By their nature, motors generate noise during operation. Because of differences in design and fabrication, even motors of equivalent frame size and horsepower may operate at different noise levels. The amount of noise produced can be affected in a number of ways when the motor is controlled by a Variable Frequency Drive (VFD). For instance, operating the motor at a lower speed will usually reduce noise level, all other factors being equal. Also, owing to a given motor’s design/construction, there may be frequencies, termed resonant frequencies, at which increased vibration and noise are created. Finally, the VFD’s switching frequency affects the amount and quality of harmonic current the drive produces; this current can create additional motor vibration and result in increased acoustic noise. Let’s look at these circumstances in more detail and discuss what can be done to address the noise produced.
Operating the motor at reduced speed is basically self-explanatory; cooling air speed and friction are reduced, resulting in less noise created. That said, a given motor may experience increased vibration at characteristic frequencies, which can increase noise to a level greater than that produced at base (i.e. rated) speed. Most VFDs can be user-programmed to “jump” those frequencies, thereby avoiding the increased vibration. Of course, this assumes that the process can accept not being driven at those speeds.
The impact of changing VFD switching frequency on motor noise requires some background explanation. Variable frequency drives operate by rectifying incoming AC voltage to DC in the “converter” section, filtering the DC to produce a relatively fixed DC voltage level in the DC link, then creating a synthesized AC waveform in the “inverter” section. The inverter typically uses Insulated Gate Bipolar Transistors (IGBTs) to generate pulses at a fixed amplitude and varying width to produce the waveform; the polarity of the pulses is dependent on which IGBTs are switched “on” at a given time, and the width of the pulses reflects the “on” and “off” durations of the IGBTs. At any specified output voltage, the waveform can be made up of many narrow pulses (fast switching with shorter “on” duration) or fewer wide pulses (slower switching with longer “on” duration). The frequency at which these devices are switched “on” and “off” is referred to as the switching frequency or the carrier frequency, and it is this frequency which can have an effect on acoustic noise generated by the driven motor. VFD specifications include available switching frequencies – for example, 1 kHz, 4 kHz, 8 kHz, 12 kHz – which are user-selectable.
Often, raising the switching frequency reduces the audible noise produced in the motor. That is because as the frequency is increased, additional acoustic power is needed to produce a sound of equal level at the eardrum. Since only the frequency, not the acoustic energy, is being increased, the ear detects the lower sound pressure as a reduction in volume. However, and again depending on the motor’s construction, there may be frequencies at which the sound is perceived to increase, so either raising or lowering the switching frequency may improve the situation. Keep in mind also that increasing the switching frequency comes at the price of reduced efficiency due to increased IGBT switching and heating, so manufacturers typically recommend operating the drive at the lowest switching frequency acceptable.
Since audible noise is somewhat subjective, and the ambient conditions of each application differ, it’s likely that readers have encountered specific challenges in reducing drive-induced motor noise. Please feel free to share them in the Comments section. And as usual, Joliet Technologies stands ready to help define your application or provide additional information. Please contact us at firstname.lastname@example.org. And remember to visit us on-line at www.joliettech.com and our blog at www.joliettech.com/blog. I hope you can join us next week.