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# Frequency of SPWM inverters

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#### mrinalmani

What is the approximate carrier frequency of SPWM inverters available in the commercial market? Is it say, 20 to 30KHz or 100KHz+
I am talking about 12V 500VA to 1000VA inverters.
Any one with experience with high frequency inverters, please share your experience.

Also what is the approx value of the filter inductor?
My calculations show that at 100KHz carrier frequency, a 100uH inductor along with a 1uF capacitor will be sufficient to lower THD below 4%
But at such small inductance, there will be rather large high frequency current (approx 3A) circulating between the inductor and capacitor, which is enough to cause excessive core loss. So it appears that THD is hardly a concern, the inductor must be larger than that required by the THD limit, to reduce circulating HF current.
Thanks

Engineering is both the science and art of trade-offs.

One must trade off cost, performance (with many sub-categories here like THD, efficiency, output impedance), size, weight, component availability and the amount of effort and expertise required to design it.

You are also correct about the recirculating currents and its effect on the inductor. But even worse, is the effect of self heating on the capacitor itself. It will have to be the stacked plate (not wound!!) film capacitor.
Your capacitor size will be dictated not by THD, but by actual ripple current capabilities.

While there are higher frequency ones available, the most common is 16kHz I believe.

mrinalmani

### mrinalmani

Points: 2
The inductor should be a value that does not restrict 50 Hz at the current level you desire. A larger Henry value restricts (chokes) current. This may be a handy method to reduce a high output voltage.

To help you calculate a ballpark figure you can use the formula for inductive impedance:

XL = 2 Pi F L

The capacitor is necessary for power factor correction. It must be customized to the load and frequency.

But at such small inductance, there will be rather large high frequency current (approx 3A) circulating between the inductor and capacitor,

If you select the inductor correctly, it should block the high frequencies.

It is unavoidable that the capacitor carries about the same current that the load carries. Its purpose is power factor correction, to adjust the phase of the supply current waveform. It must coincide with the supply voltage waveform.

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