A DC generator may be constructed in a variety of ways depending upon the relationship and location of each of the fields. Each type of construction contains certain advantages.

EO 1.10 DESCRIBE the differences in construction between a shunt-wound and a series-wound DC generator with respect to the relationship between the field and the armature.

EO 1.11 DESCRIBE the relationship between the shunt and series fields for cumulatively-compounded and differentiallycompounded DC generators.

EO 1.12 DESCRIBE the voltage-vs-load current characteristics for a flat-compounded, over-compounded, and undercompounded DC generator.

Shunt-Wound DC Generators

When the field winding of a generator is connected in parallel with the generator armature, the generator is called a shunt-wound generator (Figure 8).

The excitation current in a shunt-wound generator is dependent upon the output voltage and the field resistance. Normally, field excitation is maintained between 0.5 and 5 percent of the total current output of the generator.

Figure 8 Shunt-Wound DC Generator

The shunt-wound generator, running at a constant speed under varying load conditions, has a much more stable voltage output than does a series-wound generator. Some change in output voltage does take place. This change is caused by the fact that, as the load current increases, the voltage drop (I_aR_a) across the armature coil increases, causing output voltage to decrease. As a result, the current through the field decreases, reducing the magnetic field and causing voltage to decrease even more. If load current is much higher than the design of the generator, the drop in output voltage is severe. For load current within the design range of the generator, the drop in output voltage is minimal (Figure 9).

Figure 9 Output Voltage-vs-Load Current for Shunt-Wound DC Generator