{"id":21353,"date":"2025-05-13T09:30:50","date_gmt":"2025-05-13T07:30:50","guid":{"rendered":"https:\/\/mag.ebmpapst.com\/en\/?p=21353"},"modified":"2025-05-13T09:30:53","modified_gmt":"2025-05-13T07:30:53","slug":"fans-used-in-ventilation-detecting-and-avoiding-resonances","status":"publish","type":"post","link":"https:\/\/mag.ebmpapst.com\/en\/industries\/refrigeration-ventilation\/fans-used-in-ventilation-detecting-and-avoiding-resonances_21353\/","title":{"rendered":"Fans used in venti\u00adlation: Detecting and avoiding reso\u00adnances"},"content":{"rendered":"\n

Vibra\u00adtion is omnipresent. While some kinds of vibra\u00adtion are perceived as pleasant, such as music, others can cause major damage. This is because reso\u00adnance may occur if a system is excited peri\u00adod\u00adi\u00adcally by a force whose exci\u00adta\u00adtion frequency matches a natural frequency of the system. In this state, vibra\u00adtion ampli\u00adtudes increase sharply. Although the atten\u00adu\u00ada\u00adtion effects that are always present limit the ampli\u00adtudes within the reso\u00adnance, low atten\u00adu\u00ada\u00adtion and high exci\u00adta\u00adtion force will cause the vibra\u00adtion ampli\u00adtudes to increase very signif\u00adi\u00adcantly to the point that they may exceed the system\u2019s load limit. <\/p>\n\n\n\n

The exces\u00adsive oscil\u00adla\u00adtion then results in damage, espe\u00adcially if this state is main\u00adtained over a prolonged period of time. For example, vibra\u00adtions may loosen screw connec\u00adtions or damage ball and roller bear\u00adings. Even build\u00adings can be damaged if they are excited with the reso\u00adnant frequency, which can then lead to what is known as a reso\u00adnance cata\u00adstrophe. That is precisely what happened when the Tacoma Narrows Bridge collapsed in 1940 after being excited into oscil\u00adla\u00adtion by unusual wind condi\u00adtions just a few months after it was opened.<\/p>\n\n\n\n

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\u201eExces\u00adsive oscil\u00adla\u00adtion over a prolonged period of time result in damage. Small vibra\u00adtion ampli\u00adtudes that are not crit\u00adical in mechan\u00adical terms may cause unpleasant noise.\u201c<\/p>\nBern\u00adhard Siedler, Head of Product Manage\u00adment at ebm-papst Mulfingen<\/cite><\/blockquote>\n\n\n\n

However, reso\u00adnances do not always result in system failure. Even small vibra\u00adtion ampli\u00adtudes that are not crit\u00adical in mechan\u00adical terms may cause noise that is perceived as unpleasant. This must also be avoided.<\/p>\n\n\n\n

Exci\u00adta\u00adtion sources<\/strong><\/h2>\n\n\n\n

Vibra\u00adtion exci\u00adta\u00adtion cannot be avoided completely and has different causes: internal and external excitation.The most crit\u00adical internal exci\u00adta\u00adtion is the 1st order exci\u00adta\u00adtion (speed) caused by a rotor imbal\u00adance. Balancing reduces this exci\u00adta\u00adtion to a non-crit\u00adical level but can never elim\u00adi\u00adnate it entirely. Any rotating mass has a residual imbal\u00adance. During oper\u00ada\u00adtion, the imbal\u00adance may be wors\u00adened by dirt on the impeller. Incor\u00adrect handling of a fan (impact during trans\u00adporta\u00adtion, rolling it on its impeller) can cause plastic defor\u00adma\u00adtion, increasing the imbal\u00adance and, as a result, the exci\u00adta\u00adtion forces.<\/p>\n\n\n\n

Higher-frequency exci\u00adta\u00adtion also occurs, such as blade passing noise (e.g. 5th order exci\u00adta\u00adtion in a fan with five blades) or cogging torque (e.g. 12th order exci\u00adta\u00adtion in a 12-slot motor). In ebm-papst prod\u00aducts, these exci\u00adta\u00adtions are opti\u00admized and reduced to a minimum. External exci\u00adta\u00adtion may be broad\u00adband \u2013 like the wind that caused the Tacoma Narrows Bridge failure or flow-induced vibra\u00adtion resulting from unfa\u00advor\u00adable instal\u00adla\u00adtion condi\u00adtions (insuf\u00adfi\u00adcient distance between the impeller and wall) \u2013 or it may have a specific frequency such as the motor speed of a compressor installed close by. This can also excite natural frequen\u00adcies, resulting in reso\u00adnance.<\/p>\n\n\n\n

