DC Electrics - Generators and Alternators — Page 96, Lesson 89

I want to walk you through the alternator section now. Most modern light aircraft use an alternator instead of a DC generator to provide constant voltage electricity for the electrical system, and that's because the alternator has several clear advantages. First, the alternator has a much better power-to-weight ratio — it gives you more electrical power for less weight. Second, it will produce a stable output even at low RPM, which is important when the engine is idling or running slowly. Third, it does not suffer from the problems associated with a commutator, because instead of a commutator it uses a rectifier to convert AC to DC. Let me lay out the constructional differences between a DC generator and an alternator, because they are fundamentally different in how the parts are arranged. In a DC generator, the armature — that's the part that rotates and where voltage is induced — is the rotating component. The field, which produces the magnetic field, is stationary. And it converts AC to DC by means of a commutator. The problem is that the commutator suffers from arcing and sparking, because the high load current — the main current supplying the electrical system — has to flow through the commutator and the brushes that ride on it. In an alternator, the arrangement is reversed. The armature is stationary — it does not rotate. The field is the rotating component. And instead of a commutator, the alternator converts AC to DC by means of a rectifier. Because the high load current is taken from the stationary armature, there is no arcing and sparking — that problem is eliminated. Only a small field current flows through the slip rings, so the brushes and slip rings carry very little current and don't suffer the same wear or sparking. Now let's move to voltage control. The output voltage of a generator or alternator depends on four factors. First, the speed of rotation of the armature or field. Second, the strength of the magnetic field. Third, the number of turns in the armature. Fourth, the size and shape of the turns in the armature. Most light aircraft DC electrical systems operate at 14 volts, so all the equipment is designed to work correctly when supplied with 14 volts. That means the output of the generator or alternator must be controlled or regulated to ensure it always supplies 14 volts. Looking at those four factors, the number of turns and the size and shape of the turns are design factors — they are fixed when the machine is built, and the operator cannot alter them. The speed of rotation is linked to the engine speed, because the generator or alternator is driven by a drive belt or an engine accessory gearbox. Controlling the output voltage by controlling engine speed is not a practical solution — you can't ask the pilot to change engine RPM just to keep the electrical system at 14 volts. That leaves the strength of the magnetic field. Remember back to basic magnetism: the strength of the magnetic field produced by a coil of wire is proportional to the current flowing through the coil — that's an electromagnet. So the only practical method of controlling the output voltage is to control the strength of the magnetic field by controlling the current flow in a coil wound around the magnetic pole pieces. That coil is called the field coil or field winding. Control of that current flow is achieved by a voltage regulator. A voltage regulator consists of a variable resistance in series with the field coil. In older voltage regulators, that variable resistance was the key component — by changing the resistance in the field circuit, you change the field current, which changes the magnetic field strength, and that controls the output voltage.