Author Topic: Power Inverters  (Read 9041 times)

starfire

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Power Inverters
« on: June 01, 2015, 06:25:54 AM »
Inverters and what to look for:.... this could be getting too far off topic, I guess someone will tell me if so....

All inverters work the same. The input DC battery voltage, 12/24 volts is converted to high frequency AC, stepped up to 300 volts,rectified back to DC, then chopped into a 50 or 60 cycle  240 volt regulated AC waveform to power mains appliances.
 The important features of any inverter is the following, in order of importance.

Overload protection. The better this is. the more reliable the inverter will be. This ability to adequately protect itself is severely lacking in cheap inverters. The easy answer here is to under rate the inverter to guarantee it never approaches over half of its claimed  rated output. Pretty much all cheaper domestic type inverters are grossly misrepresented in what current they can realistically supply. Simply put, the manufacturers lie. A 1000 watt inverter can be safely rated at 500 watts continuous. Ignore any grandious claims of surge capabilty, this is largely BS. See here for a scary rundown of a really bad design.
http://ludens.cl/Electron/chinverter/chinverter.html

Undervoltage switchoff.  Generally all cheap inverters will automatically shut down when input battery voltage drops below a preset level.. This is important to prevent the inverter drawing excess current in an attempt to supply the load as the battery voltage drops. Check this before buying.

Waveform. All cheap inverters use whats called a modified sine wave. This should really be called a modified square wave. These cause severe radio and TV interference, and excess heating of some electric motors. These are fine for lighting circuits, and the use of small power tools, drills, grinders etc. For refrigeration, pumps, washing machines and any induction type electric motor that will run for longer than around 15 minutes, use a more expensive true sinewave inverter just for these  appliances. Pretty much all electronics are tolerant of modified sine inverters, as they  use internal  switch mode power supplies that work in an identical fashion. If radio interference is a problem,  additional  filtering can be attached on the outputs of each noisy inverter.

Stand by current. To have instant power on demand, all inverters must be powered continuosly, and therfore each will put  a constant power drain on the battery bank, and this quickly adds up. Many cheap inverters will not mention this drain in the spec sheet. Avoid these. A good figure to look for is .3 to .5 amps standby. This could also be written as 300 to 500 milliamp or mA. The lower this figure is, the better the inverter is likely to be. Some inverters will "pulse" the line every second to see if a load has been switched on, and if so, will "wake up" and run  until the load is disconnected again before reverting to sleep mode. These types have  very low standby current, but likely more expensive.

Many other protection circuits may or may not be incorporated, such as over temperature, overvoltage etc. These are not so important if the inverter is used well below maximum ratings. Manufacturers that try to make a cheaper product will leave everything out that they can that  does not directly or visibly  affect the normal operation of the device. Even many name brand inverters contain generic Chinese internals that are unlikely to be superior except in price.

Reverse polarity protection.  Ignore this one. Not important at all. If you need this to work, you have no business messing with this stuff  and are likely a hazard to yourself and others.

Fuse the positive input lead to the inverter with a rating twice what current is expected. This fuse will protect the wiring when the inverter eventually dies, sometimes very spectacularly, with smoke and flames.. fuses here can be rated at 100s of amps for the larger setups, welding cable is useful source  for high current DC runs.

Seperately fuse the mains output of the inverter to whatever current maximum you have allowed for that specific inverter. This will help protect the inverter from any overload that its internal circuits probably cannot handle. Accidental overloads will happen!! Fuse ratings  here will  generally below 10 amps. Remember, amps =  watts divided by volts. This information is always labeled on each appliance you intend to use.. Induction motors in general can require 5 to 10 times their running current when starting. Most inverters will attempt to supply this without failing, some wont, depending on the imagined surge current specs the advertising people have invented. Here, the only way is to try it unfortunately.. we are running blind, and trusting the manufacturer on this one.. Large industrial circuit breakers can also be used here on a lower voltage , despite being mains rated if these can be got cheaply. A 24 volt inverter supplying a 2400 watt 240 volt load, such as a two bar electric heater, is using 100 amps from a 24 volt battery bank, or 200 amps from a 12 volt battery bank.... these are very very large current demands.

