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DC Motor MCQ Questions and Answers for Electrical Engineering Exams

November 27, 2021 , ,
DC Motor MCQ Questions and Answers for Electrical Engineering Exams

DC Motor MCQ Questions and Answers for Electrical Engineering Exams

Search Description: Practice 158 DC Motor MCQ questions with answers and short explanations for Electrical Engineering, ITI, Diploma, GATE, SSC JE, RRB JE and technical interview preparation.

DC motors are one of the most important topics in Electrical Machines. They are frequently asked in competitive exams, semester exams, diploma exams, ITI exams and technical interviews. This post contains a complete DC Motor MCQ question series arranged from easy to hard level so that beginners can start from basic concepts and gradually move toward numerical and application-based questions.

The questions cover DC motor working principle, Fleming’s left-hand rule, back EMF, torque, starters, speed control, braking, losses, efficiency, applications, commutation, armature reaction and modern DC drive concepts. Each question includes the correct answer and a short explanation in simple language.

Table of Contents
Study Tip: Try to solve each MCQ first, then check the answer and explanation. This helps you remember the concept for exams and interviews.

Beginner Level DC Motor MCQs

These questions are arranged from simple concepts to exam-oriented application questions.

Question 1. A DC motor converts electrical energy mainly into:

  1. Heat energy
  2. Mechanical energy
  3. Chemical energy
  4. Light energy

Answer: B. Mechanical energy

Explanation: A motor changes DC electrical input into rotating mechanical output.

Question 2. The working principle of a DC motor is based on:

  1. Electromagnetic force on a current-carrying conductor
  2. Electrostatic induction
  3. Photoelectric effect
  4. Thermal expansion

Answer: A. Electromagnetic force on a current-carrying conductor

Explanation: A current-carrying conductor placed in a magnetic field experiences force.

Question 3. The direction of force in a DC motor is found by:

  1. Fleming's left-hand rule
  2. Fleming's right-hand rule
  3. Lenz law only
  4. Kirchhoff law

Answer: A. Fleming's left-hand rule

Explanation: Fleming's left-hand rule is used for motor action.

Question 4. In Fleming’s left-hand rule, the forefinger represents:

  1. Current
  2. Magnetic field
  3. Force
  4. Voltage

Answer: B. Magnetic field

Explanation: The forefinger shows the direction of magnetic field or flux.

Question 5. In Fleming’s left-hand rule, the middle finger represents:

  1. Motion
  2. Current
  3. Flux
  4. Speed

Answer: B. Current

Explanation: The middle finger indicates the direction of current.

Question 6. In Fleming’s left-hand rule, the thumb represents:

  1. Current
  2. Magnetic field
  3. Force or motion
  4. Resistance

Answer: C. Force or motion

Explanation: The thumb gives the direction of force or motion.

Question 7. The rotating part of a DC motor is called:

  1. Yoke
  2. Armature
  3. Pole shoe
  4. Brush holder

Answer: B. Armature

Explanation: The armature is the rotating part where torque is developed.

Question 8. The stationary outer frame of a DC machine is called:

  1. Commutator
  2. Yoke
  3. Armature core
  4. Shaft

Answer: B. Yoke

Explanation: The yoke gives mechanical support and magnetic return path.

Question 9. The function of brushes in a DC motor is to:

  1. Produce flux
  2. Collect/supply current through commutator
  3. Reduce speed
  4. Increase resistance only

Answer: B. Collect/supply current through commutator

Explanation: Brushes provide sliding electrical contact with the commutator.

Question 10. Brushes in DC machines are commonly made of:

  1. Wood
  2. Carbon/graphite
  3. Glass
  4. Rubber

Answer: B. Carbon/graphite

Explanation: Carbon or graphite brushes give good contact and less commutator wear.

Question 11. The commutator in a DC motor helps to:

  1. Reverse current in armature conductors at proper instant
  2. Store energy
  3. Cool the machine
  4. Increase frequency

Answer: A. Reverse current in armature conductors at proper instant

Explanation: Commutator keeps torque unidirectional by reversing armature current properly.

Question 12. A DC motor cannot be started directly without starter because:

  1. It has no field
  2. Back EMF is zero at starting
  3. Torque is zero always
  4. Commutator is absent

Answer: B. Back EMF is zero at starting

Explanation: At starting, back EMF is zero, so armature current can become very high.

Question 13. At the instant of starting a DC motor, back EMF is:

  1. Maximum
  2. Zero
  3. Equal to supply voltage
  4. Negative always

Answer: B. Zero

Explanation: Back EMF depends on speed, and speed is zero at starting.

Question 14. A starter is used in a DC motor mainly to limit:

  1. Speed only
  2. Starting current
  3. Field flux only
  4. Output power only

Answer: B. Starting current

Explanation: Starter resistance limits the large starting armature current.

Question 15. A DC shunt motor has approximately:

  1. Constant speed
  2. Zero speed
  3. Very unstable speed
  4. Speed independent of voltage only

Answer: A. Constant speed

Explanation: In a shunt motor, flux is nearly constant, so speed remains nearly constant.

Question 16. A DC series motor should not be started on:

  1. Full load
  2. Half load
  3. No load
  4. Rated load

Answer: C. No load

Explanation: At no load, series motor flux becomes small and speed may become dangerously high.

Question 17. The DC motor commonly used for electric traction is:

  1. DC series motor
  2. DC shunt motor
  3. Stepper motor
  4. Synchronous motor only

Answer: A. DC series motor

Explanation: DC series motor gives very high starting torque, useful in traction.

Question 18. For machine tools, the generally preferred DC motor is:

  1. Series motor
  2. Shunt motor
  3. Universal motor
  4. Repulsion motor

Answer: B. Shunt motor

Explanation: Shunt motor gives nearly constant speed, suitable for machine tools.

Question 19. For cranes and hoists, the preferred DC motor is usually:

  1. Series motor
  2. Shunt motor
  3. Single-phase induction motor
  4. Reluctance motor

Answer: A. Series motor

Explanation: Cranes and hoists require high starting torque, which series motors provide.

Question 20. For elevators, a suitable DC motor is:

  1. Cumulative compound motor
  2. Differential compound motor only
  3. Very small shunt motor
  4. Stepper motor only

Answer: A. Cumulative compound motor

Explanation: Cumulative compound motors give good starting torque and better speed control.

