Letter Re: Inverters for Disaster Preparedness

There have been several recent articles on UPS systems and inverters. Most of the current inverters use MOSFETs or similar sensitive solid state switching devices. These are the hardest stressed components of the inverter. Spares should be a consideration. In addition many modern inverters use custom chips that may not even be available now, so not at all after TEOTWAWKI. Consider this, inverters have been around for quite along time. The best design that I have worked with and designed circuitry for is the McMurray-Bedford inverter. It is a simple design that originally used thyratron tubes and later, SCRs. This inverter can be built very simple, yes you will need to hand wind the commutation coils which are air core and you will need some oil capacitors, wind or find a suitable output transformer, many options here, harmonic filters if you want the best sine wave output,
but not always required, depending what your loads require. The industry common DC buss voltages are 135 VDC, 270 VDC and 405 VDC. But you can design around most anything as long as you realize as the DC buss voltage goes down, the DC current will go up and so does the size of the power components. As far as the control electronics and oscillator are concerned you can build it with common op-amp technology, transistor technology or vacuum tubes if you want to. Since this is technology that has been around for quite some time much information can be found in older electronics and electrical engineering books and even on the Internet. If you want to prepare using this technology collect some large power SCRs and oil capacitors, look at some other people’s designs too. Some complete schematics of early Emerson Electric, through the mid 1980s are some very excellent and robust designs. That product started to suffer when microprocessors were phased in their designs. If you can get your hands on them–and I have seen them in surplus stores–any of the small 20 KVA or 40 KVA 120/208 VAC in and out inverters that use the 135V DC buss, you will have a nice UPS that will be rugged, sine wave output as they all had harmonic filtering. (Think 10 car or deep cycle batteries in series, or better yet larger single cell UPS batteries). That, along with a collection of spare SCRs, fuses and a few other components and you will be set. They are not the most efficient inverters around but they are very reliable. Hospitals and others have used them for years.

They can easily be made to run on single phase 240V AC input, built in battery charger, and you have the advantage of 3 phase output. Emerson at that time also made some very rugged variable frequency drives that run on single phase and have a 3 phase output that have standard TO-3 transistors as the output devices. Very rugged and reparable, I have fixed several I found surplus to run various machine tools that were 3 phase for people to have in their garages, i.e. Bridgeport mills, lathes etc. Once I fixed one
it never needed fixing again. Usually the surplus ones just have a blown transistor or two. I regret not obtaining some of the smaller UPS systems as they came to the surplus market a few years ago. Now that my eyes have been opened by JWR and his fantastic blog, if the price is right I may get one or later I will build a smaller, say 5-10 KVA one. I have always liked and designed things that are as simple and reliable as possible, the K.I.S.S. principle.

Around the US large amounts of electricity are transmitted by high voltage DC. The Sylmar inverter in Southern California is the closest high voltage VDC, (500,000 VDC) inverter to me connected to the Pacific Intertie originally used mercury arc rectifiers, as a kid you could look through the fence and see it and hear it. The new station now uses thyristors instead of tubes. I think Edison and Tesla are smiling and have become fast friends in Heaven today, as both AC and DC are equally important in power transmission.

I hope some day our family is blessed to move to the American Redoubt and I will gladly help anybody that is interested build and maintain rugged and reliable survival electronics. I would love to teach others especially children what I can. Sparking an interest in a child’s mind makes me so happy it’s exciting for me too. I know because that’s what started me out in electronics.

Thank you James for all you do and God Bless all. – Jimmy in California

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