Dear All,

In my Three Phase Inverter application, for closed loop control (Voltage and Current control) i need to take feedback of Voltages and Currents . My doubt is whether i need to have output LC filter and obtain sinusoidal waveforms for taking feedback OR whether the stepped waveform is sufficient.

If i use stepped waveform ie without using LC filter, and give it to ADC of microcontroller, does it affect the accuracy in calculations and control?

Put lp filter before the adc

I think that you need to look at each detail for its effect on the system or the objective specification. If a crude stepped waveform is completely adequate for the intended load and similarly the voltage specification is loose, then perhaps having the ADC sample at the peak with no filtering will be adequate. However, if more precision and accuracy are needed, then filtering for the load may be needed as well as further filtering for the ADC. Be careful with the phase shift of any filtering for the ADC that does not correspond to phase shift to the load. If enough samples are taken, perhaps some of the filtering can be done in software. Is the load linear or does it distort the intended waveform and do you need to account for that?

Dear Jim,
The load may not be linear and might distort the intended waveform....

I meant the question as one to ask yourself as to how much finesse is needed to meet the requirements of the project. Since you answered yes, the next question is how much waveform distortion is allowed and how can it be mitigated? Or, what exactly is the feedback supposed to accomplish? What is the application for the inverter? Is it a consumer item to charge an electric razor or is it for medical laboratory equipment, or spacecraft? You need to have or obtain complete details of the requirements and then to design to each and every one of them.
Back to the filter question, do be aware that filters in the feedback loop are very much a part of the analysis of that feedback loop. That may be obvious but is sometimes overlooked.

Unnikrishnan, for EMI reasons alone, one would generally not ever want to put the unfiltered output of a PWM AC sine wave inverter into a load, unless perhaps, the load is inductive and would tend to filter out the harmonics itself. Even this would require a commutation circuit to keep the EMI in check. I don't really recommend it except for the most unusual of circumstances. Why even use a PWM inverter if your not going to filter the output waveform? You'd be better off with a modified sine waveform (square wave with deadtime between half-cycles). At the very least, you could use a smaller than normal filter, but you should use a filter unless you have some extreme reason not to. Regards, Kamran Kazem, V.P. & CTO, Magnetic Design Labs, Inc.