Electrical Machines( Synchronous Machines/Alternator ):
Q- 277. 9) In
an alternator, the voltage regulation will be positive when the power factor of
the load is
a. Leading
b. Unity
c. Lagging
d. Both
( b ) & ( c )
e. None
of these
Q- 278. The
given figure shows the load characteristics of an alternator. Which among the
following is correct?
a. A – Leading p.f., B – unity p.f., C – lagging
p.f.
b. A
– Lagging p.f., B – unity p.f., C – leading p.f.
c. A
– Leading p.f., B – lagging p.f., C – unity p.f.
d. A
– Unity p.f., B – leading p.f., C – lagging p.f.
Q- 279. As
the value of load power factor of an alternator changes, the value of armature
reaction reactance
a. Remains constant
b. Changes
c. None
of these
Q- If the armature resistance
and synchronous reactance of a 2 MVA, 11 kV, 3 phase star connected alternator
are 2 ohm and 20 ohm per phase respectively. The value of full load generated
voltage per phase at unity power factor will be
a. 6888.45 V
b. 6899.65
V
c. 6999.45
V
d. 7111.45
V
Q- 280. In
a salient pole synchronous generator direct axis reactance is
a. Greater than quadrature axis reactance
b. Less
than quadrature axis reactance
c. Equal
to quadrature axis reactance
d. None
of the above
Q- 281. A
synchronous generator has a direct axis and quadrature axis reactances of 0.6
per unit and 0.3 per unit respectively. What will be its total induced emf on
open circuit if it is used to supply full load at rated voltage at 0.8 power
factor ( Assume rated terminal voltage is equal to 1 pu, full load armature
current be 1 pu and armature resistance to be 0).
a. 1.099 pu
b. 1.199
pu
c. 1.488
pu
d. 1.588
pu
Q- 282. A 3
phase star connected synchronous generator having direct axis reactance of 8
ohm and quadrature axis reactance of 6 ohm supplies current of 9 A with phase
angle of 20 degree lagging at 415 V. Then its direct axis component of armature
current and quadrature axis component of armature current are
a. 8.04 A, 4.04 A
b. 4.04
A, 6.84 A
c. 6.84
A, 8.04 A
d. 4.04
A, 8.04 A
Q- 283. The
reluctance power of a salient pole alternator is given by
a. V2[Xd - Xq /
2XdXq] sin2 δ
b. V2[Xq -
Xd / 2Xd Xq] sin2 δ
c. V2[Xd -
Xq / XdXq] sin δ
d. V[Xd -
Xq / 2Xd Xq] sin2 δ
Q- 284. In case
of salient pole alternators, maximum output power can be obtained when power
angle (d) is
a. Less than 90
degree
b. Equal
to 90 degree
c. Greater
than 90 degree
d. None
of the above
Q- 285. Two
identical alternators having impedances Z1 and Z2 are
connected in parallel produces E1 and E2 as
their induced emf’s. if no load is connected to the alternators then a
circulating current will flow in the circuit. This current is given by
a. E1 + E2 / Z1 -
Z2
b. E1 -
E2 / Z1 + Z2
c. E2 -
E1 / Z1 + Z2
d. E1 +
E2 / Z1 + Z2
Q- 286. If a 3
phase alternator is short circuited at its terminal, then the initial value of
short circuit current will be limited by
a. Synchronous reactance
b. Transient
reactance
c. Sub
transient reactance
d. All
of the above
Q- 287. he
change in excitation of the two alternators operating in parallel causes
a. Only the kVAR sharing of two alternators
without disturbing kW sharing of the two machines
b. Only
the kW sharing of two alternators without disturbing kVAR sharing of the two
machines
c. Both
the kVAR sharing and kW sharing of two alternators
d. None
of the above
Q- 288. In
case of hydro generators, the driving torque can be changed by controlling the
a. Gate opening
b. Throttle
opening
c. Both
(a) and (b)
d. None
of these
Q- 289. The
increase in mechanical torque in case of alternator will
a. Increase the armature current
and reduce the power factor
b. Increase
the armature current and improve the power factor
c. Decrease
the armature current and improve the power factor
d. Decrease
the armature current and reduce the power factor
Q- 290. When an
alternator is connected to infinite bus bar, change in excitation for
alternator will
a. Change the terminal voltage and power factor
both
b. Affect
only terminal voltage and power factor remains unaffected
c. Affect
only power factor and terminal voltage remains unaffected
d. Neither
terminal voltage will get affected nor power factor
Q- 291. The
plot of armature current verses excitation is known as V curves for synchronous
generator. Determine the correct combination of load.
