1 VLT HVAC Drive in Fan Application with Resonance vibrations in the Fan
In applications where Motor with fan mounteddirectly on the drive end shaft and where the running point is in field weakening area, a small number of customers have experienced vibrations when applying FC102 drives in the power range (7,5[kW]-18 [kW]). This Application note answers the question of the causes for these Vibrations and give solutions to solve it
Over modulation: Over modulation is a way to increasethe motor voltage delivered by the drive between fmot > 45Hz and fmot < 55Hz or fmot < 55 Hz and < 65Hz The advantage of over modulation is: Lower currents and higher efficiency at over fieldweakening area. Speed wise? That the drive can give nominal Grid voltage at nominal at nominal grid frequency If the mains voltage sometimes goes below the correct motor voltage at e.g. 43 Hz over-modulationcan again add the needed motor voltage.
1.3 Cause to the vibrations
I expect that this is just an example with one motor frequency. The vibrations occur at frequencies corresponding to the 6thharmonic of the drive output frequency, fout. The system vibrates when the motor is supplied by a output frequency of approximately 55 Hz (6 multiplied fout = 6*55=330 Hz) (Depends of the resonancefrequency of the fan) which is approximately 330 [Hz] on the fan wheel . This 6th harmonics is generated only by the over modulation. If the resonance frequency of the fan is close to this 330Hz, Theresonance will cause vibrations. Those vibrations can in the end cause cracks in the Fan and in worst cases a broken fan or /and broken Bearings
Over modulation means that a voltage ismodulated above the possible level given by the DC-link voltage. The result is non-sinusoidal output voltages, because of the shape of the sinus wave
1. The best solution is...