Question 21. The speed of a DC shunt motor can be increased by:

  1. Increasing field current
  2. Reducing field current
  3. Increasing load only
  4. Opening armature

Answer: B. Reducing field current

Explanation: Reducing field current weakens flux and increases speed.

Question 22. The speed of a DC motor is mainly controlled by changing:

  1. Flux, armature voltage or armature resistance
  2. Only temperature
  3. Only brush color
  4. Only frame size

Answer: A. Flux, armature voltage or armature resistance

Explanation: These are the main practical speed-control methods.

Question 23. Back EMF in a DC motor acts in direction:

  1. Same as supply voltage
  2. Opposite to applied voltage
  3. Perpendicular to voltage
  4. No fixed direction

Answer: B. Opposite to applied voltage

Explanation: Back EMF opposes the applied voltage as per Lenz law.

Question 24. The equation for armature current of a DC motor is:

  1. Ia=(V-Eb)/Ra
  2. Ia=V+Eb
  3. Ia=Ra/(V-Eb)
  4. Ia=Eb/V

Answer: A. Ia=(V-Eb)/Ra

Explanation: Armature current equals net voltage across armature resistance divided by Ra.

Question 25. The torque of a DC motor is proportional to:

  1. Flux × armature current
  2. Speed only
  3. Resistance only
  4. Voltage only

Answer: A. Flux × armature current

Explanation: Motor torque is proportional to product of flux and armature current.

Question 26. In a DC shunt motor, flux is practically:

  1. Zero
  2. Constant
  3. Infinite
  4. Random

Answer: B. Constant

Explanation: Shunt field is connected across supply, so flux is nearly constant.

Question 27. In a DC series motor before saturation, torque is approximately proportional to:

  1. Ia
  2. Ia²
  3. 1/Ia
  4. Speed only

Answer: B. Ia²

Explanation: In series motor, flux is proportional to Ia before saturation, so T ∝ Ia².

Question 28. The no-load speed of a DC series motor is:

  1. Very low
  2. Dangerously high
  3. Always zero
  4. Exactly rated speed

Answer: B. Dangerously high

Explanation: With very small load current, flux is weak and speed rises dangerously.

Question 29. A three-point starter is used for:

  1. DC shunt and compound motors
  2. Only AC induction motors
  3. Only transformers
  4. Only alternators

Answer: A. DC shunt and compound motors

Explanation: Three-point starter is commonly used with shunt and compound DC motors.

Question 30. A four-point starter is preferred when:

  1. Field control speed variation is required
  2. No motor is used
  3. Only AC supply is available
  4. Load is zero always

Answer: A. Field control speed variation is required

Explanation: Four-point starter separates no-volt coil from shunt field circuit.

Question 31. No-volt release in a DC motor starter protects against:

  1. High room temperature
  2. Supply failure and automatic restart
  3. Low friction
  4. Low speed only

Answer: B. Supply failure and automatic restart

Explanation: It disconnects motor when supply fails, preventing unsafe restart.

Question 32. Overload release in a starter protects the motor from:

  1. Excessive current
  2. Low voltage always
  3. Good commutation
  4. Normal load

Answer: A. Excessive current

Explanation: Overload release trips when motor current becomes excessive.

Question 33. The armature core of a DC motor is laminated to reduce:

  1. Eddy current loss
  2. Copper loss only
  3. Friction loss only
  4. Output torque

Answer: A. Eddy current loss

Explanation: Laminations increase resistance to circulating eddy currents.

Question 34. The field winding of a DC shunt motor has:

  1. Many turns of thin wire
  2. Few turns of thick wire
  3. No turns
  4. Only one copper bar

Answer: A. Many turns of thin wire

Explanation: Shunt field needs high resistance, so it uses many turns of thin wire.

Question 35. The field winding of a DC series motor has:

  1. Many turns of thin wire
  2. Few turns of thick wire
  3. Only insulation
  4. No conductor

Answer: B. Few turns of thick wire

Explanation: Series field carries load current, so it uses thick wire with fewer turns.

Question 36. A DC motor connected to AC supply may:

  1. Run normally always
  2. Overheat due to excessive losses
  3. Become a transformer
  4. Produce DC only

Answer: B. Overheat due to excessive losses

Explanation: Ordinary DC motors are not designed for AC and may heat badly.

Question 37. The nameplate power rating of a motor usually indicates:

  1. Shaft output power
  2. Input copper loss
  3. Only field loss
  4. Brush voltage drop

Answer: A. Shaft output power

Explanation: Motor rating generally refers to useful mechanical output at shaft.

Question 38. The main purpose of pole shoes is to:

  1. Spread flux and support field coils
  2. Increase brush wear
  3. Remove shaft
  4. Act as fuse

Answer: A. Spread flux and support field coils

Explanation: Pole shoes spread flux uniformly and support the field winding.

Question 39. In a DC motor, sparking at commutator is undesirable because it:

  1. Can damage commutator and brushes
  2. Always increases efficiency
  3. Stops all losses
  4. Improves cooling

Answer: A. Can damage commutator and brushes

Explanation: Sparking causes heating, pitting and wear.

Question 40. A DC shunt motor is best suited for:

  1. Constant speed drives
  2. No-load traction only
  3. Very high variable load with no starter
  4. Only lighting load

Answer: A. Constant speed drives

Explanation: Its speed changes only slightly with load.

Question 41. A DC series motor is best suited where:

  1. High starting torque is needed
  2. Constant speed at no load is needed
  3. Zero current is needed
  4. Low torque only is needed

Answer: A. High starting torque is needed

Explanation: Series motors are known for very high starting torque.

Question 42. The direction of rotation of a DC motor can be reversed by reversing:

  1. Either armature or field connections
  2. Both armature and field together only
  3. Supply frequency
  4. Frame material

Answer: A. Either armature or field connections

Explanation: Reversing either armature or field reverses torque direction.

Question 43. If both armature and field connections of a DC motor are reversed together:

  1. Direction remains same
  2. Direction reverses always
  3. Motor stops permanently
  4. Current becomes zero

Answer: A. Direction remains same

Explanation: Both current and flux reverse, so torque direction remains unchanged.

