Motor driver

Abstract

A motor driver (100) includes an inverter (32) that converts a DC voltage into an AC voltage and applies the AC voltage to an AC motor (1), a DC voltage detector (31) that detects the DC voltage applied to the inverter (32), a voltage command generator (21) that generates a voltage command based on a torque command and a detection value of the DC voltage, and a gate signal generator (22) that generates a gate signal for performing pulse width modulation control of the inverter (32) based on a comparison result between a modulated wave, which is a waveform of the voltage command, and a carrier wave.The number of slots per magnetic pole in the stator core (9) of the AC motor (1) is a natural multiple of three, and there is a relationship of Fc=6n+3 between Fc and n, where Fc represents a numerical value as carrier order obtained by normalizing the frequency of the carrier wave with the frequency of the modulated wave, and n is natural.

Description

Field of InterestThe present disclosure relates to a motor driver configured to drive an AC motor having a plurality of slots arranged at equal intervals along an inner circumferential surface of a stator core.BackgroundIt is known that in an AC motor having a plurality of slots in a stator core, a torque ripple is generated depending on the number of slots. Since this kind of torque ripple is generated due to the structure of the AC motor, a design for reducing the torque ripple is often performed by designing the structure of the AC motor.Meanwhile, it is known that the torque ripple is also generated by harmonics generated by pulse width modulation (PWM) control of an inverter that drives an AC motor. In Patent Literature 1 below, in order to reduce this type of torque ripple, the motor current is detected in a cycle longer than the switching cycle of the inverter, the motor current is estimated in a period in which the motor current is not detected, and the PWM pulse for the inverter is calculated so that the estimated motor current value matches the current target value.Citation ListPatent LiteraturePatent Literature 1: Japanese Patent No. 6407683SUMMARY OF THE INVENTIONProblem to be Solved by the InventionHowever, the calculation process of Patent Literature 1 is complicated, and there is a problem that the calculation time and the calculation amount required for the calculation increase.The present disclosure has been made in view of the above, and an object thereof is to provide a motor driver capable of reducing torque ripple while suppressing an increase in computation time and computation amount.Means for Solving the ProblemIn order to solve the above-described problems and achieve the object, a motor driver according to the present disclosure is a motor driver configured to drive an AC motor, which has a stator core in which a plurality of slots arranged at equal intervals along an inner circumferential surface are formed, and which includes an inverter, a DC voltage detector, a voltage command generator, and a gate signal generator. The inverter is configured to convert a DC voltage into an AC voltage and apply the AC voltage to the AC motor. The DC voltage detector is configured to detect a DC voltage applied to the inverter. The voltage command generator is configured to generate a voltage command based on a torque command and a detection value of the DC voltage. The gate signal generator is configured to generate a gate signal for performing pulse width modulation control of the inverter based on a comparison result between a modulated wave, which is a waveform of a voltage command, and a carrier wave. The number of slots per magnetic pole in the stator core is a natural multiple of three. There is a relationship Fc=6n+3between Fc and n, where Fc represents a numerical value as a carrier order obtained by normalizing the frequency of the carrier wave with the frequency of the modulated wave, and n is natural.Effects of the InventionThe motor driver according to the present disclosure brings about the effect of reducing the torque ripple while suppressing an increase in the computation time and the computation load.Brief Description of the DrawingsFIG. 1 is a diagram showing the configuration of a motor driver according to an embodiment. FIG. 2 is a diagram illustrating an example of the waveform of a phase of a modulated wave generated by the modulated wave generator of FIG. 1 and the waveform of a carrier wave generated by the carrier wave generator of FIG. 1. FIG. 3 is a sectional view of the AC motor according to the embodiment taken along the axial direction of a shaft. FIG. 4 is a sectional view of the AC motor shown in FIG. 3 taken along line A-A of FIG. 3. FIG. 5 is a sectional view showing a magnetic pole of the 6-pole 36-slot reluctance motor shown in FIG. 4. FIG. 6 is a waveform diagram showing torque variation at the time when the 6-pole 36-slot reluctance motor with the carrier assembly 27 shown in FIG. 4 is driven. FIG. 7 is a diagram showing a frequency analysis result of the torque fluctuation waveform shown in FIG. 6. FIG. 8 is a diagram showing a frequency analysis result of a torque fluctuation waveform at the time when the 6-pole 36-slot reluctance motor with the carrier order 17 shown in FIG. 4 is driven. FIG. 9 is a diagram showing a frequency analysis result of a torque fluctuation waveform at the time when the 6-pole 36-slot reluctance motor with the carrier order 15 shown in FIG. 4 is driven. FIG. 10 is a diagram illustrating a relationship between the carrier order and orders of the slot harmonics, the slot harmonics, and the inverter harmonics in the 36-slot 6-pole reluctance motor illustrated in FIG. 4. FIG. 11 is a diagram illustrating a relationship between the carrier order and the torque ripple in the 6-pole 36-slot reluctance motor illustrated in FIG. 4. FIG. 12 is a sectional view showing a magnetic pole of a 6-pole 54-slot reluctance motor as a reluctance motor having a different structure from that in FIG. 5. FIG. 13 is a waveform diagram showing torque variation at the time when the 6-pole 54-slot reluctance motor with the carrier assembly 27 shown in FIG. 12 is driven. FIG. 14 is a diagram showing a frequency analysis result of the torque fluctuation waveform shown in FIG. 13. FIG. 15 is a diagram showing a frequency analysis result of a torque fluctuation waveform at the time when the 6-pole 54-slot reluctance motor with the carrier order 17 shown in FIG. 12 is driven. FIG. 16 is a diagram showing a frequency analysis result of a torque fluctuation waveform at the time