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analog8484 Regular Member Joined: 11/11/2021 Location: United StatesPosts: 79
Posted: 04:30pm 31 Aug 2023
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It appears that static transformer resonance is no longer so important. So, it looks like the cap size can just be selected as part of a typical LC filter design and maybe with some consideration for inverter idle power consumption.
KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1187
Posted: 10:58pm 31 Aug 2023
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Yes, that's what it's looking like, and why it was there in the first place.
It's also appears that with respect to Idle current, the Cap value influence is dependent on the value and type of choke material, two chokes with the same inductance but different core material, will behave differently in the inverter as each will present a different value of inductance to the Inverter under varying load conditions. So in my case the cap makes almost no difference in idle current.
I spent a lot of time testing almost every previous choke design in an effort to come up with a choke that gave the best results under very high peak DC input currents up to 500A and above in some cases, I found that I only had to sacrifice a small amount of idle current and SPWM filtering to accomplish this.
I do notice that using Wiseguys Power Board design and Controller, the Idle current hardly increase even with half the choke inductance. I put that down to his design having very fast FET switching and his SPWM Symmetry drive, both reduce FET heating/current and it appears this inverter is less dependent on high choke inductance for low idle power.
Under some high current startup loads, the higher value of secondary cap appears to reduces harmonic distortion and is reflected in the inverter DC Input current sensor waveforms, at least that's what it looks like to me. There may be another explanation?It's all too hard. Mike.
wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 887
Posted: 11:00pm 31 Aug 2023
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There is a downside of making the capacitor too large and that is circulating current which is not the same as idle current. The capacitor should be selected to be the minimum size required for an acceptable level of distortion. For instance if the street power is usually 4% distortion ( and we are happy to connect to it (except for cost and maybe reliability)) why try to make the inverter achieve 0.1% distortion as there is a downside to pay for this.
The circulating current is easily seen on the 50Hz mains power side, if you put an AC clamp meter on one lead of a 10uF AC filter capacitor, you will see 1 - 1.5A of AC current which is the circulating current. Considering it is at ~ 240V and that is well over 200W of power, but hang on the DC input idling power is only ~ 20W, what gives ?
During the rise to the peaks of the 50Hz waveform current flows into the AC capacitor but after the peaks at 90 & 270 degrees the energy we just filled the AC capacitor with is "pulled out" of the AC capacitor & returned to the inverter input DC filter capacitors resulting in the low idling input power - thanks to FET switching magic. If we see 1.5A of 240VAC circulating current this energy had to come from somewhere and it came from the input DC power initially and due to say a 1:8 turns ratio the primary current will be 8 times the secondary side or 12A for the example just given (at 50Hz).
So the FETs are already switching an RMS AC current of 12A @ 48V (note the HF switching current is greater then this) with only 20W of input power which is equivalent stress on the FETs of running a 600W load but with no load connected as yet. This is why we should be happy to accept a distortion figure that is maybe equivalent to or marginally better than the mains to be kind to our FETs.
I know Mike (KeepIS) will be saying that my FETs run cold no matter what size cap I have used and that is also a fair comment but 4 x HY5608 are not what a lot of others are using so I thought a comment on capacitor size was timely, bigger is not always better.
Lastly I am fairly confident very few on the Forum here have access to a mains analyser that can show the distortion and harmonic content of the AC waveform so maybe Mike (if he has the time and inclination) can fill in a few gaps on examples of say 1uF 2uF 4u7 & 10uF caps on the inverters output with and without other filtering, and log the resulting real measured distortion for the record. Edited 2023-09-01 09:15 by wiseguyIf at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike
KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1187
Posted: 11:14pm 31 Aug 2023
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Thanks Mike, just replied and then saw your post.
Yes, I was happy when the Inverter THD was no more than the Mains THD for the same good and bad loads. Just some extra spikes under bad loads between the 15th and 25th order harmonics from the inverter, I guess due to the nature of the AC generation scheme and the sink capacity of the Mains.
I was wary about going to 12uF across the transformer but I wanted to be sure that the low resonance values produced by caps between 5uf and 12uf, each running for a few hours in the inverter, would caused no resonance problems - they did not.It's all too hard. Mike.
wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 887
Posted: 11:16pm 31 Aug 2023
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I just edited my previous post with a final paragraph..... please don't yell at me....If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike
KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1187
Posted: 11:45pm 31 Aug 2023
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No worries, I'll try and remember do that, surprisingly the THD did not show any real improvements with various filter configurations or values of inductance, capacitance and core material used. I really was not expecting that, sure there were slight AC waveform differences but THD - Only changed with Loads just like the Mains?
Inverter THD with 100W of resistive load is around 0.9% to 1.2% and the Capacitor value I selected is 4.5uF as it gave a slight visual improvement AC waveform over 3.3uf, and the inverter gives a better waveform on DC input current on some loads.
BTW I had the inverter running at 4.5kW for over an hour yesterday, once again the FETs and heat sink are just room temperature, and there is no fan cooling of the heatsink. The heatsink thermostats [on the heat transfer bars) are redundant, although still good for measuring room temperature It's all too hard. Mike.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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