1277286 (1) 玖、發明說明 【發明所屬之技術領域】 本發明是關於一種特別是作爲纖維機械等在開動時， 需要極大的驅動轉矩的產業機械的動力源的感應電動機的 控制方法。 【先前技術】 習知，作爲此種感應電動機的控制方法，有首先，將 感應電動機連接於商用電源使之開動，在該電動機達到大 致穩定轉速之後，從商用電源斷開該感應電動機，替代連 接於可任意地設定頻率的可變電壓，可變頻率反流器的輸 出者。[參照例如日本實開昭6 1 -90877號公報（第3〜8頁， 第1圖）] 然而，對於使用依上述公報的感應電動機的控制方法 等的問題點，依據表示於習知的感應電動機的方法的電路 構成，如下地說明。 亦即，第3圖是表示此種感應電動機的控制方法的習 知例的電路構成圖；1是商用電源’ 2是反流器主電路2 1與 反流器控制電路22所構成的可變電壓可變頻率（VVVF ) 反流器，3是電磁接觸器MCI、MC2所構成的切換開關電 路，4是感應電動機，5是在感應電動機4被驅動的纖維機 械等的負荷，6是A C電f几器’ 7是順序控制電路。 該VVVF反流器2是藉由反流器控制電路22的控制動作 ，能輸出與第1電流相位同步的相位的交流電壓’以下’ -4- (2) 1277286 說明表示於第3圖的電路構成的感應電動機4的控制方法。 首先，在確立商用電源1的電壓的狀態下，對於電磁 接觸器MC 1、MC2均在斷開狀態的切換開關電路3，從順 序控制電路7發出作爲切換信號的對於MC 1的投入指令， 當電磁接觸器MCI 1導通時，則從商業電源1開始供電的感 應電動機4與負荷5 —起開動，不久，以對應於商用電源1 的頻率的轉速，使得感應電動機4旋轉而完成開動。 利用對於該MCI的投入指令，也開動VVVF反流器2， 該VVVF反流器2是從容許最低電壓，容許最低頻率的輸出 電壓迅速地增大至商用電源1的額定電壓，額定頻率的輸 出電壓。 之後，在反流器控制電路22監視商用電源1的任一相 的電壓位相，及對應於VVVF反流器2的輸出電壓的上述商 用電源1的相約電壓相位，在兩者如數周期一致的時候， 將同步完成信號送出至順序控制電路7。 然後，藉由以對應於商用電源1的頻率的轉速使得感 應電動機4完成開動的時刻以後的同步完成信號’順序控 制電路7是藉由發出對於作爲切換信號的MC 1的斷開指令 ，電磁接觸器MCI是斷開，在若干空檔時間（例如約l〇ms 至20ms)之後，藉由發出對於作切換信號的MC2的導通指 令，電磁接觸器MC2是導通’使得對於感應電動機4的供 電從商用電源1切換成VVVF反流器2，惟此時’藉由設在 從VVVF反流器2至感應電動機4的路徑的AC電抗器6，抑 制從V V V F反流器2至感應電動機4的突入電流。 (3) 1277286 然後，VVVF反流器2是將VVVF2輸出電壓徐徐地變 更成感應電動機4所要求的電壓’頻率的交流電壓，使負 荷5進入運轉狀態。 在表示於第3圖的習知電路構成中，作爲VVVF反流器 2是以二極體整流器與平流電容器將商用電源1的交流電壓 變換成直流電壓’橋接將該直流電壓變換成交流電壓的電 晶體與二極管的逆並聯電路所成的反流器主電路2 1，及將 上述電晶體藉由如PWM控制成爲導通，斷開而從反流器主 電路2 1輸出所期望的電壓，頻率的交流電壓所需的控制功 能，及輸出與商用電源1的電壓相位同步的相位的交流電 壓的控制功能須具備於反流器控制電路22，結果，該 VVVF反流器2是與僅輸出所期望的電壓，頻率的交流電壓 的通用的VVVF反流器相比較’其電路構成較複雜。又， 需要切換開關電路3，AC電抗器6，順序控制電路7等，表 示於第3的電路構成整體也複雜。 又，藉由AC電抗器6抑制從VVVF反流器2至感應電動 機4的突入電流，惟此時，感應電動機4的轉速會變動，而 有該變動對於負荷5不理想的情形。 又，藉由來自商用電源1的供電來開動上述的在纖維 機械等開動時需要極大的驅動轉矩的感應電動機4之故， 因而可減輕VVVF反流器2的動作務，惟有開動完成感應 電動機4後的轉速依存於商用電源1的頻率的困難處，結果 ，在感應電動機4所要求的輸出頻率與商用電源1的頻率不 相同時，則開始依VVVF反流器2的供電，最後須成爲作成 (4) 1277286 感應電動機4所要求的頻率的交流電壓的動作，而有負荷：5 成爲運轉狀態費時的情形。 【發明內容】 本發明的目的，是在於提供一種解決上述各種問題點 的感應電動機的控制方法。 本發明的感應電動機的控制方法，其特徵爲：首先， 從輸出容許最低電壓，容許最低頻率的交流電壓的可變電 壓可變頻率反流器開始供電至一次繞組爲角接的感應電動 機而開動該電動機；然後，將供電於該開動的上述感應電 動機的上述可變電壓可變頻率反流器的輸出電流作成事先 設定的上限値以下，而以所定增加率徐徐地增大該反流器 的輸出電壓與輸出頻率；當該輸出頻率達到事先設定的頻 率設定値後，供電於上述感應電動機的上述可變電壓可變 頻率反流器是保持在上述頻率設定値之下’僅將該反流器 的輸出電壓以所定減少率徐徐地降低至事先設定的電壓設 定値；然後，上述可變電壓可變頻率反流器是將依據上述 電壓設定値，頻率設定値的交流電壓供給於上述感應電動 機。 本發明是注重在一次繞組爲角接的感應電動機中’與 一次繞組爲星接的感應電動機相比較，可更增大（具體爲 約3倍）對於從可變電壓可變頻率（VVVF )反流器所供給 的交流電壓的該感應電動機所輸出的驅動轉動’又’注重 在利用具備從VVVF反流器的輸出電流限制功能’而將在 (5) 1277286 纖維機械等開動時需要極大驅動轉矩的負荷加以驅動的上 述感應電動機快速地起動’並予以加速的動作’及在完成 該開動，加速之後’以較小驅動轉矩可運轉該感應電動機 ，藉由通用性的上述VVVF反流器’能連續地進行僅降低 該WVF反流器的輸出電壓的動作。 【實施方式】 第1圖是表示本發明的感應電動機的控制方法的實施 形態的電路構成圖；在與表示於第3圖的習知構成具有相 同功能者賦予相同記號。 亦即，在表示於第1的電路構成中，除了商用電源1 ’ 負荷5之外，是由以二極體整流器與平流電容器將商用電 源1的交流電壓變換成直流電壓，橋接將該交流電壓變換 成交流電壓的電晶體與二極體的逆並聯電路所成的反流器 主電路8 1，及檢測該反流器主電路8 1的輸出電流的分路電 阻等的電流檢測器8 1 a，及具備如以PWM控制導通，斷開 上述電晶體，而從反流器主電路8 1輸出所期望的電壓，頻 率的交流電壓所需的控制功能，及在電流檢測器8 1 a的檢 測値超過事先設定的上限値（例如爲反流器主電路8 1的額 定輸出電流値的約1 5 0 % )時，爲了作成該上限値以下的 輸出電流而節流反流器主電路8 1的輸出電壓的電流限制功 能的反流器控制電路82成的VVVF反流器8，及一次繞組爲 角接的感應電動機9，及電磁接觸器10，及電磁接觸器1〇 的打開電器1 1所構成。 -8- (6) 1277286 將表示於第1圖的電路構成動作參照表示於第2圖的波 形圖，說明如下。 首先，電磁接觸器1〇成爲斷開，在確立商用電源1的 電壓的狀態下，運轉VVVF反流器8，利用反相控制電路82 的控制動作，作成輸出VVVF反流器8事先設定的電壓，頻 率（例如，容許最低電壓，容許最低頻率）的交流電壓的 狀態。 在之後的時刻U，藉由來自打開電路1 1的打開指令導 通電磁接觸器1〇，使得感應電動機9與負荷5運轉，感應電 動機9是開始以低速旋轉，同時藉由依據上述打開指令的 反流器控制電路82的控制動作，VVVF反流器8是從上述容 許最低電壓，容許最低頻率的交流電壓，如第2圖所示地 開始輸出直線徐徐地增大電壓，頻率的交流電壓。 將徐徐地增大該電壓，頻率的期間中的反流器主電路 8 1的輸出電流，經由電流檢測器8 1 a來監視反流器控制電 路82，藉由該監視，在上述輸出電流超過事先設定的上限 値時，能使該輸出電源成爲上述上限値以下地，藉由反流 器控制電路82的控制動作來抑制反流器主電路8 1的輸出電 壓的振幅增大，而該VVVF反流器8是如第2圖所示地繼續 輸出徐徐地增大電壓，頻率的交流電壓，感應電動機9是 繼續加速。 在時刻“中，增大VVVF反流器8的輸出頻率已達到事 先設定的頻率設定値，一直到該時刻U爲止成爲終了感應 電動機9的加速動作的狀態。因此，如第2圖所示地， -9- (7) 1277286 VVVF反流器8的輸出電流也超過上述上限値的狀態，而在 纖維機械等負荷5運轉，加速時需要極大的驅動轉矩時的 感應電動機9，是在完成該加速後即以小驅動轉矩就足夠 ，因此VVVF反流器8的輸出電流也成爲減少的狀態。 