SYNCHRONOUS MOTOR :A synchronous motor is one in which the rotor normally rotates at the same speed as the revolving field in the machine. The stator is similar to that of an induction machine consisting of a cylindrical iron frame with windings, usually three-phase, located in slots around the inner periphery.
Short notes
Objective questions (Answers are given below)
- In a synchronous motor, damper winding is
provided in order to
(a) stabilize rotor motion
(b) suppress rotor oscillations
(c) develop necessary starting torque
(d) both (b) and (c) - In a synchronous motor, the magnitude of stator
back e.m.f. Eb depends on
(a) speed of the motor
(b) load on the motor
(c) both the speed and rotor flux
(d) d.c. excitation only - An electric motor in which both the rotor and
stator fields rotates with the same speed is called
a/an ……..motor.
(a) d.c.
(b) chrage
c) synchronous
(d) universal
- While running, a synchronous motor is compelled
to run at synchronous speed because of
(a) damper winding in its pole faces
(b) magnetic locking between stator and rotor
poles
(c) induced e.m.f. in rotor field winding by
stator flux
(d) compulsion due to Lenz’s law - The direction of rotation of a synchronous motor
can be reversed by reversing
(a) current to the field winding
(b) supply phase sequence
(c) polarity of rotor poles
(d) none of the above
- When running under no-load condition and with
normal excitation, armature current Ia drawn
by a synchronous motor
(a) leads the back e.m.f. Eb by a small angle
(b) is large
(c) lags the applied voltage V by a small
angle
(d) lags the resultant voltage ER by 90º. - The angle between the synchronously-rotating
stator flux and rotor poles of a synchronous
motor is called…….. angle.
(a) synchronizing
(b) torque
(c) power factor
(d) slip - If load angle of a 4-pole synchronous motor is
8º (elect), its value in mechanical degrees is
……..
(a) 4
(b) 2
(c) 0.5
(d) 0.25 - The maximum value of torque angle a in a synchronous
motor is ……..degrees electrical.
(a) 45
(b) 90
(c) between 45 and 90
(d) below 60 - A synchronous motor running with normal
excitation adjusts to load increases essentially
by increase in its
(a) power factor
(b) torque angle
(c) back e.m.f.
(d) armature current. - When load on a synchronous motor running
with normal excitation is increased, armature
current drawn by it increases because
(a) back e.m.f. Eb becomes less than applied
voltage V
(b) power factor is decreased
(c) net resultant voltage ER in armature is
increased
(d) motor speed is reduced - When load on a normally-excited synchronous
motor is increased, its power factor tends to
(a) approach unity
(b) become increasingly lagging
(c) become increasingly leading
(d) remain unchanged. - The effect of increasing load on a synchronous
motor running with normal excitation is to
(a) increase both its Ia and p.f.
(b) decrease Ia but increase p.f.
(c) increase Ia but decrease p.f.
(d) decrease both Ia and p.f. - Ignoring the effects of armature reaction, if
excitation of a synchronous motor running with
constant load is increased, its torque angle must
necessarily
(a) decrease
(b) increase
(c) remain constant
(d) become twice the no-load value. - If the field of a synchronous motor is underexcited,
the power factor will be
(a) lagging (b) leading
(c) unity (d) more than unity - Ignoring the effects of armature reaction, if
excitation of a synchronous motor running with
constant load is decreased from its normal
value, it leads to
(a) increase in but decrease in Eb
(b) increase in Eb but decrease in Ia
(c) increase in both Ia and p.f. which is
lagging
(d) increase in both Ia and φ - A synchronous motor connected to infinite busbars
has at constant full-load, 100% excitation
and unity p.f. On changing the excitation only,
the armature current will have
(a) leading p.f. with under-excitation
(b) leading p.f. with over-excitation
(c) lagging p.f. with over-excitation
(d) no change of p.f.
(Power App.-II, Delhi Univ. Jan 1987) - The V-curves of a synchronous motor show
relationship between
(a) excitation current and back e.m.f.
(b) field current and p.f.
(c) d.c. field current and a.c. armature current
(d) armature current and supply voltage.
- When load on a synchronous motor is increased,
its armature currents is increas- ed provided
it is
(a) normally-excited
(b) over-excited
(c) under-excited
(d) all of the above - If main field current of a salient-pole
synchronous motor fed from an infinite bus and
running at no-load is reduced to zero, it would
(a) come to a stop
(b) continue running at synchronous speed
(c) run at sub-synchronous speed
(d) run at super-synchronous speed - In a synchronous machine when the rotor speed
becomes more than the synchronous speed
during hunting, the damping bars develop
(a) synchronous motor torque
(b) d.c. motor torque
(c) induction motor torque
(d) induction generator torque
- In a synchronous motor, the rotor Cu losses are
met by
(a) motor input
(b) armature input
(c) supply lines
(d) d.c. source - A synchronous machine is called a doublyexcited
machine because
(a) it can be overexcited
(b) it has two sets of rotor poles
(c) both its rotor and stator are excited
(d) it needs twice the normal exciting current. - Synchronous capacitor is
(a) an ordinary static capacitor bank
(b) an over-excited synchronous motor driving
mechanical load
(c) an over-excited synchronous motor running
without mechanical load
(d) none of the above 623
(Elect. Machines, A.M.I.E. Sec. B, 1993)
- d 2. d 3. c 4. b 5. b 6. c 7. b 8. a 9. b 10. d 11. c 12.b 13. c 14. a 15. a 16. d 17. b 18. c 19. d 20. b 21. d 22. d 23c 24. c
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