Chapter 1
Understanding Alternating Current
In This Chapter
Discovering and using alternating current
Converting mechanical power to alternating current
Working with electric motors
Stepping up and down with transformers
When you paddle in a river as a child, you can feel the cool water flowing between your toes and the current flowing steadily in one direction. Water doesn’t flow uphill and so a river always flows one way, towards the sea.
Direct current (DC), as with the current you get from a battery, is like that – it flows only one way around the circuit and carries on doing so until you switch it off or turn the battery around so that the current goes the other way. For example, in Book III we focus mostly on working with DC and electric current flowing in one direction only.
Paddling in the sea, however, feels quite different to a river. You still sense the cool water around your toes, but it doesn’t just go one way. The waves sweep in over your feet, and then you feel the water pulling the sand from under you as the water recedes. Another wave comes in, and then the water recedes and so on. The water forever ebbs and flows, back and forth, first one way then the other.
In alternating current (AC), the current flows in both directions – forward and backward – just like the waves breaking on a shore. Alternating current is of vital importance in electronics for the simple reason that the electric current you access by plugging a circuit into a wall outlet happens to be AC. So if you want to free your circuits from the tyranny of batteries, which eventually die, you need to discover how to make your circuits work from an AC power supply.
In this chapter, you take a look at the nature of AC and how it delivers reliable voltage. You also examine three fundamental AC devices: alternators, which generate AC from a source of motion such as a steam turbine or windmill; motors, which turn AC into motion; and transformers, which can transfer AC from one circuit to another without any physical connection between the circuits.
What Is Alternating Current?
In a DC circuit the electric current flows continuously in a single direction, caused by electrons that tend to move in one direction. Within a wire carrying DC, a given electron that starts its trek at one end of the wire eventually ends up at the other end of the wire. But in AC, the electrons don’t tend to move in only one direction. Instead, they just move back and forth.
When the electrons in AC switch direction, the direction of current and the voltage of the circuit reverses itself. In public power-distribution systems in the UK, the voltage reverses itself 50 times per second. In some countries, such as the United States, the voltage reverses itself 60 times per second.
The rate at which AC reverses direction is called its frequency, which is expressed in hertz (Hz). Thus, standard household current in the UK is 50 Hz.
In an AC circuit, the voltage and therefore the current is always changing. The voltage doesn’t, however, instantly reverse polarity; it increases steadily from 0 volts (V) until it reaches a maximum voltage, which is called the peak voltage. The voltage then begins to decrease again to 0 V, before reversing polarity and dropping below zero, again heading for the peak voltage but of negative polarity. When it reaches the peak negative voltage, it begins climbing back again until it gets to zero. Then the cycle repeats.
The swinging change of voltage is important because of the basic relationship between magnetic fields and electric currents. When a conductor (such as a wire) moves through a magnetic field, the magnetic field induces a current in the wire. But if the conductor is stationary relative to the magnetic field, no current is induced.
Physical movement isn’t necessary to create this effect. If the conductor stays in a fixed position but the intensity of the magnetic field increases or decreases (that is, if the magnetic field expands or contracts), a current is induced in the conductor just as if the magnetic field were fixed and the conductor was physically moving across the field.
Therefore, the voltage in an AC circuit is always increasing or decreasing as the polarity swings from positive to negative and back again, which means that the magnetic field that surrounds the current is always collapsing or expanding. So, if you place a conductor within this expanding and collapsing magnetic field, AC is induced in the conductor.
As if by magic, with AC the current in one wire can induce current in an adjacent wire, even though no physical contact exists between the wires.
The bottom line is that AC can be used to create a changing magnetic field and changing magnetic fields can be used to create AC. This relationship between AC and magnetic fields makes three important devices possible:
Alternator: A device that generates AC from a source of rotating motion, such as a turbine powered by flowing water, steam or a windmill. Alternators work by using the rotating motion to spin a magnet that’s placed within a coil of wire. As the magnet rotates, its magnetic field moves, which induces an AC in the coiled wire. (Coils of wires are used instead of straight wires simply because coiling up the wire allows a greater length of wire to be exposed to the changing magnetic field.) For more details, flip to the later section ‘Understanding Alternators’.
