ELECTRICAL MACHINES 1
UNIT 1 PART A ( 1 MARKS )
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# **UNIT I – DC GENERATORS (MCQs with Answers)**
### **Basics of Electromagnetism (Q1–10)**
1. The direction of induced EMF in a conductor is given by:
A) Fleming’s left-hand rule
B) Fleming’s right-hand rule
C) Lenz’s law only
D) Ampere’s law
**Answer: B**
2. Magnetic flux is measured in:
A) Tesla
B) Gauss
C) Weber
D) Henry
**Answer: C**
3. If the flux through a coil of 50 turns changes by 0.02 Wb in 0.01 s, the induced EMF is:
A) 50 V
B) 100 V
C) 200 V
D) 20 V
**Answer: B**
4. A conductor of length 0.5 m moves at 10 m/s perpendicular to a flux density of 0.8 T. The induced EMF is:
A) 2 V
B) 4 V
C) 6 V
D) 8 V
**Answer: B**
5. The main cause of self-induced EMF in a coil is:
A) Change in external flux
B) Change in current in the coil itself
C) Static magnetic field
D) Eddy current
**Answer: B**
6. The unit of magnetic field intensity (H) is:
A) Tesla
B) A/m
C) Weber
D) Henry
**Answer: B**
7. When a magnetic circuit has an air gap, the reluctance:
A) Increases
B) Decreases
C) Remains same
D) Becomes zero
**Answer: A**
8. If flux density is doubled and area remains constant, flux:
A) Becomes half
B) Remains same
C) Doubles
D) Quadruples
**Answer: C**
9. The property of a material which opposes the creation of magnetic flux is:
A) Conductivity
B) Reluctance
C) Permeability
D) Susceptibility
**Answer: B**
10. Which material has the highest permeability?
A) Copper
B) Aluminum
C) Soft iron
D) Brass
**Answer: C**
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### **Overview & Importance of DC Generators (Q11–15)**
11. The main function of a DC generator is to convert:
A) Electrical to mechanical energy
B) AC to DC
C) Mechanical to electrical energy
D) Heat to electrical energy
**Answer: C**
12. DC generators are rarely used in large power plants because:
A) They are expensive
B) They have high losses
C) AC transmission is more efficient
D) They cannot generate high voltage
**Answer: C**
13. Which of the following industries commonly use DC generators?
A) Electroplating
B) Textile
C) Steel
D) All of the above
**Answer: D**
14. The main advantage of DC generators in industries is:
A) Low maintenance
B) Variable speed control
C) Compact size
D) Low voltage
**Answer: B**
15. In power stations, DC generators are replaced by:
A) AC generators + rectifiers
B) Batteries only
C) Transformers
D) Inverters
**Answer: A**
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### **Types of DC Generators (Q16–25)**
16. Which type of generator has field winding connected in parallel with the armature?
A) Shunt generator
B) Series generator
C) Compound generator
D) Separately excited generator
**Answer: A**
17. In a series generator, the load current is equal to:
A) Field current
B) Armature current
C) Both A and B
D) None of these
**Answer: C**
18. A generator that gives nearly constant voltage under varying load is:
A) Shunt generator
B) Series generator
C) Short-shunt compound generator
D) Over-compound generator
**Answer: D**
19. The field of a separately excited DC generator is powered by:
A) Armature
B) External DC source
C) Permanent magnets
D) Capacitor
**Answer: B**
20. Which generator is best suited for arc welding?
A) Shunt generator
B) Series generator
C) Flat-compound generator
D) Differential-compound generator
**Answer: B**
21. Which generator is used where load is variable but constant voltage is required?
A) Shunt generator
B) Cumulative compound generator
C) Series generator
D) Differential compound generator
**Answer: B**
22. A differential compound generator has:
A) Series aiding shunt field
B) Series opposing shunt field
C) Both fields in same direction
D) No series winding
**Answer: B**
23. The no-load voltage of a compound generator is 250 V. On load, the terminal voltage rises to 270 V. The generator is:
A) Flat-compound
B) Over-compound
C) Under-compound
D) Differential compound
**Answer: B**
24. Which generator is not self-excited?
A) Shunt
B) Series
C) Compound
D) Separately excited
**Answer: D**
25. The type of DC generator used in electroplating is:
A) Shunt
B) Series
C) Compound
D) Any of the above
**Answer: A**
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### **EMF Equation & Problems (Q26–32)**
26. The EMF equation of a DC generator is:
A) $E = \frac{60APN}{Z}$
B) $E = \frac{PΦZN}{60A}$
C) $E = \frac{ΦZ}{N}$
D) $E = \frac{PN}{Φ}$
**Answer: B**
27. In the EMF equation, ‘Z’ stands for:
A) Number of poles
B) Number of armature conductors
C) Flux per pole
D) Speed of rotation
**Answer: B**
28. A 4-pole, wave-wound DC generator has 400 conductors, flux per pole 0.02 Wb, speed 1500 rpm. Induced EMF is:
A) 400 V
B) 600 V
C) 800 V
D) 1000 V
**Answer: C**
29. If flux per pole is doubled, EMF will:
A) Halve
B) Double
C) Remain same
D) Reduce to zero
**Answer: B**
30. A generator has 2 parallel paths in lap winding with 8 poles. The number of parallel paths (A) is:
A) 2
B) 4
C) 6
D) 8
**Answer: D**
31. In wave winding, the number of parallel paths is:
A) Equal to number of poles
B) 2
C) Equal to number of slots
D) 4
**Answer: B**
32. If the speed of a DC generator is reduced to half, the induced EMF will:
A) Double
B) Remain same
C) Halve
D) Zero
**Answer: C**
### **Internal & External Characteristics (Q33–38)**
33. The internal characteristic of a DC generator shows relation between:
A) Induced EMF and load current
B) Terminal voltage and load current
C) Flux and field current
D) Speed and EMF
**Answer: A**
34. The external characteristic shows relation between:
A) Induced EMF and load current
B) Terminal voltage and load current
C) Flux and field current
D) Armature resistance and load
**Answer: B**
35. In a shunt generator, the external characteristic drops mainly due to:
A) Decrease in flux because of armature reaction
B) Armature resistance drop
C) Brush contact drop
D) All of the above
**Answer: D**
36. The external characteristic of a series generator has:
A) Rising curve
B) Falling curve
C) Flat curve
D) Zero slope
**Answer: A**
37. For a cumulatively compounded generator, the external characteristic lies:
A) Below that of shunt
B) Above that of shunt
C) Same as series
D) Same as differential compound
**Answer: B**
38. In a differential compound generator, terminal voltage:
A) Increases with load
B) Decreases rapidly with load
C) Remains constant
D) First increases then decreases
**Answer: B**
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### **Applications (Q39–42)**
39. Which generator is best suited for electroplating and battery charging?
A) Shunt
B) Series
C) Compound
D) Differential
**Answer: A**
40. DC series generators are used in:
A) Lighting load
B) Arc welding
C) Electroplating
D) Battery charging
**Answer: B**
41. Over-compounded generators are used in:
A) Railways for traction
B) Power supply to DC distribution lines
C) Electroplating
D) Welding
**Answer: B**
42. Flat-compounded generators are used where:
A) Constant voltage is required irrespective of load
B) Rising voltage with load is required
C) Dropping voltage is required
D) No external load is connected
**Answer: A**
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### **Process of Building up of EMF (Q43–46)**
43. The initial EMF in a self-excited generator is due to:
A) Field winding current
B) Armature reaction
C) Residual magnetism
D) External source
**Answer: C**
44. The process of voltage buildup stops when:
A) Field circuit resistance line intersects the magnetization curve
B) Armature resistance increases
C) Residual magnetism is lost
D) Speed becomes zero
**Answer: A**
45. If residual magnetism is lost, the generator fails to:
A) Supply load
B) Build up EMF
C) Rotate
D) Supply current
**Answer: B**
46. To restore residual magnetism in a generator, we can:
A) Increase load
B) Reduce speed
C) Field flashing with external DC
D) Reverse rotation
**Answer: C**
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### **Critical Field Resistance (Q47–49)**
47. Critical field resistance of a shunt generator is the resistance:
A) At which generator just excites at given speed
B) At which generator fails to supply load
C) At which efficiency is maximum
D) At which armature reaction is minimum
**Answer: A**
48. If field resistance > critical resistance, the generator will:
A) Build up normally
B) Not build up
C) Build up faster
D) Generate higher voltage
**Answer: B**
49. Critical resistance is determined from:
A) Straight line slope of field resistance line
B) External characteristic
C) Internal characteristic
D) Load test
**Answer: A**
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### **Causes of Failure to Build Up Voltage & Remedies (Q50–52)**
50. A shunt generator fails to build up if:
A) Wrong field winding connection
B) High field resistance
C) Loss of residual magnetism
D) All of the above
**Answer: D**
51. Remedy for wrong field winding connection is:
A) Reverse field connections
B) Reduce speed
C) Increase resistance
D) Increase load
**Answer: A**
52. If generator does not build up because of no residual magnetism, remedy is:
A) Provide higher load
B) Field flashing with battery
C) Increase armature resistance
D) Decrease speed
**Answer: B**
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### **Armature Reaction (Q53–55)**
53. Armature reaction in DC generator mainly causes:
A) Flux strengthening
B) Flux distortion and weakening
C) Voltage increase
D) Voltage stabilization
**Answer: B**
54. Armature reaction effect is more pronounced at:
A) No load
B) Light load
C) Full load
D) Open circuit
**Answer: C**
55. The axis of armature reaction MMF is:
A) Along field axis
B) Perpendicular to field axis
C) At 45° to field axis
D) Opposite to field axis
**Answer: B**
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### **Commutation & Improvement Methods (Q56–60)**
56. The process of reversal of current in the coil during short-circuit is called:
A) Reactance
B) Commutation
C) Compensation
D) Armature reaction
**Answer: B**
57. Poor commutation results in:
A) Sparking at brushes
B) High efficiency
C) Reduced eddy currents
D) Reduced flux
**Answer: A**
58. The main cause of sparking at brushes is:
A) Inductance of coil
B) High flux
C) Low speed
D) Mechanical vibration
**Answer: A**
59. One method of improving commutation is:
A) Using interpoles
B) Increasing armature resistance
C) Decreasing speed
D) Reducing load
**Answer: A**
60. Interpoles are connected in:
A) Series with field winding
B) Parallel with field winding
C) Series with armature
D) Parallel with armature
**Answer: C**
UNIT 1 PART B ( 2 MARKS )
# **UNIT I – DC GENERATORS (Higher Order MCQs: Q61–100)**
### **Basics of Electromagnetism (61–65)**
61. If the flux in a generator reduces by 10% but speed remains constant, the generated EMF will:
A) Increase by 10%
B) Decrease by 10%
C) Increase by 20%
D) Remain unchanged
**Answer: B**
62. A student mistakenly doubles the length of a conductor in a magnetic field but keeps velocity the same. The induced EMF will:
A) Halve
B) Double
C) Remain constant
D) Be zero
**Answer: B**
63. A coil rotates in a magnetic field. If its angular speed is doubled, the induced EMF will:
A) Halve
B) Double
C) Quadruple
D) Remain unchanged
**Answer: B**
64. If a generator with weak residual magnetism is reversed in rotation, the voltage build-up will:
A) Improve
B) Fail
C) Remain same
D) Become negative
**Answer: B**
65. You want to increase EMF of a generator without changing speed or conductors. Best way is to:
A) Increase flux per pole
B) Reduce field resistance
C) Increase armature resistance
D) Increase brush contact resistance
**Answer: A**
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### **Overview & Importance (66–69)**
