Discusion recently on inverters, "low frequency"& "high frequency"

westbranch
westbranch Solar Expert Posts: 5,183 ✭✭✭✭
In one of the longer threads we have had somebody explained the inner workings of inverters that operate at low frequency (more reliable, use transformers, heavy) and High frequency (use electronics to function, lighter).

I have done a couple of searches, but don't recognize any titles that wandered off to that explanation..

Any ideas as to which one it is?

thanks.
 
KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    Is this the thread you where looking for?

    What kind of inverter is more reliable? A transformer based one or Non Transformer one??


    Another related thread?

    PWM Inverter


    And some others with some design discussions:

    Inverter design stupid

    Fridge is surge overloading inverter

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"
    westbranch wrote: »
    In one of the longer threads we have had somebody explained the inner workings of inverters that operate at low frequency (more reliable, use transformers, heavy) and High frequency (use electronics to function, lighter).

    I have done a couple of searches, but don't recognize any titles that wandered off to that explanation..

    Any ideas as to which one it is?

    thanks.
    Have you looked at http://forum.solar-electric.com/showthread.php?18276-What-kind-of-inverter-is-more-reliable-A-transformer-based-one-or-Non-Transformer-one/page2
    ?
    SMA SB 3000, old BP panels.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    Thanks guys, it was the first one in each post. A long and interesting read.
    Last weekend I met the local (to BC) distributor for Cotek inverters and we were discussing a problem about inverter size and he mentioned that all the Cotek inverters were better because they all were low frequency inverters not hi.
    thanks again
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    Just about all the "industrial" type battery inverters are low frequency. And every one I have ever tested leaves the high frequency ones for dead when trying to start difficult loads. In most cases they can start things like fridges ,compressors as well as you can with the grid power.
    But they are also 50%or more expensive than high frequency inverters also much heavier but this is usually of no importance.
    They also tend to be made by the better manufacturers. ie Victron.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    good to hear that from Dr. Destructo...:blush: your testing experience8) holds high stead with me!
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    Westbranch Iusually use this analogy to people that have no idea of the differenvce.. OK I accept its a flawed description but it helps them understand.
    You have 2 ICE engines 1 a 1l 4cyl the other a 6l v8 . Now they both tuned to develop same max HP. so will drive a vehicle using same gearbox and wheels etc to same top speed. Now we put 10 bags cement in each vehicle the V8will pullaway from the start no effort. The 1l engine is really going to be struggling.

    Not a perfect comparison but it gives the idea.
    As for testing Idont know how many inverters ive tested but it would be well over a thousand in the last 10 years.

    My masterpiece is getting a battery powered inverter to sync as pretend grid power to keep a GTinverter going.Ido know you can buy ones to do that,, but hey whats the fun in that.
    I got it to work after only turning to scrap 3x 1500w GTinverters and 3 x1000w TSW inverters.. If at first,second and third try you dont succeed there is a better chance on the 4th try, and so it proved to be.

    On the previous if you can afford a good low frequency inverter go for it you wont regret it.. especially if you have something like a fridge to power.. A rule of toe is a 1000w low frequency inverter will start a fridge as well or better than a 2000 w TSW inverter.
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    The one downside I see with most (maybe all) LF inverters is idle power consumption. As far as I can tell they have much greater idle power consumption. With power saver some of that can be limited though. But could be over 100w idle for the larger inverters with 50watt idle (non power saver) for smaller sized inverters. Not trivial loads but depends on usage scenarios.



    I have 2 low frequency inverters. Although not much actual run time with them. Picked up a refurbished aims 1000w inverter on ebay for $70. I am looking forward to seeing what it can start.

    I see most LF inverters have surge ratings roughly 3x of continuous usage rating. The same thing responsible for large surge rating is responsible for the high idle power usage. The large iron core.

    Anyway above is from my limited knowledge. There are MANY LF inverter makers. Some names are promariner, samlex, aims, tripp lite, sterling power, and alot of generic brands as well. They also make HF inverters so be careful when buying.
    Have no idea if some are better than others.
    As a side note It appears sunelec is selling some samlex LF inverters cheap.

    While I hate the wasteful nature of the LF idle power consumption I do like the idea of a robust design and thus why I went that way. Avoiding down time due to failure will be important to me as I will be in areas with little access to any solar/ inverter products.


    matthew
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    Thanks John, I will try the analogy. My friend has a small 12V system and wants 'more things' to run off his present inverter, all at once, including some power hungry loads. The inverter is a 'Canadian Tire' weekend sale special, 1000w 12v, no idle load # or other load parameters. The main new load is the need to run a Sat. Internet connection so he can have VoIP for his business, 24/7. Then TV, Sat TV, :grrMicrowave :cry: lights, water pump, cell charger.
    He keeps telling me he is a "Mortician not an Electrician" and I believe he doesn't know a watt from an amp as he keeps giving me 'that' blank look when I try to explain things... So, we are making progress slowly, as far as I am concerned he needs a ROCK for the foundation not sand, hence a good quality TSW Inverter.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"
    john p wrote: »
    If at first,second and third try you dont succeed there is a better chance on the 4th try, and so it proved to be.

