Inverters

[mikenash]

Hi Guys

I know folks on here love to share their experience, and I'm interested in improving my thinking on inverters . . .

Firstly let me say I have been re-reading Starfire's excellent post and subsequent responses.  See link below:

http://listerengine.com/smf/index.php?topic=7460.msg84365#msg84365

I'm a big KISS fan.  I used to enjoy having cars with points ignitions and carburettors cos I could see how everything worked and diagnose a fault in 5 mins flat - but I have gotten over that particular Ludditery and these days have a couple of $1000 2.2-litre Toyota Camrys as they are the philosophical equivalent and may well outlast me

I'd like to own an inverter like the Camry - tough, simple, good, over-engineered and under-stressed . . .

The gentleman who has been advising me on solar stuff - and whose advice I have found to be good - suggests that the modern combination of inverter & charge controllers are the "way to go"

He is recommending this unit:

https://www.victronenergy.com/inverter-charger-mppt/easysolar

But I could just as easily buy whatever I can find that is a super-robust charge-controller and do things like managing charging/voltage myself, and maybe run two or three small inverters

Typically my households use bugger-all current as I tend not to have anything with an electric element apart from an electric blanket and a small toaster.  My norm/current setup is: solar hot water, woodstove, wetback, gas hob, low-output microwave, spring-fed water so no pumps etc etc - a familiar picture I am sure

I am attracted to the Outback inverters

I have a couple of 230V gen heads as works-in-progress and hope to be in a situation where one or other of the two CSs can be either putting 24VDC back into a battery bank and/or generating a few kWs AC if I need to run the small welder or the angle grinder

I'd be interested on thoughts on inverters and on modern complex/sophisticated equipment like the Victron units vs something heavy and simple with a big copper transformer and a steel frame and two big handles kind-of-setup

I have a couple of quiet, (one genuine Honda, one cheap Chinese) 2.4 kW generators with 15-litre tanks that can idle inaudibly all night if need be, plus a big 5kVA genset that can run the fast-acting 3.6kW 60-litre hot-water cylinder as a convenience if the sun isn't shining, the fire hasn't been going and herself wants a shower NOW . . . so no real need for a big-current inverter.  If I had just one unit (instead of a few small ones) it'd be a 1500/2000W job, I would say

I don't reckon I give a rat's about pure sine either.  Everything I own seems to run OK on a dirty square wave with a few corners clipped off and, in my luddite-ish way I figure cos the laptop runs off of a 19.5V (DC I assume) charger thingie, it probably doesn't care if the VAC going into it is a bit dirty?  Since I am blessed to not own a TV I don't care about that either

I might put a search on TradeMe (think ebay/gumtree) for small inverters and get an idea of what sells for what $$

Enough from me - this is turning into a Glort-length rant

I'd appreciate any thoughts

Cheers, Mike

[BruceM]

Perhaps you've mentioned it before Mike, but I don't remember the context for this proposed inverter- an off grid home? Backup power?  Grid tie?

"I tend not to have anything with an electric element apart from an electric blanket and a small toaster.  My norm/current setup is: solar hot water, woodstove, wetback, gas hob, low-output microwave"

You didn't mention refrigerators or freezers.  They are a substantial load and night time drain on batteries.

Peak load is a big design issue too. If you add up the watts for a toaster and microwave on top of refrigerator you could be getting into some serious power, inverter-wise.  Best to look carefully at your peak loads and think about how important something like a microwave (1000 watt plus, 2.4 GHz radiation device with leaky shielding) really is.   

The biggest issue of all for inverters isn't the inverter at all, but the batteries and their ongoing replacement cost.  Since you are no stranger to Lister engines and generators, that's a huge leg up.  IF you are willing to actively manage your depth of discharge by shifting loads and running the generator you can save yourself a bundle on batteries. 

I don't know of a single Outback inverter user (out of a dozen I'm aware of) who has had any problem. Their good reputation seems well deserved.  I don't think badly of Magnum's products of the last decade either.  All the current 48V systems (including Outback) are short cutting good engineering and  ignore good series string battery management and rely on tough batteries and lots of equalization charging- which means regular watering and shortened battery life.
 

If you are going to run your home on inverter power continuously, not just for backup, you should add some decent commercial level passive filtration for both AC and DC sides and think carefully about where you put the inverter, batteries and PV  and wiring.  Your home wiring is totally unshielded.  Get a cheap AM radio like the Sony ICF-S10MK2 or older Radio Shack 12-465and scan the AM band within a 4 feet of the wiring in a home with inverter power.  Likely you will find the entire AM band swamped with EMI from the inverter everywhere in the home.  Then think about if you knowingly would like to do a personal test to see if how your family's health will be affected by such an (easily avoided) high EMI environment. 


