High frequency versus low inverters

I’d appreciate knowing or being pointed to a resource that explains the difference between a high and low frequency inverter? I’m trying to understand the pros and cons.

Thanks

Bluedog225 said:

I’d appreciate knowing or being pointed to a resource that explains the difference between a high and low frequency inverter? I’m trying to understand the pros and cons.

Thanks

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There are tons of details we could go into but it boils down to this:

High-frequency inverters are generally lower-priced, lighter in weight, and can handle brief surges of 2x their wattage rating.
Low-frequency inverters are generally more expensive, weigh more, and can handle brief surges of 3x their wattage rating.

If you have power tools, Air conditioners, or other motor-driven loads.... you should seriously consider low-frequency inverters.\
Edit: Corrected the following sentence.
If you have 'simple' loads without big motors, a high frequency might be the right choice. Microwave ovens can be a problem on high-frequency inverters as well.... but I have also seen the work just fine on high-frequency inverters.

Bluedog225 said:

I’d appreciate knowing or being pointed to a resource that explains the difference between a high and low frequency inverter? I’m trying to understand the pros and cons.

Thanks

Click to expand...

I have experience with only one of each, but based on that one experience, a low-frequency inverter can have a significantly higher if for draw and can put out much more heat when unloaded than a high-frequency inverter.

So my gut feel is that a pair of 120V high-frequency inverters is likely to be more efficient that a split-phase low-frequency inverter, especially for low levels of consumption (but again, take that gut feel with a big grain of salt because if my very limited sample size).

FilterGuy said:

There are tons of details we could go into but it boils down to this:

High-frequency inverters are generally lower-priced, lighter in weight, and can handle brief surges of 2x their wattage rating.
Low-frequency inverters are generally more expensive, weigh more, and can handle brief surges of 3x their wattage rating.

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A critical difference is the definition of brief.

My HF inverters can only handle a 2x surge for milliseconds. Not useful at all.

My Sigineer LF inverter can handle a 3x surge for 20 seconds. Enough time to start dual AC motors from locked status on a boat lift. Very useful.

Edit:

After reviewing my manuals I see that my 48v, 3000W WZRELB HF inverter can do 6000W for only 5 milliseconds.

My 48v, 3000W Sigineer LF inverter can do 9000W for 20 seconds.

That is a huge difference.

And the Sigineer was more expensive and weighs MUCH more weighing in at 53 pounds as opposed to 11 pounds for the HF inverter.

Here is a thread about when I bought the Sigineer:


Edit: Note that this particular Sigineer is 120v only, same as the WZRELB.

Great info. Thanks all! I’ve got some reading to do.

JoeHam said:

A critical difference is the definition of brief.

My HF inverters can only handle a 2x surge for milliseconds. Not useful at all.

My Sigineer LF inverter can handle a 3x surge for 20 seconds. Enough time to start dual AC motors from locked status on a boat lift. Very useful.

Edit:

After reviewing my manuals I see that my 48v, 3000W WZRELB HF inverter can do 6000W for only 5 milliseconds.

My 48v, 3000W Sigineer LF inverter can do 9000W for 20 seconds.

That is a huge difference.

And the Sigineer was more expensive and weighs MUCH more weighing in at 53 pounds as opposed to 11 pounds for the HF inverter.

Here is a thread about when I bought the Sigineer:

Sigineer 48v inverter. 3kw, 120 ac

Anyone have experience with or a reasoned opinion on this inverter? https://www.sigineer.com/product/3000-watt-inverter-charger-48-volt-110v-120vac-pure-sine-wave/ The price is decent and I like that it can go up to 9000 watts for 20 seconds supposedly. I need to start/run dual 120v 3/4 HP...

diysolarforum.com

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Funny, those are the precise two inverters I have experience with (Sigineer 6kW LF and WZRELB 3kW HF).

The Sigineer is split-phase while the WZRELB is only single-phase 120V, so a true comparison would be needed against two 120V HF inverters that can be synced / paired.

But if you compare a pair of 6kW inverters against a single Sigineer 6kW LF inverter, I suspect efficiency of the HF pair is much better at low load levels and the 9kW loads that need more 5ms of startup surge becomes vanishingly small/rare.

Heck, even a pair of synced 9kW HF inverters probably delivers better efficiency than that Sigineer 6kW LF inverter for low/modest load levels…

Lifetime/reliability is an entirely different criterion, however, and HF inverters have the reputation of not lasting as long as LF inverters…

That was my next question regarding longevity. For no rational reason I suspected the LF was more robust.

fafrd said:

Lifetime/reliability is an entirely different criterion, however, and HF inverters have the reputation of not lasting as long as LF inverters…

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That is certainly the reputation, but is that a fundamental difference between LF and HF? It could be that the low end of the LF market is trying to be as low price as possible and therefore are more likely to cut corners on the design and components. Are these units tarnishing the whole LF market? (I really don't know).

While I suspect the LF inverters may last longer I will note that my WZRELB HF is 6 years old and I have not treated it well.

I have under DC voltage faulted it multiple times.

I also had it AC overload shut down multiple times trying to run the boathouse motors one at a time which sometimes worked, sometimes not.

Despite that mistreatment it lives on today.

A surge spec with no time duration specified is a useless spec.

