{"id":7753,"date":"2013-09-16T10:45:35","date_gmt":"2013-09-16T09:45:35","guid":{"rendered":"https:\/\/mag.ebmpapst.com\/en\/?p=7753"},"modified":"2025-11-11T10:06:39","modified_gmt":"2025-11-11T09:06:39","slug":"external-rotor-motors-need-no-rare-earth-magnets","status":"publish","type":"post","link":"https:\/\/mag.ebmpapst.com\/en\/products\/motors\/external-rotor-motors-need-no-rare-earth-magnets_7753\/","title":{"rendered":"External rotor motors need no rare earth magnets"},"content":{"rendered":"\n
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<\/div>\n\n\n\n

Sustain\u00adability, digi\u00adtal\u00adiza\u00adtion & inno\u00adva\u00adtion<\/h3>\n\n\n\n

With our brand claim \u201cEngi\u00adneering a better life\u201d, we express how impor\u00adtant it is to us to imple\u00adment sustain\u00adable and intel\u00adli\u00adgent solu\u00adtions.<\/p>\n\n\n\n

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Mehr Infos <\/i><\/a><\/div>\n<\/div>\n<\/div><\/div>\n\n\n\n

Perma\u00adnently excited elec\u00adtric motors rely on perma\u00adnent magnets due to their func\u00adtion. Espe\u00adcially strong magnets can be produced in the sintering process from compounds with rare earth mate\u00adrials, such as samarium cobalt or neodymium iron boron. After the arti\u00adfi\u00adcial scarcity of these mate\u00adrials and the resulting drastic rise in costs, the prices have been falling recently. However, since now as before China controls a large part of the quan\u00adtity supplied, the user must continue to reckon with extreme price fluc\u00adtu\u00ada\u00adtions. Like\u00adwise, avail\u00adability is not guar\u00adan\u00adteed.<\/p>\n\n\n\n

Also in future, the costs for rare earth magnets will be very diffi\u00adcult for the manu\u00adfac\u00adturers of elec\u00adtric motors to calcu\u00adlate. There\u00adfore perma\u00adnent-magnet excited elec\u00adtric motors, which are partic\u00adu\u00adlarly energy-effi\u00adcient, are often regarded as expen\u00adsive in user circles. It is not neces\u00adsary, however, that each elec\u00adtric drive oper\u00adating with high effi\u00adciency also really depends on strong rare earth magnets. EC motors with an external rotor design, for example, which are used in energy-saving fans, run with \u201csimple\u201d and cost-effec\u00adtive and anywhere avail\u00adable ferrite magnets, and they work at effi\u00adcien\u00adcies of over 90 % in some cases.<\/p>\n\n\n\n

What is an EC motor?<\/h2>\n\n\n\n

Since the terms in drive tech\u00adnology are not neces\u00adsarily always used with clear and unam\u00adbiguous defi\u00adn\u00adi\u00adtions, it makes sense first to clarify which motors are actu\u00adally meant in conjunc\u00adtion with the rare earth discus\u00adsion. Whether a brush\u00adless DC motor (BLDC), a brush\u00adless perma\u00adnent-magnet (BLPM) motor or an elec\u00adtron\u00adi\u00adcally commu\u00adlated (EC) motor, always means a perma\u00adnently excited synchro\u00adnous motor, which is oper\u00adated with power elec\u00adtronics \u2013 mains-powered or with a DC power supply. The so-called BLDC\/BLPM motors are usually oper\u00adated with square-wave currents (block commu\u00adta\u00adtion). The EC motors can be oper\u00adated with square-wave currents as well as with sinu\u00adsoidal currents (sinu\u00adsoidal commu\u00adta\u00adtion). Doing the latter the drive achieves a signif\u00adi\u00adcant noise reduc\u00adtion over square-wave currents. The design with sinu\u00adsoidal currents corre\u00adsponds to the classic synchro\u00adnous motor. The basic func\u00adtion of the EC motor is easy to under\u00adstand (Figure 1):<\/p>\n\n\n