Motor: The opposite of an alternator. A motor converts AC to rotating motion. In its simplest form, a motor is simply an alternator that’s connected backwards. A magnet is mounted on a shaft that can rotate; the magnet is placed within the turns of a coil of wire. When AC is applied to the coil, the rising and falling magnetic field created by the current causes the magnet to spin, which turns the shaft. Check out ‘Meeting up with Motors’ later in this chapter to find out more.
Transformer: Consists of two coils of wire placed within close proximity. When an AC is placed on one of the coils, the collapsing and expanding magnetic field induces an AC in the other coil. We discuss transformers later in the ‘Thinking about Transformers’ section.
Measuring Alternating Current
With DC, determining the voltage that’s present between two points is easy: you simply measure the voltage with a voltmeter (see Book I, Chapter 8). But with AC, measuring the voltage isn’t so simple, because the voltage in an AC circuit is constantly changing. So, for example, when people say that the voltage at a wall socket is 230 VAC, what does that mean in practice?
You can measure voltage in an AC circuit in three ways, as Figure 1-1 illustrates:
Peak voltage: A measurement of the largest voltage present between 0 V and the highest point on the AC cycle (check out the preceding section for a little more). Peak voltage is the maximum voltage that the AC voltage attains.
Peak-to-peak voltage: The difference between the highest and lowest peaks of the AC voltage. In most AC voltages, the peak-to-peak voltage is double the peak voltage.
RMS voltage: The average voltage of the circuit and also called the mean voltage. RMS voltage is far and away the most common way to specify the voltage of an AC circuit. For example, when people say that the voltage at a household electrical outlet is 230 VAC, they mean that the RMS voltage is 230 V.
If the AC voltage follows a true sine wave (which we define in Book I, Chapter 9), the RMS voltage is equal to 0.707 multiplied by peak voltage. Or to turn it around, the peak voltage is equal to about 1.4 multiplied by the RMS voltage. Thus, the actual peak voltage at a household electrical outlet is about 322 V.
Figure 1-1: Three ways to measure alternating current.
RMS stands for root mean square, but that’s important only if you’re studying for an exam. The true RMS voltage is a bit tricky to work out, because it involves some fairly complicated maths. You calculate RMS by sampling the actual voltage in very small time increments. Then, you square the sample voltages, add up the squares of the voltages and calculate the average of all the squared values. Finally, you calculate the square root of the average to obtain the actual RMS value.
For a true sine wave, the preceding calculation turns out to be very close to multiplying the peak voltage by 0.707. For AC voltages that aren’t true sine waves, however, the precise RMS value can be different from what the ‘multiply by 0.707’ shortcut indicates.
Nearly all AC voltmeters report the RMS voltage, but only more expensive AC voltmeters calculate the actual RMS by sampling the input voltage and doing the sum-of-the-squares thing. Inexpensive voltmeters simply measure the peak voltage and multiply it by 0.707. Fortunately, this result is close enough for most purposes.
Don’t try to measure a household mains voltage, not even with a voltmeter whose scale seems to cover 230 VAC. Voltmeters and other electrical measurement meters often lack the required isolation, probes and other safety measures needed to measure such a dangerous voltage.
Understanding Alternators
One good way to get your mind around how AC works is to look at the device that’s most often used to generate it: the alternator. As we describe in the earlier section ‘What Is Alternating Current?’, an alternator is a device that converts rotary motion into electric current. By its very nature, an alternator creates AC.
Figure 1-2 shows a simplified diagram of how an alternator works. Essentially, a large magnet is placed within a set of stationary wire coils. The magnet is mounted on a rotating shaft that’s connected to a turbine or windmill. When water or steam flows through the turbine or when wind turns the windmill, the magnet rotates.
As the magnet rotates, its magnetic field moves across the coils of wire. The phenomenon of electromagnetic induction means that the moving magnetic field induces an electric current within the wire coils. The strength and direction of this electric current depends on the position and direction of the rotating magnet.
Figure 1-2: An alternator generates alternating current from a rotating magnet: S = south; N = north.