66. An industry requires smooth DC supply for electroplating. Which type of DC generator is best?
A) Shunt
B) Series
C) Over-compound
D) Differential compound
**Answer: A**
67. If a DC generator is replaced by an AC alternator + rectifier, one advantage is:
A) Higher efficiency in transmission
B) Lower losses in commutator
C) Easier high voltage generation
D) All of the above
**Answer: D**
68. A railway system needs rising voltage with load. The suitable generator is:
A) Shunt
B) Differential compound
C) Over-compound
D) Series
**Answer: C**
69. A DC generator is rarely used in modern power stations because:
A) Speed cannot be varied
B) Commutator problems at high voltage
C) DC cannot be stepped up efficiently
D) Both B and C
**Answer: D**
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### **Types of DC Generators (70–77)**
70. A cumulative compound generator is used instead of a shunt generator because:
A) It reduces voltage drop under load
B) It is cheaper
C) It is simpler
D) It needs no residual magnetism
**Answer: A**
71. Which generator type would you select for supplying long-distance DC distribution lines?
A) Shunt
B) Differential compound
C) Over-compound
D) Series
**Answer: C**
72. A differential compound generator is generally not used in practice because:
A) It is unstable under load
B) It cannot generate EMF
C) It has high copper loss
D) It needs large commutator
**Answer: A**
73. If the series field of a compound generator is reversed, the generator becomes:
A) Over-compounded
B) Differential compounded
C) Shunt type
D) Separately excited
**Answer: B**
74. Which generator is best for DC welding (current increases with load)?
A) Shunt
B) Series
C) Compound
D) Over-compound
**Answer: B**
75. A student connects the shunt field winding of a generator in series with armature by mistake. The machine behaves as:
A) Series generator
B) Motor
C) Short circuit
D) Transformer
**Answer: A**
76. For constant load voltage with fluctuating current demand, you recommend:
A) Series generator
B) Shunt generator
C) Flat-compound generator
D) Differential compound generator
**Answer: C**
77. If a generator must run in parallel with another shunt generator, it is preferable to use:
A) Over-compound generator
B) Flat-compound generator
C) Series generator
D) Differential compound generator
**Answer: B**
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### **EMF Equation & Problems (78–83)**
78. A 6-pole, wave-wound generator runs at 1000 rpm with 720 conductors. Flux per pole is 0.02 Wb. Induced EMF is:
A) 144 V
B) 288 V
C) 432 V
D) 576 V
**Answer: D**
79. To double the EMF without changing speed, you should:
A) Double conductors or double flux
B) Halve armature resistance
C) Reverse polarity
D) Reduce series field turns
**Answer: A**
80. If number of poles is doubled but speed halved, EMF of wave-wound generator will:
A) Double
B) Halve
C) Remain same
D) Reduce to zero
**Answer: C**
81. An 8-pole lap-wound generator has 960 conductors and 40 A armature current. Current per conductor is:
A) 2 A
B) 4 A
C) 5 A
D) 10 A
**Answer: C**
82. A lap-wound generator has more parallel paths than wave-wound. This results in:
A) Lower current capacity
B) Higher current capacity
C) Lower EMF
D) Both B and C
**Answer: D**
83. For a given generator, doubling the speed doubles EMF. This shows EMF is directly proportional to:
A) Flux × speed
B) Speed only
C) Flux only
D) Armature resistance
**Answer: A**
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### **Characteristics & Applications (84–88)**
84. In the internal characteristic, actual induced EMF is lower than theoretical due to:
A) Armature reaction
B) Hysteresis
C) Eddy currents
D) Brush friction
**Answer: A**
85. A shunt generator loaded beyond rated value shows large voltage drop because:
A) Armature reaction weakens flux
B) Field resistance increases
C) Critical resistance is exceeded
D) All of the above
**Answer: D**
86. For traction systems requiring constant speed, the suitable generator characteristic is:
A) Rising
B) Drooping
C) Flat
D) Differential
**Answer: C**
87. For DC arc welding, the rising external characteristic of a series generator is preferred because:
A) Current increases automatically with arc length
B) Voltage is constant
C) Power is constant
D) Efficiency is maximum
**Answer: A**
88. If a compound generator supplies long feeder lines, the advantage is:
A) Voltage drop is compensated by rising characteristic
B) Flux decreases
C) Current becomes zero
D) Resistance of feeder is reduced
**Answer: A**
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### **Building up EMF & Critical Resistance (89–92)**
89. A shunt generator builds up voltage only if the field resistance line is:
A) Above magnetization curve
B) Below magnetization curve
C) Tangent to X-axis
D) Vertical
**Answer: B**
90. A generator builds up at 1000 rpm but fails at 500 rpm. This is because:
A) Critical resistance decreases with speed
B) Critical resistance increases with lower speed
C) Residual magnetism is lost
D) Armature reaction is reduced
**Answer: B**
91. If the generator is rotated in reverse direction, it may fail to build up because:
A) Flux reduces
B) Residual magnetism opposes induced EMF
C) Critical resistance increases
D) Speed reduces
**Answer: B**
92. If a generator fails to build up due to residual magnetism loss, first test should be:
A) Flashing the field
B) Increasing speed
C) Checking brushes
D) Adding load
**Answer: A**
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### **Armature Reaction & Commutation (93–100)**
93. Armature reaction shifts the magnetic neutral axis (MNA). This causes:
A) Better commutation
B) Sparking at brushes
C) Increased EMF
D) Reduced speed
**Answer: B**
94. Compensating windings are provided to:
A) Neutralize armature reaction
B) Increase voltage
C) Increase flux
D) Decrease resistance
**Answer: A**
95. If brushes are shifted in the direction of rotation, the effect is:
A) Advance of commutation
B) Delay in commutation
C) No effect
D) Increase in flux
**Answer: B**
96. Interpoles are used in DC generators to:
A) Reduce core losses
B) Improve commutation
C) Reduce hysteresis loss
D) Increase speed
**Answer: B**
97. If interpoles are absent, commutation problem can be reduced by:
A) Using high resistance brushes
B) Increasing speed
C) Using short-circuited coils
D) Increasing load
**Answer: A**
98. Armature reaction at full load reduces flux. This leads to:
A) Voltage increase
B) Voltage drop
C) Current reduction
D) Speed reduction
**Answer: B**
99. During commutation, reactance voltage appears due to:
A) Inductance of coil under commutation
B) Armature resistance
C) Brush contact drop
D) Residual magnetism
**Answer: A**
100. Best combination for sparkless commutation is:
A) Interpoles + compensating windings
B) High flux + low speed
C) Low flux + high resistance
D) High speed + weak field
**Answer: A**
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UNIT 2 - PART A - 1 MARKS MCQ
1 MARKS
Nice nanba 👍 You’ve completed **UNIT I (DC Generators)** with 100 MCQs. Now let’s move to **UNIT II – DC Motors**.
I’ll prepare **MCQs (Understand + Apply + Analyze levels)** step by step, covering all the subtopics you listed.
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# **UNIT II – DC MOTORS (MCQs with Answers)**
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### **Overview & Significance of DC Motors (Q1–5)**
1. The function of a DC motor is to convert:
A) Electrical energy to mechanical energy
B) Mechanical energy to electrical energy
C) AC to DC
D) Heat to mechanical energy
**Answer: A**
2. DC motors are widely used in industries because:
A) They are cheaper than AC motors
B) They allow easy speed control
C) They do not need maintenance
D) They always run at constant speed
**Answer: B**
3. Which of the following industries depends most on DC motors?
A) Cement
B) Textile
C) Paper
D) All of the above
**Answer: D**
4. Automation systems prefer DC motors mainly because:
A) High starting torque and smooth speed control
B) Low voltage
C) Easy commutation
D) Less cost
**Answer: A**
5. The most significant property of DC motors is:
A) Power factor
B) Speed–torque characteristics
C) High efficiency
D) Size
**Answer: B**
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### **Types of DC Motors (Q6–12)**