\n
\"Explosionsmodell_permanente\"
Figure 1: Exploded drawing of the perma\u00adnently excited synchro\u00adnous motor, also called brush\u00adless direct current motor or EC motor.<\/figcaption><\/figure>\n<\/div>\n\n\n

The rotor, which has perma\u00adnent magnets, rotates synchro\u00adnously with the rotary field of the stator. Unlike the mains-powered asyn\u00adchro\u00adnous motor, the rotor speed is not auto\u00admat\u00adi\u00adcally coupled to the frequency of the supply voltage, but it is deter\u00admined by the commu\u00adta\u00adtion elec\u00adtronics. There\u00adfore oper\u00ada\u00adtion of the EC motor always requires addi\u00adtional elec\u00adtronics. This deter\u00admines the angular speed of the stator rotating field, at which the rotor synchro\u00adnously rotates. The corre\u00adla\u00adtions between voltage and speed and between current and torque are largely linear. Conse\u00adquently, with respect to its torque-speed char\u00adac\u00adter\u00adistic, the motor acts like a DC shunt motor.<\/p>\n\n\n\n

To detect the rotor posi\u00adtion, either rotor posi\u00adtion sensors are inte\u00adgrated in the motor, or the commu\u00adta\u00adtion elec\u00adtronics measure the rotor posi\u00adtion without sensors via the para\u00adme\u00adters field EMF and motor currents. The no-load speed depends on the applied voltage and the number of wind\u00adings of the stator winding. There\u00adfore in the limits which are defined by the phys\u00adical para\u00adme\u00adters (such as output power, torque, temper\u00ada\u00adture rise etc.), nearly arbi\u00adtrary oper\u00adating speeds can be imple\u00admented slip-free (synchro\u00adnous with the stator rotating field), which can even lie above the power frequency, unlike the mains-powered asyn\u00adchro\u00adnous motor. For example, if a fan is oper\u00adated with an EC motor, the speed can always be adapted to the require\u00adments of the venti\u00adlation system or the process. In partial-load oper\u00ada\u00adtion, there\u00adfore, the energy consump\u00adtion can be signif\u00adi\u00adcantly reduced, because the required input power of a fan changes according to the speed to the power of 3.<\/p>\n\n\n

\n
\"EC-Motoren_hoehere_Wirkungs\"<\/a>
Figure 2: EC motors have a signi\ufb01\u00adcantly higher ef\ufb01ciency than compa\u00adrable asyn\u00adchro\u00adnous motors.<\/figcaption><\/figure>\n<\/div>\n\n\n

Aside from this, EC motors feature a signif\u00adi\u00adcantly higher effi\u00adciency (Figure 2) than mains-powered AC motors both in partial-load oper\u00ada\u00adtion and at full load, and they usually do this with a smaller size. One reason is that EC motors do not require a magnetising current, so that current heat losses of the rotor disap\u00adpear. Another reason is the possi\u00adbility to imple\u00adment a special winding arrange\u00adment with a low end winding (single-tooth winding \/ concen\u00adtrated winding, which reduce the winding losses). Even if the rare earth magnet discus\u00adsion does not favour these motors, they are simply the best choice in terms of energy effi\u00adciency.<\/p>\n\n\n\n

Dynamic require\u00adments deter\u00admine the choice of magnets<\/h2>\n\n\n\n

With EC motors you are not always forced to rely on the strong rare earth magnets, because their excel\u00adlent magnetic quality is really needed only for highly dynamic servo drives, such as those used in robotics. On the one hand, compact dimen\u00adsions are required; on the other hand, however, the lowest possible rotor mass is required to minimise the moment of inertia. These require\u00adments are attain\u00adable only with highly rema\u00adnent and highly-coer\u00adcive rare earth magnets. There\u00adfore, manu\u00adfac\u00adturers of such servo drives primarily concen\u00adtrate on reducing the required magnet mass and height by means of complex opti\u00admi\u00adsa\u00adtions; and they have already achieved very remark\u00adable savings here. Motor and fan specialist ebm-papst Mulfingen is not even faced by this problem with its fans, which are equipped with energy-effi\u00adcient EC motors. Despite the high effi\u00adciency, these drives run without rare earth magnets. The external-rotor motor prin\u00adciple provides the key for this:<\/p>\n\n\n\n