when the 6-pole 54-slot reluctance motor with the carrier order 15 shown in FIG. 12 is driven. FIG. 17 is a diagram illustrating a relationship between the carrier order and the torque ripple in the 54-slot 6-pole reluctance motor illustrated in FIG. 12. FIG. 18 is a diagram illustrating a relationship between the carrier order and orders of the slot harmonics, the slot harmonics, and the inverter harmonics in the 54-slot 6-pole reluctance motor illustrated in FIG. 12. FIG. 19 is a block diagram illustrating an example of a hardware configuration for implementing the functions of the control device according to the embodiment. FIG. 20 is a block diagram illustrating another example of a hardware configuration for implementing the functions of the control device according to the embodiment.DESCRIPTION OF THE EMBODIMENTSHereinafter, a motor driver according to an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. In the following embodiment, a motor driver for driving a railway vehicle will be described as an example, but it is not intended to exclude application to other uses. In the accompanying drawings, the scale of the individual parts may deviate from the actual scale for ease of understanding. Likewise, the scale of the individual elements in some drawings may be different from that in other drawings.Embodiment.FIG. 1 is a diagram illustrating a configuration of a motor driver 100 according to an embodiment. In FIG. 1, the motor driver 100 according to the embodiment includes an inverter 32 and a control device 20.In FIG. 1, an AC motor 1 is a drive motor mounted on a rail vehicle. The AC motor 1 generates torque for driving the railway vehicle by AC power supplied from the inverter 32. The AC motor 1 is an induction motor or a synchronous motor.A DC power supply 30 is a supply source of DC power supplied to the inverter 32. The DC power supply 30 includes a overhead line, a current collector, a filter capacitor, and the like. The inverter 32 converts a DC voltage applied from the DC power supply 30 into an AC voltage and applies the AC voltage to the AC motor 1.Between the DC power supply 30 and the inverter 32, a DC voltage detector 31 for detecting a DC voltage output from the DC power supply 30 is provided. The detection value of the DC voltage detected by the DC voltage detector 31 is output to the controller 20.FIG. 1 illustrates an example in which the main circuit of the inverter 32 is a 2-level inverter. The inverter 32 is provided with six semiconductor switching elements Su, Sv, Sw, Sx, Sy, and Sz, and includes the same number of arm circuits as the number of output phases. In the branch circuits, two of the semiconductor switching elements are connected in series, and the output voltage is an intermediate potential that is a potential at the connection end of the two semiconductor switching elements. In FIG. 1, for generating three-phase alternating voltages Vu, Vv, and Vw, a u-phase arm consisting of the semiconductor switching elements Su and Sx, a v-phase arm consisting of the semiconductor switching elements Sv and Sy, and a w-phase arm consisting of the semiconductor switching elements Sw and Sz are formed. Note that the main circuit of the inverter 32 does not need to be a 2-level inverter, but may be a 3-level inverter, for example.The control device 20 includes a voltage command generator 21 and a gate signal generator 22, and the gate signal generator 22 includes a modulation wave / carrier wave selector 23, a modulation wave generator 24, a carrier wave generator 25, and a comparator 26.The voltage command generator 21 generates a voltage command based on the torque command and the detection value of the DC voltage. The gate signal generator 22 generates a gate signal for performing PWM control of the inverter 32 based on the voltage command output from the voltage command generator 21. In the example of FIG. 1, gate signals for performing PWM control of the six semiconductor switching elements Su, Sv, Sw, Sx, Sy, and Sz, i.e., gate signals for the six elements, are generated and output to the inverter 32. From the inverter 32, the PWM-controlled three-phase AC voltages Vu, Vv, and Vw are generated and applied to the AC motor 1.In the gate signal generator 22, the modulation wave generator 24 generates a modulated wave based on the voltage command output from the voltage command generator 21 and the selection signal output from the modulation wave / carrier wave selector 23. The carrier wave generator 25 generates a carrier wave based on the voltage command output from the voltage command generator 21 and the selection signal output from the modulation wave / carrier wave selector 23.FIG. 2 is a diagram illustrating an example of the waveform of a phase of a modulated wave generated by the modulated wave generator 24 of FIG. 1 and the waveform of a carrier wave generated by the carrier wave generator 25 of FIG. 1. Here, the modulated wave is a waveform signal obtained by normalizing the command waveform of the voltage applied to the AC motor 1 with the DC voltage of the DC power supply 30 to generate the gate signal.As illustrated in FIG. 2, the frequency of the carrier wave is greater than the frequency of the modulated wave. FIG. 2 illustrates an example in which the frequency of the carrier wave is 27 times the frequency of the modulated wave. In this specification, a numerical value obtained by normalizing the frequency of the carrier wave with the frequency of the modulated wave is referred to as "carrier order" and represented by the symbol "Fc". That is, FIG. 2 shows an example of a case where the selection signal having the carrier order Fc of 27 is output to the modulation wave generator 24 and the carrier wave generator 25 by the modulation wave / carrier wave selector 23. Note that the example of FIG. 2 is an example, and a value other than 27 may be selected as the carrier order by the selection signal.The modulation wave / carrier wave selector 23 determines a selection signal for reducing the torque ripple generated in the AC motor 1, and outputs the selection signal to the modulation wave generator 24 and the carrier wave generator 25. The modulation wave generator 24 generates a modulated wave according to the selection signal output from the modulation wave / carrier wave selector 23. The carrier wave generator 25 generates a carrier wave according to the selection signal output from the modulation wave / carrier wave selector 23.The comparator 26 generates the gate signal described above based on the comparison result between the modulated wave