第十章無刷直流電機(jī)

1、第10章 無刷直流電動(dòng)機(jī)Chapter 10 Brushless DC MotorCooling fan - brushless electric motor, outside rotorParameters: PC power supply fan, 12V 0.12A, brushless 大功率永磁無刷直流電機(jī)（發(fā)布時(shí)間：2007-3-17 15:52:03） 10.1 概 述直流電動(dòng)機(jī)主要優(yōu)點(diǎn)是調(diào)速和起動(dòng)特性好，堵轉(zhuǎn)轉(zhuǎn)矩大，因而被廣泛應(yīng)用于各種驅(qū)動(dòng)裝置和伺服系統(tǒng)中。但是，直流電動(dòng)機(jī)具有電刷和換向器，其間形成的滑動(dòng)機(jī)械接觸嚴(yán)重地影響了電機(jī)的精度、性能和可靠性，所產(chǎn)生的火花會(huì)引起無線電干擾

2、，縮短電機(jī)壽命，換向器電刷裝置又使直流電機(jī)結(jié)構(gòu)復(fù)雜、噪音大、維護(hù)困難，因此長(zhǎng)期以來人們都在尋求可以不用電刷和換向器裝置的直流電動(dòng)機(jī)。隨著電子技術(shù)的迅速發(fā)展，各種大功率電子器件的廣泛采用，這種愿望已被逐步實(shí)現(xiàn)。本章要介紹的無刷直流電動(dòng)機(jī)利用電子開關(guān)線路和位置傳感器來代替電刷和換向器，使這種電機(jī)既具有直流電動(dòng)機(jī)的特性，又具有交流電動(dòng)機(jī)結(jié)構(gòu)簡(jiǎn)單、運(yùn)行可靠、維護(hù)方便等優(yōu)點(diǎn)；它的轉(zhuǎn)速不再受機(jī)械換向的限制，若采用高速軸承，還可以在高達(dá)每分鐘幾十萬轉(zhuǎn)的轉(zhuǎn)速中運(yùn)行。因此，無刷直流電動(dòng)機(jī)用途非常廣泛，可作為一般直流電動(dòng)機(jī)、伺服電動(dòng)機(jī)和力矩電動(dòng)機(jī)等使用，尤其適用于高級(jí)電子設(shè)備、機(jī)器人、航空航天技術(shù)、數(shù)控裝置、 醫(yī)

3、療化工等高新技術(shù)領(lǐng)域。 無刷直流電動(dòng)機(jī)將電子線路與電機(jī)融為一體， 把先進(jìn)的電子技術(shù)應(yīng)用于電機(jī)領(lǐng)域，這將促使電機(jī)技術(shù)更新、更快的發(fā)展。無刷直流電動(dòng)機(jī)是由電動(dòng)機(jī)、 轉(zhuǎn)子位置傳感器和電子開關(guān)線路三部分組成， 它的原理框圖如圖10-1所示。圖中直流電源通過開關(guān)線路向電動(dòng)機(jī)定子繞組供電，電動(dòng)機(jī)轉(zhuǎn)子位置由位置傳感器檢測(cè)并提供信號(hào)去觸發(fā)開關(guān)線路中的功率開關(guān)元件使之導(dǎo)通或截止，從而控制電動(dòng)機(jī)的轉(zhuǎn)動(dòng)。圖 10 - 1 無刷直流電動(dòng)機(jī)的原理框圖圖3.1 控制系統(tǒng)結(jié)構(gòu)框圖永磁無刷直流電機(jī)控制系統(tǒng)由控制部分、驅(qū)動(dòng)及逆變電路部分、轉(zhuǎn)子位置檢測(cè)以及電流采樣電路構(gòu)成其中以TMS320LF2407A 為核心的控制部分負(fù)責(zé)控制