By all means spend more on "better" inverters, but, trust me..... they will eventually go up in smoke too, in my experience their is little benefit in doing this. A run of the mill Chinese inverter will last usually 1 to 3 years possibly  longer if the above steps are taken. An expensive one will last around the same time , but can be run harder.... much closer  to its advertised ratings.

Once every six months, clean any  crap out of any cooling slots, check the fans, power down and in damp climates, squirt copius quantites of a CRC5/56 or a similar product into the internals to prevent corrosion and  moisture . After  a few minutes, its safe to power them back up. Just squirt into any cooling slots or openings and let it drain. Dont do this in hot dusty climates.

Run seperate circuits for large appliances, lighting, wall outlets and workshop. These all terminate into the engine shed with  mains plugs that can be alternated into any inverter for fault finding or hot swapping in case of failure. Unfortunately, some electrical wiring codes in nearly all countries must be substantially  ignored in order to make a practical off grid system.... but use grounding wherever possible. Also think of safety..... its a bugger being electrocuted even once, so no live exposed terminals. My experience indicates  pretty much all registered sparkies have very little to no working knowlege of off grid systems, in this country at least, and if consulted you will end up with a very expensive white elephant... it will be legal, but it wont work properly.
« Last Edit: June 01, 2015, 08:04:25 AM by starfire »

richardhula

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Re: Power Inverters
« Reply #1 on: June 01, 2015, 10:13:15 AM »
Interesting - all the inverters I have seen take the DC and using pairs of high current transistors switch it on/off at the desired AC output frequency of 50 or 60Hz, controlled by an oscillator. This low voltage AC is then stepped up to the desired voltage with a transformer.

I wonder if you have in mind a Dynawatt or TravelPower system. Here an engine driven alternator provides a three phase output similar to an automotive one but around 300 volts. Its rectified to DC and fed to an inverter controlled as above by an oscillator running at desired output frequency. The beauty of this means of power generation is that the engine can run at any speed as long as its fast enough to produce enough power to match the AC load. The higher initial voltage makes for greater efficiency. Schematic of Dynawatt below.

 

As can be seen a modulator running at high frequency shapes the output via second transformer to that of a pure sine wave.
« Last Edit: June 01, 2015, 10:40:50 AM by richardhula »

starfire

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Re: Power Inverters
« Reply #2 on: June 01, 2015, 10:37:33 AM »
The early inverters did operate the way you describe, and some still do, the Australian Selectronic design springs to mind. These can be identified by weight, the low frequency transformer needed was a big heavy thing. The modern inverters use a high intermediate frequency allowing the use of small cheap ferrite transformers with fewer windings and higher efficiency. Modern "caddy" inverter welders use similar technology to reduce size and weight for similar output of there older,much heavier and bulky predecessor. Times are a changin.

richardhula

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Re: Power Inverters
« Reply #3 on: June 01, 2015, 10:46:35 AM »
Thanks - yes that concurs with the schematic above.

mike90045

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Re: Power Inverters
« Reply #4 on: June 01, 2015, 05:28:12 PM »
I'd like to add here, that there are 2 different "grades" of inverters, Hobby style, which is what was covered by Starfires' post, and the "Professional" style.

Briefly, the professional style are larger, expensive and rugged units, of either 24 or 48V, with 2-8 Kw rating.  They are rated for continuous duty at some %, and 100% overload for many seconds (for starting large motors)
These are the Outback, Magnum, Radian, Xantrex & Schneider.  They weigh in from 40 - 120lbs, and carry 5-10 year warranty.

