Characteristics of DC Shunt Generator (Self Excited)

No Load Characteristics of Self Excited DC Shunt Generator

The no-load characteristic of a self excited dc shunt generator is similar to that of a separately excited dc generator. In that case, we have to disconnect the field winding from the generator. Then we have to connect this field winding with an external DC source. Then like separately excited dc generator we increase the field current from 0 to its rated value step by step with the help of a rheostat. In this case, also the generator runs with its related speed. We have to plot open circuited terminal voltage of the generator against field current of each step.

open circuit characteristics of dc generator

As a result, we get the relationship curve between field current and open circuited terminal voltage of self-excited DC shunt generator. But here we can see that the curve does not start from the origin (zero voltage). Instead, it starts from a point slightly above the origin. This is because of the residual magnetism of the field system. When we start running the generator, there will always be an induced EMF of the armature. It will be even at the zero fields current condition. Because there always a small magnetism exists in the field core due to its residual magnetism property.

Critical Resistance of a DC Shunt Generator

We have seen from the no-load curve of a self-excited dc shunt generator that it has knee point after which the generator voltage does not increase significantly further. Because here the saturation occurs in field core. The corresponding voltage of this knee point signifies the rated voltage of the generator. Now, we connect a straight line from the origin to that knee point. We will get the shunt field resistance from the slope of this line. Because in shunt dc generator the armature terminal voltage appears across the shunt field winding. Hence, this armature voltage divided by the corresponding field current is nothing but the shunt field resistance. critical resistance

Now if we slowly increase the resistance of the field coil, the slope angle of the straight line increases accordingly. After a certain slope angle, the straight line becomes exactly tangential to the active reason of the no-load characteristics curve. If we further increase the shunt field resistance, the slope angle of the straight line further increases. After a certain angle, the line becomes detached from the characteristics curve. This implies that the generator can not build any voltage. Hence the shunt resistance line exactly tangential to no load curve implies the maximum resistance. The maximum resistance below which the generator builds the armature EMF. We call this value of resistance as the critical resistance of the shunt field. So we can conclude that, above the critical resistance a dc generator fails to build up armature EMF.

Critical Speed of DC Shunt Generator

critical speed

The pattern of open circuit characteristics of DC shunt generator is similar for any speed. Although, at a lower speed (< rated speed) the characteristic curve shifts below the position of the original OCC curve of rated speed. The same for the higher speed (> rated speed), it shifts above the position of the original OCC curve of rated speed. Now let us draw the resistance line from the origin to the knee point of the original OCC curve. Now if we decrease the speed of the prime mover, the OCC curve will shift downwards. So, at a certain speed of the prime mover, the OCC will just touch the field resistance line. Below that speed, there would not be any EMF across armature. This speed for that rated field resistance of the machine is the critical speed of the machine.

External Characteristics of Shunt Field DC Generator

When we run the shunt field DC generator with its rated speed the terminal voltage will build up and will get fixed at its rated value. Now we connect the load and slowly increase the load current by introducing load resistance in parallel one by one. As a result, we get the inclined line represents the external characteristics of the shunt field DC generator.

But after certain load beyond the rated value the armature reaction becomes so prominent, it reduces significantly the internal voltage or armature generator voltage in the machine. Due to this strong armature reaction and the large armature resistance drop at the high armature current, the voltage appearing across the shunt field coil reduces significantly. As a result, the shunt field current decreases. Which again causes a reduction in armature voltage. So, due to reduced field current, due to significant armature reaction and high resistance drop, the armature EMF further decreases. This phenomenon is cumulative in nature.

external characteristics of dc shunt generator

As a result ultimately the terminal voltage of the machine becomes zero. The external characteristic for the load above the rated load will touch the horizontal zero axis as shown above.

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