Armature Reaction

ARMATURE REACTION

What is Armature Reaction??Armature reaction is the effect of armature produced flux on the main field flux. The armature mmf produces two undesirable effects on the main field flux and these are:-

Net reduction in the main field flux per pole.

Distortion of the main field flux wave along the air-gap periphery.

How it takes place??From the fig(A) it shows that the flux path for the main poles of a 2-pole DC machine at no load ie. With no armature currents.

This main field flux, produced by GNAfield mmf If Nf , is shown by horizontal MNAPhasor OA= . O A

Now when the DC machine is loaded, current flows in the armature windings.These currents are as shown in the figure (B), these currents set up armature flux shown by the vertical flux lines, with the field winding unexcited.The other figure represent armature flux a by a vertical phasor OB. GNA

O

B

Now by examination of the above two figures reveals that the path of armature flux a is perpendicular to the main flux path f .In other words, the path of the armature flux crosses the path of the main field flux. O A

BThus the effect of armature flux on the main field flux is entirely cross-magnetizing and it is for this reason that the flux produced by armature mmf is called cross-flux.

Now when the current flows in both the armature and field windings, the resultant flux distribution is obtained by superimposing the two fluxes of fig (A) and fig(B).

From the fig(C)shown below, it is seen that the armature flux aids the main field flux at the lower end of the N-pole and at the upper end of the S-pole, therefore at these two poles, the armature flux strengthens the main field flux. GNA MNA

Likewise, the armature flux weakens the main field flux at Upper end of the N-pole and at lower end of the S-pole.

Now, if there is no magnetic saturation, then the amount of strengthening and weakening of the main field flux are equal and the resultant flux per pole remains unaltered from its no load value.

Actually, the magnetic saturation does occurs and as a consequence, the strengthening effect is less as compared to the weakening effect and the resultant flux is decreased from its no-load value. This is called Demagnetizing effect of armature reaction

ConclusionIt may therefore be stated from the above that net effect of armature flux on the main field flux is:-

To distort the main field flux thereby causing non-uniform distribution of flux under the main poles.

To shift the MNA in the direction of the rotation for a generator and against the direction of rotation for a motor.

To reduce the main field flux from its no-load value due to magnetic saturation.

THANKS!!!

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