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Rotor Brake
A mechanical breaking system is besides the aerodynamic breaking function of the rotor an unavoidable component of a wind turbine. It is part of the
mechanical drive train. The first task is to keep the rotor of a wind turbine in position when it is at a standstill. Locking the rotor is a must for servicing and repair work and is generally common practice during normal down times. Moreover, most turbines have locking bolts between rotor hub and nacelle for bridging extended periods of standstill and for servicing and repair work. The
rotor can thus be secured in one or more positions.
Rotor brakes are almost always disk brakes. Suitable disk brakes can frequently be adopted cost-effectively from existing production runs intended for other machines or vehicles .Against this background, the design of the rotor brake itself poses few problems. Nevertheless, the rotor brake presents the systems designer of a wind turbine with issues which have consequences for the entire system.
The first and most important question is, which task the rotor brake is to fulfill within the operating concept. In the simplest case, its role is restricted to a mere holding function during rotor standstill. In this case, the brake must be dimensioned for the required holding torque of the rotor during standstill. This is determined in accordance with the aerodynamic forces calculated to occur at the assumed maximum wind speeds (Chapt.6.3.2).
Apart from its function as a pure rotor parking brake, the rotor brake can also be dimensioned as a service brake. As long as the braking torque and braking power(thermal loading) can be absorbed, the mechanical rotor brake can be used as a second independent raking system in addition to aerodynamic rotor braking and the operational reliability of the wind turbine is considerably improved in this way. In small wind turbines, a mechanical rotor brake, which in cases of emergency prevents rotor runaway, has proved to be extraordinarily successful and is widely used today.
With increasing turbine size, it becomes more and more difficult to meet this requirement. For a turbine with a rotor diameter of 60 to 80 m, the rotor brake takes on almost absurd dimensions if it is to brake the rotor torque and power during full-load operation. For this reason, the task of the rotor brake in large turbines is always restricted to the function of pure parking brake.
Apart from the issue of the rotor brake’s task with respect to operations, there is the question of where in the drive train the rotor brake is best installed. The alternatives are for the rotor brake to be on the” low-speed” or on the” high-speed” side of the gearbox. In most turbines, efforts to keep the brake disk diameter as small as possible lead to the rotor brake being installed on the high-speed shaft, i. e. between gearbox and generator(Fig. 8.31). Owing to the higher rotational speed, the torque is one or even two orders of magnitude lower than at the slower rotor shaft, depending on the gear ratio.
However, mounting the brake on the high-speed shaft has at least two disadvantages. It is inferior from the point of view of safety, since the braking function fails if the low-speed shaft or the gearbox break down. Moreover, the rotor must be held by the gears during a standstill. Gears react with increased wear of the tooth flanks to small oscillating movements, which are unavoidable in a stopped wind turbine due to air turbulence. In some turbines, it is attempted to solve this problem by no longer locking the rotor during standstill but by letting it” spin” at low speed.
To avoid these disadvantages, the rotor brake was installed on the low-speed rotor shaft in some earlier systems. In small wind turbines a fully effective operating brake can be implemented with justifiable effort on the low-speed side, as long as design of the rotor shaft bearing assembly does not present an obstacle. The rotor brake on the low-speed side was a common feature of many earlier stall-controlled Danish wind turbines up to a power rating of about 100 kW in the Eighties. At that time it was considered to be an extra safety
element even though the rotor brake was only designed as a parking brake.
Installing the rotor brake on the slow side is much more problematic in large wind turbines, however. Even a parking brake already assumes a considerable size (Fig. 8.32).These disadvantages have led to the rotor brake being arranged on the high-speed side behind the gearbox in almost all new systems.
8.8 Gearbox
The conversion of the greatly differing rotational speeds of the rotor and the electric generator has given the designers of the first wind turbines many headaches. This situation led to costly low-speed generator designs and to hydraulic or pneumatic transmission systems to the generator (Chapt.8.1).Aerodynamicists made efforts to drive the rotor speed as high
as possible in order to lower the gear ratio. It was assumed that costs would also increase considerably with increasing gear ratios, so that the development of rotors with extremely high tip-speed ratios was pushed forward.
