The magnetic neutral axis (MNA) is shifted due to

 Q1. The function of the commutator in a DC machine is to

A) Provide mechanical support to armature
B) Convert AC to DC
C) Reduce losses
D) Increase speed
Ans: B

Q2. The armature winding of a DC machine is placed on
A) Stator
B) Rotor
C) Field poles
D) Yoke
Ans: B

Q3. In a DC generator, the direction of induced emf is given by
A) Fleming’s right-hand rule
B) Fleming’s left-hand rule
C) Lenz’s law
D) Faraday’s law
Ans: A

Q4. The main function of brushes in a DC machine is
A) To conduct current between rotating and stationary parts
B) To provide insulation
C) To cool the armature
D) To support the shaft
Ans: A

Q5. A DC machine can be used as
A) Generator only
B) Motor only
C) Both generator and motor
D) Alternator
Ans: C

Q6. The armature reaction in a DC generator causes
A) Reduction in main flux
B) Increase in main flux
C) No change in main flux
D) Sparking at brushes
Ans: A

Q7. The number of commutator segments equals the
A) Number of turns
B) Number of coils
C) Number of poles
D) Number of slots
Ans: B

Q8. The generated emf in a DC generator is given by
A) E = 4.44 f N Φ
B) E = (PΦZN)/60A
C) E = (VIt)/t
D) E = IR
Ans: B

Q9. The back emf in a DC motor is due to
A) Supply voltage
B) Lenz’s law
C) Magnetic reluctance
D) Counter current
Ans: B

Q10. The purpose of interpoles in a DC machine is
A) To reduce armature reaction
B) To improve commutation
C) To reduce losses
D) Both A and B
Ans: D

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Q11. The voltage equation of a DC motor is
A) V = Eb + IaRa
B) V = Eb − IaRa
C) V = Eb × Ia
D) V = Eb / Ia
Ans: A

Q12. In a shunt motor, if the field current increases, the speed
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
Ans: B

Q13. The speed of a DC motor can be controlled by
A) Varying armature voltage
B) Varying field flux
C) Inserting resistance in armature circuit
D) All of the above
Ans: D

Q14. The torque in a DC motor is proportional to
A) IaΦ
B) Φ²
C) Ia²
D) Ia/Φ
Ans: A

Q15. In a DC shunt generator, if load increases, terminal voltage
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
Ans: B

Q16. A compound DC machine has
A) Series field only
B) Shunt field only
C) Both series and shunt fields
D) No field winding
Ans: C

Q17. The efficiency of a DC generator is maximum when
A) Iron loss = Copper loss
B) Armature current = Field current
C) Iron loss = Friction loss
D) Load current is zero
Ans: A

Q18. The commutation process converts
A) DC to AC
B) AC to DC
C) DC to DC
D) AC to AC
Ans: B

Q19. In a DC motor, armature current direction is determined by
A) Fleming’s right-hand rule
B) Fleming’s left-hand rule
C) Lenz’s law
D) Faraday’s law
Ans: B

Q20. If the back emf equals supply voltage, the motor
A) Draws no current
B) Draws maximum current
C) Runs at maximum speed
D) Stops
Ans: A

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Q21. The losses in a DC machine are
A) Copper losses
B) Iron losses
C) Mechanical losses
D) All of the above
Ans: D

Q22. Hysteresis loss in a DC machine depends on
A) Supply voltage
B) Frequency and flux density
C) Load
D) Speed only
Ans: B

Q23. Eddy current losses can be reduced by
A) Using laminated cores
B) Increasing flux
C) Using solid cores
D) Increasing frequency
Ans: A

Q24. The field winding of a shunt generator is connected
A) In series with armature
B) In parallel with armature
C) Across load
D) In series with load
Ans: B

Q25. The direction of rotation of a DC motor can be reversed by
A) Reversing field current
B) Reversing armature current
C) Reversing both
D) Any one of A or B
Ans: D

Q26. The no-load speed of a DC shunt motor is
A) Finite and safe
B) Zero
C) Infinite
D) Depends on load
Ans: A

Q27. In DC machines, the yoke serves as
A) Magnetic path
B) Mechanical support
C) Housing for field poles
D) All of the above
Ans: D

Q28. If a DC shunt motor runs at high speed suddenly, it may be due to
A) Field winding open
B) Armature short
C) High load
D) High voltage
Ans: A

Q29. In DC machines, armature reaction leads to
A) Flux weakening
B) Distortion of main field
C) Sparking at brushes
D) All of the above
Ans: D

Q30. The torque developed in a DC motor is directly proportional to
A) Armature current
B) Field flux
C) Product of armature current and field flux
D) Square of field current
Ans: C

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Q31. The mechanical power developed in a DC motor is
A) Eb × Ia
B) V × Ia
C) (V − Eb)Ia
D) Ia²Ra
Ans: A

Q32. The field winding resistance of a shunt motor is
A) Very low
B) Very high
C) Moderate
D) Infinite
Ans: B

Q33. The main difference between lap and wave winding is
A) Number of parallel paths
B) Wire size
C) Armature material
D) Commutator type
Ans: A

Q34. Wave winding is preferred for
A) Low current, high voltage machines
B) High current, low voltage machines
C) High torque machines
D) Small motors only
Ans: A

Q35. The armature of a DC machine is laminated to
A) Reduce weight
B) Reduce eddy current losses
C) Increase strength
D) Improve commutation
Ans: B

Q36. The residual magnetism in a generator is important for
A) Initial voltage build-up
B) Maintaining speed
C) Reducing losses
D) Commutation
Ans: A

Q37. In a DC generator, if field winding is reversed, the voltage
A) Reverses
B) Increases
C) Decreases
D) Remains same
Ans: A

Q38. A DC series motor should never be started
A) At full load
B) Without load
C) With weak field
D) With low voltage
Ans: B

Q39. The sparkless commutation is achieved by
A) Interpoles
B) Compensating winding
C) Both A and B
D) None
Ans: C

Q40. The open-circuit characteristic (OCC) of a DC generator is
A) Linear throughout
B) Initially linear, then saturates
C) Non-linear
D) Straight line
Ans: B

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Q41. For a given flux, the speed of a DC motor is directly proportional to
A) Armature voltage
B) Armature current
C) Torque
D) Resistance
Ans: A

Q42. In a DC motor, the back emf opposes
A) Supply voltage
B) Armature current
C) Field current
D) Flux
Ans: A

Q43. The characteristic of a DC shunt motor is
A) Constant speed
B) Variable speed
C) High starting torque
D) Series type
Ans: A

Q44. A cumulatively compounded generator gives
A) Higher voltage under load
B) Constant voltage
C) Decreasing voltage with load
D) Zero voltage
Ans: A

Q45. The function of compensating winding is to
A) Neutralize armature reaction
B) Reduce speed
C) Increase torque
D) Improve cooling
Ans: A

Q46. The magnetic neutral axis (MNA) is shifted due to
A) Armature reaction
B) Commutation
C) Supply voltage
D) Pole shape
Ans: A

Q47. The load characteristic of a DC shunt generator is nearly
A) Flat
B) Falling
C) Rising
D) Constant
Ans: A

Q48. The current in armature of DC generator is
A) Alternating
B) Direct
C) Alternating and then rectified
D) Zero
Ans: C

Q49. In a DC motor, back emf is directly proportional to
A) Flux and speed
B) Armature current
C) Torque
D) Supply voltage
Ans: A

Q50. The torque-speed characteristic of a DC series motor is
A) Hyperbolic
B) Linear
C) Parabolic
D) Constant
Ans: A