Noise Reduction in Switched Reluctance Motors by Selective Radial Force Harmonics Reduction

Abstract

An SR drive having an acoustic noise reduction system for reducing vibration and acoustic noise in a switched reluctance motor (SRM). The vibration and acoustic noise at a particular harmonic of current excitation in the SRM are proportional to the radial force harmonics acting on the SRM stator teeth. The acoustic noise reduction system includes a processor having an acoustic noise reduction application installed thereon that is designed to derive an optimal current waveform that produces an average torque that satisfies an optimal torque condition and generates a radial force having a minimum amplitude at a desired order harmonic of the current excitation. By utilizing the optimal current waveform to reduce the amplitude of the particular radial force harmonics, the vibration and acoustic noise in the SRM are minimized. The acoustic noise reduction system applies turn-on and turn-off angles to the optimal current waveform to improve system efficiency.

Description

[Technical field]

[0001] The present invention relates generally to a system for reducing acoustic noise in switched reluctance motors, and more particularly to an acoustic noise reduction system for reducing vibration and acoustic noise in switched reluctance motors by selectively reducing certain radial force harmonics acting on the stator teeth. [Background technology]

[0002] Electric motor performance is a key issue in industrial applications. A good quality electric motor must be efficient, reliable, and low cost. Electric machines such as industrial motors (IMs), permanent magnet synchronous motors (PMSMs), and synchronous reluctance motors have been developed to meet these quality requirements. Currently, it is believed that the IE4 standard can be reached by using permanent magnets in electric motors. This method of utilizing permanent magnets is well advanced, but is expensive due to the cost of the required components.

[0003] Switched reluctance motors (SRMs) can be used as one of the alternatives to electric motors in industrial applications. SRMs have the advantages of not using rare earth materials, low manufacturing cost, high reliability, and the possibility of operating at high temperatures and high rotational speeds. However, SRMs have the disadvantages of high acoustic noise and vibration, as well as high torque ripple. In order to reduce the torque ripple and increase the system efficiency, novel SRMs with a high number of rotor poles are being researched.

[0004] Several methods have been developed to overcome vibration and acoustic noise in SRM. In one such method, vibration and acoustic noise are reduced by improving the motor structure or by optimizing the voltage and current excitation. In another method, spacers made of diamagnetic material are inserted between the stator teeth to increase the stiffness of the stator structure. In this way, acoustic noise can be reduced. Yet another improvement introduces windowed rotor and stator structures to reduce the radial force amplitude. In this way, further reduction of peak noise is achieved. Yet another method to reduce radial forces introduces skewing rotor and stator structures that can further limit acoustic noise.

[0005] In other conventional systems, active vibration cancellation (AVC) methods have been implemented to achieve voltage and current optimization to reduce the noise at the natural frequency, with a reduction of up to -16.3 dB in one conventional case. Further research in AVC methods has been carried out by including mechanical impulses as a reference to determine the reference current. Although multiple frequency components generally exist, only one frequency component can be eliminated. Current shaping based on two-dimensional fast Fourier transform (2D-FFT) of radial force density analysis has been introduced to reduce the acoustic noise by up to -19.6 dB. Another effective method of flattening the sum of radial forces has been proposed and verified to reduce the acoustic noise in high pole count SRMs (HRSRMs). These conventional methods have been effective in reducing vibration mode 0 in high pole count SRMs, resulting in an acoustic noise reduction of up to -22 dB, but further research and progress is still needed.

[0006] FIG. 1 shows a graphical illustration of an example of radial force density of a three-phase 6 / 10 pole SRM excited by a conventional square wave current, where the radial force density acts in the air gap with respect to mechanical angle α and time t. Radial force density σ r (α,t) is calculated using Maxwell's stress tensor, which is expressed as follows:

[0007]

number

[0008] There are two peaks of radial force density in the same time period from a pair of stator teeth in each phase, as shown in Figure 1. The value of the peak radial force density is proportional to the number of stator teeth. The radial force density in each phase is produced by excitation with conventional square wave currents, which are typical of SRMs.

[0009] Figure 2 is a graph of an example of a 2D-FFT analysis of the radial force density shown in Figure 1. In this analysis, the radial force density is calculated by the formula expressed as follows:

number

number

number

[0010] Surface deformation D of the SRM in time order u and space order v uv is proportional to the amplitude of the radial force density at the same time and spatial orders. Furthermore, the acoustic noise of the SRM, Π(v,u), is proportional to the square of the surface deformation, which can be expressed as:

number

number

[0011]

number

[0012] Therefore, in summary, the radial force density amplitude at a particular time order or current harmonic is proportional to the deformation, vibration, and acoustic noise at that particular time order and current harmonic.

[0013] The coupling of the radial force density between the phases can be considered small and therefore can be neglected. The second assumption is that the currents excited in the three phases are identical. The analysis of the radial force density can then be simplified to one stator tooth since the waveforms are identical and periodic. The radial force F of one stator tooth is r (t) can be calculated using integral equation (2), which is expressed as follows:

[0014]

number

[0015] FIG. 3A shows a graph of an example of radial force acting on one stator tooth by a conventional square wave current, and FIG. 3B shows its FFT analysis. Based on this measurement, a 6 / 10 pole SRM operating at a rated speed of 1800 rpm and a rated torque of 4.8 Nm has significant acoustic noise at 600 Hz, 1200 Hz, 1500 Hz, 2100 Hz, and 2400 Hz. At a fundamental current of 300 Hz, significant noise occurs at the 2nd, 4th, 5th, 7th, and 8th harmonics of the current excitation. Therefore, the acoustic noise at the 2nd, 4th, 5th, 7th, and 8th current harmonics must be reduced.

