The circuit diagram of a DC shunt wound generator is shown in the above fig. In that the armature current Ia ruptures up into 2 parts; a little fraction shunt current Ish flowing through shunt coil whereas the main part IL goes to the external load.
Open Circuit Characteristic
The fig shows that the open circuit characteristic of a DC shunt generator, which is similar in form to it of a series generator. The line OP signifies the shunt field circuit resistance. Once the generator is run at typical speed, it'll build up a voltage OM. At no-load, the terminal voltage of the generator are going to be constant (= OM) diagrammatical by the horizontal line MR.
The internal characteristic of a shunt Generator is shown in the curve 1. As soon as the generator gets loaded, flux per pole is decreased because of armature reaction. For that reason, e.m.f. E generated on load is a smaller amount than the e.m.f. generated at no load. Thus the internal characteristics curve of shunt generator (E/Ia) drops down slightly as shown in Fig.
The external characteristic of a shunt generator is shown in curve 2. This curve shows the relation between the terminal voltage V and load current IL.
= E-(IL+Ish) Ra
Hence, the external characteristic curve can lie under the internal characteristic curve by an equal amount up to the drop in the armature circuit [i.e., (IL + Ish) Ra].