一、晶閘管保護(hù)電路
1、主電路中的晶閘管保護(hù)電路
FOXBORO COMPANY CM400YL晶閘管陽(yáng)極、陰極兩端或晶閘管電源輸入端、輸出端經(jīng)常加設(shè)相關(guān)保護(hù)電路，以對(duì)晶閘管提供過(guò)電壓、過(guò)電流等相關(guān)保護(hù)。
1）過(guò)電流保護(hù)
產(chǎn)生過(guò)載的主要原因：負(fù)荷過(guò)載、線路短路、電源缺相、晶閘管本身?yè)舸p壞或誤觸發(fā)等，因晶閘管元件體積小，過(guò)載時(shí)會(huì)造成結(jié)溫過(guò)高而燒毀，所以必須嚴(yán)格限制過(guò)載電流，除控制（電子）電路實(shí)施的保護(hù)外，在主電路中經(jīng)常采用在電源串入快速熔斷器，對(duì)晶閘管的過(guò)載進(jìn)行保護(hù)，在發(fā)生6倍晶閘管額定電流時(shí)，一個(gè)周波可以熔斷。此外，還可采用過(guò)電流繼電器、直流快速斷路器等用于過(guò)載和短路保護(hù)，但保護(hù)速度和效果不如快速熔斷器。
FOXBORO COMPANY CM400YL 快速熔斷器的額定電流值為晶閘管電流平均值的1.25~1.5倍。
下圖以直流調(diào)壓電路為例，說(shuō)明快速熔斷器在主電路中的接法。
2）過(guò)電壓保護(hù)
FOXBORO COMPANY CM400YL產(chǎn)生過(guò)電壓的原因一般因感性負(fù)載電路的開(kāi)閉、電源電壓波動(dòng)、快速熔斷器熔斷、電源側(cè)侵入的浪涌電壓等，針對(duì)形成過(guò)電壓的不同原因，可采取不同的抑制方法，如抑制過(guò)電壓能量的上升速率、增加其能量的耗散等，目前最常用的是中主電路回路中接入吸收能量的元件，使能量得以耗散，稱之為吸收回路或緩沖電路。
通常過(guò)電壓具有較高的頻率，因此常采用電容作為吸收元件，但為防止振蕩，增加阻尼電阻，構(gòu)成R、C吸收回路。阻容吸收回路可以接在電源輸入側(cè)（交流側(cè)）、輸出側(cè)（直流側(cè)）和晶閘管的陽(yáng)極和陰極之間。但R、C阻容吸收回路的時(shí)間常數(shù)是固定的，對(duì)時(shí)間短、峰值高、能量大的過(guò)電壓吸收能力有限，因而在輸入側(cè)，通常還并有硒堆、壓敏電阻等非線性元件，用以對(duì)晶閘管的過(guò)電壓進(jìn)行吸收。硒堆由多片硒片疊合而成，硒堆涌流容量大，對(duì)過(guò)電壓抵制效果好，有自恢復(fù)特性等優(yōu)點(diǎn)，但因體積大，價(jià)格高，在中、小容量的晶閘管裝置中，已經(jīng)很少應(yīng)用。
FOXBORO COMPANY CM400YL壓敏電阻的電壓與電流呈非線性關(guān)系，當(dāng)其兩端所加電壓低于壓敏電壓值時(shí)，壓敏電阻的電阻值接近無(wú)窮大，為高阻狀態(tài)，對(duì)連接電路沒(méi)有影響；當(dāng)壓敏電阻兩端電壓高于壓敏電壓值時(shí)，迅速擊穿導(dǎo)通（變?yōu)榈妥锠顟B(tài)），形成較大的泄放電流。當(dāng)其端電壓因泄放又低于壓敏電壓值時(shí)，又恢復(fù)為高阻態(tài)。當(dāng)其兩端電壓超過(guò)最大限制電壓時(shí)，壓敏電阻出現(xiàn)不可逆性擊穿損壞。壓敏電阻在電路中起到過(guò)電壓保護(hù)、抑制浪涌電流、吸收尖峰電壓、電壓限幅、穩(wěn)壓等作用。
圖2中的a電路，是并聯(lián)在晶閘管陽(yáng)極和陰極之間的RC吸收回路，對(duì)晶閘管兩端的電壓躍變產(chǎn)生抑制作用，降低閘管元件在換向時(shí)承受的過(guò)電壓沖擊。其“瞬升”電壓尖刺為電容C所吸收，電阻R為防止振蕩出現(xiàn)的阻尼電阻。
電容C的耐電壓值，取回路電壓的1.5倍左右，C的容量值在晶閘管通態(tài)平均電流值20A~200A時(shí)，取值0.1μF~0.5μF，R取值100Ω~10Ω，功率1W~3W。
圖2中b電路，是在晶閘管兩端并聯(lián)壓敏電阻，以吸收突變電壓能量，使用壓敏電阻需注意兩個(gè)參數(shù)：
1）壓敏電壓：即擊穿電壓或閥值電壓，其范圍從10~9000V不等。用于單相220V時(shí)，其正弦波電壓最大值約300V，需采用壓敏電壓值為470V的壓敏電阻器件，用于吸收470V左右的尖峰電壓；用于三相380V回路，因電源電壓的最大值為500V左右，需采用壓敏電壓值為800V左右的壓敏電阻，吸收回路中可能產(chǎn)生的異常尖峰電壓。
壓敏電壓值的選用標(biāo)準(zhǔn)，應(yīng)為回路電壓的1.2倍以上，用于交流回路時(shí)，應(yīng)取交流峰值（最大值）電壓的1.2倍以上。
2）流通容量：流通容量的范圍為2~20kA，指壓敏電阻兩端的電壓變化不超過(guò)±10%，在8~20μs內(nèi)施加的峰值電流值。從保護(hù)效果出發(fā)，F(xiàn)OXBORO COMPANY CM400YL選用流通容量大一些的器件交流較好，多只并聯(lián)使用，其流通容量增大，使擊穿時(shí)的泄放電流值上升。
圖2的c電路，當(dāng)壓敏電阻并接于L1、L2、L3電源線上時(shí)，應(yīng)選用MYG14k821（壓敏電壓值820V、流通容量14kA）型的壓敏電阻器件；當(dāng)壓敏電阻進(jìn)行星接時(shí)，可選用MYG14K471（壓敏電壓值470V、流通容量14kA）型的壓敏電阻器件。
----商務(wù)熱線---廈門興銳達(dá)自動(dòng)化設(shè)備有限公司
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