Alternating and Direct

We’re all advised to change the batteries in our smoke detectors once each year. This is truly good and affordable advice, and most of us are happy to comply.

If you were to decide on a whim to replace all of the batteries in all of your battery-powered appliances or other devices that incorporate battery backup in their design on that mandated day, however, you might find the number of replacements surprising, the day a long one and the financial impact to be staggering.

In addition to those smoke alarms, you now find batteries in clocks and watches, flashlights, calculators, TV remote controls, timers, telephones, radios, garage-door openers, tape and CD players, virtually all of the toys your children cherish, cameras, heating/air conditioning thermostats, desktop and portable computers, test equipment, countless power tools and – the granddaddy of them all – your automobile.

If an alien from a far-off galaxy were to read the above, he/she/it might assume we live on a battery-powered planet – a direct current (DC) planet. Far from it: It is the alternating current (AC) electricity distributed throughout the world that provides the huge amount of electrical energy required to power our homes and factories.

TWO CURRENT TYPES

There was a time not so long ago when DC reigned supreme and very nearly became the standard. It is not an exaggeration to state that the inventive brilliance of one man, Croatian inventor Nikola Tesla, prevented that from happening. That’s a fascinating short story I’ll get into in next month’s column; first, let’s look at some of the major differences between AC and DC.

The distinction between AC and DC didn’t arise at all until the late 1880s. Before then everything was DC and was called, simply, “electricity.”

A great deal of basic research into all things electric took place in the second half of the 19th Century. During this period, for example, the telegraph, telephone and stock ticker were perfected, and several versions of the arc lamp were being operated around the world. Electric motors began to show up in the early 1870s.

Initially, some form of chemical battery provided the electric current to operate all of these devices; later, rotating, mechanical generators called dynamos would provide the needed DC current.

The telegraph, telephone and similar low-power devices worked quite well with the electric current supplied by batteries. It was lights and motors that the world wanted most of all, however, and battery power or the DC provided by dynamos is not well suited to these applications.

The problem is distribution – getting the electric current to users in the required quantities. To illustrate the point, start with a fully charged automobile battery removed from the car and sitting on the sidewalk in front of your house. Connect one 1,000-foot-long, No. 18 AWG wire to the positive terminal of the battery and another to the negative terminal.

Now pace off about 100 feet along the wires and connect a l2-volt light bulb to the two wires. The bulb will glow brightly. Measure the voltage across the bulb and it will be somewhere in the vicinity of 12 volts. Pace off another 100 feet and connect another bulb. It won’t glow quite as brightly as the first one, and the voltage across the bulb will be less than the first reading as well. It might be 11.5 volts or thereabouts.

Continue this pacing and connecting until you get to the end of the wires. My guess is that the last bulb might not light at all and that the voltage you measure at the end of the wires will be down in the 5- or 6-volt range.

If you were attempting to sell electric lighting to people along your street, those toward the end of the circuit would be far from impressed with your service. The problem is that long length of small-gauge wire: It has relatively high resistance to the flow of current moving through the circuit and is causing a significant voltage drop in each segment of the circuit.

CONQUERING DISTANCE

In a DC system, there are two ways of solving this problem: increase the voltage of the battery, or increase the size of the wire.

Neither, however, is a perfect solution. If the battery voltage is increased to a point where the folks at the far end of the circuit have a usable voltage available to them, then the people in the beginning of the circuit – closest to the battery – will be connected to a very high voltage that would result in drastically shortened bulb lives.

(A whimsical thought – manufacturers could market different light bulbs, motors, appliances and such for people living different distances from the battery. You would find 100-yard, 60-watt bulbs; 300-yard, 60-watt bulbs; 2,000-yard, 60-watt bulbs, and so on. A retailer’s nightmare!)

Increasing the size of the wire is a practical remedy – up to a point. To service people a mile or two from the battery would require a wire about the size of your wrist.
The cost of this massive chunk of copper would be absolutely prohibitive to the profitable distribution of DC electricity.

The scenario is radically different with AC. The current in an AC system flows first in one direction and then reverses and flows in the opposite direction. It does this 60 times per second in the distribution systems used throughout North America.

This alternating allows the voltage to be stepped-up or stepped-down by a transformer and makes practical the distribution of power to customers at relatively long distances — several miles — from the AC generating plant. Also, the distribution wires can be operated at high voltage (10,000 volts and above) and low current for maximum efficiency and smallest wire size.

For each customer or group of customers, a step-down transformer is installed to reduce the voltage to a usable level, such as 115 or 120 volts. Each of the transformers installed along the distribution wire can be adjusted to provide each group of customers with the same voltage, regardless of any voltage drop that may occur in the distribution wires.

In other words, everyone in town has the same 115- or 120-volt electricity coming into their home. And the happy retailer doesn’t have to stock dozens of different products after all.

We can conclude that both forms of electric energy have their place: All those batteries work just fine when they are inside the product they are powering and the distribution distance is thereby kept to an absolute minimum. But they are no match for the AC system that provides us with huge quantities of consistent power, hundreds of miles away from the generating plant.

Next time: how a rivalry between Nikola Tesla and Thomas Edison changed the world.

Jim McNicol was a technical consultant to the swimming pool, jetted bath and spa industries. He worked on development of equipment standards for pools and spas throughout his career and was honored for his service by the National Spa & Pool Institute.