SYNCHRONOUS MACHINES QUESTION BANK
1.What are the
principal advantages of rotating field type construction?
A.Relatively small
amount of power required for field system can easily supplied to rotating
system using slip rings and brushes, more space is available in the stator part
of the machine to provide more insulation, it is easy to provide cooling
system, stationary system of conductors can easily be braced to prevent deformation.
2.What are the
advantages of salient type pole construction used in sync. Machines?
A.They
allow better ventilation; the pole faces are so shaped radial air gap length
increases from pole center to pole tips so flux distortion in air gap is
sinusoidal so emf is also sinusoidal.
3.Which
type of sync. generators are used in hydroelectric plants and why?
A.As
the speed of operation is low, for hydro turbines used in hydroelectric plants,
salient pole type sync. generator is used because it allows better ventilation
also better than smooth cylindrical type rotor
4.Why are
alternators rated in KVA and not in KW?
A.As
load increases I2R loss also increases, as the current is directly related to
apparent power delivered by generator, the alternator has only their apparent
power in VA/KVA/MVA as their power rating.
5.Why the sync. impedance method of estimating voltage
regulation is is considered as pessimistic
method?
A.Compared to
other method, the value of voltage regulation obtained by this method is always
higher than the actual value, so it is called as pessimistic method.
6.Why MMF method
of estimating voltage regulation is considered as optimistic method?
A.Compared
to EMF method, MMF method involves more no. of complex calculation steps.
Further the OCC is referred twice, and SCC is referred once while
predetermining the voltage regulation for each load condition. Reference of OCC
takes core saturation effect. As this method requires more effort, final result
is very close to actual value, hence this method is called as optimistic
method.
7.Define voltage
regulation of the alternator?
A.It
is defined as the increase in terminal voltage when full load is thrown off,
assuming field current and speed remaining the same. % reg = [(E0 – V)/V]x100
Where E0 = no terminal
voltage V = full load rated terminal voltage
8.How is arm.
winding in alternators is different from those used in dc machines?
A.The
arm. winding of the alternator is placed in the stator, but in the case of dc
machines the arm winding is placed in the rotor.
9.What is hunting
how can it be prevented?
A.When
a sync motor is used for driving a fluctuating load, the rotor starts
oscillating about its new position of equilibrium corresponding to the new
load. This is called hunting or phase swinging. To prevent hunting dampers are
damping grids are employed.
10.what are
different torques of a sync motor?
1. Starting torque
2. Running torque
3. Pull-in torque
4. Pull-out torque