Tom

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Re: Power Inverters
« Reply #5 on: June 01, 2015, 07:01:47 PM »
The Outback vfx 3648 inverters that power our home have worked reliably for 8 years now. I weld and have a small machine shop. No issues except starting the 1hp drill press when belted for top spindle speed.
Tom
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starfire

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Re: Power Inverters
« Reply #6 on: June 02, 2015, 07:18:10 AM »
Yes, the hobby type are what seem to be most commonly used. What I was putting forward here was my suggested first steps for a beginner to get off grid as cheaply as possible... many that want to commit to off grid do so with some fear and trepidation, unless there is no other option of the grid being available, in which case go for the better equipment for sure.. I do have a chinese inverter that continues to run after 13 years , so it all not doom and gloom. I can only comment on my experiences and what has worked for my situation. Boy have I had some major mistakes as well....

biggkidds

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Re: Power Inverters
« Reply #7 on: June 02, 2015, 06:10:37 PM »
My short 7 years off grid have taught me to leave the inverter powered up. Turning them on and off in my experience tends to cause more frequent problems.  Spectacularly bright and loud problems. LOL   
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rmchambers

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Re: Power Inverters
« Reply #8 on: June 20, 2015, 01:36:25 PM »
The larger inverters, mine is a Trace/Xantrex 3624 model (3600W, 24VDC input) that I bought probably 10 or more years ago from a guy along with 4 Trojan golf cart batteries for about $600 have been very handy and reliable since I got it.

The batteries were marginal but that bank limped along for another 5 years with care and equalization charging every so often.  I purchased a set of 4 new Trojan T125's and that would run my house (both banks one after the other) for around 2 days (fridge, freezer, TV, a few lights, etc) but once the batteries were low and it shut off, I was in the dark.  The old set of batteries died and I got another 4 T125's and now I can run probably 3-4 days with just the essentials.  I've switched to mostly LED lights too which help.

When Hurricane Sandy came to town we were without power for 6 days.  By that time I had also acquired an Onan single cylinder marine generator 3000W that runs on Diesel (which I have a big tank of in the basement - #2 heating oil) so fuel isn't an issue.

I would get up in the morning and fire up the generator which fed directly into the inverter, this would run the house loads and whatever power was left over went back into the batteries.  I'd also fire up the hot water heater (oil fired) so we could shower and such.  I'd shut the generator down at night and run quiet on batteries.  This inverter does do power search (pulse to see if something gets turned on, if not it doesn't invert).

The bottom line here is if you're thinking of getting some sort of inverter to work along with your generator don't get one of the cheap and nasty ones because they will fail when you need them.  Spending a bit more on a good quality (and yes they are heavy) inverter will hopefully prevent an outage at an inopportune time.

starfire

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Re: Power Inverters
« Reply #9 on: June 27, 2015, 04:15:31 PM »
I recommend replacing the input fuses to a value of 1/2 what the inverter is rated for. Pretty much all cheap Chinese inverters will never supply their continuose rated output without failing. But if limited to 50 percent of rated, in my experience they can and do last many  years, making them a cost effective option. I use these in my setup to run lighting and other non fussy loads, and a  good sine wave inverter for supplying any electronics.. A cheap 2 killowatt inverter run at a max of 1 KW, will be very reliable. Many inverters if used to supply individual circuits is also a very good option, if one blows, the result is not fatal, it will drop just one circuit, not the whole system. Doing it this way, an off grid system can be completed in small stages, meaning any high initial investment can be avoided..

bandmiller2

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Re: Power Inverters
« Reply #10 on: August 16, 2015, 01:42:52 PM »
I have been using smaller inverters for years, they are best for light loads and intermittent service. I have an outside water tube boiler and my biggest fear is power loss and the copper tube overheating. During the heating season I have a holding relay that stays open as long as theirs power. If the power goes off it makes a solenoid that powers up an inverter and runs the circulator. This buys me time to fire up the old Lister for long outages. When the power comes back on it automatically switches back and shuts down the inverter. Frank C.
Fast cheap and easy are seductive sirens,its a rare man that does not court their pleasures.