DC Electrics - Generators and Alternators — Page 88, Lesson 79

I want to walk you through the fundamental principle that makes aircraft generators and alternators work: electromagnetic induction. Let's start with the big picture. Batteries give us DC electricity by converting chemical energy, but they're not inexhaustible — they go flat and need recharging. The primary source of electricity in an aircraft is always the generator or alternator. So understanding how they produce electricity is essential. We generate electricity from magnetism by converting mechanical energy into electrical energy through a process called electromagnetic induction. Here's how it works. If you take a conductor — a wire — and move it through a magnetic field, that conductor will "cut through" the invisible lines of magnetic flux. When that happens, an Electromotive Force, which we call EMF — that's just voltage — is induced into the conductor. But here's the key: this only happens as long as the conductor keeps moving. The moment the conductor stops, the induced EMF stops. It doesn't matter whether you move the conductor or move the magnetic field — what matters is that there is relative movement between the two. Now, if that conductor is connected to a complete circuit, a current will flow in that circuit, and the amount of current is proportional to the induced EMF. Let me show you what this looks like. This is Figure 6.1 — it shows the situation with relative motion between the magnet and the coil. When the magnet moves relative to the coil, you get that induced voltage. Figure 6.2 shows the magnet at rest — no relative motion, so the meter reads zero. And Figure 6.3 shows that the direction of the relative motion determines the direction of current flow. So how do we figure out which direction that current flows? That's where Fleming's Right Hand Rule comes in. Look at Figure 6.4. Here's how you do it. Hold out your right hand. Your first finger points in the direction of the magnetic field — from the North Pole to the South Pole. Your thumb points in the direction of motion — that's the direction the conductor is moving. Your second finger then naturally points in the direction of the induced current. Let me give you the example from the book. In Figure 6.4, the first finger is aligned with the field. The thumb is pointing upward, which is the direction of rotation of the red half of the armature — that's the rotating part of the generator. The second finger then shows the current coming out of that red half, which is the negative half. The blue half of the armature is moving downward, so if you rotate your hand through 180 degrees — keeping the first finger still aligned with the field — your second finger will now show the current going into the armature. Here's an important point: if you reverse either the direction of rotation or the polarity of the magnetic field, the direction of the current reverses. But if you reverse both — both the rotation direction and the field polarity — the direction of current stays the same. Now, what about the strength of that induced voltage? There are three factors that affect the magnitude of the induced EMF, and I want you to remember all three. First, the rate of cutting of lines of force — that's the speed at which the conductor moves through the magnetic field. Increase the speed, and you increase the induced voltage. Second, the strength of the magnetic field — what we call flux density. A stronger magnetic field means a stronger induced EMF. Third, the number of turns of wire on the coil. A larger coil with more turns gives you a higher induced voltage. Figure 6.5 shows these three ways of increasing the strength of the induced EMF: increase the speed, increase the magnetic field strength, or increase the number of turns on the coil. So to summarise what we've covered: electromagnetic induction is how we convert mechanical energy into electrical energy in an aircraft. It requires relative motion between a conductor and a magnetic field. Fleming's Right Hand Rule tells us the direction of the induced current. And the strength of that induced voltage depends on speed, field strength, and the number of turns of wire. This is the foundation for understanding how generators and alternators work.