and the carrier wave. Specifically, the comparator 26 compares the modulated wave output from the modulated wave generator 24 with the carrier wave output from the carrier wave generator 25 for each phase, and outputs a gate signal indicating:(i) Upper element: ON, Lower element: OFF when amplitude of the modulated wave>Amplitude of the carrier wave(ii) Upper element: OFF, Lower element: ON when amplitude of the modulated wave<amplitude of the carrier wave. Note that the upper elements correspond to the semiconductor switching elements Su, Sv, and Sw, and the lower elements correspond to the semiconductor switching elements Sx, Sy, and Sz. The directions of the inequality in the above (i) and (ii) may be reversed.As described in the background section, the torque ripple also increases due to harmonics occurring by the PWM control of the inverter 32 driving the AC motor 1. In this specification, these harmonics are referred to as "inverter harmonics". The inverter harmonic is a harmonic that may be included by PWM-controlling the inverter 32 in the AC voltage applied to the AC motor 1. As described above, the modulation wave / carrier wave selector 23 detects the selection signal for reducing the torque ripple generated in the AC motor 1. A specific method of determining the selection signal for reducing the torque ripple generated in the AC motor 1 will be described below.FIG. 3 is a sectional view of the AC motor 1 according to the embodiment taken along the axial direction of a shaft 4. the broken line B illustrated in FIG. 3 is the axis of the shaft 4. FIG. 4 is a sectional view of the AC motor 1 illustrated in FIG. 3 taken along the line A-A in FIG. 3. FIGS. 3 and 4 illustrate the structure of a three-phase reluctance motor as an example of the AC motor 1. in FIG. 4, a frame 5 is not illustrated.The AC motor 1 includes an annular stator 6 inserted into the frame 5 and fixed thereto by a method such as press-fitting or shrinking, and a cylindrical rotor 7. the annular stator 6 and the cylindrical rotor 7 are arranged to be relatively rotatable via a magnetic gap 19 which is a mechanical gap using a bearing 8.The stator 6 is formed by applying a winding 10 to an annular stator core 9 made of an iron core. The rotor 7 is integrally formed by inserting the shaft 4 into the center of a cylindrical rotor core 11 made of an iron core by a method such as press-fitting or shrinking.The stator core 9 includes an annular core back 12 and teeth 13 protruding radially inward from the core back 12 and arranged at equal intervals. A plurality of slots 14 are formed at equal intervals between the plurality of teeth 13 provided on the radially inner side of the stator core 9. The winding 10 is accommodated in the slots 14. The teeth 13 and the grooves 14 are arranged at the same angle in the circumferential direction of the ring shape. In this specification, the entire plurality of grooves 14 may be referred to as a "groove portion".In FIG. 4, S=36 and P=6 are considered as the number of slots in the stator core 9 being "S" and the number of magnetic poles of the rotor core 11 being "P". that is, FIG. 4 shows a cross-sectional structure of a 6-pole 36-slot three-phase reluctance motor. The number of the grooves and the number of the magnetic poles illustrated in FIG. 4 are examples and are not limited to the example of FIG. 4.FIG. 5 is a sectional view showing a magnetic pole of the 6-pole 36-slot reluctance motor shown in FIG. 4, and is an enlarged view of a 1 / 6 region of FIG. 4.In FIG. 5, in the cross section of the stator core 7 and the rotor core 11, a d-axis is defined in the center line direction of the magnetic poles, and a q-axis is defined in the center line direction between the magnetic poles. The center line direction of the magnetic poles is a direction in which the magnetic flux is likely to pass, and the center line direction between the magnetic poles is a direction in which the magnetic flux is unlikely to pass. The d-axis direction may be referred to as a "salient pole direction", and the q-axis direction may be referred to as a "non-salient pole direction".The d-axis and the q-axis electrically have a phase difference of 90 degrees. The rotor 7 rotates by an induction torque generated due to an inductance difference between the d-axis direction and the q-axis direction. That is, the reluctance motor generates an output torque by using a difference in magnetic resistance in the rotational direction. Therefore, the reluctance motor can generate a higher output torque as the inductance difference between the d-axis and the q-axis is larger.In FIG. 5, the rotor core 11 is provided with a plurality of slits 15 each consisting of an arc-shaped opening that is convex toward a cylinder center O of the rotor core 11 for each magnetic pole of the rotor core 11 and has a peak positioned on the q-axis, as viewed in the direction of the central axis of the cylinder. A space is provided in the rotor core 11 by the plurality of slots 15. That is, the slits 15 cause the rotor core 11 to have a structure in which a magnetic portion including a magnetic material that is a material of an electromagnetic steel sheet and a non-magnetic portion formed of air alternately occur. The slits 15 are provided so as to be symmetrical with respect to the q-axis for each magnetic pole. In this specification, the entirety of the plurality of slits 15 may be referred to as the "slit portion".FIG. 5 shows a case where the number of slits 15 is three, but the number is not limited thereto and may be two or four or more. That is, the number of the slits 15 need only be more than one. Further, in FIG. 5, the end portions of the slits 15 are linearly formed along the side portion located on the magnetic gap 19 side of the rotor core 11, but are not limited to this shape. The end portions of the slits 15 may be arcuately chamfered. Note that, for example, an arc shape simulated by a straight line or the like may also be considered an arc shape.A center point in the circumferential direction of the arc-shaped end portion along the outer circumferential surface of the rotor core 11 in the arc-shaped opening portion of the slot 15 closest to the d-axis passing through the cylinder center point O of the rotor core 11 is defined as a center point W. An angle formed by the centers W of the slits 15 each provided in a magnetic pole with respect to the cylinder center O of the rotor core 11 between adjacent slits is defined as θ.In the examples of FIGS. 4 and 5, the slits 15 are provided so that the angle θ between adjacent slits