4、運(yùn)算模擬量采集等任務(wù)驅(qū)動(dòng)電路將控制電路輸出的弱電信號(hào)進(jìn)行功率放大輸出具有一定驅(qū)動(dòng)能力的強(qiáng)電信號(hào)去控制逆變電路中的開關(guān)管工作逆變電路由功率半導(dǎo)體器件MOSFET 搭建而成38 按一定控制規(guī)律的信號(hào)工作實(shí)現(xiàn)對(duì)直流電的逆變轉(zhuǎn)換供給電機(jī)達(dá)到對(duì)電機(jī)控制的目的位置檢測(cè)部分檢測(cè)電機(jī)轉(zhuǎn)子信號(hào)并送給控制部分處理電流采樣部分完成對(duì)直流電源母線電流的檢測(cè)控制系統(tǒng)硬件框圖如圖3.2 所示。圖3.2 控制系統(tǒng)硬件框圖10.2 無刷直流電動(dòng)機(jī)的基本結(jié)構(gòu) 無刷直流電動(dòng)機(jī)的基本結(jié)構(gòu)如圖10 - 2所示。圖中電動(dòng)機(jī)結(jié)構(gòu)與永磁式同步電動(dòng)機(jī)相似，轉(zhuǎn)子是由永磁材料制成一定極對(duì)數(shù)的永磁體，但不帶鼠籠繞組或其它起動(dòng)裝置，主要有兩種結(jié)構(gòu)型

5、式，如圖10 - 3(a)和(b)所示。第一種結(jié)構(gòu)是轉(zhuǎn)子鐵心外表面粘貼瓦片形磁鋼，稱為凸極式；第二種結(jié)構(gòu)是磁鋼插入轉(zhuǎn)子鐵心的溝槽中，稱為內(nèi)嵌式或隱極式。圖 10 - 2 無刷直流電動(dòng)機(jī)的基本結(jié)構(gòu) 圖 10 - 3 永磁轉(zhuǎn)子結(jié)構(gòu)型式 定子是電動(dòng)機(jī)的電樞。定子鐵心中安放著對(duì)稱的多相繞組，可接成星形或封閉形(角形)，各相繞組分別與電子開關(guān)線路中的相應(yīng)晶體管相連接。電子開關(guān)線路有橋式和非橋式兩種。圖10 - 4表示常用的幾種電樞繞組連接方式，其中圖(a)、 (b)是非橋式開關(guān)電路，其它是橋式開關(guān)電路。 圖10 - 4 電樞繞組連接方式 (a) 三相星形三狀態(tài)；(b) 四相星形四狀態(tài)；(c) 三相星形六

6、狀態(tài)；(d) 兩相正交四狀態(tài)；(e) 三相封閉六狀態(tài)；(f) 四相封閉四狀態(tài)位置傳感器10.2.1 電磁式 這種傳感器的結(jié)構(gòu)如圖10 - 5所示。它由定子和轉(zhuǎn)子兩部分組成。定子磁芯及轉(zhuǎn)子上的扇形部分均由高頻導(dǎo)磁材料(如軟磁鐵氧體)制成，導(dǎo)磁扇形片數(shù)等于電機(jī)極對(duì)數(shù)，放置在不導(dǎo)磁的鋁合金圓盤上制成了轉(zhuǎn)子。傳感器定子由磁心和線圈組成，磁心的結(jié)構(gòu)特點(diǎn)是中間為圓柱體，安放勵(lì)磁繞組，外施高頻電源勵(lì)磁。圓周上沿軸向有凸起的極，極上套著信號(hào)線圈產(chǎn)生信號(hào)電壓?？梢钥闯?，這實(shí)際上是一個(gè)有共同勵(lì)磁線圈的幾個(gè)開口變壓器，扇形導(dǎo)磁片的作用是使開口變壓器鐵心接近閉合，減少磁阻，使信號(hào)線圈感應(yīng)出較大的電勢(shì)。電磁式位置傳感器

7、具有輸出信號(hào)大、工作可靠、壽命長(zhǎng)、使用環(huán)境要求不高、適應(yīng)性強(qiáng)、結(jié)構(gòu)簡(jiǎn)單和緊湊等優(yōu)點(diǎn)。但這種傳感器的信噪比較低，體積較大，同時(shí)其輸出波形為交流，一般需經(jīng)過整流、濾波后才可使用。圖10-5 電磁式位置傳感器10.2.2 光電式 光電式傳感器是由固定在定子上的幾個(gè)光電耦合開關(guān)和固定在轉(zhuǎn)子軸上的遮光盤所組成，如圖10-6所示。遮光盤上按要求開出光槽(孔)，幾個(gè)光電耦合開關(guān)沿著圓周均布，每只光電耦合開關(guān)是由相互對(duì)著的紅外發(fā)光二極管(或激光器)和光電管(光電二極管，三極管或光電池)所組成。紅外發(fā)光二極管(或激光器)通上電后，發(fā)出紅外光(或激光)；當(dāng)遮光盤隨著轉(zhuǎn)軸轉(zhuǎn)動(dòng)時(shí)，光線依次通過光槽(孔)，使對(duì)著的光電

