Thank you. PWM means high frequency.
Big transformer means galvanic isolation which is a good thing. Costs money too.
Inverters High or Low Frequency ?
- Thread starter Noel
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Big transformer means galvanic isolation which is a good thing. Costs money too.
gnubie
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Strictly speaking unless it's an idiot design a 'HF' inverter, ie HV DC chopped to produce sine wave through an inductor, is just as isolated as a 'LF' inverter since there should be isolation at the HV DC production stage. In several countries, Australia included, it's illegal to sell an inverter that is not isolated (it took a couple of deaths before they were banned unfortunately).
That said there are unwise designs that tie mid points to ground because they've decided to save a few bucks by not designing a proper single phase inverter for the 120V market and just reuse an existing '230 split phase design' but with lower voltage on the HV DC side to save money with reduced inventory costs (recent thread about Reliable inverter) and probably stupid designs that just use an inductor based boost stage to produce HV DC too.
That said there are unwise designs that tie mid points to ground because they've decided to save a few bucks by not designing a proper single phase inverter for the 120V market and just reuse an existing '230 split phase design' but with lower voltage on the HV DC side to save money with reduced inventory costs (recent thread about Reliable inverter) and probably stupid designs that just use an inductor based boost stage to produce HV DC too.
If you can't ground the "Neutral" terminal of an inverter designed to sold in the USA, then that says everything you need to know about that inverter.
I am unwilling to believe there is anything the Chinese won't do if they can make a buck and get away with it.
You can buy ANY quality of product you want in China. From excellent all the way to likely to kill you. Caveat Emptor was never more true.
I am unwilling to believe there is anything the Chinese won't do if they can make a buck and get away with it.
You can buy ANY quality of product you want in China. From excellent all the way to likely to kill you. Caveat Emptor was never more true.
Definitely a LF unit, I thought when you were looking for impressive surge stats that's what you were after, my mistake.
Very definitely 49 lbs, my aching back can testify to that
I don't think I said anything about simple square or modified sine inverters, as I said the Victron has a PWM-controlled FET bridge right off the battery.
I think there is a terminology issue, you define LF as switching at AC frequency i.e. 60 Hz or some low multiple of that, and HF as anything that uses PWM. I'm not sure that's a widely accepted definition. Is there a standard definition here? If there's a reference I'd appreciate it.
I'll have to find the reference I'm thinking of but I thought there was a distinction between PWM switching large currents at low voltage direct from the battery (i.e. as Victron does), vs. first boosting to high voltage DC and then PWM switching there. The first has much larger FETs due to switching such high current, and as a result a lower switching frequency to keep switching losses down. Switching frequency in the 10 kHz range. The second switches high voltage at low current thus smaller FETs, smaller magnetics, higher switching frequency in the 100s of kHz to MHz range. That's what I meant by LF vs HF, but I certainly good be wrong there.
I don't know what all the limiting factors are on surge capability in these designs but I'm guessing thermal losses in the transformer core could be some of it, and if so then a large mass transformer will take longer to heat up and thus have longer duration longer surge than a HF inverter with small low mass magnetics.
I'll look into this some more, I am curious. I'd appreciate any references on this topology difference with respect to surge capability.
gnubie
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Any time anyone talks about LF inverters they are typically talking about current generation devices where the sine wave drive for the transformer is produced with PWM. I know there are people that get confused by the fact that 'HF' and 'LF' both use PWM at 10+ kHz so I try to refer to HF inverters as HF direct, ie they do their PWM directly on the output terminals (via an inductor).
In a low frequency inverter there is one switching point, the FETs driving the transformer (yes, there are exceptions to this where there is an intermediate boost stage but not at the sort of power levels we are talking about on this forum). This is usually 10+ kHz. In a high frequency inverter there are two. The first stage that takes the DC supply in and produces 200-400VDC. This stage runs at 100+ kHz. That HV DC is then PWMed at 10kHz to 50kHz to produce the output sine directly.
'HF' and 'LF' do not inherently have more or less surge capability. For a HF inverter it comes down to how much capacitance there is in the HV DC supply and LF comes down to being able to tap the transformers magenetic store. Either way once those precious milliseconds run out the output voltage collapses unless the energy is replaced. HF designs are usually encheapened to the point where they only just do the job and have no headroom but a decent design will do the job just as well.
If you want to see a LF inverter that can't do the job, take a look at atpowerjoke powerjack. Those things are lucky to have 1/4 of their claimed specs and in some cases their heatsinks aren't actually heatsinks in the normal sense. They are just solid blocks of aluminium (no fins, really, no fins) designed (use term as generously as possible) act as thermal mass to let their crap devices work long enough to fool people into thinking they are usable.
In a low frequency inverter there is one switching point, the FETs driving the transformer (yes, there are exceptions to this where there is an intermediate boost stage but not at the sort of power levels we are talking about on this forum). This is usually 10+ kHz. In a high frequency inverter there are two. The first stage that takes the DC supply in and produces 200-400VDC. This stage runs at 100+ kHz. That HV DC is then PWMed at 10kHz to 50kHz to produce the output sine directly.
'HF' and 'LF' do not inherently have more or less surge capability. For a HF inverter it comes down to how much capacitance there is in the HV DC supply and LF comes down to being able to tap the transformers magenetic store. Either way once those precious milliseconds run out the output voltage collapses unless the energy is replaced. HF designs are usually encheapened to the point where they only just do the job and have no headroom but a decent design will do the job just as well.
