Carrier frequency
Carrier frequency in fact causes the audible noise. You can diminish the noise with high frequency ripple filters which will add cost to the installation. This may be necessary in places where the public or employees are in close proximity to the SMPS or VFD. In most cases of a VFD you may increase the carrier frequency beyond the audible level of human hearing level, however, not without drawback. The drawback is simply that the VFD now has higher losses in the Power Stack due to higher switching frequencies and may have the need to be de-rated in terms of highest continuous current available.
Higher carrier frequency will reduce audible noise but at a cost in losses in the variable frequency drive. Lower carrier means high motor noise and higher losses in the motor. Some VFDs are rated full output at only 2.5kHZ max, others at 4kHz and higher. So, if you push the VFD carrier higher than its standard output, it has to be derated (unloaded a bit). Take the carrier down, the VFD likes it and the motor hates it. Motors like pure sinusoidal waves coming from generators - any reconstructed wave form coming off a DC bus (all VFD outputs) has harmonics that heat up the motor and the lower the carrier, the more the harmonics and motor losses. Finally, be aware these losses are significant. Derate curves in the VFD instruction books show 5 - 7% losses just to go from 2.5 to 4kHz or 10 to 14HP degradation on a 200HP motor.
Higher carrier frequency will reduce audible noise but at a cost in losses in the variable frequency drive. Lower carrier means high motor noise and higher losses in the motor. Some VFDs are rated full output at only 2.5kHZ max, others at 4kHz and higher. So, if you push the VFD carrier higher than its standard output, it has to be derated (unloaded a bit). Take the carrier down, the VFD likes it and the motor hates it. Motors like pure sinusoidal waves coming from generators - any reconstructed wave form coming off a DC bus (all VFD outputs) has harmonics that heat up the motor and the lower the carrier, the more the harmonics and motor losses. Finally, be aware these losses are significant. Derate curves in the VFD instruction books show 5 - 7% losses just to go from 2.5 to 4kHz or 10 to 14HP degradation on a 200HP motor.
