Inverters; Pure versus Modified Sine Wave

[EmeraldCoastSailor]

I appreciate the spirited discussion. Not sure what I am going to do but I do appreciate learning of everyone's experiences and understanding of the situation.

[Sailmonkey]

Originally Posted by smac999

Who would buy a non sine wave inverter in the year 2014? This isn't the 80's anymore. There is no need for decussion.... Really there isn't. I don't even know why they still make square ones.

I am a cheapskate......when I need the best I'll buy it, but when I need a low cost of AC power to occasionally run a drill or blender then I'll buy the appropriate technology. Case in point the 600 watt inverter I just bought to replace the 6 year old failed unit (msw) was $80. The sine wave version from the same manufacturer was over $300. I don't need to feed my blender a pure sine wave badly enough to justify that cost.


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[Jim Cate]

Originally Posted by smac999

Who would buy a non sine wave inverter in the year 2014? This isn't the 80's anymore. There is no need for decussion.... Really there isn't. I don't even know why they still make square ones.

Hmmm... why do they still make MSW inverters?

They are less expensive.

They will do most of the chores that they are tasked with, as evidenced by numerous real world experiences quoted in this and other threads.

They are often slightly more efficient.

They often have lower idle currents... important to those who leave them on 24/7

There are still lots of folks who will buy them, and that's why they still build them.

You may not wish to discuss this issue, but others do, and the consensus here seems to be that MSW inverters are useful on board cruising boats and represent good value for money.

Cheers,

Jim

[transmitterdan]

Originally Posted by smac999

Who would buy a non sine wave inverter in the year 2014? This isn't the 80's anymore. There is no need for decussion.... Really there isn't. I don't even know why they still make square ones.

The MSW inverters are not square wave. At least they aren't supposed to be. They should have a waveform that has at least the second, third and fourth harmonics nearly zero. This is a simple waveform to make and it can be quite efficient. It has higher peak voltage than a square wave for a given RMS voltage so more accurately simulates a sine wave than does a square wave.

As for efficiency, it's a question of cost. At the price point of most MSW and pure sine inverters they are about the same efficiency. Maybe 2 points more for the MSW over a well designed pure sine. But the cost difference is substantial. Also, the size of a pure sine unit will often be larger and heavier for the same VA rating.

As others have said, use whatever works for you. But please install it correctly and don't become another fire statistic.

[deckofficer]

A bit off topic but since this thread has attracted such expertise, here is my question.

I still have a Trace 2012 that I purchased new for $1500 back in the early 90's. Today a MSW unit of the 2000 watt rating of my old Trace can be picked up for $300 2014 dollars. Spending $1500 in 1992 was a bit of coin for the day. Is this price difference due to early adopter cost, economies of scale, or the battery charging that my old 2012 was set up to do?

It has powered many things over the last 20 years but not a induction cook top. On low volume I could hear a buzz in my consumer electronics but that was tolerable. When the time comes I'll probably go with a pure sine wave.

[transmitterdan]

The transistors and diodes available 20 years ago were not as high power or as efficient as we can get today at a given cost point. So today you can make a MSW inverter a lot smaller and cheaper and with less cooling because efficiency has gone up. Also, the modern "cheap" MSW inverters have taken a bit less care with some of the filtering. If your old unit works I would stick with it. If it's been working for 20 years then that's a sign of a high quality unit. I doubt the $300 one you buy today will be working in 20 years. And yes, some of the cost is due to the battery charge feature. That's about 30% of the cost in the materials.

[sailinglegend]

We have a small cheap MSW 100 watt inverter which is fine for phone chargers etc. but it has blown the Braun toothbrush charger three times. We keep sending them back and they replace them FOC - no receipt needed - they have a date code on them with a two year warranty. Braun admit they have a problem with some MSW inverters.

Now we are forced to use our 2.5 KVA Victron pure sine wave inverter just to charge our toothbrush!!!!!

[rwidman]

Originally Posted by sailinglegend

............. Now we are forced to use our 2.5 KVA Victron pure sine wave inverter just to charge our toothbrush!!!!!

Drugstores now sell electric toothbrushes with a built in, non replaceable battery for $6 or so. The theory is that by the time the battery fails the brush needs to be replaced anyway. This is what we use. Much simpler than trying to recharge a toothbrush, especially on a boat.

[goboatingnow]

Despite some claims to the countary, I attached a real life " modified sine wave " inverter under load,

The issues are as follows

1. total energy available is less then sine wave,

2. peak voltage is less

3. Significant transients and high frequency harmonics

4. Voltage spikes

5. Zero crossing jitter. ( or in ability to accurately determine the zero crossing position )


Hence many devices have problems with these waveforms, being originally designed for pure sine waves, in particular devices with voltages dividers based on capacitors, filters, excessive spike voltages exceeding capacitors etc and devices that detect and use the zero crossing point, ( battery chargers etc ). Any device with a SMPS ( switched mode power supply) can be affected by these waveforms.

