The three important characteristics of DC generator are
1. Open circuit characteristic or Magnetization curve or No – load saturation Curve
Open circuit characteristic is the relation between the No-load generated emf in the armature, and the field exciting current at a fixed speed. It is the magnetization curve for the material of electro- magnets. It is same for separately excited or self excited machine.
2. Internal or total characteristic
This characteristic curve gives the relation between the emf generated in the armature and the armature current.
3. External characteristic
This gives the relation between the terminal voltage and the load current. This characteristic takes into account the voltage drop due to armature circuit resistance and the effect of armature reaction. This characteristic is of importance in judging the suitability of generator for a particular purpose. This characteristic is also referred to as performance characteristics or voltage regulating curve.
The circuit diagram for obtaining the OCC is shown in figure 1.9. Irrespective of the type of the DC machine, namely, shunt, series, compound, the shunt field winding is disconnected and excited from an external source.
As iron is unsaturated under low excitation current condition, emf increases as flux increases with increase in excitation current. As the field current increases further the iron starts saturating, the emf will not increase proportionately as the flux is not varying proportionately with the current. This is shown by the knee ‘pq’ of the characteristic curve shown in figure 1.10. A further increase in field current leads to saturation of iron and the flux remains almost constant and hence the induced emf will also remain constant. This is shown by the region ‘qr’ in the figure1.10. Figure 1.10
CRITICAL RESISTANCE FOR SHUNT GENERATOR
When the armature is rotating with armature open circuited, an emf is induced in the armature because of the residual flux. When the field winding is connected with the armature, a current flows through the field winding ( in case of shunt field winding, field current flows even on No-load and in case of series field winding only with load) and produces additional flux. This additional flux along with the residual flux generates higher voltage. This higher voltage circulates more current to generate further higher voltage. This is a cumulative process till the saturation is attained. The voltage to which it builds is decided by the resistance of the field winding as shown in the figure 1.11. If field circuit resistance is increased such that the resistance line does not cut OCC like ‘om’ in the figure 1.11, then the machine will fail to build up voltage to the rated value.
The slope of the air gap line drawn as a tangent to the initial linear portion of the curve represents the maximum resistance that the field circuit can have beyond which the machine fails to build up voltage. This value of field circuit resistance is called critical field resistance. The field circuit is generally designed to have a resistance value less than this so that the machine builds up the voltage to the rated value.
CONDITIONS FOR VOLTAGE BUILD UP OF A GENERATOR
Following are the conditions necessary for the voltage build up of a self excited generator.
(i) Residual magnetism must be present.
(ii) For the given direction of rotation, the field coils must be properly connected to the armature so that the flux produced by the field current reinforces the residual flux.
(iii) Its field resistance must be less than the critical field resistance.
CRITICAL SPEED
Critical speed of a generator is that speed for which the field circuit resistance becomes the critical field resistance.
Relation between induced emf and terminal voltage will provide an insight into the performance of the machine. The terminal voltage of the machine under loaded condition reduces from the no-load induced emf value because of the armature circuit voltage drop and armature reaction. Further the contact drop of the brushes will have to be taken into account. Usually a brush contact drop of 1volt is considered for a brush. This will be constant throughout the operating range of the machine.