We know that Direct Current Generator is a machine which converts the mechanical energy into electrical energy. The generator is usually driven by a steam engine or a diesel engine or an electric motor which are called prime movers.
The principle of Direct Current Generator is it works on the principle of Faraday's Laws of electromagnetic induction. According to this law the conductors or armature are rotated in the magnetic field and electro magnetic force is induced in these conductors which is collected from the commutators fitted on the shaft of armature.
The simple loop of a Direct Current Generator when the plane of the coil is at right angles to the lines of flux, the flux linked with the coil is maximum but the rate of change of flux linkage is minimum. As coil continues to rotate further, the rate of change of flux linkage increases, till it attains maximum value of 90 degree to 180 degree, the flux linked with the coil gradually increases resulting in decrease in induced electro motive force till it reduces to zero at 180 degree. A reversal of the trend occurs during next half revolution. For unidirectional current, the ends of the coil are connected commutation whose function is to reverse the connections to the rotating coil through fixed brushes and to collect the electro motive in one direction.
Four Reasons for failure of a generator to build voltage:
1.Defective contact of brushes with conmmutator due to dirt, insufficient pressure, tight brushes, dirty or rough commutators or projecting intersegment micas.
2. High resistance or open circuit in the shunt field circuit, faulty contact or burnt resistance in shunt regulators.
3. Loss of residual magnetism.
4. Reverse field connection or reversed speed.
Ten Reasons for sparking and Bad Bommutations:
2. Projecting intersegment micas.
3. Earth fault on armature
4. Armature short circuit
5. Incorrect brush position
6. Wrong grade of brushes
7. Reversed interpole coils
8. Brushes not properly bedded
9. Brushes not equally spaced
10. Worn Brushes
Question and answer about D.C. Generator
1. The armature of a DC generator is laminated to reduce eddy current loss.
2. In a shunt generator the voltage built up is generally restricted by the saturation of iron.
3. Copper loss in DC generators varies with load.
4. Full load efficiency of the generators is 92.51 percent.
5. Shunt generators are preferred for parallel operations.
6. In DC generator the ripples in the direct electro motive force generated are reduced by using commutator with large number of segments.
7. The functions of an interpole is to neutralize crossfield of armature reaction and obtain ideal commutation.
8. Equalizer connection are required when paralleling two compound generators.
9. A simple method of increasing the voltage of DC generator is to increase the speed of rotation.
10. In the commutation process it is the current which is getting reversed.