Consider the field magnetization characteristics of a d.c. shunt generator shown in the Fig. 1.
|Fig. 1 Concept of critical resistance|
The critical resistance is the slope of the critical resistance line.
Note : If speed changes, then the O.C.C. changes hence the value of RC changes.
Now if RC is asked at speed N2, while data for O.C.C. is given at N1. It is known that,
Note : Generate the data for O.C.C. at new speed and repeat the procedure to obtain RC.
Graphically critical speed can be obtained for given Rsh. The steps are,
1. Drawn O.C.C. for given speed N1.
2. Draw a line tangential to this O.C.C. say OA.
3. Draw a line representing the given Rsh say OP.
4. Select any field current say point R.
5. Draw vertical line from R to intersect OA at S and OP at T.
6. Then the critical speed NC is,
|Fig. 2 Determine critical speed|