Hi readers, today am here with a topic armature reaction and how it will effect the generator performance, how we reduce it we will discuss here.lets start now....
Armature Reaction:
Armature Reaction is defined as the effect of magnetic field set up by the armature current on the main field pole flux of a generator. The effect of armature current flux on main pole flux are of two types
Lets take an example to illustrate the armature reaction in brief . The flux distribution of a bipolar generator when there is no current in the armature conductors is shown in above fig. Generally we use multi poles in generator for easy explanation am taking 2-pole generator with brushes connected to armature conductors.
In the above fig.1 the flux is distributed symmetrically with respect to polar axis. Polar axis is the line joining the centers of two poles.Magnetic neutral axis is defined as the axis along which no emf is induced in armature conductors because they move parallel to lines of flux.
Magnetic neutral axis coincide with geometrical neutral axis and it is perpendicular to flux passing through armature. Brushes are always placed along magnetic neutral axis and hence it is called axis of commutation as the reversal of current take place along this axis.
In fig.1 above we can see OFm is the MMF producing main flux which is represented in both direction and magnitude, and perpendicular to M.N.A.
In fig above the flux produced is due to armature conductors alone when carrying current but the field coils unexcited. The current passing through armature cnductors is same when we load the generator. The current direction under N-pole is downwards and under S-pole is upwards.
The mmf of armature conductors combine to send flux downwards through the armature. The direction of lines of force can be found by applying cork-srew rule. The armature mmf is shown in vector form with magnitude and direction OFa which is parallel to brush axis.
Up to now we discussed main field mmf and armature mmf separately but in general under loaded condition both occurs simultaneously.Due to armature effect the flux through armature is no longer uniform.
Due to armature reaction effect flux gets distorted and it is crowded at one end and thinned at other end. The flux is crowded at trailing pole tip and weakened at leading pole tip( the pole tip which met first during rotation of armature is leading pole tip and other as trailing pole tip) Thus flux density at one side is more than other side. In above fig.3 shows resultant mmf OF which is resultant of OFm and OFa.
Now the armature reaction effect the resultant mmf changes to new position then M.N.A which is always perpendicular to mmf also changes which results in changes of brushes along new M.N.A position. Due to this shifting of brushes the armature conductors also changes its position from north pole to south pole and viceversa but not all conductors only some conductors.
Due to change of conductors from one pole to another armature current redistribution takes place. All conductors which are left to new M.N.A has current in downwards and other in upwards.The resultant MMF can be given in new position of M.N.A as OFa which is not vertical but inclined with some angle. It can be resolved into two rectanglar components, OFd which is parallel to polar axis and OFc which is perpendicular to this axis.
- de magnetising effect
- cross magnetising effect
Lets take an example to illustrate the armature reaction in brief . The flux distribution of a bipolar generator when there is no current in the armature conductors is shown in above fig. Generally we use multi poles in generator for easy explanation am taking 2-pole generator with brushes connected to armature conductors.
In the above fig.1 the flux is distributed symmetrically with respect to polar axis. Polar axis is the line joining the centers of two poles.Magnetic neutral axis is defined as the axis along which no emf is induced in armature conductors because they move parallel to lines of flux.
Magnetic neutral axis coincide with geometrical neutral axis and it is perpendicular to flux passing through armature. Brushes are always placed along magnetic neutral axis and hence it is called axis of commutation as the reversal of current take place along this axis.
In fig.1 above we can see OFm is the MMF producing main flux which is represented in both direction and magnitude, and perpendicular to M.N.A.
In fig above the flux produced is due to armature conductors alone when carrying current but the field coils unexcited. The current passing through armature cnductors is same when we load the generator. The current direction under N-pole is downwards and under S-pole is upwards.
The mmf of armature conductors combine to send flux downwards through the armature. The direction of lines of force can be found by applying cork-srew rule. The armature mmf is shown in vector form with magnitude and direction OFa which is parallel to brush axis.
Up to now we discussed main field mmf and armature mmf separately but in general under loaded condition both occurs simultaneously.Due to armature effect the flux through armature is no longer uniform.
Due to armature reaction effect flux gets distorted and it is crowded at one end and thinned at other end. The flux is crowded at trailing pole tip and weakened at leading pole tip( the pole tip which met first during rotation of armature is leading pole tip and other as trailing pole tip) Thus flux density at one side is more than other side. In above fig.3 shows resultant mmf OF which is resultant of OFm and OFa.
Now the armature reaction effect the resultant mmf changes to new position then M.N.A which is always perpendicular to mmf also changes which results in changes of brushes along new M.N.A position. Due to this shifting of brushes the armature conductors also changes its position from north pole to south pole and viceversa but not all conductors only some conductors.
Due to change of conductors from one pole to another armature current redistribution takes place. All conductors which are left to new M.N.A has current in downwards and other in upwards.The resultant MMF can be given in new position of M.N.A as OFa which is not vertical but inclined with some angle. It can be resolved into two rectanglar components, OFd which is parallel to polar axis and OFc which is perpendicular to this axis.
- component OFc is at right angles to vector OFm represents main mmf. It produces distortion in main field flux and hence it is called as cross magnetising or distorting component of armature reaction
- component OFd which is in direct opposite to OFm which represent main mmf. It exerts a demagnetising influence on main flux. Hence it is called demagnetising or weaking component of main flux.
The effect of armature reaction increases with increase in armature current.
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