電機(jī)是工業(yè)生產(chǎn)領(lǐng)域中最主要的驅(qū)動(dòng)源，如何有效地監(jiān)控電機(jī)的運(yùn)行狀態(tài)，保護(hù)電機(jī)回路，提高電機(jī)的運(yùn)行時(shí)間，減少電機(jī)故障，對(duì)工廠整體電網(wǎng)的運(yùn)行十分關(guān)鍵。
電動(dòng)機(jī)保護(hù)裝置有很多種，目前使用得比較普遍的還是基于金屬片機(jī)械式的熱繼電器，它結(jié)構(gòu)簡(jiǎn)單，在保護(hù)電動(dòng)機(jī)過載方面具有反時(shí)限特性。但它的保護(hù)功能少，無(wú)斷相保護(hù)，對(duì)電機(jī)發(fā)生通風(fēng)不暢、掃膛、堵轉(zhuǎn)、長(zhǎng)期過載、頻繁啟動(dòng)等故障也不能起保護(hù)作用。此外，熱繼電器還存在重復(fù)性能差、大電流過載或短路故障后不能再次使用、調(diào)整誤差大、易受環(huán)境溫度影響而誤動(dòng)或拒動(dòng)、功耗大、耗材多、性能指標(biāo)落后等缺陷。
為響應(yīng)國(guó)家節(jié)能減排的要求，采用基于微控制器的電子式電機(jī)保護(hù)器替代現(xiàn)有熱繼電器，具有廣大的市場(chǎng)。設(shè)計(jì)采用集成豐富外設(shè)的STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" 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title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32" title="STM32">STM32系列ARM芯片為核心的智能電機(jī)保護(hù)器，具有響應(yīng)速度快、附加芯片少、生產(chǎn)調(diào)試簡(jiǎn)單、生產(chǎn)及社會(huì)效益高等優(yōu)點(diǎn)。
1 智能保護(hù)器功能及硬件架構(gòu)
電機(jī)運(yùn)行中主要發(fā)生的故障包括：起動(dòng)超時(shí)、過載、堵轉(zhuǎn)、缺相、不平衡、過熱、欠載、過壓、欠壓等。因此智能保護(hù)器需要監(jiān)測(cè)電機(jī)的工作電壓、工作電流和機(jī)殼溫度。
同時(shí)，由于電機(jī)的類型、容量和負(fù)載類型不同，電機(jī)保護(hù)的參數(shù)也不盡相同，所以需要能夠針對(duì)不同的電機(jī)設(shè)置保護(hù)參數(shù)。
再者，為了使智能保護(hù)繼電器能夠滿足當(dāng)前流行的智能電機(jī)控制中心(IMCC)的需求，智能電機(jī)保護(hù)器還需要具有網(wǎng)絡(luò)通訊功能。
圖1是智能電機(jī)保護(hù)器的硬件結(jié)構(gòu)框圖。
2 系統(tǒng)硬件設(shè)計(jì)
2．1 MCU
MCU是電機(jī)保護(hù)器的核心部分，主要負(fù)責(zé)數(shù)據(jù)采集、數(shù)據(jù)處理、輸出控制和參數(shù)設(shè)置。這里采用的是ST公司最新推出的STM32F103xD系列ARM芯片。
該系列芯片采用ARM公司32位的C0rtex M3為核心，最高主頻為72MHz，Cortex核心內(nèi)部具有單周期的硬件乘法和除法單元，所以適合用于高速數(shù)據(jù)的處理。
芯片具有三個(gè)獨(dú)立的轉(zhuǎn)換周期，最低為1μs的高速模數(shù)轉(zhuǎn)換器，三個(gè)獨(dú)立的數(shù)模轉(zhuǎn)換器帶有各自獨(dú)立的采樣保持電路，所以特別適合三相電機(jī)控制、電網(wǎng)監(jiān)測(cè)和多參數(shù)儀器設(shè)備的使用。
芯片還帶有豐富的通訊單元，包括多達(dá)5個(gè)異步串行接口、1個(gè)USB從器件、1個(gè)CAN器件、I2C和SPI等模塊。
2．2 模擬量采集單元
電機(jī)保護(hù)器主要需要采集電流、電壓和溫度這三個(gè)模擬量來對(duì)電機(jī)的運(yùn)行狀態(tài)進(jìn)行監(jiān)測(cè)和保護(hù)。
