, is completely electrically isolated from the machine. The invention may particularly be applied to an electrical circuit being part of a sensing means for sensing the position of a rotor of an electrical machine whether run as a motor or a generator. Data may be transmitted from the sensing means to the control means along a fibre optic link or other non-electrically conductive means of data transmission between the sensing means and the control means. The magnetic field excites a coil to cause a voltage to be produced in the coil to power the sensing means, the coil being positioned adjacent, but being electrically independent of, a winding of the motor of the machine such that the magnetic field is generated by the motor. Alternatively, the coil may be positioned around at least one conductor providing power to a winding of the machine such that the magnetic field is generated by the electrical current flowing in the conductor. The electrical signal generated in the coil may be rectified, filtered and/or regulated as required to provide an appropriate power supply to the sensing means, preferably by means located at or adjacent the machine so that no or relatively little cabling is required between the coil and the sensing means. The sensing means may further comprise a sensor located on or adjacent a driven shaft of the motor, the sensor being sensitive to the position of the shaft, and hence of the rotor, relative to the stator. The invention has particular merit where the control means is remote from the stator and rotor, and the sensing means are adapted to provide the data to the motor controller. Preferably, the electrical circuit powered by the derived electrical power supply is located closer to the stator and rotor than to the control means. The invention can be put into practice in various ways, some of which will now be described by way of example with reference to the accompanying drawing in which:- Figure 1 is a cross-section of a conventional reluctance machine; and Figure 2 is a schematic block diagram of a machine system incorporating the invention. Referring to the drawings, a switched reluctance machine 10 is arranged as part of a switched reluctance drive system which may be arranged to run the machine 10 as a motor or a generator as the skilled person will appreciate. As shown in Figure 1, the machine 10 comprises a laminated stator 11 which defines, in this example, six stator poles 12. A winding 14 is arranged in relation to each stator pole 12 to energise it. It will be seen that the pairs of diametrically opposed windings 14 are electrically connected together to form three phases of windings. For the sake of clarity, only the windings of one phase (phase A) are shown. The windings for each of the remaining two phases B and C are arranged in relation to the respective pairs of diametrically opposed stator poles 12. A rotor 16 is arranged to rotate within the stator 11. The rotor 16 defines four rotor poles 18. The phases of the windings 14 are energised in sequence to create a rotationally shifting magnetic field which induces the rotor 16 to move as it seeks the orientation of least reluctance to the induced flux passing between the opposed windings 14 of a phase through the diametrically opposite poles 18 of the rotor 16. Other numbers of phases and combinations of rotor and stator pole numbers will be apparent to the skilled person. It is necessary, in this kind of machine, to know the relative position of the poles 18 of the rotor 16 with respect to the stator poles 12 so that the stator poles can be energised at appropriate times and for the appropriate periods. Thus, sensing means are provided in relation to a shaft 22 on which the rotor 14 is mounted. This is illustrated in Figure 2. The sensing means sense the position of the rotor 16 relative to the stationary stator 11. The sensing means may comprise a suitable sensor 20, a disc 21 mounted to rotate with the shaft 22 and an interface unit 24, or any other suitable kind of arrangement which is sensitive to the position of the rotor. The sensor may be a Hall effect device. Transmissive- or reflective-based opto-electronics could also be used, or capacitive, magnetic or inductive sensing devices are also available. In Figure 2 the embodiment shown generally illustrates an optical system. The sensor 20 operates in conjunction with the disc 21 carried on the shaft 22. The disc 21 is slotted to induce a digital output from the sensor 20 as the shaft rotates to provide information indicative of the position of the rotor relative to the stator. The output of the sensor 20 is provided to the interface unit 24 which, in turn, provides data to a motor controller 25 which switches an available power supply 23 using power switches 26 to power the respective phases of the motor windings 14 at appropriate times to generate the required rotational force. The interface unit can be considered as part of the overall function of the motor system. Its purpose is to provide position information data readable by the controller 25 from the digital output from the sensor 20. It will be appreciated by those skilled in the art that thus far is conventional and the operation of the sensing means and controller 25 to operate the motor will be familiar. In accordance with the invention an electrical power supply for the electrical components of the sensing means 20 is derived locally of the machine 10 from a magnetic field associated with the windings 14, rather than being obtained from an output of the motor controller 25 as is conventional. Thus, there is no need to provide cabling between the motor controller 25 and the sensing means 20 in order to power the sensing means. In the embodiment illustrated in the drawing, the power supply for the sensing means is derived from the magnetic field generated by the machine windings 14. Thus, a coil 19 is positioned in that magnetic field within, or at least close enough to, the stator 11 for power to be transferred to the coil 19. It will be appreciated that the coil 19 is illustrated somewhat schematically in Figure 2 for the purposes of illustration. Preferably, where the stator comprises a plurality of windings, a coil is provided for each of several windings so as more efficiently to cooperate with the magnetic field of each. The coil 19 may comprise turns around or closely adjacent the, or the respective, winding 14, or may be more elaborate if necessary. The coil 19 is connected by a suitable, short length of cabling 27 to a power supply unit 28 where the alternating current derived by the coil 19 from the magnetic field is rectified, filtered and regulated as necessary, for a suitable power supply to be derived to power an electrical circuit of the interface unit 24 and the sensor 20 of the sensing means. The interface unit 24 transmits data to the motor controller 25 via, for example, a fibre optic link 29. Alternatively, the data may be transmitted by other means, preferably not requiring electrically conductive cabling, such as electromagnetically generated radio frequencies. Thus, the power supply for the electronic circuit of the interface unit 24 and the sensor 20 is isolated electrically from the current in the machine 