Inverters rectify ac to dc then chop it up again with transistors to provide a variable frequency, variable voltage pseudo ac to create a variable speed motor from a fixed speed ac motor.
This chopping takes place many times a second from around 1kHz to 16kHz. This is the switching frequency. At lower switching frequencies, the inverter losses reduce (lower number of power consuming transistor switches) but the motor losses increase (cruder voltage waveform). The optimum switching frequency for overall efficiency is around 4kHz.
However, at 4kHz, there is a characteristic whine generated from the motor. Most adults can hear from approximately 50Hz to 10kHz so if the switching frequency is raised to 16kHz, the whine is outside the hearing range and the noise disappears.
Eliminate Noise from a Motor with an Inverter
Often, in industrial environments, this whine does not matter. However, in other environments this noise can be an irritant for example an inverter controlling the speed of an AHU in a hospital, the whine would be very apparent.
ABB inverters have a function that changes this switching frequency. There is a downside though, increased switching means increased switching losses, so the inverter would need to be derated by around 30%.
ABB inverters have a second noise function – noise smoothing. This function rotates the switching frequency and reduces the irritation of a continuous whine.
In summary, inverter motors can be made to be silent, but they use more internal power to operate, so our advice is; if you can put up with the whine, then let the inverter run at 4kHz.
Talk to IDS
To eliminate noise from a motor with an inverter and for specific advice regarding inverter technology and to discuss solutions fit for the environment they will be situated in, talk to our knowledgeable inverter engineers. Call us on 0115 944 1036, email [email protected] or complete our online form here.