    I do like that. My favorite has always been "If at first you don't succeed, get a bigger hammer."

    And in case, as seems likely, you have some math and programming background there is this one:
    Programmers' Motto:
    "If at first you don't succeed, curse and try again.
    If again you don't succeed, re-curse."
    SMA SB 3000, old BP panels.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    Animatt, yes there is always a trade off.
    I have used MSW inverters as well as TSW and prefer the benefits of TSW LF to the others, if only one inverter, for their reliability and strength to power surge loads if needed, though I opt for more Watts to start than relying on surge capacity only. MSW has its place especially if powering lights only. But one option I see and like is the 'sleep mode' and no load idle of better higher priced inverters, old adage: you get what you pay for, especially if you suss things out first...
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    Unless they have improved those CT inverters tremendously in the past few years they are just expensive door stops. Most disappointing equipment I've ever tried out.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    Inetdog, I like that, it rings true, and true, and true.

    @##$$%^&^&** and Punch Cards? but I did enjoy the challenge ...so did my now wife (ha), when we spent Sat nights at the card puncher and compiler at Uni.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    If you are looking for a long lasting quality inverter its not going to likely come from a big box store or "Hung ones or his brother who was Hungtoo all cheapo super sale store. And some of the not as good ones come from AIMS,Canadian Tyre, the ones from "X....."
    There are sine wave and sine wave, Some manufactures actually sell MSW as TSW . Dont believe all you read on the cardboard box on items from C....,
    If its cheap it usually means "nasty"
    I dont give much out as brand names, but give you these.
    Inverters .Victron
    Generators. Northern Lights
    Charge controllers.Victron ,GSL
    As most know I sell nothing or have anyone relative or friend that does either.
    I just test things and advise manufacturers and goverment departments of my findings. Thats it last 10 yrs.
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"
    westbranch wrote: »
    Thanks John, I will try the analogy. My friend has a small 12V system and wants 'more things' to run off his present inverter, all at once, including some power hungry loads. The inverter is a 'Canadian Tire' weekend sale special, 1000w 12v, no idle load # or other load parameters. The main new load is the need to run a Sat. Internet connection so he can have VoIP for his business, 24/7. Then TV, Sat TV, :grrMicrowave :cry: lights, water pump, cell charger.
    He keeps telling me he is a "Mortician not an Electrician" and I believe he doesn't know a watt from an amp as he keeps giving me 'that' blank look when I try to explain things... So, we are making progress slowly, as far as I am concerned he needs a ROCK for the foundation not sand, hence a good quality TSW Inverter.


    I personal would probably get a DC clamp meter and measure how much power that inverter is using at idle. Then calculate how many watt hours are lost in a day just keeping the inverter going. Those large LF inverter not in power saver mode could be over 2.4kwh of power consumption. But if you had a 4-5kw inverter with peak rating of 15kw you maybe using alot of power anyway and 2.4kwh not such a big deal.

    I have no experiences with this but I would probably try the morningstar 300w inverter to power all those small 24/7 loads. This would allow idle power consumption to be saved. This would definitely not be for everyone as implementation could be tricky. Maybe those 24/7 loads are spread over several circuits in different areas of the home. But if successful could save a good bit of power. I think the LF inverter in the 1000w range are around 50 watt idle power consumption and about 20 in power saver mode. Even if using the morningstar inverter let the larger LF inverter stay in power saver mode a good bit of power could be saved. The bigger the inverter the more savings. With a larger 3000kw inverter you could save maybe 1kwh a day. A 2000 LF inverter with I believe 6kw peak could be had at the previously mentioned location at clearance for $275.00 and it is a 12v inverter. While that would be tons of amps it would allow you to stick with 12v battery array if that was important. Although if spending a good bit of money going to 24v or 48v would be advisable.

    That 2000 w inverter(6000 peak) could draw over 500 amps of current if the batteries would support it.