10. Also, the power supply to the sensor 20 and the unit 24 is completely electrically isolated from the motor supply. It will be appreciated that until a magnetic field is established, no power supply for the sensing means can be derived. Hence, preferably the power supply unit 28 includes a stand-by battery or other means of storing, or at least initially supplying, electrical power. The battery may be of the re-chargeable type, initially to power the sensing means during start-up. Such a battery may also be used in the event of any interruptions in normal operation of the machine 10. Alternatively, the controller 25 may be arranged to provide a pulse of power to the machine 10 at start-up to cause an initial magnetic field to be developed, sufficient for a voltage to be generated in the auxiliary coil 19. The invention has been developed particularly but not exclusively for use where the machine controller 25 is located substantially distant from the machine 10, such as in a train application where the motor controller 25 may be located at one end of a carriage and the machine 10 at the other end of the carriage, the machine being of the switched reluctance type which, in use, drives a compressor of an air conditioning/air ventilating system for the carriage. In this application, the saving in cabling to power the sensing means 20 by adoption of the present invention can be substantial. It will be appreciated by the skilled person that the invention is also applicable to linear machines as well as the rotary machines referred to above. The moving member of a linear motor, for example, is often referred to as a rotor. The term rotor used herein is intended to embrace such moving members of linear machines. The invention may be used in any other application where a power supply is required for an electrical circuit and it is desirable to derive this locally as no suitable local power supply is available. Thus, in the example described, the sensing means may comprise an electronic temperature sensor of the kind which requires a power supply, or any other desired electrical circuit. It will be appreciated that only the sensor 20 and its interface unit 24 are powered from the coil 19 in the above embodiment. However, other functions of the motor system could be powered in the same way. For example, the controller 25 could be powered from the power supply derived from the coil 19 as long as the power requirement of the controller 25 were adequately met by the power output derivable from the coil 19. While the invention has been described in connection with a machine that is operated to produce a force, torque or electrical power output, such that the power required by, e.g., the sensor is derived from the magnetic field under running conditions, it is also possible to energise the winding or windings partially so that a magnetic field is created prior to running the machine. In this way, a supply is available to power the sensor so that its output is available when the machine is started up thereafter. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may separately or in any combination of such features be utilised for realising the invention in diverse forms. Accordingly, the invention is to be limited only by the scope of the accompanying claims.

Claims: An electrical machine system comprising at least one electrical machine having at least one part that is movable relative to other parts of the machine and at least one first winding (14) for producing a magnetic field to achieve electromechanical energy conversion in conjunction with the movable part and control means (25) operable to control the or each machine, wherein each machine has a second winding (19) electrically isolated from the machine for interacting with the magnetic field produced by said first winding (14) to produce power for powering at least part of said control means (25), characterised in that the second winding (19) is mounted separately from the said movable part (16). A system as claimed in claim 1 in which the second winding (19) is electrically connected with the control means (25). A system as claimed in claim 1 or 2 wherein the control means (25) comprises sensor means having an output indicative of the relative movement of the said movable part, which sensor means are connected with the output of the second winding (19). A system as claimed in claim 3 in which the sensor means are operable to produce a digital signal as the output. A system as claimed in claim 3 or 4 in which the machine is a rotary machine comprising a stator (12), wherein the movable part is a rotor (16). A system as claimed in any of claims 1 to 5 in which the control means (25) are arranged to run the machine as a motor. A system as claimed in any of claims 1 to 5 in which the control means (25) are arranged to run the machine as a generator. A system as claimed in any preceding claim, including filter means arranged to filter the output of the second winding to a desired frequency range. A system as claimed in any preceding claim, including limiter means arranged to limit the voltage of the power output of the second winding to a desired range. A method for powering an electrical circuit from an electrical machine system including at least one electrical machine having at least one part that is movable relative to other parts of the machine and at least one first winding (14) for producing a magnetic field to achieve electromechanical energy conversion in conjunction with the movable part, and control means (25) operable to control the or each machine, the method comprising: energising the or each machine to produce the magnetic field; deriving an electrical power supply from the magnetic field using a second winding (19) having a power output and being electrically isolated from the machine; and powering at least part of the control means (25) from the output of the said second winding (19); characterised in that the second winding (19) is mounted separately from the said movable part. A method as claimed in claim 10 including electrically connecting the said second winding (19) with the control means (25). A method as claims in claim 9, 10 or 11 in which the control means (25) comprise sensor means having an output indicative of the said relative movement, the method further comprising using the power output of the said second winding (19) to power the sensor means. A method as claimed in claim 12 in which the sensor means produce a digital output. A method as claimed in any of claims 10 to 13 in which the control means (25) operate the machine as a motor. A method as claimed in any of claims 10 to 13 in which the control means (25) operate the machine as a generator. A method as claimed in any of claims 10 to 15 in which the power output of the said second winding (19) is filtered to a desired frequency range. A method as claimed in any of claims 12 to 15 in which the voltage of the power output of the said second winding (19) is limited to a desired range.

Inventors: WILSON MALCOLM PHILIP, WILSON, MALCOLM PHILIP
Applicants: SWITCHED RELUCTANCE DRIVES LTD, SWITCHED RELUCTANCE DRIVES LIMITED
Application year: 1996
Main classification: H02K11
IPC codes: H02P3, H02K29, H02K11