Vibra­tion is omnipresent. While some kinds of vibra­tion are perceived as pleasant, such as music, others can cause major damage. This is because reso­nance may occur if a system is excited peri­od­i­cally by a force whose exci­ta­tion frequency matches a natural frequency of the system. In this state, vibra­tion ampli­tudes increase sharply. Although the atten­u­a­tion effects that are always present limit the ampli­tudes within the reso­nance, low atten­u­a­tion and high exci­ta­tion force will cause the vibra­tion ampli­tudes to increase very signif­i­cantly to the point that they may exceed the system’s load limit.

The exces­sive oscil­la­tion then results in damage, espe­cially if this state is main­tained over a prolonged period of time. For example, vibra­tions may loosen screw connec­tions or damage ball and roller bear­ings. Even build­ings can be damaged if they are excited with the reso­nant frequency, which can then lead to what is known as a reso­nance cata­strophe. That is precisely what happened when the Tacoma Narrows Bridge collapsed in 1940 after being excited into oscil­la­tion by unusual wind condi­tions just a few months after it was opened.

„Exces­sive oscil­la­tion over a prolonged period of time result in damage. Small vibra­tion ampli­tudes that are not crit­ical in mechan­ical terms may cause unpleasant noise.“

Bern­hard Siedler, Head of Product Manage­ment at ebm-papst Mulfingen

However, reso­nances do not always result in system failure. Even small vibra­tion ampli­tudes that are not crit­ical in mechan­ical terms may cause noise that is perceived as unpleasant. This must also be avoided.

Exci­ta­tion sources

Vibra­tion exci­ta­tion cannot be avoided completely and has different causes: internal and external excitation.The most crit­ical internal exci­ta­tion is the 1st order exci­ta­tion (speed) caused by a rotor imbal­ance. Balancing reduces this exci­ta­tion to a non-crit­ical level but can never elim­i­nate it entirely. Any rotating mass has a residual imbal­ance. During oper­a­tion, the imbal­ance may be wors­ened by dirt on the impeller. Incor­rect handling of a fan (impact during trans­porta­tion, rolling it on its impeller) can cause plastic defor­ma­tion, increasing the imbal­ance and, as a result, the exci­ta­tion forces.

Higher-frequency exci­ta­tion also occurs, such as blade passing noise (e.g. 5th order exci­ta­tion in a fan with five blades) or cogging torque (e.g. 12th order exci­ta­tion in a 12-slot motor). In ebm-papst prod­ucts, these exci­ta­tions are opti­mized and reduced to a minimum. External exci­ta­tion may be broad­band – like the wind that caused the Tacoma Narrows Bridge failure or flow-induced vibra­tion resulting from unfa­vor­able instal­la­tion condi­tions (insuf­fi­cient distance between the impeller and wall) – or it may have a specific frequency such as the motor speed of a compressor installed close by. This can also excite natural frequen­cies, resulting in reso­nance.

The instal­la­tion condi­tions affect the reso­nance behavior

As each system has one or more natural frequen­cies, fans are also not immune to reso­nance-related damage, despite being balanced precisely by the manu­fac­turers during produc­tion in accor­dance with the state of the art. A crucial role is played by the condi­tions of a fan’s instal­la­tion, which unfor­tu­nately can neither be predicted nor allowed for in advance. Installing a fan in a system always creates a new reso­nant arrange­ment with several specific natural frequen­cies (Fig. 1).

With the speed-controlled oper­a­tion that is now wide­spread, the like­li­hood that a fan will at times be oper­ated at reso­nance increases consid­er­ably. The reci­p­rocal effects are often under­es­ti­mated. Here, the reso­nant system consisting of a fan and its surround­ings can lead to a consid­er­able shift in the natural frequen­cies of the overall struc­ture, thus resulting in a signif­i­cant increase in the dynamic loads caused by reso­nance.

However, the influ­ences of the surrounding struc­tures can be simu­lated very precisely using FEM and the loca­tion of the crit­ical speeds can then be predicted if the appli­ca­tion design is known. By stiff­ening the correct struc­tural compo­nents, reso­nance points can be shifted to a less crit­ical range or even elim­i­nated completely from the exci­ta­tion (no reso­nance).

