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GREENTECH EC MOTORSFigure 1: Exploded drawing of the permanentlyexcited synchronous motor, also calledbrushless direct current motor or EC motor.The GreenTech EC fansare completely independentof the market trend ofrare earth magnetsrespect to its torque-speed characteristic, themotor acts like a DC shunt motor. To detect therotor position, either rotor position sensors are integratedin the motor, or the commutation electronicsmeasure the rotor position without sensorsvia the parameters field EMF and motor currents.The no-load speed depends on the appliedvoltage and the number of windings of the statorwinding. Therefore in the limits which are definedby the physical parameters (such as output power,torque, temperature rise etc.), nearly arbitraryoperating speeds can be implemented slip-free(synchronous with the stator rotating field), whichcan even lie above the power frequency, unlikethe mains-powered asynchronous motor. Forexample, if a fan is operated with an EC motor,the speed can always be adapted to the requirementsof the ventilation system or the process.In partial-load operation, therefore, the energyconsumption can be significantly reduced, becausethe required input power of a fan changesaccording to the speed to the power of 3. Asidefrom this, EC motors feature a significantly higherefficiency (Figure 2) than mains-powered AC motorsboth in partial-load operation and at full load,and they usually do this with a smaller size. Onereason is that EC motors do not require a magnetisingcurrent, so that current heat losses of therotor disappear. Another reason is the possibilityto implement a special winding arrangement witha low end winding (single-tooth winding / concentratedwinding, which reduce the winding losses).Even if the rare earth magnet discussion doesnot favour these motors, they are simply the bestchoice in terms of energy efficiency.Dynamic requirements determine thechoice of magnets With EC motors you arenot always forced to rely on the strong rare earthmagnets, because their excellent magnetic qualityis really needed only for highly dynamic servodrives, such as those used in robotics. On theone hand, compact dimensions are required; onthe other hand, however, the lowest possible rotormass is required to minimise the moment ofinertia. These requirements are attainable onlywith highly remanent and highly-coercive rareearth magnets. Therefore, manufacturers of suchservo drives primarily concentrate on reducingthe required magnet mass and height by meansof complex optimisations; and they have alreadyachieved very remarkable savings here.Motor and fan specialist ebm-papst Mulfingenis not even faced by this problem with itsfans, which are equipped with energy-efficientGreenTech EC motors. Despite the high efficiency,these drives run without rare earth magnets. Theexternal-rotor motor principle provides the keyfor this:18techmag 01°2013