[0016] FIG. 4 shows a graphical representation of an example of radial force harmonics of a conventional square wave current in decibels and vibration harmonics being analyzed by FEA. To analyze the relationship between radial force and vibration, only the second harmonic of the radial force is changed from 100% of the original amplitude to 75%, 50%, and 25%, respectively. The reduction in radial force amplitude is performed for all stator teeth. As shown in FIG. 4, the reduction in the second harmonic of the radial force amplitude only affects the vibration at the second harmonic. When the radial force amplitude is reduced, the vibration is also reduced. Therefore, based on FEA, the radial force harmonics and the vibration harmonics have a proportional relationship.

[0017] A similar analysis is performed for the fourth harmonic of the radial force. FIG. 5 shows a graph of an example of the relationship of the FEA results of the radial force and the fourth harmonic of the vibration, where the amplitude at the fourth harmonic is changed. From FIG. 5, the fourth harmonic of the radial force also has a proportional relationship with the vibration at the fourth harmonic. FIG. 6 shows a graph of an example of the summary of the relationship between the FEA results of the radial force and the vibration at the second, fourth, fifth, seventh, and eighth harmonics. All the radial force harmonics and vibrations have the same relationship. From this analysis, it can be summarized that the vibration and acoustic noise of the SRM can be reduced by reducing a specific harmonic of the radial force.

[0018] 7A and 7B show a graphical illustration of an example of the radial force on one stator tooth in an SRM by exciting several constant DC currents in the FEA. The aligned and unaligned positions shown in FIG. 7A are 0° and 180° electrical rotor positions, respectively. A 6 / 10 pole SRM is operated in a magnetically saturated condition. As the current increases, the relationship between the radial force and the current becomes nonlinear. In this case, a radial force approximation is performed to simplify the relationship between the radial force, electrical position, and current. The following description describes a method for cancellation of harmonics up to the seventh order, but the method is applicable to any number of harmonics.

[0019] The radial force at one stator tooth with respect to the electrical rotor position is approximated by using a Fourier series equation expressed as: F r A=K r0 (i)+K r1 (i) cosθ+…+K r7 (i) cos7θ (7) where θ is the electrical rotor position and K r0 , K r1 , K r2 , K r3 , K r4 , K r5 , K r6 , and K r7is a polynomial function of the current i. The radial force with respect to the current is approximated by a polynomial function: K rn A polynomial function (n=0, 1, 2, ..., 7) can be expressed as follows: K r0 (i)=K r07 i 8 +K r06 i 7 +...+K r03 i 3 +K r02 i 2 +K r01 i (8) K r1 (i)=K r17 i 8 +K r16 i 7 +...+K r13 i 3 +K r12 i 2 +K r11 i (9) K r2 (i)=K r27 i 8 +K r26 i 7 +...+K r23 i 3 +K r22 i 2 +K r21 i (10) K r3 (i)=K r37 i 8 +K r36 i 7 +...+K r33 i 3 +K r32 i 2 +K r31 i (11) K r4 (i)=K r47 i 8 +K r46 i 7 +...+K r43 i 3 +K r42 i 2 +K r41 i (12) K r5 (i)=K r57 i 8 +Kr56 i 7 +...+K r53 i 3 +K r52 i 2 +K r51 i (13) K r6 (i)=K r67 i 8 +K r66 i 7 +...+K r63 i 3 +K r62 i 2 +K r61 i (14) K r7 (i)=K r77 i 8 +K r76 i 7 +...+K r73 i 3 +K r72 i 2 +K r71 i (15) In the formula, the coefficient K rxy (x=0, 1, 2, 3, ..., 7 and y=0, 1, 2, 3, ..., 7) are constants.

[0020] Referring to FIG. 7A, the radial force approximation for the relationship between radial force and electrical rotor position is:

number

number

[0021] Considering the above background, there remains a need for an efficient, reliable, and cost-effective system for reducing vibration and acoustic noise in an SRM. Such a system is designed to reduce specific harmonics of radial forces acting on the stator teeth to minimize vibration and acoustic noise in the SRM. Moreover, such a system is operated at high temperatures and high rotational speeds. Such a system improves the system performance level and does not require additional components to reduce acoustic noise generated in the SRM. Moreover, although such a system is described for the exemplary embodiment to reduce acoustic noise at the second, fourth, fifth, seventh, and eighth harmonics of the current excitation, where significant acoustic noise occurs, the method is not limited to only these harmonics. One skilled in the art can apply the proposed invention to reduce any other combination of harmonics of interest. Such a system implements several techniques to enhance system efficiency and reduce torque ripple. The present embodiment overcomes shortcomings in the art by achieving these important objectives. Summary of the Invention

[0022] In order to minimize the limitations found in the prior art and to minimize other limitations that will become apparent upon reading this specification, the present invention provides an acoustic noise reduction system for reducing vibration and acoustic noise in a switched reluctance motor (SRM). An SR drive with an acoustic noise reduction system derives optimal current waveforms that reduce the amplitudes of the second, fourth, fifth, seventh, and eighth harmonics of the radial force. This is only an example and not a limitation of the method. A user can select any particular harmonic of interest to them and use the method for vibration and acoustic noise reduction for their application. The acoustic noise reduction system includes an SRM having at least one stator pole and at least one rotor pole, an SRM inverter connected to the SRM, and a processor. The processor includes a digital processor, microprocessor, microcontroller, ASIC, FPGA, or any other analog or digital circuit responsible for determining and supplying voltages and currents to the motor. The processor is installed with an acoustic noise reduction application that enables the processor to derive optimal current waveforms and current regulation.

