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附錄：外文翻譯
煤礦業(yè)帶式輸送機(jī)幾種軟起動(dòng)方式的比較
el L. Nave, P.E.
統(tǒng)一公司
1800 年華盛頓路匹茲堡, PA 15241帶式運(yùn)送機(jī)是采礦工業(yè)運(yùn)輸大批原料的重要方法。從傳送帶驅(qū)動(dòng)系統(tǒng)到傳送帶紋理結(jié)構(gòu)啟動(dòng)力矩的應(yīng)用和控制影響著運(yùn)送機(jī)的性能，壽命和可靠性。本文考查了不同啟動(dòng)方法在煤礦工業(yè)帶式運(yùn)送機(jī)中的應(yīng)用。
簡(jiǎn)介
運(yùn)行帶式運(yùn)送機(jī)的動(dòng)力必須由驅(qū)動(dòng)滑輪產(chǎn)生，通過滑輪和傳送帶之間的摩擦力來傳遞。為了傳遞能量，傳送帶上面的張力在接近滑輪部分和離開滑輪部分必定存在著差別。這種差別在穩(wěn)定運(yùn)行、啟動(dòng)和停止時(shí)刻都是真實(shí)存在的。傳統(tǒng)傳送帶結(jié)構(gòu)的設(shè)計(jì)，都是根據(jù)穩(wěn)定運(yùn)行情況下傳送帶的受力情況。因?yàn)樵O(shè)計(jì)過程中沒有詳盡研究傳送帶啟動(dòng)和停止階段的受力情況，所有的安全措施都集中在穩(wěn)定運(yùn)行階段（Harrison 1987）。本文主要集中講述傳送機(jī)啟動(dòng)和加速階段的特性。傳送帶設(shè)計(jì)者在設(shè)計(jì)時(shí)必須考慮控制啟動(dòng)階段的加速狀況，以免使傳送帶和傳送機(jī)驅(qū)動(dòng)系統(tǒng)產(chǎn)生過大的張力和動(dòng)力（Suttees，1986）。大加速度產(chǎn)生的動(dòng)力會(huì)給傳送帶的紋理、傳送帶結(jié)合處、驅(qū)動(dòng)滑輪、軸承、減速器以及耦合器帶來負(fù)面影響。毫無控制的加速度產(chǎn)生的動(dòng)力能夠引起帶式傳送機(jī)系統(tǒng)產(chǎn)生諸多不良問題，比如上下曲線運(yùn)動(dòng)、過度傳送帶提升運(yùn)動(dòng)、滑輪和傳送帶打滑、運(yùn)輸原料的溢出和傳送帶結(jié)構(gòu)。傳送帶的設(shè)計(jì)需要面對(duì)兩個(gè)問題：第一，傳送帶驅(qū)動(dòng)系統(tǒng)必須能夠產(chǎn)生啟動(dòng)帶式傳送機(jī)的最小轉(zhuǎn)動(dòng)力矩；第二，控制加速度產(chǎn)生動(dòng)力在安全界限內(nèi)?？梢酝ㄟ^驅(qū)動(dòng)力矩控制設(shè)備來完成，控制設(shè)備可以是電子手段也可以是機(jī)械手段，也可以是兩者的組合（CEM，1979）。
本文主要闡述輸送機(jī)的開始和加速的過程。傳送帶設(shè)計(jì)師必須控制開始加速度防止過度張緊在傳送帶織品和力量在皮帶傳動(dòng)系統(tǒng). 強(qiáng)加速度力量可能有害地影響傳送帶織品，傳送帶接合，驅(qū)動(dòng)皮帶輪，更加無所事事的滑輪, 軸, 軸承, 速度還原劑, 并且聯(lián)結(jié)。未管制的加速度力量可能造成皮帶輸送機(jī)有垂直的曲線的系統(tǒng)性能問題,傳送帶緊線器運(yùn)動(dòng), 驅(qū)動(dòng)皮帶輪摩擦損失, 材料溢出, 并且做成花彩傳送帶織品。傳送帶設(shè)計(jì)員與二個(gè)問題被面對(duì), 皮帶傳動(dòng)系統(tǒng)必須導(dǎo)致極小的扭矩足夠強(qiáng)有力開始傳動(dòng)機(jī), 和控制了這樣加速度強(qiáng)制是在安全限額內(nèi)。光滑開始傳動(dòng)機(jī)可能由對(duì)驅(qū)動(dòng)器扭矩控制設(shè)備的用途, 或機(jī)械或電子, 或組合的二完成(CEM 1979) 。
軟起動(dòng)結(jié)構(gòu)評(píng)估標(biāo)準(zhǔn)
