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Modified Sine Inverters


edmundp
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Hi All,

This one's for Plonkster, OomD and such....

Just a bit of a technical question here. It has been said many times before that it does not matter if you run SMPS (Switch Mode Power Supply) devices of ModSine Inverters. All good.

But what about doing this permanently:

  • Any risk to such a device in doing the above long term?
  • What is the conversion DC to AC efficiency of the typical modsine vs puresine inverter? Referring to sub 600W scale devices.
  • SMPS's are inherently split in two types: Transformer based and Transformerless. What about devices such as a Hi-Fi unit that has a primary transformer first before going to the SMPS?

Thanks!

Ed

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I think the main danger of MSW inverters is that they are cheap and sometimes catch fire. Seriously, that's what my electrician acquaintances are telling me. The cheapness doesn't stop with the driving circuitry :-)

In theory -- that is to say I'm not putting money on this -- any device where the first component in the pipe is a rectifier bridge, is not going to worry what the waveform looks like. So the majority of Switch mode PSUs won't be harmed even in long term use.

As I recall, efficiency is pretty good, often better than pure sine wave. The efficiency losses are downstream, in things like electrical motors etc. knurlgnar has a youtube video where he tested this with a microwave oven. For SMPS and other resistive loads, I would expect efficiency to be very good.

https://www.youtube.com/watch?v=wimTJw_Gpgk

For a sound system, I'd probably not use MSW because of the buzzing that sometimes comes with that. Small transformers will run a little hotter than usual, but is usually not a problem in the long term. Personal experience though, we used a MSW inverter on the farm for years before my dad bought a "Nelson Adams" inverter many years later. The only things that suffered damage in the long term was small electrical motors.

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I want to agree with plonkster's post, to some extend. The moment you go DC, it doesn't matter much. since the bridge rectifier will turn AC into DC. But, depending on the design, you might have some noise and could add some filters in line to smooth it out a bit. Generally speaking, well designed SMPS would have enough filtering for you not to notice it. The hifi on the other hand probably has a very basic linear power supply

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Well the hifi is actually a Sony HDMI amp component. And I suspect it has a fairly huge mains transformer before entering the smps stage. I once opened it's much older brother to peek. Does have a bit of a hum on modsine. But nothing really noticeable.

I.t.o. the quality I am looking at something such as a Devel or Meanwell unit 300w.

What I am trying to really establish is for a particular application if it is worth it to spend 1000 to 1500 more for the same 12v inverter....

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Just speaking in theory here, I have no experience with modified sine wave inverters. Many devices these days have EMI filters on the mains input. These include some X capacitors across the line (line to neutral). These will suffer continual spikes of current due to the sharp rise and fall times of the inverter output. I'd say that this will cause those capacitors to fail, likely open circuit as designed, rendering the filter useless, but until then stressing the inverter and any chokes between the inverter and X capacitors. (Any such chokes will however mitigate the spikes somewhat).

Elcon (TCCH) chargers have a large 2.2 uF capacitor right across the mains, before any filtering. I think that this is for correcting the slight inductive power factor of the PFC (power factor correcting) front end.

Finally, that's another concern: anything with a PFC stage is not going to like MSW. I've seen some spectacular charger failures (not Elcon/TCCH) that I suspect happened from running off MSW inverters. The trouble is these days even your shaver might have a tiny PFC chip in it. Even if you can open it and read the chip number, some of the Chinese chips (and nearly everything is made in China, often with Chinese manufactured chips now) are hard to find data on.

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Just speaking in theory here, I have no experience with modified sine wave inverters. Many devices these days have EMI filters on the mains input. These include some X capacitors across the line (line to neutral). These will suffer continual spikes of current due to the sharp rise and fall times of the inverter output. I'd say that this will cause those capacitors to fail, likely open circuit as designed, rendering the filter useless, but until then stressing the inverter and any chokes between the inverter and X capacitors. (Any such chokes will however mitigate the spikes somewhat).

Elcon (TCCH) chargers have a large 2.2 uF capacitor right across the mains, before any filtering. I think that this is for correcting the slight inductive power factor of the PFC (power factor correcting) front end.

Finally, that's another concern: anything with a PFC stage is not going to like MSW. I've seen some spectacular charger failures (not Elcon/TCCH) that I suspect happened from running off MSW inverters. The trouble is these days even your shaver might have a tiny PFC chip in it. Even if you can open it and read the chip number, some of the Chinese chips (and nearly everything is made in China, often with Chinese manufactured chips now) are hard to find data on.

Very interesting...

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Why not run it straight off 12V battery + solar panel? You're going to save quite a bit of energy this way. Anytime you step up / step down energy, there's losses involved. 

Indeed would be the ideal but problem is they are all 240VAC devices such as TV's, laptops, modems and such.

