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Murphy's friend

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Joined: 04/10/2019
Location: Australia
Posts: 651

Posted: 09:11am 10 Jan 2024

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Hi Roger, you were reading correctly between the lines.
I was using my warpverter for some time to power my house, it ran OK but was noisy when backfeeding.
Then, after I had re built my 6KW nano inverter and found it did the same job quietly the warpverter was retired as a spare.
A long time later I turned the warpverter on again to see if it ran and it came on very noisy with a horrible AC waveform. While fetching the clamp meter to check current it made a 'pop' sound and died.
So I took it apart again and that is where it's now - in pieces.

By 'kick' I mean electrical, destroying mosfets. There is no soft start with these inverters so I fitted a choke.
Later I fitted a relay that first powered the electronics & drivers and only then applied power to the caps (via a resistor) so the inverter started with the raising cap voltage. Once they were fully charged that resistor was shorted out by a high current switch.

This step is VERY important, I strongly suggest you include it, even before fitting a choke should it also be required.
I did post a circuit for that soft start if I remember correctly.

rogerdw
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Joined: 22/10/2019
Location: Australia
Posts: 853

Posted: 01:21pm 10 Jan 2024

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Thanks for clarifying that Klaus.

I'm not sure if it was you or Tony that provided the guidance for me, but I am doing something similar ... just switching power to the electronics first ...

... then precharging the caps via a large wattage resistor ... and then switching in a large DC Circuit Breaker to apply full DC to the caps.

It's nice to see the AC output slowly ramping up as the caps charge and then pop up the last few volts as the switch kicks in.

Cheers, Roger

nickskethisniks
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Joined: 17/10/2017
Location: Belgium
Posts: 458

Posted: 09:20pm 11 Jan 2024

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Hi, if I remember correctly the theory was that there is a "problem" with the capacitance those big transformers have.

So di/dt is very high causing high peak currents wich make the mosfets blow up eventually. If current rises to fast and the current is not evenly shared between the parallel mosfets this can cause a cascade failure of mosfets.

RenewableMark experimented (or wanted) with a different winding approach, I think by fitting more insulation material between the metal core and the windings. I don't know if he found differences or finished his warpverter.

Warpspeed uses a 100V primary wich has different characteristics then a 48V transformer. Actually we are all making tranformers with different parameters anyway.
Some use EI material others use the toroidal cores wich can have a high impact.

Another theory could be that he uses (robust?) slow switching IGBT's so therefore di/dt is slower and the maximum peak current is far lower.
Since the semiconductors switching frequency is a lot lower, you can slow down the gatedrive by increasing the gate resistance in the mosfetdrive, no need for crazy fast speeds.

Or maybe some kind of flux walking causing the transformer to saturate? Who knows?

By adding a small series inductor you can reduce the di/dt, that's basically what we are doing in our swpwm inverters anyway, it will help to reduce the current during the turn on event.
Edited 2024-01-12 07:25 by nickskethisniks

rogerdw
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Joined: 22/10/2019
Location: Australia
Posts: 853

Posted: 01:50pm 13 Jan 2024

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Thanks for your comments Nick.

Yes there was a worry about stray capacitance in the secondary winding down to the toroid core (ground). Whenever the bridge switches, the secondary voltage creates an almost instantaneous 225 step change in voltage.

Any capacitance down to ground needs to charge or discharge in essentially zero time, which can create a huge spike in the primary.

It was particularly bad for Andrew because he was using 24 volts and he was getting 80 amp spikes in his primary. Because of the turns ratios and the fact that Tony was using 96 volts, plus differences between E-I transformers and toroids, he was seeing only tiny spikes ... perhaps 4 or 5 amps.

Mark had already wound his transformer so he wound another, but did 2 things different ... put a 5 or 6mm spacer around the core first ... then wound the primary first with multiple strands ... and put the secondary windings on top ... all to put more space between the secondary windings and the core.

As far as I know Mark has not completed his Warpverter and so we don't know if those steps were effective. He was going to do a comparison and swap between the two transformers.

In fact I held off for a long time before I wound my large toroid because I was keen to learn the outcome of that test. I also put a 6mm spacer all around the core for my large toroid ... but because I was running out of room, didn't fit an additional spacer between secondary and primary.

Now that I've run mine, I find the same effect on my output waveform. Lots of sprinkly vertical spikes dancing around randomly on the ac waveform.

Today I added an EMI filter off a 3kW Aerosharp Inverter to the output of mine ... and that cleared up all of those lines. Of course because it is only a 10 or 15 amp filter, I would need to find one big enough to handle the 60 amps this thing can put out. Now that I've written this, I realise that I need to go look at the mosfet drive waveforms and see if there are any changes there at all.

Regarding transformer flux, there was some concern that if the inverter was turned off abruptly, there may have been some remnant flux ... and then there was potential for dramas if it happened to be turned back on at the wrong part of the phase.

Tony's suggestion was for the DC supply to the caps to be turned off and as the thing winds down ... as the caps discharge, the core is effectively demagnetised, preventing remnant flux.

Cheers, Roger

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