Alternating Current

Alternating current (AC) is one of two types of electrical flow. The other, Direct current (DC), was the predecessor of alternating current. Following the invention of the electric battery by Alessandro Volta in 1800, the next eighty years saw the invention and development of numerous devices that utilized DC. It was not until 1884 that AC posed a real challenge to DC. Unlike direct current, which flows in one direction, alternating current oscillates forward and backward at a specific frequency, usually 50 or 60 cycles per second (a cycle per second is referred to as Hertz, abbreviated Hz). In other words, the current peaks first in one direction, drops to zero, peaks in the other direction, drops to zero, and then repeats the cycle. AC in North America oscillates at 60 Hz, so it takes only one-sixtieth of a second for a single cycle. In other parts of the world 50 Hz is common, and 400 Hz is common on aircraft, to make possible lighter electrical machines.

Alternating current was the brainchild of Nikola Tesla, a brilliant Croatian electrical engineer initially employed by Continental Edison in Paris. In 1884 Tesla came to the United States to work with the chief proponent of DC, Thomas Alva Edison, and to convince him of the benefits of AC. Alternating current has a number of advantages over DC. Alternators (generators designed for AC operation) did not require the slip-rings and commutators (brushes) upon which their DC cousins depended. AC operates on the process of electrical induction, which was discovered by Michael Faraday in 1831. Inducing the flow of electricity from one coil of wire into another eliminated the arcing that plagued DC generators. As mentioned above, an even greater advantage to AC is that its voltage can be stepped-up to higher levels with a transformer, sent great distances through high tension wires, and stepped-down at its destination.

Since the current of an alternating current peaks in opposite directions over one cycle, the average voltage for the cycle is zero. The voltage of the current intensity, or amplitude, as displayed on an oscilloscope, is that of a sine wave. In three phase power, as the voltage in one wire peaks, the voltage in the other two are halfway to peak (one increasing, the other decreasing).

Unfortunately for him, Edison rejected alternating current. First, he thought, the world was geared to DC; there were no AC applications so there was no need to change. Secondly, Edison considered alternating electricity to be "killer current," and would not accept arguments to the contrary. Tesla remained with Edison for only one year before quitting in disgust. The opportunity for AC to prove itself came in 1893. Tesla, with his backer George Westinghouse, underbid Edison for the contract to provide power for the Columbian Exposition in Chicago. This was the first electrical fair in history, and Tesla's polyphase (two-phase) AC system was a spectacular success. The consummate showman, Tesla put on impressive performances; at one point he sent a potential of 200,000 volts of AC through his body and challenged Edison to do the same with DC. (Edison may have had the last laugh; after the state of New York began using AC to electrocute prisoners, Tesla became convinced that Edison had helped establish the system to prove to the public that alternating current was indeed deadly.) Also during the fair, Westinghouse demonstrated a rotary converter that, ironically, changed the polyphase AC into direct current to operate a DC motor and run a railway car. The case in favor of alternating current was further advanced by Charles Steinmetz. He established the law governing hysteresis,the residual magnetism that occurs in generators and motors. Hysteresis causes a loss of power and was little understood at the time. His theoretical studies of AC in 1893 resulted in making what was then a very complex field understandable to the average engineer and electrician.