It has two main parts — Page 96, Lesson 91

I want to walk you through how a voltage regulator works in a DC generator or alternator. This is a critical piece of equipment because without it, the output voltage would vary wildly with engine speed and electrical load — and that's not acceptable in an aircraft electrical system. Let's start with what a voltage regulator consists of. It has two main parts. First, a variable resistance placed in series with the field coil. In older regulators, this variable resistance was achieved using a carbon pile — a stack of carbon discs. In modern regulators, it's done with an electronic solid-state system using transistors, diodes, and resistors. The net result is the same either way: you get a resistance that can be changed. Second, there's a control coil wired in parallel with both the field coil and the armature. This control coil senses the generator's output voltage and varies that resistance to control the current through the field coil, and therefore controls the output voltage. So the voltage regulator senses the output voltage of the generator or alternator, and adjusts the field current to maintain the correct output voltage — regardless of generator speed or electrical load. That's its job. Now let's look specifically at the carbon pile voltage regulator. The carbon pile itself is a stack of carbon discs. Its overall resistance is proportional to how much the stack is compressed. The more you compress the stack, the lower the resistance. So it acts as a variable resistor. In Figure 6.15, which I have on screen, the control coil is in parallel with the generator armature, so the generator output voltage is supplied across it. Because the control coil has a fixed resistance, and Ohm's Law tells us V equals I times R, the current through the control coil will vary in direct proportion to the generator output voltage. As that current varies, so does the strength of the magnetic field produced by the control coil. That magnetic field strength from the control coil affects the compression of the carbon pile — the variable resistance — which is in series with the field coil. As the resistance changes, again by Ohm's Law V equals I R, the current in the field coil changes. As the field coil current changes, the strength of its magnetic field changes, and therefore the EMF induced into the armature changes. That means the output voltage of the generator is controlled automatically. One important note: in Figure 6.15, the field coil is shown outside the generator for clarity. In reality, it's an integral part of the generator construction — it's built right inside. There's also another type: the vibrating contact voltage regulator. It controls the voltage output in a similar fashion, but instead of varying a resistance continuously, it rapidly switches a fixed resistance in and out of the circuit. That achieves the same net effect — controlling the average field current and therefore the output voltage — but by a different method.