Effect of Salient Poles
Effect of Salient Poles:
Fig: 1 Fig: 2
A multipolar machine with cylindrical rotor has a uniform air-gap, because of which its reactance remains the same, irrespective of the spatial position of the rotor. However, a synchronous machine with salient or projecting poles has non-uniform air-gap due to which its reactance varies with the rotor position.
Consequently, a cylindrical rotor machine possesses one axis of symmetry (pole axis or direct axis) whereas salient-pole machine possesses two axes of geometric symmetry (i) field poles axis, called direct axis or d-axis and (ii) axis passing through the centre of the interpolar space, called the quadrature axis or q axis, as shown in Fig. 1.
The treatment developed so far is applicable only to cylindrical rotor machines. In these machines, the air-gap is uniform so that the reluctance of the magnetic path is the same in all directions. Therefore, the effect of armature reaction can be accounted for by one reactance—the synchronous reactance Xs. It is because the value of Xs is constant for all directions of armature flux relative to the rotor.
However, in a salient-pole machine, the radial length of the air-gap varies [See Fig. (1)] so that reluctance of the magnetic circuit along the polar axis (called direct axis or d-axis) is much less than the reluctance along the interpolar axis (called quadrature axis or q-axis). This is illustrated in Fig. (2).
Because of the lower reluctance along the polar axis (i.e., d-axis), more flux is produced along d-axis than along the q-axis. Therefore, reactance due to armature reaction will be different along d-axis and q-axis. These are:
Xad = direct axis reactance due to armature reaction
Xaq = quadrature axis reactance due to armature reaction