Seems pure sine is the way then ne?

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Coulomb makes a good point. I thought of this PSU I removed from a Phillips Blue Ray player (the mechanical parts failed). This is a very decently put-together little board with proper isolation between the mains and the low end, and good air gaps as well.

But note the input stages. In the green, you see a capacitor. That's a capacitive dropper. From there it goes into the (rather small!) transformer in the red, and then it is turned into DC by the four diodes also in the red. Big electrolytic tank to make it smooth. Then it goes into the SMPS, which is in the blue block. That is, two stage PSU, first drop the current with a cap, then drop the voltage somewhat with a transformer, then do the rest with an SMPS.

So... how would this thing fare on a MSW inverter? I have no idea really. My gut feeling is to say that that little transformer has far too little iron for me to try it. In addition, capacitive droppers also have far worse power dissipation on MSW. So did the manufacturer take that into account when he specced that cap in green? Do I want to risk it?

:-)

psu.jpg

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19 minutes ago, edmundp said:

Indeed would be the ideal but problem is they are all 240VAC devices such as TV's, laptops, modems and such.

Seems pure sine is the way then ne?

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Oh, you just said wifi ;)

Well, you would have a good clean signal with a puresinewave. BUT  both myself and I dad have run TV's, laptops and wifi's off modified sinewave inverters for many years. He used to work in Africa for about 15 years and there everything is super expensive. For example, if a 1Kw modified inverter cost R1000 here, the exact same one there would cost USD1000 (the imported just changed the currency!)

I have an old small 300w modified inverter built into the bakkie, for camping, and on many occasions used it for my laptop. TV, charging my portable drill's 18V batteries and even used a jigsaw on it. 

The thing is, how well is the PSU built on the equipment you want to use. Worse case scenario you could put a small filter or 1:1 transformer in between. 

And, yes, most TV's and hifis will operate on 12V. They do in any case. 

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27 minutes ago, plonkster said:

Coulomb makes a good point. I thought of this PSU I removed from a Phillips Blue Ray player (the mechanical parts failed). This is a very decently put-together little board with proper isolation between the mains and the low end, and good air gaps as well.

But note the input stages. In the green, you see a capacitor. That's a capacitive dropper. From there it goes into the (rather small!) transformer in the red, and then it is turned into DC by the four diodes also in the red. Big electrolytic tank to make it smooth. Then it goes into the SMPS, which is in the blue block. That is, two stage PSU, first drop the current with a cap, then drop the voltage somewhat with a transformer, then do the rest with an SMPS.

So... how would this thing fare on a MSW inverter? I have no idea really. My gut feeling is to say that that little transformer has far too little iron for me to try it. In addition, capacitive droppers also have far worse power dissipation on MSW. So did the manufacturer take that into account when he specced that cap in green? Do I want to risk it?

:-)

psu.jpg

You will probably find that it can operate between 90 & 240V / 50-60Hz!

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Ok. Well all considered I think the safest would be pure sine. Full stop. Mod sine seems ok, but fine for more "mobile" solutions.

Anybody know where to get decent priced, decent quality 300W pure sine 12V inverters? I do not think they will ever see much more than 120W load. Need about 3 to 4....

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Thing is... there comes a point where it becomes very expensive to enlarge your solar storage capacity. If you break it down into small subsystems it is much cheaper IF you have them driven by your main pv system. And you do not have to rewire the whole house or be worried about COC's....

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Oh. And you can concoct your own storage devices...

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3 hours ago, viper_za said:

... another reason to want to run your whole house on the inverter 230v all the way ;)

@viper_za FWIW a friend of mine in Gordon's Bay has seen 380v coming in at one of his clients sites a few months ago.
Never has a online UPS gone in faster.

In my case:
All the stuff that matters like DSTV, lights, routers, Wifi, switches and fridges are all off-grid. 
All stuff like computers, alarm, TV, door interkom are daytime are on solar, evenings on Eskom protected by a online APC 1400w UPS, that I got at a HUGE bargain.
And if Eskom is off, all the above runs of the batts.

So what is left to worry about?

This drama you guys are now picking up, thanks to your systems and cool software, did not start today. Started prior 2008.

I never understood back then why my UPS'es at times started clicking until someone showed me their logs. Should have kept them.

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3 hours ago, edmundp said:

break it down into small subsystems

You know, I've thought of that too. I was saying just the other night, now that all the lighting is LED and the whole house probably needs 100W at night, it does make a lot of sense to get a smaller inverter just for the lights circuit. Then right after that I shot down my own idea because 1) it is not as if the current inverter is overloaded and 2) then there is yet more wiring and paperwork... nee wat. But it does make sense if you start looking at complete separate systems, ie separate banks that also charge separately.