Vibra­tion measure­ment in the installed state

It is there­fore neces­sary to test the reso­nance char­ac­ter­is­tics of the fans during oper­a­tion and in inter­ac­tion with the surround­ings, modi­fying them if neces­sary. This involves deter­mining the vibra­tion levels and avoiding speed ranges at which vibra­tions exceed the limit value. It is also neces­sary to iden­tify the reso­nance speeds and ensure that these are not sustained during normal oper­a­tion. Following the instal­la­tion of a fan, vibra­tion measure­ments and/or a search for reso­nance points should there­fore be performed throughout the entire speed control range.

This provides an overall impres­sion of the system’s vibra­tion char­ac­ter­is­tics and reveals any unpre­dictable effects and also possible mistakes made during the run-up to commis­sioning. These reso­nance points must also be moni­tored during oper­a­tion, at least within the interval spec­i­fied in the oper­ating instruc­tions. This is usually every six months because natural frequen­cies may shift during service life, for example, if the rigidity of the customer’s system changes. Of course, acoustic changes are always a warning sign, but damage is usually unavoid­able by that stage.

Auto­matic reso­nance detec­tion for more oper­a­tional reli­a­bility

To make it easier to handle unavoid­able reso­nance, the third gener­a­tion of centrifugal fans with inte­grated high-perfor­mance elec­tronics from ebm-papst is equipped with auto­matic reso­nance detec­tion. The inte­grated vibra­tion sensors detect the speeds at which reso­nance occurs and the asso­ci­ated soft­ware prevents oper­a­tion in the spec­i­fied crit­ical ranges.

For this purpose, a test start-up can be performed during commis­sioning in which the vibra­tion speed is analyzed over the entire speed progres­sion from stand­still to nominal speed (Fig. 4). If exces­sive vibra­tion speeds are detected in certain ranges, the control soft­ware adjusts itself after acti­va­tion by the customer so that these speed ranges are “passed over” in the future. This means that the fan passes through these speeds, but contin­uous oper­a­tion in these ranges is avoided. Oper­a­tors can manu­ally edit the soft­ware settings at any time, meaning that they always have full control.

However, the vibra­tion speed may also increase during oper­a­tion, for example due to dirt deposits on the impeller, which results in addi­tional imbal­ances. In this case, the soft­ware emits an auto­matic warning. The oper­ator can now check the system for addi­tional imbal­ances and can perform trou­bleshooting measures (e.g. cleaning and/or new start-up). Vibra­tions that are intro­duced by devices installed nearby, e.g. defec­tive compres­sors, can be detected but not avoided. In this case, the oper­ator can initiate suit­able measures (Fig. 5). This makes auto­matic reso­nance detec­tion a useful option for increasing the oper­a­tional reli­a­bility of the centrifugal fans in crit­ical appli­ca­tions.

Side note: Vibra­tion-absorbing elements and speed range

Vibra­tion-absorbing elements (Fig. 6), such as appto­pri­ately designed springs or rubber elements, help to isolate fans from vibra­tions in their surround­ings (and vice versa). However, certain aspects must be taken into consid­er­a­tion when selecting these elements. Along­side the natural frequency of the fan set-up itself, the instal­la­tion of vibra­tion-absorbing elements produces a further spring-mass system with its own reso­nant frequency. The fan moves as a rigid body on the vibra­tion-absorbing elements (fan defor­ma­tion is negli­gible).

In this case, the move­ment comes from defor­ma­tion of the vibra­tion-absorbing elements, which also causes higher vibra­tion ampli­tudes. For this reason, the fan may only be oper­ated above rigid body mode, which deter­mines the minimum speed (Fig. 7). Although the device does not suffer any direct damage during oper­a­tion in rigid body mode, prolonged oper­a­tion in this range reduces the overall service life. It can also result in high noise levels. This speed range should there­fore be passed through as quickly as possible, and sustained oper­a­tion in this range avoided alto­gether.

The speed range in which the vibra­tion level is much lower begins at a suffi­cient distance from the reso­nance peak. Only in this range, above the minimum speed, can the vibra­tion-absorbing elements isolate the fan from the vibra­tions of the system or building. To select the right vibra­tion-absorbing elements, the oper­ating speed of the fan in its appli­ca­tion must be known.

For every fan, one can find correctly sized vibra­tion-absorbing elements and the asso­ci­ated minimum speed. The above-mentioned factors must be heeded if using different elements. It is always worth consid­ering vibra­tion-related aspects when installing fans. Correctly installed fans func­tion reli­ably throughout their service lives, unex­pected fail­ures are prevented, and users also benefit from lower noise emis­sions.