Cast iron inverter

I write this after getting a free broken UPS from a datacenter and spending the weekend trying to fix it, blowing many fuses and tripping many breakers in the process, ultimately giving up.
All the iron, copper, and silicon basically good for nothing, no schematic, no factory to refurbish it, its now “obsolete” model replaced by probably the same thing with a new part number.

In contrast to PV panels which have achieved kick-ass no moving parts ultra reliability, decades long life in the field, inverters seem to suck and fail, most have a 1 year warranty and the most i’ve seen is 3. None seem to have open schematics or a maintenance routine.

Does my dream of finding the cast iron skillet of inverters exist? Or am I stuck with cheesy bullpoop with capacitors that blow out after a few years and all the valuable components are encased in epoxy and not meant to be replaced. Maybe cast iron skillet reliability is impossible, but the world is full of well designed electrical things from the 70s and 80s that are still humming away.

I don’t care about small, I don’t care about 99% efficiency, i don’t mind doing maintence, or even replacing parts which inherently have to be wear and tear but maybe design for them to be easily replaceable and PLEASE release a schematic please. Bonus points if theres some flexibility for accomodating different package sizes.

Are there any inverters designed for reliability and repairability?

Nothing I know of. Though after 4 years of beating on it, the Aims Power inverter in my office is holding up quite well.

You might look around the Fieldlines forum - there are some people doing their own inverter design/repair there, and they’d be my first step in looking for repairable inverters.

Inversion is hard on capacitors. That’s why inverters have lower lifespans overall than most other power electronics. They also have high dV/dt rates which can be hard on other components and can cause physical fatigue through rapid thermal cycling as well as magnetic physical force at high frequencies. As a result, they are basically pretty hard on the electronics all around. The reason a lot of them are potted is actually to extend their life by keeping moisture and other contaminants that may interact with those fields, and to physically stabilize some of the components and their solder joints that might be most prone to vibration.

Most power electronics in the low voltage range (that is, < 1kV or thereabouts, so anything you’d deal with on a home or light industrial basis) use either standard power MOSFET or IGBT transistors and lowish switching frequencies (from the low 100’s of Hz up to a max of about 15kHz or thereabouts, 1-5kHz is typical). These are square waves so can have not-insignificant harmonic content well past 21x the switching frequency.

If you’re concerned about maximum reliability, you’d likely find that more in the industrial side of things (vendors like Siemens/ABB/Schneider/Eaton/etc…) than in the consumer/homeowner ranges offered, but you’ll pay commensurately more as well. Industries value uptime and are willing to pay significant premiums to minimize downtime losses. DIY is of course an option, but while the basic principles of operation of these devices are fairly straightforward, the safety mechanisms and failsafe designs that the industrial stuff has (and the designs which need to pass UL listings - which, if you want your home insurance to be valid, might even extend to DIY, I’m not sure) are not so trivial to guarantee, especially under fault conditions - which is when they’re most important. My personal view is that you shouldn’t try to DIY something that deals with mains power and high frequency switching unless you both know what you’re doing from a power electronics standpoint AND from a safety and EMI compliance standpoint (causing interference at those frequencies or their higher harmonics can put you in violation of the FCC’s radio frequency regulations, by the way). If there are tried, true, proven and certified designs out there you can DIY, by all means try them out. But don’t just wing it.

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What you’re saying about capacitors makes sense. Definitely the 5kva TrippLite I took apart there was at least one bulging 450V 470uF capacitors. I appreciate that to operate at scale and make a profit, theres probably a lot of consideration that goes into component selection. Maybe known-to-fail-eventually electrolytic caps are the only reasonable way to go, i just wish the assembly would make them possible to replace, and the design would have some sort of passive protection to stop failure from cascading to other components, or maybe even have active protection that could sense them degrading. For me, I’m willing to pay higher component costs, longer assembly times, and end up with a larger/heavier end product. All of those would be terrible tradeoffs for a volume manufacturer I assume.

I do like the industrial option (besides the price of course, but watching ebay is always a possibility) in terms of proven reliability, designs by highly competent companies with lots of resources. My only complaint with this route is if it does break, there’s no schematic, no community of people to help troubleshoot, and my experience with those big manufacturers is they don’t exactly cater to the small quantity buyer. Also, I guess I wouldn’t miss the proprietary industrial software and protocol that requires a Windows XP VM and a $150 dongle to configure.