The rotor is on the outside<\/h2>\n\n\n
\n
\"Radial-ventilator_Aussenlae\"<\/a>
Figure 3: Cutaway drawing of centrifugal fan with external rotor motor \u2013 the rotor rotates not in, but about the stator.<\/figcaption><\/figure>\n<\/div>\n\n\n

At this kind of motor, the still-standing part, the stator, is located on the inside and is surrounded by the rotating part, the rotor (Figure 3). The exter\u00adnally posi\u00adtioned rotor rotates about the internal stator. Condi\u00adtioned by this arrange\u00adment, the external rotor motor can achieve a higher torque (magnet volume, air gap surface, radius) than the internal rotor motor of the same package length, the same magnet system and the same magnet thick\u00adness (reduced magnet volume, reduced air gap surface, smaller radius).<\/p>\n\n\n\n

By clev\u00aderly using the design para\u00adme\u00adters in the fan and blower area, an external rotor motor using hard ferrite magnets can attain torques and effi\u00adcien\u00adcies which the internal rotor motor can achieve only with rare earth magnets (because of the limited volume and mass). Unlike servo drives, fans do not require high dynamics. Quite the oppo\u00adsite is required: a certain moment of inertia is desir\u00adable for the fans to have smooth starting and accel\u00ader\u00ada\u00adtion behav\u00adiour. Without restric\u00adtions rare earth magnets can be given up and ferrite magnets can be used, which are not only signif\u00adi\u00adcantly more cost-effec\u00adtive, but also have stable market prices due to their avail\u00adability.<\/p>\n\n\n

\n
\"Energieef-ziente_Ventilator\"<\/a>
Figure 4: Energy-ef\ufb01cient fans whose motors make do without rare earth magnets.<\/figcaption><\/figure>\n<\/div>\n\n\n

The motor design with an external rotor is advan\u00adta\u00adgeous for fans in another regard as well. This way, the axial or centrifugal impellers can be mounted on the rotating rotor, thus directly on the \u201chousing\u201d of the motor (Figure 4). Compact dimen\u00adsions, espe\u00adcially in an axial direc\u00adtion, are the conse\u00adquence and cooling is made simpler as the moved air of the fan is also cooling the motor housing.<\/p>\n\n\n\n

The design with sinu\u00adsoidal commu\u00adta\u00adtion also provides for partic\u00adu\u00adlarly low-noise oper\u00ada\u00adtion. The energy-effi\u00adcient EC fans are there\u00adfore completely inde\u00adpen\u00addent of the market trend of rare earth magnets.<\/p>\n","protected":false},"excerpt":{"rendered":"

Sustain\u00adability, digi\u00adtal\u00adiza\u00adtion & inno\u00adva\u00adtion With our brand claim \u201cEngi\u00adneering a better life\u201d, we express how impor\u00adtant it is to us to imple\u00adment sustain\u00adable and intel\u00adli\u00adgent solu\u00adtions. Perma\u00adnently excited elec\u00adtric motors rely on perma\u00adnent magnets due to their func\u00adtion. Espe\u00adcially strong magnets can be produced in the sintering process from compounds with rare earth mate\u00adrials, such [\u2026]<\/p>\n","protected":false},"author":6,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"wp_typography_post_enhancements_disabled":false,"footnotes":""},"categories":[20],"tags":[],"class_list":["post-7753","post","type-post","status-publish","format-standard","hentry","category-motors"],"acf":[],"_links":{"self":[{"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts\/7753","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/comments?post=7753"}],"version-history":[{"count":7,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts\/7753\/revisions"}],"predecessor-version":[{"id":22259,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts\/7753\/revisions\/22259"}],"wp:attachment":[{"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/media?parent=7753"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/categories?post=7753"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/tags?post=7753"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}