8、管導(dǎo)通，相應(yīng)地產(chǎn)生反應(yīng)轉(zhuǎn)子相對(duì)定子位置的電信號(hào)，經(jīng)放大后去控制功率晶體管，使相應(yīng)的定子繞組切換電流。光電式位置傳感器產(chǎn)生的電信號(hào)一般都較弱，需要經(jīng)過放大才能去控制功率晶體管。但它輸出的是直流電信號(hào)，不必再進(jìn)行整流，這是它的一個(gè)優(yōu)點(diǎn)。圖 10 - 6 光電式位置傳感器10.2.3 霍爾元件采用霍爾元件作為位置傳感器的無刷直流電動(dòng)機(jī)通常稱為“霍爾無刷直流電動(dòng)機(jī)”。由于無刷直流電動(dòng)機(jī)的轉(zhuǎn)子是永磁的，就可以很方便地利用霍爾元件的“霍爾效應(yīng)”檢測(cè)轉(zhuǎn)子的位置。圖10-7表示四相霍爾無刷直流電動(dòng)機(jī)原理圖。圖中兩個(gè)霍爾元件H1和H2以間隔90°電角度粘于電機(jī)定子繞組A和B的軸線上，并通上控制電流，電

9、機(jī)轉(zhuǎn)子磁鋼兼作位置傳感器的轉(zhuǎn)子。當(dāng)電機(jī)轉(zhuǎn)子旋轉(zhuǎn)時(shí)磁鋼N極和S極輪流通過霍爾元件H1和H2，因而產(chǎn)生對(duì)應(yīng)轉(zhuǎn)子位置的兩個(gè)正的和兩個(gè)負(fù)的霍爾電勢(shì)，經(jīng)放大后去控制功率晶體管導(dǎo)通，使四個(gè)定子繞組輪流切換電流?；魻枱o刷直流電動(dòng)機(jī)結(jié)構(gòu)簡(jiǎn)單，體積小，但安置和定位不便，元件片薄易碎，對(duì)環(huán)境及工作溫度有一定要求，耐震差。圖10 - 7 霍爾無刷直流電動(dòng)機(jī)原理圖10.3 無刷直流電動(dòng)機(jī)工作原理無刷直流電機(jī)，其轉(zhuǎn)子采用永磁體勵(lì)磁產(chǎn)生直軸位置的勵(lì)磁磁場(chǎng)，而定子為電樞繞組通過功率控制器控制各相繞組的通斷狀態(tài)。由于定子電樞繞組通斷狀態(tài)的組合方式是有限的，因此定子電樞繞組產(chǎn)生的電樞磁場(chǎng)位置也是有限的，不可能像交流感應(yīng)電機(jī)一樣

10、產(chǎn)生幅值恒定又連續(xù)旋轉(zhuǎn)的定子磁場(chǎng)，所以無刷直流電機(jī)定子繞組產(chǎn)生的磁場(chǎng)必定是跳躍式前進(jìn)的(也稱步進(jìn)式的)，這是不同于直流電機(jī)和交流電機(jī)電樞磁場(chǎng)運(yùn)動(dòng)的特征之一。但是，這種跳躍式前進(jìn)的磁場(chǎng)仍然要與轉(zhuǎn)子磁場(chǎng)保持相對(duì)同步。如果定子磁場(chǎng)相對(duì)于轉(zhuǎn)子磁場(chǎng)始終超前90°電角左右范圍內(nèi)運(yùn)動(dòng)，那么定子電樞磁場(chǎng)總是吸引轉(zhuǎn)子永磁勵(lì)磁磁場(chǎng)，它們之間能夠產(chǎn)生正的平均電磁轉(zhuǎn)矩。雖然定轉(zhuǎn)子磁場(chǎng)之間存在相對(duì)運(yùn)動(dòng)，但是轉(zhuǎn)子能夠不停地跟隨定子磁場(chǎng)正向旋轉(zhuǎn)。同樣，如果定子磁場(chǎng)相對(duì)于轉(zhuǎn)子磁場(chǎng)滯后90°電角左右范圍內(nèi)運(yùn)動(dòng)，那么定子電樞磁場(chǎng)同樣能吸引轉(zhuǎn)子永磁勵(lì)磁磁場(chǎng)，它們之間也能夠產(chǎn)生負(fù)的平均電磁轉(zhuǎn)矩。雖然定轉(zhuǎn)子磁場(chǎng)之間

