DC generator external characteristics

Reference notes

Use Next → on the narrator above to step through the external characteristics of the four classical DC generator types.

What "external characteristic" means

Plot terminal voltage V_t on the y-axis and load current I_L on the x-axis, with the prime mover holding shaft speed constant. The shape of the resulting curve is the generator's external characteristic. It tells you, at a glance, how the generator's terminal voltage behaves as the load grows.

From the equivalent circuit, V_t = E_a − I_a·R_a (minus any armature-reaction effect). But how E_a itself responds to load depends entirely on how the field is connected.

1. Separately excited — small linear droop

Field current I_f is supplied from an external DC source — independent of the armature. Φ is constant. So E_a = K_a·Φ·ω stays constant. The only drop is I_a·R_a plus a small extra term from armature reaction:

Result: a nearly straight line with slope ≈ −R_a. Typical full-load droop is 1–3 %. Best regulation of any DC generator, but you pay the cost of a separate exciter.

2. Shunt-wound — regenerative droop

Field winding is across the armature terminals: I_f = V_t / R_f. So as V_t drops under load, I_f also drops, which weakens Φ, which lowers E_a, which drops V_t further. Regenerative droop — the voltage drop is amplified by this feedback loop.

• Light load: V_t close to no-load value (~5–8 % droop at rated load).
• Overload: voltage collapses sharply. Beyond a critical load, the operating point falls off the magnetisation curve and the generator de-excites — V_t goes to (almost) zero.

3. Series-wound — rising then falling

Field winding is in series with the armature: same current flows in both. Φ ∝ I_a (before saturation). So E_a = K_a·Φ·ω grows with load — the increase in E_a can easily exceed the I_a·R_a drop. Result:

• Voltage RISES as load grows, from a small no-load value (only residual flux).
• At saturation, Φ stops growing, but I_a·R_a keeps growing → V_t peaks and starts to fall.
• Used in series boosters (to compensate line drop on long DC feeders) and as railway dynamic braking resistors. Standalone use is rare because of the strong load-dependence.

4. Compound — three sub-flavours

A shunt field provides baseline flux; a small series field adds load-dependent flux. The series field's polarity decides the behaviour:

• Cumulative compound: shunt and series fields aid each other. Tunable by the number of series turns:
  • Over-compounded: V_t RISES with load (series boost > armature drop). Used where line losses must be compensated.
  • Flat-compounded: V_t stays approximately constant from no-load to full load. Ideal for many applications.
  • Under-compounded: V_t still droops, but less than shunt alone.
• Differential compound: shunt and series oppose. Voltage droops more than shunt; can become unstable. Rarely used.

Comparison at rated load

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