數(shù)控車床外文翻譯---數(shù)控車床實驗系統(tǒng)的主軸變頻調(diào)速系統(tǒng)設(shè)計分析

1、<p><b>　　畢業(yè)設(shè)計(論文)</b></p><p><b>　　外文文獻(xiàn)翻譯</b></p><p><b>　　信息工程學(xué)院</b></p><p><b>　　目 錄</b></p><p>　　1 Spindle variab

2、le speed system design and analysis forCNC lathe experiment System·························

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5、············1</p><p>　　2 數(shù)控車床實驗系統(tǒng)的主軸變頻調(diào)速系統(tǒng)設(shè)計分析················

6、3;·························6</p><p>　　Spindle variable speed system design and ana

7、lysis for</p><p>　　CNC lathe experiment System</p><p>　　ZHENG Yan-qin，TANG Xue-zhe</p><p>　　( School of Mechanical Engineering，Zhejiang University of Technology，Hangzhou 310014，Chin

8、a)</p><p>　　Abstract: Mainly for the demand of designing high-accuracy，fast-response CNC lathe spindle variable speed system，a speed control proposal of fitting frequency motor and transducer together was ra

9、ised via analyzing the basic principles and frequency of the variable frequency speed control．According to the CNC lathe experimental system operating conditions，the lathe cutting force and spindle motor power were calcu

10、lated，evidence for choosing spindle adjustable frequency motor of experiment system w</p><p>　　ideas for CNC lathe experimental platform were described．The frequency motor and transducer are reasonable，and i

11、ts adjustable performance and starting torque can meet the requirements of the experimental system．</p><p>　　Key words: variable frequency speed control; adjustable frequency motor; transducer; CNC lathe<

12、/p><p>　　Introduction</p><p>　　With the development of modern industrial technology, frequency conversion and speed regulation technology in mechanical processing has been widely applied. In the CN

13、C lathe spindle rotary precision of experimental system, determines the components the processing precision, frequency speed control of NC machine tool spindle speed, high efficiency, wide range, high precision, the spin

14、dle speed is more and more using variable frequency speed regulation. From the spindle speed on the overall system</p><p>　　the basic principle of frequency conversion</p><p>　　AC induction moto

15、r speed is: N0 = n ( 1 - s ) = 60F ( 1 - s ) / P ( 1)</p><p>　　Type : F - stator power supply frequency, s, slip, P - magnetic pole logarithm. By the formula (1 ) knowledge, change the frequency of the power

16、 supply F, motor speed will be adjusted, this is the variable frequency speed regulation. Converter with power semiconductor device on-off action, could be the power supply conversion to another frequency stepless speed

17、regulation of motor power, provide the required voltage, frequency, realizes the stepless speed regulation of asynchronous motor. Freque</p><p>　　2 CNC lathe spindle speed control experiment system of vari

18、able frequency motor selection</p><p>　　Due to the complexity of cutting process, the cutting force influencing factor is very much, therefore has not yet been derived and easy to calculate cutting force FX,

19、 FY, FZ theoretical formula, generally through a large number of experiments, the establishment of the experience formula to calculate cutting force. This study will back cutting depth, feed rate and cutting speed of 3 m

20、ain factors as variable factors, and other factors, correction coefficient</p><p>　　Indirect calculation, it can be FX, FY, FZ 3 component ( 6 ) empirical formula:</p><p>　　?????????????????? (

21、2) </p><p>　　Type : FZ - FY - cutting force; radial force; FX - feeding force; V - cutting speed, V = nd / ( 1000 x 60 ); N - workpiece speed; D - F - feed the workpiece diameter. When turning, FZ than FY

22、, FX is much greater, and FY, FX direction movement speed is small, therefore, FY, FX power consumption is very small (only 1% ~ 2% ), it can be ignored. Used in lathe cutting force calculation formula is as follows:<

23、/p><p>　　FZ = 9． 81CFz·axFzP ( 3)</p><p>　　According to the numerical control lathe experimental system operating conditions, to take: CFz = 40, xFz = 1, yFz = 0.75, apmax = 6 mm

24、, Fmax = 0.2 mm / R, n = 2500 mm / min, d = 30 mm. The cutting power of Pm = Fz, V x 10 - 3 = 704.13 x 3.93 x 10 - 3 = 2.765 kW. The study of machine tool transmission efficiency for M = 0.75, then the NC machine tool sp