6. In a shunt motor, the field winding is connected:
A) In series with armature
B) In parallel with armature
C) Across brushes only
D) None of the above
**Answer: B**
7. A DC motor that has constant speed irrespective of load is:
A) Shunt motor
B) Series motor
C) Compound motor
D) Stepper motor
**Answer: A**
8. A motor used for electric traction is:
A) Shunt motor
B) Series motor
C) PMDC motor
D) Stepper motor
**Answer: B**
9. Which motor can operate both as shunt and series depending on connection?
A) Compound motor
B) Universal motor
C) Stepper motor
D) Servomotor
**Answer: A**
10. Which motor is used where high starting torque is required?
A) Shunt
B) Series
C) Compound
D) BLDC
**Answer: B**
11. A cumulative compound motor has:
A) Series aiding shunt field
B) Series opposing shunt field
C) Only shunt field
D) Only series field
**Answer: A**
12. A differential compound motor is:
A) Stable
B) Unstable for load increase
C) Better than series motor
D) Used in traction
**Answer: B**
---
### **Torque Equation & Problems (Q13–18)**
13. Torque produced in a DC motor is proportional to:
A) Armature current only
B) Flux × Armature current
C) Speed × Flux
D) Flux²
**Answer: B**
14. The torque equation is:
A) $T = kΦI_a$
B) $T = \frac{ΦZN}{60A}$
C) $T = V \times I_a$
D) $T = kN²$
**Answer: A**
15. A motor takes 40 A at 220 V, armature resistance 0.5 Ω. Back EMF is:
A) 180 V
B) 190 V
C) 200 V
D) 210 V
**Answer: B**
16. If flux is halved and armature current doubled, torque will:
A) Double
B) Halve
C) Remain same
D) Be zero
**Answer: C**
17. A DC motor has flux of 0.02 Wb, 500 conductors, current per conductor 5 A, pole = 4. Torque is proportional to:
A) 0.02 × 500 × 5
B) 0.02 × 500 / 4
C) 0.02 × 5 × 4
D) 0.02 × 500
**Answer: A**
18. If armature current is 50 A and flux is reduced by 10%, torque will:
A) Increase by 10%
B) Decrease by 10%
C) Remain same
D) Double
**Answer: B**
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### **Characteristics (Load, Torque-Speed) (Q19–25)**
19. The torque–speed characteristic of a shunt motor is nearly:
A) Straight line with constant speed
B) Drooping curve
C) Rising curve
D) Hyperbolic
**Answer: A**
20. The speed–torque characteristic of a series motor is:
A) Constant speed
B) Variable speed, high torque at low speed
C) Flat curve
D) Falling straight line
**Answer: B**
21. In a shunt motor, speed remains almost constant because:
A) Flux is constant
B) Armature resistance is low
C) Series field winding is absent
D) Commutation is good
**Answer: A**
22. A series motor should not be run without load because:
A) Flux becomes very weak, causing very high speed
B) Armature current reduces
C) Efficiency increases dangerously
D) Brushes get damaged
**Answer: A**
23. The external characteristic of a shunt motor shows:
A) Speed nearly constant with load
B) Speed rising with load
C) Speed decreasing rapidly
D) Speed dropping to zero
**Answer: A**
24. For constant torque load like conveyors, the best motor is:
A) Series motor
B) Shunt motor
C) Compound motor
D) BLDC motor
**Answer: B**
25. For variable torque loads like cranes, the best motor is:
A) Series motor
B) Shunt motor
C) Stepper motor
D) PMDC motor
**Answer: A**
### **Applications of DC Motors (Q26–30)**
26. For elevators where constant speed is required, the best motor is:
A) DC shunt motor
B) DC series motor
C) DC compound motor
D) Stepper motor
**Answer: A**
27. For rolling mills requiring high starting torque, the suitable motor is:
A) DC series motor
B) DC shunt motor
C) DC compound motor
D) PMDC motor
**Answer: A**
28. Compound motors are used in:
A) Lathes
B) Shears
C) Presses
D) All of the above
**Answer: D**
29. For constant speed in paper mills, the best motor is:
A) Shunt motor
B) Series motor
C) Stepper motor
D) Servomotor
**Answer: A**
30. In traction (railways, trams), the most commonly used motor is:
A) Shunt motor
B) Series motor
C) BLDC motor
D) Stepper motor
**Answer: B**
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### **Necessity of Starters (Q31–35)**
31. A DC motor requires a starter because:
A) It starts slowly
B) At starting, back EMF is zero → high current flows
C) Speed cannot be controlled
D) To reduce efficiency
**Answer: B**
32. If no starter is used, the armature may:
A) Burn out due to excessive current
B) Run slowly
C) Stop rotating
D) Overheat slightly but remain safe
**Answer: A**
33. The main function of a starter is to:
A) Reduce applied voltage
B) Insert resistance in series with armature at starting
C) Reduce field current
D) Increase efficiency
**Answer: B**
34. In a 3-point starter, the "No-Volt Coil" is used to:
A) Protect against short circuit
B) Protect against loss of supply voltage
C) Increase torque
D) Increase speed
**Answer: B**
35. In a 4-point starter, the "No-Volt Coil" is connected:
A) Directly across supply
B) In series with shunt field
C) In series with armature
D) Across armature
**Answer: A**
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### **3-Point and 4-Point Starters (Q36–40)**
36. A drawback of 3-point starter is:
A) Speed control difficult at high resistance
B) No-volt coil may get weak at low field current (in speed control)
C) Armature current not limited
D) Cannot protect against overload
**Answer: B**
37. The overload relay in a DC starter protects against:
A) Excessive speed
B) Excessive current
C) Loss of residual magnetism
D) Reduced torque
**Answer: B**
38. In a 4-point starter, the no-volt coil remains unaffected by:
A) Supply failure
B) Change in field resistance
C) Overload
D) Back EMF
**Answer: B**
39. If supply is suddenly lost, the no-volt coil ensures:
A) Motor continues running
B) Handle of starter returns to OFF position
C) Field winding burns
D) Motor reverses
**Answer: B**
40. In modern starters, resistance steps are replaced by:
A) Choppers
B) Capacitors
C) Transformers
D) Flywheels
**Answer: A**
---
### **Speed Control of DC Motors (Q41–47)**
41. The speed of a DC motor is given by:
$N = \frac{V - I_aR_a}{kΦ}$
From this, speed can be controlled by:
A) Armature voltage control
B) Flux control
C) Armature resistance control
D) All of the above
**Answer: D**
42. Field control method is mainly used in:
A) Shunt motors
B) Series motors
C) Compound motors
D) Stepper motors
**Answer: A**
43. Armature resistance control is used for:
A) Speed above normal
B) Speed below normal
C) Constant speed
D) Efficiency improvement
**Answer: B**
44. Which method of speed control has poor efficiency?
A) Field control
B) Armature resistance control
C) Armature voltage control
D) Chopper control
**Answer: B**
45. Armature voltage control is suitable for:
A) Speed above normal
B) Speed below normal
C) Constant speed
D) Both A and B
**Answer: B**
46. In Ward Leonard method of speed control, the motor is fed from:
A) Rectifier
B) Another DC generator
C) Transformer
D) Inverter
**Answer: B**
47. The disadvantage of Ward Leonard system is:
A) Poor efficiency
B) Large size and cost
C) High maintenance
D) All of the above
**Answer: D**
---
### **Losses, Efficiency & Regulation (Q48–54)**
48. Copper losses in a DC motor vary with:
A) Current²
B) Voltage
C) Flux
D) Speed
**Answer: A**
49. Iron losses in a DC motor depend on:
A) Speed and flux
B) Current only
C) Voltage only
D) Temperature
**Answer: A**
50. Mechanical losses in DC motor are due to:
A) Hysteresis
B) Friction and windage
C) Eddy currents
D) Armature current
**Answer: B**
51. Maximum efficiency of a motor occurs when:
A) Copper loss = Iron + Mechanical losses
B) Copper loss = Iron loss only
C) Copper loss = Zero
D) Copper loss = Armature resistance
**Answer: A**
52. Stray load losses in DC machines are approximately:
A) 0.1% of load
B) 1% of load
C) 10% of load
D) Negligible
**Answer: B**
53. Efficiency of DC motors is generally in the range of:
A) 30–40%
B) 50–60%
C) 70–80%
D) 80–90%
**Answer: D**
54. Voltage regulation of a DC motor is:
A) Increase in voltage with load
B) Drop in voltage with load
C) Ratio of no-load to full-load voltage drop
D) None of the above
**Answer: C**
---
### **Special DC Machines (Q55–60)**
55. BLDC motors are preferred in computer fans because:
A) They produce no sparks
B) They are noiseless
C) They are efficient and maintenance free
D) All of the above
**Answer: D**
56. A DC servomotor is mainly used in:
A) Household appliances
B) Control systems requiring accurate positioning
C) Traction
D) Welding
**Answer: B**
57. PMDC motor stands for:
A) Permanent Magnet DC motor
B) Primary Magnet DC motor
C) Parallel Magnetic DC motor
D) Power Magnetized DC motor
**Answer: A**
58. PMDC motors are commonly used in:
A) Toys, wipers, small appliances
B) Traction
C) Heavy industries
D) Paper mills
**Answer: A**
59. Stepper motors are widely used in:
A) Traction
B) CNC machines and robotics
C) Welding
D) Textile mills
**Answer: B**
60. A stepper motor rotates:
A) Continuously like a normal motor
B) In discrete steps for each input pulse
C) At constant synchronous speed
D) Only in one direction
**Answer: B**
UNIT 2 - PART B - 2 MARKS
---
# **UNIT II – DC MOTORS (Q61 – Q100)**
---
### **Torque Equation – Apply & Analyze (61–67)**
**61.** A 220 V DC motor has Ra = 0.2 Ω. It takes 50 A at rated voltage. Calculate back EMF.
A) 210 V
B) 200 V
C) 180 V
D) 190 V
**Answer: A**
**62.** A DC motor develops torque of 80 Nm at 40 A armature current. If the flux is reduced to 80% and current increased to 50 A, new torque is:
A) 80 Nm
B) 100 Nm
C) 120 Nm
D) 128 Nm
**Answer: B**
**63.** A motor has torque proportional to flux × Ia. If flux reduces by 20% and Ia increases by 25%, net torque change is:
A) +5%
B) –5%
C) +10%
D) –10%
**Answer: B**
**64.** A 4-pole motor with 500 conductors carries 40 A, flux per pole = 0.03 Wb. Developed torque is proportional to:
A) 0.03 × 500 × 40
B) 0.03 × 500
C) 0.03 × 40
D) 0.03 × 4 × 40
**Answer: A**
**65.** If torque at full load = 200 Nm and armature current is halved with constant flux, torque becomes:
A) 100 Nm
B) 50 Nm
C) 150 Nm
D) 200 Nm
**Answer: A**
**66.** A series motor gives high torque at start because:
A) Flux ∝ Ia and Torque ∝ Ia²
B) Flux constant, Torque ∝ Ia
C) Flux decreases with load
D) Ia is small
**Answer: A**
**67.** Which test result would confirm that torque is directly proportional to flux × current?
A) Load test on shunt motor
B) Load test on series motor
C) Swinburne’s test
D) Hopkinson’s test
**Answer: B**
---
### **Load & Speed Characteristics – Analyze & Evaluate (68–75)**
**68.** You are comparing shunt and series motors for conveyor drives. Which will you recommend?
A) Series (high torque, variable speed)
B) Shunt (constant speed)