什么是最佳的皮帶輸送機(jī)驅(qū)動(dòng)系統(tǒng)? 答案取決于許多變量。最佳的系統(tǒng)是一個(gè)為開始, 運(yùn)行, 和終止提供可接受的控制在合理的費(fèi)用和以及高可靠性。皮帶傳動(dòng)系統(tǒng)為本文我們考慮的設(shè)計(jì)方案, 皮帶輸送機(jī)被電子頭等搬家工人幾乎總驅(qū)動(dòng)。傳送帶"驅(qū)動(dòng)系統(tǒng)" 將包括多個(gè)要素包括電子原動(dòng)力、電子馬達(dá)起始者以控制系統(tǒng), 馬達(dá)聯(lián)結(jié)、速度還原劑、低速聯(lián)結(jié)、皮帶傳動(dòng)滑輪、和滑輪閘 (Cur 1986) 。它重要, 傳送帶設(shè)計(jì)員審查各個(gè)系統(tǒng)要素的適用性對(duì)特殊申請(qǐng)。為本文的目的, 我們假設(shè), 所有驅(qū)動(dòng)系統(tǒng)要素設(shè)置礦的新鮮空氣, 非允許, 面積,全國電子編碼, 條款500 防爆, 礦的表面的面積。皮帶傳動(dòng)要素歸因于范圍。某些驅(qū)動(dòng)器要素是可利用和實(shí)用的用不同的范圍。為這論述, 我們假設(shè)那皮帶傳動(dòng)系統(tǒng)范圍從分?jǐn)?shù)馬力對(duì)千位的多個(gè)馬力。小驅(qū)動(dòng)系統(tǒng)經(jīng)常是在50 馬力以下。中型系統(tǒng)范圍從50 到1000 馬力。大型系統(tǒng)可能被考慮在1000 馬力之上。范圍分部入這些組是整個(gè)地任意的。必須被保重抵抗誘惑對(duì)超出馬達(dá)或在馬達(dá)之下傳送帶飛行提高標(biāo)準(zhǔn)化。驅(qū)動(dòng)器結(jié)果在粗劣的效率和在高扭矩的潛在, 當(dāng)驅(qū)動(dòng)器能導(dǎo)致破壞性超速在再生, 或過度加熱以變短的馬達(dá)壽命。扭矩控制。傳送帶設(shè)計(jì)員設(shè)法限制開始的扭矩到?jīng)]有比150% 運(yùn)行中。限額在應(yīng)用的開始的扭矩經(jīng)常是傳送帶胴體肉、傳送帶接合、滑輪絕熱材料,軸偏折評(píng)級(jí)。在更大的傳送帶和傳送帶以優(yōu)化大小的要素, 扭矩限額110% 至125% 是公用。除扭矩限額之外, 傳送帶起始者必需限制會(huì)舒展圍繞和會(huì)導(dǎo)致旅行的波浪的扭矩增量。一個(gè)理想的開始的控制系統(tǒng)會(huì)適用于資格整個(gè)傳送帶的扭矩傳送帶休息由問題的脫離決定, 或運(yùn)動(dòng), 然后扭矩相等與傳送帶的運(yùn)動(dòng)需求以負(fù)荷加上恒定的扭矩從休息加速系統(tǒng)要素的慣性對(duì)最終奔跑速度。這使系統(tǒng)臨時(shí)強(qiáng)制和傳送帶舒展。不同的驅(qū)動(dòng)系統(tǒng)陳列變化的能力控制扭矩的申請(qǐng)對(duì)傳送帶休息和以不同的速度。并且, 傳動(dòng)機(jī)陳列裝載二個(gè)極端。一條空傳送帶正常存在最小的必需的扭矩為脫離和加速度, 當(dāng)一條充分地被裝載的傳送帶存在最高的必需的扭矩。開采驅(qū)動(dòng)系統(tǒng)必須是能稱應(yīng)用的扭矩從一個(gè)2/1 比率為一個(gè)水平的簡(jiǎn)單傳送帶安排, 對(duì)一個(gè)10/1 范圍為一個(gè)傾斜、復(fù)雜傳送帶配置文件。
熱量評(píng)級(jí)
在開始和運(yùn)行期間, 各個(gè)驅(qū)動(dòng)系統(tǒng)也許消散廢熱。廢熱也許被解放在電子馬達(dá)、電子控制、, 聯(lián)結(jié)、速度還原劑, 或傳送帶制動(dòng)系統(tǒng)。各個(gè)起始時(shí)間熱量負(fù)荷依靠相當(dāng)數(shù)量傳送帶負(fù)荷和起始時(shí)間的期限。設(shè)計(jì)員必須履行被重復(fù)的起始時(shí)間的申請(qǐng)需求在運(yùn)行傳動(dòng)機(jī)以后在全負(fù)荷。典型的開采傳送帶開始的責(zé)任變化從3到10 個(gè)起始時(shí)間每時(shí)數(shù)等隔,或2到4 個(gè)起始時(shí)間在連續(xù)。被重復(fù)的開始也許要求減稅或系統(tǒng)要素。有一個(gè)直接關(guān)系在熱量評(píng)級(jí)為被重復(fù)的起始時(shí)間和費(fèi)用之間。可變速度。一些皮帶傳動(dòng)系統(tǒng)是適當(dāng)?shù)臑榭刂崎_始的扭矩和速度, 但只運(yùn)行以恒定的速度。一些傳送帶申請(qǐng)會(huì)要求一個(gè)驅(qū)動(dòng)系統(tǒng)能運(yùn)行延長的期間以較不比最高速度。這是有用的當(dāng)驅(qū)動(dòng)器負(fù)荷必須與其它驅(qū)動(dòng)器被共享,傳送帶被使用當(dāng)處理飼養(yǎng)者為被表達(dá)的物料的費(fèi)率控制, 傳送帶速度被優(yōu)選為貨車使用費(fèi)費(fèi)率,傳送帶被使用以慢速運(yùn)輸人工或材料, 或傳送帶運(yùn)行緩慢的檢驗(yàn)或移動(dòng)速度為維護(hù)目的?？勺兯俣绕鲃?dòng)將要求一個(gè)控制系統(tǒng)根據(jù)某一算法調(diào)控操作速度。