Low Frequency Inverters

[Archie]

A handful of people around here recommend low frequency inverters, I wasn't aware of the distinction before joining this forum, and most companies don't seem to push this as a marketing point and most reviews don't mention it. And I've found very few clearly marked low frequency inverters at the small (<2-3kw) end of the spectrum.

I'm wondering, are LF inverters one of those things where the saying "If you don't know if you need one, you probably don't need one" applies?

Can someone ELI5 (explain it like I'm 5 years old), the benefits of LF inverters / drawbacks of HF inverters?

What I have gathered so far:

Pros of LF inverters:

1. Longevity / reliability
2. Higher peak/surge power rating (~200% for HF >300% for LF)
3. ..?

Cons of LF inverters:

1. Cost
2. Size/Weight
3. ..?

Am I missing anything, are there loads that need or greatly benefit from low frequency, in the same way that some loads require a pure sine wave? Is LF something the average small system builder should even worry about?

How about efficiency and power consumption at idle, any differences?

If these are the main differences, it seems that priorities would have a lot to do with how much sense it makes to go LF. If you are building an off grid home and designing a system to last 20+ years, where size and weight aren't a big issue and the cost of a LF inverter relative to your overall budget is somewhat small, and your AC demands are higher, LF probably makes a lot more sense. If on the other hand you are building a small mobile system where size and weight matter and where the cost of the inverter will be a big chunk of your budget, LF might make less sense.

Low Frequency inverters tend to handle reactive loads much better. What is a reactive load? One of the best examples are motors or anything that has a motor. That would include refrigerators, saws, compressors, etc. Also any inductive or capacitive load. Any product that has a big intial power surge when starting up will be handled much better by a low frequency inverter. These LF inveters have large transformers that store energy in a magnetic field so when there is a demand for high energy (i.e. motor start-up) the magnetic field around the transformer and/or capacitors in the inverter can step in to help supply the energy needs. So the bottom line is an LF inverter will handle surge loads much better than an HF inverter and ,all else being equal, will be more reliable and last longer in a surge environment. The big transformer in these units is why they weigh so much.

 

[Archie]

Not true as HF inverters definitely use transformers. On youtube look at some info on Reliable (WZRELB) 8000W inverters. A number of people have opened them up and done comparisons. Dave Poz and another Canadian I cannot recall at this point. You will see that the old ones had 9 small transformers and the new one has 4 much larger transformers.

My child like understanding (I trust someone will correct me here if I am wrong ) is that HF inverters also provide "cleaner" electricity although I have not found an exhaustive study comparing the sine waves on a 'scope. Many electronics prefer a better sine wave and are less efficient with "dirty" waves.

Not really true. Yes, the WZRELB has some seemingly big transformers but not when you compare them to a Magna sine. Also, HF puts all kinds of voltage spikes on the output that either goes out to your loads (not good) or may be filtered out in better (i.e. more expensive) HF units. The bigger the transformer, in general, the better if you drive any kind of reactive load. The Magna Sine sports <5% THD.

 

[Dzl]

Not really true. Yes, the WZRELB has some seemingly big transformers but not when you compare them to a Magna sine. Also, HF puts all kinds of voltage spikes on the output that either goes out to your loads (not good) or may be filtered out in better (i.e. more expensive) HF units. The bigger the transformer, in general, the better if you drive any kind of reactive load. The Magna Sine sports <5% THD.

Can you define/explain the significance of THD for lay-folk like me

 

[Archie]

Can you define/explain the significance of THD for lay-folk like me

THD = Total Harmonic Distortion. The less the better. Basically it is a measure of noise on the power line. Ideally, there would be no noise, just a pure sine wave. Noise on the power line tends to be hard on electronics and can shorten life span. So anything powered by the inverter especially items with micro-controllers or other high tech semiconductors will be subject to lifespan issues. Also, HF noise on the line may interfere with other electronics nearby such as cell phone, TVs, WiFI, radios, etc.

 

[Red Squirrel]

How do the LF ones actually produce the sine wave? Are they actually driving transistors in their linear region? I know for HF they essentially use PWM at high frequency then smooth it out which creates the sine wave.

 

[BiduleOhm]

Today they use mosfets driven with PWM. But the thing is that the big iron transformer is a nice filter so you have far less noise than with HF inverters.

In the old days, and for low power, they used to drive BJTs linearly; no noise but the losses are huge of course.

It's actually the same differences between a class D and a class AB audio amplifier.

I've explained both HF and LF typical topologies in this post.

 

[Red Squirrel]

Oh ok so they still have a high frequency component then? Just that one does the boost with the transformer? I guess by sending the high frequency PWM signal through a big chunky iron core transformer it basically helps filter out the high frequencies and you end up with the clean sine wave while the HF ones are using inductors and capacitors for the filtering?

 

[BiduleOhm]

Yes, exactly.

