ANALYSIS OF HARMONICS

Abstract: Analysis of rotor slot harmonics from a motor (10) is achieved by an adaptive filter (32) controlled in accordance with an algorithm (36), such as the recursive maximum likelihood algorithm. The setting of the filter which gives rise to minimum output (34) provides useful information from which speed information, or other information such as position information or motor condition can be deduced.

Description: Analysis of Harmonics The present invention relates to the analysis of harmonics and in particular, the analysis of rotor slot harmonics of an induction motor. A conventional form of induction motor uses stator coils around a rotor. Fields created by the stator interact with the rotor to cause rotation. Longitudinal slots are formed along the outer surface of the rotor and these give rise to stator currents at a frequency which is harmonic of the fundamental drive frequency. The frequency of these harmonics contain information about rotor speed. The present invention provides a method of analysing rotor slot harmonics of an induction motor, in which a signal containing the harmonic is applied to an adaptive filter, the condition of which is monitored to provide information about the harmonic, and wherein the adaptive filter is arranged to be adjustable according to a recursive method which seeks to maximise or minimise the filter ouput. Preferably the recursive method is a recursive maximum likelihood algorithm. Preferably the induction motor is supplied with a plurality of drive current phases, at least one of which is analysed as aforesaid. The or each signal is preferably a stator current. Preferably the adaptive filter comprises a notch filter or bandpass filter having a variable centre frequency. Preferably the notch frequency is set at a harmonic of the fundamental drive frequency, preferably the harmonic at which speed information is present in the stator current. The or each signal may be pre-filtered before application to the adaptive filter. The pre-filter may be a bandpass filter set to pass the required harmonic. Preferably the pre-filter is adjustable, and is preferably set according to a measurement of the fundamental drive frequency and/or drive current magnitude. The invention also provides analysis means operable to analyse rotor slot harmonics of an induction motor, comprising adaptive filter means operable to receive a signal containing the harmonic, and means operable to monitor the condition of the adaptive filter to provide information about the harmonic, the adaptive filter being adjustable according to a recursive method which seeks to maximise or minimise the filter output. Preferably the recursive method is a recursive maximum likelihood algorithm. Preferably the apparatus supplies a plurality of drive current phases to the induction motor, at least two of which are analysed as aforesaid. The or each signal is preferably a stator current. Preferably the adaptive filter comprises a notch filter or bandpass filter having a variable centre frequency. Preferably the notch frequency is set at a harmonic of the fundamental drive frequency, preferably the harmonic at which speed information is present in the stator current. Preferably the or each signal is pre-filtered before application to the adaptive filter. The pre-filter may be a bandpass filter, set to pass the required harmonic. Preferably the pre-filter is adjustable, and is preferably set according to a measurement of the fundamental drive frequency and/or drive current magnitude. An example of the present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 is a schematic partial section transverse to the axis of an induction motor with which the present invention may be used; and Fig. 2 is a schematic block diagram of apparatus implementing the present invention. Fig. 1 shows a motor 10 comprising a rotor 12 and stator coils 14. Bars 16 of magnetic material are embedded in the rotor 12 around its periphery and under surface slots 18 which extend longitudinaUy along the outer surface of the rotor 12. The stator coils 14 are arrayed around the rotor. Alternating drive currents at a drive frequency are applied to the coils 14 to create fields which interact with the bars 16 to cause rotation of the rotor 12, in known manner. Interaction with the bars 16 also causes magnetic flux around the bars, as indicated by the arrow 20. This flux is affected by the presence of the slots 18 and as a consequence, the stator currents applied to the coils 14 are found to include harmonics of the fundamental drive frequency being applied to the coils. These harmonics contain information about the rotor speed ( and are hereafter called the rotor slot harmonics, in view of their origin). The applicant has also determined that if the slots 18 are not all identical, rotor position information can be retrieved from the rotor slot harmonics in the coil currents. Analysis of the rotor slot harmonics is achieved as indicated in Fig. 2. Fig. 2 shows the technique implemented in digital manner and accordingly, the coil current, including the