[0023] The SRM is driven by an SRM inverter. The shaft of the SRM is connected to a load. In a preferred embodiment, the load can be a fan, a pump, a blower or any other load machine.

[0024] The preferred acoustic noise reduction system utilizes a method for reducing vibration and acoustic noise in an SRM. The preferred method includes four main steps. In a first step, a number of reference currents i ref is generated by a reference current generation module in the processor using a Fourier series formula expressed as follows:

[0025]

number

[0026] Step 2 of the acoustic noise reduction method utilizes the maximum current rate calculation module in the processor to calculate the back electromotive force V emf and limited input dc voltage V dc The maximum rate of change of the current is calculated by taking into account the effect of

number

number

number

[0027] In step 3, the average torque of the SRM and the radial force harmonics of one stator tooth are calculated. A torque generating module in the processor determines the average torque of the SRM. A torque checking module in the processor checks whether the average torque satisfies an optimum torque condition. The optimum torque condition is satisfied when the average torque achieves an optimum value. In a preferred embodiment, the optimum value is 4.8 Nm. A radial force calculation module in the processor calculates the radial force at one stator tooth if the calculated average torque satisfies the optimum torque condition.

[0028] A radial force harmonic calculation module in the processor calculates the radial force harmonics. In a preferred embodiment, the radial force calculation focuses on the radial force harmonics acting on one stator tooth by FFT. Steps 1, 2, and 3 are performed using the parameters i0, i n , and φ n Finally, in step 4, an optimal current waveform selection module in the processor selects at least one current profile as an optimal current waveform I if the at least one current profile produces an average torque that satisfies the optimal torque condition and generates a radial force having a minimum amplitude at the second, fourth, fifth, seventh, and eighth harmonics of the current excitation. cal Since the vibration and acoustic noise at a particular harmonic of the current excitation in the SRM is proportional to the radial force harmonic acting on the stator teeth, the vibration and acoustic noise in the SRM is minimized by utilizing the optimum current waveform to reduce the amplitude of the particular radial force harmonic.

[0029] Optimal currents suitable for reducing the 2nd, 4th, 5th, 7th, and 8th radial force harmonics have been derived, which are the harmonics that generate significant acoustic noise when the SRM is driven by a conventional square wave current. Some experimental results show a significant reduction of 6.18 dB and 5.76 dB in the 2nd and 7th harmonics or the highest and second highest peak noise at 600 Hz and 2100 Hz, respectively. The maximum reduction in acoustic noise is achieved at -21.17 dB at the 5th harmonic or 1500 Hz.

[0030] To improve the efficiency of the acoustic noise reduction system, turn-on and turn-off angles are introduced. By adding turn-on and turn-off angles to the optimal current waveform, the efficiency is improved by up to 1.43%.

[0031] SUMMARY OF THE PRESENT EMBODIMENT It is a primary object of the present invention to provide an efficient, reliable and cost-effective acoustic noise reduction system for reducing vibration and acoustic noise in SRMs.

[0032] A second object of the present invention is to provide an acoustic noise reduction system that is designed to selectively reduce specific harmonics of the radial forces acting on the stator teeth to minimize vibration and acoustic noise in an SRM.

[0033] A third object of the present invention is to provide an acoustic noise reduction system that is adaptable to operate at high temperatures and high rotational speeds.

[0034] A fourth object of the present invention is to provide an acoustic noise reduction system that does not require additional components, thereby improving system performance.

[0035] A fifth object of the present invention is to provide an acoustic noise reduction system that reduces acoustic noise at the second, fourth, fifth, seventh, and eighth harmonics of the current excitation that generates significant acoustic noise when an SRM is driven by a conventional square wave current.

[0036] It is another object of the present invention to provide an acoustic noise reduction system that is designed to implement several techniques for enhancing system efficiency and reducing torque ripple.

[0037] These and other advantages and features of the present invention are described with particularity to make the invention understandable to those skilled in the art.

[0038] In order to enhance their clarity and improve understanding of these various components and embodiments of the present invention, elements in the drawings are not necessarily drawn to scale. Moreover, elements that are known to be common and familiar to those skilled in the art have not been drawn in order to provide a clear view of the various embodiments of the present invention. Thus, the drawings have been generalized in a manner that is in the interest of clarity and conciseness. [Brief description of the drawings]