11、存在相對(duì)運(yùn)動(dòng)，但不會(huì)影響轉(zhuǎn)子跟隨磁場(chǎng)反向旋轉(zhuǎn)。否則，定子電樞磁場(chǎng)與轉(zhuǎn)子勵(lì)磁磁場(chǎng)之間的相對(duì)運(yùn)動(dòng)將導(dǎo)致產(chǎn)生的平均電磁轉(zhuǎn)矩很小甚至為0，不能驅(qū)動(dòng)負(fù)載連續(xù)運(yùn)行，最終轉(zhuǎn)子停止不動(dòng)，這種情況稱為失步?？傊３侄ㄞD(zhuǎn)子磁場(chǎng)產(chǎn)生恒定的平均電磁轉(zhuǎn)矩，必須保證定轉(zhuǎn)子磁場(chǎng)在空間保持相對(duì)靜止。這種相對(duì)靜止有2層含義：一是恒定的平均電磁轉(zhuǎn)矩而不是恒定的瞬時(shí)電磁轉(zhuǎn)矩，即瞬時(shí)轉(zhuǎn)矩可以變化，但總體上存在一定大小的平均值，這種電磁轉(zhuǎn)矩瞬時(shí)變化由具有機(jī)械慣性的轉(zhuǎn)子起到平滑作用，即轉(zhuǎn)矩波動(dòng)隨著轉(zhuǎn)子轉(zhuǎn)動(dòng)慣量增大而減小。二是定轉(zhuǎn)子磁場(chǎng)在空間保持相對(duì)靜止而不是保持相互之間的絕對(duì)靜止，即使瞬時(shí)定轉(zhuǎn)子磁場(chǎng)之間存在相對(duì)運(yùn)動(dòng)，但總統(tǒng)上始終保持

12、同步以產(chǎn)生恒定的平均電磁轉(zhuǎn)矩。10.3.1 三相非橋式星形接法 圖10 - 8表示一臺(tái)采用非橋式晶體管開關(guān)電路驅(qū)動(dòng)兩極星形三相繞組，并帶有電磁式位置傳感器的無刷直流電動(dòng)機(jī)。轉(zhuǎn)子位置傳感器的勵(lì)磁線圈由高頻振蕩器供電，通過導(dǎo)磁片的作用使信號(hào)線圈獲得較大的感應(yīng)電壓，并經(jīng)整流、放大加到開關(guān)電路功率管的基極上使該管導(dǎo)通，因而與該管串聯(lián)的定子繞組也就與外電源接通。由于導(dǎo)磁片與電動(dòng)機(jī)轉(zhuǎn)子同軸旋轉(zhuǎn)，所以信號(hào)線圈Wa、Wb、Wc依次得電，3個(gè)功率管依次導(dǎo)通，使定子三相繞組輪流通電。當(dāng)電機(jī)轉(zhuǎn)子處于圖10-8瞬時(shí)，位置傳感器PS的扇形導(dǎo)磁片位于圖示位置處，它的信號(hào)線圈Wa開始與勵(lì)磁線圈相耦合，便有信號(hào)電壓輸出，其余

13、兩個(gè)信號(hào)線圈Wb、Wc的信號(hào)電壓為0。線圈Wa供出的信號(hào)電壓使晶體管V1開始導(dǎo)通，而晶體管V2、V3截止。這樣，電樞繞組AX有電流通過，電樞磁場(chǎng)Ba的方向如圖中所示。電樞磁場(chǎng)與永磁轉(zhuǎn)子磁場(chǎng)相互作用就產(chǎn)生轉(zhuǎn)矩，使轉(zhuǎn)子按順時(shí)針方向旋轉(zhuǎn)。圖 10 - 8 無刷直流電動(dòng)機(jī)工作原理 圖 10 - 9 電樞磁場(chǎng)與轉(zhuǎn)子磁場(chǎng)間的相對(duì)位置當(dāng)電機(jī)轉(zhuǎn)子在空間轉(zhuǎn)過2/3電角度時(shí)，位置傳感器的扇形片也轉(zhuǎn)過同樣角度，從而使信號(hào)線圈Wb開始有信號(hào)電壓輸出，Wa、Wc的信號(hào)電壓為0。Wb輸出的信號(hào)電壓便使晶體管V2開始導(dǎo)通，晶體管V1、V3截止。這樣，電樞繞組BY有電流通過，電樞磁場(chǎng)Ba的方向如圖10 - 9(a)所示。電樞