is the same. Further, an angle formed by a straight line connecting the center W of the slit closest to the d-axis with the cylinder center O of the rotor core 11 and the d-axis is set to θ / 2. When the angle θ between adjacent slits is the same, the angle θ is set to θ=6.67 (=360 / 54) degrees. In this specification, the angle θ may be referred to as "slot interval θ", and the number of angles θ on the entire circumference of the rotor core 11 may be referred to as "division number". In the case of the configuration of FIG. 5, the number of the divisions is 54. note that the configuration of FIG. 5 is an example and the slot interval θ between adjacent slots does not necessarily need to be the same.As described above, the rotor core 11 has a configuration in which the low magnetic resistance core portion through which the magnetic flux is likely to pass and the high magnetic resistance slot portion through which the magnetic flux is unlikely to pass alternately appear in the rotational direction. Due to such a variation in magnetic resistance of the rotor core 11, harmonics are superimposed on the winding 10. In this specification, harmonics resulting from the variation in the magnetic resistance in the rotational direction in the rotor core 11 are referred to as "slot harmonics".In addition, when the stator core 9 is viewed from the rotor core 11 in FIG. 5, it is found that, in the stator core 9, the low magnetic resistance core portion through which the magnetic flux is likely to pass and the high magnetic resistance groove portion through which the magnetic flux is unlikely to pass alternately occur in the rotational direction. Due to such a variation in the magnetic resistance of the stator core 9, harmonics are superimposed on the winding 10. In this specification, the harmonics generated by the variation of the magnetic resistance in the rotational direction in the stator core 9 are referred to as "slot harmonics".FIG. 6 is a waveform diagram illustrating torque variation at the time when the 6-pole 36-slot reluctance motor with the carrier assembly 27 shown in FIG. 4 is driven. In FIG. 6, the horizontal axis represents the electrical angle, and the vertical axis represents the magnitude of the torque applied to the AC motor 1 in normalized form. FIG. 7 is a diagram showing a frequency analysis result of the torque fluctuation waveform shown in FIG. 6. In FIG. 7, the horizontal axis represents the order of the torque ripple, and the vertical axis represents the torque ripple amplitude, i.e., the amplitude value of the torque ripple. The order of the torque ripple is a numerical value obtained by normalizing one of the frequencies of the torque ripple with the frequency of the modulated wave and is shown as a multiple of the normalized frequency. Moreover, the vertical axis represents a value obtained by normalizing the amplitude for each order of the torque ripple with the magnitude of the total torque ripple over the entire frequency band.First, as shown in FIG. 6, the value of the torque fluctuates when the electric angle corresponding to the rotational position of the rotor 7 is different. Moreover, FIG. 7 shows that the torque ripple of the 12th order, the 18th order, the 24th order, the 30th order, and the 36th order is large except for the sixth and lower orders. Among the torque ripple of these orders, the 12th, 24th, and 36thharmonics correspond to the slot harmonics. The AC motor 1 shown in FIG. 4 has a 6-pole 36-slot configuration, and the number of slots per pole is six. Therefore, the order of the fundamental frequency of the nut harmonic is the 12th order, and the 24th and 36th orders, which are integer multiples of the 12th order, also correspond to the nut harmonics. The number of divisions of the AC motor 1 shown in FIG. 4 is 54, which is 1.5 times the number of slots S=36 on the entire circumference of the stator core 9. Therefore, the order of the fundamental frequency of the slot harmonics is the 18th order, and the 36th order, which is an integer multiple of the 18th order, also corresponds to the slot harmonic.Through this study, the inventors of the present application have found that the orders of the inverter harmonics that greatly influence the torque ripple among the plurality of inverter harmonics are (Fc- 3), (Fc+3), and 2Fc. In the case of FIG. 7, the 24th order corresponds to the (Fc-3)th order, and the 30th order corresponds to the (Fc+3)th order. Note that the 2Fc order has a large order value and is not included in the analysis result of FIG. 7.According to the analysis result of FIG. 7 in which the 30-th order component is greatly emphasized, it is understood that the (Fc+3)-th order is the inverter harmonic. On the other hand, the 24th order corresponding to the (Fc-3) order is superimposed on the nut harmonic and is indistinguishable. Therefore, frequency analysis was performed with various carrier orders. The results of the analysis are shown in FIGS. 8 and 9. Specifically, FIG. 8 is a diagram showing a frequency analysis result of a torque fluctuation waveform at the time when the 6-pole 36-slot reluctance motor shown in FIG. 4 is operated with the carrier order 17, and FIG. 9 is a diagram showing a frequency analysis result of a torque fluctuation waveform at the time when the 6-pole 36-slot reluctance motor shown in FIG. 4 is operated with the carrier order 15.According to the analysis result of FIG. 8, similarly to FIG. 7, the 12th, 24th, and 36th slot harmonics are generated and the 18th, and 36th slot harmonics are generated. In the analysis result of FIG. 8, the 14th, 20th, and 34thharmonics are generated, but the 14thharmonic corresponds to the (Fc-3)thharmonic, the 20thharmonic corresponds to the (Fc+3)thharmonic, and the 34thharmonic corresponds to the (2Fc)thharmonic. From this, it can be understood that the 14th order is an order component belonging to neither the nut harmonic nor the slot harmonic, and the 14th order is the (Fc-3)th order of the inverter harmonic. Moreover, the 34th order is also an order component belonging to neither the slot harmonic nor the slot harmonic, and it can be understood that the 34th order component, which is the 2Fc order of the inverter harmonic, is also a component that greatly affects the torque ripple.According to the analysis result of FIG. 9, similarly to FIG. 7, the 12th, 24th, and 36th slot harmonics are generated and the 18th, and 36th slot harmonics are generated. In the analysis result of FIG. 9, the 12th, 18th, and 30th inverter harmonics are generated, and the 12th harmonic corresponds to