Question 44. The losses varying with load current in a DC motor are mainly:

  1. Armature copper losses
  2. Friction losses
  3. Windage losses
  4. Core losses only

Answer: A. Armature copper losses

Explanation: Armature copper loss varies as Ia²Ra.

Question 45. Mechanical losses in a DC motor include:

  1. Friction and windage
  2. Only copper loss
  3. Only hysteresis
  4. Only eddy current

Answer: A. Friction and windage

Explanation: Friction and windage occur due to rotation.

Question 46. Core losses include:

  1. Hysteresis and eddy current losses
  2. Only brush loss
  3. Only bearing loss
  4. Only output power

Answer: A. Hysteresis and eddy current losses

Explanation: Iron/core losses include hysteresis and eddy losses.

Question 47. Maximum efficiency of a DC motor occurs when:

  1. Variable losses equal constant losses
  2. Copper loss is infinite
  3. Output is zero
  4. Speed is zero

Answer: A. Variable losses equal constant losses

Explanation: This is the standard maximum efficiency condition.

Question 48. A DC motor used in household refrigerator is generally replaced by:

  1. Single-phase induction motor
  2. DC series motor only
  3. DC shunt motor only
  4. DC generator

Answer: A. Single-phase induction motor

Explanation: Domestic refrigerators usually use single-phase induction motors.

Question 49. The part of a DC motor that clearly identifies it as DC is:

  1. Commutator
  2. Cooling fan only
  3. Frame color
  4. Bearing cap

Answer: A. Commutator

Explanation: A commutator is a clear feature of DC machines.

Question 50. The back EMF of a DC motor increases when:

  1. Speed increases
  2. Speed becomes zero
  3. Flux becomes zero only
  4. Resistance is removed only

Answer: A. Speed increases

Explanation: Back EMF is proportional to flux and speed.

Intermediate Level DC Motor MCQs

These questions are arranged from simple concepts to exam-oriented application questions.

Question 51. If supply voltage is 220 V, back EMF is 200 V and armature resistance is 0.5 Ω, armature current is:

  1. 20 A
  2. 40 A
  3. 220 A
  4. 400 A

Answer: B. 40 A

Explanation: Ia=(220−200)/0.5=40 A.

Question 52. If a DC motor has V=250 V, Eb=230 V and Ra=0.4 Ω, Ia is:

  1. 25 A
  2. 50 A
  3. 100 A
  4. 575 A

Answer: B. 50 A

Explanation: Ia=(250−230)/0.4=50 A.

Question 53. If a DC motor develops 20 N-m torque at 10 A and flux is constant, torque at 15 A is:

  1. 20 N-m
  2. 25 N-m
  3. 30 N-m
  4. 45 N-m

Answer: C. 30 N-m

Explanation: For shunt motor with constant flux, torque is proportional to armature current.

Question 54. A DC shunt motor speed falls from 1000 rpm no-load to 950 rpm full-load. Regulation is approximately:

  1. 2.5%
  2. 5.26%
  3. 10%
  4. 50%

Answer: B. 5.26%

Explanation: Speed regulation=(1000−950)/950×100≈5.26%.

Question 55. For a DC series motor before saturation, if current is doubled, torque becomes approximately:

  1. Double
  2. Four times
  3. Half
  4. One-fourth

Answer: B. Four times

Explanation: T ∝ Ia² before magnetic saturation.

Question 56. For a DC series motor before saturation, if current is reduced to half, torque becomes:

  1. Half
  2. One-fourth
  3. Double
  4. Four times

Answer: B. One-fourth

Explanation: T ∝ Ia², so (0.5)²=0.25.

Question 57. Armature voltage control of DC motor mainly gives:

  1. Constant torque operation
  2. Constant power above base speed
  3. Zero torque
  4. Only braking

Answer: A. Constant torque operation

Explanation: Below base speed, armature voltage control gives constant torque.

Question 58. Field weakening control of DC motor mainly gives:

  1. Constant power region
  2. Zero speed region only
  3. Constant torque below base only
  4. No speed control

Answer: A. Constant power region

Explanation: Above base speed, field weakening is used for constant power operation.

Question 59. Armature resistance control has low efficiency because:

  1. Extra power is wasted in series resistance
  2. Flux becomes zero
  3. Motor becomes AC
  4. Commutator disappears

Answer: A. Extra power is wasted in series resistance

Explanation: Added resistance causes large I²R loss.

Question 60. Ward-Leonard speed control is basically:

  1. Variable voltage control
  2. Only field resistance control
  3. Mechanical braking only
  4. Frequency control

Answer: A. Variable voltage control

Explanation: Ward-Leonard system controls motor speed by varying armature voltage.

Question 61. Main drawback of Ward-Leonard control is:

  1. High cost and maintenance
  2. No speed control
  3. No smooth control
  4. Only fixed speed

Answer: A. High cost and maintenance

Explanation: It needs motor-generator set, so cost and maintenance are high.

Question 62. Dynamic braking of a DC motor means:

  1. Motor acts as generator and energy is dissipated in resistor
  2. Supply voltage is doubled
  3. Field is removed permanently
  4. Motor is disconnected from shaft

Answer: A. Motor acts as generator and energy is dissipated in resistor

Explanation: In dynamic braking, generated energy is wasted in braking resistance.

Question 63. Regenerative braking is possible when:

  1. Back EMF exceeds supply voltage
  2. Back EMF is zero
  3. Armature is open
  4. Flux is absent

Answer: A. Back EMF exceeds supply voltage

Explanation: Then the machine returns energy to the supply.

Question 64. Plugging in a DC motor is obtained by:

  1. Reversing armature supply polarity while running
  2. Reducing load
  3. Increasing ventilation
  4. Opening field

Answer: A. Reversing armature supply polarity while running

Explanation: Plugging reverses torque and gives strong braking.

Question 65. Plugging gives:

  1. High braking torque
  2. No braking torque
  3. Only field heating
  4. Only cooling

Answer: A. High braking torque

Explanation: Reverse torque is produced, so braking is strong.

Question 66. In rheostatic braking, the DC motor is disconnected from supply and connected to:

  1. A braking resistor
  2. A capacitor only
  3. A lamp only
  4. A transformer primary

Answer: A. A braking resistor

Explanation: Generated energy is dissipated in the resistor.