C) Either
D) Compound only
**Answer: B**
**69.** If a series motor is used in a lathe (constant speed required), what will happen?
A) Good operation
B) Poor operation, overspeed at light load
C) Stable speed control
D) No effect
**Answer: B**
**70.** Which curve best represents torque–speed of shunt motor?
A) Almost flat horizontal line
B) Rising parabola
C) Drooping curve
D) Hyperbola
**Answer: A**
**71.** For traction purposes, why is a series motor preferred?
A) High torque at start
B) Constant speed
C) Better commutation
D) High efficiency
**Answer: A**
**72.** A compound motor is selected for rolling mill. Reason?
A) High torque at start + good speed regulation
B) Low cost
C) High efficiency
D) No maintenance
**Answer: A**
**73.** If a series motor is suddenly unloaded, what is likely to happen?
A) Speed increases dangerously
B) Speed decreases
C) Torque increases
D) Motor stops
**Answer: A**
**74.** In a shunt motor, if load increases suddenly, speed:
A) Drops slightly
B) Drops rapidly
C) Increases
D) Remains constant
**Answer: A**
**75.** A hoist requires high starting torque and stable speed at full load. Which motor is best?
A) Series motor
B) Shunt motor
C) Compound motor
D) Stepper motor
**Answer: C**
---
### **Starters – Apply & Analyze (76–82)**
**76.** A 3-point starter cannot protect the motor in which case?
A) No-load running
B) Field winding open-circuit
C) Over-current in armature
D) Sudden overload
**Answer: B**
**77.** Why is a starter needed in DC motors?
A) To limit starting current due to low armature resistance
B) To improve commutation
C) To improve torque
D) To reduce copper loss
**Answer: A**
**78.** A 4-point starter differs from 3-point by:
A) Additional No-volt coil connection independent of field
B) More resistances
C) Overload protection
D) Automatic speed control
**Answer: A**
**79.** If field circuit of a shunt motor breaks while running with 3-point starter, motor will:
A) Overspeed
B) Stop
C) Run at constant speed
D) Burn out
**Answer: A**
**80.** In a 4-point starter, NVC is connected:
A) Across line
B) In series with field
C) Across supply independent of field
D) Across armature
**Answer: C**
**81.** If starter resistance is not inserted during starting, the motor will:
A) Take very high current
B) Run at low torque
C) Run at half speed
D) Not run
**Answer: A**
**82.** Starter protects the motor by:
A) Limiting high starting current
B) Controlling speed automatically
C) Acting as overload relay only
D) Improving efficiency
**Answer: A**
---
### **Speed Control – Evaluate & Create (83–89)**
**83.** Field control method changes:
A) Flux → affects speed inversely
B) Armature current directly
C) Torque directly
D) Voltage constant
**Answer: A**
**84.** Armature resistance control is inefficient because:
A) High I²R loss in external resistance
B) Poor torque
C) Cannot control speed above base speed
D) All of the above
**Answer: D**
**85.** For speed above base, which method is used?
A) Armature resistance control
B) Flux weakening
C) Voltage reduction
D) Brush shifting
**Answer: B**
**86.** For speed below base, best method is:
A) Field control
B) Armature resistance control
C) Voltage control
D) Permanent magnet control
**Answer: B**
**87.** Which method is most energy efficient for speed control?
A) Voltage control at armature
B) Armature resistance control
C) Field weakening
D) Series resistance control
**Answer: A**
**88.** Which modern technique is widely used for speed control in DC motors?
A) Chopper control (PWM)