the (Fc-3)th harmonic, the 18th harmonic corresponds to the (Fc+3)th harmonic, and the 30th harmonic corresponds to the (2Fc)th harmonic. The 12th and 18th orders cannot be distinguished from the slot harmonic and the slot harmonic, but the 30th order is a component of only the inverter harmonics, and from this result, it can be understood that the 30th order component, which is the 2Fc order, has a large influence on the torque ripple.FIG. 10 is a diagram showing a relationship between the carrier order Fc and the orders of the slot harmonics, the slot harmonics, and the inverter harmonics in the 6-pole 36-slot reluctance motor illustrated in FIG. 4. Note that matching orders occur in much higher orders, but these are not the major components of the torque ripple and are therefore not shown. Further, FIG. 10 shows only a case where the carrier order Fc is an odd number, i.e., a case where the frequency of the carrier wave is an odd multiple of the frequency of the modulated wave. This is because when the carrier order Fc is an even number, the N pole and the S pole of the magnetic pole are not symmetrical, and the pulse of the PWM signal is not a synchronization pulse.In FIG. 10, a circled portion indicates that the order intersects with at least one of the slot harmonics and the slot harmonics. In the case of the carrier order Fc=27, as described with reference to FIG. 7, twice (24th order) the nut harmonic Fc- 3 (24th order) corresponds to the inverter harmonic. In addition, in the case of the carrier order Fc=17, as described with reference to FIG. 8, there is no matching order. In addition, in the case of the carrier order Fc=15, as described with reference to FIG. 9, the simple (12th order) of the slot harmonic Fc- 3 (12th order) corresponds to the inverter harmonic, and the simple (18th order) of the slot harmonic corresponds to Fc+3 (18th order) of the inverter harmonic.FIG. 11 is a diagram illustrating the relationship between the carrier order Fc and the torque ripple in the 36-slot 6-pole reluctance motor shown in FIG. 4. In FIG. 11, the horizontal axis represents the carrier order Fc, and the vertical axis represents a value obtained by normalizing the 29th-order torque ripple having the largest carrier order Fc. FIG. 11 shows that the torque ripple decreases as the carrier order Fc increases. Moreover, FIG. 11 also shows that the torque ripple at a natural number n has the smallest value when the carrier order Fc is 6n+3. Specifically, the torque ripple in FIG. 11 has the smallest value when Fc=9(n=1), Fc=15(n=2), Fc=21(n=3), and Fc=27(n=4).In view of the above-described results of FIGS. 10 and 11, the following can be said. First, as illustrated in FIG. 10, when the carrier order Fc is 6n+3(n=1 to 4), there is an order corresponding to the inverter harmonic in at least one of the slot harmonic or the slot harmonic. Further, when comparing the analysis result of FIG. 8 in which the carrier order Fc is 17 with the analysis result of FIG. 9 in which the carrier order Fc is 15, and referring to the result of FIG. 11, it can be seen that the torque ripple decreases when there is an order corresponding to the inverter harmonic in at least one of the slot harmonic and the slot harmonic. When there is an order corresponding to the inverter harmonic in at least one of the slot harmonic and the slot harmonic, this means that the number of orders in which the torque ripple is generated is reduced with respect to the entire torque ripple. As a result, it is considered that the torque ripple is reduced.Moreover, in a case where the carrier order Fc is 27, although there is no matching order between the inverter harmonic and the slot harmonic, the torque ripple has the minimum value as illustrated in the result of FIG. 11, and the effect of reducing the torque ripple can be obtained. Therefore, it can be seen that the method described in the present embodiment provides an effect as long as the AC motor has a configuration including a stator core in which a plurality of slots are formed on the inner circumferential surface. Therefore, the technique of the present embodiment is not limited to the reluctance motor, and even if the technique is applied to a permanent magnet motor or an induction motor, it is possible to enjoy the effect of reducing the torque ripple.The above description relates to the 6-pole 36-slot reluctance motor shown in Figs. 4 and 5. In order to confirm the above-described contents, analyses were also performed on a reluctance motor having a different structure. The details will be described below.FIG. 12 is a sectional view showing a magnetic pole of a 6-pole 54-slot reluctance motor as a reluctance motor having a different structure from that in FIG. 5. The number of pitches of the rotor core 11 is 72, and the slot interval θ is θ=5.0 (=360 / 72) degrees. In this structure, the fundamental frequency of the slot harmonic is 4 / 3 times the fundamental frequency of the slot harmonic.FIG. 13 is a waveform diagram showing torque variation at the time when the 6-pole 54-slot reluctance motor with the carrier assembly 27 shown in FIG. 12 is driven. The notation of the vertical and horizontal axes is the same as in FIG. 6, FIG. 13 shows that the value of the torque fluctuates when the electric angle corresponding to the rotational position of the rotor 7 is different.FIG. 14 is a diagram showing a frequency analysis result of the torque fluctuation waveform shown in FIG. 13. The notation of the vertical axis and the horizontal axis is the same as in FIG. 7.FIG. 14 shows that the torque shafts of the 12th order, the 18th order, the 24th order, and the 30th order are large except for the sixth and lower orders. Among the torque ripple of these orders, the 18thharmonic corresponds to the slot harmonic and the 30thharmonic corresponds to the inverter harmonic. It is assumed that the 24thharmonic corresponds to both the slot harmonic and the inverter harmonic, but it can not be distinguished only from the analysis result of FIG. 14. Therefore, frequency analysis was performed with various carrier orders. The results of the analysis are shown in Figs. 15 and 16. Specifically, FIG. 15 is a diagram showing a frequency analysis result of a torque fluctuation waveform at the time when the 6-pole 54-slot reluctance motor shown in FIG. 12 is operated with the carrier order 17, and FIG. 16 is a diagram showing a frequency analysis result of a torque fluctuation waveform at the time when the 6-pole 54-slot reluctance motor shown in FIG. 12 is operated with the carrier order 15.According to