DC Machine(Engineering > Electrical Engineering ) Questions and Answers

Question 1. The condition for maximum efficiency for a D.C. generator is
  1.    Eddy current losses = stray losses
  2.    Hysteresis losses = eddy current losses
  3.    Copper losses = 0
  4.    Variable losses = constant losses
Explanation:-
Answer: Option D. -> Variable losses = constant losses
Question 2. If a D.C. motor is connected across the A.C. supply it will
  1.    Run at normal speed
  2.    Not run
  3.    Run at lower speed
  4.    Burn due to heat produced in the field winding by eddy currents
Explanation:-
Answer: Option D. -> Burn due to heat produced in the field winding by eddy currents
Question 3. The speed of a D.C. shunt motor can be increased by
  1.    Increasing the resistance in armature circuit
  2.    Increasing the resistance in field circuit
  3.    Reducing the resistance in the field circuit
  4.    Reducing the resistance in the armature circuit
Explanation:-
Answer: Option B. -> Increasing the resistance in field circuit
Question 4. The armature voltage control of D.C. motor provides
  1.    Constant torque drive
  2.    Constant voltage drive
  3.    Constant current drive
  4.    None of the above
Explanation:-
Answer: Option A. -> Constant torque drive
Question 5. Starters are used with D.C. motors because
  1.    These motors have high starting torque
  2.    These motors are not self-starting
  3.    Back e.m.f. of these motors is zero initially
  4.    To restrict armature current as there is no back e.m.f. while starting
Explanation:-
Answer: Option D. -> To restrict armature current as there is no back e.m.f. while starting
Question 6. If I2 be the armature current, then speed of a D.C. shunt motor is
  1.    Independent of Ia
  2.    Proportional to Ia
  3.    Varies as Ia
  4.    None of these
Explanation:-
Answer: Option A. -> Independent of Ia
Question 7. In case of D.C. shunt motors the speed is dependent on back e.m.f. only because
  1.    Back e.m.f. is equal to armature drop
  2.    Armature drop is negligible
  3.    Flux is proportional to armature current
  4.    Flux is practically constant in D.C. shunt motors
Explanation:-
Answer: Option D. -> Flux is practically constant in D.C. shunt motors
Question 8. A direct on line starter is used: for starting motors
  1.    Up to 5 H.P.
  2.    Up to 10 H.P.
  3.    Up to 15 H.P.
  4.    Up to 20 H.P.
Explanation:-
Answer: Option A. -> Up to 5 H.P.
Question 9. The ratio of starting torque to full-load torque is least in case of
  1.    Series motors
  2.    Shunt motors
  3.    Compound motors
  4.    None of the above
Explanation:-
Answer: Option B. -> Shunt motors
Question 10. For starting a D.C. motor a starter is required because
  1.    It limits the speed of the motor
  2.    It limits the starting current to a safe value
  3.    It starts the motor
  4.    None of the above
Explanation:-
Answer: Option B. -> It limits the starting current to a safe value
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