harmonics, is first applied from the motor 10 to an analogue-to-digital converter 22. The converter 22 will receive one phase of the stator currents; other phases may be analysed in like manner by further circuits of the type shown in Fig. 2. The choice of phases and harmonics for analysis will depend on the information being sought. The output of converter 22 is sent to identifier circuit 24 which acts to identify the fundamental drive frequency ω . The identifier 24 may be conventional in itself and may be based, for instance, on a Fast Fourier Transform technique, the Prony algorithm or on phase angle differentiation using two phase current signals. The output of the converter 22 is also applied to a band pass filter 26, the centre frequency of which is adjustable and set in accordance with the value of ω o received from the identifier 24 on an output 28. The purpose of the filter 26 is to provide coarse pre-filtering to pass the desired harmonic and substantially exclude other components of the signal. Accordingly, if the desired information is contained in the nth harmonic of the fundamental drive frequency, the centre frequency of the filter 26 will be set at the nth harmonic of the frequency identified by the circuit 24. The output 30 of the filter 26 therefore contains only the desired harmonic and frequencies close to it (in the frequency domain). The output 30 is applied to an adaptive notch filter 32 to provide an output at 34. The input 30 and output 34 of the filter 32 are applied to a control circuit 36 which adaptively controls the setting of the filter 32. The purpose of the adaptive control 36 is to vary the filter centre frequency to "hunt" the strongest signal within the range of frequencies received at 30. The strongest frequency will normally be the required harmonic, by virtue of the pre-filtering at 26. (The pass band of the filter 26 is preferably controlled to prevent other harmonics being passed, which could cause the hunting filter 32 to find a spurious harmonic). The signal at 30 will thus contain only the desired harmonic and noise. Thus, the control 36 is continuously monitoring the input 30 and output 34 and adjusting the setting of the filter 32 to attempt to minimise the output signal strength 34. The frequency ω Q may be supplied from the identifier 24 to aid rapid setting of the filter parameters. When minimised, the assumption can be made that the filter 32 is centred on the desired harmonic. The settings of the filter, provided as an output 38, allow this centre frequency to be analysed and thus allow the speed information (or whatever other information is sought) to be retrieved. It is important to note that it is not the output 34 of the filter which is itself important, but the setting of the filter (by the circuit 36) which gives rise to the minimum output 34. The circuit 36 operates to control the filter 32 according to an algorithm of the recursive maximum likelihood (RML) type. It is envisaged that the output 38 will be in the form of a periodic sampling of the filter parameters, thus allowing the speed measurement or other analysis to be updated at each sample point. Briefly, a recursive maximum likelihood algorithm has a convergence property governed by time- varying parameters which are continuously adjusted. This achieves a compromise between elimination of noise and tracking of time-varying parameters; for example a compromise between speed measurement accuracy and tracking of speed changes. The technique envisaged above is expected to be computationally very efficient. For instance, with a sampling rate of 5kHz, the technique is envisaged to be capable of updating speed within 0.2ms and with an accuracy of 0.1% or better. (The precise characteristics of performance will depend on the quality of the signal under analysis). Experiments using simulated data have shown that the arrangement is able to track changes of frequency at slew rates up to 600Hz per second (5kHz sampling rate) and up to 1,800Hz per second (sampling rate of 10kHz). Information obtained from one or more harmonics in the manner described can be used for a variety of purposes. Rotor speed information can be obtained from analysis of at least one phase of stator current. If the rotor slots 18 are non-uniform, position information can be deduced. Harmonics can also reveal information about the condition of the motor. Various modifications can be made to the apparatus described above, without departing from the scope of the present invention. In particular, many different forms of rotor could be envisaged. The rotor slot harmonics created will depend on the details of the design but it will be apparent to the skilled man how the converter 22 and other components shown in Fig. 2 can be modified to take account of changes of this nature. Individual components of the circuit in Fig. 2 could be implemented in a wide variety of techniques and using various different circuit designs. For instance, the detailed design of the filter 26 will be chosen according to the range of frequencies over which it is expected to operate and the pass band expected of it. Similarly, the filter 32 will be arranged to provide a notch sufficient to enable the control circuit 36 to implement the recursive maximum likelihood algorithm. Alternatively, the filter 32 could be a bandpass filter, in which case the circuit 36 will seek to maximise the filter output, as indicating that the desired harmonic has been found. It is envisaged that the filter could be controlled by other recursive methods which seek to optimise the filter output, but the recursive maximum likelihood algorithm is preferred. It may be appropriate to adjust the parameters of the algorithm, such as when changing the motor speed, to ensure best performance. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims: CLAIMS 1. A method of analysing rotor slot harmonics of an induction motor, in which a signal containing the harmonic is applied to an adaptive filter, the condition of which is monitored to provide information about the harmonic, and wherein the adaptive filter is arranged to be adjustable according to a recursive method which seeks to maximise or minimise the filter output. 2. A method according to claim 1, wherein the recursive method is a recursive maximum likelihood algorithm. 3. A method according to claim 1 or 2, wherein the induction motor is supplied with a plurality of drive current phases, at least one of which is analysed as aforesaid. 4. A method according to claim 1, 2 or 3, wherein the or each signal is a stator current. 5. A method according to any preceding claim, wherein the adaptive filter comprises a notch filter or bandpass filter having a variable centre frequency. 6. A method according to claim 5, wherein the centre frequency is adaptively set at a harmonic of the fundamental drive frequency. 7. A method according to claim 6, wherein the centre frequency is set at the harmonic at which speed information is present in the stator current. 8. A method according to any preceding claim, wherein the or each signal is pre-filtered before application to the adaptive filter. 9. A method according to claim 8, wherein the pre-filter is a bandpass filter set to pass the required harmonic. 10. A method according to claim 8 or 9, wherein the pre-filter is adjustable. 11. A method according to claim 8, 9 or 10, wherein the pre-filter is set according to a measurement of the fundamental drive frequency and/or drive current magnitude. 12. A method of analysing rotor slot harmonics, substantially as described above with reference to the accompanying drawings. 13. Analysis means operable to analyse rotor slot harmonics of an induction motor, comprising adaptive filter means operable to receive a signal containing the harmonic, and means operable to monitor the condition of the adaptive filter to provide information about the harmonic, the adaptive filter being adjustable according to a recursive method which seeks to maximise or minimise the filter output. 14. Analysis means according to claim 13, wherein the recursive method is a recursive maximum likelihood algorithm. 15. Analysis means according to claim 13 or 14, wherein the apparatus supplies a plurality of drive current phases to the induction motor, at least two of which are analysed as aforesaid. 16. Analysis means according to any of claims 13 to 15, wherein the or each signal is a stator current. 17. Analysis means according to any of claims 13 to 16, wherein the adaptive filter comprises a notch filter or bandpass filter having a variable centre frequency. 18. Analysis means according to claim 17, wherein the centre frequency is adaptively set at a harmonic of the fundamental drive frequency. 19. Analysis means according to claim 18, wherein the frequency is set at the harmonic at which speed information is present in the stator current. 20. Analysis means according to any of claims 13 to 19, comprising pre-filter means operable to pre-filter the or each signal before application to the adaptive filter. 21. Analysis means according to claim 20, wherein the pre-filter means is a bandpass filter, set to pass the required harmonic. 22. Analysis means according to claim 20 or 21, wherein the pre-filter means is adjustable. 23. Analysis means according to claim 22, wherein the pre-filter means is set according to a measurement of the fundamental drive frequency and/or drive current magnitude. 24. Analysis means operable to analyse rotor slot harmonics of an induction motor, and substantially as described above with reference to the accompanying drawings. 25. Any novel subject matter or combination including novel subject matter disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.

Inventors: ASHER GREG MARK, BRADLEY KEITH JOHN, FERRAH AZZEDINE, WOOLFSON MALCOLM STEVEN, ASHER, GREG, MARK;, BRADLEY, KEITH, JOHN;, FERRAH, AZZEDINE;, WOOLFSON, MALCOLM, STEVEN;
Applicants: ASHER GREG MARK, BRADLEY KEITH JOHN, FERRAH AZZEDINE, UNIV NOTTINGHAM, WOOLFSON MALCOLM STEVEN, ASHER, GREG, MARK;, BRADLEY, KEITH, JOHN;, FERRAH, AZZEDINE;, THE UNIVERSITY OF NOTTINGHAM, WOOLFSON, MALCOLM, STEVEN;
Application year: 1997
Main classification: G01P3
IPC codes: G01P3, H02K17