再生或翻修負(fù)荷。一些傳送帶配置文件存在翻修傳送帶系統(tǒng)用品能量對(duì)驅(qū)動(dòng)系統(tǒng)的負(fù)荷的潛在。沒有所有驅(qū)動(dòng)系統(tǒng)有能力接受被重新生成的能量從負(fù)荷。一些驅(qū)動(dòng)器可能接受能量從負(fù)荷和退回它到輸電線供其它負(fù)荷使用。其它驅(qū)動(dòng)器接受能量從負(fù)荷和消散它入選定的動(dòng)態(tài)或機(jī)械剎車的要素。一些傳送帶描出切換從開汽車對(duì)再生在運(yùn)算期間。驅(qū)動(dòng)系統(tǒng)可能接受有些巨大的被重新生成的能量為申請(qǐng)嗎? 驅(qū)動(dòng)系統(tǒng)控制或必須調(diào)整相當(dāng)數(shù)量減速的強(qiáng)制在翻修期間嗎?翻修發(fā)生當(dāng)運(yùn)行和開始? 維護(hù)和支持系統(tǒng)。各個(gè)驅(qū)動(dòng)系統(tǒng)將要求定期預(yù)防維護(hù)?？商鎿Q的項(xiàng)目會(huì)包括馬達(dá)畫筆、軸承、閘填充、散逸電阻器、油, 和涼水。如果驅(qū)動(dòng)系統(tǒng)被設(shè)計(jì)和保守地被管理, 更低的重音在可消耗導(dǎo)致更低的維修費(fèi)用。一些驅(qū)動(dòng)器要求支持系統(tǒng)譬如流通的油為潤滑油、冷卻空氣或水, 環(huán)境塵土過濾, 或計(jì)算機(jī)儀器工作。支持系統(tǒng)的維護(hù)可能影響驅(qū)動(dòng)系統(tǒng)的可靠性。
費(fèi)用
驅(qū)動(dòng)器設(shè)計(jì)員將審查各個(gè)驅(qū)動(dòng)系統(tǒng)的費(fèi)用。費(fèi)用合計(jì)是第一基建成本獲取驅(qū)動(dòng)器, 費(fèi)用安裝和委任驅(qū)動(dòng)器, 費(fèi)用運(yùn)行驅(qū)動(dòng)器, 和費(fèi)用的總和維護(hù)驅(qū)動(dòng)器。費(fèi)用使力量運(yùn)行驅(qū)動(dòng)器也許廣泛變化用不同的地點(diǎn)。設(shè)計(jì)員努力符合所有系統(tǒng)性能要求在最低的費(fèi)用合計(jì)。經(jīng)常超過一個(gè)驅(qū)動(dòng)系統(tǒng)也許滿足所有系統(tǒng)性能標(biāo)準(zhǔn)在競(jìng)爭(zhēng)費(fèi)用。更喜歡的驅(qū)動(dòng)器安排是最簡(jiǎn)單, 譬如一個(gè)唯一馬達(dá)驅(qū)動(dòng)通過一個(gè)唯一頂頭滑輪。但是, 機(jī)械, 經(jīng)濟(jì), 和功能需求經(jīng)常需要對(duì)復(fù)雜驅(qū)動(dòng)器的用途。傳送帶設(shè)計(jì)員必須平衡對(duì)優(yōu)雅的需要反對(duì)伴隨復(fù)雜系統(tǒng)的問題。復(fù)雜系統(tǒng)要求額外設(shè)計(jì)工程為成功配置。經(jīng)常被忽略的費(fèi)用在復(fù)雜系統(tǒng)是培訓(xùn)人事部的費(fèi)用, 或停工期的費(fèi)用由于不足的培訓(xùn)。
軟起動(dòng)驅(qū)動(dòng)控制邏輯
各個(gè)驅(qū)動(dòng)系統(tǒng)將要求一個(gè)控制系統(tǒng)調(diào)控開始的機(jī)制。最共同的類型控制被使用在更小對(duì)中等大小驅(qū)動(dòng)以簡(jiǎn)單的外形被命名"開環(huán)加速度控制" 。 在開環(huán), 控制系統(tǒng)早先被配置程序化開始的機(jī)制以被規(guī)定的方式, 通常準(zhǔn)時(shí)根據(jù)。 在開環(huán)控制, 駕駛使用參數(shù)譬如潮流, 扭矩, 或速度不影響序列操作。 這個(gè)方法假定, 控制設(shè)計(jì)師充分地塑造了驅(qū)動(dòng)系統(tǒng)表現(xiàn)在傳動(dòng)機(jī)。 為更大或更加復(fù)雜的傳送帶, "閉合回路" 或"反饋" 控制可以他運(yùn)用了。 在閉合回路控制, 在開始期間, 控制系統(tǒng)顯示器通過傳感器駕駛使用參數(shù)譬如馬達(dá)的當(dāng)前層, 傳送帶的速度, 或力量在傳送帶, 并且修改起動(dòng)程序控制, 極限, 或優(yōu)選或佩帶了參量。閉合回路控制系統(tǒng)修改開始的被應(yīng)用的力量在一臺(tái)空和充分地被裝載的傳動(dòng)機(jī)之間。常數(shù)在數(shù)學(xué)模型與被測(cè)量的可變物有關(guān)對(duì)系統(tǒng)驅(qū)動(dòng)反應(yīng)被命名定調(diào)的常數(shù)。 