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Tuesday, 17 December 2019

Characteristics of DC Shunt Generator

1. No load Characteristics of DC Shunt Generator

The no load characteristics of shunt generator is the same as that of a separately excited generator. This is due to the fact that a very low percentage of about 1% to 3% of armature current flows through the shunt field winding which has negligible effect on the characteristics.

2. Load Characteristics of DC Shunt Generator :

As in the case of no load characteristic the load characteristics also does not differ much from that of separately excited generator. The only difference is the armature current. In the case of separately excited generator the armature current is equal to load current whereas in the case of shunt generator the armature current is the sum of load current and shunt field current which is again a low percentage value of armature current. The connection diagram and the characteristics of shunt generator is given in Figure a and b.

3. External and Internal Characteristics of DC Shunt Generator :

The shunt generator is connected as shown in above Figure a and is run at rated speed and the field current is adjusted for rated terminal voltage at no load. Now the switch S is closed and the load is gradually increased by decreasing its resistance in steps. The reading of the field current, load current and terminal voltage is noted for each step. The graph for terminal voltage against load current is drawn with the present value of field current and speed as shown in Figure b. The curve gives the external characteristics. It is seen that the terminal voltage drops as load increases. This is due to

a. drop in the armature IaRa
b. reduction in the main flux due to armature reaction
c. reduction in field current due to fall in terminal voltage

From the curve, it is seen that for a given field current on load the voltage drop due to armature resistance is equal to CD, the voltage drop due to reduction in field current is equal to DF and the voltage drop due to armature reaction is equal to FG. Thus the total voltage drop CG amounts to the sum of the voltage drops due to armature resistance, reduction in field current and armature reaction. The terminal voltage is the difference between OA and CG. Further increase in load current drops the terminal voltage upto a point B is reached and any further decrease in load resistance momentarily increases the load current producing more armature reaction. This increases the armature drop laRa reducing the terminal voltage. The rapid reduction in terminal voltage results in decreased load current. The cumulative effect is a short circuit of armature resulting in the terminal voltage falling to zero at point H. OH is the load current due to the voltage generated by the residual flux. The point B is called breakdown point. It is evident that any further effort to decrease load resistance beyond the breakdown point B will decrease the load current due to rapid decrease in terminal voltage. Over the normal operating range the internal characteristics is obtained by adding armature drop (CD) to the external characteristics.

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