- Series Generator Definition: A series wound DC generator is defined as a generator where the field windings, armature windings, and external load circuit are connected in series, causing the same current to flow through each part.
- Magnetic Characteristic Curve: The magnetic or open circuit characteristic curve shows the relationship between no-load voltage and field excitation current, remaining linear until pole saturation.
- Internal Characteristic Curve: The internal characteristic curve depicts the voltage generated in the armature versus load current, accounting for voltage drops due to the armature reaction.
- External Characteristic Curve: The external characteristic curve illustrates how terminal voltage varies with load current, considering ohmic drops due to armature and series field resistance.
- Constant Current Generator: A series wound DC generator is known as a constant current generator because the load current remains nearly constant despite variations in load resistance.
In a series-wound DC generator, the series field winding, armature windings and external load form one series circuit.
The same current therefore flows through the armature, series field and load.
Let I = Ia = Isc = IL
Ia is armature current.
Isc is series-field current.
IL is load current.
The three characteristics of series wound DC generator relate excitation current, generated emf, terminal voltage and load current.
Magnetic or Open Circuit Characteristic of Series Wound DC Generator
The magnetic or open circuit characteristic curve relates generated no-load voltage to field excitation current at constant speed. With the load terminals open, the normal series circuit carries no field current. The test therefore separates the field winding and supplies it from an external source while the DC generator runs at the test speed.
Curve AB is the magnetisation characteristic of the series wound DC generator. Generated voltage rises almost linearly with field current while the magnetic circuit is unsaturated. The slope then falls as the poles saturate. Residual magnetism produces a small initial voltage, so point A lies above the origin.
Internal Characteristic of Series Wound DC Generator
The internal characteristic relates generated emf to load current after allowing for armature reaction. The generated voltage Eg is below the corresponding open-circuit value E0 when the armature reaction is demagnetising. Curve OC represents this internal characteristic.
External Characteristic of Series Wound DC Generator
The external characteristic relates terminal voltage V to load current IL. Terminal voltage is generated emf minus the resistive drops in armature resistance Ra and series-field resistance Rsc. The page’s simplified relation for the generated voltage Eg is:
Terminal voltage V = Eg – I(Ra + Rsc)
Curve OD lies below the internal curve because it includes these winding drops.
The characteristics of a series wound DC generator explain the unusual load response. As load resistance is reduced from open circuit, current increases the series-field flux and generated voltage. The rise slows after the magnetic field approaches saturation. At higher current, armature reaction and winding drops can make terminal voltage fall. A portion of the falling branch can give a comparatively small change in load current as resistance changes. In that limited operating region, the series DC generator can be described as a constant-current generator. The description does not apply over its whole characteristic.