In Figure 1-2, you can see how the current is induced in the wire at four different positions of the magnet’s rotation. Here’s the cycle:
1. Position a shows the magnet at its farthest point away from the coils and oriented in the same direction as the coils. At this moment, the magnetic field doesn’t induce any electric current at all. Thus, the light bulb is dark.
But as the magnet begins to rotate clockwise, the magnet comes closer to the coils, thus exposing more of its magnetic field to the coils. The moving magnetic field induces a current that gets stronger as the magnet continues to rotate closer to the coils, which causes the light bulb to glow.
2. Position b shows where the magnet reaches its closest point to the coils. At this point, the current and the voltage are at their maximum and the light bulb glows at its brightest.
As the magnet continues to rotate clockwise, it begins to move away from the coil. The moving electric field continues to induce current in the coil, but the current (and the voltage) decreases as the magnet retreats farther away from the coils.
3. Position c shows the magnet reaching its farthest point from the coils, when the current stops and the light bulb goes dark. As the magnet continues to rotate, it gets closer again to the coils.
4. Position d shows that this time the polarity of the magnet is reversed. Thus, the electric current induced in the wire by the moving magnetic field is in the opposite direction. Again, the light bulb glows as the current passing through it increases.
And so on. With each revolution of the magnet, voltage starts at zero and rises steadily to its maximum point, and then falls until it reaches zero again. Then the process is reversed, with the current flowing in the opposite direction.
Here are a few other interesting facts about alternators:
The term generator refers to any device that converts mechanical energy into electrical energy. An alternator is a specific type of generator, and so people commonly – and quite correctly – refer to an alternator as a generator.
You can generate DC from rotating magnetic fields. A DC generator is more complex than an alternator, however, and contains additional components that can wear out over time.
The rate at which the magnet rotates dictates the frequency of the AC generated by an alternator: the faster the magnet rotates, the higher the frequency of the resulting AC.
If you place two sets of coils spaced evenly around the magnet, each forming its own complete circuit, AC is induced in each set of coils. However, the polarities of the two voltages are mirror images of one another. In other words, when the voltage is positive in one of the circuits, it’s negative in the other. The relationship between the polarity of the circuits is called a phase and a power-generating system with two circuits arranged in this way is called a two-phase system. The two circuits are said to be 180 degrees out of phase with one another.
If you use three sets of coils, the system is called a three-phase system and the three circuits are 120 degrees out of phase. Most public power-generation systems are three-phase systems, because that results in the most efficient generation of power from the rotating magnetic fields.
Meeting up with Motors
An electric motor converts electrical energy in the form of electric current to rotating mechanical energy. The simplest type of electric motor is essentially the same thing as an alternator (see the preceding section). The difference is that instead of using some other mechanical force such as water or steam to turn the magnet – which in turn induces electric current in the coils – electric current is applied to the coils, which in turn causes the magnet to rotate.
We don’t need to show you a diagram depicting the operation of a motor, because it looks pretty much like the alternator diagram in the earlier Figure 1-2. The only difference is that the light bulb is replaced by an AC power source. The same phenomenon that causes an electric current to be induced in a coil when the coil passes through a moving magnetic field causes a magnetic field to be created when a current is passed through a coil. The magnetic field, in turn, causes the magnet to rotate. This rotation is transferred to the shaft to which the magnet is attached.
As with alternators, you can create motors that work with DC circuits instead of AC, but DC motors are more complicated than AC motors. In a DC motor, the polarity of the coils must be reversed every half-revolution of the magnet to keep the magnet moving in complete rotations. Usually, metal brushes are used to accomplish this task. In an AC motor, the brushes aren’t necessary because the AC reverses polarity on its own.
Thinking about Transformers
In Book II, Chapter 4, you discover the basic principles of magnetism and inductance, as follows:
Magnetism: A changing current passing through a wire creates a moving magnetic field around the wire.
Inductance: A changing current is induced in a wire that’s exposed to a moving magnetic field.
A transformer is a device that exploits and combines these two principles by placing two coils of wire in close proximity to one another, as shown in Figure 1-3. When a source of AC is connected to one of the coils, that coil creates a magnetic field that expands and collapses in concert with the changing voltage of the AC. In other words, as the voltage increases across the coil, the coil creates an expanding magnetic field. When the voltage reaches its peak and begins to decrease, the magnetic field created around the coil begins to collapse.
