Skip to main content

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

       The Fig. 1 shows that generator voltage builds in step till point A. This point is intersection of field resistance line with the open circuit characteristics (O.C.C.). The voltage corresponding to point A is the maximum voltage it can generate. If the slope of field resistance line is reduced by decreasing the field resistance, the maximum voltage generator can build will be higher than that corresponding to point A. Similarly if the slope of field resistance line is increased by increasing the field resistance, the maximum voltage generator can build will be less that that corresponding to point A i.e. corresponding to point B.
       If now the slope of the field resistance line is increased in such a way that it becomes tangential to the lower part of the open circuit characteristics. The voltage corresponding to this point is EC. 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 RC, of the shunt field circuit at given speed.
Note : If field circuit resistance is more than RC 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.

       Similar to the critical resistance there is a concept of critical speed. We know that E α N. As speed decreases the induced e.m.f. decreases and we get O.C.C. below the O.C.C. at normal speed. If we go on reducing the speed, at a particular speed we will get O.C.C. just tangential to normal field 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 NC.
1.1 Practical Determination of RC
       Generally data for plotting the open circuit characteristics is given. Plot the characteristics on the graph paper to the scale.
       Draw the tangent, to the initial part of this O.C.C. then the slope of this line is the critical resistance for the speed at which the data is given.
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.

1.2 Critical Speed
       It is known that as speed changes, the open circuit characteristics also changes, similarly for different shunt field resistances, the corresponding lines are also different. 
Note : The speed for which the given field resistance acts as critical resistance is called the critical speed, denoted as NC.
       Thus if the line is drawn representing given Rsh 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 NC.
       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
 
Labels:

Comments

Post a Comment

Comment Policy
We’re eager to see your comment. However, Please Keep in mind that all comments are moderated manually by our human reviewers according to our comment policy, and all the links are nofollow. Using Keywords in the name field area is forbidden. Let’s enjoy a personal and evocative conversation.

Popular posts from this blog

Practical Commutation

The e.m.f. induced in each coil of armature is alternating in nature. If load is connected, the current flowing will also be alternating. But the flow of current in a d.c. generator must be undirectional. This can be achieved by the use of commutator. When the armature conductors are under the influence of one pole they carry current in one direction whereas the current is reversed when the conductors are under the influence of other pole. This reversal of current takes place along the magnetic neutral axis. Fig. 1 Note : The reversal of current is likely to take place in short interval when a coil is short circuited by a brush so that transfer of current from one direction to other is carried out without any sparking. This process is called commutation.        Thus a process by which current in the short circuited coil is reversed while it crosses the MNA is called commutation. The time during which the coil remains short circuited is known as...
1 comment
Read more

Demagnetising and Cross Magnetizing Conductors

The conductors which are responsible for producing demagnetizing and distortion effects are shown in the Fig.1. Fig. 1        The brushes are lying along the new position of MNA which is at angle θ  from GNA. The conductors in the region AOC = BOD = 2θ  at the top and bottom of the armature are carrying current in such a direction as to send the flux in armature from right to left. Thus these conductors are in direct opposition to main field and called demagnetizing armature conductors.         The remaining armature conductors which are lying in the region AOD and BOC carry current in such a direction as to send the flux pointing vertically downwards i.e. at right angles to the main field flux. Hence these conductors are called cross magnetizing armature conductors which will cause distortion in main field flux.        These conductors are shown in the Fig. 2 Fig. 2  ...
Post a Comment
Read more

Winding Pitches

Now we will consider the definitions of different types of winding pitches, required in the design of the armature winding. 1.1 Back Pitch         The distance which is measured interns of armature conductors i.e. between top and bottom coil sides of a coil measured around the back of the armature i.e. away from the commutator is called back pitch and is denoted by . It is measured interms of coil sides. Note : Since it is difference between odd and even number, it is always odd number.        The size of the coil is decided by the back pitch. The back pitch is normally equal to coilsides per pole. It is shown for lap and wave type of winding in the Fig. 1. Fig. 1        The back pitch for the winding shown in the Fig. 1 is y b = 14 - 1 = 13 as coilsides 1and 14 are of coil number 1. 1.2 Front Pitch        It is defined as the distance measured between two c...
Post a Comment
Read more