Be warned Modified Sine Wave Inverters

[TaliaB]

Got a callout yesterday from a client for advice. When i got to his premises he explained that he couldn't understand his router packed up three months ago he replaced it and now his ONT(optical network terminal)stopped working and he again had no internet access. It immediately struck me i had a similar experience last year with a different client. He was using a Mecer IVR-1200MPPT modified sine wave inverter for LS that damaged his router. This client is using a Lalela trolley inverter also modified sine wave inverter. Just a warning to all that if purchasing these "cheaper" type of inverters that you are putting all your electronic devices connected  at risk. Why? Let me try to explain and i am sure @Modinawill weigh in on this. It is called THD (total harmonic distortion) and it can be as high as 40%+ on these devices. It's the total amount of distortion in an electrical signal caused by the presence of harmonics within that signal. Without getting too technical, harmonics are multiples (2x, 3x, 4x, etc.) of the signal's fundamental frequency - the lowest frequency of a repeating waveform and measured in %. Just to make a comparison depending on where and how your power is produced, the load on the power grid, temperature, the age of your home's wiring, and many other factors, the THD in your home's power may vary usually, it's about 2% to 3% on any given day for most municipal locations. Stay away from modified sine wave inverters for sensitive electronics like routers and ont's rather use DC-DC inverters or then true sine wave inverters where THD is at least much lower. 

Edited March 1, 2023 by TaliaB

[P1000]

THD will not influence any of the devices mentioned - they all have SMPSs, so your modified sine gets converted to DC in the first step, and then it does not matter. Pretty much all affordable consumer UPSs for computers are modified sine and they work perfectly.

Edited March 1, 2023 by P1000

[TaliaB]

21 minutes ago, P1000 said:

THD will not influence any of the devices mentioned - they all have SMPSs, so your modified sine gets converted to DC in the first step, and then it does not matter. Pretty much all affordable consumer UPSs for computers are modified sine and they work perfectly.

On many low power power supplies and in these 2 instances smps were not use but TPS supplies. I tend to totally disagree with your statement. Modified sine wave will long term damage electronic devices using smps, tps or transformer based supplies. The fast rising and falling edges create noise that will be coupled to the DC busses. It is also a stress riser for capacitors and silicon components since there is a resulting current spike. That means it is shortening the life of the SMPS while generating noisier DC.

Edited March 1, 2023 by TaliaB

[P1000]

TPS is not an accepted acronym. You are talking about linear power supply. I will believe that when I see it - they are much more expensive to make and ship, I have not seen access points or routers with one of these in a long time.

5 minutes ago, TaliaB said:

It is also a stress riser for capacitors and silicon components since there is a resulting current spike.

You have the same issue with a sine wave -> rectifier -> cap, which is why active PFC became a requirement in the EU.

 

[TaliaB]

TPS- transformerless power supplies, capacitor(x rated) dropper supply if you like they are becoming the norm

[BrettB]

6 minutes ago, TaliaB said:

TPS- transformerless power supplies, capacitor(x rated) dropper supply if you like they are becoming the norm

They are becoming far too common! They work, they are cheap,  but man they are not safe! They heavily rely on the sine waveform from an AC supply to function properly. The sudden steep rise of the modified sine waveform actually causes a much higher voltage on output side than its rated for. 

A well designed switching DC supply shouldn’t be affected by modified sine wave. But how is the average consumer supposed to know what will with modified sine wave. With that said, I agree with you, stay clear unless you know exactly what you are plugging into it. 

[JustinSchoeman]

29 minutes ago, TaliaB said:

TPS- transformerless power supplies, capacitor(x rated) dropper supply if you like they are becoming the norm

I sincerely doubt you will ever see one on a router or ONT though. Power demands are too high, and the isolation requirements mean that you can't use them with standard DC jacks.

[Bobster.]

22 minutes ago, BrettB said:

They are becoming far too common! They work, they are cheap,  but man they are not safe! They heavily rely on the sine waveform from an AC supply to function properly.

And that's too much to expect, is it? 

[WannabeSolarSparky]

I have 4 mecer ups's (all modified sinewave - 3000va, 2000va, 1000va, 800va ) and have been running 3 of them for almost 6 years now with zero damage to any of my electronics devices, pc's, routers, tv's etc.
Maybe I have just been lucky

[Modina]

Wow, within 2 hours a lot of comments.... you guys are obviously not retired like I am.  Retired people are busy and don't have so much time to read and respond to messages.  LOL.  Some interesting comments.  This is how I see it:

@TaliaB is absolutely correct that modified sine wave inverters have very poor THD and high orders of harmonics.  These high frequency components are not doing anyone any good.  These frequencies radiate (using your house wiring, extension cords, or whatever wiring you use to get the juice to your load), they cause EMI/RFI noise that can affect audio, visual and communication  processes.  What is also REALLY bad is that these high frequency components can cause extra heating in motors and many other wound components such as transformers.  Extra heating firstly wastes energy that must come from somewhere (your batteries) and it will quite possibly lead to failures or at least reduce the life-expectancy of equipment.  Just don't ever use a modified Sine inverter to power motors.  That will simply have a bad outcome.