這些常數(shù)必須適當(dāng)?shù)乇徽{(diào)整為成功的應(yīng)用對(duì)各臺(tái)傳動(dòng)機(jī)。 最共同的計(jì)劃為傳動(dòng)機(jī)開始閉合回路控制是車頭表反饋為速度控制和壓電池力量或驅(qū)動(dòng)力反饋為扭矩控制。在一些復(fù)雜系統(tǒng), 它是中意安排閉合回路控制系統(tǒng)調(diào)整自己為各種各樣的遇到的傳動(dòng)機(jī)情況。 這被命名"能適應(yīng)的控制" 。這些極端可能介入浩大的變異在裝貨, 圍繞的溫度, 裝貨的地點(diǎn)在外形, 或多個(gè)驅(qū)動(dòng)選擇在傳動(dòng)機(jī)。 有三個(gè)共同的能適應(yīng)的方法。介入決定做在開始之前,如果控制系統(tǒng)能知道傳送帶是空的, 它會(huì)減少最初的力量和會(huì)加長加速度力量的應(yīng)用對(duì)最高速度。如果傳送帶被裝載, 控制系統(tǒng)會(huì)應(yīng)用資格力量在攤位之下使較少時(shí)刻和供應(yīng)充足的扭矩及時(shí)地充分地加速傳送帶。 因?yàn)閭魉蛶е怀蔀榱搜b載在早先賽跑期間由裝載驅(qū)動(dòng), 平均驅(qū)動(dòng)潮流可能被抽樣當(dāng)連續(xù)和被保留在反射傳送帶搬運(yùn)器時(shí)間的緩沖記憶。然后在停工平均也許是預(yù)先處理一些開環(huán)和閉合回路為下個(gè)開始。第二個(gè)方法介入根據(jù)驅(qū)動(dòng)觀察發(fā)生在最初開始或"行動(dòng)期間證明'' 的決定。這及時(shí)驅(qū)動(dòng)潮流的或力量通常介入比較對(duì)傳送帶速度。 如果驅(qū)動(dòng)潮流或力量必需及早在序列是降低并且行動(dòng)被創(chuàng)始, 傳送帶必須被卸載。如果驅(qū)動(dòng)潮流或力量必需是高的。在開始, 傳動(dòng)機(jī)必須被裝載。 這個(gè)決定可能被劃分在區(qū)域和使用修改起動(dòng)程序控制的中部和結(jié)束。第三個(gè)方法介入傳送帶速度的比較對(duì)時(shí)刻為這個(gè)開始反對(duì)傳送帶加速度歷史極限, 或'' 加速度信封監(jiān)視''。在開始, 傳送帶速度被測(cè)量對(duì)時(shí)間。這與被保留在控制系統(tǒng)記憶的二限制的傳送帶速度曲線比較。第一曲線描出空的傳送帶加速, 并且第二個(gè)充分地被裝載的傳送帶。 因而, 如果當(dāng)前的速度對(duì)時(shí)間比被裝載的外形低, 它也許表明, 傳送帶被超載, 妨礙, 或驅(qū)動(dòng)故障。 如果當(dāng)前的速度對(duì)時(shí)間比空間的外形高級(jí), 它也許表明一條殘破的傳送帶結(jié)合或驅(qū)動(dòng)故障。
無論如何，當(dāng)前的起飛中止并且警報(bào)運(yùn)行。
結(jié)論
最好的傳送帶啟動(dòng)系統(tǒng)要求在不同的傳送帶負(fù)載條件下，能夠以合理的代價(jià)帶來可靠性高的可以接受的運(yùn)行性能。但是至今沒有一個(gè)啟動(dòng)系統(tǒng)能夠達(dá)到這樣的要求。傳送帶設(shè)計(jì)者必須為每個(gè)傳送帶設(shè)計(jì)啟動(dòng)系統(tǒng)屬性?？偟脕碚f，全電壓交流發(fā)動(dòng)機(jī)啟動(dòng)適合于簡(jiǎn)單結(jié)構(gòu)的小型傳送帶。減電壓SCR交流發(fā)動(dòng)機(jī)啟動(dòng)是地下中、小型傳送帶的基本啟動(dòng)方法。最新的進(jìn)展顯示，固定液體填充耦合系統(tǒng)的交流發(fā)動(dòng)機(jī)是簡(jiǎn)單結(jié)構(gòu)中、大型傳送帶基本啟動(dòng)方法。對(duì)于那些大、中型而且需要重復(fù)啟動(dòng)的復(fù)雜結(jié)構(gòu)傳送帶，繞線轉(zhuǎn)子發(fā)動(dòng)機(jī)驅(qū)動(dòng)是常用的選擇。在結(jié)構(gòu)特別復(fù)雜，運(yùn)行需要不同速度的傳送帶啟動(dòng)中，傳送帶直流發(fā)動(dòng)機(jī)驅(qū)動(dòng)、不同填充液體驅(qū)動(dòng)、和相異機(jī)械傳遞驅(qū)動(dòng)系統(tǒng)一直實(shí)力相當(dāng)?shù)暮蜻x者。具體選擇哪個(gè)啟動(dòng)方式由使用環(huán)境，相對(duì)價(jià)格，運(yùn)行能耗，反應(yīng)速度和使用者習(xí)慣來決定。變頻交流驅(qū)動(dòng)和非電刷直流驅(qū)動(dòng)主要限制于中型傳送帶，這些中型傳送帶需要精確的速度控制，高代價(jià)和復(fù)雜性。但是，隨著持續(xù)的競(jìng)爭(zhēng)和技術(shù)進(jìn)步，波形綜合技術(shù)的電子驅(qū)動(dòng)器的使用將越來越廣。