25、indle motor power PE > Pm / M = 3.687 kW.</p><p>　　Fed by inverter asynchronous motor running condition produced fundamental change ( 7 ), when the motor runs, along with the frequency changer style press

26、ure control, mechanical characteristic curve can be arbitrary translation, which enable asynchronous motor performance is improved greatly. Therefore, CNC Experiment System of variable frequency motor selection, also sho

27、uld be considered : the lathe spindle motor and power characteristics matching problem; the frequency conversion motor and moto</p><p>　　Selection of experimental system of Xinxiang City special motor manufa

28、cturing company limited series of 4 YVP variable frequency motor rated power, rated torque of 5.5 kW, 30 N m, the rated current of 12 A, with similar frequency conversion device matching, interchangeability, general stro

29、ng, its power, foot mounting dimensions and center of Higher Correspondence index and Y series asynchronous motor is completely consistent, and experimental system</p><p>　　Entry requirements. The main techn

30、ical indexes for the three-phase voltage: 380 V / 50 Hz; variable frequency speed regulation range: 5 Hz ~ 100 Hz stepless speed regulation. The low speed torque smoothing, without creeping phenomenon; through the conver

31、sion device of voltage boosting, ensure the motor in 5 Hz rated output torque and the motor for heating and burning.</p><p>　　3 inverter type selection</p><p>　　n the CNC lathe spindle speed c

32、ontrol experimental system, inverter selection should pay attention to : 1 converter is to realize constant pressure control and constant current control; the inverter load performance curve; the frequency converter and

33、load matching problem of the rated voltage, current, frequency and load rated voltage, current match, for the special load needs to reference the motor performance parameters to determine the maximum current of inverter,

34、 current and overload capacit</p><p>　　In the CNC lathe in the experiment system, selection of inverter, inverter current rating is a key quantity. The 4 pole above the motor or motor connected in parallel,

35、must with total current does not exceed the rated current for the principle of frequency converter. CNC machine tool belongs to continuous operation, continuous operation to control the rated capacity and rated current s

36、hould satisfy the type:</p><p>　　?????????????????????( 4)</p><p>　　Type : PM - load required motor power output shaft; the ETA, the efficiency of the motor ( usually about 0.85 ); Cos - motor p

37、ower factor ( usually about 0.75 ); UM - motor voltage, V; IM: A; K motor current, current waveform correction coefficient ( PWM 1.05 ~ 1 ); PCN inverter rated capacity, kVA; ICN, the rated current of the inverter, A.<

38、;/p><p>　　Frequency converter and asynchronous motor is composed of different speed regulation system, the converter capacity calculation methods are also different. The above applies to single inverter for sin

39、gle motor supply and continuous operation, the three are unified, selection of frequency inverter capacity should meet 3 formula.</p><p>　　In the CNC lathe spindle speed control system in the experimental sy

40、stem, frequency converter is selected in accordance with the motor models identified, motor rated current of IM for 12 A, rated voltage 380 V, rated at 5.5 kW, select the converter capacity and rated current should satis

41、fy the type:</p><p>　　???????????????????????????????( 5)?</p><p>　　On the dynamic performance requirements of high CNC experiment system, the original use of DC transmission. At present, vector

42、 control inverter has been universal, for the requirements of high precision, fast response CNC experiment system, the use of vector control of high performance general-purpose inverter is a kind of very good plan ( 11 )

43、, and, in small capacity inverter with voltage type frequency converter, the system selects the Mitsubishi 7K-CHT series FR-A740-3. Frequency converter. I</p><p>　　This study selected variable frequency moto

44、r and inverter configuration is reasonable, and its adjustment performance and the starting torque can meet the system requirements.</p><p>　　4 CNC lathe experimental device</p><p>　　Numerical

45、 control experimental device is a small experiment machine, speed range is not big, do not need to shift gears. Spindle speed control experimental device wiring diagram as shown in Figure 1, the TA, TB, TC terminal conne

46、cted to an external control circuit, PA, PB terminal used to brake, SVC, 0V terminal used to change the frequency, COM, SCW, SCCW terminal control motor positive inversion, QF air switch, with isolated power supply, circ

47、uit breaker, too current, voltage and other protect</p><p>　　CNC Experiment Platform diagram as shown in figure 2.. </p><p>　　CNC lathe experimental system is the use of experimental control pan

