Generators and their Characteristics

    Direct Current generators are provided with the armature winding ang one or two field windings . Depending on the type of the armature winding and field winding interconnection d.c. generators may be recognized as those of separate excitation , of shunt excitation , of series excitation , and of compound excitation .  

    In a separately excited generator , the field winding obtains its supply from a separate current source . It is connected in series with the armature winding in a series-wound generator in parallel - in a shunt-wound generator . A compound-wound generator has two field windings positioned on the main poles , one of them being connected in shunt , the other - in series with the armature winding . The parallel field winding concentrates the current ranging from 1 to 6 % of the rated armature current . It is made from copper conductors provided wity a large number of turns of relatively small section . The series field winding carries the entire armature current and hence its conductors are of large section .

     Shunt-wound generators , series-wound generators and compound-wound generators are self-excited ; that is to say they don't require a separate current source for their excitation . The  current supplying the winding is derived from the generator armature .

     The generator properties depend upon the method of the generator excitation . They may be expressed by definite characteristics , that is , the relationships between the e.m.f. , the voltage , the armature current and the excitation , all of them being responsible for the operation of a d.c. machine on different loads . The most important of the indicated magnitudes is the voltage which is dependent on the above mentioned excitation current , the armature current and the rotational speed .The main characteristics which reveal the properties of d.c. generators may be described as follows .  

     No-load characteristic is defined as the relationship between the generator e.m.f. E and the excitation current  Iex  with the armature current  I=0 : E=f(Iex) . This characteristic lets us judge the saturation of the magnetic circuit and may be applied for plotting the other characteristics .

     Short-circuit characteristic shows the relationship between the armature current Ish  and the excitation current with the short-circuited armature and the voltage equal to zero : Ish = F(Iex) . This characteristic is plotted as a straight line since under short-circuit conditions the generator magnetic circuit is practically not saturated .

     External characteristic is the relationship between the voltage U and the armature current  I with resistance in the excitation circuit Rex =const. The equation is U=f(I) .

     Regulation characteristic makes up the relationship between the excitation current and the armature current with the generator voltage U=const : Iex=f(I).

     Load characteristic is the relationship between the voltage U and the excitation current  Iex:U=f(Iex) with I=const.