What's an Inverter? or Why Can't the World Run on DC and Make Life Easier for Everyone? Elliot Josephson c.1992 Elliot Josephson Alternating current or direct current? How shall electricity be sold to the public? Over a century ago the battle raged, George Westinghouse versus Thomas Edison. Edison had electrified New York City with DC power, to the wonderment of the world. Then along came this upstart Westinghouse, with his foreign friend Nikola Tesla, who wanted to change everything. Everybody knew DC was simpler. Direct current flowed from positive to negative continuously and did its job very nicely. What foolishness it was to talk of alternating the direction of current flow sixty times a second. No good could come of that! Editorials were written. Lawsuits were argued. Millions of dollars were at stake. In the end, the battle was decided by economics. No amount of philosophizing could overcome the fact that alternating current was just plain cheaper to make and distribute than direct current. Why Not DC? But why? Doesn't the simplicity of DC electricity make it easy to work with? Yes, as long as you're reasonably close to the source, and as long as you're happy with the voltage that's available. But suppose that your source of power is a great big 12 Volt battery, located a mile away. You've seen how heavy the battery cables are in your car just to carry the current a few feet to your starter and engine. Imagine how heavy (and costly) the wire would have to be to carry DC current a mile and still have enough voltage left to light a headlight! Or suppose 12 Volts DC were available, but you really needed 120 Volts to light your bulb. In Edison's time, the only way to make this transformation was to use the 12 Volts to run a 12 Volt motor, and then to use that motor to drive a 120 Volt generator. It worked, but it was terribly expensive and wasted a lot of power. Transformer Magic! A bit of magic was needed to overcome these problems. Ac provided that magic because, unlike DC, it could operate transformers. Transformers are devices that can change the voltage up or down easily, inexpensively, and efficiently. To appreciate the importance of the transformer, put yourself in the position of the electric power utility. It costs you money to generate power at your power plant, and you can sell that power only by delivering it to the user, wherever he may be. Any power that gets lost along the way comes out of your pocket. The user will only pay for the power he actually receives. Suppose that a reasonably sized power transmission line loses 60 volts per mile. If you start with a 120 volt signal, you'll lose half of the voltage sending it one mile. But if you transform the signal up to 600 volts and send it a mile, the 60 volt loss is only 10% of the total. And if you transform the signal up to 6000 volts and send it a mile, you only lose 1% of the voltage. In fact, the utilities transform electricity as high as 500,000 volts to send power over long distances, and then transform it back down to 120 volts to supply their customers. Standards Once the ac versus DC battle was settled, it took a long time before the voltage, frequency, and socket spacing was standardized, but finally a manufacturer could build a toaster and know that it would plug in and work anywhere in the United States. Appliances of all sorts were designed and built to operate on AC power, and DC was used primarily for automobiles and flashlights. The First Inverters Where it was necessary to convert DC to ac, a rotary inverter was used. It consisted of a DC motor driving an ac alternator at the proper rotational speed to create AC at 60 Hertz (cycles per second). Some of these inverters are still being sold under the trade name Redi-Linetm and are primarily used in utility vehicles. As automobiles became more sophisticated, it became desirable to install a radio, first for police and emergency vehicles, and then for the general public. Automobiles used 6 Volt batteries then, and the transistor hadn't been invented yet. Radios ran on vacuum tubes which needed over 100 Volts to operate. Clearly, a device was required to change the 6 Volts DC into ac so it could be transformed to a higher voltage. One of the earliest inverters, used in the radio for the family car, was an electromechanical vibrator. The vibrator was a type of buzzer, with contacts that opened and closed many times a second, to switch DC into ac. This ac could then be transformed up in voltage. After transformation, another set of contacts switched the high voltage ac back to DC to be used by the radio. Although effective and relatively cheap, it had very poor reliability and had to be replaced fairly often. Square Wave Inverters When the transistor appeared on the scene, it replaced the vacuum tube. Now you could operate a car radio directly from the battery voltage, so radio vibrators were not needed. At the same time, it was also possible to build a transistorized inverter (also called a static inverter because it had no moving parts) for general purpose use, getting rid of the unreliable vibrator. This type of inverter consisted of little more than a transformer and a pair of transistors. It is still being sold today as part of the Tripp-Lite inverter line. Instead of the sine wave delivered by an electric utility, this inverter produces a square wave. Because current is a switched, or turned on and off, the generated current had very abrupt changes and over time looks like a wave with square corners. This is very unlike what comes from the power company which has much more gradual changes that look like sine waves. With this type of inverter neither the frequency nor output voltage is regulated, the inverter has little surge capability, and it is not protected against overloads except by a fuse or circuit breaker. However, it is simple and low cost and suitable for many non-critical applications. The shortcomings of the square-wave inverter become most evident when you try to use it to run a motor. Ac induction motors draw a substantial current surge on startup. They really prefer sine-waves to square-waves and may overheat on the latter. Ac induction motors also store energy during a portion of each cycle, which will create problems if not returned to the battery or otherwise controlled. Enter the Modified Sine Wave Inverter In response to these shortcomings, Heart Interface pioneered the development of the "modified sine wave" inverter, and today most of the inverters sold are of this type. The "modified sine wave" is neither a sine wave nor a square wave, but a moderate-cost compromise which has been found to run most loads in an acceptable manner. The inverters described up to this point are fairly large and heavy. This is simply because the power they generate is delivered through a 60 Hertz transformer, which, by the laws of physics, must weigh about 30 pounds for 1000 watts. A smaller, lighter alternative is found in the inverters pioneered by PowerStar and now offered by both PowerStar and Statpower. Instead of converting 12 Volts DC directly to 120 volts 60 Hertz ac, they use a multi- step process. First twelve Volts DC is changed to 160 volts peak high frequency ac (25 kiloHertz), This is converted to 160 Volts DC, and finally inverted to 120 volts rms, 60 Hertz ac. At the heart of this process is the high frequency transformer, which is less than one-tenth the size and weight of a 60 Hertz transformer for the same power level. Thus a 5 pound inverter can do the job of a 50 pound one. Sine Wave A few true sine wave inverters have appeared on the market, notably from Dynamote and Exeltech. This kind of inverter will run motors cooler and may offer less interference with radio and TV. However, they are inherently more complex and less efficient, so they may not be justified in many installations. Ask your local dealer If you are confused as to which is the best inverter for you, nothing can beat a truly knowledgeable dealer who carries a broad line of inverters. He will know what's available, what works, and what it costs, and he can translate your present and future projected needs into a solid recommendation. Meanwhile, look for technology to progress and for inverters to become more efficient, better protected, more reliable, smaller, lighter, less expensive and easier to use. This trend will undoubtedly continue for many years to come. Access Author: Elliot Josephson, PowerStar Products Inc., 10011 North Foothill Boulevard, Cupertino, CA 95014 ù 408-973-8502 ù FAX 408-973-8573