Synchronous motors objective questions with explanations

Synchronous motors objective questions with explanations for ESE,GATE,SSC,RRB,Electricity board &other exams

Q no 1 . Under which of the following conditions hunting of synchronous motor is likely to occur ?

              a ) periodic variation of load 

              b ) over -excitation

              c ) over -loading for long periods

              d ) small and constant load

explanation 

                 answer   a ) periodic variation of load 

Q no 2. When V is the applied voltage , then the breakdown torque of synchronous motor varies as 

A ) V

B ) 1/V

C ) V2

D ) V3

explanation 

                 answer   A ) V

Q no 3. The power developed by a synchronous motor will be maximum when the load angle is 

A ) zero

B ) 45 

C ) 90

D ) 120

explanation 

            answer  C ) 90

Q no 4 . A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased 

A ) power factor as well as armature current will decrease 

B ) power factor as well as armature current will increase 

C ) power factor will increase but armature current will decrease 

D ) power factor will decrease and armature current will increase 

explanation 

            answer D ) power factor will decrease and armature current will increase 

Q no 5. As the load is applied to a synchronous motor, the motor takes more armature current because 

A ) the increased load has to take more current 

B )the rotor by shifting its phase backward cause motor to take more current 

C ) the back emf decrease causing an increase in motor current 

D ) the rotor strengthens the rotating field causing more motor current 

explanation 

            answer B )the rotor by shifting its phase backward cause motor to take more current 

Q no 6 . The minimum armature current of the synchronous motor corresponds to operation at 

A ) zero power factor leading 

B ) unity power factor 

C ) 0.707 power factor lagging 

D ) 0.707 power factor leading 

explanation 

            answer B ) unity power factor

Q no 7. The back emf of a synchronous motor depends on 

A ) speed 

B) load

C ) load angle 

D ) all of the above 

explanation 

            answer C ) load angle 

Q no 8 . The oscillations in 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 

explanation 

            answer C ) providing damper bars in the rotor pole faces

Q no 9 . The shaft of synchronous motor is made of 

 A ) mild steel

B ) chrome steel 

C ) alnico

D ) stainless steel 

explanation 

            answer  A ) mild steel

Q no 10. When load on an over excited or under - excited synchronous motor is increased, rate of change of its armature case compared with that of power factor is

A ) more 

B ) less

C ) equal 

D ) twice

explanation 

            answer B ) less