- Performance Curves Definition: Performance curves show how a DC generator’s output voltage changes with different load currents, indicating voltage regulation efficiency.
- Separately Excited DC Generator: This type of generator, though costly, can maintain constant terminal voltage by increasing field excitation to counteract voltage drops.
- Series Wound DC Generator: The terminal voltage starts at zero with no load and increases with load, but is affected by armature reaction and resistance drops.
- Shunt Wound DC Generator: This generator has a rapid voltage drop as the load increases due to strong armature reactions, leading to poor performance.
- Compound Wound DC Generator: Combines shunt and series field effects, maintaining constant voltage by balancing voltage drops and rises in different windings.
Performance curves of a DC generator show how terminal voltage changes with load current at a stated speed and field setting. They are also called external characteristic curves. Their slope and shape show the voltage regulation and usable operating range. A flatter curve means less voltage change, but the required characteristic depends on the application.
Performance Curve of Separately Excited DC Generator
This type of DC generators requires an independent DC field supply, which adds equipment but also allows field current to be controlled without direct dependence on terminal voltage. In separately excited DC generators, terminal voltage normally falls as load current increases at fixed speed and excitation.
Armature reaction reduces the effective flux, while armature and brush resistance cause internal voltage drops. Increasing field excitation can compensate for part of this decline and regulate terminal voltage within the machine’s thermal and magnetic limits. Curve AB shows the separately excited characteristic.
Performance Curve of Series Wound DC Generator
In series DC generators, no-load field current is nearly zero because the field winding carries load current. Residual magnetism may still produce a small no-load voltage. As load rises, series-field flux and generated voltage increase until saturation, armature reaction and internal resistance limit the rise. At high current, terminal voltage may flatten or fall because of the voltage drop in the armature winding and series field.
Performance Curve of Shunt Wound DC Generator
In shunt wound DC generators, residual flux starts voltage build-up and the shunt winding establishes the no-load voltage. Increasing load produces armature-reaction and resistance drops. Falling terminal voltage also reduces shunt-field current, so a self-excited shunt generator droops more than a separately excited machine. Past its maximum-current point, this feedback can make voltage and current collapse. The curve therefore has a stable operating region and a breakdown region rather than uniformly poor performance for this type of generators.
Performance Curve of Compound Wound DC Generator
At no load, a cumulative compound generator behaves like a shunt generator because little load current flows through its series field. As load rises, armature and shunt-field effects make terminal voltage drops, while the aiding series field increases generated emf. The balance is set by the series-field ampere-turns. Under-compounding gives a lower full-load voltage, flat compounding gives similar no-load and full-load voltages, and over-compounding gives a higher full-load voltage. Curve FG represents the selected compound characteristic.