 

[mikenash]

Hi Bruce

I should have explained myself better, sorry

I have an off-grid property some 400Ks from where I live now, which is a “retire in the sun” project for me (if there is such a thing as retirement for our generation)

It’s a few acres of rural peace-and-quiet in the province that gets the most sunshine hours nationally – so “off grid” is a no-brainer

I spend weekends and the odd week or two up there working away on developing it, so when I am there I’m “camping” with a chilly-bin full of ice instead of a fridge and a few other compromises like that

I don’t want to buy expensive deep-cycle batteries until I’m actually living up there – they won’t enjoy sitting around – so for the moment I am just “shopping” for parts and thinking about what I need.  Also, battery tech and pricing keeps changing – that “big breakthrough” in battery tech has been just-around-the-corner for twenty years now . . .

But I probably have a sort-of-adequate handle on kW/h load calculations, on the idea of how many days worth of electricity usage my battery bank has to represent, on the relationship between depth-of-discharge and the number of cycles a battery will survive, on the idea of just running one major load item at a time, and on the judicious use of a generator – short-term – in load situations

When I’m there on my own, I’m quite capable of having a “day” when all the load stuff gets done and the genset just runs hard for a few hours – washing machine, big hot-water heater etc

EMFs?  I just last summer poured a concrete pad and set the RSJ base for the #2 Lister into it.  I’ll build a shed on it sooner or later.  It’s about 13 metres from my shed/house/workshop and I hope it will eventually house a battery bank, inverter etc – that’ll keep some of the “nasties” at arm’s length

(And I’m no big fan of the microwave, either.  It gets run for five minutes at a time any evening when Herself is there as she uses it to heat up wheat packs (heat for muscular pain-relief stuff).  Apart from that it’s just something on a high kitchen shelf for dust-bunnies to form on top of

So, last night I was thinking about inverters (I had put in a bid on a big Victron one on TradeMe but missed out – too lean) and I thought that, instead of re-inventing the wheel, I would tap into some of the experience and expertise here

So that’s the background  

Cheers, Mike

[BruceM]

A minimalist PV/inverter system; a throw away for temporary use ala Starfire, might make sense if you are there fairly regularly and if the convenience of some lights, etc, is worth it over a small inverter/generator.  An AGM battery that is mostly on float will last 8 years. It doesn't need water, which is why I suggested it if it might be a few months between visits.  My home system has 12v power at my shop, also distributed for for very low power use in the house.  That uses a 110AH 12V AGM battery.  It is also used for my solar hot water pump and in floor heat pumps.  It's charged by a single 150W 12V panel on the shop roof, oriented for winter, and uses a linear charge regulator of my own design.  When I camped in the shop while building the house, I used 12V for minimalist lighting. 

Because of the potential for a true battery breakthrough- the one we've been waiting for nearly 90 years for, stalling on getting the big inverter/charger unit is wise.  Gods only know what battery management might be needed for a future battery.  The current inverters are designed for charging a nominal 48V battery bank but do no battery management to speak of.  Not a good setup for Lithium at all. 

For the inverter power issue-  distance removes the direct emissions of the inverter but does not much at all for the conducted emissions- that which is on the wires and brought into the home and wrapped around you. Adding a decent commercial filter at your remote shed, with the filter in a metal junction box and it's output run in metal conduit for a distance away from the inverter/generator shed is what you'd like. The PV wiring and panels will likewise radiate inverter EMI unless filtered.  If the panels are distant from the house, not a problem but if the panels are on the roof of the house it must be filtered.

[ajaffa1]

Hi Guys for what it`s worth, I have a grid tied inverter with solar array and etc. It`s only been running for about a year and has produced about 7 megawatt hours of electricity. It provides all our requirements during the day and we even export about 60% to the grid. I have looked at the price of batteries with regards to getting off grid completely but it is cheaper to pay the electric bill for the next twenty years than to buy the batteries.

I have a cunning and completely illegal plan. I have just purchased a 5KVA ST generator head, I plan to drive that off my Lister CS 6/1 running on WVO once the sun goes down. I will rectify the 240v AC to approximately 380 volt DC and use it to feed the same grid tied inverter I already have, I will need some serious capacitance to smooth the output. This will provide all the electricity we need during the evenings and any excess will be sold to the grid.

If the energy company ever question why my energy usage has dropped to nearly nothing I`ll just tell them that I now work away during the week and am only home at the weekend.

Bob

[mikenash]

G'day Bob.

Is that an ST or a Chinese ST clone?  I ask because I have one of the Chinese ones sitting here I bought a couple years ago waiting for my #1 CS to go north and get settled in and do some work

I'd be very interested in your experience with it as time goes by'

Thanks, Mike

[ajaffa1]

Hi Mike, yes it`s a China knock off and non compliant with Australian regs. That said if you put some decent bearings in it, give the slip rings a quick skim on the lathe and soak the windings in a decent electrical varnish It should be good for at least ten years. All up it`ll cost me less than $200, I`ll pick it up at the weekend and post some pictures.

