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The details of synchronous generators developed so far is applicable to only round rotor or non-salient pole alternators.

**Salient pole alternators and
Blondel's Two Reaction Theory**

The
details of synchronous generators developed so far is applicable to only round
rotor or non-salient pole alternators. In such machines the air gap is uniform
throughout and hence the effect of mmf will be same whether it acts along the
pole axis or the inter polar axis. Hence reactance of the stator is same
throughout and hence it is called synchronous reactance. But in case salient
pole machines the air gap is non uniform and it is smaller along pole axis and
is larger along the inter polar axis. These axes are called direct axis or
d-axis and quadrature axis or q-axis. Hence the effect of mmf when acting along
direct axis will be different than that when it is acting along quadrature
axis. Hence the reactance of the stator cannot be same when the mmf is acting
along d - axis and q- axis. As the length of the air gap is small along direct
axis reluctance of the magnetic circuit is less and the air gap along the q -
axis is larger and hence the along the quadrature axis will be comparatively
higher. Hence along d-axis more flux is produced than q-axis. Therefore the
reactance due to armature

reaction
will be different along d-axis and q-axis. These reactances are,

*Xad *= direct axis reactance;* Xaq *= quadrature axis reactance

Hence the
effect of armature reaction in the case of a salient pole synchronous machine
can be taken as two components - one acting along the direct axis (coinciding
with the main field pole axis) and the other acting along the quadrature axis
(inter-polar region or magnetic neutral axis) and as such the mmf components of
armature-reaction in a salient-pole machine cannot be considered as acting on
the same magnetic circuit. Hence the effect of the armature reaction cannot be
taken into account by considering only the synchronous reactance, in the case
of a salient pole synchronous machine.

In fact,
the direct-axis component Fad acts over a magnetic circuit identical with that
of the main field system and produces a comparable effect while the
quadrature-axis component Faq acts along the interpolar axis, resulting in an
altogether smaller effect and, in addition, a flux distribution totally
different from that of Fad or the main field m.m.f. This explains why the
application of cylindrical-rotor theory to salient-pole machines for predicting
the performance gives results not conforming to the performance obtained from
an actual test.

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