Page:Electricity (1912) Kapp.djvu/205

Rh forms an electrical shunt to the external circuit, and machines of this kind are therefore called "shunt machines."

Some of the e.m.f. induced in the armature is necessarily lost in overcoming the internal resistance of the machine, so that the e.m.f. available at the terminals is a little smaller than the induced e.m.f., the difference being the greater, the greater is the current the machine is called upon to furnish. Since the excitation of a shunt machine is proportional to the potential difference at its brushes, and since this decreases with the external current, such a machine cannot give an absolutely constant voltage. Its voltage will slightly drop with an increase of external current. On the other hand, a series machine is excited by the external current, and if this increases the excitation also increases, so that within certain limits the voltage rises with the external current. We have thus in the two types contradictory working conditions. If the machine is "series excited," the terminal e.m.f. rises with an increase of load; if it is "shunt excited" the terminal e.m.f. drops with an increase of load. If then we combine these two methods of excitation, that is, if we magnetise the field system both by a shunt