Introduction:

A DC generator is the curve that provides the relationship between field current and armature terminal voltage on an open circuit.
When a DC generator is driven by the prime mover, an emf is then induced in the armature. The emf induced in the armature is given by an expression.

Now, from the equation we can clearly see that the emf produced per pole is directly proportional to the product of the flux and the speed of the armature.

It is clear that, as the excitation current or field current (if) increases from its initial value, the flux and hence the induced emf is increased along with the field current.

If we plot the induced voltage on the Y-axis and the field current on the X-axis, the magnetization curve is as shown in the figure below.

The magnetization curve of a DC generator is of great importance because it represents the saturation of the magnetic circuit. For this reason, this curve is also called a saturation curve.
According to the molecular theory of magnetism, the molecules of magnetic materials, which are not magnetic, do not arrange or align in a definite order. When a current passes through a magnetic material, its molecules get arranged in a certain order. More and more molecules are arranged up to a certain value of the field current. In this phase the pole-based flux increases directly with the field current and the generated voltage also increases. Here, in this curve, from point B to point C these phenomena are visible and this part of the magnetization curve is almost a straight line. Above a certain point (point C in this curve) the newly magnetized molecules become very scarce and it becomes very difficult to further increase the polar flux. This point is called saturation point. as you know that the Point C is also known as the knee of the magnetization curve. A small increase in magnetization requires a much larger field current above the saturation point. This is why the upper part of the curve (from point C to point D) bends as shown in the figure.
The magnetization curve of a DC generator does not initially start at zero. It starts from the value of voltage produced due to residual magnetism.

Residual Magnetism:

In ferromagnetic materials, the magnetic strength and induced voltage increase with increasing current flow through the coils. When the current is reduced to zero, the magnetic force still remains in the core of these coils. This phenomenon is called residual magnetism. The core of the DC machine is made of ferromagnetic material.