DC Electrics - Generators and Alternators — Page 88, Lesson 86

I want to walk you through the shunt wound DC generator — a key configuration you'll see in aircraft electrical systems. Let's start with the name itself: "shunt wound" tells us how the field winding is connected. In a shunt wound DC generator, the field winding is connected in parallel with the armature. The word "shunt" simply means a parallel connection. So instead of all the current going through the field coils first, the field winding sits across the same two points as the armature — it shares the terminal voltage. Now, because the field winding is in parallel with the armature, the current flowing through those field coils is determined by two things: the terminal voltage across the generator output, and the resistance of the field winding itself. That's a direct application of Ohm's law — current equals voltage divided by resistance. Here's an important design detail: the shunt field windings have a large number of turns. That means they can produce the necessary magnetic field flux with a relatively small current. So even though the field current is small, the magnetic field is strong enough to generate the rated output. Let's talk about what happens when you start a shunt generator. When it's first started, the build-up time to reach rated terminal voltage is very rapid. Rated terminal voltage is defined here as the maximum voltage at which the generator can continuously supply its rated load current. And why does it build up so fast? Because field current flows even though the external circuit is open — meaning even with no load connected, the field winding is already drawing current from the armature's own generated voltage, and that positive feedback causes the voltage to rise quickly. Now look at the characteristic curve — Figure 6.12 shows both a schematic diagram and the load curve for the shunt generator. Over the normal operating range from no load to full load, the drop in terminal voltage as the load current increases is relatively small. That's a critical performance feature. Because the voltage stays nearly constant, the shunt generator is used where a virtually constant voltage is desired, regardless of how the load changes. Finally, how do you control the terminal voltage? By a variable resistance connected in series with the shunt field coils. By adjusting that resistance, you change the field current, which changes the magnetic flux, which changes the generated voltage. That's your voltage control method. So to summarise the key points: shunt wound means field in parallel with armature; large number of turns means small field current; rapid voltage build-up because field current flows even with no load; small voltage drop from no load to full load; and voltage control via a variable resistor in series with the field. That's the shunt wound DC generator.