I need to calculate an LC filter that will be connected to the output of a single phase inverter (in order to filter the PWM voltage). the switching is 8Khz. the foundamental is 400Hz. Any thought how to calculate the L,C? THANK YOU.

 

f=1/(sqrt(L*C)*2*pi)

f should be at some suitable point between the frequencies you want to pass and those you want to block, in your case I would say around 1kHz or somewhat lower.

Increasing inductance and capacitance both give a lower frequency and you will need (of course) a coil capable of handling the current and the ripple current (ripple current is the 8kHz and higher frequency variations in current) and with a thick enough insulation to handle any peak voltage (can be higher than what you output
actually). You will also need a capacitor with low enough ESR (equivalent serial resistance) to actually do some useful work at 8kHz, capable of handling any peak voltage and the ripple current - and of course bipolar.

As a general rule of thumb I usually try to spend approximately as much money on the inductor as on the capacitor - that usually gives the lowest overall cost and generally a good enough mix of filtering with the inductor and capacitor. (This simple approach seems to be good enough for this kind of applications - but is of course not good enough when you deal with sound, radio waves and so on...).

/Johan Bengtsson

 

Of particular interest is the inductor. I have used a ferrite inductor and reduced the inductance value by increasing airgap. This causes no effect on the filtered waveshape. However, if I reduce the inductance value by going to a smaller core, I seem to notice increased ripple in the output voltage. Any comments?

Mahesh

 

Sounds a lot like you are saturating the smaller core. The airgap not only decreases the inductance, it greatly increases the saturation current. This is why many inductors for DC filtering service have an air gap.

Regards

cww