[0039] [Figure 1] FIG. 2 is a graph of an example of radial force density of a three-phase 6 / 10 pole SRM excited by a conventional square wave current. [Diagram 2] FIG. 2 is a graphical representation of an example of a 2D-FFT analysis of the radial force density shown in FIG. 1. [Figure 3A] FIG. 2 is a graph of an example of the radial force acting on one stator tooth due to a conventional square wave current. [Figure 3B] FIG. 3B is a graphical representation of an example of an FFT analysis of the radial force shown in FIG. 3A. [Figure 4] FIG. 13 is a graph of an example of a relationship between FEA results of second harmonic of radial force and second harmonic of vibration. [Diagram 5] FIG. 13 is a graph of an example of a relationship between FEA results of the fourth harmonic of radial force and the fourth harmonic of vibration. [Figure 6] FIG. 13 is a graphical representation of an example summary of the relationship between radial force and vibration FEA results at 2nd, 4th, 5th, 7th, and 8th harmonics. [Figure 7A]FIG. 2 is a graph of an example of a radial force approximation of the relationship between radial force and electrical rotor position. [Figure 7B] FIG. 13 is a graph of an example of a radial force approximation in the relationship between radial force and current. [Figure 8] FIG. 13 is a graph of an example of approximation of radial force by polynomial functions Kr0 and Kr1 of current i. [Figure 9A] FIG. 1 is a block diagram of an acoustic noise reduction system for experimental purposes to reduce vibration and acoustic noise in a switched reluctance motor (SRM). [Figure 9B] FIG. 2 shows an SR drive with a block diagram of a processor with an acoustic noise reduction system according to a preferred embodiment of the present invention. [Figure 10] 4 is a flow chart of a method for reducing vibration and acoustic noise in an SRM utilizing an acoustic noise reduction system according to a preferred embodiment of the present invention. [Figure 11] FIG. 2 is a graphical representation of an example summary of a method for reducing vibration and acoustic noise in an SRM according to a preferred embodiment of the present invention. [Figure 12] FIG. 1 is a graph of an example comparison of FEA results of a conventional square wave current and an optimal current for reducing vibration and acoustic noise in an SRM according to a preferred embodiment of the present invention. [Figure 13] FIG. 13 is a graph of an example comparison of FEA results of radial force harmonics for a conventional square wave current and an optimized current according to a preferred embodiment of the present invention. [Figure 14] FIG. 13 is a graph of an example comparison of FEA results of vibration and acoustic noise for a conventional square wave current and an optimized current according to a preferred embodiment of the present invention. [Figure 15] FIG. 2 is a graphical illustration of an example comparison between current values ​​measured by a conventional square wave current and an optimal current generated by an acoustic noise reduction system in accordance with a preferred embodiment of the present invention. [Figure 16] 2 is a graph of an example of vibration measured by a conventional square wave current and an optimal current generated by an acoustic noise reduction system according to a preferred embodiment of the present invention; [Figure 17] 2 is a graph of an example of acoustic noise measured by a conventional square wave current and an optimal current generated by an acoustic noise reduction system according to a preferred embodiment of the present invention; [Figure 18] 2 is a graphical illustration of an example of a comparison between a conventional square wave current being measured and an optimal current produced by an acoustic noise reduction system in accordance with a preferred embodiment of the present invention; [Figure 19] FIG. 13 is a graph of an example of an optimum current improvement in terms of root mean square (RMS) value reduction according to a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] In the following discussion dealing with certain embodiments and applications of the present invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced It is to be understood that other embodiments may be utilized and changes may be made without departing from the scope of the present invention.

[0041] Various inventive features are described below, each of which can be used independently of one another or in combination with other features. However, any single inventive feature may not address any of the problems discussed above, or may address only one of the problems discussed above. Furthermore, one or more of the problems discussed above may not be fully addressed by any of the features described below.

[0042] As used herein, the singular forms "a", "an" and "the" include plural references unless the context clearly dictates otherwise. As used herein, "and" is used interchangeably with "or" unless the context clearly dictates otherwise. As used herein, the term "about" means ±5% of the stated parameter. All embodiments of any aspect of the invention can be used in combination unless the context clearly dictates otherwise.

[0043] Unless the context clearly requires otherwise, throughout this specification and the claims, words like "comprise", "comprising" and the like are to be construed in an inclusive sense, i.e., in the sense of "including but not limited to", as opposed to an exclusive or exhaustive sense. Words using the singular or plural also include the plural and singular, respectively. In addition, the words "herein", "wherein", "whereas", "above" and "below" and words of similar import when used herein shall refer to this application as a whole and not to any particular portions of this application.

[0044] The description of the embodiments of the present disclosure is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. While specific embodiments and examples of the present disclosure are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the present disclosure, as those skilled in the art will recognize.

[0045] Referring to FIG. 9A, an experimental setup 10 for reducing vibration and acoustic noise in a switched reluctance motor (SRM) 30 is shown. In FIG. 9B, an SR drive 10' is shown. The SR drive includes a processor, an SRM inverter, an SR motor, and a rotational encoder. The encoder detects the rotational position of the SR motor shaft. The encoder can be replaced by a module for estimating the rotational position based on the voltage and current at the SR motor terminal. The SRM inverter is fed by a DC power source, which may be composed of a diode rectifier, a PWM rectifier, a battery, etc. The SRM inverter detects the currents and provides them to the processor 12. The processor provides a voltage switching signal v*. The SRM inverter provides the voltage and current to the SR motor 30. In the processor, a current regulation module generates a voltage switching signal v* based on the detected current i and a reference current generated by a reference current generation module 14. The other modules 16, 18, 20, 22, 24, 26, and 28 can perform calculations offline when computing power is needed. The calculations can be done in real time if the processor power is high enough. The processor 12 derives the optimal current waveforms that reduce the amplitudes of the 2nd, 4th, 5th, 7th and 8th harmonics of the radial force.

[0046] The experimental setup 10 includes an SRM 30 having at least one stator pole and at least one rotor pole, a load 32 connected to the SRM 30 through a torque meter 34, a three-phase inverter 36 connected to the load 32 for controlling the rotation speed of the load 32, an SRM inverter 38 connected to the SRM 30, and a processor 12. The processor 12 is installed with an acoustic noise reduction application that enables the system 10 to derive an optimal current waveform. The processor includes a reference current generation module 14, a maximum current rate calculation module 16, a current profile calculation module 18, a torque generation module 20, a torque check module 22, a radial force calculation module 24, a radial force harmonic calculation module 26, and an optimal current waveform selection module 28. The experimental setup 10 further includes a microphone 40 and a vibration sensor 42.