B) Armature resistance
C) Field weakening only
D) Starter control
**Answer: A**
**89.** An elevator requires constant speed under varying load. Which control is best?
A) Ward Leonard system
B) Armature resistance
C) Field weakening
D) No control needed
**Answer: A**
---
### **Losses, Efficiency & Testing – Apply & Analyze (90–95)**
**90.** Mechanical losses in a DC motor include:
A) Friction and windage
B) Armature copper loss
C) Field copper loss
D) Iron loss
**Answer: A**
**91.** Iron losses depend on:
A) Voltage and flux
B) Speed and flux density
C) Armature resistance
D) Load
**Answer: B**
**92.** Copper loss varies with:
A) Square of current
B) Voltage
C) Speed
D) Flux
**Answer: A**
**93.** Efficiency of a DC motor is maximum when:
A) Variable losses = constant losses
B) Constant losses are zero
C) Variable losses are zero
D) Load is maximum
**Answer: A**
**94.** Swinburne’s test gives:
A) Constant losses of machine
B) Efficiency at full load
C) Direct measurement of torque
D) Speed–torque curve
**Answer: A**
**95.** Hopkinson’s test requires:
A) Two identical machines
B) One machine only
C) Alternator and motor pair
D) DC supply with alternator
**Answer: A**
---
### **Special DC Machines – Evaluate & Create (96–100)**
**96.** BLDC motors are widely used in:
A) Computer fans and drones
B) Heavy traction
C) Hoists
D) Rolling mills
**Answer: A**
**97.** A stepper motor is best suited for:
A) Precise position control
B) High starting torque loads
C) Constant speed drive
D) Traction
**Answer: A**
**98.** Servomotors are mainly applied in:
A) Control systems requiring accuracy
B) Heavy load cranes
C) Rolling mills
D) Conveyors
**Answer: A**
**99.** PMDC motors eliminate:
A) Field winding losses
B) Armature reaction
C) Back EMF
D) Torque ripple
**Answer: A**
**100.** If you are designing a CNC machine, which DC motor will you prefer?
A) Stepper motor for position control
B) Shunt motor for constant speed
C) Series motor for high torque
D) PMDC motor for simplicity
**Answer: A**
UNIT III - PART A - 1 MARKS-
# ✅ UNIT III – SINGLE PHASE TRANSFORMERS (MCQs)
### Basics & Ratings
**Q1.** A transformer works on the principle of:
A) Self induction
B) Mutual induction
C) Electromagnetic attraction
D) Capacitive effect
**Ans: B) Mutual induction**
**Q2.** The rating of a transformer is expressed in:
A) kW
B) kVA
C) Amperes
D) Ohms
**Ans: B) kVA**
**Q3.** Why transformer rating is in kVA, not kW?
A) Power factor is constant
B) Losses are independent of power factor
C) Only copper loss is considered
D) Only iron loss is considered
**Ans: B) Losses are independent of power factor**
**Q4.** A 2 kVA transformer has primary voltage 400 V. If secondary is 200 V, then max secondary current is:
A) 10 A
B) 5 A
C) 2 A
D) 20 A
**Ans: A) 10 A**
**Q5.** Power transformers are designed for:
A) Maximum efficiency at 50% load
B) Maximum efficiency at full load
C) All-day efficiency
D) High leakage reactance
**Ans: B) Maximum efficiency at full load**
---
### EMF Equation
**Q6.** EMF equation of transformer is:
A) 4.44 f N Φm
B) 2π f N Φm
C) 0.159 f N Φm
D) f N / Φm
**Ans: A) 4.44 f N Φm**
**Q7.** In deriving EMF equation, we assume:
A) Leakage flux is zero
B) Iron loss is included
C) Resistance of winding is considered
D) Harmonics included
**Ans: A) Leakage flux is zero**
**Q8.** If EMF per turn is 5 V, then induced secondary voltage for 200 turns =
A) 200 V
B) 500 V
C) 1000 V
D) 600 V
**Ans: C) 1000 V**
**Q9.** Which factor decides induced EMF in a transformer?
A) Flux per pole
B) Maximum flux in core
C) Leakage reactance
D) Resistance of winding
**Ans: B) Maximum flux in core**
**Q10.** In an ideal transformer:
A) No copper & no iron loss
B) Only copper loss exists
C) Only iron loss exists
D) Leakage flux is maximum
**Ans: A) No copper & no iron loss**
---
### Phasor Diagrams & Power Factor
**Q11.** In a no-load phasor diagram, no-load current lags supply voltage by:
A) 0°
B) 45°
C) 90°
D) 180°
**Ans: C) 90°**
**Q12.** For inductive load, power factor is:
A) Unity
B) Lagging
C) Leading
D) Zero
**Ans: B) Lagging**
**Q13.** For capacitive load, power factor is:
A) Lagging
B) Leading
C) Unity
D) Zero
**Ans: B) Leading**
**Q14.** At unity power factor, current in transformer is:
A) In phase with voltage
B) Lagging by 90°
C) Leading by 90°
D) Zero
**Ans: A) In phase with voltage**
**Q15.** The effect of leading power factor on regulation is:
A) Increases regulation
B) Decreases regulation
C) Regulation becomes negative
D) No effect
**Ans: C) Regulation becomes negative**
---
### Equivalent Circuit & Tests
**Q16.** Open circuit test on transformer gives:
A) Copper loss
B) Iron loss
C) Efficiency
D) Voltage regulation
**Ans: B) Iron loss**
**Q17.** Short circuit test on transformer gives:
A) Iron loss
B) Copper loss
C) Efficiency only
D) Magnetizing current
**Ans: B) Copper loss**
**Q18.** Why open circuit test is performed at rated voltage?
A) To heat transformer
B) To get full iron loss
C) To measure copper loss
D) To determine leakage reactance
**Ans: B) To get full iron loss**
**Q19.** Why short circuit test is performed at reduced voltage?
A) To minimize copper loss
B) To reduce iron loss
C) To avoid overheating
D) To reduce magnetizing current
**Ans: C) To avoid overheating**
**Q20.** Equivalent resistance and reactance are obtained from:
A) Open circuit test
B) Short circuit test
C) Load test
D) Efficiency test
**Ans: B) Short circuit test**
---
### Voltage Regulation
**Q21.** Voltage regulation of transformer is defined as:
A) (V1 − V2) / V1
B) (V2 − V1) / V1
C) (V1 − V2) / V2
D) (V2 − V1) / V2
**Ans: A) (V1 − V2) / V1**
**Q22.** Voltage regulation is negative when:
A) Load is lagging pf
B) Load is leading pf
C) Load is unity pf
D) Load is zero
**Ans: B) Load is leading pf**
**Q23.** A transformer has 2% regulation at full load unity pf. At 0.8 pf lagging, regulation is:
A) 0%
B) More than 2%
C) Less than 2%
D) Negative
**Ans: B) More than 2%**
**Q24.** To reduce regulation, transformer should have:
A) High leakage reactance
B) Low resistance and reactance
C) High resistance only
D) High reactance only
**Ans: B) Low resistance and reactance**
**Q25.** If a transformer is loaded with capacitive load, the voltage regulation becomes:
A) Positive
B) Negative
C) Zero
D) Infinite
**Ans: B) Negative**
---
### Efficiency
**Q26.** Efficiency =
A) (Input/Output) × 100
B) (Output/Input) × 100
C) (Loss/Input) × 100
D) (Output/Loss) × 100
**Ans: B) (Output/Input) × 100**
**Q27.** Condition for maximum efficiency:
A) Copper loss = 0
B) Copper loss = Iron loss
C) Iron loss = 0
D) Copper loss = twice iron loss
**Ans: B) Copper loss = Iron loss**
**Q28.** Distribution transformers are designed for:
A) Maximum efficiency at full load
B) Maximum efficiency at 75% load
C) Maximum efficiency at half load
D) Maximum all-day efficiency
**Ans: D) Maximum all-day efficiency**
**Q29.** All-day efficiency is important for:
A) Distribution transformer
B) Power transformer
C) Generator transformer
D) Isolation transformer
**Ans: A) Distribution transformer**
**Q30.** If input = 1000 W, efficiency = 95%, then output =
A) 950 W
B) 900 W
C) 1050 W
D) 1000 W
**Ans: A) 950 W**
### Parallel Operation of Transformers
**Q31.** Two transformers in parallel will share load in proportion to their:
A) Voltage ratings
B) kVA ratings
C) Copper losses
D) Efficiency
**Ans: B) kVA ratings**
**Q32.** Condition for parallel operation is:
A) Equal efficiency
B) Same percentage impedance
C) Equal leakage reactance
D) Equal size of core
**Ans: B) Same percentage impedance**
**Q33.** If transformers of unequal impedance run in parallel, the one with lower impedance will:
A) Take less load
B) Take more load
C) Share equally
D) Not operate
**Ans: B) Take more load**
**Q34.** In parallel operation, for proper sharing:
A) Same polarity
B) Same phase sequence
C) Same voltage ratio
D) All the above
**Ans: D) All the above**
**Q35.** If polarity of one transformer is reversed in parallel, the result will be:
A) Overheating
B) Short circuit
C) No load sharing
D) Higher efficiency
**Ans: B) Short circuit**
---
### Auto Transformer
**Q36.** An auto transformer uses:
A) One winding
B) Two windings
C) Three windings
D) Four windings
**Ans: A) One winding**
**Q37.** Copper saving in an auto transformer is proportional to:
A) Voltage ratio
B) Current ratio
C) kVA rating
D) Efficiency
**Ans: A) Voltage ratio**
**Q38.** For small transformation ratio, auto transformer is preferred because:
A) Efficiency is low
B) Copper saving is high
C) Cost is high
D) Regulation is poor
**Ans: B) Copper saving is high**
**Q39.** Disadvantage of auto transformer:
A) High copper loss
B) No electrical isolation
C) Bulky construction
D) Low efficiency
**Ans: B) No electrical isolation**
**Q40.** Auto transformer is NOT preferred when:
A) Ratio is close to 1
B) High voltage isolation is required
C) Cost is important
D) Copper saving is required
**Ans: B) High voltage isolation is required**
---
### Problem-Based (Apply Level)
**Q41.** A 230/115 V transformer has primary turns = 400. Then secondary turns = ?
A) 100
B) 200
C) 400
D) 800
**Ans: B) 200**
**Q42.** A transformer has iron loss = 100 W, copper loss = 100 W. Efficiency at that load = ?
A) 100%
B) 50%
C) Maximum efficiency
D) Zero
**Ans: C) Maximum efficiency**
**Q43.** A 5 kVA transformer has efficiency 95%. Output = 4.75 kW. Input = ?
A) 4.75 kW
B) 5.0 kW
C) 5.25 kW
D) 4.5 kW
**Ans: C) 5.25 kW**
**Q44.** If EMF per turn = 2 V, turns = 500, flux = 0.01 Wb, frequency = ?
A) 25 Hz
B) 50 Hz
C) 100 Hz
D) 60 Hz
**Ans: B) 50 Hz**
**Q45.** A transformer has efficiency = 98%. If output = 49 kW, then loss = ?
A) 1 kW
B) 2 kW
C) 0.98 kW
D) 0.5 kW
**Ans: A) 1 kW**
---
**Q46.** Distribution transformer efficiency is measured by:
A) Wattmeter method
B) All-day efficiency
C) Regulation method
D) Direct loading
**Ans: B) All-day efficiency**
**Q47.** Compare two winding transformer with auto transformer:
A) Auto is smaller and cheaper
B) Auto has no isolation
C) Two winding has isolation
D) All the above
**Ans: D) All the above**
**Q48.** No-load current in transformer is small because:
A) High resistance winding
B) Low leakage reactance
C) Core is highly permeable
D) Secondary winding is shorted
**Ans: C) Core is highly permeable**
**Q49.** Transformer copper loss depends on:
A) Load current
B) Voltage ratio
C) Iron loss
D) Flux density
**Ans: A) Load current**
**Q50.** Transformer iron loss depends on:
A) Frequency & flux density
B) Load current
C) Power factor
D) Leakage reactance
**Ans: A) Frequency & flux density**
-
**Q51.** Open circuit test measures:
A) Copper loss at no load
B) Iron loss at rated voltage
C) Stray loss
D) Short circuit loss
**Ans: B) Iron loss at rated voltage**
**Q52.** Short circuit test measures:
A) Copper loss at rated current
B) Iron loss
C) All-day efficiency
D) Regulation
**Ans: A) Copper loss at rated current**
**Q53.** Which test combination gives full equivalent circuit?
A) O.C test only
B) S.C test only
C) Both O.C & S.C tests
D) Direct loading
**Ans: C) Both O.C & S.C tests**
**Q54.** If copper loss > iron loss at rated load, then maximum efficiency occurs:
A) Below rated load
B) At rated load
C) Above rated load
D) Never
**Ans: A) Below rated load**
**Q55.** Which condition gives negative regulation?
A) Unity pf
B) Lagging pf
C) Leading pf
D) Zero pf
**Ans: C) Leading pf**
-
**Q56.** You need to design a transformer for rural areas (long idle hours, short load hours). Which efficiency to optimize?
A) Maximum efficiency
B) All-day efficiency
C) Regulation
D) Voltage ratio
**Ans: B) All-day efficiency**
**Q57.** If you want minimum copper usage in a 11 kV/10 kV transformer, which design is better?
A) Two winding
B) Auto transformer
C) Stepper transformer
D) Isolation transformer
**Ans: B) Auto transformer**
**Q58.** A transformer is overheating without load. The cause is mainly:
A) Copper loss
B) Iron loss
C) Load current
D) Voltage regulation
**Ans: B) Iron loss**
**Q59.** If you parallel connect two transformers of different kVA ratings, load sharing will be:
A) Equal kVA
B) Equal voltage
C) Proportional to kVA ratings
D) Random
**Ans: C) Proportional to kVA ratings**
**Q60.** For sensitive electronic equipment, which transformer is preferred?
A) Auto transformer
B) Isolation transformer
C) Power transformer
D) Distribution transformer
**Ans: B) Isolation transformer**
---
UNIT 3 - PART B - 2 MARKS
### Higher Apply & Analyze
**Q61.** Transformer core is laminated to reduce:
A) Copper loss
B) Hysteresis loss
C) Eddy current loss
D) Leakage reactance
**Ans: C) Eddy current loss**
**Q62.** If supply frequency increases, transformer iron loss:
A) Decreases
B) Increases
C) Remains same
D) Becomes zero
**Ans: B) Increases**
**Q63.** If flux density is increased, core loss will:
A) Increase
B) Decrease
C) Remain same
D) Zero
**Ans: A) Increase**
**Q64.** Which material is used for transformer core?
A) Silicon steel
B) Cast iron
C) Aluminum
D) Copper
**Ans: A) Silicon steel**
**Q65.** Transformer oil is used for:
A) Cooling only
B) Insulation only
C) Both cooling & insulation
D) Increasing flux
**Ans: C) Both cooling & insulation**
---
### Situational & Evaluate
**Q66.** If a transformer designed for 50 Hz is operated at 25 Hz, flux will:
A) Double
B) Halve
C) Remain same
D) Zero
**Ans: A) Double**
**Q67.** Transformer noise mainly due to:
A) Eddy currents
B) Hysteresis
C) Magnetostriction
D) Copper loss
**Ans: C) Magnetostriction**
**Q68.** Buchholz relay is used for:
A) Short circuit protection
B) Oil level indication
C) Gas protection in transformer
D) Voltage regulation
**Ans: C) Gas protection in transformer**
**Q69.** For parallel operation, why % impedance should be same?
A) For same polarity
B) For proper load sharing
C) To reduce copper loss
D) To improve pf
**Ans: B) For proper load sharing**
**Q70.** Tap changers are provided in transformers to:
A) Improve efficiency
B) Regulate secondary voltage
C) Reduce copper loss
D) Reduce leakage reactance
**Ans: B) Regulate secondary voltage**
---
### Final 71–100 (Revision + Mix of Apply/Evaluate/Create)
**Q71.** All-day efficiency is ratio of:
A) Output / Input
B) kWh output / kWh input
C) Loss / Input
D) Output / Loss
**Ans: B) kWh output / kWh input**
**Q72.** Condition for zero regulation:
A) Unity pf
B) Lagging pf
C) Leading pf = cos⁻¹(X/R)
D) Leading pf = cos⁻¹(R/X)
**Ans: C) Leading pf = cos⁻¹(X/R)**
**Q73.** Copper loss is proportional to:
A) I
B) I²
C) V²
D) f²
**Ans: B) I²**
**Q74.** Iron loss is proportional to:
A) f
B) f²
C) V
D) I²
**Ans: A) f**
**Q75.** Efficiency of transformer at 50% load = full load efficiency when:
A) Iron loss = Copper loss
B) Iron loss = ½ Copper loss
C) Copper loss = 2 × Iron loss
D) Iron loss = 4 × Copper loss
**Ans: C) Copper loss = 2 × Iron loss**
**Q76.** Why transformers are rated in kVA not kW?
A) Independent of pf
B) Because pf is always unity
C) Because efficiency is low
D) To reduce cost
**Ans: A) Independent of pf**
**Q77.** If secondary is open, primary draws:
A) Full load current
B) Magnetizing current only
C) Zero current
D) Copper loss current
**Ans: B) Magnetizing current only**
**Q78.** Which test is used to determine efficiency without loading?
A) Open circuit test
B) Short circuit test
C) Sumpner’s test
D) Direct load test
**Ans: C) Sumpner’s test**
**Q79.** A distribution transformer operates mostly:
A) At no load
B) At full load
C) At 50% load
D) At unity pf
**Ans: A) At no load**
**Q80.** In power transformers, design priority is:
A) All-day efficiency
B) Maximum efficiency
C) Voltage regulation
D) Cooling
**Ans: B) Maximum efficiency**
---
### Creative / Industry-Oriented
**Q81.** If you need a transformer for railway traction (frequent overloads), design should focus on:
A) Voltage ratio
B) Overload capacity
C) No-load loss
D) Magnetizing current
**Ans: B) Overload capacity**
**Q82.** If a hospital requires isolation, which transformer is best?
A) Auto transformer
B) Two winding transformer
C) Isolation transformer
D) Power transformer
**Ans: C) Isolation transformer**
**Q83.** For renewable energy inverter step-up, transformer must:
A) Have low core loss
B) Handle harmonics
C) Be light weight
D) All the above
**Ans: D) All the above**
**Q84.** Dry type transformers are used when:
A) Oil is dangerous
B) High voltage required
C) Efficiency is high
D) Power factor correction is needed
**Ans: A) Oil is dangerous**
**Q85.** Transformer size reduces with:
A) High frequency
B) Low frequency
C) High voltage drop
D) High resistance
**Ans: A) High frequency**
---
### Wrap-up (Revision Mixed)
**Q86.** Transformer equivalent circuit has:
A) R only
B) R & X
C) R, X & core loss
D) All the above
**Ans: D) All the above**
**Q87.** Condition for no load on transformer:
A) Secondary shorted
B) Secondary open
C) Primary disconnected
D) Core removed
**Ans: B) Secondary open**
**Q88.** Transformer’s primary and secondary power (ideal) are:
A) Equal
B) Unequal
C) Primary > Secondary
D) Secondary > Primary
**Ans: A) Equal**
**Q89.** Iron loss occurs due to:
A) Hysteresis & eddy currents
B) Copper heating
C) Stray fields
D) Armature reaction
**Ans: A) Hysteresis & eddy currents**
**Q90.** Transformer efficiency at no load is:
A) 100%
B) Less than 100%
C) Zero
D) Maximum
**Ans: B) Less than 100%**
**Q91.** Load sharing between parallel transformers depends on:
A) % impedance
B) Core loss
C) Power factor
D) Turns ratio only
**Ans: A) % impedance**
**Q92.** Harmonics in transformer are reduced by:
A) Using CRGO steel
B) Distributed winding
C) Step-lap core
D) All the above
**Ans: D) All the above**
**Q93.** For large power stations, transformers are connected:
A) Delta-Delta
B) Star-Delta
C) Delta-Star
D) Any depending on application
**Ans: D) Any depending on application**
**Q94.** Which factor is NOT affecting transformer efficiency?
A) Load power factor
B) Core loss
C) Copper loss
D) Leakage flux
**Ans: A) Load power factor**
**Q95.** Transformer cooling is necessary because:
A) To reduce iron loss
B) To reduce copper loss
C) To maintain insulation life
D) To increase voltage
**Ans: C) To maintain insulation life**
**Q96.** When two transformers with unequal voltage ratios are connected in parallel:
A) Proper sharing
B) Circulating current flows
C) No load sharing
D) Efficiency improves
**Ans: B) Circulating current flows**
**Q97.** Which test is NOT conducted on transformer?
A) Open circuit
B) Short circuit
C) Blocked rotor
D) Sumpner’s
**Ans: C) Blocked rotor**
**Q98.** Copper loss is maximum when transformer is:
A) At no load
B) At full load
C) At half load
D) Open circuited
**Ans: B) At full load**
**Q99.** Iron loss occurs:
A) Only on load
B) Only at no load
C) At all conditions (load or no load)
