External rotor motors need no rare earth magnets

Perma­nently excited elec­tric motors rely on perma­nent magnets due to their func­tion. Espe­cially strong magnets can be produced in the sintering process from compounds with rare earth mate­rials, such as samarium cobalt or neodymium iron boron. After the arti­fi­cial scarcity of these mate­rials 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 quan­tity supplied, the user must continue to reckon with extreme price fluc­tu­a­tions. Like­wise, avail­ability is not guar­an­teed.

Also in future, the costs for rare earth magnets will be very diffi­cult for the manu­fac­turers of elec­tric motors to calcu­late. There­fore perma­nent-magnet excited elec­tric motors, which are partic­u­larly energy-effi­cient, are often regarded as expen­sive in user circles. It is not neces­sary, however, that each elec­tric drive oper­ating with high effi­ciency 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 “simple” and cost-effec­tive and anywhere avail­able ferrite magnets, and they work at effi­cien­cies of over 90 % in some cases.

What is an EC motor?

Since the terms in drive tech­nology are not neces­sarily always used with clear and unam­biguous defi­n­i­tions, it makes sense first to clarify which motors are actu­ally meant in conjunc­tion with the rare earth discus­sion. Whether a brush­less DC motor (BLDC), a brush­less perma­nent-magnet (BLPM) motor or an elec­tron­i­cally commu­lated (EC) motor, always means a perma­nently excited synchro­nous motor, which is oper­ated with power elec­tronics – mains-powered or with a DC power supply. The so-called BLDC/BLPM motors are usually oper­ated with square-wave currents (block commu­ta­tion). The EC motors can be oper­ated with square-wave currents as well as with sinu­soidal currents (sinu­soidal commu­ta­tion). Doing the latter the drive achieves a signif­i­cant noise reduc­tion over square-wave currents. The design with sinu­soidal currents corre­sponds to the classic synchro­nous motor. The basic func­tion of the EC motor is easy to under­stand (Figure 1):

Figure 1: Exploded drawing of the perma­nently excited synchro­nous motor, also called brush­less direct current motor or EC motor.

The rotor, which has perma­nent magnets, rotates synchro­nously with the rotary field of the stator. Unlike the mains-powered asyn­chro­nous motor, the rotor speed is not auto­mat­i­cally coupled to the frequency of the supply voltage, but it is deter­mined by the commu­ta­tion elec­tronics. There­fore oper­a­tion of the EC motor always requires addi­tional elec­tronics. This deter­mines the angular speed of the stator rotating field, at which the rotor synchro­nously rotates. The corre­la­tions between voltage and speed and between current and torque are largely linear. Conse­quently, with respect to its torque-speed char­ac­ter­istic, the motor acts like a DC shunt motor.

To detect the rotor posi­tion, either rotor posi­tion sensors are inte­grated in the motor, or the commu­ta­tion elec­tronics measure the rotor posi­tion without sensors via the para­me­ters field EMF and motor currents. The no-load speed depends on the applied voltage and the number of wind­ings of the stator winding. There­fore in the limits which are defined by the phys­ical para­me­ters (such as output power, torque, temper­a­ture rise etc.), nearly arbi­trary oper­ating speeds can be imple­mented slip-free (synchro­nous with the stator rotating field), which can even lie above the power frequency, unlike the mains-powered asyn­chro­nous motor. For example, if a fan is oper­ated with an EC motor, the speed can always be adapted to the require­ments of the venti­lation system or the process. In partial-load oper­a­tion, there­fore, the energy consump­tion can be signif­i­cantly reduced, because the required input power of a fan changes according to the speed to the power of 3.

Aside from this, EC motors feature a signif­i­cantly higher effi­ciency (Figure 2) than mains-powered AC motors both in partial-load oper­a­tion 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 disap­pear. Another reason is the possi­bility to imple­ment a special winding arrange­ment with a low end winding (single-tooth winding / concen­trated winding, which reduce the winding losses). Even if the rare earth magnet discus­sion does not favour these motors, they are simply the best choice in terms of energy effi­ciency.

Figure 2: EC motors have a signifi­cantly higher efficiency than compa­rable asyn­chro­nous motors.

Dynamic require­ments deter­mine the choice of magnets

With EC motors you are not always forced to rely on the strong rare earth magnets, because their excel­lent magnetic quality is really needed only for highly dynamic servo drives, such as those used in robotics. On the one hand, compact dimen­sions are required; on the other hand, however, the lowest possible rotor mass is required to minimise the moment of inertia. These require­ments are attain­able only with highly rema­nent and highly-coer­cive rare earth magnets. There­fore, manu­fac­turers of such servo drives primarily concen­trate on reducing the required magnet mass and height by means of complex opti­mi­sa­tions; and they have already achieved very remark­able savings here. Motor and fan specialist ebm-papst Mulfingen is not even faced by this problem with its fans, which are equipped with energy-effi­cient Green­Tech EC motors. Despite the high effi­ciency, these drives run without rare earth magnets. The external-rotor motor prin­ciple provides the key for this:

The rotor is on the outside

Figure 3: Cutaway drawing of centrifugal fan with external rotor motor – the rotor rotates not in, but about the stator.

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 exter­nally posi­tioned rotor rotates about the internal stator. Condi­tioned by this arrange­ment, 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 thick­ness (reduced magnet volume, reduced air gap surface, smaller radius).

By clev­erly using the design para­me­ters in the fan and blower area, an external rotor motor using hard ferrite magnets can attain torques and effi­cien­cies 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 oppo­site is required: a certain moment of inertia is desir­able for the fans to have smooth starting and accel­er­a­tion behav­iour. Without restric­tions rare earth magnets can be given up and ferrite magnets can be used, which are not only signif­i­cantly more cost-effec­tive, but also have stable market prices due to their avail­ability.

Figure 4: Energy-efficient fans whose motors make do without rare earth magnets.

The motor design with an external rotor is advan­ta­geous 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 “housing” of the motor (Figure 4). Compact dimen­sions, espe­cially in an axial direc­tion, are the conse­quence and cooling is made simpler as the moved air of the fan is also cooling the motor housing.

The design with sinu­soidal commu­ta­tion also provides for partic­u­larly low-noise oper­a­tion. The energy-effi­cient Green­Tech EC fans are there­fore completely inde­pen­dent of the market trend of rare earth magnets.