In this post we will discuss how Voltage is Build up in DC Shunt generator at No Load. Consider a DC Shunt Generator at no load as shown in figure below. The switch in the field circuit is supposed open and the armature of DC Shunt Generator is driven at rated speed.
As there is small voltage is there across the terminals of DC Shunt Generator and Switch S is closed, therefore a small current will start flowing through the field circuit of DC Shunt Generator which in turn will produce magnetic flux and if the produced magnetic flux adds the residual magnetic flux then net flux will increase and the generated voltage (Ea = KaØωm) will increase corresponding to point J on the Magnetization curve as shown in figure below.
Since the generated voltage has increased, therefore the field current will also increase to OK corresponding to which the Generated Voltage across the Terminals of DC Shunt Generator will increase to point L. In the same manner the voltage will continue to build up till the point of intersection of Field Resistance Line and Magnetization curve / Open Circuit Characteristics of DC Shunt Generator. Beyond point of intersection of Field Resistance Line and Magnetization curve / Open Circuit Characteristics the voltage won’t build up as in that case the generated voltage Ea will not be able to drive the required field current. Thus the stable point at which the voltage will remain fix is the voltage Ea corresponding to point of intersection of Field Resistance Line and Magnetization curve / Open Circuit Characteristics.
If the voltage build up fails then what could be the cause?
As discussed, the small generated voltage because of residual pole flux will derive field current OD which in turn will produce magnetic flux. If it happens that the flux due to field current OD is in opposition with the residual flux then voltage will reduce which will reduce the filed current. Thus no cumulative increase in field current and hence there will not be any voltage build up in DC Shunt generator. So what to be done to overcome this? We just need to change the direction of current flowing through the field coil for which we will Reverse the connection of Filed Coil.
Hope you understand the voltage build up process in DC Shunt generator. If you have any doubt, ask please.
Effect of variation of field resistance of DC Shunt Generator in its Voltage Build up:
Carefully observe the figure below.
If the field resistance is increased to OA, then Field Resistance line intersect the OCC curve at point p, and hence there will not be voltage build up beyond point p.
Now, if shunt field resistance is such that OB represents the Field resistance line then as clear from the figure above, the lone is intersecting the OCC curve at many points between q and r, therefore the field current will fluctuate between s and t and hence the voltage generated at the terminals of DC Shunt Generator will vary from qs to rt resulting in unstable condition. If we find the slope (tanƟ) of the Field Resistance Line then we will get Field Resistance value which is known as Critical Filed Resistance. Here Critical Field Resistance = tanƟ = qs/Os
What is the significance of Critical Field Resistance?
As clear from the figure above, if the field resistance is more than the Critical Field Resistance then there will not be voltage build up in DC Shunt Generator. See in the figure OA is shunt field resistance which is more than Critical Field Resistance OB (check by slope, slope of OA is more than slope of OB), hence there is no voltage build up in DC Shunt Generator.
Effect of variation of speed of rotation of DC Shunt Generator in its Voltage Build up:
Suppose the field resistance is OC and DC Shunt Generator is running at a speed of n1 for which the stable point of its terminal voltage is C. Now the speed of DC Shunt Generator is reduced to n2 therefore the OCC curev will also move downward as shown in figure. It should be noted here that the same field resistance line OC is now tangent to the new OCC curve and therefore will create an unstable condition of operaton of DC Shunt Generator. This speed n2 is hence called Critical Speed. Thus Critical Speed is that speed at which the DC Shunt Generator just fails to buil up voltage with no external resistance in the field circuit.
Hope you enjoyed the topic. In next post we will discuss on the different causes due to which DC Shunt Generator fails to build up voltage. So be there and follow me. Thank you!!!