D) During short circuit only
**Ans: C) At all conditions (load or no load)**
**Q100.** Which device protects transformer against internal faults?
A) MCB
B) RCCB
C) Buchholz relay
D) Surge arrester
**Ans: C) Buchholz relay**
UNIT 4 - PART A - 1 MARKS
## **UNIT IV – THREE PHASE TRANSFORMER (MCQs for Diploma Students)**
---
### **PART A – Understand & Apply Level (Q1–60)**
1. The primary function of a transformer is to
a) Convert DC to AC
b) Change voltage levels
c) Convert electrical to mechanical energy
d) Store energy
**Ans: b**
2. A three-phase transformer can be constructed by using:
a) Three single-phase transformers
b) One three-phase unit
c) Either (a) or (b)
d) None
**Ans: c**
3. The most common core used in three-phase transformers is:
a) Shell type
b) Core type
c) Spiral type
d) Toroidal
**Ans: b**
4. The star connection is preferred on the \_\_\_\_\_\_\_ side to reduce insulation requirements.
a) Primary
b) Secondary
c) Both
d) None
**Ans: a**
5. In a delta connection, line voltage is equal to:
a) Phase voltage
b) √3 × phase voltage
c) 3 × phase voltage
d) Phase current
**Ans: a**
6. In star connection, line current =
a) Phase current
b) √3 × phase current
c) Phase voltage
d) Line voltage
**Ans: a**
7. A Y-Δ connection is often used for:
a) Transmission lines
b) Step-down applications
c) Step-up applications
d) Motor control
**Ans: c**
8. In Δ-Y connection, the secondary voltage is:
a) Higher than primary
b) Lower than primary
c) √3 times primary
d) Depends on turns ratio
**Ans: d**
9. The grouping of three-phase transformers ensures:
a) Correct polarity
b) Proper phase sequence
c) Parallel operation feasibility
d) All of the above
**Ans: d**
10. The three-phase transformer winding that provides neutral availability is:
a) Star
b) Delta
c) Open-delta
d) Zig-zag
**Ans: a**
11. Which connection provides a path for third harmonic currents?
a) Star
b) Delta
c) Zig-zag
d) Open-delta
**Ans: b**
12. In three-phase banks, open-delta (V-V) connection provides \_\_\_\_% of the original capacity.
a) 57.7%
b) 66.6%
c) 87%
d) 100%
**Ans: a**
13. Buchholz relay is used for:
a) Detecting gas formation due to internal faults
b) Cooling
c) Reducing harmonics
d) Voltage regulation
**Ans: a**
14. A conservator tank is provided in transformers to:
a) Prevent core losses
b) Allow oil expansion
c) Store spare oil
d) Increase efficiency
**Ans: b**
15. The breather in transformer contains:
a) Activated carbon
b) Silica gel
c) Mercury
d) Mica
**Ans: b**
16. The explosion vent in a transformer is used for:
a) Cooling
b) Releasing pressure during faults
c) Oil filtration
d) Reducing eddy currents
**Ans: b**
17. Which type of cooling uses external fans?
a) ONAN
b) ONAF
c) OFAF
d) Air natural
**Ans: b**
18. In ONAN cooling, heat is removed by:
a) Oil and natural air
b) Oil and forced air
c) Oil and water
d) Only air
**Ans: a**
19. OFAF cooling stands for:
a) Oil Forced Air Forced
b) Oil Free Air Flow
c) Over Forced Air Fan
d) Oil Flow Air Flow
**Ans: a**
20. Tap changing under load is done by:
a) Off-load tap changer
b) On-load tap changer
c) Conservator
d) Breather
**Ans: b**
Ok nanba 👍🔥 I’ll give you **Q21–60 (Understand & Apply level)** for **UNIT IV – Three Phase Transformers**.
Earlier I gave Q1–20 and Q61–100. Now this fills the middle gap.
**21.** A 3-phase transformer is usually constructed by connecting:
a) Three single-phase units
b) One 3-phase core unit
c) Either (a) or (b)
d) None
**Ans: c**
**22.** The advantage of using a single 3-phase transformer instead of 3 single-phase units is:
a) Cheaper and compact
b) Less reliable
c) More losses
d) Difficult to transport
**Ans: a**
**23.** Which connection provides a neutral point for grounding?
a) Δ–Δ
b) Δ–Y
c) Y–Δ
d) Y–Y
**Ans: d**
**24.** In Y–Δ connection, line voltage ratio is related to phase voltage ratio by:
a) 1:1
b) √3:1
c) 1:√3
d) 3:1
**Ans: b**
**25.** If the primary of a transformer is Δ-connected and secondary is Y-connected, the line current relation is:
a) IL1 = IL2
b) IL1 = √3 × IL2
c) IL2 = √3 × IL1
d) IL2 = 3 × IL1
**Ans: c**
**26.** Which transformer connection eliminates third harmonics in line voltage?
a) Y–Y
b) Δ–Δ
c) Δ–Y
d) Y–Δ
**Ans: b**
**27.** A Δ–Δ transformer is supplying a balanced load. If one transformer fails, the bank can continue as:
a) Y–Δ
b) Open-delta (V–V)
c) Scott connection
d) Parallel
**Ans: b**
**28.** The load handling capacity in open-delta (V–V) connection is:
a) 100%
b) 86.6%
c) 66.6%
d) 50%
**Ans: b**
**29.** Which connection is preferred for step-up transformers in power stations?
a) Δ–Y
b) Y–Δ
c) Δ–Δ
d) Y–Y
**Ans: a**
**30.** Which connection is preferred for step-down distribution transformers?
a) Δ–Y
b) Y–Δ
c) Δ–Δ
d) Open-delta
**Ans: a**
---
**31.** The main purpose of using conservator tank in a transformer is:
a) Oil cooling
b) Oil expansion and contraction
c) Voltage regulation
d) Pressure relief
**Ans: b**
**32.** The function of a breather is:
a) Prevent oil leakage
b) Prevent moisture entry
c) Provide cooling
d) Reduce harmonics
**Ans: b**
**33.** Which material is used in breather?
a) Activated carbon
b) Silica gel
c) Cotton
d) Asbestos
**Ans: b**
**34.** What does a pink color of silica gel indicate?
a) Dry condition
b) Moisture absorbed
c) Oil leakage
d) Fault in core
**Ans: b**
**35.** Explosion vent is provided in transformer to:
a) Release pressure due to internal faults
b) Reduce harmonics
c) Provide insulation
d) Cool the core
**Ans: a**
**36.** Buchholz relay is used for:
a) Cooling control
b) Internal fault detection
c) Preventing oil leakage
d) Improving efficiency
**Ans: b**
**37.** Which type of fault is mainly detected by Buchholz relay?
a) Overcurrent fault
b) Core heating fault
c) Incipient (slow-developing) fault
d) Overvoltage
**Ans: c**
**38.** Tap changers are used in transformers to:
a) Change frequency
b) Control output voltage
c) Improve power factor
d) Reduce iron loss
**Ans: b**
**39.** Off-load tap changers are operated when:
a) Transformer is under load
b) Transformer is without load
c) Transformer is overheated
d) Transformer is in parallel
**Ans: b**
**40.** On-load tap changers are used in:
a) Small distribution transformers
b) Large power transformers
c) Instrument transformers
d) Isolation transformers
**Ans: b**
---
**41.** ONAN cooling stands for:
a) Oil Natural Air Natural
b) Oil Natural Air Nominal
c) Oil Normal Air Normal
d) Oil Normal Air Natural
**Ans: a**
**42.** ONAF cooling includes:
a) Fans with natural oil circulation
b) Water cooling
c) Pumping of oil
d) Conservator cooling
**Ans: a**
**43.** OFAF cooling indicates:
a) Oil Forced Air Forced
b) Oil Flow Air Flow
c) Oil Filled Air Flow
d) Oil Forced Air Fan
**Ans: a**
**44.** OFWF cooling is mainly used in:
a) Small distribution transformers
b) Indoor substations
c) Large EHV transformers
d) Auto transformers
**Ans: c**
**45.** Which cooling method allows maximum rating of transformer?
a) ONAN
b) ONAF
c) OFAF
d) OFWF
**Ans: d**
**46.** The main function of transformer oil is:
a) Insulation and cooling
b) Reducing copper loss
c) Improving voltage regulation
d) Reducing harmonics
**Ans: a**
**47.** Oil used in transformers should have:
a) Low dielectric strength
b) High viscosity
c) High dielectric strength
d) High moisture content
**Ans: c**
**48.** The instrument used to check dielectric strength of oil is:
a) Voltmeter
b) Megger
c) Oil tester
d) Ammeter
**Ans: c**
**49.** Which test indicates moisture content in oil?
a) BDV test
b) Acidity test
c) Winding resistance test
d) Open-circuit test
**Ans: a**
**50.** The main reason for transformer oil deterioration is:
a) High load
b) Absorption of moisture and oxidation
c) Magnetic flux leakage
d) Copper loss
**Ans: b**
---
**51.** Parallel operation of 3-phase transformers is necessary when:
a) Load is small
b) Load is large and variable
c) Transformer is isolated
d) Efficiency is poor
**Ans: b**
**52.** For successful parallel operation, the voltage ratios must be:
a) Unequal
b) Equal
c) Very different
d) Zero
**Ans: b**
**53.** If % impedance of two parallel transformers is unequal, then:
a) Load is shared equally
b) Load is shared in inverse ratio of impedances
c) Load is shared in ratio of impedances
d) Transformers cannot run in parallel
**Ans: b**
**54.** Incorrect polarity during parallel operation causes:
a) Circulating currents
b) Overvoltage
c) Reduced losses
d) Increased efficiency
**Ans: a**
**55.** If two transformers with different vector groups are paralleled:
a) They share load equally
b) Circulating current flows
c) No problem occurs
d) Both improve efficiency
**Ans: b**
**56.** For parallel operation, frequency of transformers must be:
a) Different
b) Equal
c) Higher for one
d) Lower for one
**Ans: b**
**57.** The main advantage of parallel operation is:
a) Reliability and flexibility
b) Increased copper loss
c) Reduced cooling
d) More harmonics
**Ans: a**
**58.** If one transformer fails in parallel operation:
a) Entire system shuts down
b) Remaining transformer(s) continue to supply load
c) Load is lost permanently
d) Efficiency increases
**Ans: b**
**59.** During parallel operation, unequal phase sequence leads to:
a) Short-circuit
b) Reduced load
c) Improved voltage regulation
d) Stable operation
**Ans: a**
**60.** The grouping of three-phase transformers depends on:
a) Voltage ratio
b) Vector group (phase displacement)