14、磁場(chǎng)Ba與永磁轉(zhuǎn)子磁場(chǎng)相互作用所產(chǎn)生的轉(zhuǎn)矩，使轉(zhuǎn)子繼續(xù)沿順時(shí)針方向旋轉(zhuǎn)。 若轉(zhuǎn)子繼續(xù)轉(zhuǎn)過2/3電角度，回到原來的起始位置(如圖10 - 9(c)，通過位置傳感器將重復(fù)上述的換流情況，如此循環(huán)下去，無刷直流電動(dòng)機(jī)在電樞磁場(chǎng)與永磁轉(zhuǎn)子磁場(chǎng)的相互作用下，能產(chǎn)生轉(zhuǎn)矩并使電機(jī)轉(zhuǎn)子按一定的轉(zhuǎn)向旋轉(zhuǎn)。可以看出，在三相星形非橋式的無刷直流電動(dòng)機(jī)中，當(dāng)轉(zhuǎn)子轉(zhuǎn)過2電角度時(shí)，定子電樞繞組共有3個(gè)通電狀態(tài)；每一狀態(tài)僅有一相導(dǎo)通，定子電流所產(chǎn)生的電樞磁場(chǎng)在空間跳躍著轉(zhuǎn)動(dòng)， 相應(yīng)地在空間也有3個(gè)不同的位置，稱為三個(gè)磁狀態(tài)；每一狀態(tài)持續(xù)2/3電角度， 這種通電方式稱為一相導(dǎo)通三相星形三狀態(tài)。每一晶體管導(dǎo)通時(shí)轉(zhuǎn)子所轉(zhuǎn)過的空間

15、電角度稱為導(dǎo)通角c。顯然，轉(zhuǎn)子位置傳感器的導(dǎo)磁扇形片張角p至少應(yīng)該等于導(dǎo)通角c。通常為了保證前后兩個(gè)導(dǎo)通狀態(tài)之間不出現(xiàn)間斷，就需要有個(gè)短暫的重疊時(shí)間，必須使p略大于c。電樞磁場(chǎng)在空間保持某一狀態(tài)時(shí)轉(zhuǎn)子所轉(zhuǎn)過的空間電角度，即定子上前后出現(xiàn)的兩個(gè)不同磁場(chǎng)軸線間所夾的電角度稱為磁狀態(tài)角，或稱狀態(tài)角，用m來表示。三相星形非橋式無刷直流電動(dòng)機(jī)各相繞組與各晶體管導(dǎo)通順序的關(guān)系如表10-1所示?？梢钥闯?，由于一個(gè)磁狀態(tài)對(duì)應(yīng)一相導(dǎo)通，所以角c和m都等于2/3。當(dāng)電機(jī)是p對(duì)極時(shí)，位置傳感器轉(zhuǎn)子沿圓周應(yīng)有p個(gè)均布的導(dǎo)磁扇形片， 每個(gè)扇形片張角p2/(3p)。表 10 - 1 星形三相三狀態(tài)導(dǎo)通順序表電角度 0 2

16、/3 4/3 2定子繞組的導(dǎo)通角ABC導(dǎo)通的晶體管V1V2V310.3.2 三相星形橋式接法 若定子繞組仍為三相，而功率晶體管接成橋式開關(guān)電路如圖10-10所示，相應(yīng)的位置傳感器原理圖如圖10-11所示。三相電樞繞組與各晶體管導(dǎo)通順序的關(guān)系如表10-2所示?？梢钥闯?，電機(jī)應(yīng)有6個(gè)通電狀態(tài)，每一狀態(tài)都是兩相同時(shí)導(dǎo)通，每個(gè)晶體管導(dǎo)通角仍為c=2/3，位置傳感器扇形片張角p2/(3p)。電樞合成磁場(chǎng)是由通電的兩相磁場(chǎng)所合成。若每相磁密在空間是正弦分布，用向量合成法可得合成磁密Ba的幅值等于每相磁密幅值的倍，它在空間也相應(yīng)有6個(gè)不同位置，磁狀態(tài)角m=/3。三相星形橋式電路的通電方式稱為兩相導(dǎo)通星形三相