This situation has changed with the progress which has been made in gearbox technology. Today, high-performance gearboxes with gear ratios of up to 1:100 and more are available. In many areas of mechanical engineering, gearboxes are used which are suitable for deployment in wind turbines, as regards their technical concept, their efficiency and their operating life. The gearbox for the wind turbine has become a” vendor-supplied component”, which, with certain adaptations, can be taken from the standard product range of the gearbox manufacturers.
Regardless of this favorable situation, the gearbox has been and still is a source of failures and defects in many wind turbines. The cause of these
“gearbox problems” is not so much the gearbox itself, rather the correct dimensioning of the gearbox with regard to the load spectrum. In wind turbines, it is easy to underestimate the high dynamic loads to which the gearbox is subjected. Thus, in the early phase, many turbines had gearboxes which
were undersized. Having learned their lessons, successful manufacturers equipped their turbines with ever stronger gearboxes and thus, in the course of development, empirically arrived at the right dimension.
8.8.1Gearbox Configurations
Toothed-wheel gearboxes are constructed in two different forms. One is the parallel shaft or spur-gear system, the other is the technically more elaborate planetary gearing. The gear ratio per single reduction is limited, so that the difference in diameter between the small and the large wheel does not become too unfavorable. Parallel-shaft-gear stages are built with a gear ratio of up to 1 :5, whereas planetary stages have a gear ratio of up to 1 : 12. Wind turbines generally require more than one stage. Fig. 8.33 shows what effects different
designs have on gearbox size, mass and relative cost [11].
It is noteworthy that the three-stage planetary design has only a fraction of the overall mass of a comparable parallel shaft system. The relative costs are reduced to about one half. In the megawatt power class, the multi-stage planetary gearbox is, therefore, clearly superior. In smaller power classes, the comparison is not quite as unambiguous. In the range up to about 500 kW, parallel-shaft gear designs are often preferred for cost reasons.
Small wind turbines are equipped with parallel-shaft gear systems.Theprevailingmodels are two-stage gearboxes which are commercially available from numerous manufacturers as modified universal transmissions (Fig. 8.34).
In larger wind turbines, the planetary design definitely prevails. For outputs of several megawatts, two- or three-stage models are used (Fig. 8.35). Large gearboxes of this type are used, for example, in ship-building and several other fields of mechanical engineering, so that suitable gearboxes for large wind turbines can be derived from these production sources. Gearboxes with one planetary stage and two additional parallel-shaft stages are used in many late-model turbines (Fig. 8.36).With the additional parallel shaft, the primary and secondary shafts are no longer coaxial. This has the advantage that a hollow through shaft can be implemented more easily. In this way, power supply lines supplying power to the blade pitch drive, as well as measurement and control signals for the rotor, can be routed through the gearbox.
In larger gearboxes, an auxiliary rotor drive is frequently flanged to the gearbox housing. Using this electric motor, the rotor can be turned slowly. Such an auxiliary unit is indispensable for assembly and maintenance work in large rotors. Gearbox lubrication is usually carried out via a central oil supply in the nacelle. As a rule, it also contains an oil cooler and a filter.
In spite of indisputable advances having been achieved in the durability of the gearboxes, there is still “trouble with the gears” being experienced even in the latest wind turbines. Although it is possible to adapt gearboxes for wind turbines from other types of machine, they are subject to special demands which are often not encountered in other applications. Much negative experience in recent years has provided important insights into this issue:
– Special attention must be devoted to the smooth running of the tooting. Particularly prominent gear meshing frequencies can cause resonances in the drive train.“Cheap” transmissions with simple tooting are unsuitable for use in wind turbines.
– Oil leaks in the transmission are a particular problem. Labyrinth seals have proven more reliable than slipping type seals. In many cases, the housing flanges also showed leaks after some time. A box design with a top flange is apparently more advantageous than gearbox housings with flanges on the input and output side.
– The quality of the lubrication has been found to be a decisive factor for the service life of the gearbox. Oil temperatures which are too high cause just as much damage as does contamination in the oil. Oil coolers and filters are indispensible for large gearboxes ands is the careful observance of oil change intervals.
