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Inverters

Inverters

Richard Perez

©1993 Richard Perez Six years ago a working inverter

was a marvel. Now inverters are

virtually standard equipment in renewable energy systems. Inverters are the magical black boxes that convert direct current (DC) electricity into 120 volt alternating current (vac), 60 cycle power just like the power company rents out. Here is a quick guide to the high technology packed into those small expensive boxes known as inverters.

Why use an inverter?

Many renewable energy systems have survived quite nicely for years on specialized DC appliances. Most of these old-timer systems now use inverters to convert battery stored low voltage DC into 120 volts of 60 cycle per second alternating current. There are two reasons why inverters are used in modern stand-alone RE systems. The first reason is access to full featured, inexpensive appliances, some of which are not available in low voltage DC models. The second reason is built into the physics of electric power transmission. Grid-connected RE systems have their own reason — inverters are essential to interface a renewable energy source, which usually produces DC power, with commercial 120/240 volt alternating current.

Let’s look at appliances first. Consider a common kitchen appliance — the blender. A 12 VDC blender costs about twice as much as a conventional 120 vac blender. The 12 Volt blender has two speeds (on & off) while the 120 vac blender has twelve speeds or more. The DC blender is a special order item from a catalog while the 120 vac blender is available at the local discount store. The DC blender requires special heavy wiring and sockets while the 120 vac blender uses standard house wiring. Get the picture? For years Karen and I didn’t even look at appliances that didn’t have a cigar lighter plug. Now we can shop the sales at the discount stores. Access to mainstream consumer appliances offers RE users more function for their appliance buck. One step further are appliances with no low voltage DC counterparts. Consider the Macintosh computer I’m typing on right now. When I bought my

first Mac (April 1983), I took it apart before I ever plugged it in. I wanted to convert it to 12 VDC power. The project proved difficult, specialized and expensive. I bought our first inverter instead — a 1,000 watt Heart Interface. It ran not only the Mac, but also its printer. Today’s full featured and inexpensive appliances like compact fluorescent lighting, full featured TV/video, VCR, FAX, computers, and many others, are all powered by 120 vac. This is not to say that 12 VDC models of the above appliances do not exist. In some cases they are available, but they are more expensive and limited in performance.

Next consider the physics of moving electric power through wires. Consider a 120 Watt load located in a barn 300 feet from the main system’s batteries (that’s 600 feet of wire, round-trip). Ohm’s Law tells us that watts is equal to volts times amps. In order to move 120 watts of power at 12 volts, we must move 10 amperes of current. The same 120 watts of power can be moved at 120 volts with 1 ampere of current. This is a ten fold reduction in the amount of current flowing through the wires. The more current that flows through a wire, the more voltage, and thereby power, is lost. Bottom line is that powering the 120 watt load on 12 volts would require 600 feet of massive 1/0 gauge copper cable for an efficiency of 95% and a cost of about $650 for the cable. The same level of efficiency can be obtained at 120 volts with 18 gauge wire! At 120 volts, a sensible person would install 600 feet of 12 gauge wire, get an efficiency of 99% and pay only about $50 for the wire. Basic physics and our wallets limit the distance we can move electric power at low voltages. If you look deeper into Ohm’s Law, then you’ll find that the amount of power lost in wires is equal to the resistance of the wire times the current squared. Physics makes moving electric power at 120 volts 100 times more efficient than moving the same amount of power at 12 volts.

Some renewable energy systems put their power on the utility grid. Here the inverter is essential in changing the DC power produced by photovoltaics (PVs), and wind generators into 60 cycle alternative current acceptable to the utility grid. These utility intertie inverters make sine wave power that is in lockstep (in phase) with the utility power. This type of inverter is called “synchronous” because it can synchronize its power output with the grid’s.

Inverter Wave Forms

An inverter makes one of three different types of alternating current wave forms—sine wave, modified (quasi) sine wave, and square wave. While we talk about ac as alternating current, what we actually mean is that the voltage of the wave form is regularly changing. It is voltage (electronic pressure) that drives

34 Home Power #36 • August / September 1993

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