In this type of dc machine, the field winding is connected parallel to the alternator. The simple parallel connection plays a major role in voltage built up in the shunt generator. we will see about what are the interesting things happen in magnetization and load characteristics of self excitation machine.
Applying kirchoff voltage law to the circuit,
Vt = Ea - IaRa - Vb
Vt- Terminal voltage
Ea- generated or induced voltage
Ia- Armature current
Ra- Armature resistance
Applying kirchoff current law,
Ia = IL + Ish
Practically, there will be voltage drop due to brush but it may be around one or two volt per brush. So, we neglect the Vb.
Vt = Ea - IaRa
Observing the shunt generator, we can classify into open circuit or magnetization characteristics and terminal characteristics
Open circuit or magnetization characteristics of dc shunt generator
In this case, the load of the dc machine is removed. At this point the load current flowing through the circuit is zero.(IL= 0)
We can say,
Ia=Ish
Ea = Vt - IshRa
The terminal voltage is equal to the field winding of the machine. because it is connected across the armature. When the rotor is rotated, the residual magnetic flux present in the poles induces emf in that armature.
Because of the induction of emf, there will be minimum amount of current flowing through the field winding and this current aid the magnetic flux in the air gap. So, the voltage develops like a loop up to the saturation of point.
The minimum value of resistance to excite the shunt field is known as critical resistance. If the resistance of field winding is beyond the critical resistance there will be no further increase in induced emf.
Rsh < Rcritical
The shunt resistance should less than the critical resistance.
Terminal or load characteristics of dc shunt generator
In practical if we increase the load, the demand of load current is high results in decrease in the terminal voltage Vt. because of armature voltage drop IaRa across the generator. Under no load conditions the terminal voltage is nearly equal to the induced emf because of IaRa drop on no load is small.
The drop due to demagnetization effect of armature reaction. Because of increase in load current causes the flux weakening there will be reducing magnetic flux.
To neglect the armature reaction the compensating winding is provided in between the field winding of self excited machine.
The load current increases with decrease in terminal voltage due to decrease in field current Ish. In separate excited dc generator the field current is kept as constant in case of self excitation field current is variable.
Problem
For example,
A 10kw 250v dc shunt machine has armature field resistance of 0.1 ohm and 125 ohm.
If we analyse the total armature power,
Ia = IL + Ish
Ish = 250/125 = 2A
IL = 40A
Ia = 42A
Ea = Vt + IaRa
=250 + (42*0.1)
Ea = 254.20 volt
Conclusion
Notice that only field current is 2A and load current is 40A
The total Armature power developed Pa = 10.67 KW
comments
What is the advantage of shunt machine while working as motor?
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