After the discovery that an electric current flowing through a conductor creates a magnetic field around the conductor, there was considerable scientific speculation about whether a magnetic field could create a current flow in a conductor. In 1831, Faraday discovered that this could be accomplished. To show how an electric current can be created by a magnetic field, a demonstration similar to Figure 12-104 can be used. Several turns of a conductor are wrapped around a cylindrical form, and the ends of the conductor are connected together to form a complete circuit, which includes a galvanometer. If a simple bar magnet is plunged into the cylinder, the galvanometer can be observed to deflect in one direction from its zero (center) position. [Figure 12-104A]
When the magnet is at rest inside the cylinder, the galvanometer shows a reading of zero, indicating that no current is flowing. [Figure 12-104B]
In Figure 12-104C, the galvanometer indicates a current flow in the opposite direction when the magnet is pulled from the cylinder.
The same results may be obtained by holding the magnet stationary and moving the cylinder over the magnet, indicating that a current flows when there is relative motion between the wire coil and the magnetic field. These results obey a law first stated by the German scientist, Heinrich Lenz. Lenz’s Law states that the induced current caused by the relative motion of a conductor and a magnetic field always flows in such a direction that its magnetic field opposes the motion.
When a conductor is moved through a magnetic field, an emf is induced in the conductor. [Figure 12-105]
The direction (polarity) of the induced emf is determined by the magnetic lines of force and the direction the conductor is moved through the magnetic field. The generator left-hand rule (not to be confused with the left-hand rules used with a coil) can be used to determine the direction of the induced emf. [Figure 12-106]
The left-hand rule is summed up as follows:
The first finger of the left hand is pointed in the direction of the magnetic lines of force (north to south), the thumb is pointed in the direction of movement of the conductor through the magnetic field, and the second finger points in the direction of the induced emf.
When a loop conductor is rotated in a magnetic field, a voltage is induced in each side of the loop. [Figure 12-107]
The two sides cut the magnetic field in opposite directions, and although the current flow is continuous, it moves in opposite directions with respect to the two sides of the loop. If sides A and B and the loop are rotated half a turn and the sides of the conductor have exchanged positions, the induced emf in each wire reverses its direction, since the wire formerly cutting the lines of force in an upward direction is now moving downward.
The value of an induced emf depends on three factors:
- Number of wires moving through the magnetic field
- Strength of the magnetic field
- Speed of rotation
Generators of Alternating Current
Generators used to produce an alternating current are called AC generators or alternators.
The simple generator constitutes one method of generating an alternating voltage. [Figure 12-108] It consists of a rotating loop, marked A and B, placed between two magnetic poles, N and S. The ends of the loop are connected to two metal slip rings (collector rings), C1 and C2. Current is taken from the collector rings by brushes. If the loop is considered as separate wires A and B, and the left-hand rule for generators is applied, then it can be observed that as wire A moves up across the field, a voltage is induced which causes the current to flow inward. As wire B moves down across the field, a voltage is induced which causes the current to flow outward. When the wires are formed into a loop, the voltages induced in the two sides of the loop are combined. Therefore, for explanatory purposes, the action of either conductor, A or B, while rotating in the magnetic field is similar to the action of the loop.
Figure 12-109 illustrates the generation of AC with a simple loop conductor rotating in a magnetic field. As it is rotated in a counterclockwise direction, varying values of voltages are induced in it.
The conductor A moves parallel to the lines of force. Since it cuts no lines of force, the induced voltage is zero. As the conductor advances from position 1 to position 2, the voltage induced gradually increases.
The conductor is now moving perpendicular to the flux and cuts a maximum number of lines of force; therefore, a maximum voltage is induced. As the conductor moves beyond position 2, it cuts a decreasing amount of flux at each instant, and the induced voltage decreases.
At this point, the conductor has made one-half of a revolution and again moves parallel to the lines of force, and no voltage is induced in the conductor. As the A conductor passes position 3, the direction of induced voltage now reverses since the A conductor is moving downward, cutting flux in the opposite direction. As the A conductor moves across the south pole, the induced voltage gradually increases in a negative direction, until it reaches position 4.
Like position 2, the conductor is again moving perpendicular to the flux and generates a maximum negative voltage. From position 4 to 5, the induced voltage gradually decreases until the voltage is zero, and the conductor and wave are ready to start another cycle.
The curve shown at position 5 is called a sine wave. It represents the polarity and the magnitude of the instantaneous values of the voltages generated. The horizontal base line is divided into degrees, or time, and the vertical distance above or below the base line represents the value of voltage at each particular point in the rotation of the loop.
Cycle and Frequency
A cycle is a repetition of a pattern. Whenever a voltage or current passes through a series of changes, returns to the starting point, and then again starts the same series of changes, the series is called a cycle. The cycle is represented by the symbol of a wavy line in a circle.In the cycle of voltage shown in Figure 12-110, the voltage increases from zero to a maximum positive value, decreases to zero; then increases to a maximum negative value, and again decreases to zero. At this point, it is ready to go through the same series of changes. There are two alternations in a complete cycle: the positive alternation and the negative. Each is half a cycle.
The frequency is the number of cycles of AC per second (1 second). The standard unit of frequency measurement is the hertz (Hz). [Figure 12-111] In a generator, the voltage and current pass through a complete cycle of values each time a coil or conductor passes under a north and south pole of the magnet. The number of cycles for each revolution of the coil or conductor is equal to the number of pairs of poles. The frequency, then, is equal to the number of cycles in one revolution multiplied by the number of revolutions per second (rps).
Expressed in equation form:
where P⁄2 is the number of pairs of poles, and rpm/60 the number of revolutions per second. If in a 2-pole generator, the conductor is turning at 3,600 rpm, the revolutions per second are:
Since there are 2 poles, P⁄2 is 1, and the frequency is 60 cycles per second (cps). In a 4-pole generator with an armature speed of 1,800 rpm, substitute in the equation:
The time required for a sine wave to complete one full cycle is called a period. [Figure 12-110] The period of a sine wave is inversely proportional to the frequency: the higher the frequency, the shorter the period.
The mathematical relationship between frequency and period is given as:
The distance that a waveform travels during a period is commonly referred to as a wavelength and is indicated by the Greek letter lambda (l). The measurement of wavelength is taken from one point on the waveform to a corresponding point on the next waveform. [Figure 12-110]