What I'm actually thinking about is dissimilar battery banks that charge separately during the day, but are latched together at night. If my research on this pans out (I've posted a link to the paper before), this will avoid sulphation because the batteries are charged separately, and at night they will contribute according to their capacities. The only problem with such an arrangement, is a shallow depth of discharge: Because the stronger battery takes the first shift, so to speak, so that one works harder.

Basically, the idea is to solve the problem of incremental battery upgrades. :-)

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You know, I've thought of that too. I was saying just the other night, now that all the lighting is LED and the whole house probably needs 100W at night, it does make a lot of sense to get a smaller inverter just for the lights circuit. Then right after that I shot down my own idea because 1) it is not as if the current inverter is overloaded and 2) then there is yet more wiring and paperwork... nee wat. But it does make sense if you start looking at complete separate systems, ie separate banks that also charge separately.

What I'm actually thinking about is dissimilar battery banks that charge separately during the day, but are latched together at night. If my research on this pans out (I've posted a link to the paper before), this will avoid sulphation because the batteries are charged separately, and at night they will contribute according to their capacities. The only problem with such an arrangement, is a shallow depth of discharge: Because the stronger battery takes the first shift, so to speak, so that one works harder.

Basically, the idea is to solve the problem of incremental battery upgrades. :-)

Agreed, but not if your not using lead acid....[emoji41]

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Agreed, but not if your not using lead acid....[emoji41]

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As long as the chemistry is the same, dissimilar banks behave predictably on discharge. Each string contributes according to its own capacity. The only caveat is that the strongest string takes the first shift, so at less than 30% DoD the best string will do most of the work and degrade much faster. At deeper levels of discharge the stronger string will get close to its specification life. According to this one paper at least. They did test it with actual batteries :-)

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6 hours ago, plonkster said:

You know, I've thought of that too. I was saying just the other night, now that all the lighting is LED and the whole house probably needs 100W at night, it does make a lot of sense to get a smaller inverter just for the lights circuit. Then right after that I shot down my own idea because 1) it is not as if the current inverter is overloaded and 2) then there is yet more wiring and paperwork... nee wat. But it does make sense if you start looking at complete separate systems, ie separate banks that also charge separately.

What I'm actually thinking about is dissimilar battery banks that charge separately during the day, but are latched together at night. If my research on this pans out (I've posted a link to the paper before), this will avoid sulphation because the batteries are charged separately, and at night they will contribute according to their capacities. The only problem with such an arrangement, is a shallow depth of discharge: Because the stronger battery takes the first shift, so to speak, so that one works harder.

Basically, the idea is to solve the problem of incremental battery upgrades. :-)

The biggest problem you're going to face with this, will be to connect / disconnect the batteries under load. Unless off cause you don't mind shutting down the inverter every night and every morning to safely connect / disconnect the batteries. In my case, I simple can't be home at those times every evening and I can't expect my wife to be there either. I have seen a small system (4Kw) under slight load (probably 200w) blow 300A fuses on the 48V battery bank. Some ideas some come mind:

1) Use very big & expensive SSR's like this: http://za.rs-online.com/web/p/solid-state-relays/7761412/

2) If the battery bank is in series (i.e. 4x 12V @ 48V) then you could connect a charger per battery. But you might have to put a "blocking diode" between each charger and battery to prevent any possible cross charge between the chargers. 

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21 hours ago, SilverNodashi said:

The biggest problem you're going to face with this, will be to connect / disconnect the batteries under load

The idea is to keep the inverter permanently connected to one of the banks. The other extras are latched on at night. That might indeed be a challenge under load. Even a solid state switch (aka mosfet) would probably need two in series, because the body diode will conduct in one direction, so it won't be possible to disconnect it in both directions using just one. Haven't thought the whole thing through just yet :-)

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20 minutes ago, plonkster said:

The idea is to keep the inverter permanently connected to one of the banks. The other extras are latched on at night. That might indeed be a challenge under load. Even a solid state switch (aka mosfet) would probably need two in series, because the body diode will conduct in one direction, so it won't be possible to disconnect it in both directions using just one. Haven't thought the whole thing through just yet :-)

DC volts & current isn't as "forgiving" as AC volts and current, especially when connecting / disconnecting. 

 

One way, perhaps, is to use a microcontroller (or PC / raspberry Pi) to momentarily switch off the load / inverter, then engage the SSR and switch it back on again. In theory most SSR's should cary many more Amps when switched, than it can  switch under load. i.e. If it can switch 100A, it can carry upto 200A - https://www.futurlec.com/Relays/SSRDC30V100A.shtml

SO, if your system is 12V and you thought of running multiple batteries in parallel, this could work with some careful planning and proper equipment. 

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