【正文】 oftware. The test results obtained on induction motor performance show improved efficiency and increased accuracy in variableload constantspeedcontrolled operation. Thus, the PLC correlates and controls the operational parameters to the speed set point req uested by the user and monitors the induction motor system during normal operation and under trip conditions. II. PLC AS SYSTEM CONTROLLER A PLC is a microprocessorbased control system, designed for automation processes in industrial environments. It uses a programmable memory for the internal storage of userorientated instructions for implementing specific functions such as arithmetic, counting, logic, sequencing, and timing [23], [24].A PLC can be programmed to sense, activate, and control industrial equipment and, therefore, incorporates a number of I/O points, which allow electrical signals to be interfaced. Input devices and output devices of the are connected to the PLC and the control program is entered into the PLC memory. In our application, it controls through analog and digital inputs and outputs the varying loadconstant speed operation of an induction motor. Also, the PLC continuously monitors the inputs and activates the outputs according to the control program. This PLC system is of modular type posed of specific hardware building blocks (modules), which plug directly into a proprietary bus: a central processor unit (CPU), a power supply unit, inputoutput modules I/O, and a program terminal. Such a modular approach has the advantage that the initial configuration can be expanded for other future applications such as multimachine systems or puter linking. III. CONTROL SYSTEM OF INDUCTION MOTOR In Fig. 2, the block diagram of the experimental system is illustrated. The following configurations can be obtained from this setup. a) A closedloop control system for constant speed operation, configured with speed feedback and load current feedback. The induction motor drives a variable load, is fed by an inverter, and the PLC controls the inverter V/F output. b) An openloop control system for variable speed operation. The induction motor drives a variable load and is fed by an inverter in constant V/F control mode. The PLC is inactivated. c) The standard variable speed operation. The induction motor drives a variable load and is fed by a constant voltageconstant frequency standard threephase supply. The openloop configuration b) can be obtained from the closedloop configuration a) by removing the speed and load feedback. On the other hand, operation c) results if the entire control system is bypassed. IV. HARDWARE DESCRIPTION The control system is implemented and tested for a wound rotor induction motor, having the technical specifications given in Table I. The induction motor drives a dc generator, which supplies a variable load. The threephase power supply is connected to a threephase main switch and then to a threephase thermal overload relay, which provides protection against current overloads. The relay output is connected to the rectifier, which rectifies the threephase voltage and gives a dc input to the insulated gate bipolar transistor (IGBT) inverter. Its technical specifications [25] are summarized in Table II. The IGBT inverter converts the dc voltage input to threephase voltage output, which is supplied to the stator of the induction motor. On the other hand, the inverter is interfaced to the PLCbased controller. This controller is implemented on a PLC modular system [5], [26]–[28]. The PLC architecture refers to its internal hardware and software. As a microprocessorbased system, the PLC system hardware is designed and built up with the following modules [29]–[37]: ? central processor unit (CPU)。同時(shí)，在獲得異步電動(dòng)機(jī)運(yùn)行參數(shù)的前提下，實(shí)現(xiàn)了電動(dòng)機(jī)保護(hù)和速度控制的硬件和軟件設(shè)計(jì)。因此，可以說明 PLC 在電力拖動(dòng)的工業(yè)控制方面是一個(gè)通用且有效的工具。許多廠家都將 PLC運(yùn)用到自動(dòng)化過程中，降低生產(chǎn)成本且提高了 質(zhì)量和可靠性。 很少有人發(fā)表關(guān)于用 PLC控制直流電機(jī)的文章。開關(guān) 磁阻電動(dòng)機(jī)可以選擇用直流或是交流加速驅(qū)動(dòng)，它是用可實(shí)現(xiàn)數(shù)字邏輯的 PLC和電源控制器做成的單片邏輯控制器來控制電機(jī)的轉(zhuǎn)矩和速度。一個(gè)三相異步電動(dòng)機(jī)功率控制器利用 PLC來提高功率因數(shù)，且通過調(diào)整頻率占空比來調(diào)整電壓。這篇文章講述了基于 PLC的三相電動(dòng)機(jī)監(jiān)督控制系統(tǒng)。 II． PLC作為系統(tǒng)控制器 PLC是為工業(yè)環(huán)境下自動(dòng)化生產(chǎn)設(shè)計(jì)的基于微處理器的控制系統(tǒng)。 圖 1 PLC 的控制過程 在我們的應(yīng)用中， PLC控制異步電機(jī)恒速變負(fù)載的邏輯和數(shù)字輸入輸出信號(hào)。 a)恒速閉環(huán)控制系統(tǒng)由速度反饋和負(fù)載電流反饋組成。 PLC 輸入信號(hào) 輸出信號(hào) 控制命令 圖 2 實(shí)驗(yàn)系統(tǒng)電氣原理圖 c)標(biāo)準(zhǔn)變速系統(tǒng) .由三相標(biāo)準(zhǔn)恒壓恒頻電源提供給異步電機(jī)來驅(qū)動(dòng)變負(fù)載。異步電機(jī)驅(qū)動(dòng)作為變動(dòng)負(fù)載的直流電機(jī)。 表 1 異步電機(jī)技術(shù)參數(shù) 接線方式 △ /Y 輸入電壓 380/660 AC 輸入電流 額定功率 輸入頻率 50HZ 磁極數(shù) 4 額定速度 1400rpm 表 2 逆變器技術(shù)參數(shù) 輸出電壓 380， 460 AC 輸出頻率 0， 480 HZ 輸出電流 2， 5 A 輸出負(fù)載 150%/60S 電源電壓 38046010% V AC 輸入電流 3 A 功耗 46 W IGBT逆變器將直流電壓輸入轉(zhuǎn)變成三相電壓輸出，提供給異步 電機(jī)的定子。作為基于微處理器的系