## Critical Field Resistance in D.C. Shunt Generator

Consider the field magnetization characteristics of a d.c. shunt generator shown in the Fig. 1.

Fig. 1 Concept of critical resistance |

_{C}. This voltage is just sufficient to drive the current through field resistance so that cumulative process of building the voltage starts. This value of field resistance is called critical resistance denoted as R

_{C}, of the shunt field circuit at given speed.

**Note**: If field circuit resistance is more than R

_{C}at start then induced e.m.f. fail to drive current through field circuit and generator fails to excite at given speed.

The critical resistance is the slope of the critical resistance line.

**Note**: This speed at which the machine just excites for the given field circuit resistance is called the critical speed of a shunt generator denoted as N

_{C}.

__1.1 Practical Determination of R__

_{C}**Note **: If speed changes, then the O.C.C. changes hence the value of R_{C} changes.

Now if R_{C} is asked at speed N_{2}, while data for O.C.C. is given at N_{1}. It is known that,

**Note **: Generate the data for O.C.C. at new speed and repeat the procedure to obtain R_{C}.

__1.2 Critical Speed__

**Note**: The speed for which the given field resistance acts as critical resistance is called the critical speed, denoted as N

_{C}.

_{sh}then O.C.C. drawn for such a speed to which this line is tangential to the initial portion, is nothing but the critical speed N

_{C}.

Graphically critical speed can be obtained for given R_{sh}. The steps are,

1. Drawn O.C.C. for given speed N_{1}.

2. Draw a line tangential to this O.C.C. say OA.

3. Draw a line representing the given R_{sh} 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 N_{C} is,

Fig. 2 Determine critical speed |

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