INVERTER, CONVERTER, TRANSFORMER: What's the difference?
While Inverters turn DC electricity into AC electricity,
Converters change the voltage of DC electricity (up or down), and
Transformers change the voltage of AC electricity (up or down).
Comparing Inverter Features:
1) WAVE FORM:
Modified Sine Wave is the wave form most inverters produce. It is more accurately called Mod square wave, since it more closely resembles a square wave than a sine). Since this wave form isn't fluid in shape and has large "corners," it sometimes causes problems with certain appliances.
Ultramodern washing machines and a few other appliances with electronically controlled, variable-speed motors won't work at all on mod-sine power, and some people in the RE field come down hard on mod-sine inverters because of this. However, mod-sine isn't as bad as all this might make it seem.
Most people experience no problems whatsoever
with their mod-sine system: We have found mod-sine
inverters to be great for low cost, off-grid homes and grid backup systems. Despite
the claims, mod-sine inverters work perfectly well with the vast majority of appliances
including most stereos and computers. We ran our entire shop off a Trace DR 2424
(the earlier version of the Xantrex TR) for years, and many of our more affordable
systems still use TR inverters.
The next step up from mod-sine is
the wave form generated by the Xantrex/Trace SW and SW+ series. Trace calls it
a sine wave, but we call it a stepped-sine
wave, since it is composed of 34 to 52 little "steps", depending
upon the amount of load placed on the inverter at the time. The stepped sine is
utility grade and can be fed (sold) to the grid, although it may not be clean
enough for some critical applications. Stepped-sine inverters can cause the same
problems as mod-sine inverters, though less pronounced. (see side bar of
Mod sine problems).
Pure sine wave inverters are made by a few manufacturers. As mentioned above, the power provided by them can be "cleaner" than grid power. Exeltechs are the cleanest of the clean, being the preferred inverter for off-grid recording studios. High-end recording studios with grid power are sometimes set up with pure sine inverters for the musical equipment and recording gear, so as to guard against noise and voltage dips and spikes from the grid. Sometimes people will dedicate a small pure sine inverter for their stereo, computer, or other critical load, while using a larger, less expensive mod-sine inverter for all the other house loads.
2) GRID-INTERTIE vs. OFF-GRID:
Inverters for off-grid battery-based systems are designed for
stand-alone power systems that can operate independently of the electric
grid. Many of these inverters have AC inputs for grid power as well; so,
if grid power is available to the site, it can be used for back-up power and emergency
battery charging the way one would use a generator. In this way, an "off-grid"
inverter can have the option of grid back-up. This is different than what
is called grid-tie. Grid-tie
systems can be of two types: batteryless and battery-based.
A batteryless grid-tie inverter, like the SMA SunnyBoy, is able to accept a wide range of Solar module power that can be much higher than the voltages standard with battery-based inverters (up to 600V with the SunnyBoy 2500). Wiring a solar array for a higher voltage saves money on wire and reduces the power losses associated with lower voltages.
The biggest drawback to a batteryless grid-tie system is the fact that the system will shut down when the grid goes down and will not provide back-up power during a power outage. This is done in order to protect utility workers who will be trying to restore grid power.
Battery-based grid-tie systems combine the best of both worlds: surplus power can be sold to the utility company, and power can be bought from the utility's electric grid when needed. Plus, if grid power fails, the system does not shut down with it. Programming is still being developed to keep up with all the choices possible with this arrangement. When to sell, when to charge the battery, etc.
Note: the rest of this guide does not apply to batteryless inverters.
4) SIZING THE INVERTER:
If your inverter is undersized, a combination of loads such as this will trip the inverter's output breaker; this is known as nuisance tripping, and it truly is a nuisance.
Inverter Power Rating to start an AC well pump (with no additional loads)
An inverter sized by these minimum guidelines will dip its voltage during the starting surge. This is not harmful, but it will cause lights to dim. Fluorescents may blink off, and desktop computers may crash. To eliminate voltage dips, oversize the inverter by an additional 50% minimum plus the watts capacity required to handle other household loads at the same time.
Inverter Sizing Chart Copyright ©2002 by Dankoff Solar
There is a very important distinction to be made between the many cheapo inverters, which are sold in import tool catalogs and big box stores, and the high quality inverters made specifically for renewable energy applications. They're both called inverters, but they're very different products, each with its own purpose. The cheapo inverters are meant to be hooked up to a car's battery for occasional use: to run a power tool or tv out in the field for example. The inverters we sell are designed to run an independent power system 24 hours a day, 7 days a week, for many years. The cheapo inverters simply are not designed for continual use or made of components that can hold up to it.
Let's take a closer look at a couple of mod-sine inverters with comparable wattage ratings:
The above chart is misleading in so far as the "cost per year" is based only on the inverter's price, which, in the case of the offbrand 2500, does not include a battery charger or transfer switch. So, for the comparison to be truly equal, both of these should be added
A 90amp, 12 volt battery charger would cost upwards of $300 (the DTR 2412 has a 120 amp charger), and a 30 amp transfer switch would cost around $100. Now, since the charger and transfer switch are already incorporated into the TR, labor and materials to wire the cheapo inverter, transfer switch, and charger together should also be factored into the equation. Thus, the true price of the cheapo inverter setup would be $650, plus wire, conduit and labor, which will add up to a similarly priced system of much lower quality that will last a fraction of the time that the TR will!
But you could also just grab that Brand X 2500 (13.5 lbs.) with one hand and sling a Xantrex TR 2412 (45 lbs.) under the other arm, and you'll feel another big difference--the TR and other quality inverters are transformer based, whereas the Coleman, and other such inverters, use high-frequency switching (transistor based), which is much more prone to burn out.
A word to the thrifty: when price shopping, make sure you compare apples to apples. And get some advice from a professional before you buy. If you are planning on living with this equipment and want it to last, don't skimp; in the long run, buying quality is the best deal.
Next: Inverter Comparison Charts.
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