DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
• There are two practical ways by which commutation may be improved. These methods are, 1. Resistance commutation and 2. E.M.F. commutation.
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
Key Point: 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.
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
It is seen that, the e.m.f. induced in the conductors is always sinusoidal and commutator converts this sinusoidal e.m.f. to unidirectional e.m.f. Let us see, how it happens.
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
In order to reduce the effect of armature reaction following methods are used.
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
The armature reaction always results in reduction of generated e.m.f. due to decrease in value of flux per pole.
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
Thus these conductors are in direct opposition to main field and called demagnetising armature conductors.
D.C. Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
To understand the concept of armature reaction, consider a two pole d.c. generator. For simplicity we will assume that the brushes are touching the armature conductors directly, although, they touch commutator segments in actual practice.
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
Key Point: Thus the armature current will set up its own magnetic field. The effect of this armature flux on the distribution of main field flux is called armature reaction.
with Solved Example Problems
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
Now e.m.f. gets induced in the conductor according to Faraday's law of electromagnetic induction. Hence average value of e.m.f. induced in each armature conductor is,e = Rate of cutting the flux = dϕ / dt
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
Now we will consider the definitions of different types of winding pitches, required in the design of the armature winding.
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
e have seen that there are number of armature conductors, which are connected in specific manner as per the requirement, which is called armature winding.
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
As stated earlier, whether a machine is d.c. generator or a motor the construction basically remains the same as shown in the Fig. 3.3.1.