If you want to see a LF inverter that can't do the job, take a look at at
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The simplest way I would explain the difference is this: HF inverter = inverter who uses a small light ferrite transformer working at high frequency, LF inverter = inverter who uses a big heavy iron transformer working at line frequency.
How does Sigineer do 300% surge for 20 seconds (not milliseconds).
Since I have only seen that on LF inverters I assumed it was inherent with the heavy toroid design.
But my in depth knowledge of inverters wouldn’t fill a thimble ?. Just trying to learn.
Electric Personality
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So at the low end of the market (read: cheap) what would be the best line of cheap 2K inverters? Microwave and hair dryer are the only serious loads.
I think that question is best asked elsewhere, lets try to keep this thread on topic, its a pretty productive discussion so far.
You can see what will recommends on his website. And/or search in the beginners section, or inverter section of this website. Giandel and Reliable/Wzrelb are two of the more popular cheap high frequency inverters. Aims is one of the more popular cheaper low frequency options. Can't comment personally on the quality of any of these.
I've got a 5kW Reliable that has been under test for a while now (as part of this thing I'm doing). It's handled everything I threw at it for the time being, including a washing machine, microwave oven, etc. I'll test some bigger loads like a pump soon, but I also have a soft starter for those in any case.
I'm hoping that we (meaning you all who are educated enough to have an informed opinion) can determine some agreed upon terminology to distinguish between the two classes of inverters (that up until now we have tended to refer to as HF vs LF).
There clearly is some meaningful difference between the two types in form and in function "LF" (heavy, large, big ass transformer, high surge current and duration) and "HF" (smaller, lighter, often cheaper, often lower idle(?), limited surge rating).
Can we discuss this, I am under the impression that what are generally called low frequency inverters on this forum (Victron Multiplus, Samlex Evo, etc) tend to have significantly higher idle power consumption than comparably sized high frequency inverters. This is an impression that I first got from the opinions/advice of others (so in that sense, I'm just reflecting what I've heard), but also seems to hold true when comparing against datasheets.
You are the first person I've come across who says HF inverters have higher idle power consumption. Can you explain this, or provide a reference or example for me to learn more?
Does anyone else have any input on this?
edit: I suppose a third option would be that there is no correlation between idle power consumption and HF/LF
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The explaination I gave is historically accurate.
However low frequency inverters have a reputation for large surge capability (due to the ferroresonant transformers). Subsequent marketing people gladly slap the term low frequency on high frequency inverters that include transformers, because of customer expectations.
I am sure you can still buy true low frequency inverters just like you can still buy tube audio amplifiers and turntables. And for much the same reason.
Modern (as in designed in the 21st century) inverters all use high frequency PWM to convert DC voltage into AC voltage because it is more efficient and creates less electrical noise and heat.
Biggest difference (quality issues aside) I have seen is does the inverter use an isolation transformer between the DC power supplies and the AC output or not?
The higher quality inverters use isolation transformers because they are lower noise and electrically safer. They also don't push components to their limits and as a result have adequate margin to handle surge loads. It is not the transformer that makes this surge capability happen it is conservative electronic design.
A transformer has to drop to zero volts 120 times per second in order to output 60 Hz AC. It is not the electronic equivalent of a flywheel that keeps things turning after the input goes away.
A ferroresonant transformer IS the equivalent of a flywheel, but I would be surprised if many inverter manufacturers are still using them.
However low frequency inverters have a reputation for large surge capability (due to the ferroresonant transformers). Subsequent marketing people gladly slap the term low frequency on high frequency inverters that include transformers, because of customer expectations.
I am sure you can still buy true low frequency inverters just like you can still buy tube audio amplifiers and turntables. And for much the same reason.
Modern (as in designed in the 21st century) inverters all use high frequency PWM to convert DC voltage into AC voltage because it is more efficient and creates less electrical noise and heat.
Biggest difference (quality issues aside) I have seen is does the inverter use an isolation transformer between the DC power supplies and the AC output or not?
The higher quality inverters use isolation transformers because they are lower noise and electrically safer. They also don't push components to their limits and as a result have adequate margin to handle surge loads. It is not the transformer that makes this surge capability happen it is conservative electronic design.
A transformer has to drop to zero volts 120 times per second in order to output 60 Hz AC. It is not the electronic equivalent of a flywheel that keeps things turning after the input goes away.
A ferroresonant transformer IS the equivalent of a flywheel, but I would be surprised if many inverter manufacturers are still using them.
Thanks. Most of this makes good sense to me except the 3rd paragraph on surge. I would think that thermal effects are more important than electromagnetic energy storage for surge capability. For example the Victron Multiplus 3000 does 2400W continuous but 6000W surge for 2 minutes. I'm sure that isn't electromagnetic storage whether capacitive or inductive, my bet is that it is how long it can go without overheating the transformer core or the big FETs or both.
And my hypothesis on why the Victron-type inverters (which most seem to refer to as LF, with large heavy magnetics, but still PWM) have better surge is as follows:
- they PWM switch very high current and low voltage directly from the DC source
- they need large heavy transformers to handle the high current
- those large heavy transformers take longer to heat up than the small magnetics in high(er) frequency inverters which convert to a couple hundred volts DC first then PWM switch at the output
As I said, it's a hypothesis not a theory at this point
What do you call an inverter that uses a big heavy iron transformer working at 20 kHz not line frequency? I.e. a Victron Multiplus.
I pointed out above that the Sigineer site says they are LF inverters... and they weigh ~50lbs
The transformer doesn't work at that frequency, the PWM feeding it may, but I can assure you a big iron transformer like that will not work at 20 kHz.