a. A – full load, B – half load, C – no load
b. A
– full load, B – no load, C – half load
c. A
– no load, B – half load, C – full load
d. A
– half load, B – full load, C – no load
Q- 292. If a
three phase winding is displaced in space by 120 degree in a synchronous motor
then the magnitude of resultant flux when it is rotated by Θ = 0 degree and Θ =
60 degree is
a. 0, 1.5 times the magnitude of maximum flux
b. 1.5
times the magnitude of maximum flux, 0
c. 0.866
times the magnitude of maximum flux, 1.5 times the magnitude of maximum flux
d. 1.5
times the magnitude of maximum flux, 1.5 times the magnitude of maximum flux
Q- 293. For a
half cycle of fluxes in a synchronous motor, the resultant flux has rotated
through
a. 0 degree
b. 90
degree
c. 180
degree
d. 360
degree
Q- 294. For
a two pole, 50 Hz, 3 phase synchronous motor the speed of rotating magnetic
field is
a. 1000 rpm
b. 2000
rpm
c. 3000
rpm
d. 3500
rpm
Q- 295. A
synchronous motor draws 2 MVA at a power factor of 0.8 leading. If the
efficiency developed in the motor is 90% then the power developed is equal to
a. 1212 kW
b. 1440
kW
c. 1676
kW
d. 1767
kW
Q- 296. For
a synchronous motor, as the load angle reaches up to 90 ° electrical i.e. half
a pole pitch, then the magnetic locking between the stator and rotor
a. No longer exist
b. Will
be strongest
c. Will
be weakest
d. None
of these
Q- 297. Practically
in a synchronous motor, the pull-out torque will occur when the torque angle is
nearly equal to
a. 30° electrical
b. 45
° electrical
c. 60
° electrical
d. 75
° electrical
Q- 298. In
a synchronous motor the torque produced will be maximum when load angle is
equal to
a. 0° electrical
b. 90°
electrical
c. 180°
electrical
d. 360°
electrical
Q- 299. f the
load on a synchronous motor increases then its
a. Current drawn by motor increases and load angle
decreases
b. Current
drawn by motor decreases and load angle remains constant
c. Current
drawn by motor remains constant and load angle increases
d. Current
drawn by motor increases and load angle also increases
Q- 300. Back
emf in case of synchronous motor depends on the
a. Excitation given to the field winding
b. Speed
c. Both
(a) and (b)
d. None
of the above
Q- 301. With
increase in load, the speed of synchronous motor
a. Increases
b. Decreases
c. Remains
constant
d. None
of the above
Q- 302. A
synchronous motor is started by using damper winding. When the motor runs at
synchronous speed then the induced emf in the damper winding will be
a. Zero
b. Maximum
c. Half
of the terminal voltage
d. None
of these
Q- 303. The
method used for starting synchronous motor is/are
a. Using pony motors
b. Using
damper windings
c. As
a slip ring induction motor
d. All
of the above
Q- 304. Can
synchronous motor rotate at any speed other than the synchronous speed?
a. Yes
b. No
Q- 305. Synchronous
motors are
a. Self starting
b. Not
self starting
c. Both
(a) and (b)
d. None
of these
Q- 306. Synchronous
motor works on the principle of
a. Mutual induction
b. Electromagnetic
induction
c. Magnetic
locking
d. None
of these
Q-307. The
impedance angle or internal machine angle (°) of a synchronous motor is equal
to
a. tan-1Xs / Ra
b. tan-1 Rs /
Xa
c. tan-1 Xs /
Za
d. tan-1 Zs /
Ra
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