The instal\u00adla\u00adtion condi\u00adtions affect the reso\u00adnance behavior<\/h2>\n\n\n\n

As each system has one or more natural frequen\u00adcies, fans are also not immune to reso\u00adnance-related damage, despite being balanced precisely by the manu\u00adfac\u00adturers during produc\u00adtion in accor\u00addance with the state of the art. A crucial role is played by the condi\u00adtions of a fan\u2019s instal\u00adla\u00adtion, which unfor\u00adtu\u00adnately can neither be predicted nor allowed for in advance. Installing a fan in a system always creates a new reso\u00adnant arrange\u00adment with several specific natural frequen\u00adcies (Fig. 1).<\/p>\n\n\n\n

Fig. 1: Vibra\u00adtion char\u00adac\u00adter\u00adis\u00adtics of the fan installed in an AHU. (Image | ebm-papst)<\/figcaption><\/figure>\n\n\n\n

With the speed-controlled oper\u00ada\u00adtion that is now wide\u00adspread, the like\u00adli\u00adhood that a fan will at times be oper\u00adated at reso\u00adnance increases consid\u00ader\u00adably. The reci\u00adp\u00adrocal effects are often under\u00ades\u00adti\u00admated. Here, the reso\u00adnant system consisting of a fan and its surround\u00adings can lead to a consid\u00ader\u00adable shift in the natural frequen\u00adcies of the overall struc\u00adture, thus resulting in a signif\u00adi\u00adcant increase in the dynamic loads caused by reso\u00adnance.<\/p>\n\n\n\n

However, the influ\u00adences of the surrounding struc\u00adtures can be simu\u00adlated very precisely using FEM and the loca\u00adtion of the crit\u00adical speeds can then be predicted if the appli\u00adca\u00adtion design is known. By stiff\u00adening the correct struc\u00adtural compo\u00adnents, reso\u00adnance points can be shifted to a less crit\u00adical range or even elim\u00adi\u00adnated completely from the exci\u00adta\u00adtion (no reso\u00adnance).<\/p>\n\n\n\n

Vibra\u00adtion measure\u00adment in the installed state<\/h2>\n\n\n\n

It is there\u00adfore neces\u00adsary to test the reso\u00adnance char\u00adac\u00adter\u00adis\u00adtics of the fans during oper\u00ada\u00adtion and in inter\u00adac\u00adtion with the surround\u00adings, modi\u00adfying them if neces\u00adsary. This involves deter\u00admining the vibra\u00adtion levels and avoiding speed ranges at which vibra\u00adtions exceed the limit value. It is also neces\u00adsary to iden\u00adtify the reso\u00adnance speeds and ensure that these are not sustained during normal oper\u00ada\u00adtion. Following the instal\u00adla\u00adtion of a fan, vibra\u00adtion measure\u00adments and\/or a search for reso\u00adnance points should there\u00adfore be performed throughout the entire speed control range. <\/p>\n\n\n\n

<\/div>\n\n\n\n
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Fig. 2: Vibra\u00adtion measure\u00adment of a RadiPac fan in cube design. (Image | ebm-papst)<\/figcaption><\/figure>\n<\/div>\n\n\n\n
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Fig. 3: For the vibra\u00adtion analysis of RadiPac centrifugal fans, ebm-papst recom\u00admends measuring the vibra\u00adtion in all three axes. (Image | ebm-papst)<\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n

This provides an overall impres\u00adsion of the system\u2019s vibra\u00adtion char\u00adac\u00adter\u00adis\u00adtics and reveals any unpre\u00addictable effects and also possible mistakes made during the run-up to commis\u00adsioning. These reso\u00adnance points must also be moni\u00adtored during oper\u00ada\u00adtion, at least within the interval spec\u00adi\u00adfied in the oper\u00adating instruc\u00adtions. This is usually every six months because natural frequen\u00adcies may shift during service life, for example, if the rigidity of the customer\u2019s system changes. Of course, acoustic changes are always a warning sign, but damage is usually unavoid\u00adable by that stage.<\/p>\n\n\n\n