the analysis result of FIG. 15, the 18th slot harmonic and the 24th slot harmonic are generated. In the analysis result of FIG. 15, the 14th, 18th, 20th, 24th, 30th, and 34thharmonics are generated; and the 14thharmonic corresponds to the (Fc-3)thharmonic, the 20thharmonic corresponds to the (Fc+3)thharmonic, and the 34thharmonic corresponds to the (2Fc)thharmonic. From this, it can be understood that the 14th order is an order component belonging to neither the nut harmonic nor the slot harmonic, and the 14th order is the (Fc-3)th order of the inverter harmonic. Moreover, it can be understood that the 20th order is also an order belonging to neither the slot harmonic nor the slot harmonic, and the 20th order is the (Fc+3) order of the inverter harmonic. Further, it can be understood that the 34th order is also an order belonging to neither the nut harmonic nor the slot harmonic, and the 34th order is the (Fc+3) order of the inverter harmonic.According to the analysis result of FIG. 16, the 18th and 36th slot harmonics and the 24th slot harmonic are generated. Moreover, in the analysis result of FIG. 16, harmonics of the 12th, 18th, 24th, 30th, and 36th orders are generated; and the 12th order corresponds to the (Fc-3)th order and the 30th order corresponds to the (2Fc)th order. From this, it can be understood that the 12th order is an order component belonging to neither the slot harmonic nor the slot harmonic, and the 12th order is the (Fc-3)th order of the inverter harmonic. Moreover, it can be understood that the 30th order is also an order component belonging to neither the slot harmonic nor the slot harmonic, and the 30th order is the (2Fc)th order component of the inverter harmonic. It is also understood from the above description that the 18th order is the fundamental wave component of the nut harmonic and is also the (Fc-3)th order component of the inverter harmonic.FIG. 17 is a diagram illustrating a relationship between the carrier order Fc and the torque ripple in the 54-slot 6-pole reluctance motor illustrated in FIG. 12. As in FIG. 11, the horizontal axis represents the carrier order Fc, and the vertical axis represents a value obtained by normalizing the 29th-order torque ripple having the largest carrier order Fc. As in FIG. 11, FIG. 17 shows that the torque ripple decreases as the carrier order Fc increases. As in FIG. 11, when n is a natural number, the relationship in which the torque ripple has the lowest value is maintained when the carrier order Fc is 6n+3.FIG. 18 is a diagram illustrating a relationship between the carrier order Fc and orders of the slot harmonics, the slot harmonics, and the inverter harmonics in the 54-slot 6-pole reluctance motor illustrated in FIG. 12. Similar to FIG. 10, the higher order components are not shown. In addition, similar to FIG. 10, only the case where the carrier order Fc is an odd number will be described.In view of FIGS. 10, 11, 17 and 18, the following can be said. First, regardless of whether the structure of the stator core 9 is the 6-pole 36 slot or the 6-pole 54 slot, the relationship in which the torque ripple has the smallest value when the carrier order Fc is 6n+3 is maintained. This relationship is because the number of slots per pole is a natural multiple of three for both the 6-pole 36 slot and the 6-pole 54 slot. In fact, the number of slots per pole of the 6-pole 36 slot is six, the number of slots per pole of the 6-pole 54 slot is nine, and the number of slots per pole is a natural multiple of three. When the stator core 9 is viewed from the rotor core 11, since the low magnetic resistance core portion and the high magnetic resistance groove portion are alternately arranged in the rotational direction, it can be understood that there is periodicity in which the torque ripple has the smallest value at a position that is a natural multiple of six.From the above description, the gate signal generator 22 included in the control device 20 according to the embodiment performs the following control. Here, a numerical value obtained by normalizing the frequency of one harmonic of the plurality of inverter harmonics with the frequency of the modulated wave is referred to as the "primary order"; a numerical value obtained by normalizing the frequency of one harmonic of the plurality of slot harmonics with the frequency of the modulated wave is referred to as the "secondary order"; and a numerical value obtained by normalizing the frequency of one harmonic of the plurality of slot harmonics with the frequency of the modulated wave is referred to as the "tertiary order".First, when generating a gate signal for performing the PWM control of the inverter 32, the gate signal generator generates the gate signal so that the primary order coincides with the secondary order. With this control method, the frequency of at least one harmonic of the slot harmonics and the frequency of at least one harmonic of the inverter harmonics can be matched to each other. As a result, the number of orders in which the torque ripple occurs can be reduced, and thus the torque ripple can be reduced.In the above control method, since the frequency of the modulated wave is determined by the voltage command value, a gate signal that matches the primary order with the secondary order can be generated by appropriately adjusting the frequency of the carrier wave. Moreover, this control method does not significantly affect the existing control and does not require complicated calculations unlike Patent Literature 1. By using this control method, it is therefore possible to reduce the torque ripple while suppressing an increase in the computation time and the computation amount. Moreover, by using this control method, since it is not necessary to change the structure of the AC motor, it is possible to meet the torque ripple requirements by inverter control and at the same time promote the use of existing AC motors in various applications having different torque ripple requirements.In addition, in a case where the AC motor is a reluctance motor including a rotor core provided with a plurality of slots, the gate signal generator 22 generates a gate signal such that the primary order coincides with at least one of the secondary order and the tertiary order when a gate signal for performing the PWM control of the inverter 32 is generated. With this control method, the frequency of at least one harmonic of the slot harmonics and the slot harmonics and the frequency of at least one harmonic of the inverter harmonics can be matched to each other. As a result, the