Question 67. Commutation in a DC motor is the process of:

  1. Reversing current in short-circuited armature coil
  2. Increasing shaft length
  3. Changing DC to AC supply
  4. Removing flux

Answer: A. Reversing current in short-circuited armature coil

Explanation: Commutation reverses coil current while it passes neutral zone.

Question 68. Interpoles in DC motors are connected:

  1. In series with armature
  2. In parallel with shunt field only
  3. Across supply with high resistance only
  4. Mechanically only

Answer: A. In series with armature

Explanation: Interpoles carry armature current to counter commutation effects.

Question 69. Compensating winding is used to reduce:

  1. Armature reaction
  2. Bearing friction
  3. Windage only
  4. Supply voltage

Answer: A. Armature reaction

Explanation: It neutralizes armature reaction under pole faces.

Question 70. Armature reaction in DC motor causes:

  1. Flux distortion
  2. Increase in frame size
  3. Zero armature current
  4. No heating

Answer: A. Flux distortion

Explanation: Armature current produces its own magnetic field and distorts main flux.

Question 71. Brushes are placed near the magnetic neutral axis to improve:

  1. Commutation
  2. Field resistance
  3. Bearing life only
  4. Shaft diameter

Answer: A. Commutation

Explanation: At neutral axis, coil EMF is minimum during commutation.

Question 72. If brushes are incorrectly positioned, the motor may show:

  1. Sparking
  2. Perfect efficiency
  3. No copper loss
  4. Zero noise

Answer: A. Sparking

Explanation: Wrong brush position causes poor commutation and sparking.

Question 73. The electromagnetic torque equation of a DC motor is generally:

  1. T ∝ ΦIa
  2. T ∝ N only
  3. T ∝ Ra only
  4. T ∝ 1/V only

Answer: A. T ∝ ΦIa

Explanation: Torque depends on flux per pole and armature current.

Question 74. The speed equation of a DC motor is:

  1. N ∝ (V−IaRa)/Φ
  2. N ∝ IaRa only
  3. N ∝ Φ only
  4. N ∝ 1/Rsh only

Answer: A. N ∝ (V−IaRa)/Φ

Explanation: Speed is proportional to back EMF divided by flux.

Question 75. If flux of a DC shunt motor is reduced while voltage is constant, speed:

  1. Increases
  2. Decreases to zero
  3. Does not change at all
  4. Becomes negative always

Answer: A. Increases

Explanation: N ∝ Eb/Φ, so lower flux increases speed.

Question 76. If armature resistance drop increases in a DC motor, speed generally:

  1. Decreases
  2. Increases infinitely
  3. Becomes exactly zero always
  4. Unaffected always

Answer: A. Decreases

Explanation: Higher IaRa drop reduces back EMF and speed.

Question 77. For a DC shunt motor, torque-current characteristic is approximately:

  1. Straight line
  2. Parabola
  3. Hyperbola only
  4. Circle

Answer: A. Straight line

Explanation: With constant flux, torque is proportional to armature current.

Question 78. For a DC series motor, torque-current characteristic before saturation is:

  1. Parabolic
  2. Straight line through origin only
  3. Constant
  4. Negative

Answer: A. Parabolic

Explanation: Since flux also varies with current, T ∝ Ia² before saturation.

Question 79. For a DC series motor after saturation, torque is nearly proportional to:

  1. Armature current
  2. Square of current always
  3. Zero
  4. Speed squared

Answer: A. Armature current

Explanation: After saturation, flux is nearly constant, so T ∝ Ia.

Question 80. The speed-torque characteristic of a DC series motor is:

  1. Highly drooping
  2. Perfectly constant
  3. Rising straight line
  4. Zero at all torques

Answer: A. Highly drooping

Explanation: Series motor speed falls sharply as load torque increases.

Question 81. The speed-torque characteristic of a DC shunt motor is:

  1. Nearly constant speed
  2. Very steep rising speed
  3. No speed
  4. Random

Answer: A. Nearly constant speed

Explanation: Shunt motor has good speed regulation.

Question 82. A cumulatively compounded DC motor has:

  1. Series field aiding shunt field
  2. Series field opposing shunt field
  3. No field
  4. Only permanent magnet

Answer: A. Series field aiding shunt field

Explanation: In cumulative compounding, fluxes aid each other.

Question 83. A differentially compounded DC motor has:

  1. Series field opposing shunt field
  2. Series field aiding shunt field
  3. No armature
  4. Only AC winding

Answer: A. Series field opposing shunt field

Explanation: Differential compounding reduces net flux as load increases.

Question 84. Differential compound motors are rarely used because they may become:

  1. Unstable
  2. Perfectly constant at all loads
  3. Lossless
  4. Brushless

Answer: A. Unstable

Explanation: Opposing series field may cause unstable speed behavior.

Question 85. A cumulative compound motor is useful where:

  1. High starting torque and fairly constant speed are needed
  2. No load operation only
  3. Zero torque is needed
  4. Only synchronous speed is needed

Answer: A. High starting torque and fairly constant speed are needed

Explanation: It combines series and shunt motor advantages.

Question 86. A DC series motor is suitable for traction because:

  1. High starting torque and self-relieving nature
  2. Very poor starting torque
  3. Constant speed at no load
  4. No need of current

Answer: A. High starting torque and self-relieving nature

Explanation: High torque helps vehicles start under load.

Question 87. Self-relieving property is strongest in:

  1. DC series motor
  2. DC shunt motor
  3. Synchronous motor
  4. Transformer

Answer: A. DC series motor

Explanation: As load increases, speed falls and power demand is moderated.

Question 88. If a DC shunt motor field opens at no load, the speed may:

  1. Rise dangerously
  2. Fall safely to zero immediately
  3. Remain exactly rated
  4. Become synchronous

Answer: A. Rise dangerously

Explanation: Loss of flux causes very high speed.

Question 89. If a DC shunt motor field opens at full load, armature current may:

  1. Increase heavily
  2. Become zero safely
  3. Remain exactly constant
  4. Become AC

Answer: A. Increase heavily

Explanation: Reduced back EMF can cause large armature current.

Question 90. A motor for intermittent heavy loads with flywheel is often:

  1. Cumulative compound motor
  2. Differential compound motor only
  3. Tiny shunt motor
  4. No motor

Answer: A. Cumulative compound motor

Explanation: Flywheel and compound motor help manage load peaks.

