- Magnetization Curve Definition: The magnetization curve of a DC machine shows the relationship between field current and armature terminal voltage on an open circuit.
- Importance: The magnetization curve indicates the saturation of the magnetic circuit, crucial for understanding the generator’s efficiency.
- Saturation Point: This point, also known as the knee of the curve, shows where further increases in field current yield minimal increases in flux.
- Molecular Alignment: As field current increases, magnetic molecules align, increasing flux and generated voltage until saturation.
- Residual Magnetism: Even when current is zero, some magnetism remains in the generator’s core, influencing the magnetization curve.
The magnetization curve of a DC generator relates field current to generated open-circuit voltage at constant armature speed.
A prime mover rotates the armature, and the field flux induces emf. The machine equation expresses generated emf in terms of flux, speed, conductors, poles and parallel paths.
For one machine, the fixed construction terms may be combined into a constant K.
In the equation:
φ is flux per pole.
P is the number of poles.
N is armature speed in revolutions per minute.
Z is the total number of armature conductors.
A is the number of parallel paths.
Generated emf is proportional to flux per pole multiplied by armature speed.
At constant speed, generated emf is directly proportional to flux per pole.
Increasing field current (If) increases the flux and generated emf. Near magnetic saturation, each further increase in field current produces a smaller increase in flux.
Plotting generated voltage on the Y-axis against field current on the X-axis gives the magnetization curve shown below.
The magnetization curve of a DC generator shows how the magnetic circuit approaches saturation, so it is also called the saturation curve.
In an initially weakly magnetized magnetic material, applying field current moves domain walls and turns magnetic domains towards the applied field. The resulting increase in pole flux makes the generated voltage rise. The B-to-C region is nearly straight because flux responds strongly to field current. Near point C, called the knee, much of the available domain alignment has occurred. Further increases in field current then produce smaller increases in flux, so the C-to-D region bends towards saturation.
The magnetization curve of a DC generator usually has a small generated voltage when field current is zero. Residual flux in the poles produces this voltage while the armature is turning.
Residual Magnetism
In ferromagnetic materials, magnetic domains can retain partial alignment after the magnetizing current is removed. The remaining flux is residual magnetism. In a DC generator, it produces the small open-circuit voltage at zero field current and helps a self-excited generator begin building voltage.