[0047] As shown in FIG. 9A, the shaft 44 of the SRM 30 is connected to a torque meter 34. In a preferred embodiment, the load 32 is an induction motor designed to operate as a generator. The induction motor is connected to the shaft 44 of the SRM 30 and is driven by a three-phase inverter 36. The SRM 30 is driven by an SRM inverter 38. Preferably, the SRM inverter 38 is a unidirectional inverter. A microphone 40 is positioned near the SRM 30 and positioned radially one meter from the center of the SRM 30 to measure the acoustic noise generated in the SRM 30. A vibration sensor 42 is positioned on the surface of the SRM 30 to detect and measure the vibrations generated in the SRM 30. An oscilloscope 46 is provided to record and monitor the voltages and currents in the various coils on the experimental setup 10.

[0048] In a preferred embodiment, the SR drive 10' utilizes four main steps to calculate the optimal current waveform. In the first step, a number of reference currents i ref is generated by the reference current generating module 14 using a Fourier series formula expressed as follows:

[0049]

number

[0050] Step 2 is the back electromotive force V emf and limited input dc voltage V dc The maximum current rate calculation module 16 calculates the maximum rate of change of the current, expressed as

number

number

[0051] The radial force harmonic calculation module 26 calculates the radial force harmonics. In a preferred embodiment, the radial force calculation focuses on the radial force harmonics acting on one stator tooth by FFT. Steps 1, 2, and 3 are performed using the parameters i0, i n , and φ n Finally, in step 4, the optimal current waveform selection module 28 selects at least one current profile as the optimal current waveform I if the at least one current profile produces an average torque that satisfies the optimal torque condition and generates a radial force having a minimum amplitude at a desired harmonic for a particular application of the current excitation. cal Since the radial force acting on the stator teeth is proportional to the vibration and acoustic noise at a particular harmonic of the current excitation, by utilizing an optimal current waveform to reduce the amplitude of the particular radial force harmonic, the vibration and acoustic noise in the SRM 30 is minimized.

[0052] 10 shows a flow chart of a method for reducing vibration and acoustic noise in an SRM 30 utilizing the experimental setup 10. In block 50, the amplitudes of the current components i0 and i n and phase shift φ nA combination of parameters (C) including: ref As shown in block 54, the nonlinear inductance L ref (θ) is calculated. Next, as shown in block 56, the current i cal The initial condition for (θ) is calculated. Initially, the value of n is assigned as 0. In this step, i ref The value of (n) is i cal (n) is assigned to L ref The value of (n) is L cal (n) is assigned to the i cal (n)=i ref (n) (19) L cal (n)=L ref (n) (20)

[0053] Then, as shown in block 58, the maximum rate of change of the current

number

number

number

number

number

number

number

number

number

number

[0054] FIG. 11 shows a graphical representation of an example summary of a method for reducing vibration and acoustic noise in an SRM 30. Step 1 shows candidates for reference currents that are generated using equation (15). Step 2-1 shows the V emf Step 2-2 shows the calculation of the high V emf and limited input DC voltage V dc Current waveform i acted on by calStep 3-1 shows the calculated average torque. Then, the radial force generated by the current waveform in step 2 is calculated in step 3-2. Step 3-3 shows the FFT result of the calculated radial force for the analysis of radial force harmonics. In this example, all the current waveforms meet the target torque of 4.8 Nm. For radial force harmonics, current candidate 1 is not recommended because it has high radial force amplitudes at the 4th and 5th harmonics compared to the conventional square wave current. The best candidate is number 3, which has low amplitudes of radial force harmonics at the 2nd, 4th, 5th, and 7th orders compared to the conventional square wave current. Finally, the optimized current waveform is selected in step 4.

[0055] FIG. 12 shows a graphical example of a comparison of drive currents in a FEA transient analysis at 1800 rpm. The conventional square wave current and the optimum current produce an average torque of 4.8 Nm. The conventional square wave current is applied at turn-on and turn-off angles of electrical rotor position of 178° and 303°, respectively, with a peak reference current of 5.2 A. The optimum current is generated using the preferred method shown in FIG. 10. The optimum current is selected based on low amplitude radial force harmonics at the 2nd, 4th, 5th, 7th, and 8th harmonics.

[0056] As shown in FIG. 12, the optimal current has an RMS value of 3.51 A, which is higher than the conventional current of 2.93 A. The preferred optimal current has a high RMS value because it is also provided in the negative torque region, which spans the electrical rotor position of 0° to 180°, to obtain low amplitude radial force harmonics. Therefore, the current in the positive torque region must be increased to compensate for the negative torque. On the other hand, the conventional square wave current is excited optimally in the positive torque region of the electrical rotor position of 0° to 180°. A small portion of the conventional current is excited in the negative torque region, which has a high back-EMF effect. Because the back-EMF is high, the conventional current cannot become zero instantly.

