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 the motion of electrons (current). Check out the illustration. Here the voltage of the waveform is graphed on the y-axis against time on the x-axis.

Inverter Waveforms

—— Square Wave

Sine Waves

The sine wave form is what the utilities rent. The smoothness of the sine wave is due to its mechanical origins. The rotary alternators used by utilities and even small engine-fired power plants produce a smooth sine wave. Now we have electronic inverters that can synthesize sine wave power.

Modified (Quasi) Sine Waves

The modified sine wave (and the square wave) are technically ac wave forms, but obviously different from the smoother sine wave. The modified sine wave is capable of having its pulse width (duration in time) expanded and contracted. This is how modified sine wave inverters are able to deliver their incredible 90+% efficiencies. Varying the width of the power pulse allows the inverter to only produce as much power as is being consumed. This further increases the power output range at high efficiencies. Varying the pulse width also allows the inverter to maintain a more constant output voltage regardless of type and amount of loading.

In physics and reality, the power content of an electrical waveform is equivalent to the area under its wave form. This fact allows the modified sine wave inverters to replicate the power content of a radically different sine wave. The graphic illustrates this concept.

Square Waves

The square waveform takes advantage of the high efficiency of rapid voltage/time transitions. It, however, lacks the pulse width modulation offered by the modified sine wave. This renders the square wave form inherently unable to meet both high efficiency and voltage regulation (average and peak) criteria at the same time.

How Inverters Are Compared

These high tech boxes are maturing quickly. Advances in transistors and power circuit design give us a new generation of inverters every six months. What is not changing is the ideal sine wave. Every inverter is attempting to mimic the sine wave generated by power utilities. Why? Well, there is nothing electrically sacred about the sine wave, it is merely a standard. It is however, the standard to which all 120 vac appliances are constructed. We compare and rate inverters by how close they replicate utility produced sine wave power. We do this not because this form of electricity is the best, but because all of our appliances are designed to feed on 120 vac, 60 Hz sine wave ac electricity. The performance specifics to watch for in the tables ahead are listed under the headings of Output RMS voltage and Peak Voltage. The definitions of these appear later in this text.

Different Inverters for Different Uses

Technology and the fertile imaginations of inverter makers have provided us with two basic methods of upconverting voltage from a lower DC voltage to 120 vac 60 Hz. In order to appreciate the differences between the two basic schemes of voltage upconversion, a little basic physics is required.

60 Cycle Voltage Upconversion

Direct current electricity will not operate a transformer. A transformer is a device that accepts alternating current electricity in one side and disgorges alternating current of a different voltage on the other side. The constantly changing voltage and current of the ac waveform produces the constantly changing magnetic fields necessary to operate a transformer. (Editor's note: for those wishing to enter the world of transformers and electromagnetic induction there will be a complete beginners techie article next issue by our esteemed colleague, Herr Docktor Kluge.) In order to convert the direct current produced by PVs and wind machines or the DC stored in batteries into any higher voltage it must be changed into alternating current. This is accomplished in every inverter by semiconductor switches (transistors). Inverters use transistors to switch DC into ac and then feed the low voltage ac to the transformer for voltage upconversion. The major question here is at what frequency is the voltage upconversion accomplished? The first modified sine wave inverters developed by Heart and Trace opted for a 60 Hz switching frequency. The reason for this is obvious — we want 60 Hz electric power from the inverter. These 60 Hz inverters use large transformers. A 2,000 watt inverter of this type is bigger than a bread box and weighs in at 40 to 75 pounds.

25,000 Cycle Voltage Upconversion

A few years ago designers from PowerStar and Statpower applied the same high frequency switching power supply design used in space-going electronics and computers to inverters. Instead of operating the switch at 60 Hz, they operated the switch at much higher frequencies — 25,000 cycles per second or more. It is a happy fact of physics that as the frequency of an ac wave form increases the size of the transformer shrinks. This allows smaller and less expensive inverters. A 1,300 watt inverter of this type is smaller than a loaf of Velveeta cheese and weighs in at less than five pounds. Higher frequency voltage upconversion also increases the efficiency of the inverter at low power loadings. Since a 25,000 Hz wave form is useless to the appliances, these types of inverters convert the high voltage, high frequency waveform back into DC and then chop it up at 60 Hz for use by the appliances.

I know that these concepts are slippery, even for techies and electroweenies. The illustration below shows the essential differences between straight 60 Hz upconversion and higher frequency upconversion.

All the inverters on the following tables use 60 Hz voltage upconversion except the following makes: Exeltech, PowerStar, and Statpower. The higher

60 cycles per second voltage conversion

Battery Power

60 Hz. Transformer 12 vac in & 120 vac out

60 Hz. Switching 12 VDC in & 12 vac out

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