48、el on the working table control, the panel can reflect each part of the operation condition. Failure in the panel show, make the system more intuitive, convenient for CNC lathe detection, repair and troubleshooting, and

49、other aspects of learning. The design of variable frequency speed regulation system of NC lathe experiment is an important part of the system, the experimental system in CNC lathe, spindle speed by frequency conv</p&g

50、t;<p>　　5 ending</p><p>　　Application of frequency control of motor speed range is very wide, in the following 50 Hz can be used as a constant torque operation, more than 50 Hz for constant power op

51、eration, is an ideal speed control mode. For NC lathe spindle speed control system of VVVF, because of its superior performance, high efficiency and energy saving starter, wide speed range, high precision, stepless speed

52、 regulation has been widely used, and gradually replace the DC motor speed control. AC frequency converter a</p><p>　　References</p><p>　　［1］ Ding Jinfu, Yu Fujin. The frequency conversion and s

53、peed regulation technology in CNC lathe application [ J ]. New technology and new process, 2005 ( 5): 13-15.</p><p>　?。?］ Zhou Hong, Zhou Wenjun. Numerical control lathe spindle speed realization [ J ]. Mach

54、ine tool & hydraulics, 2007, 35 ( 1): 60-62.</p><p>　?。?］ Zhu Quanyin, Han Laiji, Fu Yunqiang, et al. Numerical control machine tool spindle speed control system theory and application [ J ]. Coal mine m

55、achinery, 2009, 30 ( 9): 194-196.</p><p>　　［4］ Chen Hequan. Frequency basic method ( J ). Industrial automation, 2010 ( 2): 122-125.</p><p>　?。?］ Huang Guanyao, Liu Baohe. Mechanical manufacturi

56、ng technology base [ M ]. Tiianjin: Tianjin University press, 1999</p><p>　　［6］ Helped Yu Peng, Huang Shou. The three-phase induction motor variable frequency speed regulating effect ( J ). The explosion-pro

57、of motor, 2000 ( 3): 6-9.</p><p>　　數(shù)控車床實驗系統(tǒng)的主軸變頻調(diào)速系統(tǒng)設(shè)計分析</p><p><b>　　鄭艷琴，唐學(xué)哲</b></p><p>　　( 浙江工業(yè)大學(xué)機械工程學(xué)院，浙江杭州310014)</p><p>　　摘要:主要針對要求高精度、快響應(yīng)的數(shù)控車床主軸調(diào)速系統(tǒng)的設(shè)計

58、要求，通過對變頻調(diào)速的基本原理和變頻方式的分析，提出了</p><p>　　變頻電機與變頻器組合的調(diào)速方案。根據(jù)數(shù)控車床實驗系統(tǒng)的運行條件，計算車床的切削力和主軸電機的功率，為選擇實驗系統(tǒng)</p><p>　　的主軸變頻調(diào)速電機提供依據(jù)。根據(jù)選定的變頻電機的特性，提出了變頻器的選擇原則，進(jìn)而選擇了額定容量和額定電流均與變</p><p>　　頻電機匹配的，且采用矢量

59、控制高性能型通用電壓型變頻器。最后，簡要介紹了數(shù)控車床實驗平臺的設(shè)計思路。研究結(jié)果表明，選</p><p>　　擇的變頻電機與變頻器配置合理且其調(diào)整性能和啟動轉(zhuǎn)矩均能滿足實驗系統(tǒng)的要求。</p><p>　　關(guān)鍵詞:變頻調(diào)速; 變頻電機; 變頻器; 數(shù)控車床</p><p><b>　　0 引言</b></p><p>

60、　　隨著現(xiàn)代工業(yè)技術(shù)的發(fā)展，變頻調(diào)速技術(shù)在機械加工中已得到廣泛的應(yīng)用。在數(shù)控車床實驗系統(tǒng)中，主軸旋轉(zhuǎn)精度決定了零件的加工精度，變頻調(diào)速調(diào)節(jié)數(shù)控機床主軸的轉(zhuǎn)速，具有高效率、寬范圍、高精度的特點，主軸調(diào)速越來越多地采用變頻調(diào)速。從主軸調(diào)速系統(tǒng)的總體性能指標(biāo)出發(fā)，如何正確選擇電機和變頻器，求得電機與變頻器的最佳配合，是主軸變頻調(diào)速系統(tǒng)設(shè)計的關(guān)鍵。本研究主要探討數(shù)控車床實驗系統(tǒng)的主軸變頻調(diào)速系統(tǒng)的設(shè)計。</p><p>

