DC GENERATOR BASIC THEORY
A electric machine has two parts, stator and rotor, separated by a air gap. The stator of the machine does not move and normally is the outer frame of the machine. The rotor is free to move and normally is the inner part of the machine. Both stator and rotor are made of ferromagnetic materials. Slots are cut on the inner periphery of the stator and the outer periphery of the rotor. Conductors are placed in the slots of the stator or rotor. They are interconnected to form windings. The winding in which voltage is induced is called armature winding. The winding through which a current is passed to produce the main flux is called the field winding. Permanent magnets are used in some machines to provide the main flux of the machines. There are two types of d.c machines, the d.c generator and the d.c motor. The d.c generator converts mechanical energy into electrical energy. The d.c generator converts mechanical energy into electrical energy. The d.c motor converts electrical energy into mechanical energy. The d.c generator is based on the principles that when a conductor is rotated in a d.c magnetic field, a voltage will be generated in the conductor.
LOAD MAGNETIZATION CURVE
The load magnetization curve shows the variation of terminal voltage with the field current for a particular value of load current. This characteristic is more or less a replica of the no-load magnetization characteristics but with ordinates reduced by an amount corresponding to the voltage drop in the armature for the particular load. Curve N3 in the figure shows the load magnetization
characteristics and curve N1 shows the no load magnetization characteristics. Curve N2 is deduced by adding armature and brush drop, corresponding to the load current at which the curve has been drawn, to each order of curve N3.the difference between curve N1 and curve N2 is the voltage drop due to armature reaction. For a machine with brushes in geometrical neutral plane, this drop is negligibly small in unsaturated region but becomes appreciable as saturation increases. This is due to the increased demagnetizing armature reaction.
The load magnetization characteristics for
the separately and shunt excited generators is almost the same. The slight difference is due to the fact for shunt machine, the armature current IA = Il- It , whereas for separately excited machine IA = Il. This results in a slightly different armature reaction and voltage drop in the two cases.
The external characteristics for a shunt generator are determined for a fixed resistance in the field circuit and at a constant speed. The shape of these characteristics for a shunt generator is similar to that for a separately excited machine but is more drooping because the field current also gets influenced as the load increases. Due to armature reaction and armature resistance drop the terminal voltage is reduced on load and is the field current since the field winding is connected across the armature, thereby reducing the terminal voltage still further. The effect of reduction in field current on induced voltage becomes predominant at higher values of load current especially those greater than rated current of the machine. If the load resistance is continuously decreased, the characteristics actually turns back indicating that the total voltage drop is so large that there is a net decrease in the load current even though the load resistance is decreased.