Question 91. For rotary compressors, the commonly preferred motor type is:

  1. Synchronous motor
  2. DC series motor only
  3. Tiny DC shunt motor
  4. Universal motor only

Answer: A. Synchronous motor

Explanation: Rotary compressors often use constant-speed AC/synchronous drives.

Question 92. For hazardous explosive atmosphere, preferred drive may be:

  1. Air motor
  2. Open DC motor
  3. Sparking commutator motor
  4. Unprotected series motor

Answer: A. Air motor

Explanation: Air motors avoid electrical sparking.

Question 93. The brush voltage drop is usually lowest in:

  1. Metal graphite brushes
  2. Plain carbon brushes
  3. Wooden brushes
  4. Mica strips

Answer: A. Metal graphite brushes

Explanation: Metal-graphite brushes have lower contact voltage drop.

Question 94. High mica between commutator segments can cause:

  1. Brush wear and sparking
  2. Perfect commutation
  3. Zero friction
  4. No current

Answer: A. Brush wear and sparking

Explanation: High mica prevents smooth brush contact.

Question 95. Undercutting mica in commutator is done to:

  1. Allow brushes to contact copper properly
  2. Increase insulation above copper
  3. Stop all current
  4. Increase eddy current

Answer: A. Allow brushes to contact copper properly

Explanation: Mica is undercut so brushes ride on copper segments.

Question 96. An open-circuited armature coil may be indicated by:

  1. Sparking/scarring near connected commutator segments
  2. Zero brush wear always
  3. No voltage ever
  4. Perfect quiet running

Answer: A. Sparking/scarring near connected commutator segments

Explanation: Open coil disturbs commutation and causes local sparking.

Question 97. Short-circuit in armature winding may occur due to:

  1. Insulation failure between turns or commutator bars
  2. Good insulation only
  3. Low load only
  4. Correct ventilation

Answer: A. Insulation failure between turns or commutator bars

Explanation: Insulation failure can short turns or commutator segments.

Question 98. Field control method is more efficient than armature resistance control because:

  1. Less power is wasted in series resistance
  2. It removes the motor
  3. It stops rotation
  4. It needs no supply

Answer: A. Less power is wasted in series resistance

Explanation: Field current is small, so field control losses are lower.

Question 99. For speed below base speed without large wastage, preferred method is:

  1. Armature voltage control
  2. Field weakening only
  3. Opening field
  4. Increasing brush pressure

Answer: A. Armature voltage control

Explanation: Reducing armature voltage gives efficient below-base speed control.

Question 100. For speed above base speed, preferred method is:

  1. Field weakening
  2. Adding armature resistance only
  3. Increasing load only
  4. Reducing voltage to zero

Answer: A. Field weakening

Explanation: Weakening flux raises speed beyond base speed.

Question 101. Retardation test on DC machines is used to find:

  1. Stray losses
  2. Only copper loss
  3. Only output torque
  4. Only field resistance

Answer: A. Stray losses

Explanation: Retardation test estimates rotational/stray losses.

Question 102. Swinburne’s test is mainly suitable for:

  1. DC shunt machines
  2. DC series motors only
  3. Transformers only
  4. Induction motors only

Answer: A. DC shunt machines

Explanation: It is a no-load test commonly used for DC shunt machines.

Question 103. Hopkinson’s test is also called:

  1. Regenerative test
  2. Blocked rotor test
  3. Open circuit test only
  4. Short circuit test of transformer

Answer: A. Regenerative test

Explanation: Two DC machines are tested together with power circulation.

Question 104. Hopkinson’s test is economical because:

  1. Only losses are supplied from mains
  2. Full input power is wasted
  3. No machine is loaded
  4. It uses no electricity

Answer: A. Only losses are supplied from mains

Explanation: Most power circulates between two machines; supply covers losses.

Question 105. Field test is suitable for:

  1. Two similar DC series machines
  2. Single transformer
  3. Only alternator
  4. No-load shunt motor only

Answer: A. Two similar DC series machines

Explanation: Field test can load two series machines efficiently.

Question 106. A DC motor drive is preferred over AC motor where:

  1. Wide and smooth speed control is required
  2. Only lowest cost is required
  3. No speed control is needed
  4. No starting torque is needed

Answer: A. Wide and smooth speed control is required

Explanation: DC motors are traditionally used for smooth variable speed control.

Question 107. If terminal voltage of a DC motor is increased, no-load speed generally:

  1. Increases
  2. Decreases
  3. Stays zero
  4. Becomes negative

Answer: A. Increases

Explanation: Higher voltage increases back EMF and speed for same flux.

Question 108. The armature torque of a DC shunt motor is proportional to:

  1. Armature current
  2. Square of armature current always
  3. Field resistance only
  4. Brush length only

Answer: A. Armature current

Explanation: Flux is nearly constant, so T ∝ Ia.

Advanced Level DC Motor MCQs

These questions are arranged from simple concepts to exam-oriented application questions.

Question 109. A 220 V DC motor has armature resistance 0.2 Ω and takes 40 A. Back EMF is:

  1. 212 V
  2. 220 V
  3. 228 V
  4. 8 V

Answer: A. 212 V

Explanation: Eb=V−IaRa=220−40×0.2=212 V.

Question 110. A 250 V DC motor takes 60 A and has Ra=0.25 Ω. Back EMF is:

  1. 235 V
  2. 250 V
  3. 265 V
  4. 15 V

Answer: A. 235 V

Explanation: Eb=250−60×0.25=235 V.

Question 111. A DC motor has Eb=240 V and Ia=50 A. Gross mechanical power developed is:

  1. 12 kW
  2. 4.8 kW
  3. 290 W
  4. 24 kW

Answer: A. 12 kW

Explanation: Power developed EbIa=240×50=12000 W.

Question 112. A 200 V shunt motor takes 25 A. Armature resistance is 0.4 Ω and shunt current is 1 A. Back EMF is:

  1. 190.4 V
  2. 200 V
  3. 210 V
  4. 180 V

Answer: A. 190.4 V

Explanation: Ia=25−1=24 A, Eb=200−24×0.4=190.4 V.