自上述時刻q稍經過時間的時刻t2，藉由反流器控制 電路82的控制動作，VVVF反流器8的輸出頻率是維持上述 頻率設定値之狀態下，如第2圖所示地一直到時刻t3爲止 ，僅將VVVF反流器8的輸出電壓，開始輸出大約直線徐徐 地減少的交流電壓。在這裏，開始減少VVVF反流器8的輸 出電壓的時刻t2，是該反流器的輸出電流減少成事先設定 値的情形，反流器控制電路82經由電流檢測器81a的檢測 値加以確認的時刻，或是經由事先實驗求出時刻“至時刻 t2爲止的時間，利用未圖示的定時器測出達到時刻t2相當 的時刻也可以。亦即，該時刻是時刻h以後就可以。 在上述時刻t2至t3的期間中，在纖維機械等感應電動 機9的驅動轉矩較少就可以的時候，VVVF反流器8是將該 輸出頻率維持在頻率設定値之下，藉由僅將該感應電動機 8的輸出電壓的振幅減少至電壓設定値[例如，時刻h的輸 出電壓的{ 1 /( 3的平方根）}倍，或可得到負荷5所需要的 轉矩的輸出電壓値]，對於時刻U的上述輸出電壓時的上述 輸出電流，時刻13的上述輸出電壓時的上述輸出電流，是 改善了感應電動機9的效率及功率，因此較少就可以。亦 即，在一次繞組爲角接的感應電動機與星接的感應電動機 相比較，驅動轉矩成爲大約3倍之故，因而時刻t3以後的 -10- (8) 1277286 輸出電壓是可作成時刻“的輸出電壓的大 根）]倍。所以，時刻t3以後的感應電動機 源運轉狀態。 由上述的動作說明可知，一次繞組爲角 機9的連續額定値是由上述的時刻t2以後的 或是感應電動機9的運轉，加速時的驅動轉 輸出頻率（V/F )比是可輸出作爲短時間額） 依照本發明的控制方法，自運轉感應電 反流器進行供電壓感應電動機之故，因而運 流器的輸出頻率直接移行至對應於所期望的 故，因此具有可縮短自運轉感應電動機至所 度的移行時間的優點。 (發明之效果） 依照本發明，藉由僅來自.通用的可變電 VVVF )反流器的供電，將一次繞組爲角接 ，與一次繞組爲星接的感應電動機相比較， 一次電流進行運轉、加速，又，在完成該運 在纖維機械等可用較小驅動轉矩運轉該感應 因而僅降低VVVF反流器的輸出電壓，就可 動機的運轉效率與功率之故，因而可響應於 源的要求。 【圖式簡單說明】 ί [1/ ( 3的平方 >，是成爲省能 接的感應電動 數値才設定， 矩及輸出電壓/ 定値就可以。 :動機時就經由 轉後，可將反 速度的頻率之 期望的運轉速 壓可變頻率（ 的感應電動機 成爲以更少的 轉、加速後， 電動機之故， 改善該感應電 近年來的省能 -11 - (9) 1277286 第1圖是表示本發明的實施形態的電路構成圖 第2圖是表示說明第1圖的動作的波形圖。 第3圖是表示習知例的電路構成圖。 【符號說明】 1 商用電源 2、8 VVVF反流器 3 切換開關電路 4、9 感應電動機 5 負荷 6 AC電抗器 7 順序控制電路 10 電磁接觸器 11 打開電路 21、81 反流器主電路 22 > 82 反流器控制電路 8 1a 電流檢測器 -12-1277286 (1) Technical Field of the Invention The present invention relates to a control method of an induction motor which is a power source of an industrial machine which requires a large driving torque when the fiber machine or the like is started. [Prior Art] As a control method of such an induction motor, first, an induction motor is connected to a commercial power source to be activated, and after the motor reaches a substantially stable rotation speed, the induction motor is disconnected from a commercial power source instead of the connection. The variable voltage can be arbitrarily set to the output of the variable frequency inverter. [Reference, for example, Japanese Laid-Open Patent Publication No. SHO 61-90877 (pages 3 to 8, first figure)] However, the problem of using the control method of the induction motor according to the above publication is based on the conventional induction. The circuit configuration of the method of the motor will be described below. That is, Fig. 3 is a circuit configuration diagram showing a conventional example of the control method of the induction motor; 1 is a commercial power supply '2 is a variable constituted by the inverter main circuit 2 1 and the inverter control circuit 22 Voltage variable frequency (VVVF) Inverter, 3 is a switching switch circuit composed of electromagnetic contactors MCI, MC2, 4 is an induction motor, 5 is a load of a fiber machine driven by the induction motor 4, and 6 is an AC power. f Several '7 is the sequence control circuit. The VVVF inverter 2 is an AC voltage that can output a phase synchronized with the phase of the first current 'below' by the control operation of the inverter control circuit 22. -4- (2) 1277286 The circuit shown in FIG. A method of controlling the constructed induction motor 4. First, in the state where the voltage of the commercial power source 1 is established, the switching switch circuit 3 in which the electromagnetic contactors MC1 and MC2 are both in the off state issues an input command to the MC 1 as a switching signal from the sequence control circuit 7. When the electromagnetic contactor MCI 1 is turned on, the induction motor 4 that is powered from the commercial power source 1 starts up with the load 5, and soon, the induction