Cool quietly with air

Improving the oper­ating noise of fans

In many areas, current tech­nology relies on targeted air cooling. In addi­tion, modern DC axial fans score highly in many ways: they are energy effi­cient, durable, and do not require main­te­nance. Depending on the version, they can also be inte­grated into networks. Yet, one disad­van­tage of many fans is the unpleasant oper­ating noise. With new fan concepts, noise emis­sions can be substan­tially reduced depending on the oper­ating point; fans with low noise, which are also less both­er­some, are partic­u­larly suit­able for appli­ca­tions in areas in which people live and work.


Figure 1: In many areas, tech­nology is advancing ever closer to people. In this regard, quiet-running fans can improve the well-being of the envi­ron­ment. (Photo |

Many appli­ca­tions require quiet fans with high flow rates. For example, if decen­tral­ized venti­la­tion or a solar inverter is installed in a house, the living envi­ron­ment should not be nega­tively affected by excess noise (Figure 1). The same applies in automa­tion tech­nology, e.g., in frequency converters (Figure 2). For medical appli­ca­tions, a low noise level in the cooling of elec­tronics is impor­tant in order to prevent addi­tional distur­bance to patients. In many appli­ca­tions – whether one considers refrig­er­ated display cases in super­mar­kets, IT & telecom­mu­ni­ca­tions with switch cabi­nets, IT servers, or network equip­ment – tech­nology is moving ever closer to people. Fans from ebm-papst, which cool the air without un­necessary noise, make a substan­tial contri­bu­tion to making everyday tech­nology quieter.

Causes of noise


Figure 2: The number of frequency converters is steadily increasing; quiet cooling improves working condi­tions. (Photo |

In general, fan noise can be caused by one of two mech­a­nisms. The first is noise gener­ated directly by air move­ment and the asso­ci­ated pres­sure fluc­tu­a­tion (aeroa­coustics). The second is noise resulting from the contact or rubbing of solid bodies (impact sound). In addi­tion to these two main mech­a­nisms, noise may arise from the inte­grated power and control elec­tronics. When designing fans, aero­dy­nam­i­cally gener­ated noise poses a partic­ular chal­lenge. Even the smallest changes in blade design or in the struts or housing can have a consid­er­able effect on the noise level. For example, the air flow on the outer and trailing edges of the blades can be inter­rupted, and the resulting turbu­lence increases the gener­a­tion of noise. The air that flows over the struts, which keep the rotor in the housing, also creates turbu­lence. Pres­sure fluc­tu­a­tions also result when a blade sweeps over a strut. Impact sound is the second compo­nent that contributes signif­i­cantly to oper­ating noise: for example, these vibra­tions of the fan struc­ture may be gener­ated by an imbal­ance of the rotor or an elec­tric or magnetic impulse in the engine, which is similar to a familiar trans­former hum. The struc­ture then acts like a loud­speaker. The vibra­tions are partic­u­larly strong if the reso­nance frequen­cies in the fan struc­ture are excited. However, begin­ning at a certain speed, the noise emis­sion of the fan will be domi­nated by aeroa­coustic effects.

Quiet oper­a­tion can be devel­oped


Figure 3: The in-house testing labo­ra­tory and an air perfor­mance test rig facil­i­tate the testing of new devel­op­ments.

To mini­mize these aeroa­coustic effects, the rotating blades are fash­ioned in a complex manner. The aim of this process of aero­dy­namic opti­miza­tion is to achieve low noise emis­sions with a high effi­ciency. One way in which this goal is achieved is by aligning blades and struts with each other (e.g., coor­di­nating the geome­tries of the back edge of the blade and the front edge of the strut). Through continual improve­ments, fan specialist ebm-papst has obtained optimal results with its latest fan. This fan design was calcu­lated with the help of specific simu­la­tion soft­ware and was veri­fied in prac­tice at our in-house testing station (Figure 3). Finely balanced rotors with main­te­nance-free ball bear­ings mini­mize vibra­tion and reduce impact sound. Together with a vibra­tion-opti­mized struc­ture (designed with modern finite-element methods), the smallest possible impulse is achieved. The design engi­neers responded to the magnetic and elec­tric exci­ta­tion with opti­mized magnetic flux in the stator and rotor of the motor, as well as with opti­mized elec­trical connec­tions and select elec­tronic compo­nents.

Measures in prac­tice


Figure 4: The new S-Panther: high perfor­mance and quiet oper­a­tion.

In order to mini­mize oper­ating noise, the devel­oper must imple­ment a whole series of improve­ments. For example, let’s take the new S-Panther model (Figure 4). An opti­mized current and magnetic flux as well as a precisely balanced rotor assembly ensure quiet oper­a­tion. The aero­dy­nam­i­cally designed outer edges of the rotor blades with so-called winglets, which are specially arranged struts (Figure 5), mini­mize turbu­lence between the blade and the housing. Together with the newly designed struts, the improve­ments enable the fan to work 3% more effi­ciently than its prede­cessor (Figure 6).

An opti­mized current and magnetic flux ensure quiet oper­a­tion.


Figure 5: Winglets on the blade tips reduce turbu­lence and thus noise levels.

At ebm-papst, the modern design of fans involves the phys­ical design, the specific drive concept, and control elec­tronics, as well as the selec­tion of mate­rials and the construc­tion of the fan itself. Thanks to injec­tion-molding tech­niques, plas­tics allow for a wide variety of designs, that are light­weight, and (depending on the type of plastic used) offer great resis­tance against corro­sion and wear. This is why the 3250J features select plas­tics that not only ensure a precise contour in injec­tion molding but also provide a high degree of atten­u­a­tion that is inherent in the mate­rial. The housing is made of glass-fiber-rein­forced PBT (poly­buty­lene tereph­tha­late), and the impeller is made of glass-fiber-rein­forced PA (polyamide). The drive motor is opti­mally inte­grated in the impeller, and the stator coils and elec­tronics are also fully cast in plastic. This ensures good protec­tion against water, dust, and salt spray, among other things.

Excel­lent combi­na­tion of perfor­mance and reduced noise


Figure 6: Perfor­mance chart of the new S-Panther

Although the tech­nical effort involved in designing such a quiet but powerful fan is consid­er­able, it does yield signif­i­cant advan­tages in many areas. The 92 × 92 × 38 mm (H × W × D) S-Panther is avail­able for 12, 24, and 48 VDC oper­ating voltage, each with IP68 and IP54 protec­tion (optional). Depending on the variant (7, 24, or 35 W power is avail­able), the fan conveys between 145 and 270 m3/h with an oper­ating noise of only 47 or 63 dB(A). The lifespan is over 85,000 hours (L10, 40 °C) according to the strict in-house test config­u­ra­tion and approx­i­mately 150,000 hours according to stan­dard eval­u­a­tions. Various optional features, such as speed signal output, a go/no-go alarm, or an internal/external temper­a­ture sensor are also avail­able, as is PWM control input or analog input. The molded drive and specially sealed bear­ings provide protec­tion against mois­ture and salt spray; the high effi­ciency results in lower elec­trical energy require­ments. Those who value the highest possible air flow rate, and are willing to accept slightly increased noise, will be well served by fans of the S-Force series. Up to 280 m3/h at 73 dB(A) is possible! The pres­sure increase also improves from 340 up to 700 Pa. This ensures a high air flow rate even with greater air resis­tance in the unit (e.g., through filters or the compact arrange­ment of compo­nents).

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The new generation of compact fans