we strongly recommend you to go through the armature reaction in dc machines to understand better - the need and the methods of limiting armature reaction. Also having concept fresh in your mind will make understanding easier.
The following six techniques are commonly used.
(1) By using eccentric poles :- A dc machine fitted with eccentric pole face has short air-gap length at the pole centre and longer air gap lengths under the pole tips. This increases the reluctance of the pole tips which reduces the magnitude of armature cross flux and hence armature reaction is minimised.
The following six techniques are commonly used.
(1) By using eccentric poles :- A dc machine fitted with eccentric pole face has short air-gap length at the pole centre and longer air gap lengths under the pole tips. This increases the reluctance of the pole tips which reduces the magnitude of armature cross flux and hence armature reaction is minimised.
(2) Using laminated poles :- If magnetic pole used is assembled
as shown, the laminations will sandwich air between them and therefore
reluctance of armature cross flux is
increased due to low permeability of air.
(3) Punching rectangular holes in field pole :- This also increases reluctance of armature cross flux with main flux remaining almost unaffected.
The constructional techniques mentioned above reduce the main flux also to some extent and therefore main field mmf must be raised accordingly. But the effect is more pronounced on armature flux and therefore the methods are used.
(4) Stronger main field flux as compared to armature flux :- From
phasor diagram in armature reaction, we found that the distortion θ of main field was given by,
So, greater the ratio of main flux to full load armature
flux, lesser is the distortion produced by armatur cross flux.
(5) Compensating windings :-
Using compensating winding to
neutralise armature reaction effect is the best and the most expensive method.
Compensating windings are coils embedded in slots cut in pole faces of
dc machine. Compensating windings
are connected in series with the armature so that they can adjust their mmf in accordance with armature mmf.
Compensating windings are embedded in pole faces, therefore they can counteract only flux magnitude reduction effect
of armature reaction and not
the distortion effect. For distortion effect of armature reaction, interpoles
are used.
(6) Interpoles :- To neutralise the distortion effect, poles called
interpoles are crafted in betwen
main field poles. These poles are energised by connecting them in series with the armature. Thus, distortion of main flux is corrected.