c) Cooling method
d) Oil type
**Ans: b**
### **PART B – Apply, Analyze, Evaluate, Create Level (Q61–100)**
61. If a Δ-Δ transformer is replaced by an open-delta (V-V), what happens to the kVA capacity?
a) Remains same
b) Reduces to 57.7%
c) Increases by √3
d) Becomes zero
**Ans: b**
62. A star-delta transformer is used to:
a) Reduce starting current of induction motor
b) Provide isolation
c) Suppress harmonics
d) None
**Ans: a**
63. A transformer designed for ONAN cooling is operating at higher load. Suggest a method to improve cooling.
a) Increase oil volume
b) Add forced fans → ONAF
c) Reduce tap setting
d) Use smaller tank
**Ans: b**
64. Which accessory provides early warning of internal transformer faults?
a) Tap changer
b) Buchholz relay
c) Explosion vent
d) Breather
**Ans: b**
65. Why is Δ connection preferred on low-voltage side of distribution transformers?
a) Provides higher insulation requirement
b) Provides neutral
c) Can carry unbalanced loads
d) Provides path for triplen harmonics
**Ans: d**
66. During parallel operation, if polarity of one transformer is reversed, then:
a) Output doubles
b) Output becomes zero
c) Heavy circulating current flows
d) No effect
**Ans: c**
67. If two transformers of unequal percentage impedance are connected in parallel, load sharing will be:
a) Equal
b) Proportional to their kVA ratings
c) Inversely proportional to their impedances
d) Random
**Ans: c**
68. Evaluate the main reason why three-phase transformer is preferred over three single-phase units.
a) Higher cost
b) Requires less space and lighter
c) More maintenance
d) Less efficient
**Ans: b**
69. For a 3-phase transformer operating with Δ-Y connection, if one phase of primary is lost, what happens?
a) It still runs with reduced load
b) It fails completely
c) Neutral shifts
d) Efficiency increases
**Ans: b**
70. A 3-phase transformer of 500 kVA, 11 kV/415 V has impedance of 5%. Calculate short circuit kVA.
a) 10000 kVA
b) 500 kVA
c) 25000 kVA
d) 1000 kVA
**Ans: a**
**71.** Which factor decides load sharing in parallel operation of transformers?
a) Polarity
b) % Impedance
c) Core losses
d) Type of oil
**Ans: b**
**72.** Two transformers connected in parallel have different voltage ratios. This will result in:
a) Circulating current
b) Equal load sharing
c) Reduced core loss
d) Increased efficiency
**Ans: a**
**73.** Why is it necessary to use same vector group in parallel operation?
a) To reduce oil heating
b) To ensure phase angle matching
c) To avoid tap changing
d) To reduce copper loss
**Ans: b**
**74.** A Δ-Y transformer supplies a balanced load. What will be the phase shift between primary and secondary voltages?
a) 0°
b) 30°
c) 60°
d) 90°
**Ans: b**
**75.** Which connection is best suited for large power transmission step-up transformer?
a) Y–Δ
b) Δ–Y
c) Y–Y
d) Δ–Δ
**Ans: b**
**76.** In three-phase transformer testing, the short-circuit test is used to determine:
a) Iron losses
b) Copper losses
c) Efficiency
d) Magnetizing reactance
**Ans: b**
**77.** If Buchholz relay gives an alarm, it indicates:
a) Overload
b) Overheating due to core loss
c) Minor internal fault or gas accumulation
d) Overvoltage
**Ans: c**
**78.** If the transformer oil becomes moist, the breather silica gel turns:
a) Blue → Pink
b) Pink → Blue
c) Red → Yellow
d) White → Black
**Ans: a**
**79.** The tap changer helps in:
a) Changing frequency
b) Controlling secondary voltage
c) Reducing harmonics
d) Preventing heating
**Ans: b**
**80.** Which type of tap changer is preferred in power transformers?
a) Off-load
b) On-load
c) Both
d) None
**Ans: b**
---
**81.** Why is ONAN cooling not suitable for very high rating transformers?
a) High oil loss
b) Insufficient heat dissipation
c) High harmonics
d) Expensive design
**Ans: b**
**82.** In ONAF cooling, if fans fail:
a) Transformer continues with reduced load
b) Transformer trips immediately
c) Oil circulates faster
d) Neutral shifts
**Ans: a**
**83.** During parallel operation, unequal phase sequence will cause:
a) Reduction in efficiency
b) Short-circuit between transformers
c) More core loss
d) Reduced cooling
**Ans: b**
**84.** Zig-zag connection is mainly used for:
a) Neutral grounding
b) Increasing efficiency
c) Step-up application
d) Motor starting
**Ans: a**
**85.** Which connection is not possible in a single 3-phase transformer but possible in bank of 3 single-phase transformers?
a) Y–Y
b) Δ–Δ
c) Open-delta
d) Δ–Y
**Ans: c**
**86.** Advantage of Δ–Δ connection:
a) Provides neutral
b) Works even if one transformer fails (V–V)
c) Eliminates 3rd harmonics
d) Step-up suitable
**Ans: b**
**87.** A 3-phase transformer is rated 1000 kVA, efficiency 98%. If copper loss = iron loss, find each loss at full load.
a) 5 kW
b) 10 kW
c) 12 kW
d) 20 kW
**Ans: b**
**88.** Parallel operation improves:
a) Cooling
b) Reliability and flexibility
c) Iron losses
d) Impedance
**Ans: b**
**89.** In Δ–Y transformer, triplen harmonics circulate in:
a) Primary winding
b) Secondary winding
c) Core
d) Neutral
**Ans: a**
**90.** Evaluate the reason why transformer efficiency is maximum when iron loss = copper loss.
a) Voltage stability
b) Minimum total loss
c) Better regulation
d) Reduced harmonics
**Ans: b**
---
**91.** Create a solution: If a 3-phase transformer operates continuously under light load, which design modification improves efficiency?
a) Reduce copper cross section
b) Reduce core area
c) Use higher grade core material
d) Use thicker insulation
**Ans: c**
**92.** If cooling fails in a large transformer, first indication is:
a) Silica gel color change
b) Oil temperature rise alarm
c) Explosion vent operates
d) Buchholz relay trip
**Ans: b**
**93.** Analyze why large power transformers use forced oil and water cooling.
a) Cost effective
b) Compact size with higher kVA
c) Easy construction
d) No need of fans
**Ans: b**
**94.** A transformer has percentage impedance 6%. What will be the maximum load it can withstand without excessive voltage drop in parallel operation with another 4% impedance transformer?
a) 40%–60% sharing
b) 60%–40% sharing
c) Equal sharing
d) No operation possible
**Ans: b**
**95.** For emergency supply, one transformer in Δ–Δ connection fails. Remaining two run in V–V connection. Maximum load they can supply is:
a) 100%
b) 87%
c) 57.7%
d) 66.6%
**Ans: c**
**96.** Evaluate: Why is conservator tank placed above main tank?
a) For oil circulation
b) To reduce losses
c) To allow expansion and contraction of oil
d) For cooling fans
**Ans: c**
**97.** Create: Suggest a way to detect winding temperature inside transformer without removing oil.
a) Infrared thermometer
b) Winding resistance test
c) Winding temperature indicator with embedded sensors
d) Oil color test
**Ans: c**
**98.** If one phase winding of Δ side transformer is shorted, then:
a) Whole transformer fails
b) Neutral shifts
c) Harmonics reduce
d) It continues at half load
**Ans: a**
**99.** Analyze which factor mainly limits the rating of large 3-phase transformers.
a) Copper cost
b) Insulation level and cooling
c) Silica gel capacity
d) Tap changer size
**Ans: b**
**100.** Create: A factory requires frequent voltage regulation under load. Which transformer accessory must be installed?