Question 113. A 4-pole lap-wound DC motor has number of parallel paths equal to:

  1. 2
  2. 4
  3. 6
  4. 8

Answer: B. 4

Explanation: For simplex lap winding, A=P, so A=4.

Question 114. A simplex wave-wound DC motor has number of parallel paths equal to:

  1. 2
  2. Number of poles
  3. 4
  4. 8

Answer: A. 2

Explanation: Simplex wave winding has two parallel paths.

Question 115. If a 6-pole lap-wound armature has 600 conductors and carries 120 A armature current, current per path is:

  1. 10 A
  2. 20 A
  3. 60 A
  4. 120 A

Answer: B. 20 A

Explanation: A=P=6, so current per path=120/6=20 A.

Question 116. For a wave-wound armature carrying 80 A, current per parallel path is:

  1. 20 A
  2. 40 A
  3. 80 A
  4. 160 A

Answer: B. 40 A

Explanation: Simplex wave winding has two paths; 80/2=40 A.

Question 117. Generated/back EMF of a DC machine is proportional to:

  1. ΦZN/A
  2. Only Ra
  3. Only brush pressure
  4. Only frame size

Answer: A. ΦZN/A

Explanation: E = PΦZN/(60A), so it depends on flux, conductors, speed and paths.

Question 118. If flux is reduced by 20% and back EMF remains nearly same, motor speed becomes approximately:

  1. 1.25 times
  2. 0.8 times
  3. 0.64 times
  4. Same

Answer: A. 1.25 times

Explanation: Speed is inversely proportional to flux; 1/0.8=1.25.

Question 119. If armature voltage is reduced to 50% with constant flux and same load torque, speed approximately:

  1. Reduces nearly to half
  2. Doubles
  3. Becomes infinite
  4. Unaffected

Answer: A. Reduces nearly to half

Explanation: In armature voltage control, speed is roughly proportional to voltage.

Question 120. For maximum mechanical power in a DC motor, the theoretical condition is:

  1. Eb=V/2
  2. Eb=V
  3. Eb=2V
  4. Eb=0

Answer: A. Eb=V/2

Explanation: Mechanical power EbIa is maximum theoretically when Eb is half supply voltage.

Question 121. At maximum mechanical power condition, theoretical efficiency is about:

  1. 50%
  2. 90%
  3. 100%
  4. 10%

Answer: A. 50%

Explanation: At Eb=V/2, efficiency based on conversion is about 50%, not practical.

Question 122. Why is maximum power condition not used in normal DC motor operation?

  1. Efficiency is poor and current is high
  2. Speed becomes exactly zero
  3. Torque is always zero
  4. No current flows

Answer: A. Efficiency is poor and current is high

Explanation: The condition causes large current and poor efficiency.

Question 123. A DC motor has constant losses of 500 W. Maximum efficiency occurs when variable losses are:

  1. 500 W
  2. 250 W
  3. 1000 W
  4. Zero

Answer: A. 500 W

Explanation: For maximum efficiency, variable losses equal constant losses.

Question 124. If armature copper loss is 800 W at full load, it becomes at half-load current:

  1. 200 W
  2. 400 W
  3. 800 W
  4. 1600 W

Answer: A. 200 W

Explanation: Copper loss varies with current squared; (1/2)²×800=200 W.

Question 125. If armature current doubles, armature copper loss becomes:

  1. Double
  2. Four times
  3. Half
  4. Unchanged

Answer: B. Four times

Explanation: I²R loss varies with square of current.

Question 126. In a DC motor, iron losses are mainly located in:

  1. Armature core
  2. Yoke only
  3. Brush holder only
  4. Shaft only

Answer: A. Armature core

Explanation: Armature core experiences magnetic reversals during rotation.

Question 127. Hysteresis loss can be reduced by using:

  1. Low hysteresis coefficient steel
  2. Thick copper frame
  3. High brush pressure
  4. Large air gap only

Answer: A. Low hysteresis coefficient steel

Explanation: Good magnetic steel reduces hysteresis loss.

Question 128. Eddy current loss can be reduced by:

  1. Laminating the core
  2. Using solid core
  3. Increasing conductor thickness
  4. Removing insulation

Answer: A. Laminating the core

Explanation: Laminations restrict circulating eddy currents.

Question 129. If a DC series motor drives a belt load and belt breaks, the motor may:

  1. Overspeed dangerously
  2. Stop safely immediately
  3. Run at synchronous speed
  4. Become generator at zero speed

Answer: A. Overspeed dangerously

Explanation: Sudden loss of load reduces current and flux, causing dangerous speed rise.

Question 130. A DC shunt motor is accidentally connected with reversed supply polarity. Direction of rotation will:

  1. Remain same
  2. Reverse always
  3. Become zero
  4. Depend only on load

Answer: A. Remain same

Explanation: Both armature and field current reverse, so torque direction remains same.

Question 131. To reverse a DC shunt motor safely, reverse:

  1. Either armature or field, not both
  2. Both armature and field together
  3. Only supply fuse
  4. Only bearing

Answer: A. Either armature or field, not both

Explanation: Reversing either one reverses torque direction.

Question 132. In a differentially compounded motor, increasing load can cause speed to:

  1. Increase dangerously
  2. Remain perfectly constant
  3. Become zero immediately
  4. Always decrease safely

Answer: A. Increase dangerously

Explanation: Series field weakens net flux as load increases, so speed may rise.

Question 133. The main reason DC motors need maintenance is:

  1. Brushes and commutator wear
  2. No rotating parts
  3. No copper winding
  4. No magnetic field

Answer: A. Brushes and commutator wear

Explanation: Sliding contact requires inspection and maintenance.

Question 134. A DC motor with poor commutation may have:

  1. Excessive sparking and heating
  2. Perfect waveform
  3. Zero brush current
  4. No torque ripple

Answer: A. Excessive sparking and heating

Explanation: Poor commutation leads to sparks and local heating.

Question 135. Compensating winding is placed in:

  1. Pole faces
  2. Shaft center
  3. Brush handle only
  4. Bearing housing

Answer: A. Pole faces

Explanation: It is embedded in pole faces to oppose armature reaction.

Question 136. Interpoles are placed:

  1. Between main poles
  2. Inside shaft
  3. Outside frame only
  4. Across bearings

Answer: A. Between main poles

Explanation: Interpoles lie between main poles to improve commutation.