And Psynoyks link actually led me to the project of my dreams, at least so far it seems that way, even better than I would have imagined. I feel like this project totally gets me, SO MANY cool design elements I wouldn’t have thought of. I don’t know if I should weep or immediately buy 100 copies of this guy’s book.
I love that its designed around scavenging parts, I have no problem digging through trash, actually sometimes I have a problem throwing “so many good parts” into the trash (like this free UPS). I’m definitely not competent to DIY with “D = design”, but maybe with “D = do the assembly,testing, routine inspection and maintenence”

I’m sorry, there’s really no P there. :stuck_out_tongue:

However, I do agree that DIY and mains power is a bad idea if you don’t know what you’re doing in rather great detail already. My observation on the little bit of solar stuff I’ve seen on YouTube is that about 90% of it is fine, 10% is hazardous. The problem is, unless you already know the stuff well and often have some sort of EE background, it’s impossible to tell which 10% is hazardous and will burn your house down. On the other hand, if you delve into lithium pack stuff on YouTube, it’s flipped and 90% is terrible advice.

One thing that I would suggest looking into with inverters, and one of the reasons I’m using an Aims inverter, is that they’re fairly low power to weight ratios. The things are heavy. And that tends, in a lot of people’s experience, to be generally correlated with longevity in inverters. Thermal mass, bulky components, etc. They’re not light, but they are strong.

I might pull the 6kVA unit I’m ordering apart at some point, since I won’t be daily driving it. I would like to see how they’re built.

Sorry about that butchering of your name, I try to be a little more careful wiring than i do typing or at least drink less caffeine beforehand… hopefully.

I have an EE degree but have kind of gone down the jack of all master of none career path, i’m really at a point where i’m learning how much I still don’t know more than anything. I would approach this as long term tinkering hobby that might one day be good enough for real use, probably after repeated testing of conservatively implemented overload and overheating protection. And I’m okay with a big heavy humming thing in an even bigger paranoid fireproof enclosure. I think I’ll read through the OzInverters book if its really over my head maybe just keep my eyes open for something low power density like the AIMS

Well electrolytic capacitors are sometimes the ONLY choice, especially for power electronics, as they have significantly greater charge storage per volume than the other major capacitor types. For power use, they’re not going away, and even in the highest quality gear there is often nothing that can provide that level of capacitance along with the other necessary performance criteria needed. So, don’t look down on a cap just because it’s an electrolytic. They’re necessary, useful, and - on their own - no mark of quality, good or bad or otherwise. There have been some well-known terrible runs of bad quality from certain manufacturers, especially with wet electrolytics, but that’s as much due to them (and solid state switching power electronics in general) being a relatively new invention with respect to many other long-term durable industrial goods. They’re also a very physical component, in that they’re an assembled composite of layered disparate chemical substrates that require precise assembly. They’re bound to be a wear item, almost inherently.

And, fortunately, they’re typically not that hard to replace either.

Do they often just fail and need replaced, without taking other parts of the circuit with them when they die?

They were hard for me to remove, there were 2 groups of 5, each group in 2 rows, totally packed together each capacitor glued to at least 2 neighbors. So it was hard to melt one lead at a time and rock it out lifting one side at a time, like id normally do on a 2-lead through-hole component. I probably didn’t have optimal tools or techniques, but it was hard to do without flexing or burning the board too much.

As mentioned i’m not a calibre of engineer to do competent designs myself from scratch but I try to read some smarty blogs/forums that i don’t fully understand in hopes of absorbing somthing through osmosis. In these I’ve seen electrolytic caps kind of disparaged, I’m not sure if thats relevant to average consumer designs at this point or if its just bragging along the lines of “on my high end government funded project we only install titanium screws with $500 torque wrenches and use caviar for heat sinks” type of thing.

But anyways, heres a few links i was able to find quickly that are where I got some of these ideas in my head:

Selecting capacitors is also somewhat of a black art. Electrolytic capacitors are mostly resistors at high frequencies, and a poor implementation of an electrolytic DC link capacitor can be worse than nothing (the energy stored helps blow up the devices in case of a failure). On the other hand, insufficient DC link capacitance leads to high ripple current in the DC link cables (which could be long, and therefore potential sources of EMI) and high voltage transients (switching spikes aside, the average DC link voltage + half the peak to peak ripple needs to remain under the voltage rating of the devices). For large high-voltage inverters, the cost and weight of the capacitors is often equal to that of the switching devices.
Isopack: Freefly Systems ARC200 teardown