number of orders in which torque ripple occurs can be reduced, and thus torque ripple can be reduced. Moreover, this control method does not significantly affect the existing control and does not require complicated calculations unlike Patent Literature 1; therefore, it is possible to reduce the torque ripple while suppressing an increase in the computation time and the computation amount.Next, hardware configurations for implementing the above-mentioned functions of the control device 20 will be described with reference to the drawings of FIGS. 19 and 20. FIG. 19 is a block diagram illustrating an example of a hardware configuration for implementing the functions of the control device 20 according to the embodiment. FIG. 20 is a block diagram illustrating another example of a hardware configuration for implementing the functions of the control device 20 according to the embodiment.Some or all of the functions of the control device 20 according to the embodiment may be implemented with a configuration including, as illustrated in FIG. 19, a processor 300, a memory 302, and an interface 304. The processor 300 performs calculations. Programs read by the processor 300 are stored in the memory 302. Signals are input and output via the interface 304.The processor 300 is a computing means. The processor 300 may be a computing means referred to as a microprocessor, microcomputer, central processing unit (CPU), or digital signal processor (DSP). The memory 302 may be, for example, a nonvolatile or volatile semiconductor memory, a magnetic disk, a flexible disk, an optical disk, a compact disk, a mini disk, a digital versatile disk (DVD), or the like. Examples of nonvolatile or volatile semiconductor memories include a random access memory (RAM), a read only memory (ROM), a flash memory, an erasable programmable ROM (EPROM), an electrically EPROM (EEPROM, registered trademark), and the like.In the memory 302, a program for executing the functions of the control device 20 according to the embodiment is stored. Necessary information is transmitted and received via the interface 304, the processor 300 executes the program stored in the memory 302, and the processor 300 refers to data stored in the memory 302, whereby the processor 300 can perform the above-described processing. The calculation result of the processor 300 may be stored in the memory 302.For implementing some of the functions of the control device 20 according to the embodiment, a processing circuit 303 illustrated in FIG. 20 may also be used. The processing circuit 303 is, for example, a single circuit, a composite circuit, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or a combination thereof. The information input to the processing circuit 303 and the information output from the processing circuit 303 can be fetched via the interface 304.Note that some processings in the control device 20 may be performed by the processing circuit 303, and that processings not performed by the processing circuit 303 may be performed by the processor 300 and the memory 302.As described above, the motor driver according to the embodiment includes an inverter, a DC voltage detector, a voltage command generator, and a gate signal generator. The inverter converts a DC voltage into an AC voltage and applies the AC voltage to the AC motor. The DC voltage detector detects a DC voltage applied to the inverter. The voltage command generator generates a voltage command based on a torque command and a detection value of the DC voltage. The gate signal generator generates a gate signal for performing pulse width modulation control of the inverter based on a comparison result between a modulated wave, which is a waveform of a voltage command, and a carrier wave. The AC motor includes an annular stator core in which a plurality of slots arranged at equal intervals along an inner circumferential surface are formed. The number of slots per magnetic pole in the stator core of the AC motor is a natural multiple of three. A numerical value obtained by normalizing the frequency of the carrier wave with the frequency of the modulated wave is represented by Fc as a carrier order, and n is natural. At this time, the gate signal generator generates the gate signal so that there is a relationship of Fc=6n+3between Fc and n. When the inverter is controlled with this gate signal, the frequency of at least one harmonic of the nut harmonics and the frequency of at least one harmonic of the inverter harmonics can be matched to each other. As a result, it is possible to obtain a motor driver that can reduce torque ripple while suppressing an increase in computation time and computation load.Moreover, in the motor driver according to the embodiment, the primary order is a numerical value obtained by normalizing the frequency of one harmonic from a plurality of inverter harmonics that may be included in the AC voltage applied to the AC motor by performing pulse width modulation control of the inverter with the frequency of the modulated wave. The secondary order is a numerical value obtained by normalizing the frequency of one harmonic of a plurality of slot harmonics generated by fluctuation of the magnetic resistance in a rotational direction in the stator core with the frequency of the modulated wave. At this time, the gate signal generator included in the motor driver generates the gate signal so that the primary order coincides with the secondary order. As a result, the frequency of at least one harmonic of the slot harmonics and the frequency of at least one harmonic of the inverter harmonics can be matched with each other, whereby the number of orders in which torque ripple occurs can be reduced and torque ripple can be reduced.In the above control, the primary order is an arbitrary order obtained by subtracting -3 from the carrier order, an order obtained by adding +3 to the carrier order, or an order obtained by doubling the carrier order. Moreover, the secondary order is any one of an order corresponding to the frequency of a fundamental wave among a plurality of nut harmonics, an order corresponding to twice the frequency of the fundamental wave among a plurality of nut harmonics, and an order corresponding to three times the frequency of the fundamental wave among a plurality of nut harmonics. These components are major components in the inverter harmonics and the slot harmonics. Therefore, the control for reducing the torque ripple can be effectively performed.Moreover, in the motor driver according to the embodiment, the AC motor to be driven is a reluctance motor