A one msecs surge spec is not really good for much of anything, maybe the initial surge current of a cold incandescent light bulb or a power supply with simple rectifier/filter cap with no inrush current limiter to initially charge the filter cap when switched on (poor power factor design).

A typical single phase induction motor startup surge duration is 400 msecs so look for surge current capability for half a second in time. This is where HF inverters fall short,.

Many of the cheap inverter do not have specifications, they have 'as good as' or 'as much as' statements. This is marketing and Chinese products are gettng quite good at deceptive specmanship.

A LF hybrid inverter is single stage with PWM MOSFET switching battery voltage to a large LF power transformer. The transformer is most of the inverter weight, and component expense. They are inherently bidirectional requiring no immediate controller intervention adjustment to handle a momentary backfeed surge current.

A HF inverter is two stages. First is HF battery DC to high voltage DC converter with MOSFET's battery side switching and possibly output MOSFET or IGBT HV synchronous DC rectifiers. Second stage has another set of MOSFET's or IGBT's for PWM sinewave generation. The weak link is first stage that cannot take surge current well without saturating its small HF ferrite transformer. When HF transformer hard saturates from current surge the input MOSFET's current shoots to destructive levels within a fraction of msec making it difficult for any overload protection circuit to be fast enough to detect overload and save the MOSFET's from destruction.

Hybrid HF inverters first stage must be bidirectional adding another layer of complexity. HF DC to DC first stage converter do not change power direction instantly and switchover must be a controlled process. Many of the cheap hybrid HF inverters don't even bother trying to do this. They just put a second HV DC to battery buck converter circuit in parallel to handle reverse power to charge batteries and absorb backfeed surges to protect the first stage forward converter. They also may just use rectifier diodes to take AC input to HV DC node for battery charging which gives charging a poor AC power factor which is something you should be concerned about if charging from an AC generator. All in one HF hybrid inverter often inject their PV power to the HV DC node. Their efficiency number on their spec sheet is based on PV power injected to HV DC point. Battery to AC efficiency going through both stages is much less but they don't talk about that.

A 240/120vac split phase HF inverter is essentially two 120 vac HF inverter in series. A LF 240/120 vac inverter is usually a center tapped single LF transformer with 120vac to the center tap of transfomer secondary. The LF inverter can usually share more of inverter power to a single 120vac leg, limited by wire gauge used in LF transformer secondary side. There are options where two 120vac HF or two 120 vac LF inverters are used in series for 240/120vac split phase, requiring purchase of two 120vac inverters. Both HF and LF inverters may have paralleling options to increase power capablity.

HF inverter are much lighter in weight but have a great deal more electrical power semiconductor devices. More power stressed parts normally means reduced reliability. LF freq inverter has few to no semiconductors on AC side of large transformer which gives some isolation protection to grid surge voltages and grid lightning strikes. Most failures on HF inverters are battery side MOSFET's due to overload. IGBT for second stage HV PWM sinewave creation are quite rugged and if they get damaged it is usually from a grid input lightning strike since they are electrically connected to AC input.

no sure on your application, but 1 fundamental difference also is that LF inverters can be ac coupled and will charge your batteries with excess solar / wind power ... HF cannot... also as pointed out, LF surge ratings are pretty much what your battery system, input mosfets etc can handle, a lot closer to the grid for impedance...

Some HF inverters can do AC coupling to GT inverters but I would not recommend it. SolArk HF inverter does AC coupling but highly recommends putting GT inverters in through AC gen input. This allows them to open generator input relay quickly to dump GT inverters if backfeed power gets out of control endangering the first stage HV DC converter stage.

ok, ill correct the above, they ac couple easily...

RCinFLA said:

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Hi @RCinFLA ,

Do you think I can trust the overload and surge capacity in the spec below? Product is a 3.5KW SNADI hybrid offgrid (China made). It looks to be a copy of Axpert VMII inverter. There are other brands of the same design based on the component placement I've seen. Thanks for your inputs.

"Overload Protection -> 5S@>=150% load"

I'll take that to mean it should not trip off inverter until load gets above at least 150% rated max inverter power for 5 seconds.


"Surge Capablity -> 2x * rated power for 5 seconds"

Guess that assumes the overload protection did not trip at 150% in 5 seconds. How much do you trust self-protection when max overload protection trip is open ended?

Just an example of sloppy specs.

If I put together the two specs, protection may not happen at 200% loading at 5 seconds and going greater than 200% load for greater than 5 seconds may damage inverter.

Translation: Inverter does not have good self protection against overloading,

HF inverters do not like poor power factor loads. A single phase induction motor will have power factor between 0.5 and 0.9 depending on mechanical load on motor. The poor power factor is inductive and will have a negative current flow during a portion of the AC cycle. HF inverters cannot tolerate this well. It can cause HV DC to exceed its design ratings. Electrolytic HV DC filter cap vulnerable component. HF inverters first stage, battery to HV DC converter, cannot quickly reverse power flow to push reverse-surge current to batteries.

Second item that can cause overload damage is saturation of ferrite transformer in first stage battery DC to HV DC converter. When its core saturates, due to high overload, the driving MOSFETs' currents shoots up to destructive levels. Blown MOSFET's is a common failure mode.

Don't go by wattage overload, go by AC current overload based on V*A load. 30A max for less than 5 seconds.