A Comparison of Soft Start Mechanisms for Mining Belt Conveyors
Michael L. Nave, P.E.
CONSOL Inc.
1800 Washington Road Pittsburgh, PA 15241 Belt Conveyors are an important method for transportation of bulk materials in the mining industry. The control of the application of the starting torque from the belt drive system to the belt fabric affects the performance, life cost, and reliability of the conveyor. This paper examines applications of each starting method within the coal mining industry.
INTRODUCTION
The force required to move a belt conveyor must be transmitted by the drive pulley via friction between the drive pulley and the belt fabric. In order to transmit power there must be a difference in the belt tension as it approaches and leaves the drive pulley. These conditions are true for steady state running, starting, and stopping. Traditionally, belt designs are based on static calculations of running forces. Since starting and stopping are not examined in detail, safety factors are applied to static loadings (Harrison, 1987). This paper will primarily address the starting or acceleration duty of the conveyor. The belt designer must control starting acceleration to prevent excessive tension in the belt fabric and forces in the belt drive system (Suttees, 1986). High acceleration forces can adversely affect the belt fabric, belt splices, drive pulleys, idler pulleys, shafts, bearings, speed reducers, and couplings. Uncontrolled acceleration forces can cause belt conveyor system performance problems with vertical curves, excessive belt take-up movement, loss of drive pulley friction, spillage of materials, and festooning of the belt fabric. The belt designer is confronted with two problems, The belt drive system must produce a minimum torque powerful enough to start the conveyor, and controlled such that the acceleration forces are within safe limits. Smooth starting of the conveyor can be accomplished by the use of drive torque control equipment, either mechanical or electrical, or a combination of the two (CEM, 1979).