IN some cases a better approximation is achieved by inserting more "steps", right up to using enough to completely model a proper sine wave ( as just one of the methods)

Equally some pure sine wave inverters suffer output distortion under load as well

PS

As to the reliability of "older electronics ", todays MTBFs are far higher dollar for dollar and assembly techniques etc have created far higher reliability devices today then ever before. However people start comparing $200 inverters with devices they bought 20 years ago for $1000 !!!. and then winder why the cheapie fails . !!!


dave

[transmitterdan]

Originally Posted by goboatingnow

Despite some claims to the countary, I attached a real life " modified sine wave " inverter under load,

The issues are as follows

1. total energy available is less then sine wave,

Not really. The RMS voltage of the MSW and pure sine are theoretically the same assuming both are designed properly. They will deliver the same energy into a resistive load. Some pure sine units also become badly distorted when driving non-resistive loads.

Originally Posted by goboatingnow

2. peak voltage is less

Yes, that has already been stated and is probably the primary reason some mains powered devices don't like MSW.

Originally Posted by goboatingnow

3. Significant transients and high frequency harmonics

4. Voltage spikes

5. Zero crossing jitter. ( or in ability to accurately determine the zero crossing position )

As you correct stated many (not all) "pure sine wave" units also exhibit voltage spikes and transients. The pure sine wave units are using a high frequency switcher to create the sine wave. Some of these switching transients can make it to the output.

The real problem I have with MSW inverters is in the "whole boat" case. The waveform that Dave has posted is on every AC hot lead in the boat thus it goes everywhere. It has significant high frequency energy due to the fast leading and trailing edges (that's also where it gets its efficiency). Those frequencies (harmonics of 50/60Hz) will often radiate off the non-shielded AC wiring and can affect many systems such as auto-pilot, NMEA 0183, Wi-Fi, depth and wind sensors to name a few. This is the main reason I don't have (nor recommend) MSW as a whole boat inverter. For a small hand held unit it will be ok in most cases. But I can't bring myself to distribute that waveform all over the boat.

[Sailor Bob 350]

Very interesting, but if these are big problems, how come millions of whole-boat MSW inverters have been sold? In fact, I'd be surprised if more than 10% of installed inverters are true sine wave. Perhaps ignorance is bliss for those like me who think our MSW inverters work just fine...

[transmitterdan]

Originally Posted by Sailor Bob 350

Very interesting, but if these are big problems, how come millions of whole-boat MSW inverters have been sold? In fact, I'd be surprised if more than 10% of installed inverters are true sine wave. Perhaps ignorance is bliss for those like me who think our MSW inverters work just fine...

In the beginning all inverters were MSW. At least any that were affordable. Now days the costs have come down but still MSW are often 20% of the price of a pure sine of similar specifications. Thus MSW own the majority of the market.

Another possibility is that problems caused by MSW inverters are either tolerated (due to the lower cost) or not attributed to the inverter. It isn't always obvious that a glitch in a heading sensor is due to the inverter. Don't forget that pure sine units also can cause interference. There is no perfect solution.

[goboatingnow]

Originally Posted by transmitterdan

Not really. The RMS voltage of the MSW and pure sine are theoretically the same assuming both are designed properly. They will deliver the same energy into a resistive load. Some pure sine units also become badly distorted when driving non-resistive loads.

What I meant is that in reality a MSW invertor strives to maintain a consistent RMS . what it doesn't do is maintain a consistent peak voltage, this is often a function of load and battery state.

Microwave ovens are particularly sensitive to peak voltages, ( they are not simple resistive loads.) Hence the invertor is unable to transfer the same energy at lower battery voltages or sometimes very high loads, resulting in poor efficiency and lower microwave operating performance.


comparing the RMS is only really useful in resistive loads.

[socaldmax]

Originally Posted by sailinglegend

We have a small cheap MSW 100 watt inverter which is fine for phone chargers etc. but it has blown the Braun toothbrush charger three times. We keep sending them back and they replace them FOC - no receipt needed - they have a date code on them with a two year warranty. Braun admit they have a problem with some MSW inverters.

Now we are forced to use our 2.5 KVA Victron pure sine wave inverter just to charge our toothbrush!!!!!

I prefer the Sonicare toothbrush from Phillips. I believe it works better and the battery can go for what seems like forever between charges. It also works on both of my MSW inverters.