    Matthew
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    West branch ,no type of inverter will run loads constantly much better than any other type. if you have 2000 w of loads you need at least a 2500w inverter.. . Inverters like the low frequency type are only superior at starting heavy difficult loads,and vastly superior.
    You cant get a Toyota Camry to carry the same load as a Mack truck can...Sooner or later you will get big failure. guaranteed.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    What we are looking for is a small < 1000w (600W ?) inverter for use on a weekend/summer cabin so I believe the choices are limited to a few brands with the previously mentioned goal of a TSW with low standby consumption, sleep mode, LF for greater surge (2x) load capability.
    Primary loads are communication, then TV dish, modem,etc, lights and lastly appliances. Only Telephone is needed on 24hrs while there. 5-10W
    Agree that if he wanted to be full time OG, 2 inverters would be better.
    The 4 x 140W panels will be on a shed roof ~30' from the cabin, recommending 24V config to an MPPT CC, could go 48v if no shade issues. batteries/electronics in shed. 120V AC to cabin.

    Waiting for the snow to disappear...

    John, CTire also randomly sells a line of "X..." MSW inverters as 'Special buy" I suspect they are #2's they are so cheap. We don't have any true Big Box stores within 150 miles so most RE stuff has to be mail ordered, or a 7 hr drive to the Big Smoke is needed.

    Loos like I should get a Clamp meter too.
    we are getting there.
    cheers
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    I think LF and low idle power consumption are mutually exclusive. The large iron core of transformer allows for larger surges, but also takes energy to keep it just sitting there powered. You can have one or the other but not both. The way to limit LF idle consumption is using the smallest possible. So smaller iron core, also reduces surge capabilities. And a major savings can be had if you put it into power saver mode, but that will not work with 24/7 loads.
    So I recommend using a small inverter to cover the 24/7 loads. As I think a LF inverter running a 10w load will itself be consuming +30w. Although I could be wrong.

    Another possible solution is if the 24/7 loads could be run directly from the battery. via very small and cheap Inverter/ power brick. Something like a 30w inverter for the phone. etc.

    matthew
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"
    westbranch wrote: »
    What we are looking for is a small < 1000w (600W ?) inverter for use on a weekend/summer cabin so I believe the choices are limited to a few brands with the previously mentioned goal of a TSW with low standby consumption, sleep mode, LF for greater surge (2x) load capability.

    Victron do a range of < 1000W inverters and inverter/chargers starting at 800W. Looks like they have resellers in Canada too: http://www.victronenergy.com/where-to-buy/?continentid=1
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    Thanks Stephen, I have been in touch with Victron before about exactly this type of app. for my own place as well.
    What used to be their 600VA has been replaced with a 800VA and the do have a 180VA and 350VA models, so there is a stretch from 350 to 800. We're still pulling the consumption figures together.... so plan on max of all simultaneous loads being ~= 80% of the inverters nominal rating.
    Nearest seller is 'only' 6 1/2 hrs away, almost in the big smoke...
    Animatt. that idea of a small 20W brick may work OK, have to run a separate line though.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"
    westbranch wrote: »
    Inetdog, I like that, it rings true, and true, and true.

    @##$$%^&amp;^&** and Punch Cards? but I did enjoy the challenge ...so did my now wife (ha), when we spent Sat nights at the card puncher and compiler at Uni.
    I do. I remember being in the keypunch room late one night frantically trying to finish up a programming assignment when a guy rushed in with his shoebox full of cards, which he had neglected to rubberband together in order. He tripped on the doorsill and spilled the cards all over the floor. He sat down and cried.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"
    ggunn wrote: »
    I do. I remember being in the keypunch room late one night frantically trying to finish up a programming assignment when a guy rushed in with his shoebox full of cards, which he had neglected to rubberband together in order. He tripped on the doorsill and spilled the cards all over the floor. He sat down and cried.

    I often had to cut my own holes with a broken rasor blade.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"
    NorthGuy wrote: »
    I often had to cut my own holes with a broken rasor blade.
    Oh, you were lucky to have a razor blade! Our instructor would assign us projects two weeks after they were due and force us to chew our holes in our cards with our teeth, and then beat us with a wet chain and send us off to bed without our supper! We would have gleefully killed one another just to catch a glimpse of a razor blade through a broken window on a neighbor's telly! You hoity-toity whippersnappers with your razor blades...
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    You guys aren't from Yorkshire by any chance? :p
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"

    Kind of here to update a few of my statements.

    Came across a thread about modifying one of those Chinese LF inverters. It appears great savings in power can be had by changing out the transformer to a toroidal design. It appears to be a fairly expensive solution but that can save a good bit of power. Not only with saving ideal power consumption, but actual efficiency while using the device will be increased as well.

    While I am sure most of the electronics fans here already knew that I was updating for the rest of us.