[0057] FIG. 13 shows a graphical illustration of an example of a comparison between radial force harmonics of a conventional square wave current and the preferred optimal current in decibels. The analysis was performed by FEA. In this case, the fundamental frequency of the current harmonics is 300 Hz. This comparison focuses on the most dominant acoustic noises in the 6 / 10 pole SRM, which are the 2nd, 4th, 5th, 7th, and 8th harmonics of the exemplary embodiment. The 2nd and 4th harmonics of the radial force of the preferred optimal current are reduced by -0.16 pu and -0.17 pu, respectively. Meanwhile, the 5th and 7th harmonics of the radial force are reduced by -0.07 pu and -0.18 pu with the preferred optimal current. The radial force at the 8th harmonic is reduced by -0.27 pu. In this case, the 3rd and 6th harmonics of the radial force are increased with the optimal current. However, the third and sixth harmonics do not generate significant acoustic noise based on this example, so the increase in radial force harmonics is tolerable.

[0058] FIG. 14 shows a graphical example of a comparison of vibration and acoustic noise FEA results from a conventional square wave current and a preferred optimal current. The analysis was performed using ANSYS Mechanical. The acoustic noise was measured at 1 meter from the center of the SRM 30 in the radial direction. As shown in FIG. 14, the vibration and acoustic noise of the SRM 30 at the 2nd, 4th, 5th, 7th, and 8th harmonics are reduced by the optimal current for this exemplary embodiment. Based on the FEA results, the reduction in acoustic noise is proportional to the reduction in vibration. The maximum reduction in vibration and acoustic noise using the preferred optimal current is achieved at the 8th harmonic at 0.21 pu and -0.18 pu, respectively.

[0059] FIG. 15 shows a graph diagram of an example of the measured values ​​of the conventional square wave current and the optimal current. The conventional square wave current and the optimal current were excited using hysteresis control. The conventional square wave current has an RMS value of 3.01 A, while the RMS value of the optimal current is 3.76 A. Both RMS values ​​have higher RMS values ​​than the RMS value of the conventional square wave current analyzed in the FEA results shown in FIG. 12. There are certain possible factors that cause this mismatch. First, there is a mismatch of the arcs of the 10 rotor poles of the manufactured machine, which varies with the electrical rotor position from 0.6° to 3.2°. The second reason is that there is a mismatch of the stator pole arcs between the phases, which varies with the electrical rotor position from 0.6° to 1.3°. These two factors directly affect the generated torque and radial force waveforms. Therefore, an adjustment of the reference current is required to obtain the target torque of 4.8 Nm.

[0060] [Table 1]

[0061] Table I shows a comparison of copper loss, core loss, and efficiency of SRM with conventional square wave current and the preferred optimum current. Cu is expressed as follows: P Cu =i A 2 R+i B 2 R+i C 2 R (21) In the formula, i A , i B , and i C are the phase currents in phases A, B, and C, respectively. The coil resistance of one phase coil in the 6 / 10 pole SRM30 is 1.1 Ω.

[0062] SRM30 efficiency η e is expressed as follows:

number

[0063] From Table I, the conventional square wave current has copper and core losses of 29.98 W and 86.55 W, respectively. The preferred optimum current has copper and core losses of 47.12 W and 85.81 W, respectively. The optimum current has high copper losses due to high RMS current. The efficiency of the optimum current is 1.4% lower than the single pulse current.

[0064] FIG. 16 shows a graph of an example of vibration measurements with a conventional square wave current and a preferred optimal current. As shown in FIG. 16, for this exemplary embodiment, the dominant vibrations occur at the 2nd, 4th, 5th, 7th, and 8th harmonics of the current excitation with the conventional square wave current. Compared to the conventional square wave current, the highest and second highest vibrations of the optimal square wave current at the 5th and 7th harmonics or 1200 Hz and 1500 Hz are reduced by -0.56 pu and -0.39 pu. In addition, compared to the conventional square wave current, the vibrations of the optimal square wave current at the 2nd and 4th harmonics or 600 Hz and 1200 Hz are reduced by -0.25 pu and -0.14 pu, respectively. Similarly, the vibrations of the optimal waveform at the 8th harmonic or 2400 Hz are reduced by -0.21 pu.

[0065] FIG. 17 shows a graphical illustration of an example of acoustic noise measured with conventional current and the preferred optimal current. For this exemplary embodiment, acoustic noise reduction is seen at the 2nd, 4th, 5th, 7th, and 8th harmonics of the current excitation. Using the optimal current, the highest peak noise at the 2nd harmonic of the current frequency or 600 Hz is reduced by -0.06 pu. The maximum reduction in noise with the optimal current is up to -0.21 pu at the 5th harmonic or 1500 Hz, which is the third highest peak noise. The peak noise at 1200 Hz, 2100 Hz, and 2400 Hz are also reduced by -0.03 pu, -0.06 pu, and -0.12 pu, respectively. This measurement verifies that selective radial force harmonic reduction is effective in reducing acoustic noise and vibration at specific harmonics.

[0066] FIG. 18 shows a graph of an example of radial force and torque waveforms acting on the stator teeth when a constant DC current is excited. The radial force has a minimum value at the misaligned position while it has a maximum value at the aligned position. For the torque waveform, maximum torque occurs at the motoring and generating positions. Based on this characteristic, the optimal current to reach the target torque and low radial force at a particular harmonic is derived.

[0067] As shown in FIG. 18, the peak of the reference current is excited in the motoring position to optimally obtain the target torque. In addition, current is also excited in the matched and generating positions to generate low radial force harmonics. On the other hand, the current excited in the mismatched positions does not contribute significantly to the torque and radial force generation. In the mismatched positions, the radial force is low due to the high reluctance in the air gap. However, current excitation is necessary in the mismatched positions to compensate for the effect of back EMF, especially in the low speed and high torque region, the medium speed region, and the high speed region. In this case, the current in the mismatched positions can be optimized while compensating for the effect of back EMF.

