- Shunt Wound DC Generator Definition: A shunt wound DC generator has its field windings connected in parallel with the armature conductors, splitting the armature current into shunt field current and load current.
- Magnetic Characteristic: The curve between shunt field current and no-load voltage shows how the generated emf varies with armature speed.
- Critical Load Resistance: This is the minimum resistance needed for the generator to start and keep running.
- Internal Characteristic: The relationship between generated voltage and load current shows a drop in voltage under load due to armature reaction.
- Breakdown Point: The maximum current output point of the generator is known as the breakdown point.
In shunt wound DC generators, the field winding is connected in parallel with the armature terminals. In these types of generators, armature current (Ia) divides into shunt field current (Ish) and load current (IL).
The three principal characteristics of shunt wound DC generators are the open-circuit, internal and external characteristics.
Magnetic or Open Circuit Characteristic of Shunt Wound DC Generator
The open-circuit characteristic relates shunt field current (Ish) to no-load generated voltage (E0) at constant speed. For a given field current below saturation, no-load emf changes approximately in proportion to armature speed. The diagram shows curves for several speeds.
Residual magnetism produces the small voltage at point A when field current is zero. The curves flatten at higher field current as the magnetic circuit approaches saturation. For voltage to build up, total shunt-field circuit resistance must be below the critical field resistance at that speed. The lines AB, AC and AD are tangent resistance lines for speeds N1, N2 and N3, where N1 > N2 > N3.
Critical Field Resistance of Shunt Wound DC Generator
At a specified speed, the tangent drawn from the origin to the open-circuit characteristic gives the critical field resistance. Total field-circuit resistance must stay below this maximum value for self-excitation.
Internal Characteristic of Shunt Wound DC Generator
The internal characteristic relates internally generated voltage (Eg) to load current (IL). Armature reaction weakens the effective field as load increases, so generated emf falls below the no-load value. In the diagram, AD represents the reference no-load characteristic and AB represents the internal characteristic.
External Characteristic of Shunt Wound DC Generator
Curve AC is the external characteristic of shunt wound DC generators. It relates terminal voltage to load current. The armature-resistance drop makes terminal voltage lower than internally generated voltage, so the external curve lies below the internal curve.
Within the machine’s operating limits, terminal voltage may be regulated by adjusting field excitation or speed.
As load resistance decreases, the load current of a shunt wound DC generator initially increases. At point C it reaches a maximum. A further reduction in load resistance lowers terminal voltage and shunt-field current, which weakens the field and causes both generated voltage and load current to fall.
The terminal-voltage relation is:
Before point C, increasing IL increases the armature-resistance drop and armature reaction, so terminal voltage falls. Beyond point C, the lower terminal voltage also reduces shunt excitation. This feedback causes voltage and current to collapse even as the load resistance continues to fall.
The operating load resistance must keep current and voltage within the generator’s ratings. Point C, where the external characteristic reaches maximum load current, is called the breakdown point.