61、　　1 變頻調(diào)速的基本原理</p><p>　　交流感應(yīng)電動機的轉(zhuǎn)速為:n0 = n( 1 － s) = 60f( 1 － s) /p ( 1)</p><p>　　式中: f—定子電源頻率，s—轉(zhuǎn)差率，p—磁場極對數(shù)。由式( 1) 知，改變電源的頻率f，電動機轉(zhuǎn)子轉(zhuǎn)速就隨之得到調(diào)節(jié)，這就是變頻調(diào)速。變頻器利用電力半導(dǎo)體器件的通斷作用，可以將工頻電源變換為另一頻率的電源，提供電動機無級調(diào)速

62、所需要的電壓、頻率，實現(xiàn)異步電動機的平滑調(diào)速。變頻器實現(xiàn)變頻的方式主要有交流-交流變頻和交流-直流-交流變頻( 直接轉(zhuǎn)矩控制變頻或矢量控制變頻) 。交流-交流變頻就是改變晶閘管導(dǎo)通控制角α的大小和變化的頻率，實現(xiàn)交流-交流的電壓和頻率的調(diào)整。利用晶閘管的導(dǎo)通性能，本研究先把工頻交流電源轉(zhuǎn)換成直流電源，再利用由晶閘管構(gòu)成的逆變器主電路，輸出三相頻率和電壓均可調(diào)整、等效于正弦波的脈寬調(diào)制波( SPWM 波) ，實現(xiàn)交流-直流-交流變頻，拖動

63、三相異步電機調(diào)速運轉(zhuǎn)［4-5］。</p><p>　　2 數(shù)控車床實驗系統(tǒng)主軸調(diào)速的變頻電機選擇</p><p>　　由于切削過程的復(fù)雜性，影響切削力的因素非常多，因此目前尚未導(dǎo)出簡便的計算切削力FX，F(xiàn)Y，F(xiàn)Z的理論公式，一般都是通過大量實驗，建立經(jīng)驗公式來計算切削力。本研究將背吃刀量、進(jìn)給量及切削速度3 個主要因素作為可變因素，而其他影響因素則用修正系數(shù)</p><

64、p>　　間接計算，可以得出FX，F(xiàn)Y，F(xiàn)Z 3 個分力的經(jīng)驗公式［6］:</p><p>　　? ( 2) </p><p>　　式中: FZ—切削力; FY—徑向力; FX—進(jìn)給力; V—切削速度，V = πnd

65、/( 1 000 × 60) ; n—工件轉(zhuǎn)速; d—工件直徑; f—進(jìn)給量。車削時，F(xiàn)Z比FY，F(xiàn)X大得多，而且FY，F(xiàn)X方向上的運動速度又很小，因此，F(xiàn)Y，F(xiàn)X所消耗的功率很小( 僅為1% ～ 2%) ，故可忽略不計。適用于車床的切削力的計算公式如下:</p><p>　　FZ = 9． 81CFz·axFzP ( 3)</p><p>　　根據(jù)

66、數(shù)控車床實驗系統(tǒng)的運行條件，取: CFz = 40，xFz = 1． 0，yFz = 0． 75，apmax = 6 mm，fmax = 0． 2 mm/r，n = 2 500 mm/min，d = 30 mm。則切削功率Pm = Fz·V × 10 － 3 = 704． 13 × 3． 93 × 10 － 3 = 2． 765 kW。本研究取機床的傳動效率ηm = 0． 75，則數(shù)控機床主軸電機

67、功率PE≥Pm /ηm = 3． 687 kW。</p><p>　　變頻器供電下的異步電機運行條件發(fā)生了根本的變化［7］，電機運行時，隨著變頻器的調(diào)調(diào)壓控制，機械特性曲線可以任意平移，這使異步電機在性能上大大改善。所以，數(shù)控車床實驗系統(tǒng)變頻電機的選擇，還應(yīng)考慮: ①電機與車床主軸功率特性匹配問題; ②變頻電機與普通電機機座號盡量相同; ③強力切削和加工材料的不均勻性，主電機功率應(yīng)有一定的儲備［8-9］。<