17、六狀態(tài)。圖10 - 11 三相橋式電路的位置傳感器圖10 - 10 三相星形橋式開關(guān)電路V1 V6 (A+C-)V6V2 (B+C-)V2 V4 (B+A-)V4 V3 (C+A-)V3 V5 (C+B-)V5 V1(A+ B-)V1 V6 (A+C-)表10 - 2 兩相導(dǎo)通星形三相六狀態(tài)導(dǎo)通順序表10.3.3 三相封閉形橋式接法 封閉式定子繞組只能與橋式晶體管開關(guān)電路相組合。圖10-12表示三相封閉形(三角形)橋式接法的原理線路圖。三相電樞繞組與各晶體管導(dǎo)通順序的關(guān)系如表10 - 3所示，可以看出，它與星形接法的區(qū)別在于任何磁狀態(tài)中電樞繞組全部通電，總是某兩相繞組串聯(lián)后再與另一相繞組并聯(lián)。

18、在各狀態(tài)中僅是各相通電順序與電流流過的方向不同。電樞合成磁場(chǎng)是由通電的三相磁場(chǎng)所合成。圖10-13表示B相繞組與C相繞組串聯(lián)再與A相繞組并聯(lián)，電流由B相流向C相(符號(hào)為ABC)時(shí)的磁密向量圖?？梢姡ㄗ雍铣纱琶蹷a的幅值等于每相磁密幅值的1.5倍。三相封閉形橋式接法也有6個(gè)通電狀態(tài)，磁狀態(tài)角m=/3，導(dǎo)通角c=2/3，位置傳感器導(dǎo)磁扇形片張角p2/(3p)。這些都與三相星形橋式接法相同。三相封閉形橋式電路的通電方式也稱為封閉形三相六狀態(tài)。圖10 - 12 三相封閉形橋式開關(guān)電路V1 V6 A/(CB)，A+B-C-V6V2 B /(CA)，A+B-C+V2 V4 C /( BA)，A-B-C+

19、V4 V3 A /( BC)，A-B+C+V3 V5 B / ( AC)A，A-B+C-V5 V1C / ( AB)，A+B+C-V1 V6 A/(CB) ，A+C-B-當(dāng)V1 V6導(dǎo)通時(shí)，表 10 3 封閉形三角六狀態(tài)導(dǎo)通順序表圖10 13 對(duì)應(yīng)ABC時(shí)電樞磁密向量圖Brushless DC motorBrushless DC motors were developed from conventional brushed DC motors with the availability of solid state power semiconductors. So, why do we dis

20、cuss brushless DC motors in a chapter on AC motors? Brushless DC motors are similar to AC synchronous motors. The major difference is that synchronous motors develop a sinusoidal back EMF, as compared to a rectangular, or trapezoidal, back EMF for brushless DC motors. Both have stator created rotati

21、ng magnetic fields producing torque in a magnetic rotor. Synchronous motors are usually large multi-kilowatt size, often with electromagnet rotors. True synchronous motors are considered to be single speed, a submultiple of the powerline frequency. Brushless DC motors tend to be small a few watts to

22、 tens of watts, with permanent magnet rotors. The speed of a brushless DC motor is not fixed unless driven by a phased locked loop slaved to a reference frequency. The style of construction is either cylindrical or pancake. (Figures and below) Cylindrical construction: (a) outside rotor, (b) inside

23、rotor.The most usual construction, cylindrical, can take on two forms (Figure above). The most common cylindrical style is with the rotor on the inside, above right. This style motor is used in hard disk drives. It is also possible the put the rotor on the outside surrounding the stator. Such is the

24、 case with brushless DC fan motors, sans the shaft. This style of construction may be short and fat. However, the direction of the magnetic flux is radial with respect to the rotational axis. Pancake motor construction: (a) single stator, (b) double stator.High torque pancake motors may have stator

25、coils on both sides of the rotor (Figure above-b). Lower torque applications like floppy disk drive motors suffice with a stator coil on one side of the rotor, (Figure above-a). The direction of the magnetic flux is axial, that is, parallel to the axis of rotation. The commutation function may be pe

26、rformed by various shaft position sensors: optical encoder, magnetic encoder (resolver, synchro, etc), or Hall effect magnetic sensors. Small inexpensive motors use Hall effect sensors. (Figure below) A Hall effect sensor is a semiconductor device where the electron flow is affected by a magnetic fi

