How to Draw O.C.C. at Different Speeds?
If we are given O.C.C. of a generator at a constant speed N1 then we can easily draw the O.C.C. at any other constant speed N2.Fig (3.11) illustrates the procedure. Here we are given O.C.C. at a constant speed N1.It is desired to find the O.C.C. at constant speed N2 (it is assumed that n1 < N2)For constant
excitation,E α N
As shown in Fig. (3.11), for If = OH, E1 = HC. Therefore, the new value of e.m.f. (E2) for the same If but at N2i.
E2=HC ×( N2/N1) =HD
This locates the point D on the new O.C.C. at N2. Similarly, other points can be
located taking different values of If . The locus of these points will be the O.C.C. at N2.
Critical Speed (NC )
The critical speed of a shunt generator is the minimum speed below which it fails to excite. Clearly, it is the speed for which the given shunt field resistance represents the critical resistance. In Fig. (3.12), curve 2 corresponds to critical speed because the shunt field resistance (Rsh) line is tangential to it. If the generator runs at full speed N, thenew O.C.C. moves upward and the R’sh line represents critical resistance for this speed.
Therefore , Speed α Critical resistance
In order to find critical speed, take any convenient point C on excitation
axis and erect a perpendicular so as to cut Rsh and R’sh lines at points B and
A respectively. Then,
or NC = N ×(BC/AC)
Conditions for Voltage Build-Up of a Shunt Generator
The necessary conditions for voltage build-up in a shunt generator are:
(i) There must be some residual magnetism in generator poles.
(ii) The connections of the field winding should be such that the field current strengthens the residual magnetism.
(iii) The resistance of the field circuit should be less than the critical resistance. In other words, the speed of the generator should be higher than the critical speed.