Question 137. The polarity of interpoles in a DC motor is generally:

  1. Same as main pole behind it
  2. Always north
  3. Always south
  4. No polarity

Answer: A. Same as main pole behind it

Explanation: For motors, interpole polarity follows the main pole behind, aiding commutation.

Question 138. The most economical braking when energy can be returned to supply is:

  1. Regenerative braking
  2. Plugging
  3. Mechanical braking only
  4. Friction braking only

Answer: A. Regenerative braking

Explanation: Regenerative braking recovers energy.

Question 139. Why is plugging less efficient?

  1. Energy from supply and rotor is dissipated as heat
  2. It returns all energy
  3. It needs no current
  4. It has zero braking torque

Answer: A. Energy from supply and rotor is dissipated as heat

Explanation: Plugging wastes energy in resistance and machine losses.

Question 140. A DC shunt motor running at 1200 rpm has Eb=240 V. If flux is unchanged and Eb becomes 220 V, speed is:

  1. 1100 rpm
  2. 1200 rpm
  3. 1309 rpm
  4. 2400 rpm

Answer: A. 1100 rpm

Explanation: N ∝ Eb, so N=1200×220/240=1100 rpm.

Question 141. A DC shunt motor speed is 1000 rpm at flux Φ. If flux becomes 0.8Φ with same Eb, speed is:

  1. 1250 rpm
  2. 800 rpm
  3. 1000 rpm
  4. 640 rpm

Answer: A. 1250 rpm

Explanation: N ∝ 1/Φ; 1000/0.8=1250 rpm.

Question 142. A DC motor takes 50 A from 250 V supply. If back EMF is 230 V, armature copper loss is:

  1. 1 kW
  2. 11.5 kW
  3. 12.5 kW
  4. 230 W

Answer: A. 1 kW

Explanation: IaRa drop = V−Eb =20 V; copper loss = Ia×drop=50×20=1000 W.

Question 143. The mechanical power converted in a DC motor is:

  1. EbIa
  2. VIa only
  3. Ia²Ra only
  4. V/Rsh only

Answer: A. EbIa

Explanation: Converted mechanical power before mechanical losses is Eb times armature current.

Question 144. Shaft output power equals:

  1. Mechanical power developed minus rotational losses
  2. Electrical input plus losses
  3. Copper loss only
  4. Back EMF only

Answer: A. Mechanical power developed minus rotational losses

Explanation: Rotational and stray losses are subtracted from developed power.

Question 145. If Eb is high in a running DC motor, armature current is:

  1. Reduced for given supply
  2. Increased infinitely
  3. Always zero
  4. Independent of Eb

Answer: A. Reduced for given supply

Explanation: Ia=(V−Eb)/Ra; larger Eb reduces current.

Question 146. Why is starting current high in DC motor?

  1. Eb is zero and Ra is small
  2. Flux is zero always
  3. Load torque is zero
  4. Voltage is absent

Answer: A. Eb is zero and Ra is small

Explanation: Small armature resistance with zero Eb causes large current.

Question 147. For safe DC motor starting, starter resistance is gradually:

  1. Cut out as speed and back EMF rise
  2. Increased to infinity
  3. Kept permanently full
  4. Connected across field only

Answer: A. Cut out as speed and back EMF rise

Explanation: As Eb builds up, less external resistance is needed.

Question 148. If no-volt coil is connected in series with shunt field, heavy field weakening may:

  1. Release starter handle unintentionally
  2. Increase torque perfectly
  3. Make speed zero safely
  4. Stop all losses

Answer: A. Release starter handle unintentionally

Explanation: In three-point starter, field weakening may reduce no-volt coil current.

Question 149. A four-point starter solves this problem by connecting no-volt coil:

  1. Directly across supply through protective resistance
  2. In series with field only
  3. In series with armature only
  4. Across brushes only

Answer: A. Directly across supply through protective resistance

Explanation: No-volt coil is independent of field control current.

Question 150. In DC motor drives, closed-loop speed control uses feedback to:

  1. Maintain desired speed under load changes
  2. Increase losses only
  3. Remove starter always
  4. Avoid sensors always

Answer: A. Maintain desired speed under load changes

Explanation: Feedback compares actual speed with reference and corrects voltage/current.

Question 151. Modern DC motor speed control commonly uses:

  1. Power electronic chopper drive
  2. Only manual rheostat
  3. Only mechanical pulley
  4. Only water cooling

Answer: A. Power electronic chopper drive

Explanation: Choppers efficiently vary average armature voltage.

Question 152. In a DC chopper drive, average output voltage is controlled by:

  1. Duty ratio
  2. Brush grade only
  3. Bearing size
  4. Ambient humidity

Answer: A. Duty ratio

Explanation: Changing duty cycle changes average DC voltage.

Question 153. If chopper duty ratio increases, armature average voltage generally:

  1. Increases
  2. Decreases
  3. Becomes zero
  4. Unaffected

Answer: A. Increases

Explanation: Average output voltage is proportional to duty ratio in buck chopper.

Question 154. For EV and traction drives today, DC motor concepts are still useful because:

  1. Torque-speed, braking and drive-control ideas remain important
  2. DC machines have no theory
  3. Only brushes matter
  4. No power electronics is used

Answer: A. Torque-speed, braking and drive-control ideas remain important

Explanation: Modern drives still use similar torque, speed and braking principles.

Question 155. Which fault can cause excessive brush heating?

  1. Poor contact or overload current
  2. Perfect commutation
  3. Low current always
  4. Clean commutator only

Answer: A. Poor contact or overload current

Explanation: Poor contact and high current increase heat at brushes.

Question 156. Which condition improves commutation?

  1. Correct brush position and interpoles
  2. Rough commutator
  3. High mica
  4. Loose brush spring

Answer: A. Correct brush position and interpoles

Explanation: Good mechanical and magnetic conditions reduce sparking.

Question 157. Why are DC motors less common in many modern low-maintenance drives?

  1. Brush and commutator maintenance
  2. No speed control possible
  3. No torque available
  4. No starter possible

Answer: A. Brush and commutator maintenance

Explanation: Brushless AC/BLDC drives reduce maintenance.

