Page 755 - Basic Electrical Engineering

It can been calculated that for P = 4, N = 1500 rpm; for P = 6, N = 1000 rpm
s
s
and so on.
The construction of the rotor forming the magnetic poles which are rotated
by the turbine are of two types. One type is of projected pole-type rotor

construction where number of poles are made by passing field current
through the windings of the pole cores. Where a large number of poles are

required to be formed, this type of projected (or salient) poles are made.
In hydroelectric generating stations, the speed of the turbines is
comparatively lower than the steam turbines used in thermal power stations.

For example, the turbine speed in Bhakra Hydroelectric power generating
station is only 167 rpm. To generate electricity at 50 Hz, the number of

magnetic poles required on the rotor is as many as 36. Projected type of poles
are used in the rotors when a large number of poles are to be fixed on the

rotor. Such rotors are called salient-type rotors. When P = 36 and the
frequency of the induced EMF is to be 50 Hz, then the turbine speed, N is
s

In thermal power stations the turbine speed is usually maintained at 3000
rpm so that the number of rotor poles is only 2.

High-speed rotors are made cylindrical type, or non-projected type and are
also called non-salient type. Thus, two types of rotor construction are made,

one is the salient-type rotor and the other is the non-salient or cylindrical-type
rotors. The stator construction is the same in both the cases. Three-phase
windings, displaced at 120° apart are made on the stator slots. Cross-sectional

view of the two types of synchronous machines are shown in Fig. 10.1. The