including a rotor core provided with a plurality of slots each of which is composed of an arc-shaped opening that is convex toward a cylinder center for each magnetic pole as viewed in a direction of a central axis direction of a cylinder, and a peak positioned on a q-axis. The tertiary order is a numerical value obtained by normalizing the frequency of a harmonic of a plurality of slot harmonics generated by fluctuation of the magnetic resistance in a rotational direction in the rotor core with the frequency of the modulated wave. At this time, the gate signal generator included in the motor driver generates the gate signal such that the primary order coincides with at least one of the secondary and tertiary orders. As a result, the frequency of at least one harmonic of the slot harmonics and the slot harmonics and the frequency of at least one harmonic of the inverter harmonics can be matched with each other, whereby the number of orders in which torque ripple occurs can be reduced and torque ripple can be reduced.In the above control, the primary order is an arbitrary order obtained by subtracting -3 from the carrier order, an order obtained by adding +3 to the carrier order, or an order obtained by doubling the carrier order. Moreover, the secondary order is any one of an order corresponding to the frequency of a fundamental wave among a plurality of nut harmonics, an order corresponding to twice the frequency of the fundamental wave among a plurality of nut harmonics, and an order corresponding to three times the frequency of the fundamental wave among a plurality of nut harmonics. The tertiary order is any one of an order corresponding to the frequency of a fundamental wave among a plurality of slot harmonics and an order corresponding to a frequency twice a frequency of the fundamental wave among a plurality of slot harmonics. These components are the main components of the inverter harmonics, the slot harmonics and the slot harmonics. Therefore, the control for reducing the torque ripple can be effectively performed.The configurations described in the above-mentioned embodiment indicate examples. The configurations may be combined with another known technique, and some of the configurations may be omitted or changed in a range that does not deviate from the core.List of reference characters1 AC motor; 4 shaft; 5 frame; 6 stator; 7 rotor; 8 bearing; 9 stator core; 10 winding; 11 rotor core; 12 core back; 13 teeth; 14 slot; 15 slot; 19 magnetic gap; 20 controller; 21 voltage command generator; 22 gate signal generator; 23 modulation wave / carrier wave selector; 24 modulation wave generator; 25 carrier wave generator; 26 comparator; 30 DC power supply; 31 DC voltage detector; 32 inverter; 100 motor driver; 300 processor; 302 memory; 303 processing circuit; 304 interface; Su, Sv, Sw, Sx, Sy, Sz semiconductor switching element.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedJP 6407683

[0004]

Claims

A motor driver configured to drive an AC motor having a stator core in which a plurality of slots arranged at equal intervals along an inner circumferential surface are formed, the motor driver comprising: an inverter configured to convert a DC voltage to an AC voltage and apply the AC voltage to the AC motor; a DC voltage detector configured to detect the DC voltage applied to the inverter; a voltage command generator configured to generate a voltage command based on a torque command and a detection value of the DC voltage; and a gate signal generator configured to generate a gate signal for performing pulse width modulation control of the inverter based on a comparison result between a modulated wave that is a waveform of the voltage command and a carrier wave, wherein a number of the slots per magnetic pole in the stator core is a natural multiple of three, and a relationship of Fc=6n+3 exists between Fc and n, wherein Fc represents a numerical value as a carrier order obtained by normalizing the frequency of the carrier wave with the frequency of the modulated wave, and n is natural.The motor driver according to claim 1, wherein the gate signal generator is configured to generate the gate signal such that a primary order coincides with a secondary order, the primary order being a numerical value obtained by normalizing the frequency of a harmonic of a plurality of harmonics of the inverter that may be included in the AC voltage applied to the AC motor by performing pulse width modulation control of the inverter with the frequency of the modulated wave, and the secondary order being a numerical value obtained by normalizing the frequency of a harmonic of a plurality of slot harmonics generated by fluctuation of a magnetic resistance in a rotational direction in the stator core with the frequency of the modulated wave.The motor driver according to claim 2, wherein the primary order is any one of an order obtained by subtracting -3 from the carrier order, an order obtained by adding +3 to the carrier order or an order obtained by doubling the carrier order, and the secondary order is any one of an order corresponding to the frequency of a fundamental wave among a plurality of nut harmonics, an order corresponding to a frequency twice the fundamental wave among a plurality of nut harmonics, and an order corresponding to a frequency three times the fundamental wave among a plurality of nut harmonics.The motor driver according to claim 2 or 3, wherein the AC motor is a reluctance motor comprising a rotor core, the rotor core having a cylindrical shape; is disposed on an inner surface side of the stator core; and is provided with a plurality of slots each consisting of an arc-shaped opening that is convex to a cylinder center as viewed in a direction of a central axis direction of a cylinder for each magnetic pole and has a peak positioned on a q-axis, and the gate signal generator is configured to generate the gate signal such that the primary order coincides with the tertiary order, the tertiary order being a numerical value obtained by normalizing the frequency of a harmonic of a plurality of slot harmonics generated by fluctuation of a magnetic resistance in a rotational direction in the rotor core with the frequency of the modulated wave.The motor driver according to claim 4, wherein the tertiary order is any one of an order corresponding to the frequency of a fundamental wave among a plurality of slot harmonics and an order corresponding to a frequency twice a frequency of the fundamental wave among a plurality of slot harmonics.

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