DC Electrics - Generators and Alternators — Page 88, Lesson 83

I want to walk you through how we turn the AC output from a simple generator into DC — direct current — which is what aircraft electrical systems need. We start with the simple AC generator we just looked at. That produced an alternating EMF — electromotive force — in the armature coil as it rotated through the magnetic field. But aircraft DC systems need a steady DC output at the generator terminals. So we have to change that AC EMF into DC. The component that does this is the split ring commutator. Let me be precise about what that is. A split ring commutator is constructed from a single ring of conductive material, but with an insulator electrically separating each half of the ring. So you have two half-rings — segment A and segment B — that are not electrically connected to each other. The armature is built so that one end of the wire loop connects to one conductor half of the split ring, and the other end of the loop connects to the other half. The commutator rotates together with the armature — they are on the same shaft. Now, how do we get electrical continuity from the rotating armature out to the fixed external circuit? That's done using carbon brushes. These are stationary blocks of carbon that press against the rotating commutator. They provide the electrical connection from one side of the armature, through the armature circuit, and out to the other side. Here is the key action — what happens as the armature rotates. Look at the voltage output graph in Figure 6.9. As the armature rotates from 0° to 180°, the positive brush is in contact with commutator segment A, and the negative brush is in contact with commutator segment B. Then, as it rotates from 180° to 360°, the positive brush is now in contact with commutator segment B, and the negative brush is in contact with commutator segment A. So every 180° of rotation, the connections to the armature terminals are effectively reversed. What does this reversal achieve? It means that the current and voltage in the armature circuit, after passing through the commutator and brushes, become DC — direct current. The alternating waveform that was induced in the armature gets "flipped" every half-cycle so that the output at the brushes always has the same polarity. The result is a pulsating DC output, not a smooth one yet, but it is unidirectional. Let me show you the diagrams that illustrate this. That's Figure 6.8 — the simple DC generator with the split ring commutator labelled. And that's Figure 6.9 — the DC voltage output waveform showing how the AC gets rectified into a series of positive half-cycles. So to summarise: the split ring commutator replaces the slip rings of an AC generator. It rotates with the armature and, together with the stationary carbon brushes, mechanically reverses the armature connections every 180°. This process is called commutation, and it converts the AC EMF induced in the armature into a DC output at the generator terminals.