68、/p><p>　　該實驗系統(tǒng)選用新鄉(xiāng)市特種電機制造有限公司的YVP 系列4 級變頻電機，額定功為5． 5 kW，額定轉(zhuǎn)矩30 N·m，額定電流為12 A，可與同類變頻裝置配套，互換性、通用強，其功率、底腳安裝尺寸和中心高等對應(yīng)指標(biāo)與Y 系列異步電機完全一致，與實驗系統(tǒng)環(huán)</p><p>　　境要求相符。主要技術(shù)指標(biāo)為電壓: 三相380 V/50 Hz; 變頻調(diào)速范圍: 5 Hz ～

69、100 Hz 無級調(diào)速。低速時轉(zhuǎn)矩平滑，無爬行現(xiàn)象; 能通過變頻裝置的電壓提升，保證電動機在5 Hz 時輸出額定轉(zhuǎn)矩而不致使電機因發(fā)熱而燒毀。</p><p>　　3 變頻器類型的選擇</p><p>　　在數(shù)控車床實驗系統(tǒng)中，主軸調(diào)速變頻器的選型應(yīng)注意: ①變頻器是實現(xiàn)恒壓控制還是恒流控制; ②變頻器的負(fù)載性能曲線; ③變頻器與負(fù)載的匹配問題，變頻器的額定電壓、電流應(yīng)與負(fù)載的額定電壓、電

70、流相符，對于特殊的負(fù)載則需要參考電機性能參數(shù)，以最大電流確定變頻器電流和過載能力，在恒轉(zhuǎn)矩負(fù)載或有減速裝置時還應(yīng)注意轉(zhuǎn)矩匹配; ④考慮到高次諧波會導(dǎo)致輸出電流增大，變頻器驅(qū)動電機，特別是電抗小的高速電機，其容量應(yīng)略大于普通電機的選型; ⑤電纜、電機對地耦合電容的存在，也會引起電流增大，變頻器的容量應(yīng)適當(dāng)提高; ⑥采用PWM 控制逆變器，整流器不可控的變頻器，功率因數(shù)相對較低、網(wǎng)側(cè)諧波污染嚴(yán)重、無法實現(xiàn)能量的再生利用，在要求較高的場合，整

71、流和逆變過程都采用PWM 控制的雙PWM 變頻器［10］。</p><p>　　在數(shù)控車床實驗系統(tǒng)中，選擇變頻器時，變頻器的額定電流是一個關(guān)鍵量。對于采用4 極以上電動機或者多電動機并聯(lián)時，必須以總電流不超過變頻器的額定電流為原則。數(shù)控機床屬于連續(xù)運轉(zhuǎn)，連續(xù)運轉(zhuǎn)時所需的變頻器額定容量和額定電流應(yīng)滿足下式:</p><p>　　??????????????????????( 4)?</

72、p><p>　　式中: PM—負(fù)載所要求的電動機的軸輸出功率; η—電動機的效率( 通常約0． 85) ; cos —電動機的功率因數(shù)( 通常約0． 75) ; UM—電動機電壓，V; IM—電動機電流，A; k—電流波形的修正系數(shù)( PWM 方式取1． 05 ～1． 0) ; PCN—變頻器的額定容量，kVA; ICN—變頻器的額定電流，A。</p><p>　　變頻器與異步電動機組成不同的

73、調(diào)速系統(tǒng)時，變頻器容量的計算方法也不同。上述適用于單臺變頻器為單臺電動機供電連續(xù)運行的情況，三者是統(tǒng)一的，選擇變頻器容量時應(yīng)同時滿足3 個算式。</p><p>　　在數(shù)控車床實驗系統(tǒng)中，主軸調(diào)速系統(tǒng)變頻器是按照選定的電動機型號確定的，電動機的額定電流IM為12 A，額定電壓380 V，額定功率為5． 5 kW，選擇的變頻器容量和額定電流應(yīng)滿足下式:</p><p>　　?????????