Transformerless power supplies use a high voltage bipolar capacitor to act as a serial impedance that will drop the supply voltage.  Typically they use 220nF to 680nF depending on how much current is required.  During design, one assumes that the input is a (true) sinewave of known frequency.   The capacitor will have an Impedance Z = 1 / [2*pi*f*C).  So a 220nF capacitor at 50Hz will have a impedance of 14 468 ohms.  As this impedance is reactive, it will effect the power factor but will NOT waste power as heat (unlike a resistor).  If the applied AC is no longer a proper sinewave, but this "thing" that looks like a square wave with a dead period in between, those high freq. harmonics will reduce the capacitor's impedance and put the typical zener diode that follows the cap under strain.  If the device contains a small microprocessor it might only need 1 or 2mA and use a 220nF cap or less.  Those circuits I would expect to be OK.  If the device has to drive a high number of LEDS (for instance a 7-segment display) or drive a relay (a geyser timer) then very large capacitors are needed and components are more likely to be stressed and over-stressed by modified sinewave inverters.   Note that many, many low wattage (up to about 3W) LEDs on the SA market, use transformerless supplies.

I do however agree with @P1000argument.  Just about ALL modern electronics use proper switch-mode power supplies (SMPS).  Here, the AC is taken through a common mode filter and X and Y-capacitors and some protective fuse/current limit resistor, or NTC resistor and then immediately rectified with a full-wave rectifier and smoothed.  Cheap, low grade products will normally have the common-mode filter and X and Y-caps removed to save costs.  Often provision is made for these components on the PCB, but not populated.  Note that these filter components work in both directions.  They protect the device to a degree, of any incoming spikes/noise and they also block switching noise from the SMPS to climb onto the feeding wires.  I have a Lead Acid charger that doesn't have these filter components, and my PC speakers immediately start humming if I switch that charger on.

Generally speaking, a SMPS should be fine with a modified square wave input.  So why did those routers blow up?  Well, firstly I think the router is still working but that the power supply packed up.  My hunch is that the power supply might have an X-capacitor after a fuse.  The X-capacitor sits right across L & N.  It sees the high harmonic content and becomes warm under the collar...  It's reactive impedance drops and might lead to the fuse aging and finally failing.  It is ironic that a cheap SMPS that doesn't use the input filter components might be much happier with modified sinewaves than the properly designed SMPS that includes the input filter.  It would be cheap and easy to repair such a SMPS, but that is no good for the end-consumer.

If my above hunch is correct, it would mean that low-power SMPS are more prone to "blow" than higher powered SMPS.  The X-capacitor doesn't need to be increased for a high-power SMPS.  The higher reactive current due to the high frequency components over the X-cap, will be proportionally larger in a small power supply.  The fuse will be dimensioned for the overall power take-up required and thus more likely to fail.

I do not want to be an alarmist, but high frequency components on a less-than-decent X-cap can even lead to a fire hazard.  A true X-cap is marked as such.  There are strict requirements for these components.  Just popping in a normal high-voltage cap to save a few cents is not a good idea.  Stelzner is a US company selling "dirty" electricity "filters".  These filters are nothing else than a large capacitor that is plugged over L-N.  Some have caught fire.  I would never use them.  Noise should be reduced with LC (inductor/capacitor) combinations and not just with a capacitor alone.  

Modified sine-wave inverters are cheap and nasty.  But they are the norm for low wattage inverters.  The electric noise is a problem and as you might have seen me say elsewhere, have potential negative consequences on our health. 
 

[Gnome]

4 hours ago, P1000 said:

THD will not influence any of the devices mentioned - they all have SMPSs, so your modified sine gets converted to DC in the first step, and then it does not matter.

What he said.  A switch mode power supply's first step is a rectifier.  And in those cases you can run it on any frequency down from DC and up to near 1kHz and it would make absolutely no difference.  As eluded to some very old PFC circuits can get damaged by non-sine wave inputs but small AC-DC converters and power supplies like those made by Meanwell, etc. are exempt so they simply don't put PFC on those.  Mostly older computer power supplies and TVs used those and they have long since gone away.