    The thread below has the whole modifications. It does not seem to be too tricky but there are some things to think about.
    http://forums.energymatters.com.au/solar-wind-gear/topic3344.html


    Part of the reason I post this here is maybe if you are looking to buy one of those Chinese inverters directly you can see if anyone of them is willing to make a toroidal model. While I am sure it will cost more money, it does save a bit of material, makes product more efficient and I am sure cheaper than doing the modification yourself afterwards and having a large E I transformer left over after you are done.

    matthew
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Discusion recently on inverters, "low frequency"& "high frequency"
    animatt wrote: »
    The thread below has the whole modifications. It does not seem to be too tricky but there are some things to think about.
    http://forums.energymatters.com.au/solar-wind-gear/topic3344.html

    Lot's of bad info in this video. Starting with 48v current reading on idle. The current profile on this type of inverter can be quite complicated and requires a true rms reading meter in DC mode. True rms in DC is usually only found on high end meters. A true rms meter for a small hand held DVM usually means true rms for AC mode only, not for DC mode setting.

    For thoughs thinking that 'what are you talking about referring to rms on a DC current', a true sinewave inverter has a sine squared current profile on the battery current. At no load, this type of inverter (same scheme used by Xantrex and Outback inverters) can have a some energy 'kick back' of reverse DC current over portion of the sinewave cycle period. This is due to the particulars of the low frequency transformer designed specifically with 'more then normal' leakage inductance. This causes the transformer to store and return more energy in the transformer core compared to a normal designed power transformer. The reason for this I will explain later. At no load some of this stored energy is returned to the battery during some portion of the sinewave cycle period making the current measurement very prone to error unless you have the proper test equipment.

    Second, putting a small torroid transformer, guessing from the physical size in the video to be about a 50 VA capable transformer, is totally comparing apples and oranges. A 50 watt inverter will be expected to have a lot less idle current then a 6000 watt inverter. There is more to it then just the transformer size. The torroid transformer for a 60 Hz transformer will likely be totally unsuitable for this type of inverter. 50/60 Hz power torroids are normally designed for very very low leakage inductance. They are often used in audio amplifiers because they leak less field then a regular 'E-I' laminated silicon-iron core transformer reducing possible low freq hum in the audio.

    Now that is not to say that the Chinese copy of this type of inverter has a correctly designed low frequency transformer. More then likely the transformers are not properly designed for this application, contributing to more wasted power. If regular normal power transformers are used, a larger then optimal secondary output filter capacitor will be required to get acceptable high freq PWM removal from output making the system have much higher losses and higher resulting idle current from the battery.

    The reason for desiring a higher leakage inductance in the transformer for this type of inverter, which is normally what you don't want in a power transformer, is as followed.

    A true power transformer, likely what you are most familar with, will not store much energy in the core. It doesn't have to as its job is to tightly couple the power from the primary to secondary of the transformer. The B-H magnetization characteristics of a silicon-iron power transformer is very abrupt, requiring very little magnetizing current in the primary to drive the core into saturation. This does not matter because the tightly coupled secondary provide a back emf to counter the higher primary current when the transformer is loaded.

    The magic of this type of sinewave inverter is how the high frequency PWM primary side excitation is filtered out (with the help of only a capacitor across secondary of transformer). To do this requires the low frequency transformer be designed with a controlled amount of low loss leakage inductance. (low loss leakage inductance is somewhat a contradiction in itself) This is the 'special sauce' magic truely understood by only a few specialist in this kind of transformer design and fabrication. The leakage inductance provides the series filter inductance to help remove the high frequency PWM applied to primary of transformer. This eliminates the need for a separate expensive filter inductor.

    The large heavy special transformer could be replace with a small high frequency switching transformer design that would provide the primary to secondary isolation just like the large heavy transformer. The problem with this approach is it would also require a very large additional inductor in the output filter to remove the high frequency PWM.

    The problem with 50/60 Hz single phase AC is it has a long period of high current that must be supported in a transformer which is not possible with a small transformer core. Many of you know that a 60 Hz transformer used in a 50 Hz system may have overheating problems. This is because the core is not big enough to support the slightly longer period of peak current at 50 Hz.

    Back to the large filter inductor required to remove the high frequency PWM removal on the high frequency PWM transformer design. The filter consist of L-C, series inductor - shunt capacitor, to remove most of the 20-25 KHz PWM chopping squarewave that creates the average 60 Hz sinewave. The problem is this filter inductor must provide the correct inductance for the filter at 25 KHz AND must support the high long period 60 Hz peak current of the output AC load. To the high frequency inductor, it is just like requiring the inductor to maintain its high frequency inductance with 25 amps of DC bias flowing through inductor wire for a 6 kW inverter. This is an expensive and large inductor, more then defeating the benefit of the small high frequency transformer.

    So this type of inverter with a big heavy transformer has a high freq PWM primary amd low frequency secondary output. To the originator subject line, is it a high frequency inverter (yes) or a low frequency inverter (yes)... ;)

    This discussion is about bi-directional inverters. This is an inverter that can flow power in both ways, acting as and AC inverter and a charger. The 'Power-to-Go' type small sinewave inverters are not bi-directional inverters.