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Learn About Inverters


An inverter is an electronic device which inverts DC energy (The type of energy found in batteries) into AC energy (The type of energy that is produced by the power company and is found in your home) Household appliances such as refrigerators, TVs, lighting, stereos, computer etc., all run off of AC electricity. 


Modern DC to AC inverters are very reliable, quiet, and require virtually no maintenance. There are two different types of DC to AC inverters in common use today. The first type of inverter is known as a modified sine wave inverter. This type of inverter is very high in efficiency and produces a waveform which is an approximation of the pure sine wave waveform that is produced by the utility company. 


Modern modified sine wave inverters come in two varieties.


1. High frequency conversion units which is typical of the design that you will find in inverters that are manufactured overseas in countries like Taiwan and China. 


High frequency units take the incoming 12 Volts DC and will step up that voltage to approximately 200 volts DC through a high frequency DC to DC converter circuit and then will take the 200 Volts and will wave shape it into a modified sine wave using a using a device called a high voltage H-bridge. The high voltage H-bridge is basically a group of field effect transistors that are arranged in such a way as to form the necessary half cycles that create the modified sine wave at the 60 Hz frequency required for US appliances.


By utilizing high frequency, the need for a large iron core output transformer is eliminated and much smaller transformers can be used. As a result of this, high frequency inverters tend to be much lighter but do have a lower surge capacity because they lack the fly wheel effect found in heavy iron core output transformer based inverters. 


The technology was originally developed for the space program where weight is a major consideration. This type of inverter tends to be less expensive to manufacture and are considered a lighter duty type of product than their US made low frequency counterparts.  


2. Low frequency or 60 Hz based conversion units that are typical of US designed inverters. 


As mentioned above these inverters use large heavy iron core transformers that tend to provide a flywheel effect which yields a much greater surge capacity than do high frequency inverters. 


Low frequency units take the incoming 12 Volts DC (direct current) and converts it into AC (alternating current), using a multi-vibrator or microprocessor based circuit. This 12 volts 60 Hertz AC is fed to an iron core step up transformer which converts the 12 volts AC into 120 volts AC . 


Wave shaping and the increased current that is needed to drive the transformer is performed again by an H-bridge which is a group of field effect transistors that are arranged in such a way as to feed high current pulses to the primary windings of the transformer at precise moments of each wave form half cycle. 


The transformer converts the lower voltage which was fed to its primary windings into 120 Volts AC at its secondary windings using simple transformer step up principles involving a 10 to 1 ratio, converting 12 Volts AC to 120 AC. This type of inverter is considered more durable than their high frequency counterparts and have a much higher surge capacity.


You might be asking why anyone would choose a high frequency inverter when low frequency units are more durable ? The answer is cost. 


Low frequency units tend to cost two to five times more than do high frequency units, another consideration is of course the difference in weight. For example a typical 2000 watt high frequency based inverter may weigh 13 Lbs, where a typical low frequency inverter can weigh as much as 50 Lbs.


A modified sine wave looks more like a square wave that has been time shifted in order to produce the energy content and waveform which is found in power produced by the utility company. It's frequency and voltage is rock solid which prevents the brownouts and spikes typically seen with utility company power. Most appliances such as TVs, Lighting, stereos, computers, inkjet printers and power tools run find on modified sine wave power. 


About the only appliances that you may see a problem with are some Laser printers and some of the cheaper types of battery powered tool chargers. In fact 95% of the inverters in RV's today are of the modified sine wave type.


The second type of inverter is known as a pure sine wave inverter and you guessed it, it produces power that is exactly like the power which is produced by the utility company without the spikes and brownouts of course. This type of inverter produces pure sine waves much the same way as the two different technologies mentioned above, but at the cost of some efficiency loss and at a much higher price. In fact most pure sine wave inverters are typically priced at least 75% higher than their modified sine wave counterparts and in some cases do not have as high of a surge capability as do modified sine wave units. See the illustration below to visualize the difference between a sine wave and a modified sine wave






Sine wave and modified sine wave inverters are available as inverter units only, or may have additional circuits added that allows them charge batteries when an external AC source is fed into the inverter. This type of configuration is know as an inverter/charger. In addition to the charger circuit, these units will typically include a device known as an AC transfer switch.


