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Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780
furnace wrote ...
I tried using a class d amp and you don't get a pure sine wave at the low frequencies like 50 or 60 hz it only gets pure sine at 150 hz and up and than you have to use large caps for filtering and adding the inductor does not make much of a difference to the wave, so your inverter would be very bulky and inefficient. To my experience the best way would be to feed the mosfets a pure sine wave at the gate to achieve a perfect sine wave at the output? Please correct me if I'm wrong but after testing this seems to be the only way for efficiency?
did you put a transformer at the output? with no filter and just a small AC transformer at the output you should get AC at the output of the transformer
Registered Member #4992
Joined: Wed May 23 2012, 03:57PM
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Posts: 108
The top one is the drive signal and the bottom one is the output of my secondary, did try and put a cap in series ,parallel and also added inductance to the primary but that's the best I get.
If that's the secondary side output then the transformer is still passing some harmonic components, you need an output filter. Try parallel C, series L for a basic low pass filter. If you can, design the LC for resonance at mains, then only the reflected primary and inverter impedance will limit power throughput.
Registered Member #4992
Joined: Wed May 23 2012, 03:57PM
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If I understand you correctly Shrad! It doesn't matter what frequincy you use at your primary as long as you get a clean sine wave at your secondary? This is very easy accomplished by adding a pulsed ac cap at your primary as a dc blocking cap and it lets your circuit resonate with a cool sine wave at a specific frequincy according to the size of your cap.
If this is what you mean than you are not in the 50 to 60 hz anymore and in the kHz range?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
furnace wrote ...
If I understand you correctly Shrad! It doesn't matter what frequincy you use at your primary as long as you get a clean sine wave at your secondary? This is very easy accomplished by adding a pulsed ac cap at your primary as a dc blocking cap and it lets your circuit resonate with a cool sine wave at a specific frequincy according to the size of your cap.
If this is what you mean than you are not in the 50 to 60 hz anymore and in the kHz range?
What Shrad means is that a class D amp will give a kHz pulsed 'simulation' of a sine wave, a 'digital' sine wave, which will give an 'analogue' sine wave when filtered.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600
Yes, you need to use Pulse Width Modulation:
to make a weird looking on-off voltage waveform, and then the transformer will pretty much get rid of the high frequencies and make it into a sine wave of current that will be forced back into the mains.
So the PWM cycle might be at 10 khz, but it's producing an on-off waveform with a period of 20ms, that when smoothed, will give you a pretty good sinewave.
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Grid tie inverter