SOFT START MECHANISM EVALUATION CRITERION
What is the best belt conveyor drive system? The answer depends on many variables. The best system is one that provides acceptable control for starting, running, and stopping at a reasonable cost and with high reliability (Lewdly and Sugarcane, 1978). Belt Drive System For the purposes of this paper we will assume that belt conveyors are almost always driven by electrical prime movers (Goodyear Tire and Rubber, 1982). The belt "drive system" shall consist of multiple components including the electrical prime mover, the electrical motor starter with control system, the motor coupling, the speed reducer, the low speed coupling, the belt drive pulley, and the pulley brake or hold back (Cur, 1986). It is important that the belt designer examine the applicability of each system component to the particular application. For the purpose of this paper, we will assume that all drive system components are located in the fresh air, non-permissible, areas of the mine, or in non-hazardous, National Electrical Code, Article 500 explosion-proof, areas of the surface of the mine.
Belt Drive Component Attributes Size.
Certain drive components are available and practical in different size ranges. For this discussion, we will assume that belt drive systems range from fractional horsepower to multiples of thousands of horsepower. Small drive systems are often below 50 horsepower. Medium systems range from 50 to 1000 horsepower. Large systems can be considered above 1000 horsepower. Division of sizes into these groups is entirely arbitrary. Care must be taken to resist the temptation to over motor or under motor a belt flight to enhance standardization. An over motored drive results in poor efficiency and the potential for high torques, while an under motored drive could result in destructive overspending on regeneration, or overheating with shortened motor life (Lords, et al., 1978).
