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As a child I considered the reindeer Rudolph, with his nose so bright, to be a marvel of engineering. Now an adult, I remain perplexed as to the mystery behind the self-generating power source behind his nose. Did it ever overheat? I wondered. Perhaps today’s discussion can shed some light on the matter. During the course of our discussion of electricity certain terms have been tossed about, like voltage and current. For some the distinction between the two may be unclear, and that is what we’ll be addressing today. Electricity is a rather abstract phenomenon, but you may consider the flow of electrical current through a wire to be much like water flowing through a garden hose. The water won’t flow unless there’s sufficient pressure behind it, and that pressure is supplied by pumps, either at your city water works or your personal well. Take away the pressure, and the water stops flowing through the hose. Electricity flows in much the same manner. It requires a pushing pressure to get it on its journey from power plant to home, and that pressure is voltage. Take away voltage, and the current stops flowing through the wire. Voltage is, of course, produced by an electrical generator at the power plant. Last time we saw how an electrical transformer can reduce high voltage to low voltage and how this process also works in reverse. But how can that be? How can low voltage be turned into high? Is it really possible to get “something from nothing”? Let’s take a closer look. When a light bulb burns out in your home, you routinely look at the bulb to see how many watts it is so you can replace it with the same type. But what exactly is a “watt”? It’s a unit of power, and the markings on the bulb tell you how much electrical power it consumes when you use it. Generally speaking, this electrical power is related to voltage and current by this formula: Power = Volts × Electrical Current Knowing this, if I have a 60 watt bulb in a table lamp, and I plug it into a 120 volt wall outlet, then how much electrical current is the lamp going to draw from the outlet? Using the formula above and a little algebra, we get: Electrical Current = Power ÷ Volts Electrical Current = 60 watts ÷ 120 volts = 0.5 amperes And believe it or not, this same formula that’s used to assess power of a light bulb also applies to electrical transformers. Basically, the power going into the transformer is equal to the power coming out. To see how this works, consider the example step-up transformer shown in Figure 1, which converts a low voltage to a higher one. By the way, “step up” transformers have all sorts of applications. For example, they are used by electric utilities to raise the voltage produced by a power plant to make it more economical to transmit to far away customers. We’ll get into that in another article.
Figure 1 – A Step-Up Transformer In this example the input voltage on the primary coil is stepped up from 120 volts to 480 volts on the secondary coil, and this works according to the formula we learned about in last week’s blog: NP ÷ NS = VP ÷ VS where NP and NS are the number of turns of wire in the primary and secondary coils respectively, and VP and VS are the voltages of the primary and secondary coils respectively. Plugging in the numbers we get: 50 turns ÷ 200 turns = 120 volts ÷ VS [(200 turns ÷ 50 turns) × 120 volts] = VS = 480 volts Okay, for the sake of our example let’s say that an electric motor is connected to the 480 volt secondary coil. We have an electric meter hooked up to the primary coil and we measure a 2 ampere (a.k.a. “amps”) electrical current flowing through it. Without having the benefit of another electric meter positioned at the secondary coil, how can we measure how much electrical current is flowing through it? The current flowing through the secondary coil is found by equalizing the power in the primary and secondary coils: PowerP = PowerS Another way of stating this is to say that electrical power is equal to volts times current, so the equation becomes: VP × IP = VS × IS where IP and IS are the primary coil and secondary coil currents, respectively. Plugging in the numbers and working a little algebra we get the electrical current in the secondary coil: 120 volts × 2 amps = 480 volts × IS IS = (120 volts × 2 amps) ÷ 480 volts = 0.5 amps This shows us that the current flowing in the secondary coil is lower than that of the primary coil. It is therefore obvious that the voltage increase in the secondary coil comes at the expense of electrical current that can flow through the secondary coil. Squeeze down on current, voltage goes up. Squeeze down on voltage, current goes up. The power flowing through the transformer stays the same. Conversely, step-down transformers reduce the voltage coming in, and thereby produce the reverse effect. There is an actual increase in current that can flow through the secondary coil. This principle exemplifies the tradeoff process which is often present in science and engineering. Next time we’ll explore how both step-up and step-down transformers are used by electric utilities to transmit power from power plants to its customers tied into the utility grid. As for Rudolph and his power source, that’s still under investigation. _____________________________________________
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Posts Tagged ‘power’
Transformers – The Voltage/Current Trade-Off
Sunday, December 26th, 2010
Transformers and The Magic of Electricity
Sunday, December 5th, 2010| No, the next series of articles is not about those talking, morphing, gigantic killing machines that children love to play with. We’re going to talk about the type that adults just can’t live without.
