Pure or modified sine inverters

[Asross72]

Looking at replacing the inverter in the truck and adding one to the trailer.

Are modified sine wave inverters fine for General RV use, or is it worth upgrading to the true sine wave?

[Ronbo]

I have had both. Most electronics work better on pure sine wave. Clocks keep the right time, run cooler ext. imho worth the extra money.

[skp51443]

A pure sine won't burn up anything you plug into it, a modified sine may or may not cause problems. For the modest price difference I'm for the full sine units.

[RickS]

I agree with Stan and would go with PS inverter. I had a year of problems even with my 5500 Onan MSW generator.....finally changed it out to PSW diesel genset and have not had any problems since. I use Magnum MS2012 inverters in the RV and truck and if had any issues one way or the other could swap out easily.

[Nuke-E]

I'd definitely go PSW. The cost difference is minimal, and you have to think about what you plug in a lot less.

 

One thing that I've found frequently struggling with MSW inverters are capacitive touch screens on tablets, phones, etc. The electrical noise causes them to be somewhat erratic, inevitably causing you to hit the "Buy Now" button or "Exit Navigation" at a very inconvenient time.

 

As far as keeping time, electric clocks work with pure sine inverters OK, but most will drift from the correct time more than they would when grid connected. Some a tad fast, others a tad slower than 60Hz. In three months, both the microwave and my alarm clock have lost about 10 minutes; with the inverter I took out they would gain just a tad.

 

As far as that goes, the old inverter (Xantrex 2500W PSW) is sitting on a shelf. It was decomissioned when the 48V lithium battery bank came on board. PM me if that might be of interest.

[Asross72]

Thanks everyone.

 

Hi David, is it an inverter/charger or just an inverter?

 

I know I want an inverter/charger in the truck, like Gregg puts in his trucks, although when I looked up the one he uses, it appears to be a MSW.

 

For the trailer, it seems to me that an inverter/charger is best as well.

 

I guess now I'm asking what the best setup is, lol. Without going to David's fancy 48V system, that is. Which I've been reading, looks awesome.

 

So, inverter/charger, inverter powering converter or inverter/charger and converter?

Want to put a battery monitor on as well. I hate not knowing my battery state. I had a nice one on the sailboat from blue sea. Worked great.

[Nuke-E]

Hi David, is it an inverter/charger or just an inverter?

It's an inverter charger, but it doesn't detect incoming power so it doesn't charge. It was that way when I bought the rig 5-1/2 years ago, with a separate 3-stage charger right next to it. Over time, I developed a strong preference for running that way, with no transfer switching--even though I bought an inverter/charger, I'm just using the inverter.

 

Evidently there's a small 10:1 transformer that feeds the logic board that fails, but Xantrex wouldn't even look at it for repair. I've been meaning to take a look inside, but that was impossible where it was mounted in the rig (plus I needed it running...), and I haven't bothered since it came out. Maybe I'll pull the cover and see if it's obvious. I also have two remote control panels for it, one brand new.

[oletimer]

David touched on something I found interesting. In the last few years we have had 3 Sharp Carousel Micros. The first two we used a MSW inverter, and everything worked great, but with this last one, in our new 5th with a PSW, the clock more than just drifts, it will loose 1 1/2 to 2 minutes every 24 hours. On shore power it keeps time correctly. Go figure?? We have found however most everything else works better. Dick T

[Nuke-E]

David touched on something I found interesting. In the last few years we have had 3 Sharp Carousel Micros. The first two we used a MSW inverter, and everything worked great, but with this last one, in our new 5th with a PSW, the clock more than just drifts, it will loose 1 1/2 to 2 minutes every 24 hours. On shore power it keeps time correctly. Go figure?? We have found however most everything else works better. Dick T

 

So am I to read that (3 microwaves in a few years) as the clocks were accurate, but microwaves bit the dust prematurely? Haha.

 

I've never put a scope on the output, but I have noticed that certain devices even on pure sine wave inverters expose the fact that "pure" is a relative definition. They are using a bunch of high frequency switching to create the sine wave, which is inherently more noisy than a smoothly spinning generator.

[Yarome]

Something like that may be just a brand/model specific issue. Not all MSW or PSW's are created equal. As with any inverter.. there is no industry standard for benchmarking or rating a specific units output. As such.. some have a much "cleaner" output than others. Frequency rates may vary slightly at various loads on lower end inverters which is probably what is happening with Dicks nuke'r.

[oletimer]

David,

My wife loves that Micro, but when I installed the inverter in my Teton in 2003, I couldn't find a good place for a subpanel, so I split the 50 amp box, and had a 25/30 amp leg for shore power, and one leg for inverter, even though folks in the know, like Jack doesn't recommend that. It worked great for years, until the neutral wire loosened behind the entry plug. I had not isolated all the grounds, and ended up with 240V inside the camper, I was so stupid, I kept turning the power off and on until I fried Microwave, TV, and much more. Our microwave in our S&B in almost new (45years old).