3 hours ago, TaliaB said:

TPS- transformerless power supplies, capacitor(x rated) dropper supply if you like they are becoming the norm

This is not legal in most electronics.  It has no isolation.  There are few exceptions where the device is completely enclosed and there is no possibility of interfacing with it.  ie. it is wifi only.  But if it allows something to be plugged into it and it isn't isolated it is not legal to sell in South Africa, US or EU.  It is easier to list the countries where that would be legal.

Also "transformerless power supplies" is a marketing wank name.  I've not heard it before.  I would refer to the topology used or non-isolated.  The problem with using the term "transformerless" is that marketing wanks like to use that in relation to inverter also and there aren't any inverters that don't use transformers.

[Gnome]

1 hour ago, Modina said:

My hunch is that the power supply might have an X-capacitor after a fuse.  The X-capacitor sits right across L & N.  It sees the high harmonic content and becomes warm under the collar...  It's reactive impedance drops and might lead to the fuse aging and finally failing.  It is ironic that a cheap SMPS that doesn't use the input filter components might be much happier with modified sinewaves than the properly designed SMPS that includes the input filter.  It would be cheap and easy to repair such a SMPS, but that is no good for the end-consumer.

If my above hunch is correct, it would mean that low-power SMPS are more prone to "blow" than higher powered SMPS.  The X-capacitor doesn't need to be increased for a high-power SMPS.  The higher reactive current due to the high frequency components over the X-cap, will be proportionally larger in a small power supply.  The fuse will be dimensioned for the overall power take-up required and thus more likely to fail.

I doubt this is true.  THD in AC is measured expecting a sine wave.  So obviously modified sine wave will have high THD.  It doesn't look anything like a sine wave.  But to somehow claim that because of that it has some crazy rogue output doesn't ring true to what I've observed from most tear downs, even of really low quality "inverters".

The wave form basically becomes a complete square wave but I don't buy that it would be a problem for a capacitor.

When you read the word "total harmonic distortion", it creates this idea of crazy spikes.  Instead it is a measure of how it measures against the expected sine wave.

It seems much more likely that when the power goes on and off from the utility there are surges on the line and/or the brown-out.  Both things that really cheap UPS and modified sine wave inverters don't filter nor deal with.

Edited March 1, 2023 by Gnome

[PowerUser]

4 hours ago, P1000 said:

 

THD will not influence any of the devices mentioned - they all have SMPSs, so your modified sine gets converted to DC in the first step, and then it does not matter. Pretty much all affordable consumer UPSs for computers are modified sine and they work perfectly.

 

Yes, I have also been running multiple computer devices on modified sine wave inverter for very long time without any issues. 
 

The company has also been running the whole server room with all servers and devices on modified sine wave inverter for years without issues. 
 

Many other companies do the same. In fact larger percentage of businesses have modified sine wave inverters, compared to those with pure sine wave inverters.

 

 

 

Edited March 1, 2023 by PowerUser

[Bobster.]

36 minutes ago, Gnome said:

It seems much more likely that when the power goes on and off from the utility there are surges on the line and/or the brown-out.  Both things that really cheap UPS and modified sine wave inverters don't filter nor deal with.

I was thinking about this. But me can't even spel "enjineer". But why would a surge be more likely to reach the router's power supply if there is an inverter in the path? Or would the failure have happened anyway?

If there's 3 months between the two failures, and the inverter was present all that time, it doesn't sound like the inverter introduced a problem. Sometimes we mistake correlation for causation. 

Like other commenters on this thread, I have run a mod sine inverter for over a year without anything blowing up. One difference between then and now is that then load shedding was not sustained at 3 times a day as it is now.

[Bobster.]

1 hour ago, Modina said:

Wow, within 2 hours a lot of comments.... you guys are obviously not retired like I am.  Retired people are busy and don't have so much time to read and respond to messages.

[Gnome]

48 minutes ago, Bobster. said:

I was thinking about this. But me can't even spel "enjineer". But why would a surge be more likely to reach the router's power supply if there is an inverter in the path? Or would the failure have happened anyway?

Well the inverter isn't running from inverter power 24/7 (such inverters/UPS do exist, called double conversion)

Instead the inverter has a change over switch.  When the grid loses power, the inverter switches a relay between utility and inverter.  So while the power is on from Eskom, the router power supply is connected to the grid.  Once the grid connection failure occurs it takes around 25ms for most inverters to detect and switch.  That is essentially a full 50hz sine wave cycle.  So during this time, it is considered a brown out, power was missing for a full cycle.  In addition to that, transients on the network due to the loss of power are fully transmitted to all consumers on that leg of the grid.  So if there are surges due to the power loss, it ripples across that part of the grid.  Because they are basically disconnecting your area from the grid.  Any inductors would discharge back into the grid at that point.  It is how a boost converter works.  You power up an inductor and then disconnect it, it then discharges current back causing a momentary spike in voltage.  Done in a controlled manner you can boost voltage.  Done in an uncontrolled manner you have a surge generator.

