AC vs. DC Electricity
Did you know that in the late 1880’s the standard for power in large parts of this country was DC (direct current)? Thomas Edison had invested in electrifying some major cities with DC power. It was suitable for light bulbs and the most common type of motors in those days. George Westinghouse and some parts of Europe were making advances and investments in AC (alternating current) technology. There was a “War on Currents” going on. The winner would make a lot of money and drive the competition out of business.
Batteries use DC technology. It is very easy to parallel or series DC power sources or loads. If the neighbor decides to electrify their house, so they can have incandescent bulbs instead of gas lights in the evening, the power company could just wire up a parallel generator back at the power station. Since light bulbs used 100 VDC the power station generated 110 VDC power to accommodate some line loss. The trouble with DC is that if you needed a higher voltage for running large motors then the power company would need a separate power generator at the new voltage and all new wires would need to be laid next to the standard voltage wires. There was no economical way to step the voltage up or down after it had been generated.
The other big drawback of DC voltage is that the power plant needed to be within a mile or so of your house. Since the distribution lines were powered at the same voltage that entered the house, the voltage drop in the distribution system was significant past a mile or so. But at the time DC was first being wired into houses and businesses in certain cities the technology for AC power distribution was not much better.
When sizing a modern power distribution system the wire is sized for how much current it will carry. The insulation on the wire is sized for the voltage. The invention of transformers allow the power company to transmit power at greater than 100,000 volts AC and low current. When the power gets to a distribution point a transformer drops the voltage which increases the current since power-in is equal to power-out of a transformer minus some minor inefficiencies. The power is then distributed locally at a medium voltage and is transformed lower as it gets closer to the final customer. This allows smaller wires in the distribution system, less line loss due to the resistance to current flow, and the power stations can be built far from the load.
Remnants of the “Ware on Currents” were still common into the 1960’s and even on to today. There were sections of cities and old business sections that were powered on DC voltage and everyone who lived in those sections needed to buy special appliances and lights. Before the invention of modern electronics most stepping motors were DC. There are still elevators in the hearts of cities that require DC power provided from the power company.
Until next month,
Self proclaimed Automation Expert, CAP, and PE