a) Explosion vent
b) On-load tap changer
c) Breather
d) Zig-zag connection
**Ans: b**
UNIT 5
# **UNIT V – MAINTENANCE OF DC MACHINES AND TRANSFORMERS**
---
## **PART A (1–20) – REMEMBER LEVEL**
**1.** The main purpose of maintenance of electrical machines is:
a) Reduce losses
b) Increase efficiency
c) Ensure long life and reliability
d) Improve voltage
**Ans: c**
**2.** Preventive maintenance is carried out:
a) After failure occurs
b) Before failure occurs
c) During breakdown
d) Never
**Ans: b**
**3.** Breakdown maintenance is also known as:
a) Routine maintenance
b) Emergency maintenance
c) Predictive maintenance
d) Preventive maintenance
**Ans: b**
**4.** In DC machines, sparking occurs mainly at:
a) Field winding
b) Commutator
c) Armature core
d) Shaft
**Ans: b**
**5.** Which brush material is commonly used in DC machines?
a) Copper
b) Graphite
c) Aluminium
d) Zinc
**Ans: b**
**6.** The main cause of sparking at commutator is:
a) Open circuit
b) Poor commutation
c) Short circuit
d) High frequency
**Ans: b**
**7.** If brushes are shifted from the neutral axis, it results in:
a) Smooth commutation
b) Sparking at brushes
c) Reduced speed
d) Reduced voltage
**Ans: b**
**8.** The function of interpoles in a DC machine is:
a) Reduce armature reaction
b) Improve commutation
c) Reduce sparking
d) All of the above
**Ans: d**
**9.** Resurfacing of commutator is done by:
a) Turning in lathe
b) Sandpaper polishing
c) Stone grinding
d) Any of the above
**Ans: d**
**10.** If commutator surface is rough, it leads to:
a) Good commutation
b) Increased sparking and brush wear
c) Reduced heating
d) None
**Ans: b**
---
**11.** Brush tension in DC machine should be:
a) Too high
b) Too low
c) Optimum
d) Zero
**Ans: c**
**12.** A flat commutator segment indicates:
a) Normal condition
b) Excessive wear
c) Brush vibration
d) Moisture in oil
**Ans: b**
**13.** Which defect is caused due to high mica between commutator segments?
a) Sparking
b) Open circuit
c) Flashover
d) Heating
**Ans: a**
**14.** The process of undercutting in commutator maintenance is done on:
a) Brush
b) Armature winding
c) Mica insulation
d) Shaft
**Ans: c**
**15.** In DC machines, shorted turns in armature winding cause:
a) Voltage rise
b) Heating and sparking
c) Smooth operation
d) Reduced current
**Ans: b**
**16.** Open circuit fault in armature winding leads to:
a) Flashover
b) Heating of coil ends
c) Sparking at commutator
d) None
**Ans: c**
**17.** Inter-turn short circuit in armature winding is detected by:
a) Growler test
b) Insulation test
c) High voltage test
d) Load test
**Ans: a**
**18.** The dielectric strength of fresh transformer oil is approximately:
a) 10 kV
b) 30 kV
c) 60 kV
d) 100 kV
**Ans: c**
**19.** BDV test of oil measures:
a) Breakdown voltage of oil
b) Breakdown current of oil
c) Acidity
d) Moisture content
**Ans: a**
**20.** Acidity test of transformer oil is done to measure:
a) Oxidation level
b) Moisture content
c) Gas content
d) Conductivity
**Ans: a**
**21.** Why is preventive maintenance preferred in industries?
a) Reduces production cost
b) Avoids unexpected breakdowns
c) Increases machine downtime
d) Increases failure rate
**Ans: b**
**22.** Which maintenance method is costlier in the long run?
a) Preventive
b) Breakdown
c) Predictive
d) Routine
**Ans: b**
**23.** In preventive maintenance, inspection is carried out:
a) Daily
b) Weekly
c) Periodically
d) Never
**Ans: c**
**24.** Armature reaction in DC machines affects:
a) Efficiency
b) Commutation
c) Brush wear
d) All of the above
**Ans: d**
**25.** Sparking at brushes during load is mainly due to:
a) Armature reaction
b) Field weakening
c) Commutator vibration
d) All of the above
**Ans: d**
**26.** Remedy for sparking at brushes is:
a) Proper brush position
b) Use of interpoles
c) Good commutator surface
d) All of the above
**Ans: d**
**27.** When commutator is blackened, it indicates:
a) Proper lubrication
b) Carbon deposition
c) Normal operation
d) Oxidation of copper
**Ans: b**
**28.** Commutator segments are insulated by:
a) Asbestos
b) Mica
c) Cotton
d) Glass
**Ans: b**
**29.** In commutator resurfacing, why is mica undercutting necessary?
a) Prevents short circuit between segments
b) Reduces sparking
c) Provides brush seating
d) All of the above
**Ans: d**
**30.** The effect of worn-out brushes is:
a) Increased sparking
b) High contact resistance
c) Voltage drop
d) All of the above
**Ans: d**
---
**31.** Which tool is used to detect shorted turns in armature?
a) Voltmeter
b) Growler
c) Megger
d) Wattmeter
**Ans: b**
**32.** In a growler test, a piece of steel strip vibrates because of:
a) Magnetic induction in shorted coil
b) High resistance
c) Open winding
d) Mechanical vibration
**Ans: a**
**33.** Routine maintenance of transformer oil includes:
a) Filtering
b) Replacing
c) Moisture removal
d) All of the above
**Ans: d**
**34.** If transformer oil shows low BDV value, it means:
a) Oil is pure
b) Oil is contaminated with moisture/impurities
c) Transformer is overloaded
d) Core is saturated
**Ans: b**
**35.** What is the minimum BDV value acceptable for transformer oil in service?
a) 10–15 kV
b) 20–25 kV
c) 30–40 kV
d) Above 60 kV
**Ans: c**
**36.** Which test is performed to know chemical deterioration of oil?
a) BDV test
b) Acidity test
c) Viscosity test
d) Megger test
**Ans: b**
**37.** In acidity test, a reddish-brown color indicates:
a) Fresh oil
b) Acid formation
c) Moisture
d) Gas presence
**Ans: b**
**38.** The presence of sludge in transformer oil causes:
a) Reduced cooling efficiency
b) Increased insulation
c) Smooth operation
d) Better BDV
**Ans: a**
**39.** Earthing of transformer tanks is required to:
a) Prevent corrosion
b) Protect from lightning and leakage currents
c) Increase efficiency
d) Reduce noise
**Ans: b**
**40.** The method used for measuring earth resistance is:
a) Potentiometer method
b) Megger earth tester
c) Wheatstone bridge
d) Ohmmeter
**Ans: b**
---
**41.** In a 3-terminal earth tester, the auxiliary electrodes are used for:
a) Current and potential measurement
b) Voltage regulation
c) Insulation test
d) Commutation test
**Ans: a**
**42.** Regular earth testing ensures:
a) Proper insulation
b) Safety of equipment and personnel
c) Higher efficiency
d) All of the above
**Ans: b**
**43.** Breakdown maintenance of transformer is usually due to:
a) Oil failure
b) Winding short circuit
c) Bushing failure
d) All of the above
**Ans: d**
**44.** Which of the following is NOT a cause of sparking in commutator?
a) Armature reaction
b) Brush vibration
c) High resistance brushes
d) Strong field winding
**Ans: d**
**45.** What is the primary purpose of brush shifting?
a) Reduce copper losses
b) Improve commutation
c) Increase flux
d) Reduce hysteresis
**Ans: b**
**46.** Overheating of brushes is caused by:
a) High spring tension
b) High current density
c) Poor ventilation
d) All of the above
**Ans: d**
**47.** A DC armature coil open circuit can be detected by:
a) Multimeter continuity test
b) Growler test
c) Both a and b
d) None
**Ans: c**
**48.** During oil filtration, the main aim is to:
a) Increase viscosity
b) Remove moisture and dirt
c) Increase acidity
d) Reduce flash point
**Ans: b**
**49.** Which of the following reduces maintenance frequency?
a) Proper ventilation
b) Regular cleaning
c) Use of good quality materials
d) All of the above
**Ans: d**
**50.** Transformer oil gets deteriorated mainly due to:
a) Oxidation
b) Moisture absorption
c) High temperature
d) All of the above
**Ans: d**
---
**51.** In earthing measurement, resistance should be kept as:
a) Less than 1 Ω (for substations)
b) 10–20 Ω
c) Above 50 Ω
d) Infinite
**Ans: a**
**52.** Which safety device detects gas accumulation in transformer oil?
a) Breather
b) Buchholz relay
c) Conservator
d) Explosion vent
**Ans: b**
**53.** Preventive maintenance of DC machines includes:
a) Cleaning commutator
b) Checking insulation resistance
c) Checking bearing lubrication
d) All of the above
**Ans: d**
**54.** Which instrument is used to measure insulation resistance?
a) Voltmeter
b) Megger
c) Ammeter
d) Wattmeter
**Ans: b**
**55.** The resistance of earth electrode is inversely proportional to:
a) Soil resistivity
b) Electrode diameter
c) Depth of electrode
d) Both b and c
**Ans: d**
**56.** Earthing of transformer neutral is provided for:
a) Voltage stabilization during faults
b) Reduce noise
c) Increase efficiency
d) Save copper
**Ans: a**
**57.** Transformer maintenance schedule usually involves:
a) Oil testing
b) Winding resistance measurement
c) Visual inspection of bushings
d) All of the above
**Ans: d**
**58.** Sludge formed in transformer oil is removed by:
a) Filtering
b) Replacing oil
c) Centrifuging
d) All of the above
**Ans: d**
**59.** A routine preventive maintenance reduces:
a) Forced outages
b) Efficiency
c) Power factor
d) Reliability
**Ans: a**
**60.** In a transformer, the most common fault detected during maintenance is:
a) Core saturation
b) Winding insulation failure
c) Shaft bending
d) Pole slipping
**Ans: b**
# **UNIT V – PART B (61–100) – APPLY, ANALYZE, EVALUATE & CREATE LEVELS**
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**61.** A commutator shows heavy sparking even after brush adjustment. What should be the next action?
a) Replace brushes
b) Check interpoles
c) Resurface commutator
d) All of the above
**Ans: d**
**62.** While testing a DC motor, vibration and noise are observed. Likely cause?
a) Loose commutator segments
b) Worn bearings
c) Misaligned shaft
d) All of the above
**Ans: d**
**63.** Transformer BDV test shows 18 kV. What action should be taken?
a) Continue operation
b) Filter or replace oil
c) Increase load
d) Ignore
**Ans: b**
**64.** During a growler test, no vibration occurs. What does it mean?
a) Armature is healthy
b) Shorted turn exists
c) Open circuit coil
d) Faulty growler
**Ans: a**
**65.** You observe excessive brush wear. Which maintenance step is most suitable?
a) Replace brushes and check spring tension
b) Only polish commutator
c) Increase current rating
d) Reduce load
**Ans: a**
---
**66.** Transformer oil acidity is found high. Which immediate step is correct?
a) Replace oil
b) Neutralize acidity
c) Filter oil
d) All of the above depending on condition
**Ans: d**
**67.** Breakdown maintenance of a DC generator is generally caused by:
a) Overloading
b) Poor preventive care
c) Ageing of insulation
d) All of the above
**Ans: d**
**68.** Why is a Buchholz relay tested periodically?
a) To check insulation
b) To ensure fault detection ability
c) To measure oil level
d) To cool oil
**Ans: b**
**69.** A transformer shows overheating even at normal load. The most probable cause is:
a) Poor cooling
b) Winding short circuit
c) High ambient temperature
d) All of the above
**Ans: d**
**70.** Sparking at commutator continues even after undercutting mica. What should be checked next?
a) Field winding
b) Interpoles
c) Load fluctuations
d) All of the above
**Ans: d**
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**71.** Suggest one preventive measure to avoid commutator blackening.
a) Use good quality carbon brushes
b) Increase field flux
c) Run at higher speed
d) Reduce brush pressure
**Ans: a**
**72.** A maintenance engineer is asked to check earth resistance. Which instrument should he select?
a) Tong tester
b) Earth tester megger
c) CRO
d) LCR meter
**Ans: b**
**73.** If transformer oil moisture content is not removed, it will:
a) Increase BDV
b) Reduce BDV and cause insulation failure
c) Increase efficiency
d) Prevent sparking
**Ans: b**
**74.** Analyze: Why preventive maintenance is cheaper than breakdown maintenance?
a) Minimizes downtime
b) Reduces repair cost
c) Extends life of equipment
d) All of the above
**Ans: d**
**75.** A transformer tripped by Buchholz relay. The FIRST step of maintenance should be:
a) Re-energize immediately
b) Inspect oil for gas formation
c) Replace conservator
d) Change tap setting
**Ans: b**
---
**76.** When brushes spark more on one side, it indicates:
a) Armature eccentricity
b) Misaligned shaft
c) Uneven brush pressure
d) All of the above
**Ans: d**
**77.** After oil filtration, BDV remains low. What is the next step?
a) Replace oil completely
b) Ignore
c) Heat transformer
d) Add silica gel
**Ans: a**
**78.** A DC motor commutator becomes rough. What maintenance is required?
a) Resurfacing with emery paper
b) Replacing commutator
c) Undercutting mica
d) Any of the above depending on severity
**Ans: d**
**79.** Analyze: Which test ensures both insulation and oil quality of transformer?
a) BDV + Acidity test
b) Open circuit test
c) Short circuit test
d) Earth resistance test
**Ans: a**
**80.** A maintenance schedule includes IR test, oil test, and visual inspection. The purpose is:
a) Detect faults early
b) Reduce repair cost
c) Increase machine reliability
d) All of the above
**Ans: d**
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**81.** A DC generator is taken for servicing. Which test will you do first after servicing?
a) No-load test
b) Load test
c) IR test
d) BDV test
**Ans: c**
**82.** Transformer winding short circuit is suspected. Which method is used to confirm?
a) Resistance measurement
b) Tan delta test
c) Open circuit test
d) Both a and b
**Ans: d**
**83.** A commutator shows pitting (small holes). What is the remedy?
a) Light turning on lathe
b) Mica undercutting
c) Polishing with sandpaper
d) All of the above
**Ans: d**
**84.** Evaluate: Which maintenance practice increases reliability most?
a) Regular inspection + preventive action
b) Breakdown repair
c) Ignoring minor defects
d) Only replacing parts
**Ans: a**
**85.** Transformer earthing resistance is measured at 12 Ω. What action is required?
a) Nothing
b) Improve earthing by adding electrodes
c) Remove transformer
d) Increase load
**Ans: b**
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**86.** If a DC motor shows flashing at commutator, the probable cause is:
a) Overload
b) Improper brush contact
c) Weak interpoles
d) All of the above
**Ans: d**
**87.** While testing transformer oil, sludge is found. Best corrective action is:
a) Filtering oil
b) Replace oil
c) Clean conservator tank
d) All of the above
**Ans: d**
**88.** Evaluate: Which of the following is most critical in transformer maintenance?
a) Oil condition
b) Tap changer operation
c) Winding temperature
d) All of the above
**Ans: d**
**89.** A DC machine shows unequal sparking on brushes. Which defect is most likely?
a) Shorted armature coil
b) Open circuit field
c) Mechanical vibration
d) All of the above
**Ans: a**
**90.** If transformer breather silica gel becomes pink, it indicates:
a) Oil oxidation
b) Moisture absorption
c) Overheating
d) Normal condition
**Ans: b**
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**91.** Create: Suggest one way to increase reliability of earthing system in rural areas.
a) Use multiple earth electrodes in parallel
b) Use thin wire
c) Avoid moisture in soil
d) Reduce electrode depth
**Ans: a**
**92.** A preventive maintenance chart must include:
a) Frequency of checks
b) Type of test
c) Responsible person
d) All of the above
**Ans: d**
**93.** While analyzing sparking patterns, a zigzag mark is seen on commutator. What does it mean?
a) Wrong brush grade
b) High mica level
c) Shorted coil
d) All of the above
**Ans: d**
**94.** Transformer oil acidity increased. Which long-term solution is better?
a) Regular oil testing and timely replacement
b) Only filtering oil
c) Ignoring test results
d) Replacing conservator
**Ans: a**
**95.** A maintenance logbook is important because:
a) Helps in planning future maintenance
b) Records fault history
c) Improves accountability
d) All of the above
**Ans: d**
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**96.** If earth resistance is very high, the transformer:
a) May be unsafe during lightning/fault
b) Will have higher efficiency
c) Will run cooler
d) No effect
**Ans: a**
**97.** Create: Suggest a design improvement to minimize commutator sparking in DC machines.
a) Use interpoles and compensating windings
b) Use steel commutator
c) Reduce brush contact
d) Remove interpoles
**Ans: a**
**98.** Evaluate: Why is BDV test done before acidity test?
a) BDV test is simple and quick to indicate oil condition
b) Acidity test is costly
c) BDV test does not damage oil
d) Both a and c
**Ans: d**
**99.** A DC motor shows sparking under load but runs smoothly at no-load. The fault is:
a) Weak interpoles
b) Armature reaction
c) Poor commutation
d) All of the above
**Ans: d**
**100.** Create: If you were an engineer, how would you design a maintenance plan for a 10-year-old transformer?
a) Monthly oil test + yearly IR test
b) Regular breather check + tap changer inspection
c) Complete oil replacement every 3–5 years
d) All of the above combined
**Ans: d**