Insurance companies have indicated load shedding is causing a significant upturn in surge related claims, so this isn't some hypothetical I'm making up, there is data to suggest surges are experienced during grid loss and reconnection to the grid.

Only double conversion UPS are protected against surges and the instantaneous loss of power caused by a brown out.  All other UPS are completely susceptible to surges and to some degree brown outs (those that offer brown-out protection have a reaction time greater than 0ms and are thus at least partially susceptible to brown out).

48 minutes ago, Bobster. said:

Sometimes we mistake correlation for causation.

I highly suspect that is the problem.

Edited March 1, 2023 by Gnome

[TaliaB]

Thanks guys for all the inputs. I stand firm on this view, i will not promote nor install any modified sine wave inverters at any of my clients after all they rely on your expertise as electrician and or installer to make the right decision and to guide the client on best options available regardless of price. At the end of the day you need to walk away and know your advice to your client was correct and sound.

[Gnome]

2 minutes ago, TaliaB said:

Thanks guys for all the inputs. I stand firm on this view, i will not promote nor install any modified sine wave inverters at any of my clients after all they rely on your expertise as electrician and or installer to make the right decision and to guide the client on best options available regardless of price. At the end of the day you need to walk away and know your advice to your client was correct and sound.

Don't get me wrong, installing a "modified sine wave" inverter for someone who is a layman is a bad idea.  Like really bad.  Because there are a LOT of devices that WILL get damaged by them.  Not to mention that modified sine wave inverters are the lowest of low quality products, they offer no extra protection.

That should be the reason not to install them.  But I have my doubts as to the validity that they damage SMPSs.  Old school 50Hz transformer/linear supplies, vacuum cleaner, fridge, microwave, etc. couldn't agree more that running that on your "modified sine wave" is just asking for it to break down.  I shudder to think what happens to a microwave on a "modified sine wave" inverter.  That idea scares me.

I've installed inverters now for a number of friends and family and basically they don't walk around thinking to themselves.  Hey, you know what, I shouldn't plug my microwave into this plug.  Or I shouldn't put everything on all at once.  Sure later when it all goes wrong, they are like "oh yeah".  But they aren't the problem there, in my opinion.  They are reacting how a normal human would react.  Your install should behave such that anything that goes into a plug will not break your install or the device or you are doing it wrong (in my opinion).  If not you are setting your customer up for failure and yourself.

I did those installs so it would at worst shutdown with an overload and restart or trip a breaker.  And I would sincerely hope that installers think the same about it.

[Bobster.]

I must thank all the experts here for sharing their knowledge & thoughts. 

[Bobster.]

1 hour ago, Gnome said:

When the grid loses power, the inverter switches a relay between utility and inverter.  So while the power is on from Eskom, the router power supply is connected to the grid.  Once the grid connection failure occurs it takes around 25ms for most inverters to detect and switch.  That is essentially a full 50hz sine wave cycle.  So during this time, it is considered a brown out, power was missing for a full cycle.  In addition to that, transients on the network due to the loss of power are fully transmitted to all consumers on that leg of the grid.  So if there are surges due to the power loss, it ripples across that part of the grid.  Because they are basically disconnecting your area from the grid.  Any inductors would discharge back into the grid at that point.  It is how a boost converter works.  You power up an inductor and then disconnect it, it then discharges current back causing a momentary spike in voltage.  Done in a controlled manner you can boost voltage.  Done in an uncontrolled manner you have a surge generator.

Aha! This is an explanation for a problem I heard about a few years ago: DStv decoders burning out because of load shedding. 

I think it also explains the surge protection devices that were added at install time on the AC input to my Goodwe inverter. 

[Bobster.]

1 hour ago, Gnome said:

Any inductors would discharge back into the grid at that point.

Ok. And since all houses have inductors in them, all our houses are generating surges into all our neighbours houses? 

Does this explain the advice given to turn stuff off during shedding and wait a few minutes after restoration before turning back on (a bit of a bugger at 4:30 in the morning)? 

[TaliaB]

1 hour ago, Bobster. said:

Ok. And since all houses have inductors in them, all our houses are generating surges into all our neighbours houses? 

Does this explain the advice given to turn stuff off during shedding and wait a few minutes after restoration before turning back on (a bit of a bugger at 4:30 in the morning)? 

It is called inductor kickback,  but a flyback diode is connected across an inductor to eliminate kickback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. It is used in circuits in which inductive loads are controlled by switches, and in switching power supplies and inverters.

Edited March 1, 2023 by TaliaB