S-Force: compact fans for high output

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1 Press release S-Force: compact fans for high output by Dr. Walter Angelis and Dr. Siegfried Harmsen Cooling and ventilating devices increasingly require large air flow. This is true of IT and telecommunications applications and well as those in climate control systems and printing machinery. Because the installation space for the necessary fans is often scarce, axial compact fans are normally used, as they require the least space for any given output. Larger blower outputs can be achieved with correspondingly higher speeds, and these can be calculated using the similarity indices of fans. Raising the blower output Image 1 shows the loss curve of a device and the desired operating point for sufficient flow of air. In this example, however, the standard fans in question based on these dimensions does not attain nearly the required output. However, this output is possible if the speed of the fan selected here, Type 4100NHX, is increased from 6,000 to 11,000 revolutions per minute. Happily, the pressure drops of the device and the pressure increases of a fan increase equally. The pressure increases are higher with the square of the fan speed, while the air flow is proportional to the speed. The device losses increase with the square of the air flow, and thus also rise with the second power of the speed. This relationship has been illustrated in image 1, where the optimum operating range of the fan was particularly emphasized. S-Force series Based on a market analysis, ebm-papst St. Georgen has developed high-performance fans in five different sizes. These types, called "S-Force", are shown in image 2. Their dimensions and output are compiled in table form in image 5. In addition to

2 these very high-speed configurations, other types are available with nominal speeds graduated between standard fans and the most powerful S-Force fans. Accordingly, image 1 also shows the curve for such a variant with a medium speed, which, in similar form, also applies for the other S-Force sizes. However, the increases in speed and performance also mean additional mechanical load on the fan, particularly on the bearings, external housing and impeller. Furthermore, the greater power density also causes higher electrical currents and operating temperatures, especially in the drive motor, bearing system and electronics. Compensating for these higher loads was the most important task when developing the S-Force series, requiring new designs and other special measures. Evolution of the drive motors In the S-Force fan 4100NH8, the blower output - the product of the air flow and pressure increase - is nearly 6 times greater than that of the previous standard design. The only way to attain the required performance increase of the motor was to use a new design: - Three-phase motor instead of single-phase - Nine-slot stator core instead of four-slot - Stator pack 14 mm high instead of 10 mm - SmFeN magnet with high residual magnetism instead of rubber-ferrite magnet - Ten-pole magnetized rotor instead of four-pole The motor electronics were adapted to the more stringent requirements with a special design that includes new integrated circuits and sensorless commutation. Overall, these measures resulted in a substantial improvement in motor efficiency to 85%. This successful motor and electronics development was also used, with similar concepts, for the four other S- Force series, for which the special circumstances of each specific basic motor and the varying requirements had to be considered. As a result, rubber-ferrite magnets can also be used even for the two smaller fans, while the two larger types require ceramic magnets.

3 Owing to the higher power density, the particular problem of greater heating in the interior of the motors arose. Using the flow simulation, the flow of air through the motor can be calculated for different operating points and optimized for more intensive cooling. In addition, components with low thermal resistance were used and critical components were arranged in a favorable position for cooling. These measures, and above all the high motor efficiency, made it possible to limit the heating to permissible values. Reinforcing the mechanical components The high speeds also mean greater loads on the bearing, external housing and impeller. In each case, special measures must be put in place to absorb the higher forces, torques and vibrations reliably. For example, the S- Force fans do not use any sleeve bearings, but only ball bearings. In previous standard fans, the two ball bearings were combined in the so-called floating arrangement, which is frequently encountered in precision engineering. For the new high-performance fans, however, it was necessary to use the normal arrangement for mechanical engineering. In doing so, the inner races of the two bearings were connected to the shaft using interference fits; the outer race of one bearing is fastened via a bond in the bearing tube, the other via a sliding fit and an axial flat coil as a connecting element. Moreover, for the two smaller fans with plastic housing, it was necessary to use metal bearing tube to ensure the required strength. For these plastic housings, additional reinforcements of the motor flange and fixed connecting struts proved necessary in order to prevent unwanted vibrations in these areas. Image 4 shows how the 3200JH fan is equipped with eight struts, which made this component substantially more rigid. The aerodynamic design of the new fixed struts was perfected enough so that the blower output is not impaired, but actually improved in some cases. As the comparison of speed-dependent structure-borne noise values in image 5 shows, this increased rigidity causes a significant reduction in excitation. New impeller The objective of the redesign was to attain the specified blower output while simultaneously minimizing the noise level. This optimization, which took place using computer calculations and simulation methods, was very successful. In one case, it resulted in a blade with a sickle-shaped outline. To reduce noise,

4 "winglets" were provided on the blade ends, similar to those used on modern airplanes to prevent wake turbulence. In fans, these end strips can reduce so-called blade tip noise. Image 6 shows a comparison of the measured noise levels on fan type 3200JN with and without winglets. We can see that in this example, the winglets provide a noise reduction of up to four decibels with increasing air flow. The high speeds result in greater centrifugal forces on the impellers. To prevent voltage peaks in the area of the blade roots, during the redesign, the material thickness of the impeller hub and the adjacent blade segments were optimized according to the specifications of an extensive FEM simulation (image 7). Effects on service life Because of the high outputs and high speeds, special measures were taken for the S-Force fans to ensure the reliability of these products. For example, the ball bearing system was improved and the plastic housings were reinforced in the flange and fixed strut area and equipped with a metal bearing pipe. The material thickness was increased for the two larger impellers. Increasing operating temperatures caused by the dissipated energy would have a negative impact on reliability. Therefore, new motor designs and electronic circuits were developed to minimize the dissipation with efficiency values of up to 89%. The aerodynamic optimization of the impeller and external housing had a similar effect. Finally, targeted cooling air flows in the motor and electronics reduced the temperatures at critical elements. The successful result of these combined measures was that at an ambient temperature of 40 degrees celsius, an expected L10 service life of 60,000 hours is attained. Variants of the S-Force series The S-Force fans are available for DC voltages of 24 and 48 volts, and the two smaller types are also available for 12 volts. In addition to the designs with high speeds mentioned above, other types were established with graduated moderate speeds. All fans of this new series benefit from the outstanding high efficiencies, the extremely favorable noise-to-output ratio and the especially pleasing long service life. All fan types are also available with the usual intelligent features you expect from ebm-papst St. Georgen, such as speed and operation monitoring, closed loop speed control and motor management for software-supported operation.

5 EPS_08-002_Bild1e.jpg Image 1: Fan curve EPS_08-002_Bild2.jpg

6 Image 2: S-Force fans, overview of dimensions EPS_08-002_Bild3e.jpg Image 3: Technical data for S-Force fans EPS_08-002_Bild4.jpg

7 Image 4: Fan housing with eight fixed links EPS_08-002_Bild5e.jpg

8 Image 5: Comparison of structure-borne noise with 3 and 8 fixed links EPS_08-002_Bild6.jpg Image 6: Comparison of winglet noise

9 EPS_08-002_Bild7.jpg Image 7: Impeller reinforcements and blades with winglets EPS_08-002_Bildintro.jpg Image 8: Intro

10 Text and images/graphics are stored on the accompanying CD-ROM. ebm-papst St. Georgen will provide paper copies by request. Contact for editorial staff: ebm-papst St. Georgen GmbH & Co. KG Hubert Goetjes Phone: / Fax: /