Methods of Limiting the Effects of Armature Reaction in D.C Machine

Methods of limiting the effects of armature reaction –

The cross-magnetizing effect of armature m.m.f. can be minimised at the design and construction stage of a D.C. machine. Various methods of mitigating the effects of armature
reaction are discussed below :
(a) High-reluctance pole tips – If the
reluctance of the pole tips is increased, then the magnitude of armature cross-flux is reduced and the distortion of the resultant flux density wave is minimised. The
reluctance of the pole tips can be augmented by using chamfered or eccentric pole shoes. A machine fitted with chamfered or eccentric
pole face has short air-gap length at the pole centre and longer air gap lengths under the pole tips, i.e. the proflle of the pole shoe is not
concentric with the armature core as shown in Fig. 4.19 (a).
Another method reluctance to cross-flux is to assemble alternatively the pole laminations depicted in Fig. 4.19 (b). That is, if the first lamination has the pole tip to the left, the second lamination has its pole tip to the right, the third lamination pole tip to the left and so on, until the required pole depth is developed. Since the iron area under the pole tips is almost halved, the
reluctance under the pole tips is considerably increased.
The two constructional techniques mentioned above reduce the main field flux to some extent. In order to maintain it constant, the main field m.m.f. must be raised accordingly. But the influence of increased pole-tip reluctance is more pronounced on the cross-flux than on the main field flux.
In D.C. machines, the short air gap at the pole centre and longer air gaps at the pole tips are kept only to limit the effect of cross-magnetizing armature m.m.t. on the main pole
flux. The distribution of the flux density wave along the air-gap periphery need not be a sine wave in D.C. machines. But in synchronous machines, the air gap at the pole centre is short and at the pole tips it is larger from the view point of obtaining sine wave for the flux density wave. In synchronous machines of the salient-pole type, the non-uniform air gap under the pole faces has nothing to do with the armature reaction.

(b) Reduction in armature flux – Another constructional technique of reducing the armature cross-flux is to create more reluctance in the path of armature flux without reducing
the main field flux noticeably. This is achieved by using field-pole laminations having several rectangular holes punched in them. One such lamination having four holes or slots is shown in Fig. 4.20 (a). It is seen from Fig. 4.20 (b) that reluctance offered to armature flux is increased due to four air-gap openings introduced in the path of cross-flux. As a result, armature cross-flux is reduced considerably, whereas the main-field flux remains almost uneffected.

Combination of the constructional features described in Figs. 4.19 and 4.20 may be used most effectively in reducing the armature cross flux.
(c) Strong main-field flux – During the design of a D.C. machine, it should be ensured that the main field m.m.f. is sufficiently strong in comparison with full-load armature m.m.f. Greater the ratio of main field m.m.f. to full-load armature m.m.f, less is the distortion produced by armature cross flux and predominant would be the control of field m.m.f. over the air-gap flux. Actually, this ratio depends on the type of duty cycle the D.C. machine has to perform.
(d) Interpoles – The effect of armature reaction in the interpolar zone can be overcome by interpoles, placed in between the main poles. The magnetic axis of interpole winding is in line with the quadrature axis. Interpole winding is connected in series with armature so that
interpole m.m.f is able to neutralize the effect of armature m.m.f. in the interpolar zone at all levels of load current not exceeding the safe limit.

(e) compensating winding – The effect of armature reaction under the pole shoes can be limited by using compensating winding. This winding is embedded in slots cut in the pole is a D.C. machines this is the best but the most expensive method.