You’ve probably heard the term “electrical transformer” at some point, but you may not be entirely sure what is meant by it. Most don’t realize that they use them all the time, like when they recharge their cell phone battery. That little black box that plugs into the wall outlet is one of them, and what they do is perform the important task of reducing the 120 volts of power that fuels your home’s outlets down to a lower voltage level, for example, 12 volts, which can be used by small electronic devices. But before I explain how this reduction process works we need to understand magnets. Magnets, those wonderful curiosities that mesmerized you as a kid, keeping you busy for hours picking up nails and paper clips, have many practical applications, although they weren’t considered to be anything more than amusing novelties until the early 19th Century. That’s when a French scientist by the name of André-Marie Ampére studied the relationship between magnetism and electricity. What he found was that when an electrical current is run through a wire it turns into a magnet. Ampere’s work was then built upon by British scientist Michael Faraday. He discovered that electric current passing through wire creates magnetic lines of flux that encircle the wire as shown in Figure 1 (a). Faraday also discovered that if you move a magnet back and forth near a wire, as shown in Figure 1 (b), you can generate an electrical current in the wire.
Figure 1 – Relationships Between Electricity and Magnetism Why does this happen? Well, magnets work as they do because they have a magnetic north (N) and south (S) pole, and lines of magnetic flux extend from one pole to the other. You can actually see these lines of flux if you sprinkle iron filings between the poles. The iron filings are attracted to the magnet and align themselves along the lines as shown in Figure 2. When the lines of flux move through the wire, they induce an electrical current in it. As long as you keep the magnet moving back and forth, lines of flux will continue to pass through the wire, and the current will keep flowing. When the magnet stops moving, the current in the wire also stops.
Figure 2 – Iron Filings Aligned Along Lines of Magnetic Flux Faraday soon began experimenting with coiled wires and iron rods. He wanted to see how electrical current flowing through one coiled wire would affect another coiled wire in close proximity. His basic experimental setup is shown in Figure 3.
Figure 3 – Michael Faraday’s Experiment Faraday’s experiment consisted of two insulated wires, each coiled around an iron rod. The first coiled wire ran to a battery and then a switch. The switch enabled Faraday to connect and disconnect the battery to the first coil during his experiments. The second coiled wire was connected to an instrument called a Galvanometer, which measures the amount of electricity flowing through the wire. When the switch was closed, connecting the first coil to the battery, Faraday noticed that the Galvanometer’s indicator needle moved, then returned to zero. Somehow the electricity flowing from the battery to the first coil was causing an electric current to momentarily flow in the second coil. But how does electricity flow from one coil to the other if they’re not connected? It doesn’t. What’s actually taking place is known as “electromagnetic induction.” Faraday’s experiment enabled him to conclude that current flowing through the first coil set up lines of magnetic flux in the iron rod to which both coiled wires were attached. When the switch was closed, the lines of magnetic flux built in intensity until they induced a current in the second coil. But when the magnetic flux reached its full intensity, and stayed at full intensity, the current induced in the second coil stopped flowing. Faraday’s initial confusion as to the state of affairs soon changed into the Eureka! moment of discovery, and he was able to conclude that current will flow in the second coil only if the lines of magnetic flux are fluctuating in intensity. Next week we’ll see how an as yet undiscovered young inventor used the results of Faraday’s experiment to build the first electrical transformer. _____________________________________________ |