Figure 1-3: A transformer uses magnetic induction to pass current from one circuit to another.
The second coil is located within the magnetic field created by the first coil. As the magnetic field expands, it induces current in the second coil. The voltage across the second coil increases as long as the magnetic field expands. When the magnetic field begins to collapse, the voltage across the second coil begins to decrease.
Thus, the current induced in the second coil mirrors the current that’s passed through the first coil. A small amount of energy is lost in the process, but if the transformer is well constructed the strength of the current induced in the second coil is very close to the strength of the current passed through the first coil.
Coiling up to create energy
All transformers have a primary and a secondary coil:
Primary coil: The first coil in a transformer – the one that’s connected to the AC voltage.
Secondary coil: The second coil – the one in which an AC voltage is induced.
One of the most useful characteristics of a transformer is that the voltage induced in the secondary coil is equal to the voltage applied to the primary coil multiplied by the ratio of the number of turns in the primary coil and the second coil.
In the simplest case, where the primary and secondary coils have the same number of turns, the voltage induced in the secondary coil is the same as the voltage applied to the primary coil.
If the primary coil has more turns than the secondary coil, the voltage induced in the secondary coil is less than the voltage applied to the primary. How much less depends on the ratio of the turns in the primary and secondary coils. If the secondary coil has half as many turns as the primary coil, the voltage induced in the secondary coil is half the voltage applied to the primary coil. For example, if you apply 230 VAC to the primary coil, 115 VAC is induced in the secondary coil.
If the secondary coil has more turns than the primary coil, the induced voltage is more than the voltage applied to the primary coil. For example, suppose that the primary coil has 1,000 turns and the secondary coil has 2,000 turns. In this case, if you apply 115 V to the primary coil, 230 V is induced in the secondary coil.
A transformer whose primary coil has more turns than its secondary coil is called a step-down transformer because it reduces voltage – that is, the voltage at the secondary coil is less than the voltage at the primary coil. Similarly, a transformer that has more turns in the secondary than in the primary is called a step-up transformer because it increases voltage.
Although the voltage increases in a step-up transformer, the current is reduced proportionately. For example, if the primary coil has half as many turns as the secondary coil, the voltage induced in the secondary coil is twice the voltage that’s applied to the primary coil, but the current that flows through the secondary coil is half the current flowing through the primary coil.
Similarly, when the voltage decreases in a step-down transformer, the current increases proportionately. Thus, if the voltage is cut in half, the current doubles.
This effect makes perfect sense, because a transformer can’t just conjure up power out of thin air (otherwise the planet’s energy problems would have been solved long ago). Unfortunately, free energy simply doesn’t exist.
The basic formula for calculating electric power is power in watts equals voltage multiplied by current in amperes (as we discuss in Book I, Chapter 2):
P = V × I
A transformer transfers power from the primary coil to the secondary coil. The power must stay the same, and so if the voltage increases, the current has to decrease. Likewise, if the voltage decreases, the current has to increase.
Producing huge power efficiently with AC
Transformers are the main reason why people use AC instead of DC in large power-distribution systems. When you send large amounts of power over a long distance, transmitting that power in the form of high voltage and low current is much more efficient – which is why overhead power-transmission lines often carry voltages as high as 400,000 VAC. Such high voltages allow the electrical power to be transmitted using much smaller wires than would be required if the same amount of power were transmitted at 230 VAC.
Power-distribution systems use large step-up transformers to increase voltages generated at power plants to hundreds of thousands of volts. Then, as the power gets closer to its final destination (such as your house), a series of step-down transformers drops the voltage down to more manageable levels, until the voltage is dropped to its final level (230 VAC) before it enters your house.
Transformers work only with AC, because the change of the magnetic field created by the primary coil is what induces voltage in the secondary coil. To create a changing magnetic field, the voltage applied to the primary coil must be constantly changing. But DC is a steady, fixed voltage and so creates a fixed magnetic field that doesn’t induce voltage in the secondary coil.