motor 4 is rotated to complete the actuation at a rotational speed corresponding to the frequency of the commercial power source 1. The VVVF inverter 2 is also activated by the input command for the MCI, and the VVVF inverter 2 is rapidly increased from the allowable minimum voltage, the output voltage of the lowest allowable frequency to the rated voltage of the commercial power source 1, and the output of the rated frequency. Voltage. Thereafter, the inverter control circuit 22 monitors the voltage phase of any phase of the commercial power source 1 and the phase voltage phase of the commercial power source 1 corresponding to the output voltage of the VVVF inverter 2, when the two cycles coincide. The synchronization completion signal is sent to the sequence control circuit 7. Then, the synchronization completion signal after the timing when the induction motor 4 is started to be started by the rotation speed corresponding to the frequency of the commercial power source 1 'the sequence control circuit 7 is the electromagnetic contact by issuing the OFF command for the MC 1 as the switching signal. The MCI is off, and after a few neutral time (eg, about 10 〇 to 20 ms), the electromagnetic contactor MC2 is turned on by issuing a turn-on command for the MC2 for the switching signal so that the power supply to the induction motor 4 is The commercial power source 1 is switched to the VVVF inverter 2, but at this time, the inrush from the VVVF inverter 2 to the induction motor 4 is suppressed by the AC reactor 6 provided in the path from the VVVF inverter 2 to the induction motor 4. Current. (3) 1277286 Then, the VVVF inverter 2 is an alternating voltage that gradually changes the VVVF2 output voltage to the voltage 'frequency required by the induction motor 4, and causes the load 5 to enter the operating state. In the conventional circuit configuration shown in FIG. 3, the VVVF inverter 2 converts the AC voltage of the commercial power source 1 into a DC voltage by a diode rectifier and a smoothing capacitor, and bridges the DC voltage into an AC voltage. a inverter main circuit 2 1 formed by an inverse parallel circuit of a transistor and a diode, and the transistor is turned on by a PWM control, and is turned off to output a desired voltage from the inverter main circuit 2 1 . The control function required for the AC voltage and the control function of the AC voltage outputting the phase synchronized with the voltage phase of the commercial power source 1 are required to be provided in the inverter control circuit 22, and as a result, the VVVF inverter 2 is output only The expected voltage, frequency of the AC voltage of the universal VVVF inverter compared to 'its circuit composition is more complicated. Further, it is necessary to switch the switching circuit 3, the AC reactor 6, the sequence control circuit 7, and the like, and the entire circuit configuration shown in the third is complicated. Further, the AC reactor 6 suppresses the inrush current from the VVVF inverter 2 to the induction motor 4. However, at this time, the number of revolutions of the induction motor 4 fluctuates, and the fluctuation is not ideal for the load 5. Further, by the power supply from the commercial power source 1, the above-described induction motor 4 which requires a large driving torque when the fiber machine or the like is started is activated, so that the operation of the VVVF inverter 2 can be reduced, and only the induction motor can be started. The rotational speed after 4 depends on the difficulty of the frequency of the