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Rules for vibra\u00adtion measure\u00adment<\/h3>\n\n\n\n

A number of aspects must be consid\u00adered in vibra\u00adtion measure\u00adment. The first is avoiding disrup\u00adtion to the air intake system, i.e. there must be enough space in front of the impeller to ensure unob\u00adstructed air flow. For the vibra\u00adtion analysis of centrifugal fans, ebm-papst recom\u00admends measuring vibra\u00adtion in all three axes (Figs. 2 and 3), but at least in two axes \u2013 radial and axial to the axis of rota\u00adtion \u2013 using a typical vibra\u00adtion measuring unit fitted close to the motor mount. This provides the effec\u00adtive value of the vibra\u00adtion speed (mm\/s) but does not auto\u00admat\u00adi\u00adcally iden\u00adtify poten\u00adtially crit\u00adical speeds.<\/p>\n\n\n\n

To do this, the fan must be oper\u00adated at various speeds and data recorded, for example, by logging in an external device. In the simplest case, the speeds are noted in a table, indi\u00adcating where reso\u00adnant frequen\u00adcies occur. The best approach is to increase the speed by 5% incre\u00adments, for example. The start-up time can also play a role. The best approach is a linear start-up, with the speed increasing from minimum to maximum over a five-minute period.<\/p>\n\n\n\n

<\/div>\n<\/div><\/div>\n\n\n\n

Auto\u00admatic reso\u00adnance detec\u00adtion for more oper\u00ada\u00adtional reli\u00ada\u00adbility<\/h2>\n\n\n\n

To make it easier to handle unavoid\u00adable reso\u00adnance, the third gener\u00ada\u00adtion of centrifugal fans with inte\u00adgrated high-perfor\u00admance elec\u00adtronics from ebm-papst is equipped with auto\u00admatic reso\u00adnance detec\u00adtion. The inte\u00adgrated vibra\u00adtion sensors detect the speeds at which reso\u00adnance occurs and the asso\u00adci\u00adated soft\u00adware prevents oper\u00ada\u00adtion in the spec\u00adi\u00adfied crit\u00adical ranges.<\/p>\n\n\n\n

For this purpose, a test start-up can be performed during commis\u00adsioning in which the vibra\u00adtion speed is analyzed over the entire speed progres\u00adsion from stand\u00adstill to nominal speed (Fig. 4). If exces\u00adsive vibra\u00adtion speeds are detected in certain ranges, the control soft\u00adware adjusts itself after acti\u00adva\u00adtion by the customer so that these speed ranges are \u201cpassed over\u201d in the future. This means that the fan passes through these speeds, but contin\u00aduous oper\u00ada\u00adtion in these ranges is avoided. Oper\u00ada\u00adtors can manu\u00adally edit the soft\u00adware settings at any time, meaning that they always have full control.<\/p>\n\n\n

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<\/div>

Fig. 4: Vibra\u00adtion behavior with barred speed range. (Image | ebm-papst)<\/p><\/div>\n

<\/div>

Fig. 5: Vibra\u00adtion char\u00adac\u00adter\u00adis\u00adtics in the AHU with addi\u00adtional external vibra\u00adtion source, e.g. a compressor. (Image | ebm-papst)<\/p><\/div>\n<\/div>\n\n\n

However, the vibra\u00adtion speed may also increase during oper\u00ada\u00adtion, for example due to dirt deposits on the impeller, which results in addi\u00adtional imbal\u00adances. In this case, the soft\u00adware emits an auto\u00admatic warning. The oper\u00adator can now check the system for addi\u00adtional imbal\u00adances and can perform trou\u00adbleshooting measures (e.g. cleaning and\/or new start-up). Vibra\u00adtions that are intro\u00adduced by devices installed nearby, e.g. defec\u00adtive compres\u00adsors, can be detected but not avoided. In this case, the oper\u00adator can initiate suit\u00adable measures (Fig. 5). This makes auto\u00admatic reso\u00adnance detec\u00adtion a useful option for increasing the oper\u00ada\u00adtional reli\u00ada\u00adbility of the centrifugal fans in crit\u00adical appli\u00adca\u00adtions.<\/p>\n\n\n\n