– The stiffness of the gearbox housing is an important criterion for its service life if the housing is integrated into the static design of the nacelle.
Apart from these constructional measures, of course, the correct dimensioning has a decisive influence.
風(fēng)機(jī)剎車裝置
機(jī)械制動(dòng)裝置作為主傳動(dòng)鏈的一部分，同具有氣動(dòng)剎車功能的轉(zhuǎn)子一樣，是風(fēng)力發(fā)電機(jī)一個(gè)不可或缺的組成部分。其首要任務(wù)是當(dāng)風(fēng)機(jī)停轉(zhuǎn)時(shí)，使風(fēng)機(jī)轉(zhuǎn)子處于適當(dāng)?shù)奈恢?。在維修工作的過(guò)程中，鎖定主軸是必須的，而且在正常的停工期間，鎖定主軸也是慣常的做法。更為重要的是，為了渡過(guò)持續(xù)的主軸停轉(zhuǎn)時(shí)期以及維修工作的進(jìn)行，大多數(shù)風(fēng)機(jī)都會(huì)在輪轂和機(jī)艙之間安裝鎖緊螺栓，從而使主軸在一個(gè)或者多個(gè)位置得到保護(hù)。
主軸剎車通常采用盤式剎車，適當(dāng)?shù)谋P剎可以頻繁采用為其它的機(jī)器或裝置設(shè)計(jì)的現(xiàn)存的生產(chǎn)線，這樣就可以節(jié)約成本。在這樣的背景下，主軸剎車的設(shè)計(jì)本身不會(huì)產(chǎn)生很多問(wèn)題。然而，主軸剎車也會(huì)給風(fēng)機(jī)系統(tǒng)的設(shè)計(jì)者帶來(lái)一些問(wèn)題，而這些問(wèn)題可能給整個(gè)系統(tǒng)帶來(lái)一些后果。
最為重要的問(wèn)題是，在經(jīng)營(yíng)理念之內(nèi)，剎車裝置要完成哪項(xiàng)任務(wù)。在最簡(jiǎn)單的情況下，在主軸停轉(zhuǎn)的情況下，剎車裝置的尺寸必須滿足所需的支持轉(zhuǎn)矩。這是由假設(shè)風(fēng)速達(dá)到最大時(shí)所計(jì)算的空氣動(dòng)力所決定的（第6.3.2節(jié)）。
剎車裝置除了單純地具有主軸駐車制動(dòng)功能外，尺寸合適時(shí)，其也可以作為停車制動(dòng)裝置。只要制動(dòng)力矩和制動(dòng)力（熱負(fù)載下）能夠被吸收，主軸機(jī)械剎車也可以用作除了氣動(dòng)主軸剎車外的第二獨(dú)立剎車系統(tǒng)，這樣一來(lái)，風(fēng)機(jī)的運(yùn)作可靠性得到大幅提高。對(duì)于小型風(fēng)機(jī)，在緊急情況下，主軸機(jī)械剎車會(huì)防止主軸失控，這一點(diǎn)被證實(shí)是非常成功的，并得到了廣泛的應(yīng)用。
隨著風(fēng)機(jī)尺寸的增大，這一需求會(huì)越來(lái)越難以得到滿足。對(duì)于主軸直徑為60米到80米的風(fēng)機(jī)來(lái)說(shuō)，如果是在滿載運(yùn)轉(zhuǎn)期間制動(dòng)主軸轉(zhuǎn)矩和主軸功率，主軸剎車裝置幾乎承擔(dān)了離譜的規(guī)模。因此，在大型風(fēng)機(jī)中主軸剎車裝置的作用總是被限制在單純的進(jìn)行駐車制動(dòng)的上。
