DC Electrics - Generators and Alternators — Page 88, Lesson 81

I want to walk you through the fundamental principles that govern how generators and alternators work. We're starting with two laws that are the absolute bedrock of everything that follows: Faraday's Law and Lenz's Law. Faraday's Law states that when the magnetic flux through a coil is made to vary, a voltage is set up. The magnitude of this induced voltage is proportional to the rate of change of flux. In plain language, if you have a coil of wire and you change the magnetic field passing through it, you'll create a voltage across that coil. The faster you change that magnetic field, the higher the voltage you get. This is the principle of electromagnetic induction, and it's how every generator produces electricity. Now, Lenz's Law adds the direction of that effect. It states that a change of flux through a closed circuit induces a voltage and sets up a current. The direction of this current is such that its magnetic field tends to oppose the change in flux. So the current that gets induced creates its own magnetic field that fights against whatever caused the change in the first place. This opposition produces what we call a back EMF — back electromotive force. We'll see that term again when we get to the chapter on motors. Let's apply these laws to the simplest form of generator. Picture a single loop of wire turning in a fixed magnetic field produced by a permanent magnet. The closed circuit is made by attaching rotating slip rings to both ends of the loop, and these slip rings are in contact with stationary carbon brushes. Continuous contact between the slip rings and the brushes is maintained by spring pressure. The brushes are attached to cables which form a closed circuit. Let me define the key parts here. The rotating loop is known as the armature. The magnetic field is termed the field. In a simple generator, the armature rotates in the field. An EMF is induced in the armature by electromagnetic induction. The slip rings, brushes, and cables complete the closed circuit, and current will flow. Now, because this setup uses slip rings — which maintain continuous electrical contact as the loop rotates — this type of generator produces an AC voltage in the armature and therefore an Alternating Current in the external circuit. That means the current first flows one way, then changes direction and flows the opposite way. If you look at the voltage output as the armature rotates, you get a sine wave. The maximum voltage is induced when there is maximum cutting of lines of flux — that's when the armature sides are moving perpendicular to the magnetic field lines. The position where no voltage is induced, when the armature is moving parallel to the lines of flux, is known as the neutral plane. In the sine wave cycle, that corresponds to positions 1, 3, and 5 on the waveform. Now here's an important refinement. Instead of using a permanent magnet, we can wrap a coil of wire around the two poles of the magnet. Passing a current through this coil will allow the magnetic field strength to be increased, and that increases the voltage output of the generator. This coil is termed the field coil, and it's used to control the voltage to a fixed value irrespective of the generator speed. That's a critical capability — in an aircraft, the generator speed varies with engine RPM, but we need a stable voltage output. The field coil gives us that control.