 

Yarome,

I think you're correct about the cheaper inverters, but we do have a good inverter/charger (Magnum) because even though I'm not a big spender, I buy top of the line on important items. I've been told it has something with the way some clocks read the currant/wave?? Guess I'll have to get a watch.

 

Dick T

[Yarome]

but we do have a good inverter/charger (Magnum)

 

Where's John when you need him!?

 

It's likely not an issue with the inverter output then, but I don't know about the 120v run. There might be something along there that is affecting the current frequency, but that's not really in my wheelhouse.

[Jack Mayer]

David, I've scoped a couple of inverters (PSW). They all looked VERY good. I did it under almost max load, and also with a battery bank that was around 50% depleted. Thinking I'd see some "disruption" on that bank....nope. Pretty much a spot on sine wave.

[RandyA]

OK - sorry - but the devil made me write this..............

A decade or more back most electronic devices depended upon transformers to increase or decrease AC voltages to a level needed to power their appendages. Transformers were common in television sets, early computers, phone chargers, etc. Today, such electronic devices rarely depend upon purely inductive transformers for power conversion. They now rely on their own version of internal "inverters" or switching power supplies that operate off of a wide range of input voltages and frequencies - a typical laptop switching power supply will automatically work off of anything from 90 VAC to 200 VAC in the 40 to 70 Hz range. New TV's, phone chargers, etc. with switching power supplies could care less about power coming from a MSW converter or PSW converter. "Dirty Power" is no longer the issue it was once believed to be.

 

A pure sine wave rated at 120 VAC RMS with a frequency of 60 Hz generates a PEAK voltage equal to (approximately) 1.4 times the RMS measurement. The peak voltage is a critical part of the inductive power produced by a transformer. A 1:1 transformer with a bridge rectifier and capacitor output with a 120 volt PSW input will have an (unloaded) output of 173 volts DC. One with a MSW input will have an unloaded output of, well........ 120 volts DC. The BEST you can expect from some MSW inverters is an average voltage equal to approximately 133 VAC, which will translate to 120 volts RMS on an inexpensive voltmeter and look real good for product sales - but the lack of power does not change despite the voltmeter display. Devices like most inexpensive microwave ovens still use transformers and simply cannot produce the same power on a MSW signal as a PSW signal - that is, unless it is one of the newer "inverter technology" microwave ovens and then it could care less.

 

This leaves electric motors - things like refrigeration compressors, fans, synchronous motor devices (like clocks with no independent crystal oscillator) to deal with PSW and peak voltage outputs or MSW output. "Generally Speaking" AC motors will work "OK" on a MSW but WILL NOT produce the same power as a PSW signal would. This means the motor, like in a refrigeration compressor, will need to run longer to produce the same amount of BTU's per hour. The motor may also run at a higher temperature with a MSW inverter (so don't leave the door open too long). This does not necessarily mean that a MSW inverter is bad - it just does not have the same power for an "assumed" voltage as a PSW.

 

I personally run a MSW inverter anytime my truck is disconnected from shore power primarily to run the 120 VAC dorm size refrigerator. It will freeze a water bottle solid or explode a coke can (by freezing) just as easily as it would on a pure sine wave. And, ironically, the compressor runs cool and should last just as long (provided it is not overheated). I also use a MSW inverter to run our auxiliary electric refrigerator in the camper while traveling, power the TV/ Entertainment center, sat dish, coffee maker, Nancy's hair dryer, and keep things like the iPhone and my toothbrush charged when off grid and experience NO noise or noticeable degradation in quality (well, it takes a few seconds longer to make coffee). "If" we use the microwave off of the 3000 watt MSW inverter in the camper it can take twice as long to produce the same amount of energy as a PSW - or two minutes to make a cup of tea compared to the usual 1 minute. I've learned how to wait or use the "PSW inverter generator" rather than the battery powered MSW inverter.

 

I do have a small (300 watt) PSW inverter that can clip to a battery but have truthfully not had the need to use it just because it is PSW. There is no reason to be afraid to use a MSW inverter in your truck or camper - just understand that there are some differences and limitations.

[Nuke-E]

David, I've scoped a couple of inverters (PSW). They all looked VERY good. I did it under almost max load, and also with a battery bank that was around 50% depleted. Thinking I'd see some "disruption" on that bank....nope. Pretty much a spot on sine wave.