Side note: Vibra\u00adtion-absorbing elements and speed range<\/h2>\n\n\n\n

Vibra\u00adtion-absorbing elements (Fig. 6), such as appto\u00adpri\u00adately designed springs or rubber elements, help to isolate fans from vibra\u00adtions in their surround\u00adings (and vice versa). However, certain aspects must be taken into consid\u00ader\u00ada\u00adtion when selecting these elements. Along\u00adside the natural frequency of the fan set-up itself, the instal\u00adla\u00adtion of vibra\u00adtion-absorbing elements produces a further spring-mass system with its own reso\u00adnant frequency. The fan moves as a rigid body on the vibra\u00adtion-absorbing elements (fan defor\u00adma\u00adtion is negli\u00adgible). <\/p>\n\n\n\n

Fig. 6: Vibra\u00adtion-absorbing elements, i.e. appro\u00adpri\u00adately designed springs or rubber elements, help to isolate the fan from vibra\u00adtion in the surrounding area. (Image | ebm-papst)<\/figcaption><\/figure>\n\n\n\n

In this case, the move\u00adment comes from defor\u00adma\u00adtion of the vibra\u00adtion-absorbing elements, which also causes higher vibra\u00adtion ampli\u00adtudes. For this reason, the fan may only be oper\u00adated above rigid body mode, which deter\u00admines the minimum speed (Fig. 7). Although the device does not suffer any direct damage during oper\u00ada\u00adtion in rigid body mode, prolonged oper\u00ada\u00adtion in this range reduces the overall service life. It can also result in high noise levels. This speed range should there\u00adfore be passed through as quickly as possible, and sustained oper\u00ada\u00adtion in this range avoided alto\u00adgether.<\/p>\n\n\n\n

The speed range in which the vibra\u00adtion level is much lower begins at a suffi\u00adcient distance from the reso\u00adnance peak. Only in this range, above the minimum speed, can the vibra\u00adtion-absorbing elements isolate the fan from the vibra\u00adtions of the system or building. To select the right vibra\u00adtion-absorbing elements, the oper\u00adating speed of the fan in its appli\u00adca\u00adtion must be known. <\/p>\n\n\n\n

Fig. 7: Rigid body move\u00adment on springs (example of a RadiPac fan in size 710 with DV280 motor). (Image | ebm-papst)<\/figcaption><\/figure>\n\n\n\n

For every fan, one can find correctly sized vibra\u00adtion-absorbing elements and the asso\u00adci\u00adated minimum speed. The above-mentioned factors must be heeded if using different elements. It is always worth consid\u00adering vibra\u00adtion-related aspects when installing fans. Correctly installed fans func\u00adtion reli\u00adably throughout their service lives, unex\u00adpected fail\u00adures are prevented, and users also benefit from lower noise emis\u00adsions.<\/p>\n","protected":false},"excerpt":{"rendered":"

Vibra\u00adtion is omnipresent. While some kinds of vibra\u00adtion are perceived as pleasant, such as music, others can cause major damage. This is because reso\u00adnance may occur if a system is excited peri\u00adod\u00adi\u00adcally by a force whose exci\u00adta\u00adtion frequency matches a natural frequency of the system. In this state, vibra\u00adtion ampli\u00adtudes increase sharply. Although the atten\u00adu\u00ada\u00adtion [\u2026]<\/p>\n","protected":false},"author":42,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"wp_typography_post_enhancements_disabled":false,"footnotes":""},"categories":[16,9],"tags":[],"class_list":["post-21353","post","type-post","status-publish","format-standard","hentry","category-fans","category-refrigeration-ventilation"],"acf":[],"_links":{"self":[{"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts\/21353","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\/42"}],"replies":[{"embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/comments?post=21353"}],"version-history":[{"count":21,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts\/21353\/revisions"}],"predecessor-version":[{"id":21384,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/posts\/21353\/revisions\/21384"}],"wp:attachment":[{"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/media?parent=21353"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/categories?post=21353"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mag.ebmpapst.com\/en\/wp-json\/wp\/v2\/tags?post=21353"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}