[转帖] [2011.11.17] Reluctant heroes 磁抗英雄

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http://www.ecocn.org/thread-172076-1-1.html
Electric motors 电动机
Reluctant heroes 磁抗英雄
An electric motor that does not need expensive rare-earth magnets
一种不再需要昂贵稀土磁铁的电动机
Nov 17th 2012 | From The Economist print edition



DYSPROSIUM and neodymium are not exactly the best-known elements in the periodic table, but for makers of high-end electric motors they have become vital. Both are strongly magnetic and thus crucial to the construction of powerful motors of the sort used, for example, in electric cars. Unfortunately, they lurk in the part of the table known as the rare-earth metals and, as that name suggests, workable deposits of them are scarce. At the moment, the main source of supply is in China, whose government has used its near-monopoly to restrict availability and push up the price. So there is a lot of interest in inventing motors that can do without them. And several groups of researchers think they have come up with one.

在元素周期表上，镝和钕的确不是最为人知的元素，但是对于那些高档电动机制造者来说，他们却极端重要。他们都有很强的磁性因此对于制造现在已经应用的功能强大电动机来说非常重要。不幸的是，他们属于稀土并且正如他们的名字暗示的那样，可供开采的镝矿和钕矿很稀少。现在，中国是最大的原产地，而当局又利用近乎垄断的特权限制其产量从而推高价格。所以发明一种不需要镝和钕的发动机有很大的利润，并且几个研发组织认为他们已经想出了一个。

The device in question is known as a switched reluctance motor. The idea behind it is over 100 years old, but making a practical high-performance version suitable for vehicles has not been possible until recently. A combination of new motor designs and the advent of powerful, fast-switching semiconductor chips, which can be used to build more sophisticated versions of the electronic control systems required to operate a reluctance motor, is giving those motors a new spin.

上文提到的这种不需要稀土的发动机就是开关磁阻发动机。虽然这个想法已经有100多年的历史了，可是直到最近，人们才制造出一个高性能且适合汽车的实用电动机。由性能强大，可快速转换的半导体芯片构成的，足以操控一个磁阻电动机复杂电控系统的出现，加上全新的电动机设计，给予了电动机新生。

One of the leading contenders is Inverto, a research and development company based in Ghent, Belgium. Inverto’s engineers, led by John De Clercq, the firm’s research director, are collaborating with the University of Ghent and the University of Surrey, in Britain, and also with an unnamed carmaker. They already have a motor running in a car. At Newcastle University, also in Britain, researchers are working with several companies to produce reluctance motors for both cars and lorries. And studies are being carried out in America and Japan too. A team led by Nobukazu Hoshi of the Tokyo University of Science, for example, has experimented with a reluctance motor in a Mazda sports car.

Inverto作为其中主要竞争者，是一家坐落在比利时根特（Ghent）的发展研究公司。由研发总监John De Clercaq 领导的Inverto的工程师们和根特大学（University of Ghent）、英国萨里大学（University of Surrey）以及不知名的汽车制造商合作已经造出了一个用于汽车的电动机。在英国纽卡斯尔大学（Newcastle University），研究者们和几个公司合作正在生产一种同时用于汽车和卡车的磁阻电动机。日本和美国也在进行类似的研究，东京理工大学（Tokyo University of Science）Nobukazu Hoshi领导的小组已经开始在一个安装了磁阻发动机的马自达跑车上做实验。

Let’s twist again 让发动机再次转动起来

The main practical difference between a reluctance motor and a normal one is that the reluctance motor has no permanent magnets. A normal motor (one that runs on direct current, at least) has two parts. One, the rotor, moves. The other, the stator, does not. The stator usually forms the casing and houses a set of permanent magnets. The rotor, which turns inside that casing, is surrounded by copper windings that act as electromagnets. The motor works by the periodic reversal of the current running through these windings. That reverses the polarity of the electromagnets and causes the rotor to be pulled around by attraction and repulsion between the electromagnets and the permanent magnets.