Torque Control.
Belt designers try to limit the starting torque to no more than 150% of the running torque (CEMA, 1979; Goodyear, 1982). The limit on the applied starting torque is often the limit of rating of the belt carcass, belt splice, pulley lagging, or shaft deflections. On larger belts and belts with optimized sized components, torque limits of 110% through 125% are common (Elberton, 1986). In addition to a torque limit, the belt starter may be required to limit torque increments that would stretch belting and cause traveling waves. An ideal starting control system would apply a pretension torque to the belt at rest up to the point of breakaway, or movement of the entire belt, then a torque equal to the movement requirements of the belt with load plus a constant torque to accelerate the inertia of the system components from rest to final running speed. This would minimize system transient forces and belt stretch (Shultz, 1992). Different drive systems exhibit varying ability to control the application of torques to the belt at rest and at different speeds. Also, the conveyor itself exhibits two extremes of loading. An empty belt normally presents the smallest required torque for breakaway and acceleration, while a fully loaded belt presents the highest required torque. A mining drive system must be capable of scaling the applied torque from a 2/1 ratio for a horizontal simple belt arrangement, to a 10/1 ranges for an inclined or complex belt profile.
Thermal Rating.
During starting and running, each drive system may dissipate waste heat. The waste heat may be liberated in the electrical motor, the electrical controls,, the couplings, the speed reducer, or the belt braking system. The thermal load of each start Is dependent on the amount of belt load and the duration of the start. The designer must fulfill the application requirements for repeated starts after running the conveyor at full load. Typical mining belt starting duties vary from 3 to 10 starts per hour equally spaced, or 2 to 4 starts in succession. Repeated starting may require the dreading or over sizing of system components. There is a direct relationship between thermal rating for repeated starts and costs. Variable Speed. Some belt drive systems are suitable for controlling the starting torque and speed, but only run at constant speed. Some belt applications would require a drive system capable of running for extended periods at less than full speed. This is useful when the drive load must be shared with other drives, the belt is used as a process feeder for rate control of the conveyed material, the belt speed is optimized for the haulage rate, the belt is used at slower speeds to transport men or materials, or the belt is run a slow inspection or inching speed for maintenance purposes (Hager, 1991). The variable speed belt drive will require a control system based on some algorithm to regulate operating speed. Regeneration or Overhauling Load. Some belt profiles present the potential for overhauling loads where the belt system supplies energy to the drive system. Not all drive systems have the ability to accept regenerated energy from the load. Some drives can accept energy from the load and return it to the power line for use by other loads. Other drives accept energy from the load and dissipate it into designated dynamic or mechanical braking elements. Some belt profiles switch from motoring to regeneration during operation. Can the drive system accept regenerated energy of a certain magnitude for the application? Does the drive system have to control or modulate the amount of retarding force during overhauling? Does the overhauling occur when running and starting? Maintenance and Supporting Systems. Each drive system will require periodic preventative maintenance. Replaceable items would include motor brushes, bearings, brake pads, dissipation resistors, oils, and cooling water. If the drive system is conservatively engineered and operated, the lower stress on consumables will result in lower maintenance costs. Some drives require supporting systems such as circulating oil for lubrication, cooling air or water, environmental dust filtering, or computer instrumentation. The maintenance of the supporting systems can affect the reliability of the drive system.
Cost.
The drive designer will examine the cost of each drive system. The total cost is the sum of the first capital cost to acquire the drive, the cost to install and commission the drive, the cost to operate the drive, and the cost to maintain the drive. The cost for power to operate the drive may vary widely with different locations. The designer strives to meet all system performance requirements at lowest total cost. Often more than one drive system may satisfy all system performance criterions at competitive costs.
Complexity.
The preferred drive arrangement is the simplest, such as a single motor driving through a single head pulley. However, mechanical, economic, and functional requirements often necessitate the use of complex drives. The belt designer must balance the need for sophistication against the problems that accompany complex systems. Complex systems require additional design engineering for successful deployment. An often-overlooked cost in a complex system is the cost of training onsite personnel, or the cost of downtime as a result of insufficient training.
