Multi pulse width modulation - eliminating harmonics

Thread Starter

Aljaž Krašna

Joined May 8, 2016
14
Hey
can someone could explain me how to eliminate certain harmonics in output of inverter. The inverter is controled with Multi-Pulse Width Modulation.

Thank you
 

dendad

Joined Feb 20, 2016
3,176
When you switch something, there are harmonics produced, the sharper the edge the higher the harmonics go to.
The coils and capacitors are there to slow down the rise and fall of the switching edges. Also, sometimes tuned as flters to resonate at the unwanted harmonic frequencies to either block them from flowing out or shorting them out.
Imaging a sine wave. A pure frequency. When you add a harmonics, the wave shape changes, and to generate a square wave, many harmonics must be added to the pure sine wave to get the shape. If you want just a pure sine wave inverter, without harmonics, you would need to run a linear amplifier to feed the sine wave to the transformer. This is very inefficient, so the normal way is to switch the power, and produce a lot of harmonics, than filter them out.
The True Sine Wave inverters actually run a form of Pulse Width Modulation at a much higher frequency that your desired output, the PWM pattern being designed to produce a sine wave AFTER the now much higher frequency harmonics are filtered out. It can be easier to remove higher frequencies, and any properly designed transformer will be a lot physically smaller the higher the frequency you run it.
So the sort answer is, no, you cannot eliminate harmonics by proper switching of the transistors. You may be able to reduce them a bit but it is an inherent property of switching, or at least non sine wave shape signals. It is how they are "made" to be non sine wave. Any distortion from the sine wave generates harmonics.
To get rid of a particular frequency, a series tuned circuit at that frequency will let it flow so can be used to short it out, while a parallel tuned circuit will block it from flowing.
It can get pretty complicated!
 
Last edited:

MrChips

Joined Oct 2, 2009
19,768
That is how a Class-D audio amplifier works. The PWM clock frequency is much higher than the maximum sine wave frequency. Hence it becomes easier to remove the clock signal using inductors and capacitors.
 
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