Separately excited dc generator
In a separately excited dc generator, the excitation produced in the dc generator is by an external source. the separately excited field winding consists of several hundred turns of fine wire.
The magnitude of voltage generated depends upon the speed of armature rotation and field current.
Separately excited dc generator characteristics are described in detail.
Characteristics of separately excited dc generator
Open circuit characteristics
This characteristic is also known by a few more terms such as no-load characteristics, saturation curve, magnetization curve, or no-load magnetization curve.
Open circuit characteristics show the curve between armature generated e.m.f Ea with field current If, for constant speed and null armature current.
A circuit diagram with the open-circuit characteristics of separately excited dc generator is shown below:
The armature is driven at rated speed by the prime mover and switch s is in the open position.
When the field winding is not energized, due to the residual flux in the main poles the voltmeter indicates some voltage.
When the field winding is energized the field current is increased. Field current increases till Ea becomes 1.25 times at rated voltage.
Internal and external characteristics of separately excited dc generator
The internal characteristics give the variation of armature generated no-load voltage excluding the voltage drop due to armature reaction.
It gives the curve of generated emf and load current Il with constant speed and field current.
The internal characteristics are indicated by XY (in red color) in the diagram.
External characteristics give the variation between the armature terminal voltage with load current Il for constant speed and fixed field current.
The external characteristic is given by equation
V = Eg – IaRa
The decrease in terminal voltage with the increase in load current is because of the voltage drop caused by the armature reaction.