Deck 4: Synchronous Motor Characteristics and Applications

Question 1

A synchronous motor can be made self starting by providing

A)damper winding on rotor poles
B)damper winding on stator
C)damper winding on stator as well as rotor poles
D)none of the above

Answer 1

none of the above

Question 2

The oscillations in a synchronous motor can be damped out by

A)maintaining constant excitation
B)running the motor on leading power factors
C)providing damper bars in the rotor pole faces
D)oscillations cannot be damped

Answer 2

providing damper bars in the rotor pole faces

Question 3

The shaft of synchronous motor is made of

A)mild steel
B)chrome steel
C)alnico
D)stainless steel

Answer 3

mild steel

Question 4

When the field of a synchronous motor is under-excited, the power factor will be

A)leading
B)lagging
C)unity
D)zero

Question 5

The speed regulation of a synchronous motor is always

A)1%
B)0.5%
C)positive
D)zero

Question 6

The percentage slip in case of a synchronous motor is

A)1%
B)100%
C)0.5%
D)zero

Question 7

The operating speed of a synchronous motor can be changed to new fixed value by

A)changing the load
B)changing the supply voltage
C)changing frequency
D)using brakes

Question 8

A synchronous motor will always stop when

A)supply voltage fluctuates
B)load in motor varies
C)excitation winding gets disconnected
D)supply voltage frequency changes

Question 9

riunting in a synchronous motor takes place

A)when supply voltage fluctuates
B)when load varies
C)when power factor is unity
D)motor is under loaded

Question 10

When load on an over-excited or under excited synchronous*motor is increased, rate of change of its armature current as compared with that of power factor is

A)more
B)less
C)equal
D)twice

Question 11

The rotor copper losses, in a synchronous motor, are met by

A)d.c. source
B)armature input
C)motor input
D)supply lines

Question 12

The maximum power developed in a synchronous motor occurs at a coupling angle of

A)30°
B)60°
C)90°
D)180°

Question 13

When the stator windings are connected in such a fashion that the number of poles are made half, the speed of the rotor of a synchronous motor

A)remains same as the original value
B)decreases to half the original value
C)tends to becomes zero
D)increases to two times the original value

Question 14

In which of the following motors the stator and rotor magnetic field rotate at the same speed ?

A)Universal motor
B)Synchronous motor
C)Induction motor
D)Reluctance motor

Question 15

Synchronsizingpower of a synchronous machine is

A)direcly proportional to the synchronous reactance
B)inversely proportional to the synchronous reactance
C)equal to the synchronous reactance
D)none of the above

Question 16

Synchronous motors are

A)not-self starting
B)self-starting
C)essentially self-starting
D)none of the above

Question 17

The standard full-load power factor ratings for synchronous motors are

A)zero or 0.8 leading
B)unity or 0.8 lagging
C)unity or 0.8 leading
D)unity or zero

Question 18

A synchronous motor running with normal excitation adjusts to load increases essentially by increase in

A)back e.m.f.
B)armature current
C)power factor
D)torque angle

Question 19

A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because

A)synchronous motor has no slip
B)stator supply is not required to produce magnetic field
C)mechanical load on the rotor remains constant
D)synchronous motor has large airgap

Question 20

A synchronous motor working at leading power factor can be used as

A)voltage booster
B)phase advancer
C)noise generator
D)mechanical synchronizer

Question 21

Slip rings are usually made of

A)carbon or graphite
B)brass or steel
C)silver or gold
D)copper or aluminium

Question 22

An over excited synchronous motor is used for

A)fluctuating loads
B)variable speed loads
C)low torque loads
D)power factor corrections

Question 23

When the voltage applied to a synchronous motor is increased, which of the following will reduce ?

A)Stator flux
B)Pull in torque
C)Both (a) and (b)
D)None of the above

Question 24

The efficiency of a properly designed synchronous motor will usually fall in range

A)60 to 70%
B)75 to 80%
C)85 to 95%
D)99 to 99.5%

Question 25

To limit the operating temperature an electrical machine should have proper

A)voltage rating
B)current rating
C)power factor
D)speed