除了主軸剎車裝置的運(yùn)轉(zhuǎn)任務(wù)的問(wèn)題外的另一個(gè)問(wèn)題是，剎車裝置安裝在主傳動(dòng)鏈的哪個(gè)位置是最合適的。兩種可以選擇的方案分別是安裝在齒輪箱的低速軸和高速軸。在大多數(shù)的風(fēng)機(jī)中，為了使得剎車盤的直徑盡可能地小，常將剎車裝置安裝在高速軸上，比如安裝在如齒輪箱和發(fā)電機(jī)之間（圖8.31）。根據(jù)齒輪的傳動(dòng)比，由于轉(zhuǎn)速較高，轉(zhuǎn)矩會(huì)比在低速軸上低一個(gè)甚至兩個(gè)數(shù)量級(jí)。
然而，將剎車裝置安裝在高速軸上至少有兩點(diǎn)缺點(diǎn)。由于如果低速軸或者齒輪箱出現(xiàn)故障時(shí)，剎車功能就會(huì)失效，所以從安全的角度上，這種方案是處于劣勢(shì)的。再者，在主軸停轉(zhuǎn)的過(guò)程中，其必須由齒輪支撐。隨著齒輪齒側(cè)的逐漸磨損，齒輪會(huì)產(chǎn)生一些小的震動(dòng)，由于空氣擾動(dòng)，對(duì)于一個(gè)一個(gè)停止工作的風(fēng)機(jī)來(lái)說(shuō)，這一點(diǎn)是不可避免的。在有些風(fēng)機(jī)中，為了試圖解決這一問(wèn)題，主軸停轉(zhuǎn)時(shí)，不再鎖定主軸，而是允許它在低速下轉(zhuǎn)動(dòng)。為了避免以上缺點(diǎn)，早期的主軸剎車系統(tǒng)被安裝在低速軸上。在小型風(fēng)機(jī)中，只要主軸軸承裝置的設(shè)計(jì)不會(huì)出現(xiàn)問(wèn)題，也可以通過(guò)無(wú)可非議的努力在低速端實(shí)施一個(gè)完全有效可操作的剎車方案。80年代，丹麥許多功率高達(dá)100kw的失速控制型風(fēng)機(jī)普遍采用主軸剎車安裝在低速軸的方案。那是，即使主軸剎車裝置僅僅被設(shè)計(jì)為駐車剎車裝置，這一方案也被看作是一個(gè)額外的安全因素。然而，在大型風(fēng)機(jī)中，將主軸剎車裝置安裝在低速軸是非常有爭(zhēng)議的。即使假設(shè)剎車裝置有一個(gè)相當(dāng)大的尺寸（圖8.32）。在幾乎所有新的系統(tǒng)中，這些缺點(diǎn)已經(jīng)引導(dǎo)主軸剎車裝置被安裝在齒輪箱后部的高速軸上。
圖8.31NORDEX N-80型風(fēng)機(jī)中的主軸制動(dòng)裝置安裝在齒輪箱的高速軸上
圖8.32早期HOEDEN HWP-1000型風(fēng)機(jī)中
主軸制動(dòng)裝置直接安裝在輪轂后的低速軸上
8.8齒輪箱
齒輪箱的布置
齒輪箱有兩種布置方式。一種是平行軸傳動(dòng)或直齒輪傳動(dòng)系統(tǒng)，另一種是技術(shù)上更為精確的行星輪傳動(dòng)。單級(jí)減速的傳動(dòng)比是有限制的，所以大小齒輪直徑的之間的差異并沒(méi)有十分不利。平行軸齒輪傳動(dòng)級(jí)的傳動(dòng)比最高達(dá)1:5，而行星輪傳動(dòng)比可高達(dá)1:12?？偟膩?lái)說(shuō)，風(fēng)機(jī)不只需要一個(gè)傳動(dòng)級(jí)。圖8.33顯示不同的設(shè)計(jì)對(duì)齒輪箱的尺寸，質(zhì)量以及相應(yīng)的花費(fèi)有什么影響。
圖8.33 不同齒輪箱設(shè)計(jì)的總體質(zhì)量和相對(duì)費(fèi)用
