In a shunt wound DC generator, the field winding is connected in parallel (shunt) with the armature winding. Both the field circuit and the external load receive voltage from the same armature terminals, with the field winding drawing a small current to create the magnetic field while the armature supplies the bulk of current to the load. This parallel arrangement is the defining feature that separates a shunt generator from series and compound types.
Basic Winding Connections
A shunt DC generator has two main circuits: the armature circuit and the shunt field circuit. The armature is the rotating component that generates voltage, and it has two terminals typically labeled A1 and A2. The shunt field winding, which wraps around the pole pieces of the stator, has two terminals labeled F1 and F2.
To connect the generator, the shunt field winding (F1 and F2) is wired directly across the armature terminals (A1 and A2), forming a parallel path. Because the field winding is designed with many turns of relatively fine wire, it has high resistance and draws only a small percentage of the total armature current. The external load also connects across the armature terminals, in parallel with the field winding. So the armature current splits: a small portion flows through the field to maintain the magnetic field, and the rest flows through the load to do useful work.
If the machine has no series field winding, the series terminals (sometimes labeled S1 and S2) are disregarded entirely.
The Role of the Field Rheostat
In practice, a variable resistor called a field rheostat is connected in series with the shunt field winding before this combination is placed in parallel with the armature. The rheostat lets you adjust the current flowing through the field coils, which directly controls the strength of the magnetic field and, in turn, the generator’s output voltage.
To bring a shunt generator up to its rated voltage, you typically start with the rheostat at maximum resistance, then gradually decrease it. As resistance drops, more current flows through the field winding, the magnetic field strengthens, and the terminal voltage rises. A lab procedure from Southern Illinois University demonstrates this clearly: the field rheostat is turned counterclockwise to reduce resistance until the output reaches the desired voltage, such as 125 volts DC.
How Voltage Builds Up
A shunt generator can only produce voltage if its pole pieces retain a small amount of residual magnetism from previous operation. When the armature spins, this residual magnetism induces a tiny voltage across the armature terminals. Once the field winding is connected, that small voltage pushes a small current through the field coils, which strengthens the magnetic field slightly. A stronger field induces a higher armature voltage, which pushes even more current through the field, and the cycle continues until the voltage stabilizes at its operating level.
This self-excitation process has one important limit: the total resistance of the field circuit (the winding plus any rheostat) must stay below a value known as the critical field resistance. This is the maximum field circuit resistance at which the generator can still build up voltage at a given speed. If the rheostat is set too high or the field winding resistance is too large, the voltage buildup stalls and the generator produces little more than the tiny residual voltage. The critical resistance is essentially a tangent line to the generator’s open-circuit characteristic curve, and it shifts depending on rotational speed.
What Happens Under Load
When you connect a load to a shunt generator, the terminal voltage drops somewhat compared to the no-load voltage. Two things cause this. First, current flowing through the armature winding creates a voltage drop across the armature’s own internal resistance. Second, the load current flowing through the armature creates a magnetic effect called armature reaction, which weakens the main field. The greater the load current, the more the field is weakened, and the lower the voltage actually induced in the armature.
This means a shunt generator’s output voltage gradually decreases as you draw more current from it. The relationship between terminal voltage and load current is called the external characteristic curve, and it slopes downward. For applications requiring tighter voltage regulation, you can manually adjust the field rheostat to compensate, increasing field current to bring the voltage back up as load increases.
Connecting Shunt Generators in Parallel
When a single generator can’t handle the full load, two or more shunt generators can be connected in parallel on the same set of busbars. The process requires careful steps to avoid damaging equipment or creating dangerous circulating currents between machines.
- Check polarity. The positive terminal of the incoming generator must connect to the positive busbar, and the negative to the negative busbar. This is especially important if the generator is newly installed or recently repaired.
- Match voltage. Close the field switch, start the prime mover, and adjust the incoming generator’s field rheostat until its terminal voltage equals the bus voltage. Raise it slightly above bus voltage so the generator immediately picks up load when connected.
- Close the breaker. Once voltages match, close the load breaker to bring the generator on-line.
- Balance the load. After both generators are running, adjust each machine’s field rheostat to share the load as desired.
Shunt generators are well suited to parallel operation because their drooping voltage characteristic provides natural stability. If one machine tries to take more than its share of the load, its terminal voltage drops, which automatically reduces its output current and rebalances the system.
Why Connection Polarity Matters
If the field winding is accidentally connected with reversed polarity relative to the residual magnetism, the field current will oppose the residual flux instead of reinforcing it. The result is that the generator fails to build up voltage, or the voltage collapses instead of rising as you reduce the rheostat. Correcting this is straightforward: either swap the field connections (reverse F1 and F2) or, if the residual magnetism has been lost entirely, briefly “flash” the field with an external DC source to re-establish it in the correct direction.
Getting the connections right from the start, with correct terminal identification and proper polarity, is what makes the difference between a shunt generator that smoothly builds to rated voltage and one that sits there producing nothing.