ever, the frequency adjustment may increase the vibration level of an original system if the high level vibration response is just a forced vibration phenomenon, not resonance as shown in Fig. 3b. In the next section, reasons causing these errors are discussed and a right dynamic absorber design scheme is suggested. 8.4 8.45 8.5 8.55 8.6 -1.5 -1 -0.5 0 0.5 1 1.5 x 10-5 Time(sec) Response(Acceleration) 8.4 8.45 8.5 8.55 8.6 -1 -0.5 0 0.5 1 x 10-5 Time(sec) Response(Acceleration) 3rd model 4th model 3rd model 4th model Fig. 3 Level change in time responses of the 3rd and 4th models depending on excitation force frequency: (a) 3 * 2 Ω =Ω =Ω c (b) 3 * 2 Ω ≠Ω =Ω c . Practical Scheme for Dynamic Absorber Design As summarized above, the natural frequency of an additional system must properly be tuned so that an anti-resonance frequency should coincide with an excitation force frequency. To the end, one must check two things: the first one is to check if the high level response is due to resonance or just a forced vibration response; and the second one is to check if a used theoretical well represents vibration characteristics of a real mechanical device. Fig. 4 compares response change in a main system when an additional system is attached for two cases: resonance and just a forced vibration response. As shown in the Fig. 4b, the system has a high level response due to a single excitation frequency force, which does not coincide with its natural frequency. This case corresponds to just a forced vibration phenomenon, not resonance. Nevertheless, if an additional mass-spring-damper system is designed on the basis of dynamic absorber design theory and is attached to an original system, the response at the excitation frequency may increase as shown in Fig. 3b. An improper theoretical model may result in vibration increase in a main system. A general way to operate a linear compressor most efficiently is to tune the natural frequency of a pump in a linear compressor to the motor current frequency mω using the following equation [10]: 278
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