電流傳感器類型眾多，主要包括磁芯電流互感器、霍爾傳感器、分流電阻。而電機(jī)保護(hù)器所連接的電機(jī)容量主要以幾千瓦至數(shù)十千瓦的電機(jī)為主，因此電機(jī)相電流主要在數(shù)安培至幾十安培。因此采用電流互感器為電流采集單元，具有量程寬、發(fā)熱小、隔離電壓高等優(yōu)點(diǎn)。同時(shí)在不改變處理電路的參數(shù)的同時(shí)，采用不同變比的電流傳感器可以方便地改變電機(jī)保護(hù)器的電流檢測(cè)量程，從而可以方便地用于更大容量的電機(jī)保護(hù)。
電壓直接通過電阻分壓獲得，因此整個(gè)電機(jī)控制器是共熱地的系統(tǒng)。電阻采用的是高阻抗高耐壓類型電阻，同時(shí)為了提高電壓采集回路的過電壓能力，電壓分壓電路采用多電阻串聯(lián)形式，從而降低每一個(gè)電阻上的額定壓降，同時(shí)提高整個(gè)支路的最高耐壓。
溫度傳感器采用常見的鉑電阻傳感器或者NTC熱敏電阻，保護(hù)器硬件上設(shè)計(jì)有對(duì)應(yīng)的熱電阻信號(hào)調(diào)理電路。由于熱電阻都是非線性器件，因此溫度采集處理通道都需要進(jìn)行非線性處理，為了減少硬件電路的復(fù)雜程度，因此實(shí)際熱電阻調(diào)理單元只是設(shè)計(jì)采用一個(gè)儀表放大器，而熱電阻的非線性處理由MCU完成。另外還有一路MCU芯片內(nèi)置的半導(dǎo)體溫度傳感器，用于檢測(cè)保護(hù)器內(nèi)的溫度，從而防止由于系統(tǒng)過熱而導(dǎo)致控制出錯(cuò)。
2．3 液晶顯示
對(duì)于獨(dú)立應(yīng)用的電機(jī)保護(hù)器，需要能夠設(shè)置保護(hù)參數(shù)，顯示當(dāng)前的運(yùn)行狀態(tài)，當(dāng)發(fā)生故障時(shí)候還需要顯示故障類型。因此電機(jī)保護(hù)器需要有顯示單元。
系統(tǒng)設(shè)計(jì)采用點(diǎn)陣形式STN黑白液晶顯示(LCD)模塊，相比TFT彩色LCD模塊，具有使用溫度范圍寬、壽命長(zhǎng)、強(qiáng)光下仍可以閱讀的優(yōu)點(diǎn)。
LCD模塊內(nèi)置的控制器采用的并行數(shù)據(jù)通訊接口，包括數(shù)據(jù)總線、讀寫控制線、器件選通和復(fù)位引腳。系統(tǒng)設(shè)計(jì)時(shí)，使用STM32F103xD芯片的多功能靜態(tài)存儲(chǔ)器控制器(FSMC)與LCD模塊相連。
STM32F芯片的FSMC模塊是一個(gè)支持靜態(tài)存儲(chǔ)器(SRAM)、NOR F1ash和PSRAM的多功能靜態(tài)存儲(chǔ)器控制器?？梢灾С?位或者16位寬度的存儲(chǔ)器。
LCD模塊的訪問時(shí)序與SRAM的相同，而且可以通過配置引腳選擇為8080或者6800類型的接口時(shí)序。圖2是STM32芯片的FSMC接口與LCD的電氣連接，這里的液晶是采用的8080接口時(shí)序。
2．4 通訊電路
智能電機(jī)控制中心(IMCC)的控制結(jié)構(gòu)大都是總線型分布式網(wǎng)絡(luò)結(jié)構(gòu)，系統(tǒng)中有中央控制器負(fù)責(zé)調(diào)度和監(jiān)控所有電機(jī)的運(yùn)行。根據(jù)所使用的中央控制器(大多為PLC)的不同，系統(tǒng)的通訊協(xié)議有MODBUS、Fieldbus和以太網(wǎng)等。其中最常見的是MODBUS協(xié)議。MODBUS協(xié)議的物理層是基于RS485的半雙工通訊網(wǎng)絡(luò)，電機(jī)保護(hù)器在其中是處于從機(jī)地位。
由于電機(jī)保護(hù)器內(nèi)部是共熱地系統(tǒng)，所以RS485遠(yuǎn)程通訊需要與控制器主回路隔離。對(duì)于RS485收發(fā)器的隔離，需要對(duì)通訊信號(hào)和收發(fā)器的供電進(jìn)行隔離。電機(jī)保護(hù)器的通訊接口設(shè)計(jì)的通訊波特率最高需要達(dá)到57．6kbps，因此需要使用高速光耦或者數(shù)字隔離芯片完成對(duì)通訊信號(hào)的隔離。