Question 158. The main reason DC motors were historically popular in variable-speed drives is:

  1. Simple and smooth speed control
  2. Lowest maintenance always
  3. No losses
  4. No controller needed

Answer: A. Simple and smooth speed control

Explanation: DC motor speed is conveniently controlled by voltage and field.

Quick Answer Key

Question No.Correct Answer
1B. Mechanical energy
2A. Electromagnetic force on a current-carrying conductor
3A. Fleming's left-hand rule
4B. Magnetic field
5B. Current
6C. Force or motion
7B. Armature
8B. Yoke
9B. Collect/supply current through commutator
10B. Carbon/graphite
11A. Reverse current in armature conductors at proper instant
12B. Back EMF is zero at starting
13B. Zero
14B. Starting current
15A. Constant speed
16C. No load
17A. DC series motor
18B. Shunt motor
19A. Series motor
20A. Cumulative compound motor
21B. Reducing field current
22A. Flux, armature voltage or armature resistance
23B. Opposite to applied voltage
24A. Ia=(V-Eb)/Ra
25A. Flux × armature current
26B. Constant
27B. Ia²
28B. Dangerously high
29A. DC shunt and compound motors
30A. Field control speed variation is required
31B. Supply failure and automatic restart
32A. Excessive current
33A. Eddy current loss
34A. Many turns of thin wire
35B. Few turns of thick wire
36B. Overheat due to excessive losses
37A. Shaft output power
38A. Spread flux and support field coils
39A. Can damage commutator and brushes
40A. Constant speed drives
41A. High starting torque is needed
42A. Either armature or field connections
43A. Direction remains same
44A. Armature copper losses
45A. Friction and windage
46A. Hysteresis and eddy current losses
47A. Variable losses equal constant losses
48A. Single-phase induction motor
49A. Commutator
50A. Speed increases
51B. 40 A
52B. 50 A
53C. 30 N-m
54B. 5.26%
55B. Four times
56B. One-fourth
57A. Constant torque operation
58A. Constant power region
59A. Extra power is wasted in series resistance
60A. Variable voltage control
61A. High cost and maintenance
62A. Motor acts as generator and energy is dissipated in resistor
63A. Back EMF exceeds supply voltage
64A. Reversing armature supply polarity while running
65A. High braking torque
66A. A braking resistor
67A. Reversing current in short-circuited armature coil
68A. In series with armature
69A. Armature reaction
70A. Flux distortion
71A. Commutation
72A. Sparking
73A. T ∝ ΦIa
74A. N ∝ (V−IaRa)/Φ
75A. Increases
76A. Decreases
77A. Straight line
78A. Parabolic
79A. Armature current
80A. Highly drooping
81A. Nearly constant speed
82A. Series field aiding shunt field
83A. Series field opposing shunt field
84A. Unstable
85A. High starting torque and fairly constant speed are needed
86A. High starting torque and self-relieving nature
87A. DC series motor
88A. Rise dangerously
89A. Increase heavily
90A. Cumulative compound motor
91A. Synchronous motor
92A. Air motor
93A. Metal graphite brushes
94A. Brush wear and sparking
95A. Allow brushes to contact copper properly
96A. Sparking/scarring near connected commutator segments
97A. Insulation failure between turns or commutator bars
98A. Less power is wasted in series resistance
99A. Armature voltage control
100A. Field weakening
101A. Stray losses
102A. DC shunt machines
103A. Regenerative test
104A. Only losses are supplied from mains
105A. Two similar DC series machines
106A. Wide and smooth speed control is required
107A. Increases
108A. Armature current
109A. 212 V
110A. 235 V
111A. 12 kW
112A. 190.4 V
113B. 4
114A. 2
115B. 20 A
116B. 40 A
117A. ΦZN/A
118A. 1.25 times
119A. Reduces nearly to half
120A. Eb=V/2
121A. 50%
122A. Efficiency is poor and current is high
123A. 500 W
124A. 200 W
125B. Four times
126A. Armature core
127A. Low hysteresis coefficient steel
128A. Laminating the core
129A. Overspeed dangerously
130A. Remain same
131A. Either armature or field, not both
132A. Increase dangerously
133A. Brushes and commutator wear
134A. Excessive sparking and heating
135A. Pole faces
136A. Between main poles
137A. Same as main pole behind it
138A. Regenerative braking
139A. Energy from supply and rotor is dissipated as heat
140A. 1100 rpm
141A. 1250 rpm
142A. 1 kW
143A. EbIa
144A. Mechanical power developed minus rotational losses
145A. Reduced for given supply
146A. Eb is zero and Ra is small
147A. Cut out as speed and back EMF rise
148A. Release starter handle unintentionally
149A. Directly across supply through protective resistance
150A. Maintain desired speed under load changes
151A. Power electronic chopper drive
152A. Duty ratio
153A. Increases
154A. Torque-speed, braking and drive-control ideas remain important
155A. Poor contact or overload current
156A. Correct brush position and interpoles
157A. Brush and commutator maintenance
158A. Simple and smooth speed control

Important Notes on DC Motors

  • A DC motor converts DC electrical energy into mechanical rotational energy.
  • Back EMF opposes supply voltage and limits armature current during running.
  • DC series motors provide very high starting torque but should not be started without load.
  • DC shunt motors have nearly constant speed and are suitable for machine tools.
  • Starters are required because armature current is very high at starting.
  • Speed can be controlled by armature voltage control, field control and armature resistance control.

Frequently Asked Questions on DC Motor MCQs

Which DC motor has the highest starting torque?

A DC series motor has the highest starting torque because its torque is approximately proportional to the square of armature current before saturation.

Why is a starter required for a DC motor?

At starting, back EMF is zero and armature resistance is very small. A starter limits the starting current to a safe value.

Which rule is used for DC motor direction?

Fleming’s left-hand rule is used to find the direction of force or rotation in a DC motor.

Why should a DC series motor not be started on no load?

At no load, the current and flux are very small. This can make the speed rise dangerously high and damage the motor.

Which DC motor is used for traction?

DC series motors are widely used in traction because they provide high starting torque and suitable speed-torque characteristics.

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Conclusion

This DC Motor MCQ question series is useful for quick revision and exam practice. The questions start from basic concepts and move toward numerical, application-based and advanced topics. Keep practicing these objective questions to improve your understanding of DC machines and electrical engineering fundamentals.

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