Skip to main content ↑ ↓
Topic: Problem with Modified Sine Wave Inverters (Read 629 times) previous topic - next topic

Problem with Modified Sine Wave Inverters

Found this article in my archives that addressed why it may be a good idea to replace modified sine wave inverters with true sine wave inverters.

Re: Problem with Modified Sine Wave Inverters

Reply #1 –
Barry, I do not agree with the notion expressed in this article that all "modified" sine wave inverters will produce the same result when a capacitor is included on the power in. Here is an article which I wrote explaining the range of modified sine wave inverters available today.

Inverters from 20 years ago were heavy, tranformer based, and very expensive. They produced high distortion 120 VAC.
Inverters to day are mostly solid state and readily available. Inverters are designated as "modified sine" or "pure sine", however, inverters are available with many different wave forms from a single step spiked wave ($100), a 2 step square wave ($200), a 3 step square ($300), 4 step square ($400), 5 step square ($500), as well as high distortion "pure sine" ($750) and finally a low distortion "pure sine" ($1000). (Dollars in brackets are for a 2000 watt inverter). Unfortunately, "modified sine wave" can be put on any inverter even if it just produces a square wave. The important thing to understand is that looking at "total harmonic distortion"  will provide some information to help you purchase a good inverter. Anything from 3% distortion which is excellent, to 8% distortion which is very good, to 20% distortion which is good, to 100% distortion which is not good. If you want to really know what you are buying, look at the technical information and look for "total harmonic distortion (THD)" even on "pure sine wave" inverters. The Freedom 25 inverter/converter in my 1996 Foretravel was a triple step square wave inverter rated at 45% THD (I scoped it to see). This is a decent, reliable inverter/converter, which was quite expensive in 1996. I replaced it with a Xantrex 1800 "pure sine" inverter with THD of 3%.

If you are considering purchasing an inverter which does not show "Total Harmonic Distortion", buyer beware, there is a reason why it is not shown. Also if it is from China and only costs $100 do not expect a quality "pure sine" inverter.



[Corrected "sign" to "sine" to help for future searches - Michelle]
Wyatt
96 U320 40 WTFE, build 4943
84 Toyota Supra towd
2015 Jeep Wrangler towd
Victoria, BC, Canada

Re: Problem with Modified Sine Wave Inverters

Reply #2 –
Wyatt,  wicked helpful information for my circumstance.  I was under the impression at any modified sine inverter was bad; and any pure sine was good. From what you say that may not be so.

The 1993 Fortravel I bought had sat for 2 years. Its ME2012 modified sine inverter is only 3 years old.  All the stuff I have been reading says pure sine of die  :)    Well I really didn't want to chuck this inverter and based on your info above I may not have to.  Its spec is <5% THD.  Magnum Energy ME2012 - 2000 Watt 12 Volt Off-Grid Inverter

Am I maybe ok to charge laptops, run TV's microwave off of this device by chance?  Thoughts?  Thank you  -Nick
1993 U300 40' Limited Edition
Build 4183

Re: Problem with Modified Sine Wave Inverters

Reply #3 –
Magnum Energy inverters that I have seen are excellent, and they are expensive.
It is difficult for me to believe that a "Modified Sign Wave" inverter could have THD <5%, they are normally THD of 15% to 50%.
If you have an oscilloscope available, scope it with no load and with a load on it.
A friend of mine has a Magnum Energy 3000 watt inverter which is labeled "pure sign" and it has THD<8%.
I scoped his and mine and we could see the difference in the waveform between my 3% and his 8%.
He has never had a problem and he runs everything with it.

If you want to sell your Magnum inverter, how much to you want for it?
Wyatt
96 U320 40 WTFE, build 4943
84 Toyota Supra towd
2015 Jeep Wrangler towd
Victoria, BC, Canada

Re: Problem with Modified Sine Wave Inverters

Reply #4 –
I still have the original Freedom inverter/charger on my coach and have used it to power flat screen TV's, a laptop, phones, and other stuff with out any problem.  I once plugged the coach into an outlet that I thought was 120, but was really 240 and the only bad thing that happened to the inverter was a small blown fuse in the inverter.  For awhile though, I thought I needed a new inverter, until I finally found the blown fuse.
Jerry Whiteaker former owner 96 U270  36' #4831 Austin,TX-Owner Mods LCD TV w/front cabinet rebuild - LCD TV bedroom - Dual Central AC, either can cool coach w 30 amp - Skylights at roof AC openings - Drop ceiling for ducting of AC - Shower skylight white gelcoat/wood/epoxy frame - Air Springs/Shocks replaced - 2014 CRV - 8K Home Solar - Chevy Volt

Re: Problem with Modified Sine Wave Inverters

Reply #5 –
You got lucky.  I did the same thing and fried the Heart, the microwave and the tv.

Re: Problem with Modified Sine Wave Inverters

Reply #6 –
Thanks Wyatt.  Appreciate having a better understanding about inverters.

Our personal experience with our Freedom 25 was our 120v remote switch was failing and replacements were failing.  Looking inside, I could see a melted capacitor looking component.  Remote switch manufacturer finally figured out that the device would not work on our modified sine wave. 

Then a few years later our Freedom 25 failed at the same time our two Tripp Lite surge protectors (that had more than MOV's inside) completely burned black inside their metal box.  We were fortunate that Tripp Lite did not cause anything around them to catch on fire.  Tripp Lite surge protectors had been in service for many years.

That said, we really don't know if shore power was bad, Tripp Lite failed or Freedom 25 failed.  Since both Tripp Lite failed, we figured it was not Tripp Lite that initiated the damage.  But since surge protectors may destroy themselves while protecting downstream circuits, maybe we had a shore power surge that did the damage.  But that was the day we converted to true sine wave and have not looked back.

Re: Problem with Modified Sine Wave Inverters

Reply #7 –
When arriving at a campground which I have not been at before, I will plug in an inexpensive watt meter which displays voltage, amps and watts before plugging in power cord. I have seen some surprises, especially in Mexico.
Wyatt
96 U320 40 WTFE, build 4943
84 Toyota Supra towd
2015 Jeep Wrangler towd
Victoria, BC, Canada
 

Re: Problem with Modified Sine Wave Inverters

Reply #8 –
Quote from: P. Wyatt Sabourin –
When arriving at a campground which I have not been at before, I will plug in an inexpensive watt meter which displays voltage, amps and watts before plugging in power cord.
X2.  I use the product linked below, which is not inexpensive, but does the job.  I consider it cheap insurance.  8)

RV Campground Voltage Tester Protects Your RV
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"