數(shù)字隔離芯片是一種新型的器件，TI、ADI和Silicon Lab等公司都有推出擁有各自專利的數(shù)字隔離器件，但各家芯片的引腳封裝和引腳功能大部分都是兼容的，可以直接替換。相對(duì)傳統(tǒng)的高速光耦，數(shù)字隔離器件具有功耗低、傳輸速率高、兼容3V／5V系統(tǒng)和外設(shè)簡(jiǎn)單等優(yōu)點(diǎn)。實(shí)際連接電路如圖3所示。
3 系統(tǒng)軟件設(shè)計(jì)
3．1 監(jiān)控程序
智能電機(jī)保護(hù)器中，MCU只要完成電流和電壓的采樣、計(jì)算、分析來實(shí)現(xiàn)各種保護(hù)功能，并且實(shí)時(shí)顯示線路的參數(shù)和記錄故障狀態(tài)。
系統(tǒng)軟件中，全局監(jiān)控軟件用來循環(huán)掃描各個(gè)單元子程序的狀態(tài)，包括模數(shù)轉(zhuǎn)換器的數(shù)據(jù)采樣，數(shù)據(jù)采集完成后對(duì)數(shù)據(jù)進(jìn)行計(jì)算，而后根據(jù)計(jì)算結(jié)果和預(yù)先設(shè)置的電機(jī)參數(shù)，結(jié)合保護(hù)策略判斷出當(dāng)前電機(jī)的狀態(tài)。最后通過LCD顯示器和輸出控制接口發(fā)出相應(yīng)的狀態(tài)信息和控制功能。
3．2 ADC的配置和使用
由于電機(jī)保護(hù)器采集的是50Hz工頻電壓電流，同時(shí)為了能夠監(jiān)測(cè)到電網(wǎng)中的高次諧波成分(主要是3次諧波)，因此在ADC的采樣頻率需要設(shè)置在工頻的整數(shù)倍，從而使得采樣快速傅立葉(FFT)運(yùn)算時(shí)得到最準(zhǔn)確的精度。同時(shí)，數(shù)據(jù)采樣時(shí)，由于計(jì)算功率需要同時(shí)采集電壓、電流值，因此在系統(tǒng)設(shè)計(jì)時(shí)，將三相的參數(shù)分別分配到兩個(gè)模數(shù)轉(zhuǎn)換器，而將溫度量放置在第三個(gè)模數(shù)轉(zhuǎn)換器。所有的模數(shù)轉(zhuǎn)換均采用內(nèi)部定時(shí)器中斷觸發(fā)。
對(duì)于電壓和電流的采集，系統(tǒng)采用同步采樣模數(shù)，即同時(shí)采集同一個(gè)通道的電壓和電流值；同時(shí)在采樣通道配置寄存器中，將三個(gè)電壓電流通道依次排列，使得在一次定時(shí)器觸發(fā)下，一次性完成所有通道的轉(zhuǎn)換。
由于FFT需要一組數(shù)據(jù)進(jìn)行計(jì)算，所以為了最低程度的CPU干預(yù)，系統(tǒng)設(shè)計(jì)使用了DMA來完成轉(zhuǎn)換結(jié)果的傳遞。程序設(shè)計(jì)時(shí)，需要設(shè)置兩塊數(shù)據(jù)存放區(qū)，用于采樣數(shù)據(jù)的交替存放；同時(shí)DMA傳遞的字節(jié)計(jì)數(shù)的預(yù)置值即為傅立葉轉(zhuǎn)換數(shù)組長(zhǎng)度乘以采集的通道數(shù)。
3．3 遠(yuǎn)程通訊
電機(jī)保護(hù)器設(shè)計(jì)為MODBUS從機(jī)，所有的電機(jī)運(yùn)行狀態(tài)、控制狀態(tài)等參數(shù)都是放置在系統(tǒng)約定地址的寄存器中。同時(shí)MODBUS從機(jī)需要給每一個(gè)電機(jī)控制器預(yù)置一個(gè)網(wǎng)絡(luò)中唯一的從機(jī)地址，所以還需要使用按鍵和LCD屏幕來設(shè)置從機(jī)地址。
4 結(jié)語(yǔ)
本文提出一種基于STM32系列新品的智能電機(jī)保護(hù)器，本設(shè)計(jì)充分利用了STM32芯片的資源，提供必要的外圍器件構(gòu)成了一個(gè)完整的系統(tǒng)。實(shí)際應(yīng)用證明，此系統(tǒng)能有效保護(hù)電機(jī)，且具有結(jié)構(gòu)簡(jiǎn)單、功能完善、接口豐富等優(yōu)點(diǎn)，根據(jù)實(shí)際需要還可以開發(fā)出諸如USB、CAN open等接口，因此此系統(tǒng)可以更廣泛地應(yīng)用于工業(yè)生產(chǎn)各個(gè)領(lǐng)域。