RV Electricity – What’s a modified/stepped inverter?

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By Mike Sokol

Dear Mike,
I own a 2005 Winnebago Vectra that has a 2000-watt Dimension Inverter. I need to decide whether to repair ($400 to $800) or get a new one (2000-watt Magnum $1100) or (2000-watt Dimension exact replacement $1900). I am not sure what goes wrong with them, but maybe repairing would be OK? And what’s the difference between a pure-sine and a modified-sine inverter? Thanks in advance. —Ralph Meraz 

Dear Ralph,
First of all, I would like everyone to review my previous article on general inverter theory here. That’s so I won’t have to repeat myself like “Groundhog Day” (what a great and dark movie). Oh, you can also listen to this great song about Signs (not Sines) by the Five Man Electrical Band here. But I digress…

To the first part of your question. I don’t think I would spend $400 to $800 repairing a 14-year old inverter when I can buy a new one (with more modern technology) for $1,100 or so. Don’t throw good money after bad (as they say). Inverter design has come a long way in the last decade with much more digital controls to not only make cleaner power, but also to protect itself from overloads and such. Plus the newer ones will be more efficient, giving you longer battery life.

Making Waves

RV Electricity – What's a modified/stepped inverter?Now, to the second part of your question about pure-sine and modified-sine wave inverters. This is best explained with a diagram (on the left). And since I like to draw diagrams, you’re in luck. If you look at the top waveform labeled “Pure Sine Wave” you’ll see what the power company supplies to you. If you listen to 120-volts AC over a loudspeaker (and I have), it would sound like a very pure tone. In fact, it’s right around B-flat (60 Hz) on a bass guitar if you happen to have one laying around.

All modern electrical appliances including motors and digital power supplies are designed around a 60 Hz (50 Hz in Europe) pure-sine waveform without any harmonics. Any waveform other than a sine wave create harmonic energy which sounds like a buzz when you listen to it. And motors (especially refrigerator and air conditioner compressors) don’t like these harmonics, because that’s extra energy not used to spin the motor – it just causes things to heat up. More on that in a moment.

RV Electricity – What's a modified/stepped inverter?

RV Electricity – What's a modified/stepped inverter?When the first 12-volt DC to 120-volt AC inverters were designed, they output something called a square wave (seen above diagram). The square wave inverter was cheap to build, but you can see that the waveform sort of hammered rapidly up and down, not the smooth curve of a pure-sine wave. And with every hammer there was a burst of extra harmonic energy. In fact, you could hear it hammering the filaments in a light bulb back in the day. And you never wanted to use this square wave inverter to power a motor because now you would be hammering the windings, which sounded like a buzz and would cause the wires to rub against each other, eventually destroying the insulation and shorting out the motor. See the picture above right. Ouch!

RV Electricity – What's a modified/stepped inverter?

So the inverter manufactures came up with a modified- or stepped-sine wave inverter, seen in waveform graphic above. Not quite as clean (or expensive to build) as a pure-sine wave inverter, but not as dirty as a square-wave inverter, so most modern electronics should be fine being powered by a stepped/modified-sine wave inverter. The Stepped/Modified Sine Wave inverter is the most common (and least expensive) on the market. These work great as long as you understand their limitations.

RV Electricity – What's a modified/stepped inverter?

But it wasn’t long before companies like Xantrex and Honda began to build cost-effective pure-sine wave inverters that rival (and sometimes surpass) the cleanliness of 60-Hz power supplied by the Electric Company. And if you want the longest life out of your residential refrigerator compressor or have sensitive electronics (like a computer) that needs power, then you really should use a pure-sine wave inverter to power it, not a stepped-wave inverter.

Final Analysis 

RV Electricity – What's a modified/stepped inverter?If you’re only running a bunch of non-motorized consumer electronics (televisions, microwaves, etc…) and not a residential refrigerator, then an inexpensive stepped/modified-wave inverter should be OK. But it’s always best to use a modern pure-sine wave inverter if you can afford it.

 

UPDATE: Watch my webcast

RV Electricity – What's a modified/stepped inverter?BTW – If anyone missed my one-hour interview on the New England RV Dealers Association live talk show, RVing in New England, on Wednesday night, March 6, you can watch/listen to it here. Lots of important and useful information for RVers, whether newbies or veterans.

Let’s play safe out there….

RV Electricity – What's a modified/stepped inverter?

 

RV Electricity – What's a modified/stepped inverter?

Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

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Bob Love

Hey, Mike, I really appreciate your wealth of information. I have a related question on electrical supplies that my undergraduate physics never answered. Nearly everyone today uses multiple transformers to run electronics and charge batteries. The question — do they (or some types?) still use electricity when the output is disconnected? Should all be disconnected when not in use, As I remember transformers, two circuits are joined by coils with different numbers of wraps — yes, I realize this is a gross simplification, but the best I can do. Does the primary still draw when the secondary circuit is broken? Sometimes they buzz, and I assume still draw. Others are silent and cool down.

Tom Herd

I understand the concept of inverters, what about generators? Does a construction-type generator put out a clean sine-wave, or do I need an inverter generator? Noise isn’t an issue, as I’m looking for a back-up power supply for use if the power goes out, and would be running a fridge and freezer, along with a few lights and maybe a microwave.

Thanks, in advance.

Scott Macklin

Very nice article Mike. You have great knowledge and know how to bring your discussions down to our level.

Jeannie

Most modern desktop computers have PFC (Power Factor Correction) PSUs (Power Supply Units) that will not work well, if at all with modified sine waves. It’s best to assume that all computers need to be run on pure sine wave power.

UPSes (Uninterruptible Power Supplies), often used to allow computers to run long enough after a power outage occurs to safely save one’s data, also come with options for pure sine wave or modified sine wave. The recommendations for which to use are the same as for inverters.

Pure sine wave inverters (and UPSes) cost more than ones that output a modified sine wave but are well worth the cost considering the cost of the damage modified sine wave units could cause. Better safe than sorry.