Induction motors important questions with explanations

Induction motors important questions with explanations for ESE , GATE ,ssc ,rrb ,electricity board &other exams

Q no 1 . The shaft of an I.M is made of

               a ) high speed steel

               b ) stainless steel

               c ) carbon steel

               d ) cast iron

explanations

                   answer :  c ) carbon steel

Q no 2. In an induction motor , no - load the slip is generally

A ) less than 1 %

B ) 1.5 %

C ) 2 %

D ) 4 %

Q no 3. In case the air gap in an induction motor is increased 

A ) the magnetising current of the rotor will decrease

B ) the power factor will decrease

C ) speed of motor will increase

D ) the winding losses will increase

Q no 4. Slip rings are usually made of 

A ) copper

B ) carbon

C ) phosphor bronze

D ) aluminium

Q no 5. The efficiency of an induction motor can be expected to be nearly

A ) 60 to 90 %

B ) 80 to 90 %

C ) 95 to 98 %

D ) 99 %

Q no 6. The starting torque of a squirrel cage induction motor is 

A ) low 

B ) negligible

C ) same as full load torque

D ) slightly more than full load torque

Q no 7. A double squirrel - cage induction motor has 

A ) two rotors moving in opposite direction

B ) two parallel winding in stator

C ) two parallel winding in rotor

D ) two series winding in stator

Q no 8. An induction motor with 1000 r.p.m speed will have

A ) 8 poles

B ) 6 poles 

C ) 4 poles 

D ) 2 poles

Q no 9. The injected emf in the rotor of induction motor must have

A ) zero frequency

B ) the same frequency as the slip frequency

C ) the same phase as the rotor emf

D ) high value for the satisfactory speed control

Q no 10. The auto - starters ( using three auto transformers ) can be used to start cage induction motor of the following type

A ) star connected only

B ) Delta connected only

C ) A and B both

D ) none of the above