[0068] FIG. 19 shows the improvement of the preferred optimum current with respect to RMS value reduction. A turn-on angle, which is a typical command in a conventional square wave current, is introduced. The turn-on angle is applied at the optimum current to optimize the current waveform in the misaligned position. In this case, the electric rotor position of 175° is selected as the optimum angle. For further improvement, a turn-off angle is also applied to the preferred current. The negative torque is reduced by adding a turn-off angle to the generating position. An adjustment of the peak reference current is required to maintain the target torque.

[0069] [Table 2]

[0070] Table II shows the efficiency comparison of the conventional square wave current, the optimal current, and the optimal current with turn-on angle and various turn-off angles. By adding the turn-on angle, the RMS value of the optimal current is reduced by -4.25%. This improvement improves the efficiency by 0.83%. In addition, the efficiency of the preferred current is further improved by adding turn-off angles of 10°, 20°, and 30° electrical rotor positions. The RMS values ​​are reduced to 3.52A, 3.47A, and 3.43A, respectively. Therefore, the efficiency is improved to 88.31%, 88.59%, and 88.62% due to the reduction of copper loss and magnetic core loss.

[0071] [Table 3]

[0072] Table III shows a comparison of the measured acoustic noise of the conventional square wave current, the optimal current, and the optimal current with turn-on angle and various turn-off angles. By adding turn-on and turn-off angles, the acoustic noise at the second harmonic does not change significantly. It changes within 1 dB with respect to the acoustic noise of the optimal current. In addition, when the turn-on and turn-off angles are implemented, the acoustic noise at the seventh harmonic is reduced. By adding a turn-off angle of 10°, the maximum reduction is up to -2.68 dB. However, the acoustic noise at the fourth, fifth, and seventh harmonics tends to increase compared to the optimal current. The maximum increase of the fourth harmonic is higher than the conventional square wave current, which increases by 0.1 dB. Meanwhile, at the fifth harmonic, the maximum increase of the acoustic noise occurs at the optimal current with a turn-off angle of 20°, which is 0.06 dB. The maximum increase of the acoustic noise at the fifth harmonic is 69.05 dB, which is still lower than the conventional square wave current.

[0073] From these examples, the addition of turn-on and turn-off angles is effective in reducing the RMS value of the optimal current. However, this also contributes to an increase in acoustic noise. Although the acoustic noise of the optimal current due to the turn-on and turn-off angles is increased, it is still below the amplitude of the acoustic noise of the conventional square wave current. The optimal performance is obtained at the optimal current with a turn-off angle of 20°, which has superior RMS values ​​and low acoustic noise at the 2nd, 4th, 5th, 7th, and 8th harmonics compared to the conventional square wave current.

[0074] The foregoing description of the preferred embodiment of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. It is intended that the scope of the invention be limited not by this detailed description, but by the claims appended hereto and their equivalents.

Claims

1. 1. An SR drive having an acoustic noise reduction system, the acoustic noise reduction system comprising: a switched reluctance motor (SRM) having at least one stator pole and at least one rotor pole; an SRM inverter connected to the SRM; a processor having an acoustic noise reduction application installed thereon, the processor comprising: Using the Fourier series formula expressed as follows, multiple reference currents i ref a reference current generating module for generating a reference current [0010] In the formula, n=1, 2,...,i 0 and i n is the amplitude of the current component, and φ n is a phase shift; and Back electromotive force V emf and the limited input dc voltage V dc The maximum rate of change of the current is calculated by taking into account the effect of [0025] a maximum current rate calculation module that calculates: [0030] where R is the coil resistance and ω n a maximum current rate calculation module, where x represents the shaft rotation angular velocity, θ denotes the electrical rotor position, and L(θ,i) denotes the nonlinear inductance of the SRM; The back electromotive force V emf and the limited input dc voltage V dc At least one current profile i cal a current profile calculation module that generates a current profile (θ); a torque generating module that determines an average torque of the SRM; a torque check module for checking whether the average torque satisfies an optimum torque condition; a radial force calculation module that calculates the radial force on one stator tooth if the average torque being calculated satisfies the optimal torque condition; a radial force harmonics calculation module for calculating radial force harmonics; If at least one current profile produces the average torque that satisfies the optimal torque condition and generates the radial force having a minimum amplitude at a desired harmonic of the current excitation to be reduced, then the at least one current profile is designated as an optimal current waveform I. cal and an optimal current waveform selection module that selects as the optimal current waveform an optimal current waveform that is optimal for the radial force harmonics acting on the stator teeth, the optimal current waveform being selected to select an optimal current waveform that is optimal for the radial force harmonics acting on the stator teeth.

2. The SR drive with acoustic noise reduction system of claim 1 , wherein the optimal current waveform is applied with turn-on and turn-off angles to improve system efficiency.

3. 2. The SR drive with acoustic noise reduction system of claim 1, wherein the optimal current waveform is adaptable to achieve a maximum reduction in the acoustic noise.

4. The SR drive having an acoustic noise reduction system as recited in claim 1 , wherein the optimum torque condition is met when the average torque achieves an optimum value.