commercial power source 1. As a result, when the output frequency required by the induction motor 4 is different from the frequency of the commercial power source 1, the power supply according to the VVVF inverter 2 is started, and finally (4) 1277286 The operation of the AC voltage at the frequency required by the induction motor 4, and the load: 5 is a situation in which the operation state takes time. SUMMARY OF THE INVENTION An object of the present invention is to provide a control method of an induction motor that solves the above various problems. A method of controlling an induction motor according to the present invention is characterized in that first, a variable voltage variable frequency inverter that allows an AC voltage of a lowest frequency is outputted from an output of a permissible minimum voltage, and the power supply to the induction motor in which the primary winding is a corner is started. The motor; then, the output current of the variable voltage variable frequency inverter supplied to the driven motor is set to a predetermined upper limit 値 or less, and the reflux is gradually increased at a predetermined increase rate. Output voltage and output frequency; when the output frequency reaches a preset frequency setting ,, the variable voltage variable frequency inverter supplied to the induction motor is kept under the above frequency setting ' 'only the reverse flow The output voltage of the device is gradually reduced to a preset voltage setting 以 at a predetermined reduction rate; then, the variable voltage variable frequency inverter is supplied with the AC voltage according to the voltage setting 値 and the frequency setting 値 to the induction motor. . The present invention is directed to an induction motor in which the primary winding is angularly connected, which can be increased (specifically about 3 times) in comparison with an induction motor in which the primary winding is star-connected, for the variable voltage variable (VVVF). The drive rotation output of the induction motor supplied by the inverter is focused on the use of the output current limiting function from the VVVF inverter, which requires a large drive when the (5) 1277286 fiber machine or the like is started. The above-mentioned induction motor driven by the moment load rapidly starts 'and accelerates the action' and after completing the start and acceleration, the induction motor can be operated with a small driving torque, by the versatile VVVF inverter described above 'The operation of reducing only the output voltage of the WVF inverter can be continuously performed. [Embodiment] Fig. 1 is a circuit configuration diagram showing an embodiment of a method for controlling an induction motor according to the present invention, and the same reference numerals are given to those having the same functions as those of the conventional configuration shown in Fig. 3. That is, in the first circuit configuration, in addition to the commercial power supply 1' load 5, the AC voltage of the commercial power source 1 is converted into a DC voltage by a diode rectifier and a smoothing capacitor, and the AC voltage is bridged. A current transformer 8 1 formed by an inverse parallel circuit of a transistor and a diode which is converted into an AC voltage, and a current detector 8 1 which detects a shunt resistance of an output current of the inverter main circuit 8 1 a, and having a control function required to output the desired voltage, the frequency of the AC voltage from the inverter main circuit 8 1 by turning on the transistor, and