Torsional Vibration

Paper No. 106

Variable frequency drives (VFDs) are used to control motors over a wide operating speed range. VFDs can create torsional excitation that in some instances cause torsional fatigue failures of couplings and shafts [1,2]. This case study deals with repeated failures of disc pack couplings on three newly commissioned combustion air blowers (fans).
Field tests indicated that at certain speeds, the dynamic torque was significantly amplified by excitation of the first torsional natural frequency (TNF). The alternating torque reached 800% of full-load torque (FLT), which explained the coupling damage and could have also resulted in fatigue cracks in the motor and/or fan shafts if not corrected.
The source of the excitation was electrical energy from the VFD, and not due to turbulence or pressure pulsation from the fan. During the testing, attempts were made to reduce the electrical energy by adjusting (tuning) various VFD parameters. Although these efforts were not completely successful in solving the problem, a correlation was established with the alternating torque when VFD parameters were changed.
As a temporary solution, the VFD was reprogrammed to limit the maximum operating speed and to prevent excitation of the first TNF. The long-term solution primarily involved replacing the disc pack couplings with an alternate coupling design utilizing rubber blocks in compression. An analytical torsional vibration analysis was performed to evaluate several different coupling sizes and rubber blocks of various hardness values (durometers). It was predicted that the alternating torque amplitudes would be significantly reduced once the new couplings were installed.
Follow-up field tests confirmed that the recommended coupling reduced the alternating torque to an acceptable level over the entire operating speed range. The torsional stiffness of the rubber block coupling was lower compared to the disc pack coupling, which in turn lowered the first TNF and eliminated the coincidence with the VFD excitation within the fan operating speed range. The rubber coupling also had more damping which further reduced the overall torsional vibration levels and improved VFD stability.

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