Direct current generator || Part-02 ~ MONU TUTORIALS ACADEMY

Armature reaction in DC generator

Armature reaction is the effect of magnetic flux setup by armature current upon the distribution of flux under the main poles.

In figure show a two pole DC generator. when there is no load connected to the generator ok, the current in the armature conductors is zero. Under this condition there exist in it only the MMF of the main falls which produced the main flux.this flux is distributed symmetrically with respect to the polar Axis, that is, the central line of the north and the South poles. The direction of flux m is shown by an arrow. the magnetic neutral axis or plane is a plane perpendicular to the axis of the flux.

The m n A coincides with the geometrical neutral axis aur plane. Brushes are always placed along MNA. Hence, MNA is always called the axis of commutation.

in this figure show armature conductors carrying current with no current in field coils. The direction of current in the armature conductors may be determined by Fleming's right hand rule. the current flow in the same direction in all the conductor line under one pole.The direction of flux produced by armature conductors may be determined by corkscrew rule. The conductors on the left hand side of the armature carrying current in the direction into the paper. The flux produced by current in this armature conductor in sun in figure.

this conductor combined their MMF to produce a flux through the armature in the downward direction. similarly, the conductors on the right-hand side of the armature carrying current in the direction out of the paper. this conductor also come by their MMF to produce a flux through the armature in the downward direction.

in this figure show the condition when the field current and armature current are acting simultaneously. This occur when the generator is on load. now there are two fluxes inside the machine, one produced by the main field poles of the generator and the other by the current in the armature conductors. This two fluxus now combined to form a result flux as shown in figure.

it is seen that the field flux entering the armature is not only sifted but also distorted. The distortion produced crowding of the flux in the upper pole tip in the northpnorth and in the lower pole tip in the south pole. similarly, there is a reduction of flux in the lower tip of the n pole and in the upper pole tip of the South pole. the direction of the resultant flux has shifted in the direction of rotation of the generator. Since the m n A is always perpendicular to the axis of the resultant flux, the m n A is also sifted.

Effects of armature reaction

The effect of armature reaction are summarised below;

magnetic flux density is increased over one half of the pole and decrease over the other half. But the total flux produced by each pole is slightly reduced and, therefore the terminal voltage is slightly reduced. the effect of total flux reduction by armature reaction is known as the magnetizing effect.
the flux wave is distorted and there is shift in the position of the magnetic neutral axis in the direction of rotation for the generator and against the direction of rotation for the motor.
Armature reaction stabilize a flux in the neutral zone. armature reaction flux in the neutral zone will induce conductor voltage that aggravates the commutation problem.

Commutation

The current induced in the armature conductor of DC generator are alternating in nature.the commutation process involves the change from generated alternating current to an externally applied direct current. this induced current flow in one direction when the armature conductor are under North pole. They are in opposite direction when they are under South pole. as conductor pass out of the influence of north pole and enter the South pole, the current in them is reversed. The reversal of current take place along the m n A aur brush axis. whenever a brush spend two commutator segment, the winding element connected to those segment is short circuited. bi communication women the change that takes place in a winding element during the period of short circuit by a brush. This change as shown in figure for simplicity, consider a simple ring winding.

In position shown in figure, the current I flow to what the bus from the left hand side passes around the coil in a clockwise direction.

in position shown in figure, this call courage the same current in the same direction, but the coil is too short circuited by the brushes.

in position shown in figure, the bus mix contact with bass A and b, thereby short circuiting coil first. The current is still I from the left hand side and I from the right hand side ful Store it is seen that this to current can reach the bus without passing through a coil first.

from the above discuss and it is seen that during the period of short circuit of an armature coil by a brass the current in a coil must be reversed and also brought up to its full value in the reverse direction. The time of short circuit is called the period of commutation.

Figure 6.15 show how the current in the short circuited coil where is during the brief interval of the short circuit. Curve B show that the current change from positive I to negative Ilinearly in the commutation period. Such a commutation is called ideal commutation or state line commutation.

If the current through the coil 1 has not reached its full value in the position shown in figure then since call to is caring full current, the difference between the current, throw element to and one has to jump from commutator bar b to the brush in the form of spark. Thus,the cause of sparking at the commutator is the failure of the current in the short circuited element to reach the full value in the reverse direction by the end of short circuit.

This is known as under commutation or delay commutation.the curve of current against time in such a case is shown in figure by curve a. in ideal commutation the current of the communicating cal change linearly from positive I to negative I in the computation period. Curve represent over commutation aur accelerated commutation when the current rich its final value with zero rate of change at the end of the computation period. Usually this will result in a satisfactory commutation.

Method of improving commutation

There are three method of obtaining sparkless commutation;

Resistance commutation
Voltage commutation
Compensating winding

1. Resistance commutation

This method of improving commutation consists of using carbon brushes. This makes the contact resistance between commutator segment and brushes high.these hai contact resistance has the tendency to force the current in the short circuited coil to change according to the competition requirements, namely, two rivers and then build up in the reversed directions.

2. Voltage commutation

in this method arrangement is made to induced voltage in the coil undergoing commutation, which will neutralize the reactance voltage. This is injected voltage is in opposition to the reactance voltage. If the value of the injected voltage is made equal to the reactance voltage coma quick reversal of current in the short circuited coil will take place and there will be sparkless competition.

Two methods may be used to produce the injected voltage in a position to the reactance:

Brush shift
Commutating poles or interpoles.

Brush shift :-

The effect of armature reaction is to shift the magnetic neutral axis m n A in the direction of rotation for the generator and against the direction of rotation for the motor. Armature reaction is stabilizers reflects on the neutral zone. a small voltage is induced in the commutating cail since it is cutting the flux.

Commutating poles or interpoles:-

Interpoles are narrow pores attached to the stator yoke, and please exactly midway between the main poles. Interpoles are also called commutating poles aur compoles. The Interpol winding are connected in series with the armature comma because the Interpol must produce flux that are directly proportional to the armature current. The nature and Interpol MMF are affected simultaneously by the same armature current. consequently, the armature flux in the commutating zone which tend to shift the magnetic neutral axis, is neutralized by an appropriate component of Interpol flux. The neutral plane is, therefore, fixed in position regardless of the load.

Characteristics of DC generator

at present time bulb of electrical energy is generated in the form of alternating current. Dc generators are no more used in modern power systems. For the sake of continuity, the characteristics of DC generator briefly given here. characteristic is the graph between two dependent quantities.

following are the three important characteristics of DC generator:-

Magnetisation characteristics :- magnetization characteristics gives the variation of generated voltage with field current at a constant speed. It is also called no load or open circuit characteristics.
Internal characteristics :- it is the plot between the generated voltage and load current.
External characteristics of load characteristics :- it is a graph between the terminal voltage and current at a constant speed.

About Admin MC3

This is dummy text. It is not meant to be read. Accordingly, it is difficult to figure out when to end it. But then, this is dummy text. It is not meant to be read. Period.