turning on the transistor, and the current detector 8 1 a When the detection 値 exceeds a predetermined upper limit 値 (for example, about 150% of the rated output current 反 of the inverter main circuit 8 1 ), the choke main circuit 8 is throttled in order to create an output current of the upper limit 値 or lower. The VVVF inverter 8 formed by the inverter control circuit 82 of the current limiting function of the output voltage of 1 and the induction motor 9 of the corner winding, and the electromagnetic contactor 10, and the open electrical appliance 1 of the electromagnetic contactor 1 1 composition. -8- (6) 1277286 The circuit configuration shown in Fig. 1 is referred to the waveform diagram shown in Fig. 2, and the following description will be given. First, the electromagnetic contactor 1A is turned off, and the VVVF inverter 8 is operated while the voltage of the commercial power source 1 is established, and the voltage set in advance by the VVVF inverter 8 is output by the control operation of the inverter control circuit 82. The state of the alternating voltage of the frequency (for example, the lowest allowable voltage, the lowest allowable frequency). At a subsequent time U, the electromagnetic contactor 1 is turned on by the opening command from the open circuit 1 1 so that the induction motor 9 and the load 5 are operated, and the induction motor 9 starts to rotate at a low speed while being reversed by the above-described opening command. In the control operation of the flow controller control circuit 82, the VVVF inverter 8 is an AC voltage that allows the lowest frequency and allows the lowest frequency AC voltage, as shown in Fig. 2, to gradually increase the voltage and the frequency of the AC voltage. The voltage is gradually increased, and the output current of the inverter main circuit 8 1 during the frequency period is monitored by the current detector 8 1 a to monitor the inverter control circuit 82. By the monitoring, the output current exceeds When the upper limit 事先 is set in advance, the output power source can be set to the upper limit 値 or less, and the amplitude of the output voltage of the inverter main circuit 8 1 is increased by the control operation of the inverter control circuit 82, and the VVVF is increased. The inverter 8 continues to output an AC voltage which gradually increases the voltage and frequency as shown in Fig. 2, and the induction motor 9 continues to accelerate. At the time "in time, the output frequency of the VVVF inverter 8 is increased to a predetermined frequency setting 値, and the acceleration operation of the induction motor 9 is completed until the time U. Therefore, as shown in Fig. 2 -9- (7) 1277286 The output current of the VVVF inverter 8 also exceeds the above-mentioned upper limit ,, and the induction motor 9 is required to be operated at a load 5 such as a fiber machine and requires a large driving torque during acceleration. After the acceleration, the small driving torque is sufficient, so that the output current of the VVVF inverter 8 is also reduced. By the time t2 when the time q is slightly elapsed, the control operation of the inverter control circuit 82 is performed. The output frequency of the VVVF inverter 8 is maintained in the above-described frequency setting ,. As shown in Fig. 2, up to the time t3, only the output voltage of the VVVF inverter 8 is started to decrease approximately linearly. Here, at time t2 at which the output voltage of the VVVF inverter 8 is started to decrease, the output current of the inverter is reduced to