27、eld perpendicular to the direction of current flow. It looks like a four terminal variable resistor network. The voltages at the two outputs are complementary. Application of a magnetic field to the sensor causes a small voltage change at the output. The Hall output may drive a comparator to provide

28、 for more stable drive to the power device. Or, it may drive a compound transistor stage if properly biased. More modern Hall effect sensors may contain an integrated amplifier, and digital circuitry. This 3-lead device may directly drive the power transistor feeding a phase winding. The sensor must

29、 be mounted close to the permanent magnet rotor to sense its position. Hall effect sensors commutate 3- brushless DC motor.The simple cylindrical 3- motor Figure above is commutated by a Hall effect device for each of the three stator phases. The changing position of the permanent magnet rotor is se

30、nsed by the Hall device as the polarity of the passing rotor pole changes. This Hall signal is amplified so that the stator coils are driven with the proper current. Not shown here, the Hall signals may be processed by combinatorial logic for more efficient drive waveforms. The above cylindrical mot

31、or could drive a harddrive if it were equipped with a phased locked loop (PLL) to maintain constant speed. Similar circuitry could drive the pancake floppy disk drive motor (Figure below). Again, it would need a PLL to maintain constant speed. Brushless pancake motorThe 3- pancake motor (Figure abov

32、e) has 6-stator poles and 8-rotor poles. The rotor is a flat ferrite ring magnetized with eight axially magnetized alternating poles. We do not show that the rotor is capped by a mild steel plate for mounting to the bearing in the middle of the stator. The steel plate also helps complete the magneti

33、c circuit. The stator poles are also mounted atop a steel plate, helping to close the magnetic circuit. The flat stator coils are trapezoidal to more closely fit the coils, and approximate the rotor poles. The 6-stator coils comprise three winding phases. If the three stator phases were successively

34、 energized, a rotating magnetic field would be generated. The permanent magnet rotor would follow as in the case of a synchronous motor. A two pole rotor would follow this field at the same rotation rate as the rotating field. However, our 8-pole rotor will rotate at a submultiple of this rate due t

35、he the extra poles in the rotor. The brushless DC fan motor (Figure below) has these feature: Brushless fan motor, 2-.· The stator has 2-phases distributed between 4-poles · There are 4-salient poles with no windings to eliminate zero torque points. · The rotor has four main drive pol

36、es. · The rotor has 8-poles superimposed to help eliminate zero torque points. · The Hall effect sensors are spaced at 45o physical. · The fan housing is placed atop the rotor, which is placed over the stator.The goal of a brushless fan motor is to minimize the cost of manufacture. Th

37、is is an incentive to move lower performance products from a 3- to a 2- configuration. Depending on how it is driven, it may be called a 4- motor. You may recall that conventional DC motors cannot have an even number of armature poles (2,4, etc) if they are to be self-starting, 3,5,7 being common. T

38、hus, it is possible for a hypothetical 4-pole motor to come to rest at a torque minima, where it cannot be started from rest. The addition of the four small salient poles with no windings superimposes a ripple torque upon the torque vs position curve. When this ripple torque is added to normal energ

39、ized-torque curve, the result is that torque minima are partially removed. This makes it possible to start the motor for all possible stopping positions. The addition of eight permanant magnet poles to the normal 4-pole permanent magnet rotor superimposes a small second harmonic ripple torque upon t

40、he normal 4-pole ripple torque. This further removes the torque minima. As long as the torque minima does not drop to zero, we should be able to start the motor. The more successful we are in removing the torque minima, the easier the motor starting. The 2- stator requires that the Hall sensors be s

41、paced apart by 90o electrical. If the rotor was a 2-pole rotor, the Hall sensors would be placed 90o physical. Since we have a 4-pole permanent magnet rotor, the sensors must be placed 45o physical to achieve the 90o electrical spacing. Note Hall spacing above. The majority of the torque is due to t

42、he interaction of the inside stator 2- coils with the 4-pole section of the rotor. Moreover, the 4-pole section of the rotor must be on the bottom so that the Hall sensors will sense the proper commutation signals. The 8-poles rotor section is only for improving motor starting. Brushless DC motor 2- push-pull drive.In Figure above, the 2- push-pull drive (also known as 4- drive) uses two Hall effect sensors to drive four windings. The sensors are spaced 90o electrical apart, which is 90o physical for a single pole rotor. Si