These added devices allows the inverter to charge your batteries from a generator or utility AC power source and passes the AC power through the inverter to your loads, allowing you to operate your appliances while the batteries are being charged. As soon as the external AC power is turned off or is removed from the inverter, the inverter/charger automatically returns to functioning as an inverter, outputting its own AC power.


HOT TIP !  Never wire a generator or utility company's power directly to the AC output of any inverter. Doing so will result in damage to your inverter and or your generator. Always wire the generator or utility company's power output to the inverters AC input though an external AC transfer switch if your inverter includes that option.


An example of this is a customer who brought his inverter in to us for repair. He explained that he did not want to add an external transfer switch to his inverter nor did he want to wire up a separate sub panel. What he did was wire two switches to his main electrical panel, one which connected the utility power and one that connected his inverter. In the event of a power failure, he would throw the switch that would disconnect his utility power and then would throw the switch that would connect his inverter. He would then reverse the procedure when the utility power came back on. Well needless to say, he got confused and threw the swithes in the wrong sequence an bang went his inverter ! The lesson here is unless your inverter is specifically designed to function as a backup unit and has its own transfer switch, never mix utility or generator power with an inverter !


Backup Power 


Another advantage that an inverter/charger with transfer switch offers is that it can function as a highly reliable automatic power backup unit or UPS (uninterruptible Power Supply).


When the utility company is operating normally the inverter/charger passes the utility company power through its internal transfer switch to your appliances and maintains a charge on your battery bank. As soon as the utility power fails, the inverter automatically stops charging the battery bank and begins producing its own AC power which is passed on to your appliances through its internal AC transfer switch.


When the utility power returns the inverter goes back to charging the batteries and again passes the utility power though the transfer switch to your appliances. Most inverter/chargers switch from utility power to inverter power and back again so fast that most of your appliances will hardly miss a beat.


HOT TIP ! Unless your inverter is specifically designed to do so and very few are,  Never ever attempt to wire two inverters in parallel to double your current or in series to double your voltage. Doing so will instantly result in the immediate destruction of both inverters. If you need more power, buy a bigger inverter to begin with. If you need 240 volts, buy a step up transformer such as the Xantrex/Trace T-240 which allows you to feed 120 in and get 240 out.


Grid Intertie Inverters


A grid intertie or grid interactive inverter is a pure sine wave inverter that has had additional circuits added that allows it to synchronize with and feed power back to the grid or utility company. This type of inverter may operate in conjunction with batteries which would provide backup power as described above or may function without batteries allowing power to be fed directly from solar modules or a wind generator. 


Many states are now allowing utility companies to purchase power from such systems allowing customers to receive credits for the excess power that they produce. Grid intertie inverters are specially designed units that have the capability of interacting with the grid. Never connect the output of a non grid intertie or modified sine wave inverter to the grid, unless you enjoy seeing a lot of smoke and an expensive repair bill. Believe me we've had customers that have tried it !


Inverter brands that are recommended for multiple use: You can never go wrong with the Xantrex line of products which include Xantrex/Trace Engineering, Xantrex/Heart Interface and Xantrex/Statpower branded products. Xantrex/Trace Engineering, Xantrex/Statpower, OutBack Power and Exeltech all manufacture excellent sine wave products and Xantrex/Trace Engineering and Xantrex/Heart Interface both manufacture the preferred line of modified sine wave products.  If you're in the market for less expensive product, Cobra, Vector and Coleman make excellent high frequency based inverters.


HOT TIP ! Stick with name brand inverters. We can't tell you how many times, we've been approached by surplus dealers who try to dump their junk inverters which came from overseas manufacturers who have gone out of business. When it comes to inverters, you get what you pay for, and when it comes to off brand cheap inverters, many times you'll be stuck with what you paid for regardless of a manufacturers warranty because the overseas manufacturers that built them simply go out of business, change their name and start all over again, relieving them from any responsibility to honor your warranty.



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