Electrolytic caps are low cost which makes them attractive for first consideration. It will be interesting to see if, at the end of the evaluation, if they are still considered cheap(er) than other technology. Electrolytics have high ESR/ESL so the limiting factor in the design is how much heat is generated due to ripple current because the capacitor life is cut in half for ever 5degC rise and so the calculation tends towards how many caps in parallel are required to share ripple current rather than how much total uF cap value would be appropriate for desired ripple voltage. This is a significant statement… due to cost requirement, what is being said is that the DC link capacitor uF value cannot be chosen optimally for the application. So straight off we recognize not to have an optimal solution and thus we keep this in mind to look closely at what we must pay for in order to have benefit of low cost electrolytics… there are always trade-offs.
The other dielectric to consider for DC link cap in VSI application is Metalized Polypropelene Film due to its low ESR/ESL and self healing properties which makes is vastly superior. I’ll make the argument that it is CHEAPER solution too.

the analysis continues at Time for MORE POWER! 1400a 1200v IGBT controller build! - Endless Sphere

If you’re replacing them halfway often, I’d look pretty hard at socketing them, or running to some sort of larger capacitor bank you can mount remotely. Though if they’re killing caps that often, it’s likely just a bad design in the first place. Or good design, if they last to the end of warranty…

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I was going to suggest moving the most fragile components out to a daughter board of some kind to make replacement easier, but I’m not sure about the implications of non-soldered connections and longer traces…

The OzInverter book + PCBs arrived, I’ve skimmed it and now am reading through more thoroughly, and reading some of the big forum threads about it too.
It’s incredible the amount of thought and design that went in to making it simple and repairable. And its gone through a lot of iterations to make it as generic as possible, no single source suppliers, there is one special IC, EG8010 that is surface mount and on a socketed daughterboard, everything else is throughhole. And there’s already a hobbyist who was able to replace the one IC with a more generic 8 bit AVR microcontroller.

The book isn’t quite a textbook on theory but its very thorough as a build guide. The forseeable challenge is sourcing the transformer core. All of the leads in the book are for suppliers in Europe. So its either finding a suppliers for the US or finding some old broken equipment with at least the same size toroidal transformer core. Unfortunately the dead Tripplite UPS must have used a totally different inverter topology. It had numerous (6 of one size, 4 of another) much smaller toroidal transformers, not one big one like OzInverter uses…

Are you sure the toroids in the Tripp-Lite are transformers and not just inductors? Smallish ones like that are very commonly used for boost converters/inverters and that would be a very ordinary component in a battery-backed UPS.

I’m not sure, but there are 4 leads off most of them, and measured continuity between 2 leads of each pair. It looks like a primary and secondary winding. Although both sets of windings are about the same gauge wire. IMG_071


Learned something new with that one.

WIsh my ability to reverse engineer power electronics by looking at a PCB was better. The other thing about the Tripplite UPS that might suggest it using one topology or the other - its designed around 240VDC lead acid battery modules.
So it doesn’t need as much voltage step up as a 12/24/48 volt inverters . Also it’s usual operation would be just maintaining a battery charge, while AC->DC->AC rectifying/inverting to clean up incoming power and only use the battery during a power outage.
Also, the author of the OzInverter manual apparently had a Tripplite UPS previously, and it consumes about 150w in standby. So even if it wasn’t dead, its not necessarily great equipment if you’re concerned about efficiency.

I don’t know if any of these details would imply one topology or another.

What?! While I’m not outright saying you’re wrong here… that’s a bit of a shock. Most household UPS series are designed around ~6V security system batteries or at most 12V ordinary sealed-lead-acid types. I’m sure there are SOME massive rack-sized datacentre UPS setups that use serious battery voltages, but a home unit is definitely NOT one of those.

150W of standby use doesn’t add up either. I have two off-brand and one APC-brand UPS and even in active use they don’t get past “maybe it’s slightly warmer than ambient, but I really can’t tell”… you’d NOTICE 150W of power being dissipated as heat.

I’m not sure what sort of UPS this guy is talking about but it’s clearly not a home-sized typical desk or small server UPS rig.

Edit: Mind posting up the battery info card from the battery that was in your unit, or the model number?

I was very surprised too, I said I’d take the whole thing even though its dead since I figured it would at least have a large 24V or 48V battery that could be used with something else.
I belive its designed for a server rack so each module is in those standard data center rack mount dimensions, rated for 5kva .

For the battery i have 2x of bp240v10rt3u and 1x of BP240V5RT2U
The UPS is SU5000RT3UPM and also have a step down transformer SU5000XFMRT2U.

It’s a lot of stuff, pretty much needs 2 people to lift the bigger battery modules. I did check with multimeter and the batteries do have ~150 volts +/- 20v each, the thing has been dead for a while so I’m not sure if the batteries were at the end of their life too and/or have just permanently damaged due to discharging too low.

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My impression was that the UPS in question was a datacenter unit, and they do use really high voltage strings - 120V or 172V or something. And, for a fully online UPS, double converting (which a lot of those do), 150W idle isn’t insane either for a 5-10kVA unit.