The iron of the stator poles is still magnetized from its previous use. This is called residual magnetism. A few lines of force between poles are created by
The resistance of the control lowers the number of shunt field amperes— which lowers the number of lines of force (flux) cut by the armature conductors, resulting in a lower voltage.
The shunt field can be excited separately with another power source. Separate excitation reduces the voltage drop common to the self-excited design.
The Series Generator
The series generator has two circuits that are connected in series with each other, the armature and interpoles (Al and A2) and the series field (Sl and S2). Figure 1.23 shows a standard connection for counterclockwise rotation facing the end opposite the shaft.
All of the amperes produced by the armature pass through the series field. The series field coils are constructed with a few turns of wire that are large enough to carry the full ampere output of the generator. The large wire makes it a very low-resistance circuit compared to the shunt field circuit.
Operation of the Self-Excited Series Generator
The field pole iron has residual magnetism from the previous operation. The conductors of the armature cut lines of force of the residual magnetism and generate an excitation voltage. The output voltage will remain at the excitation voltage value until a load is applied. The excitation voltage will cause a small amount of current to flow through the load. This current goes through the series field, strengthening its magnetism and creating more lines of force.
When the armature conductors cut more lines of force, they generate higher voltage and amperes until the load demand is stabilized. The voltage output will remain at this value until more load is added. The voltage and ampere output will increase with the load increase until full power output of the generator is reached.
At this time, the amperes in the series field create full magnetic saturation in the pole iron.
The series generator with no load will create only excitation voltage. The load regulates the voltage output. This characteristic limits its use.