That's good to know. I need to hook up to the inverter I have now and see what it looks like. It'll tend to depend on the load though--most I've seen do OK with "nice" loads, like relatively pure inductive and resistive loads. Large half-wave loads (e.g. heating elements that are only using half of the sine wave) as found in some heaters and laser printers, usually tend to make things get a little uglier.

 

 

OK - sorry - but the devil made me write this..............

A decade or more back most electronic devices depended upon transformers to increase or decrease AC voltages to a level needed to power their appendages. Transformers were common in television sets, early computers, phone chargers, etc. Today, such electronic devices rarely depend upon purely inductive transformers for power conversion. They now rely on their own version of internal "inverters" or switching power supplies that operate off of a wide range of input voltages and frequencies - a typical laptop switching power supply will automatically work off of anything from 90 VAC to 200 VAC in the 40 to 70 Hz range. New TV's, phone chargers, etc. with switching power supplies could care less about power coming from a MSW converter or PSW converter. "Dirty Power" is no longer the issue it was once believed to be.

Absolutely. For most of them, it doesn't matter--even the big 12kW charger I was looking at (same basic architecture, but microprocessor controlled) handles something like 90Hz to DC at its input, and the peak voltage is a true peak voltage, i.e. sqrt(2)*RMS for a sine wave.

 

 

A pure sine wave rated at 120 VAC RMS with a frequency of 60 Hz generates a PEAK voltage equal to (approximately) 1.4 times the RMS measurement. The peak voltage is a critical part of the inductive power produced by a transformer. A 1:1 transformer with a bridge rectifier and capacitor output with a 120 volt PSW input will have an (unloaded) output of 173 volts DC. One with a MSW input will have an unloaded output of, well........ 120 volts DC. The BEST you can expect from some MSW inverters is an average voltage equal to approximately 133 VAC, which will translate to 120 volts RMS on an inexpensive voltmeter and look real good for product sales - but the lack of power does not change despite the voltmeter display. Devices like most inexpensive microwave ovens still use transformers and simply cannot produce the same power on a MSW signal as a PSW signal - that is, unless it is one of the newer "inverter technology" microwave ovens and then it could care less.

 

This leaves electric motors - things like refrigeration compressors, fans, synchronous motor devices (like clocks with no independent crystal oscillator) to deal with PSW and peak voltage outputs or MSW output. "Generally Speaking" AC motors will work "OK" on a MSW but WILL NOT produce the same power as a PSW signal would. This means the motor, like in a refrigeration compressor, will need to run longer to produce the same amount of BTU's per hour. The motor may also run at a higher temperature with a MSW inverter (so don't leave the door open too long). This does not necessarily mean that a MSW inverter is bad - it just does not have the same power for an "assumed" voltage as a PSW.

 

What I've found odd here is that the cheapo microwave that was in my RV before the kitchen remodel had no trouble, obviously with its own oscillator, but the Whirlpool Gold series convection/microwave doesn't--fast on the old inverter, slow on this one. I also have an alarm clock that sets itself when you plug it in to the atomic clock in CO, but then uses the 60Hz line frequency to keep time.

 

I personally run a MSW inverter anytime my truck is disconnected from shore power primarily to run the 120 VAC dorm size refrigerator. It will freeze a water bottle solid or explode a coke can (by freezing) just as easily as it would on a pure sine wave. And, ironically, the compressor runs cool and should last just as long (provided it is not overheated). I also use a MSW inverter to run our auxiliary electric refrigerator in the camper while traveling, power the TV/ Entertainment center, sat dish, coffee maker, Nancy's hair dryer, and keep things like the iPhone and my toothbrush charged when off grid and experience NO noise or noticeable degradation in quality (well, it takes a few seconds longer to make coffee). "If" we use the microwave off of the 3000 watt MSW inverter in the camper it can take twice as long to produce the same amount of energy as a PSW - or two minutes to make a cup of tea compared to the usual 1 minute. I've learned how to wait or use the "PSW inverter generator" rather than the battery powered MSW inverter.

 

I do have a small (300 watt) PSW inverter that can clip to a battery but have truthfully not had the need to use it just because it is PSW. There is no reason to be afraid to use a MSW inverter in your truck or camper - just understand that there are some differences and limitations.

 

One of the things that I think tends to trip up some devices with MSW is when there's switching circuitry that attempts to do the switching at the zero crossing on the AC signal. Many solid-state relays do this, and I think that the MSW profile can cause problems with that detection circuitry. I had a little plug-in flashlight from the Red Cross that I plugged in to an inverter outlet in my old RV (MSW) that melted its plastic case over about a 5-hour period. I got two new ones, and ran them side-by-side--the MSW outlet (which checked out in every other respect) consistently melted the things (I was running them until the plastic was charred and smoke was leaking out), while the grid-connected outlets charged them just fine. I found it incredibly ironic that the people who would show up to help after you burn your place down would be the same people that supplied the firestarters...

