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Power Inverter Basics

 


The power inverter is a key component in most alternative energy systems. Inverters rarely receive much discussions, if any at all. The focus is usually directed toward more glamorous production equipment such as the solar array or wind turbine. In the meantime, the inverter is quietly humming along doing what it is designed to do. In most applications it is at work 24 hours a day and requires no intervention to do its job.

off grid inverter system



There may be some off grid systems that are totally powered by direct current (DC) from a battery where an inverter is not needed but that is normally not the case. In typical applications, household or business, appliances require alternating current (AC) to operate. This makes it an essential component to have in order to conduct most household chores.





power inverter Styles

There are a variety of choices in the inverter market:



  • Some only invert DC to AC while others are integrated with battery chargers

  • A wide range of power output capacity makes it easy to match the inverter with expected load demand

  • Basic Modified Sine Wave or a more versatile True Sine Wave

  • Dedicated use for Off Grid or Grid Tie applications

power inverter Brands

The most widely available brands available in the U.S.:












Current Basics




To understand why a power inverter is needed in our alternative energy system let's look at the very basic flow scheme of electricity.



Electricity is available in two forms, AC (alternating current) and DC (direct current). Both AC and DC have their useful place in various applications and devices. Most of us are familiar with the AC side of delivery, it is the power running our household loads.



Since we are accustom to plugging into the AC wall outlet we tend to forget that many electronics are actually powered by DC. Battery chargers, cordless phones, cell phones, wireless routers, DSL and cable modems, laptops, desktops, stereos and radios, these are some examples of DC powered devices. In these cases we are taking AC power and converting it to DC. This process is called rectification. 




In the case of commercial grid power there is no need for a power inverter. The AC supplied from utility companies is already in a useable form and phase. Our appliances either use the AC in its supplied form or they have a converter to make the proper current.



Converting Household AC




The small electronics previously listed have a converter, also know as a rectifier, either built in or supplied in the form of a external power supply. These power supplies come in various shapes and forms of which we are all familiar with. However we do not always make a connection with the purpose they serve. A computer is a good example to unpack with which we are all familiar.



It would be safe to say that many people using a desktop or laptop computer assume it is running directly off of household AC. This is both true and untrue. At a glance we see an ordinary looking power cable coming from the computer case that plugs into an AC wall outlet. The mystery is hidden inside the case and unless you open it you will not see the power supply which is rectifying AC to DC. Here we are inputting 120 volts AC and getting low voltage DC, typically under 24 volts, which the computer components need to operate. This is an example of a built-in power supply.



Built-in power supplies are also in many other appliances, even those with motors requiring AC . A washing machine is an example of this mixed power requirement. The motor may use AC but the control panel will need DC to operate. If you dig through the wires in a washer you will eventually find some sort of rectifier. 



The laptop on the other hand has an external power supply. All of the converting takes place in the "brick". Other devices have various types of external power supplies. We are all familiar with the big cubes, commonly called "wall warts", which plug directly into a wall outlet. When you start looking around the house it is amazing the number of appliances and electronics running on converted electricity.




Power Point

Having an external power supply for a laptop has two advantages for this application.

  1. It adds to the reduced size of the laptop unit.
  2. The heat generated in the rectifying process is dissipated outside the case, reducing the need for increased, power intensive cooling capacity



Making the Right Current


The other side of rectifying AC to DC is to change DC to AC. This is called inverting the current. In the case of rectifying AC to DC we have the common everyday occurrences as described in the previous examples. Inverting DC to AC is not so common since the commercial power grid already supplies useable AC.



Living on the grid, most people will never have the need to use a  power inverter. Where the inverter does play a important role is when alternative energy generation is involved. Sources such as a solar panel or wind turbine will either produce DC or a form of AC that can be rectified into useable DC. 



The solar panel is an example where DC is the form of current flowing directly from the generation device. A charge controller is typically installed between the solar array and the battery but it is for regulating, not changing the current. On the other hand, most wind turbines produce variable voltage or "wild AC". The charge controller for a wind turbine may have several functions with the main job being to rectify the wild AC to DC. In both cases the end result is DC. 



If the system is on the grid, the DC goes through a power inverter that can produce a synchronized AC which is properly matched to the grid voltage and phase. These inverters have safety controls which will disconnect the system from the grid if issues arise as to the integrity of the power output or the grid goes down. 



The  power inverter used in an off grid system is considerably less complicated. There is no need to synchronize to the grid in this case. The main requirement of the inverter is to take DC from the battery and invert it to AC for a stand a lone application.




Electrical Mysteries

.

When we are connected to the commercial power grid and employ no alternative energy generation, electrical life is simple. We plug in and pay the bill. We have no care or concern for things such as the rectifying and inverting electricity. Plug n Play, it's that easy.



If using electricity required a general understanding of how it is generated, transformed and rectified, most people would probably choose to give it up. Every day, multiple times a day, we find ourselves using electricity from a wall outlet or tapping into stored energy from a battery. Most of the time we give no conscience thought about what we are doing and why it works, we just know that it does. On one hand, as mysterious as it is, we really do not need to know the intricate details of power transmission from beginning to end. If a switch if flipped and the appliance runs or light bulb glows, we have been a success at using electricity. The bottom line is achieved when either power comes out of a wall outlet or flow through a switch.



On the other hand, some people are taking a much more active role in the understanding and usage of electrical energy. If you are investigating renewable energy generation especially for an off the grid system then you are one of these people. We do not have to become an electrical science major to use alternative energy generation, but it is helpful to understand some of the basics of power generation and energy storage.



Taking the time to investigate the components necessary in a alternative energy system, on or off the grid will not be a waste of time. After the actual generating devices such as the solar panels or the wind energy system, the inverter is one of the more expensive pieces of the remaining components. To build an efficient and cost effective system, it is important to understand the difference two basic features of an inverter. First is the difference between inverters that produce a modified sine wave and a true sine wave. Second, it is essential to match the power inverter capacity to the electrical loads it will be powering. Giving proper attention to these basics will be one step in building a successful alternative energy system.












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