实际上磁抗发动机和普通发动机主要的区别是前者并不包含永磁铁。普通发动机（至少是直流驱动的）包含两部分：旋转的转子和固定的定子。定子包含一组永磁铁，常常组成发动机的外壳，缠绕着铜线圈相当于电磁铁的转子在定子组成的外壳内转动。电动机的工作原理是流经线圈的电流方向的周期性改变。线圈电流方向的周期性改变使得电磁铁南北两极发生周期性的改变，使得转子在电磁铁和永磁铁的吸引和排斥之间不停的转动。

Traditionally, this current reversal was achieved with a rotary switch known as a commutator, which transmitted electricity to the windings through brushes that made contact with conductive strips which passed under them as the rotor turned. Modern motors use electronic control systems rather than a commutator, but the principle is the same.

通常这种电流换向是通过一种叫做换向器的旋转开关而获得的。换向器通过与导电片接触的电刷将电流导入线圈，导电片在转子转动时会在电刷的下方通过。虽然现代电动机更多的使用电子控制系统而非换向器，但是其原理一致。

A switched reluctance motor is like a normal modern motor in this respect—it is also brushless. But unlike a normal motor it has no need of permanent magnets, rare-earth or otherwise, for it works on a different principle: least magnetic reluctance.

在这方面，除了没有电刷外开关磁阻发动机和普通电动机是一样的。但是跟普通发动机不同的是，开关磁阻电动机不需要永磁铁，也不需要稀土等其他材料，这是因为两者工作原理完全不同，前者采用的是最小磁抗原理。

Reluctance in magnetism is analogous to resistance in electricity. Just as a current travels along the path of least resistance, so the flux of a magnetic field (the “lines of force”, for those who remember childhood experiments with bar-magnets, sheets of paper and iron filings) takes the path of least reluctance. And iron, a material that is nice and cheap, has very low reluctance.

磁阻和电阻比较类似。正如电流会通过电阻最小的线路，磁通量（就如小时候用磁铁条、一张纸和一些铁粉做的那个“力量的线条”的实验）也选择最小磁抗的路径。而铁就恰是又便宜又好的低磁抗材料。

Inverto’s reluctance motor has a rotor made of iron sheets, while the inside of the stator is covered with copper windings. Current is fed to these windings on the say-so of the control system, generating flux. That flux then follows the path of least reluctance—ie, through the sheet-iron rotor. The rotor attempts to align itself to the flux in a way that reduces reluctance to a minimum, and that causes it to turn.

Inverto公司的磁抗电动机的转子是由铁片制成，定子里边同样覆盖着铜线圈。在控制系统的操作下，电流导入线圈中从而产生磁通量。这些磁通量选择磁抗最小的道路，也就是由铁片组成的转子。转子尝试者改变自己内铁片的排列使得磁抗最小从而使得磁通量通过，就是这个过程使得转子转起来。

The control system, however, constantly anticipates the rotor’s movement and switches the current between windings so as to stop the rotor settling into its preferred alignment. As a result, it keeps on turning.

然而，控制系统通过不断的预判转子的运动，改变线圈的电流方向从而阻止转子固定在它的最佳排列上，这样，转子就不停的转动起来。

Reluctance motors still have disadvantages, even now that the control problem has been solved. For example, to deliver a given amount of twisting force—or torque—a reluctance motor has to be larger than an equivalent permanent-magnet motor. But the materials needed to build them are significantly cheaper. Moreover, according to Dr De Clercq, the motors’ torque characteristics make them particularly suitable for cars.