74、???????????????????????( 5)</p><p>　　對動態(tài)性能要求較高的數(shù)控車床實驗系統(tǒng)，原來多采用直流傳動方式。目前，矢量控制型變頻器已經(jīng)通用化，對于要求高精度、快響應(yīng)的數(shù)控車床的實驗系統(tǒng)，采用矢量控制高性能型通用變頻器是一種很好的方案［11］，而且中、小容量變頻器以電壓型變頻器為主，此實驗系統(tǒng)選用三菱FR-A740-3． 7K-CHT 系列變頻器。其基本參數(shù): 額定電流為12． 6 A

75、，額定容量為9 kVA，允許電動機最大功率為5． 5 kW，額定過載電流為150%，額定輸出電壓為3 相380 V ～ 480 V，電壓允許波動為±5%，需強制風(fēng)冷。</p><p>　　本研究選擇的變頻電機與變頻器配置合理，且其調(diào)整性能和啟動轉(zhuǎn)矩均能滿足系統(tǒng)要求。</p><p>　　4 數(shù)控車床實驗裝置</p><p>　　數(shù)控實驗裝置屬于小型實驗機床

76、，調(diào)速范圍不大，不需要換擋。主軸調(diào)速的實驗裝置接線圖如圖1 所示，TA、TB、TC 端子接外部控制電路，PA、PB 端子用來制動，SVC、0V 端子用來改變頻率，COM、SCW、SCCW端子用控制電動機的正反轉(zhuǎn)，QF 是空氣開關(guān)，有隔離電源、斷路、過電流、欠電壓等保護(hù)作用。</p><p>　　數(shù)控車床實驗平臺示意圖如圖2 所示。</p><p>　　數(shù)控車床的實驗系統(tǒng)是利用實驗控制面板對

77、工作臺進(jìn)行控制，在面板上能體現(xiàn)各部分的操作情況。出現(xiàn)的故障能在面板上體現(xiàn)出來，使系統(tǒng)更加直觀，便于數(shù)控車床的檢測、維修及故障排除等方面的學(xué)習(xí)。主軸變頻調(diào)速系統(tǒng)設(shè)計是數(shù)控車床實驗系統(tǒng)的一個重要部分，在數(shù)控車床實驗系統(tǒng)中，主軸調(diào)速采用變頻電機與變頻器配合使用的變頻調(diào)速，較電磁調(diào)速電機結(jié)構(gòu)更加簡單，維修方便，且效率高、節(jié)能效果顯著; 調(diào)速平滑，范圍廣，能在5 Hz ～ 100 Hz 范圍內(nèi)無級調(diào)速; 起動電流小，不用附加起動設(shè)備，低頻起動時力

78、矩對負(fù)載沖擊小; 體積小、重量輕、安裝尺寸和Y 系列相同，特別適合在數(shù)控車床實驗系統(tǒng)中使用。</p><p><b>　　5 結(jié)束語</b></p><p>　　變頻調(diào)速的應(yīng)用范圍非常廣泛，在50 Hz 以下可作恒轉(zhuǎn)矩運行，在50 Hz 以上可作恒功率運行，是一種理想的調(diào)速控制方式。用于數(shù)控車床主軸調(diào)速控制系統(tǒng)的變頻調(diào)速，其因起動性能優(yōu)越、高效節(jié)能、調(diào)速范圍大、精度高

79、、無級調(diào)速等優(yōu)點而得到廣泛的應(yīng)用，并逐步取代直流調(diào)速。交流變頻器和變頻電機組成的變頻調(diào)速系統(tǒng)，應(yīng)用在數(shù)控車床的實驗系統(tǒng)中，更便于檢測、維修以及故障排除等方面的學(xué)習(xí)，具有很高的實用價值。</p><p><b>　　參考文獻(xiàn)</b></p><p>　　［1］丁金福，虞付進(jìn)． 變頻調(diào)速技術(shù)在數(shù)控車床中的應(yīng)用［J］．新技術(shù)新工藝， 2005( 5) : 13-15．<

80、;/p><p>　　［2］ 周虹，周文軍． 數(shù)控車床的主軸變頻調(diào)速實現(xiàn)［J］． 機床與液壓， 2007， 35( 1) : 60-62．</p><p>　　［3］朱全印，韓來吉，付云強，等． 數(shù)控機床主軸變頻調(diào)速控制系統(tǒng)原理及應(yīng)用［J］． 煤礦機械， 2009， 30( 9) : 194-196．</p><p>　?。?］ 陳和權(quán)． 變頻調(diào)速的基本方法［J］． 工礦