What is an Oscillator?
Principle of DC Generator
Construction of DC Generator | Yoke Pole Armature Brushes of DC Generator
Characteristics of Series Wound DC Generator
Characteristic of Separately Excited DC Generator
EMF Equation of DC Generator
Parallel Operation of DC Generators
Self Excited DC Generators
Phasor Diagram for Synchronous Generator
DC Generators Performance Curves
Types of DC Generators
Characteristic of Shunt Wound DC Generator
Magnetization Curve of DC Generator
Applications of DC Generators
Alternator Synchronous Generator | Definition and Types of Alternator
Working Principle of Alternator
Construction of Alternator
Armature Reaction in Alternator or Synchronous Generator
Armature Winding of Alternator
Rating of Alternator
Derivation of Various Power Conditions in Alternators and Synchronous Motors
Induction Generator | Application of Induction Generator
Motor Generator Set | M G set
Armature Winding | Pole Pitch Coil Span Commutator Pitch
Winding Factor | Pitch Factor | Distribution Factor
Frog Leg Winding | Drum Winding | Gramme Ring Winding
Characteristics of Series Wound DC Generator
Magnetic or Open Circuit Characteristic of Series Wound DC GeneratorThe curve which shows the relation between no load voltage and the field excitation current is called magnetic or open circuit characteristic curve. As during no load, the load terminals are open circuited, there will be no field current in the field since, the armature, field and load are series connected and these three make a closed loop of circuit. So, this curve can be obtained practically be separating the field winding and exciting the DC generator by an external source. Here in the diagram below AB curve is showing the magnetic characteristic of series wound DC generator. The linearity of the curve will continue till the saturation of the poles. After that there will be no further significant change of terminal voltage of DC generator for increasing field current. Due to residual magnetism there will be a small initial voltage across the armature that is why the curve started from a point A which is a little way up to the origin O.
Internal Characteristic of Series Wound DC GeneratorThe internal characteristic curve gives the relation between voltage generated in the armature and the load current. This curve is obtained by subtracting the drop due to the demagnetizing effect of armature reaction from the no load voltage. So, the actual generated voltage ( Eg) will be less than the no load voltage (E0). That is why the curve is slightly dropping from the open circuit characteristic curve. Here in the diagram below OC curve is showing the internal characteristic or total characteristic of the series wound DC generator.
External Characteristic of Series Wound DC GeneratorThe external characteristic curve shows the variation of terminal voltage (V) with the load current ( IL). Terminal voltage of this type of generator is obtained by subtracting the ohomic drop due to armature resistance (Ra) and series field resistance ( Rsc) from the actually generated voltage ( Eg). Terminal voltage V = Eg - I(Ra + Rsc)
The external characteristic curve lies below the internal characteristic curve because the value of terminal voltage is less than the generated voltage. Here in the figure OD curve is showing the external characteristic of the series wound DC generator.
It can be observed from the characteristics of series wound DC generator, that with the increase in load (load is increased when load current increases) the terminal voltage of the machine increases. But after reaching its maximum value it starts to decrease due to excessive demagnetizing effect of armature reaction. This phenomenon is shown in the figure by the dotted line. Dotted portion of the characteristic gives approximately constant current irrespective of the external load resistance. This is because if load is increased, the field current is increased as field is series connected with load. Similarly if load is increased, armature current is increased as the armature is also series connected with load. But due to saturation, there will be no further significance raise of magnetic field strength hence any further increase in induced voltage. But due to increased armature current, the affect of armature reaction increases significantly which causes significant fall in load voltage. If load voltage falls, the load current is also decreased proportionally since current is proportional to voltage as per Ohm’s law . So, increasing load, tends to increase the load current, but decreasing load voltage, tends to decrease load current. Due these two simultaneous effects, there will be no significant change in load current in dotted portion of external characteristics of series wound DC generator. That is why series DC generator is called constant current DC generator.