5. 1. A method for reducing vibration and acoustic noise in a switched reluctance motor (SRM) utilizing an acoustic noise reduction system, comprising: a. providing the acoustic noise reduction system, the acoustic noise reduction system comprising: an SRM having at least one stator pole and at least one rotor pole, an SRM inverter connected to the SRM, and a processor having an acoustic noise reduction application installed; b. A plurality of reference currents i ref a reference current generating module in the processor, the reference current being generated using a Fourier series equation expressed as follows: [0045] In the formula, n=1, 2,...,i 0 and i n is the amplitude of the current component, and φ n is a phase shift; c. In the processor, a maximum current rate calculation module is used to calculate the back electromotive force V emf and the limited input dc voltage V dc The maximum rate of change of the current is calculated by taking into account the effect of [0050] Calculating The maximum rate of change of the current [006] where R is the coil resistance and ω n where x represents the shaft rotation angular velocity, θ denotes the electrical rotor position, and L(θ,i) denotes the nonlinear inductance of the SRM; d. In the processor, a current profile calculation module calculates the back electromotive force V emf and the limited input dc voltage V dc At least one current profile i cal (θ); e. calculating, in said processor, an average torque of said SRM by utilizing a torque generating module; f. enabling a torque checking module in said processor to check whether said average torque satisfies an optimum torque condition; g. calculating, by a radial force calculation module in said processor, said radial force at one stator tooth if said average torque being calculated satisfies said optimum torque condition; h. calculating radial force harmonics in said processor by a radial force harmonics calculation module; i. Steps b to h are set as parameter i 0 , i n , and φ n repeating for a number of combinations of j. If at least one current profile produces the average torque that satisfies the optimal torque condition and generates the radial force having a minimum amplitude at a desired harmonic of the current excitation to be reduced, then the at least one current profile is designated as an optimal current waveform I. cal and selecting as wherein the radial force harmonics acting on the stator teeth are proportional to vibration and acoustic noise at specific harmonics of current excitation, and by utilizing the optimal current waveform to reduce the amplitude of specific radial force harmonics, vibration and acoustic noise in the SRM are minimized.

6. 6. The method of claim 5, wherein the efficiency of the SR drive having the acoustic noise reduction system is improved by applying turn-on and turn-off angles within the optimal current waveform.

7. 6. The method of claim 5, wherein the SR drive having the acoustic noise reduction system further comprises a microphone positioned radially near the SRM for measuring acoustic noise generated within the SRM.

8. The method of claim 5 , wherein the SR drive having the acoustic noise reduction system further comprises a vibration sensor positioned on a surface of the SRM to detect and measure vibrations occurring within the SRM.

9. The method of claim 5 , wherein the optimal current waveform is adaptable to achieve a maximum reduction in the acoustic noise.

10. The method of claim 5 , wherein the optimum torque condition is satisfied when the average torque achieves an optimum value.

11. 1. A method for reducing vibration and acoustic noise in a switched reluctance motor (SRM) utilizing an SR drive having an acoustic noise reduction system, comprising: a. providing the acoustic noise reduction system, the acoustic noise reduction system comprising: an SRM having at least one stator pole and at least one rotor pole, an SRM inverter connected to the SRM, and a processor having an acoustic noise reduction application installed; b. A plurality of reference currents i ref a reference current generating module in the processor, the reference current being generated using a Fourier series equation expressed as follows: [0070] In the formula, n=1, 2,...,i 0 and i n is the amplitude of the current component, and φ n is a phase shift; c. In the processor, a maximum current rate calculation module is used to calculate the back electromotive force V emf and the limited input dc voltage V dc The maximum rate of change of the current is calculated by taking into account the effect of [0080] Calculating The maximum rate of change of the current [0097] where R is the coil resistance and ω n where x represents the shaft rotation angular velocity, θ denotes the electrical rotor position, and L(θ,i) denotes the nonlinear inductance of the SRM; d. In the processor, a current profile calculation module calculates the back electromotive force V emf and the limited input dc voltage V dc At least one current profile i cal (θ); e. calculating, in said processor, an average torque of said SRM by utilizing a torque generating module; f. enabling a torque checking module in said processor to check whether said average torque satisfies an optimum torque condition; g. calculating, in said processor, by a radial force calculation module, said radial force at one stator tooth if said average torque being calculated satisfies said optimum torque condition; h. calculating radial force harmonics in said processor by a radial force harmonics calculation module; i. Steps b to h are set as parameter i 0 , i n , and φ n repeating for a number of combinations of j. If at least one current profile produces the average torque that satisfies the optimal torque condition and generates the radial force having a minimum amplitude at a desired harmonic order of the current excitation, then the at least one current profile is designated as an optimal current waveform I. cal wherein the radial force harmonics acting on the stator teeth are proportional to vibration and acoustic noise at a particular harmonic of current excitation, and by utilizing the optimal current waveform to reduce the amplitude of a particular radial force harmonic, vibration and acoustic noise in the SRM are minimized; k) applying turn-on and turn-off angles within the optimal current waveform to improve the efficiency of the acoustic noise reduction system.

12. The method of claim 11 , wherein the efficiency of the acoustic noise reduction system is improved by applying the turn-on and turn-off angles within the optimal current waveform.

13. The method of claim 11 , wherein the optimal current waveform is adaptable to achieve a maximum reduction in the acoustic noise.

14. The method of claim 11 , wherein the optimum torque condition is satisfied when the average torque achieves an optimum value.

15. the acoustic noise harmonics include acoustic noise harmonic components, the radial force harmonics include radial force harmonic components, and the acoustic noise harmonic components decrease as the corresponding radial force harmonic components decrease; 12. The method of claim 11, wherein adjusting the current harmonics reduces the radial force harmonic components.

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