Answer / man

difference is in the design of rotor being used in these two
cases.
1)In hydro plants since water falls on the turbine ,the
speed of water is less (as compared to steam in case of
thermal plants).
according to the relation, we have
f=(p*n)/120

p= no of poles
f= frequency
n= speed
so in order to have fixed frequency product of 'p' and 'n'
should be a constant.so p should be more(as n is less).

in order to accommodate more number of poles the diameter of
rotor should be large.

2)for thermal we have high speed of steam, so a long core
and small diameter rotor is required to limit the
centrifugal forces developed in the high sped rotor.

Answer / gopal manda

in hydro power we use salient pole type alternetor are use
because hydro turbine(Prime mover of alternator)is rotating
at 750 RPM.so it can't possible to manufacture/make 6 pole
on cylendrical rotor to maintain the power frequency.and in
thermal power plant steam tubine rotate at 3000 rpm so we
construct 2pole on its rotor/field winding for maintain the
frequency.so we use cylendrical rotor are use

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