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Two Wave Forms: Modified Sine Wave and Pure Sine Wave

The Inverter is a device that changes DC (direct current) current into AC (alternating current) current. Alternating current comes in many wave forms. The first type of AC current was a square wave form inverter. A really crude wave form compared to todayís true sine wave also known as pure sine wave. However in the mid point there is a modified wave which is just a step above the square but not as good as the true sine wave. A True sine wave inverter is used for fine or delicate instruments such as used in a hospital or lab work where you donít want the power floating. But has turned in to the standard inverter because it is the closest thing to grid power and in a lot of cases better then grid power.

DC voltage does not have a wave form. It is a direct line there and a direct line back so it is called Direct Current. The sine wave of AC voltage goes above the zero voltage mark to a set positive voltage and then goes below the zero voltage mark to a set negative voltage. So it is called Alternating Current because it is alternating from a positive to a negative.

Looking at the two inverter wave forms below. The power inverter wave form starts at zero voltage going up, it then starts back down passes zero to the negative peak and then on to the next zero voltage going up. This is called one cycle or a hertz. Normally there are 60 of these cycles or 60 hertz per second in a sine wave. Some inverter sine wave forms might be 50 cycles or they could be 60 or greater or even float. This odd wave form and cycle count confuses clocks, bread makers and battery chargers. It is normally found in a cheap inverter.



The picture above is an AC inverter modified sine wave form. If you notice it has a step up, step across, and step down form. It is like the crude square wave form because it clips the tops and bottoms off the wave making a flat and is either strait up or down when going to the positive or negative side ends. It just has an extra step at zero voltage. These flat spots in the wave form is where the hammering sound comes from in electric motors and the noise you hear in cheap radios. An electric motor or compressor will not have as long of a life using this type of inverter wave form.


This picture is from a true sine wave power inverter. It is the type of wave form you would see from the city electric grid. It is what that you find in your home. This is the type of wave form you find in a True sine wave inverter. Smooth and rounded all the way. Motors will run quiet and radios will not have the interference like with a modified sine wave inverter. Just looking at the two wave forms you can see how much better things would run with a true sine wave inverter.



Two Types of Inverters: Modified Sine Wave and Pure Sine Wave

There are two general types of power inverters: true-sine wave or modified-sine wave (square wave). True-sine wave inverters produce power that is either identical or sometimes slightly better to power from the public utility power grid system. The power wave when viewed through an oscilloscope is a smooth sine wave.

Modified-sine wave and square wave inverters are the most common types of power inverters on the market. Modified-sine wave power inverters produce a power wave that is sufficient for most devices. The power wave is not exactly the same as electricity from the power grid. It has a wave form that appears as a choppy squared-off wave when viewed through an oscilloscope.

What does that mean to the everyday user? Not much. Most household electrical devices will run perfectly fine on either type of wave form. Most of our customers who are using a power inverter to run a laptop, a/c cell phone charger, fan, or camera find that a modified-sine wave power inverter that operates through the cigarette lighter socket the easiest to use. We usually suggest choosing power inverters that are rated under 300 watts when using the 12-volt cigarette lighter socket found in most vehicles. We suggest this because after reaching 300 watts of draw on the inverter, the fuses in your car will begin to blow.

The problem with wave form only comes into play when specialized pieces of equipment need to be powered. Here are a few devices which could have problems when they are connected to an inverter producing a modified-sine wave signal: oxygen concentrators, fax machines, laser printers, high voltage cordless tool chargers, equipment with variable speed motors, electric shavers, and garage door openers.

There are a few other applications -- high-end audio video units, plasma displays, gaming systems, and certain scientific testing equipment -- for which true-sine wave is not usually required. Even so, these applications can usually benefit from the improved clarity of the electrical signal produced by a true-sine wave power inverter. Users of these particular items have usually spent a lot of money to achieve optimal results from their equipment, and it would be a shame to have a cheaper modified-sine wave signal cause inaccurate readings on a piece of scientific equipment. It would be equally disheartening to have small distortion lines appear on a $3000 plasma TV because the user saved $250.00 by buying a modified-sine wave power inverter.

It is also important to understand that there is no way to upgrade or clean a modified-sine wave signal. If your item does not work on a modified-sine wave inverter, you will need to purchase a new true-sine wave power inverter. We often recommend that users on a tight budget purchase only enough true-sine wave power to run required equipment and purchase a less expensive modified-sine wave inverter to run the rest of the load. The true-sine wave power inverter is often used to power only the audio video loads in RV applications. The rest of the RV's electrical loads are often powered by a larger modified-sine wave power inverter.

Many people are surprised at the overall improvement in signal quality when using inverters on audio/video applications. They notice that there are fewer distortions and few if any interference lines. While we don't recommend true-sine wave inverters to most of our customers, we do advise customers with no budgetary concerns to choose a true-sine wave product. They can then rest assured that their inverter will be able to handle anything they plug into it.

Many stores do not carry true-sine wave power inverters because the price is often significantly higher than their modified-sine cousins -- usually two to five times more. Generally, expect to pay $200 to $3,000 for pure-sine wave inverters depending upon how many output watts are needed.


Inductive Load


If connecting with inductive loads (e.g. Compressor, Pump, old CRT TV, Refrigerator, Ice conditioner, Air conditioner, Relays,Fluorescent lamp, Vacuum cleaner), please choose inverter whose rate power is 3-7 times higher than the load's rate power. For example, for a 150w refrigerator, please choose a 1000w inverter or higher; for a 800w air conditioner, please choose 2500w inverter or higher.
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