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Two types of construction employed in synchronous machine are salient pole and
smooth cylindrical rotor type.
(i) Salient pole type or Projecting Pole Type:
It is used in low and medium speed (engine driven) alternators. It has a large
number of projecting (Salient) poles, having their cores bolted or dove tailed
onto a heavy magnetic steel of cast iron or steel of good magnetic quality.
Such generators are characterized by their large diameters and short axial
lengths. The poles and pole shoes (which cover 2/3 of pole–pitch) are
laminated to minimize heating due to eddy currents.
(ii) Smooth Cylinder Type:
Used for steam turbine driven alternators i.e. turbo alternators which run at
very high speeds. Rotor here is deigned mostly for 2-pole (or 4 pole) turbo
generators running at 3600rpm (or1800 rpm).
The central polar area is surrounded by the field windings placed in slots and
the field coils are so arranged around these polar areas that flux density is
maximum on the central polar line and gradually falls away on either side. It
should be noted that poles are non-salient i.e., they do not project out from the
surface of rotor. To avoid excessive peripheral velocity, such rotors have very
small diameters (about 1m or so). Hence, turbo generators are characterized by
small diameters and very long axial (or rotor) length. The cylindrical
construction of rotor gives letter balance and quieter-operation and less
windage losses.
Cylindrical types are much easier to analyze than salient pole types because
they have uniform air gap, whereas in salient pole type air gap is much greater
between the pole than along the poles. Fortunately cylindrical rotor theory is
reasonably accurate in predicting the steady state performance of salient pole
type. Hence salient pole theory is required only when very high degree of
accuracy is needed or when problems concerning transients or power system
stability are not handled
gregorymasila answered the question on February 22, 2018 at 16:45
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