True Sine Wave
versus
Modified Sine Wave

Inverters come in two different flavors - modified sine wave and true sine wave. But power is power, right? Wrong! There are distinct differences between the two waveforms and, as applied to an RV application, there are places for both. Let's look at the differences between the two different waveforms.

What is a Waveform:

AC electrical power can be displayed on an oscilloscope. It will appear as an S curved rope or line that whips back and forth. From the center line you can measure up to the peak of the hump. This represents voltage. The higher the volts, the higher the hump. It then completes a cycle when it returns to the baseline. How often this happens per second is the frequency, which is measured in hertz. For power used in the United States 60 hertz is the standard frequency and it is determined by how fast the generator spins at your local power plant. This waveform is created by the spinning magnets of a generator's rotor as the magnetic field collapses across the stator windings of the generator. That's why we have those up and downs. The peak voltage is only there for a short time. Much of the time it is either rising to meet that point or falling as it leaves that point, which is determined by the position of the magnet as it crosses each field wire. That's why it's not a steady value. Most generator sets operate the same way. A small portable generator will have 2 field windings in it's rotor. This makes for a small and compact package but requires the engine to turn at 3,600 RPM in order to produce 60 hertz power. A larger RV generator will typically use 4 magnets and only needs to turn 1,800 RPM to produce 60 hertz power. But, these generators are larger and heavier so they're not typically used as portable units. Some generators, such as the popular Onan 7500 watt series are actually not AC generators at all. They are DC generators that run their DC output into an inverter portion within the generator, which then inverts the power to AC current. The advantage of this method is that the generator can vary it's RPM so that it runs slower (and quieter) at low loads and then increases RPM as the loads increase.

Inverter Waveforms:

Inverters do not generate power. Instead they transform power from DC to AC and bump up the voltage. They do this with electronic components rather than spinning magnets. They are solid state with no moving parts, other than a cooling fan or two. The vast majority of inverters are produced in the modified sine wave version. This is the least expensive way to make an inverter. However, the waveform that it makes does not replicate true utility power. It's an attempt to make a "somewhat close" version of it that'll get you by. The quality of this waveform will vary greatly between inverter brands, depending upon the quality of the inverter. Better quality inverters are now available at reasonable prices that are now labeled as true sine wave inverters. The true sine wave inverters still use solid state circuitry to create this waveform but advances in technology now allow that waveform to be equal to utility grade power. So, what's the difference? Following is a graph showing the two waveforms.

Let's look at the true sine wave first because it's what you get at home and with shore power. As the above graph shows, the peak voltage is actually around 170 volts. But, it only touches 170 volts for a brief instant. Electrical equipment needs to see that voltage for a longer time in order to use it. Therefore electrical equipment is designed to run on 120 volts RMS voltage. RMS stands for Root Mean Square and is a true measurement of power. All of your electronic voltmeters and test instruments will look at the RMS when registering. This RMS is measured at a lower point than peak on the waveform. This is done by squaring the value to make sure it is always positive, then averaging it, then taking the square root of the average to make up for having squared it in the first place. The above graph does show a 120 volt RMS sine wave.

However, an inverter has no magnets. Instead, it switches the DC power on and off very rapidly. In the above graph the dashed line shows the modified sine wave. It too, has 120 volts RMS as you can see where the two sine waves cross each other but the peak voltage is only 145 volts. Without magnets there is no gradual curve to the rise and fall of the voltage. Instead, the instant switching on and off gives a flat line and then an instant switch to zero while it rests. Theoretically, this works but it does have it's drawbacks. For one thing, your multimeter is calibrated to function on a true sine wave. If you check your inverter's AC output voltage you will not get an accurate reading unless you have specialized equipment designed to read modified sine wave voltage.

How it Affects Electrical Devices:

Remember, all of the electrical devices made are designed to operate on a true sine wave. As the device grows more sophisticated, it's reliance on a true sine waveform also increases. Therefore, there are some drawbacks to the modified sine wave.

If you are running simple things, such as lighting, you won't really have an issue with modified sine wave. Modified sine wave inverters will power toasters, lights, microwaves, any electrical heating element, as well as most motor loads such as refrigerators, vacuums, etc. Electronics do not fare as well on modified sine wave. Digital clocks will not keep accurate time. Many small battery chargers, such as used on cordless power tools will not function correctly. And any electronic device is subject to failure. The biggest issue is that all major appliances are now becoming heavily interfaced with electronics. While your refrigerator's compressor motor may run fine on modified sine wave, the electronic control panel in the door that controls the water dispenser and ice maker may not like it and will fail. Microwaves ovens will function but the electronic control panel may not hold up that long. Laptop computers will run fine because the inverter is merely powering a battery charger. The battery itself acts as a buffer and runs the laptop on DC power, even when plugged into the AC adaptor. Desktop computers are not in the same category though and you will probably smoke them pretty quick. Modified sine wave inverters will also generate RF noise in the line so you will have issues if you try to power a ham radio with one. Motor speed controllers using triacs won't work properly either. If there is a "wall wart" power supply box plugged into the wall you'll be fine though. Inkjet printer models can be either way on this so check first. Generally, if you see the current transformer on the power supply, you'll be fine.

With today's improvement in inverter technology it's really not that big of a deal to upgrade to a true sine wave inverter. You'll probably find that the life expectancy of your coach's electrical components will improve and save you money in the long run.

Submitted by Mark Quasius - 9/21/06
 

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