Guru
Joined: 26/07/2007
Location: AustraliaPosts: 1686
| Posted: 12:20pm 07 Mar 2017 |
Nice to see some empirical testing on this.... not sure of the real relevance of the uh in this case.
Why?
It depends on what you think the inductor is actually doing, and at this stage the jury is out as far as I know.... someone must know, but it is not me.
I think now that it has more to do with the rise time of the current into the transformer. So it is probably the first 200ns or so that is important in this instance... we need to get the thing driven, but the primary looks like a severe short circuit to a DC voltage.... 50mmsq wire is pretty hard on a 50-60v battery through a sub R load.
As we are switching at 20khz, should we worry about that..... no, I calculate he turns for 50hz, so that can't be it.
So I am left to look at the fast rise time of the DC, which gets faster the less resistance is in the transformer..... ie I am shorting the battery into the primary, the rise time will be a factor of the impedance... which is darn small.
Working with electric fence units, I had the same trouble, and needed many triacs to cover the dumping of 60uf@ 700v into a short circuit primary of only 18t of 1.8mm (x2)wire.... here we were talking hundreds of thousands of watts, and a small coil of 30 turns on air or ferrite to take the shock out of it, and allows for the same performance with only a single triac.
So it looks like the fast rise time of the input current is what we are trying to curtail....
In the fencer example, the air coil can be replaced with a smaller ferrite ring, and the same results are gained... so I also know that the saturation is immaterial to the outcome, and probably helps to keep the impedance down once the current really flows... ie the air coil was no different in the end to the ferrite torroid.
So why did I put the inductor in there.... to keep the idle current down.... nothing else.
I think it keeps it down because in the first few hundred nano seconds, the inductor actually works on the fast rising wave front of the DC into the transformer primary, as though it feels like a multi megahertz wave front ( 100mhz?).... it is not?.
It is the leading edge of the rise of the pwm wave of the 20khz, and may be only <.1% of the 20khz time window... so it may be say 1000 x 20khz or less.. or more..... but whatever it is, it is very very fast, and it is related to the transformer primary impedance.. ( as was the time width of the wave from the fencers.... high impedance = wide waveform)
So if we can garner a few uh for a few nano seconds, it 's job is done. After that we don't need to care, and in the real world this seems to be the case.
The other boards you refer to 40uh if you look at the picture of the one they recommend, it is a ferrite ring... it would saturate in a heart beat, so they also seem to agree that it is useful for only a small part of the pwm wave.
It has been tested , and found that more uh is not necessarily better, and Andrew on another forum who also wound inductors on the original W7 on energy matter site, had a inductor he calculated on the 20khz..... and it was much worse than the 3.5 turns on the ecore.
Like Tinker, I like the ferrite ring, as it is very quiet, and I am prepared to try the aerosharp if we can get the wire into it easily.
But I am still not sure we need to do much better with the ferrite inductor, as I still don't really have a good idea what I was doing with it in the first place.
I do really like the research you have put into it though, and it explains to me better what I may think I have seen to date.
And I whole heartedly agree with you that the inductors I use in particular are not quite right.. or maybe even remotely right, but they seem to work fine for what they are purposed for.
If we can find a better alternative through your efforts, it will be a big bonus.
I do know it effects the wave form of the sine wave sometimes.... we need to know a lot more about this, and I am hoping you can do it too. The ringing does not look good
........oztules
Village idiot...or... just another hack out of his depth