HomeFAQWhy asynchronous motor with VFD startup causes noise?

Why asynchronous motor with VFD startup causes noise?

By using some sort of power electronics (like variable frequency drive) to adjust the line frequency seen by the asynchronous motor, rather than having a dedicated rotating supply capable of producing a variable frequency output.

The VFD takes the input waveform from the line, which is nominally a pure AC sinusoid. It then rectifies it to a DC signal. This is then chopped up into varying time intervals and reconstitutes it back into an approximation of a sinusoidal form. However, the output waveform is definitely only an approximation; there are considerable harmonics of different amplitudes and frequencies are embedded within it.

Unfortunately, the VFD ALSO puts out harmonics on the incoming side, which means those harmonics are now circulating on the main bus ... which pretty much means anything also attached to the bus is going to see those same harmonics.

The lower the desired output frequency (relative to the input line frequency), the higher the harmonic content is likely to become. Another way to say this is that the signal has more "noise" when operated away from line frequency - both on the input and output side of the variable frequency drive.

The upstream impact on other equipment can be alleviated to some extent by any or all of the following: isolating the VFD from the line through a dedicated transformer, adding appropriate harmonic filters on the system, using a VFD with a higher pulse count, and by using a VFD with an active front end. If none of these - singly or in combination - adequately solves the problem, then the other equipment having the troubles should be replaced with ones that are relatively insensitive to the harmonic content of the incoming line.

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