The transformer acts like a very big inductor and filters a lot of the HF noise.

The HF inverters use LC low-pass filters but of course the inductors are a lot smaller so they can't filter the noise as well as the transformer of the LF ones.

 

[kernel]

Today they use mosfets driven with PWM. But the thing is that the big iron transformer is a nice filter so you have far less noise than with HF inverters.

In the old days, and for low power, they used to drive BJTs linearly; no noise but the losses are huge of course.

It's actually the same differences between a class D and a class AB audio amplifier.

I've explained both HF and LF typical topologies in this post.

Reportedly you can feed an audio amp 60hz amd power tools amd stuff.....

 

[BiduleOhm]

If it's a powerful enough amp (if not you'll not have enough voltage) then yes, it'll work. But the amp not being designed for this may not like [at all] some loads (like the inductive ones for example).

 

[kernel]

Yes, exactly.

The transformer acts like a very big inductor and filters a lot of the HF noise.

The HF inverters use LC low-pass filters but of course the inductors are a lot smaller so they can't filter the noise as well as the transformer of the LF ones.

This is what i dont get. The magnas, outbacks, zantrex etc, bread and butter most heroic capable amd reliable RE inverters of all time are using hf topology and uuuge transformers, an outback is basically a heat sink, a scad of giant capacitors and the biggest transformers in any consumer electronics i know of aside from an old sma smartformer....
A couple circuit boards and jacks round out the device. Thing is 90% heat sink amd transformer.

 

[BiduleOhm]

I'm not sure what you don't get, can you explain please?

 

[kernel]

Its like saying that hf inverters are having a transformer deficit causing them to be inferior.

 

[BiduleOhm]

They are inferior in the noise domain (and surge capability), but they are superior in other domains, like weight and cost for example.

 

[Weez]

I don't have much knowledge about inverters but I'll share to you as I'm using LF inverters for almost 3 years now. I'm not professional and still much lack knowledge about inverters. What I know about LF (based of my experience using China's inverter SUOER for model PL5KVA [2.4KW] )...

1. LF inverter (LFI) is very powerful to drives extremely high surge power electric tools such as arc welding, circular saw and so on. HF inverter (HFI) also can do this job but it's life span will be shorter due burn or overheat MOSFETs.
2. Higher surge power than HFI (3 times peak power of it's actual power).
2. More power consumption in no loads/idle/standby mode compare to HFI. But my LFI is only draw about 90W~110W in a day (full standby mode, not in power saving mode). Some LFI (especially EI transformer) consumes more power up to more than 1KW whole day in standby mode.
3. LFI signal wave (frequency) is less "clean" than HFI (measure/determine by THD: Total Harmonic Distortion). Clearly, HFI has more efficient and "clean" signal wave especially when it supply power to small sensitive devices such as mobile phones, laptop, speakers and so on. Some said that this "clean" wave able to prolong small device's battery life span and it's electronic components inside due to less signal wave interference.
4. LFI has more magnetic field that is produced by huge transformer inside it. So never place any devices which is sensitive to magnetic near LF inverter. Many manufacturers stated this in their manual/guidance book. LFI for UPS (Uninterruptible Power Supply) usually has EI type transformer (I used to have this type LF inverter with 2 massive EI transformers, it's quite hefty!) This EI type transformer has stronger magnetic field than LFI which is has toroidal type transformer. I can hear some "high tone" sound inside my previous inverter (EI type). I expected the transformer make this sound but this is less sound inside the toroidal ones. My previous LFI (EI type) has 2 cooling fans running constantly due to higher temperature (toroidal always has cooler temperature). It's very annoying! So I made up a long "exhaust" tube attached from the inverter itself to the outside to make it more silent. What a waste to build that useless stuff! But when it comes to power supply, LFI with EI type transformer is much better and stable than toroidal ones. And LFI with toroidal type is vice versa. When using high surge power tools like arc welding or circular saw, power supply will be (almost each time turning on the tools) interrupted in certain situation that will make lamps "blinking" and some sensitive devices like mobile phone or laptop will turn on, off and on again. That's quite annoying. But this never happened with EI type transformer. That's why UPS inverter usually (maybe always) using EI transformer to avoid "interruption" power supply. But for HFI, maybe there's still magnetic field occurs but not as strong as LFI.

In my opinion, there are many advantages to use LFI than HFI. The only drawback is the price. LFI always expensive than HFI. Minor drawback is idle/standby mode power consumption. But this can be solved by using higher/maximum capacity of battery storage. For my current LFI (48V) I'm using 48V/400AH lead acid batteries. 400AH is the max capacity for my current LFI. Another aspect to take count is the type of it's transformer. If your priority is for uninterrupted power supply, better choose for EI type and if your priority is just for "normal" usage, go for toroidal type. Besides, toroidal ones has cooler temperature due to less magnetic field occurred inside the transformer itself when turn on. Hope this can help. ?