DC Electrics - Generators and Alternators — Page 96, Lesson 87

I want to walk you through the compound wound DC generator and the concept of flashing the field. Let's start with the compound wound generator itself. A compound wound generator is a DC generator that combines both series and shunt windings. That means it has two sets of field coils. One set is connected in series with the armature — so all the armature current flows through it. The other set is connected in parallel with the armature — that's the shunt winding. One shunt coil and one series coil are always mounted together on a common pole piece, and sometimes they're even enclosed in a common covering. Now, why would we build a generator this way? The compound wound design was created to overcome a specific problem that occurs in a shunt wound generator. In a shunt wound generator, when you increase the electrical load — meaning you draw more current from it — the terminal voltage drops. That voltage drop is undesirable when you're powering loads that need a constant voltage, which is exactly what we have on an aircraft. Here's how the compound wound generator fixes that. By adding the series field coil, when the load current increases, that series field strengthens the total magnetic field. This increase in magnetic field strength counteracts the voltage drop that would otherwise be caused by the increased load current flowing through the resistance of the armature windings. The result is that we can obtain an almost constant voltage output, even as the load changes. Now let's move to a different but related topic: flashing the generator field. A DC generator is normally what we call self-excited. That means it doesn't need an external power source to start generating electricity. Why? Because of residual magnetism — a small amount of magnetism that remains in the field pole pieces even when the machine is inactive or static. Here's how self-excitation works: as soon as the generator starts rotating, that residual magnetism produces a small voltage. Some of that voltage is fed back to the field coil, which increases the magnetic field strength, which in turn causes the voltage to increase further. This process continues until the voltage reaches its controlled, regulated value. But there's an alternative: an externally excited generator. This is a generator that has no residual magnetism at all. In that case, you need a battery to supply current to the field coil to start the generating process. Now, here's the important practical point. Residual magnetism can be lost, destroyed, or even reversed. This can happen due to several causes: the passage of time, the effects of heat, exposure to an AC field, physical hammering or shock, or the application of a reversed polarity. If a DC generator loses its residual magnetism, it won't be able to build up any output voltage — it simply won't start generating. How do we fix that? We correct it by momentarily passing a current through the field in the normal direction. This procedure is known as "flashing the field." In practice, some aircraft might have a button or a switch in the cockpit that allows the pilot to carry out this procedure from the flight deck.