SOFT START DRIVE CONTROL LOGIC
Each drive system will require a control system to regulate the starting mechanism. The most common type of control used on smaller to medium sized drives with simple profiles is termed "Open Loop Acceleration Control". In open loop, the control system is previously configured to sequence the starting mechanism in a prescribed manner, usually based on time. In open loop control, drive-operating parameters such as current, torque, or speed do not influence sequence operation. This method presumes that the control designer has adequately modeled drive system performance on the conveyor. For larger or more complex belts, "Closed Loop" or "Feedback" control may he utilized. In closed loop control, during starting, the control system monitors via sensors drive operating parameters such as current level of the motor, speed of the belt, or force on the belt, and modifies the starting sequence to control, limit, or optimize one or wore parameters. Closed loop control systems modify the starting applied force between an empty and fully loaded conveyor. The constants in the mathematical model related to the measured variable versus the system drive response are termed the tuning constants. These constants must be properly adjusted for successful application to each conveyor. The most common schemes for closed loop control of conveyor starts are tachometer feedback for speed control and load cell force or drive force feedback for torque control. On some complex systems, It is desirable to have the closed loop control system adjust itself for various encountered conveyor conditions. This is termed "Adaptive Control". These extremes can involve vast variations in loadings, temperature of the belting, location of the loading on the profile, or multiple drive options on the conveyor. There are three common adaptive methods. The first involves decisions made before the start, or ''Restart Conditioning''. If the control system could know that the belt is empty, it would reduce initial force and lengthen the application of acceleration force to full speed. If the belt is loaded, the control system would apply pretension forces under stall for less time and supply sufficient torque to adequately accelerate the belt in a timely manner. Since the belt only became loaded during previous running by loading the drive, the average drive current can be sampled when running and retained in a first-in-first-out buffer memory that reflects the belt conveyance time. Then at shutdo wn the FIFO average may be use4 to precondition some open loop and closed loop set points for the next start. The second method involves decisions that are based on drive observations that occur during initial starting or "Motion Proving''. This usually involves a comparison In time of the drive current or force versus the belt speed. if the drive current or force required early in the sequence is low and motion is initiated, the belt must be unloaded. If the drive current or force required is high and motion is slow in starting, the conveyor must be loaded. This decision can be divided in zones and used to modify the middle and finish of the start sequence control. The third method involves a comparison of the belt speed versus time for this start against historical limits of belt acceleration, or ''Acceleration Envelope Monitoring''. At start, the belt speed is measured versus time. This is compared with two limiting belt speed curves that are retained in control system memory. The first curve profiles the empty belt when accelerated, and the second one the fully loaded belt. Thus, if the current speed versus time is lower than the loaded profile, it may indicate that the belt is overloaded, impeded, or drive malfunction. If the current speed versus time is higher than the empty profile, it may indicate a broken belt, coupling, or drive malfunction.
In either case, the current start is aborted and an alarm issued
.
CONCLUSION
The best belt starting system is one that provides acceptable performance under all belt load Conditions at a reasonable cost with high reliability. No one starting system meets all needs. The belt designer must define the starting system attributes that are required for each belt. In general, the AC induction motor with full voltage starting is confined to small belts with simple profiles. The AC induction motor with reduced voltage SCR starting is the base case mining starter for underground belts from small to medium sizes. With recent improvements, the AC motor with fixed fill fluid couplings is the base case for medium to large conveyors with simple profiles. The Wound Rotor Induction Motor drive is the traditional choice for medium to large belts with repeated starting duty or complex profiles that require precise torque control. The DC motor drive, Variable Fill Hydrokinetic drive, and the Variable Mechanical Transmission drive compete for application on belts with extreme profiles or variable speed at running requirements. The choice is dependent on location environment, competitive price, operating energy losses, speed response, and user familiarity. AC Variable Frequency drive and Brush less DC applications are limited to small to medium sized belts that require precise speed control due to higher present costs and complexity. However, with continuing competitive and technical improvements, the use of synthesized waveform electronic drives will expand.
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