The difference between Power inverters HF or LF!

[flibbidah]

Many buyers do not have the knowledge in Electronics about Power Inverters used to convert Solar Panel power (12 or 24V) to mains supply 110 or 240V AC. A 12 Volt system needs about 18V from a solar system to work effectively.  To run devices with clocks and timers a Pure Sine Wave is essential not a modified sine wave which has a sawtooth output. 

If your coffee maker is rated at 1.5KW then you will need up to 8 times that figure to handle switching surges, so do not buy a 3KW inverter because you are going to blow it apart after a short while.

The biggest question is HF or LF (high or low frequency)  Many sellers do not disclose this so "always ask"  an HF inverter uses FETs (Field effect transistors) to switch the DC input and convert it to an AC wave form this is stepped up by using a bank of HF chokes to 240volts, which is then processed by the following circuitry.   These FETs are often under-rated and may only last about 3 to 4 years if you are lucky,  often the TVS diodes (transient voltage suppression) with blow apart if the FETs are overloaded, also taking out the fuses.  Importantly an HF inverter can only handle up twice the continuous rated power  ie 1KW continuoous 2KW peak.

Basically they are a waste of money.

LF inverters are are the only way to go.  OK you will pay more, but instead of a bank of small chokes and FETS they use a huge toroid step-up transformer, which can weigh many kilograms, an 8KW continuous rated unit can weigh in at 23 Kg! So do not be surprised the postage is higher!   LF inverters can also handle up to 4 times the rated power ie. 8kw continuous 32kw peak. 

An 8KW unit will handle most kitchen appliances, TV and small fridges provided the total consumption at any one time does not exceed 2KW. 

Conclusion: Be very careful what you buy so as not to be disappointed.  Parts are readily available to repair these inverters, but should only be carried out by a qualified and experience technician.  Dangerous high voltages are present inside the units, (More so in HF units in places you would not expect) that can kill or serious burn or cause batteries to catch fire.  Solar batteries are usually rated at 100Ah (Amp hours) and if shorted can melt a screw driver tip or burn heavy cables if shorted out!!!   Do not open the cases if you do not have the knowledge.

I suggest that before you buy, you contact the seller and tell them what your needs are and what you are going to run on the inverter and ask them to suggest an approriate unit.  In any case always go for the LF version and not be tempted by the cheaper "throw away' HF versions.  Any seller worth his salt will be only too happy to assist you.  If you get a nondescript reply, then look elsewhere.   In all cases good solid Power inverters do not come cheap and avoid the HF versions they are inefficient, whereas the LF inverters can be over  90% energy transfer efficient. .

I hope this will assist you and save you headaches down the track.

(65 years experience in the electronics industry) ...DLS.

Thought of the day: Caveat Emptor  (Buyer beware) .  

[i-love-my-sheep · Accepted solution]

Great information. Have you added it as an eBay guide to potentially help buyers? I think this info would be useful as a guide.

Thanks for posting it.

[flibbidah]

No its not in the guide.  But yes, I felt it was a  necessary post as this is a very grey  and confusing area..

I originally bought one 3 years ago,  an HF 3KW inverter and it was pathetic.  They were the first of the Asian made inverters and had many problems, not the least allowing entry of a cockroach which walked over the overload sensing circuit, fried the beast and placed a short across the circuit, shutting the unit down!!

I have used those toroid transformers in Ham Radio applications, expensive,  but will beat any switching power supplies hands down, which can create a lot of spectrum interference, esp the HF versions.

I need to correct the typos LOL, my ancient hands  are somewhat arthritic! 🙂

[i-love-my-sheep]

I'm not sure how you create a user guide on eBay, but there's a few of the regulars who know. I think this is definitely good information that could help buyers choose wisely. It's surprising how many buyers will read the guides for info before buying.

Unfortunately, with the nature of the forums, unless people keep commenting on this thread, it will eventually fall down the list into the big, black hole.

[2106greencat]

I am not sure how to add a guide either, but this post will bump it up so other more experience than I can help.

[audistarelectronics]

90% of the ebay sellers of invertors would not know what you are talking about, so waste of time asking those questions. They only know how to import them, store them, and pull them out of their other toys, tools, and garden stock, and post them out to the buyer......hopefully.

[woodypm]

Thanks very much that is very help full.

[flibbidah]

I agree with your comments about people on EBay who buy in maybe 10 items and resell or dropship. But there are sellers that specialise in Solar equipment, if a seller cannot answer your questions, look elsewhere. Proof of the pudding is since I posted this, the 8000 watt Pure Sine Wave inverter I purchased has not missed a heartbeat. I run individually in my kitchen, the toaster, electric kettle, a coffee machine, and expresso machine or a microwave. Plus sundry small tools like blenders and "whizz" sticks. During some dark cloudy winter days, I was obliged to switch the inverter to the domestic "Charge Mode" at night to maintain battery strength especially if my demand was greater than input. But with the Summer sunshine approaching in the Southern Hemisphere. By 9am each day (October) my battery indicators are showing fully charged at 14.1V (no load, in the save power mode) I have also used it to power portable fluoro lights when the mains power failed in a storm. ATM I am running 3 x 120W panels and 300 Amps of deep cycle gel batteries. Which will be added to over the coming months . My system is very minimal, but over the past two years has just about repaid my initial outlay. (Not including the garbage HF inverter, which was allocated to the "learning Curve department" junk bin. ) Late News: There has been in the past few months breaking ground in new types of solar batteries. This is making whole of house supply more affordable and independent of the public supply. Here we are looking at an initial outlay of around $12,000 (AUD) a lot yes, but recoverable, and well worth the investment by young couples building a new or an existing dwelling. Plus prices of panels and storage batteries are dropping as the technology and demand improves. Go for it... Save a tree or two!

[mrasmussendonn]

You speak of transients in your description and refer to a coffee maker.  A coffee maker is a resistive load so I would not expect much problem there with a spike over inverter rating.  I see more problems with inductive loads like motors or florecent lights. 

I have had good luck with HF units as long as I have confirmed inductive loads (starting) would not bust the "rated" limit.  Forget the surge rating that is what blows these up.  Never blown one on a resistive load.

I bought my first LF unit and know it will be better at the starting motors becouse it comes down to an impedence matching game and FETs aint got the inductive impedence a transformer has.

[davewil1964]

i would not expect the OP to come back and even read your post after this length of time. I reckon they might have sorted it by now.