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why low frequency inverters can be the better option


TheButcher
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I've been meaning to do this for a while for other forums, and thought people here might like, or dislike, what I found.  This is a grab of my cheap Chinese high frequency inverter's AC output.  This is pretty normal behaviour for this class of inverters.  It's their way of avoiding blowing up the output (usually H bridge) transistors.

You might think that I've just dropped a large motor or the like onto the output but I haven't.  This is the result of a 12V 2A switching power supply being connected.  Timing is a bit critical as it has to be connected near the peak (crest) of the AC waveform to get 'best' results.  As you can see, it didn't like it.  The bulk capacitor in the switching power supply charging up destroyed the sine wave and then the inverter itself severely reduced the output voltage and slowly crept it back up over the next few seconds.  The wave on the left equates to 240VAC, on the right side, 165VAC.  The actual high draw by the capacitors was satisfied in about one cycle (50Hz) but the reduced voltage from the inverter lasted several seconds.

I don't really have a hefty motor like a well pump, the best I can do in terms of motors is my standard AC motor fridge.  The inverter copes much better with that showing only a few volts drop in output while the motor comes up to speed.  I can't test compressor stall since this fridge is operated by a microcontroller with compressor lockout timers.

So what does the nasty waveform and reduction in output voltage mean in practical terms.  For many things these days that employ switching (SMPS) / non-linear power supplies not a lot.  Well not a lot unless that power supply is being run near capacity.  The reduction in AC voltage could well be enough to cause a SMPS supporting, for it, a high load to drop out of regulation or shut down.  If you are a gamer playing away and someone plugs in their iphone charger, you may not be all that impressed.

A low frequency inverter will roll on as if nothing happened.

SDS00001.jpg.7948b454fa956efde49a1aa9b290d8fa.jpg

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2 hours ago, TheButcher said:

A low frequency inverter will roll on as if nothing happened.

Almost...there'll be a brief dip in the output voltage as the inverter reacts to pop the voltage up.

 

Chintzy Chinese HF inverters based on the EG8010 drive board (in my case, SWIPower) make a horrid mess of the AC wave with any sort of inductive load, which is partly why they're absolutely horrible for starting any sort of motor.  Let's first read the beautiful promise given by this company...

  swipowersmall.jpg.9a2c49249c26b30fbb4d2dae4fabd405.jpg

I won't mention that it overloads and shuts down under 2,700W.  Contacted them, and they informed me that it was within the 10% tolerance.  So much for the promise.

So at no load, it provides a nice and pretty sine wave output (partial trace due to this being a screengrab from a video I took...not to mention that this is the 20MHz 'tube scope we had for the longest time!):

Untitled.thumb.jpg.74ee5e43931db648e64ba7a4fcdce11f.jpg

How about we try to start a 15A circular saw?  AC output wave turns into this beauty:

1270637683_VIDEO0010_0000004573lowvoltageterriblewaveform.thumb.jpg.671ffdc4b830a6526af365f1b126971e.jpg

The blanks spots aren't the 'scope trace...that's the actual output AC wave.  Notice the AC output voltmeter reading 58v.  It takes about 4 seconds to speed up this circular saw...

 

My next inverter was a PJ "9kw" LF inverter.  Never looking back.

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Really meant that there is no prolonged drop of the output voltage, probably only a moderate notch of the waveform on a LF inverter rather than the complete mess as in my picture.  I have to say that the AC on your HF inverter into an inductive load is a shocker.  At least mine doesn't do that, so something to be thankful for.

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  • 2 weeks later...

Hey Sid, just saw this review of a SUNGOLDPOWER 6000Watt inverter and it showed some video of the inside.  Thought you might be interested even though it looks just like a lot of the Chinese inverters.  

 

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Yeah it looks identical to a Sigineer 6000 watt unit I had installed for a friend in his RV last year.  I was mostly interested in the transformer design and seeing how they laid it out.  I also like the remote control panel but it needs a screen with data to be useful.

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A heck of a lot of these things are copies, of copies, of copies, of copies, right down to the text overlay on the PCBs.  It's completely shameless but also rife.  Occasionally you can follow their evolution like following changes in DNA.  Somewhere, long ago, someone did actually design an inverter.  Ever since most of the manufacturers just pump out copies, with occasional silly changes* that slowly propagate as the updated design is copied, but ultimately their differentiation comes down to price and that means just how bad the components they put in the things can get and not have the inverter burst into flames at initial power up.

*Have you noticed how practically overnight every portable HF inverter has a volt meter / amp meter on it?  That's not by co-incidence.  One manufacturer added it, so all the others did too.  It might seem like a good idea but none of the meters I've seen have any safety considerations at all.  There's no creepage nor clearance and the components sure as hell aren't rated for the voltage so all that is preventing the AC output from appearing on the LV DC side is sheer good luck.

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28 minutes ago, TheButcher said:

A heck of a lot of these things are copies, of copies, of copies, of copies, right down to the text overlay on the PCBs.  It's completely shameless but also rife.  Occasionally you can follow their evolution like following changes in DNA.  Somewhere, long ago, someone did actually design an inverter.  Ever since most of the manufacturers just pump out copies, with occasional silly changes* that slowly propagate as the updated design is copied, but ultimately their differentiation comes down to price and that means just how bad the components they put in the things can get and not have the inverter burst into flames at initial power up.

*Have you noticed how practically overnight every portable HF inverter has a volt meter / amp meter on it?  That's not by co-incidence.  One manufacturer added it, so all the others did too.  It might seem like a good idea but none of the meters I've seen have any safety considerations at all.  There's no creepage nor clearance and the components sure as hell aren't rated for the voltage so all that is preventing the AC output from appearing on the LV DC side is sheer good luck.

I think you summed up most of the Chinese inverter market pretty succinctly 😉.

The "long ago" inverter design reportedly was an older Xantrex inverter (at least so I'm told from a Chinese source).  So there was no originality to start with anyhow...

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38 minutes ago, Sid Genetry Solar said:

I think you summed up most of the Chinese inverter market pretty succinctly 😉.

The "long ago" inverter design reportedly was an older Xantrex inverter (at least so I'm told from a Chinese source).  So there was no originality to start with anyhow...

The Chinese seem to be good at making copies of things already designed. Like the old Russian government did. Want a plane to carry your atomic bomb, copy the B-29. Want a train to pull your goods, copy an ALCo RSD1. Want a supersonic passenger plane, copy Concorde. Want a space shuttle, copy the US Space Shuttle. The Chinese did it first with steam locomotives and have been doing it ever since including also copying the B-29.

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4 hours ago, Sid Genetry Solar said:

The "long ago" inverter design reportedly was an older Xantrex inverter

Interesting to note that a whole lot of those old Xantrex/Trace inverters are still running fine!  My first solar inverter was a Trace 1500w Grid tie.  It worked flawlessly for years (until the house burned down - not because of the inverer before someone asks!) 

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