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What’s an Inverter?
or Why can’t the world run on DC and make life easier for everyone?
©1992 Elliot Josephson
A lternating 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, and changed 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.
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.
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.
22 Home Power #32 • December 1992 / January 1993
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