顯然，三級(jí)行星輪系的的質(zhì)量只是相應(yīng)的平行軸傳動(dòng)的總體質(zhì)量的一小部分。相應(yīng)的費(fèi)用也縮小的接近一半左右。因此，在兆瓦級(jí)風(fēng)機(jī)中，多級(jí)行星輪系占有明顯的優(yōu)勢(shì)。而在小型分幾種，這種比較的結(jié)果則不是那么明顯。在500kw以下的風(fēng)機(jī)中，平行軸齒輪箱因其花費(fèi)較少而多被采用。
小型風(fēng)機(jī)中采用平行軸齒輪傳動(dòng)系統(tǒng)。普遍采用的模型是兩級(jí)傳動(dòng)的齒輪箱，其是經(jīng)改造而成的一種通用的尺寸（圖8.34），由很多制造商提供。
圖8.34.200-500kw級(jí)風(fēng)機(jī)的兩級(jí)平行軸齒輪箱
而在大型風(fēng)機(jī)中，行星輪系的設(shè)計(jì)則非常普遍。對(duì)于產(chǎn)電量為幾兆瓦的風(fēng)機(jī)，常采用兩級(jí)或者三級(jí)行星輪系（圖8.35）。例如，這種大型齒輪箱在制船業(yè)以及機(jī)械工程的其它幾個(gè)領(lǐng)域都有采用，因此，大型風(fēng)機(jī)中的合適的齒輪箱都可以以這些產(chǎn)品為參照。在很多后現(xiàn)代的風(fēng)機(jī)中，多采用由一級(jí)行星兩級(jí)平行軸組成的齒輪箱（圖8.36）。由于有多加的平行軸，一級(jí)和二級(jí)的軸不再同軸。這樣布置的好處是可以很容易的應(yīng)用一個(gè)中空的軸。這樣，向葉片供電的供電線路以及主軸的控制和測(cè)量信號(hào)可以通過(guò)齒輪箱路由。
在大型風(fēng)機(jī)中，一個(gè)輔助的主軸裝置常通過(guò)法蘭和齒輪箱體連接。用這種點(diǎn)機(jī)，主軸轉(zhuǎn)速會(huì)變慢。這樣一個(gè)輔助裝置對(duì)大型主軸的組裝和維修工作是必不可少的。通常來(lái)說(shuō)，它也帶有一個(gè)油液冷卻裝置和一個(gè)濾油器。
圖8.35 2-3MW級(jí)封系的三級(jí)行星輪系齒輪箱
盡管齒輪箱在耐久性上已經(jīng)取得了無(wú)可爭(zhēng)議的優(yōu)勢(shì)，即使在最新型的風(fēng)機(jī)中，齒輪的問(wèn)題仍然存在。雖然從其它類型的機(jī)器中改編為機(jī)的齒輪箱是可以做到的，但是這些常用于特殊需求，在其它的應(yīng)用中并不常見(jiàn)。近幾年來(lái)，很多失敗的經(jīng)驗(yàn)已經(jīng)對(duì)這個(gè)問(wèn)題提供了重要的見(jiàn)解。我們一定要特別注意輪齒的平穩(wěn)運(yùn)行。特別是齒輪嚙合的頻率可能造成主傳動(dòng)鏈共振。制造簡(jiǎn)單的變速器，其輪齒很簡(jiǎn)單，并不適用于風(fēng)機(jī)中。箱體的漏油是個(gè)特殊的問(wèn)題。經(jīng)證實(shí)，迷宮型密封比滑環(huán)型密封更可靠。很多情況下，有時(shí)箱體的法蘭也會(huì)造成漏油。將法蘭安裝在上部明顯比安裝在輸入或輸出端更有優(yōu)勢(shì)。人們發(fā)現(xiàn)，潤(rùn)滑油的質(zhì)量是影響齒輪箱壽命的決定性因素。油溫過(guò)高和油被污染會(huì)造成同樣多的損害。對(duì)于大型風(fēng)機(jī)，油溫降溫裝置和濾油器是必不可少的，所以要小心遵守?fù)Q油期。如果箱體和機(jī)艙的靜態(tài)設(shè)計(jì)結(jié)合，箱體的硬度是其壽命的一個(gè)重要指標(biāo)。當(dāng)然，除了這些構(gòu)造措施，正確的尺寸標(biāo)注有這決定的影響。
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