Classification of DC machines
The field circuit and armature circuit are connected in various ways to produce a good type of performance characteristics, an excellent advantage of DC machines. Furthermore, the field poles are energized by two field windings, a shunt field coil and a series field coil, as shown. The shunt winding contains a sufficient number of turns and consumes only a very small current (less than 5% of the coil's rated current). This winding is connected across the coil (i.e. in parallel with it), hence the name shunt winding. The series winding has fewer turns but carries a high current. It is not connected in parallel to the coil, hence the name series winding. When there are shunt and series windings, the series winding is wound on top of the shunt winding.
- Causes and effects of power grid failures
The various connections of the field circuit and armature circuit are shown above. In externally excited DC machines, the field winding is powered by a separate source.
In self-excited DC machine, the field winding is connected in three different ways. The field winding is also not connected in parallel to the coil, resulting in a series DC machine; can be connected through the coil, resulting in a shunt machine; or shunt and series windings are used, resulting in a compound machine. If the shunt winding is connected across the coil, it is called a short shunt machine. In an armature circuit, the shunt winding is connected through series connection of armature and series winding and hence the machine is considered as long shunt machine. There is no important difference between these two compounds. Within the compound machine, the series-winding MMF can either support or neutralize the shunt-winding MMF, resulting in completely different performance characteristics.
Generally, a resistor is built into the shunt winding circuit to regulate the field current and thus vary the field MMF.
Field excitation can also be done using permanent magnets. You can think of it as a kind of single excitation machine, where the magnet provides separate but constant excitation.