a previously set 値, and the inverter control circuit 82 passes the current detector. The detection of 81a is confirmed at a time when the time is confirmed, or the time until the time t2 is obtained by a prior experiment, and the time corresponding to the time t2 is measured by a timer (not shown). That is, the time is after the time h. During the period from time t2 to time t3, when the driving torque of the induction motor 9 such as a fiber machine is small, the VVVF inverter 8 maintains the output frequency below the frequency setting , by only The amplitude of the output voltage of the induction motor 8 is reduced to a voltage setting 値 [for example, { 1 / (square root of 3) of the output voltage at time h} or an output voltage 値] of torque required to obtain the load 5, The output current at the time of the output voltage at the time U is such that the output current at the output voltage at the time 13 is improved in efficiency and power of the induction motor 9, and therefore it is less. That is, the driving torque of the primary winding is angularly compared with the star-connected induction motor, and the driving torque is about three times, so that the output voltage of -10- (8) 1277286 after time t3 can be made into a moment. The large value of the output voltage is ]. Therefore, the induction motor source operation state after time t3. It can be seen from the above operation description that the primary winding is the continuous rated 値 of the horn machine 9 or the induction motor after the time t2 described above. The operation of 9 and the drive-to-output frequency (V/F) ratio at the time of acceleration can be output as a short time amount. According to the control method of the present invention, the self-operated induction current inverter is used for the voltage induction motor, and thus the flow is performed. The output frequency of the device is directly shifted to correspond to the desired one, and therefore has the advantage of shortening the self-operating induction motor to the desired travel time. (Effect of the invention) According to the present invention, only the variable power from the general purpose VVVF) The power supply of the inverter is to connect the primary winding to the corner. Compared with the induction motor whose primary winding is connected to the star, the primary current is operated and accelerated. In the completion of the operation, the induction motor can operate the induction with a small driving torque, and thus only reduce the output voltage of the VVVF inverter, so that the operation efficiency and power of the motive can be used, and thus it can respond to the requirements of the source. Brief description] ί [1/ (the square of 3) is set to be the inductive motorized number that can be connected to the province. The moment and the output voltage can be set. If the motive is turned, the frequency of the anti-speed can be adjusted. The desired speed of operation of the variable frequency (the induction motor becomes a motor with less rotation and acceleration, and the motor is improved in recent years) - (9) 1277286 FIG. 1 is a view showing the present invention Fig. 2 is a waveform diagram showing the operation of Fig. 1. Fig. 3 is a circuit diagram showing a conventional example. [Description of symbols] 1 Commercial power supply 2, 8 VVVF inverter 3 Switching circuit 4,9 Induction motor 5 Load 6 AC reactor 7 Sequence control circuit 10 Magnetic contactor 11 Open circuit 21, 81 Inverter main circuit 22 > 82 Inverter control circuit 8 1a Current detection -12-