即使控制问题已经解决，磁阻发动机依然有一些缺陷。比如说，对于产生一定的扭矩，磁阻电动机的体积要比含有永磁铁的电动机大一些，但是用于制造磁阻电动机的材料会大大的便宜。另外，De Clercq博士说，电动机的扭矩特点使得磁抗发动机更适合汽车。

At high speeds, for example, they do not lose torque as quickly as a permanent-magnet motor would. That helps during overtaking. They are also safer when they fail. If a permanent-magnet motor loses power suddenly it slows down rapidly, creating an unexpected braking effect. That might also damage the motor. A reluctance motor, by contrast, freewheels if the power is cut off. Despite that, it can still act as a generator when slowing down, as permanent-magnet motors do in electric cars. (Generators are, basically, electric motors in reverse. They convert motion into electricity instead of electricity into motion. A slowing motor can thus be used to top up a car’s batteries with energy that would otherwise be lost in braking.)

在高速行驶中，磁抗发动机不像永磁铁发动机那样快的失去扭矩，这在超车时候很有用。在发动机失灵时候，他们也更加安全。对于永磁铁发动机，如果失去动力，转速会迅速下降，将会造成不可预期的刹车后果，而且这样也会损伤发动机。与之形成鲜明对比的是，磁抗电动机在失去动力时候将会靠惯性滑行。除此之外，在减速的时候，他们可以变成发电机，正如电动汽车中永磁铁做的那样。所谓发电机，电动机反过来就是了。相对于电动机把电能转化成动能，他们把动能转化成电能。电动机可以把刹车过程中由于转速降低而损失的动能用于给汽车电池充电。

The researchers at Newcastle are working with Tata Steel, part of an Indian conglomerate, on using special steels to exploit the magnetic flux more effectively and hence extract more power. They are also collaborating with a local firm, Sevcon, which makes power electronics, and Cummins Generator Technologies, part of an American group which builds engines, on reluctance motors for hybrid trucks. James Widmer, who leads the Centre for Advanced Electrical Drives, one of the reluctance-engine-research groups at Newcastle, says that by running their new motors at high speeds it should be possible to outperform the best fixed-magnet motors, and to do so at lower cost.

纽卡斯尔的研究者们跟印度企业巨头的子公司塔塔钢铁（Tata Steel）合作，用特制的钢铁来寻求更高效的磁通量以便获得更大的功率。他们同时与当地的生产电力电子的Sevcon公司以及制造发动机的美国财团的一部分的Cummins Generator Technologies公司在研制适合混合动力卡车的磁抗电动机。领导位于纽卡斯尔的一个磁抗发动机研究团队之一的高级电驱中心（Centre for Advanced Electrical Drives）的James Widmer说：高速运行他们的新发动机，其性能远超目前最好的永磁铁发动机，而且成本还要更低廉。

The chances are good, therefore, that powerful switched reluctance motors will emerge. That does not automatically mean they will be commercialised, but if the price of rare earths remains high, there is a strong chance that will happen. And if it does, the need for dysprosium and neodymium will diminish quite a lot. China may thus find that, like the farmer in the fable who killed the goose that laid golden eggs, its desire for short-term advantage brings long-term regret.

有这么多的优势，更高效的开关磁抗发动机将会出现。那并不意味着他将马上商业化，但是如果稀土价格依旧高位运行，那磁抗发动机就有很大的机会。如果真的那样，对于镝和钕和需求就会显著下降。中国就发现自己像寓言故事里边杀死会下金蛋的鹅的农夫一样：对短期利益的渴望导致其长期的悔恨。

老程

磁阻电动机不是什么新玩意，80年代我公司就大量使用了（现在已用带闭环控制的异步电机取代），汽用电动机的对电机的同步性要求不高，目前还有使用异步电机的。对调速精度高的场合还是得用永磁同步电机。
另外大型车用电动机采用永磁电机还有另外一个重要因素，在减速或制动工作状态下，电动机处于发电机工作状态，就是动能制动模式。
再多说估计就没人感兴趣了。

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老程兄多虑了，至少我对新知很有兴趣，呵呵。
这段时间概率没有继续深入，转攻线性代数了，也是别有一番风景。