Bob

[BruceM]

Ajaffa1-  Big capacitors are only the secondary solution.  It is common for small linear DC supplies to rectify the AC with a bridge rectifier, then feed it to an electrolytic capacitor.   For serious power like you're talking about, that's a serious no-no.  The power factor is a horror as when the rectified voltage raises above the big capacitor voltage, it's like hitting a brick wall; BIG current spike.  Instead, you need to rely on inductance for the first stage of filtering that DC.  A big fat choke- the kind you don't see very often these days.  If you use a choke with a value above what is called the "critical inductance" you will have a near perfect PF, and the output DC voltage is the same as the input AC RMS voltage.  The capacitors don't heat up since the ripple current they see is now smoothed by the inductor.

What I like best for this application now is a DIY gapped toroidal transformer core.  For my new inverter, for the DC side filter I tried making one made from a 450 watt transformer core, cut (with a hack saw), gap filled with epoxy, and hand wound with 130 turns (77 feet) of 12 awg magnet wire. It tests at a whopping 14 Mh.  The performance under a 10 amp load matches the test value; it's a long way from saturation, still.

I also found a surplus Hammond E-I core rated 5 millihenry (mH) and 20 amps. I use that for the second stage,
I'm also using a total of 30,000 uF of electrolytic capacitors. 

I agree with your assessment of ongoing battery cost.  It can be a financial catastrophe as commonly implemented.  My strategy for my very sunny climate was to militantly design out night time electrical loads. This allowed for a much, much smaller battery and gives me great battery life.  My ongoing battery replacement cost has been $18.50 /month.  ($1000 per set of 10 batteries, every 4.5 years).  My operational strategy is to top off the batteries EVERY day.  Since PV is cheap and my battery is small, and my night time use is low, in the past this meant running the Listeroid for an hour in dark winter mornings, about 2-3 times a year.  I only did the bulk charge via Lister CS, then let the PV finish the job. Now that I've added more PV, I doubt that I will ever need that.

It's a brave new world; shifting your big loads to daytime sun power is the game.  Daytime power is a bargain.

[BruceM]

Chokes are measured in Henry's of inductance, usually miliHenries or micro-Henries, and rated current in amps.

For filtering 60Hz power in the 10 amp range fairly large values are needed,  a few mH or more.

Your GIT's do use moderate sized DC chokes for smoothing the 10KHz PWM'd waveform into the sine.  But they would be way to small (micro Henry) to use for filtering 60Hz.

Inductors are the workhorses of all power conversion.  They store electrical energy as magnetism in the core, and then convert it back to electrical energy.  All voltage conversion and smoothing functions depend on them. Transformers are just another type of inductor.

The design of power inductors to do their work efficiently is a very complex specialty field within electrical engineering.  It's some hairy shit!  Fortunately there are programs to do standard designs for mere mortals, using different types of materials in the cores, and often giving you a part number for the core, size of wire, length of wire, number of turns.   Power inductors are often made with powdered iron composite in a toroidal or doughnut shape.  Part of the design process is to select the material, and size, and number of wire turns, to avoid exceeding the flux carrying capacity of the magnetic core material. That is called saturation.  Like any other complex field, there's lots of specialty lingo and terminology. 

There are also lots of different types inductors used for high frequency filtering-  common mode chokes are one frequently used form. They typically have ferrite cores, and because of the way they are wound can handle huge currents in a small package.  But they are only for filtering out high frequencies. 


 

[BruceM]

Your basic idea (rectify and filter via capacitors) was right except for adding a big inductor (choke) before the big cap.  Try for at least 2 mH and rated for the current you need or better.  They are rarely used these days in those big sizes so bargains can be found. Making one from a cut toroidal transformer seems harder than it is.  Honest.  You will find the large, higher voltage electrolytic capacitors are quite expensive, also.  So much so that you will want to do smaller caps and two inductors, aka a 2 stage filter.  This is an experimental hack - I have no idea what the input DC voltage ripple allowable is for unknown GTIs. 

I know nothing of the internals of typical DC welders. 

[BruceM]

I forgot to mention- a 3 phase generator head would be a much better source for DC to be fed a GTI.  A much smaller choke for filtering, much less capacitors.  Set it up so the filtered output voltage is directly OK for the GTI.

[ajaffa1]

Very interesting stuff gentlemen, a bit worried about rectified three phase as it would give nearly 600 volt DC and most GTI are only rated to a maximum 600 VDC input. I will start looking for a spare GTI and some toroidal cores, I`ll talk to the solar installer who is coming to fix my hot water timer. I bet he`s got a yard full of defunct inverters and etc.

Bob

[BruceM]

I'm sure you can find a 3 phase head with lower voltages. There certainly are here in the US. There are various wiring configurations - Wye would keep each phase within the same voltage range relative to the neutral.  That's what you'd want.  You don't want to do three phase step down transformers if you can help it!