[RandyA]

Yep - there are differences. The zero crossing detectors - especially those in older Ferro-resonant converters used in RV's, had fits with synchronous generators that used capacitor regulators. The regulator diode would go into avalanche mode just before the normal zero crossing and the converter (and some microwaves) would see it as 120 Hz, ending up doing double time and overheating.

 

A "Pure Sine Wave" is really a natural electrical phenomenon that was here long before we were. Just spin a magnet in a coil of wire and that is what you get. Easy to predict and analyze with Vector Analysis. On the other hand, there is noting natural about creating a square wave from a DC source to cause the inductive collapse of an electromagnetic field necessary to use in an iron core transformer for step-up or step-down. The "Q" of the core is determined by the frequency of the signal and as you alluded 60 Hz transformers have to be bigger and thus heavier for 60 Hz than they would for 120 Hz. Get off frequency and the suckers get really hot!

 

I remember my '53 Studebaker car radio. Mechanical vibrator producing a square wave for a transformer to hop up the voltage for the plate and grid of vacuum tubes. Sweet when parked with your girl up on Dark Ridge - that is until the electron sucker ran the car battery down and you were stuck with a 11:00 curfew you couldn't meet.

 

I guess it was in the late 60's that Heathkit came out with a "new" type of inverter that used two switching transistors to flip-flop the DC voltage and produce an alternating current for a transformer that stepped the switched DC up to what they called 120 VAC. Boy, that was amazing. I had to buy and build one for my boat - why, I guess only because I could. Much more efficient than the dynamotors that used a DC motor to power an AC generator (I still have one and the signal is as pure as the power company - but efficiency is only about 35%). BTW - the military had adopted 600 Hz for AC on ships and airplanes to reduce transformer size, heat and improve efficiency. Plug one of those transformers into 60 Hz and you could fry eggs off the case.

 

Then, we got the bright idea of the "modified sine wave" over a simple square wave. It was really a modified square wave, but that didn't sound as good. It wasn't until the advent of true digital signal processing that we could treat a switched DC current like we did the tunes that came off of a CD. That is when prices began to fall. Before that plotting the sine wave was both expensive and tricky using an extensive array of capacitors and transistors sort of like a cascading waterfall as one transistor after the other went into saturation and cut off - turning on yet another.

 

Yea, with digital it is now much, much less expensive to plot a sine wave from a DC power source. PSW inverter prices have dropped like a rock when compared to the early TrippLite models costing thousands for only a few hundred watts of power - and weighing as much as the battery that powered it.

 

My whole point was that MSW inverters are not all that bad. They do work, and actually work well in the majority of environments - like what we do in our RV's. But as I noted in my last sentence - you've got to understand their differences and limitations so you know where they are best used or not used. But, as Jack noted, with PSW inverters now costing what a MSW inverter did a decade ago they are more affordable even if they are not always necessary to get the job done.

[Nuke-E]

PSW inverters now costing what a MSW inverter did a decade ago they are more affordable even if they are not always necessary to get the job done.

 

That's the important part I think. If it were a significant premium, really think over what you need to use and whether it's necessary. If it's not much of a premium, go PSW and avoid the possible but unlikely problem that something doesn't work right or springs a smoke leak.

 

I gave it some serious thought when making the switch to 48V. Ultimately, I'm going to have "inverter"-type air conditioners running from the battery bank, which rectify to DC and then re-invert at a variable frequency, so MSW would be just fine. But until then, I have dinosaurs on the roof that aren't the most reliable to begin with.

[RandyA]

 

That's the important part I think. If it were a significant premium, really think over what you need to use and whether it's necessary. If it's not much of a premium, go PSW and avoid the possible but unlikely problem that something doesn't work right or springs a smoke leak.

 

I gave it some serious thought when making the switch to 48V. Ultimately, I'm going to have "inverter"-type air conditioners running from the battery bank, which rectify to DC and then re-invert at a variable frequency, so MSW would be just fine. But until then, I have dinosaurs on the roof that aren't the most reliable to begin with.

Funny you mention 48V - that is what my Golf Cart is (six 8V batteries). We usually carry it on the back of the Volvo. I have a surplus commercial 48V 3000 VA UPS that I can run off the GC batteries for what seems like forever. It is PSW UPS/inverter and now wired as an on-off device. I forgot about that earlier....... Also three 72 volt BP solar panels with a 48V Xantrex controller stored in the barn - I had one on top of the GC for a long time. Our personal "Solar Buggy". Never had much luck getting the smoke back into something so it would work again

 

Not trying to hijack the thread - just answer the original questions. For me, MSW works fine.