NSK - TAMILAN ULAGAM

NSK - TAMILAN ULAGAM
NSK - DIPLOMA

Monday, August 4, 2025

EEE MACHINES 1 UNIT 5 UNIT 3 and Unit 4MCQ QUESTIONS AND ANSWERS


Here are 30 MCQs from UNIT V – Maintenance of DC Machines and Transformers based on Understand and Apply levels.



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1. Importance of Maintenance


1. The primary goal of preventive maintenance in DC machines is to:

A) Increase load capacity

B) Avoid unexpected breakdowns

C) Reduce voltage regulation

D) Improve commutator size

Answer: B



2. Which of the following is NOT an advantage of proper maintenance?

A) Reduced downtime

B) Increased life of equipment

C) Higher operating cost

D) Improved reliability

Answer: C



3. In maintenance schedules, inspection frequency is usually decided based on:

A) Operator’s convenience

B) Manufacturer’s guidelines

C) Commutator size

D) Number of poles in the machine

Answer: B





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2. Preventive and Breakdown Maintenance


4. Preventive maintenance is carried out:

A) Only after a failure occurs

B) At predetermined intervals

C) When load is maximum

D) Without any schedule

Answer: B



5. Breakdown maintenance is:

A) Cheaper in the long run

B) Carried out before any faults occur

C) Carried out after a fault occurs

D) Not required for transformers

Answer: C



6. Which type of maintenance generally results in longer downtime?

A) Preventive

B) Predictive

C) Breakdown

D) Routine

Answer: C





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3. Causes of Sparking in Commutator


7. Excessive sparking in a DC machine commutator may be caused by:

A) Correct brush pressure

B) Open circuit in armature winding

C) Balanced load

D) Proper commutator alignment

Answer: B



8. High mica between commutator segments can cause:

A) Smooth brush contact

B) Poor insulation

C) Sparking during operation

D) Reduced armature current

Answer: C



9. Poor brush contact is often due to:

A) Worn out brushes

B) Correct brush spring tension

C) Overheated armature core

D) Perfectly smooth commutator

Answer: A





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4. Defects in Commutator and Remedies


10. A flat spot on the commutator surface is usually corrected by:

A) Increasing brush pressure

B) Turning and polishing the commutator

C) Increasing armature voltage

D) Adding more brushes

Answer: B



11. Uneven wear of brushes is mainly due to:

A) Even brush pressure

B) Incorrect brush spring tension

C) Proper commutator alignment

D) Good cooling system

Answer: B



12. If a commutator is out-of-round, the most effective remedy is:

A) Mica undercutting

B) Resurfacing by turning on a lathe

C) Increasing field current

D) Replacing all armature windings

Answer: B





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5. Resurfacing of Commutator and Brushes


13. The main purpose of resurfacing a commutator is to:

A) Increase armature speed

B) Improve brush contact

C) Reduce machine size

D) Increase insulation thickness

Answer: B



14. Which tool is commonly used for polishing commutators after turning?

A) Sandpaper

B) Metal file

C) Commutator stone

D) Hacksaw blade

Answer: C



15. While resurfacing brushes, the shape of the brush face should match:

A) Brush holder

B) Armature winding

C) Commutator curvature

D) Shaft diameter

Answer: C





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6. Defects in DC Armature Winding


16. An inter-turn short circuit in an armature winding may cause:

A) Reduction in load

B) Local heating and sparking

C) Improved commutation

D) Increased insulation resistance

Answer: B



17. Which test is often used to detect armature winding faults?

A) Earth resistance test

B) Growler test

C) BDV test

D) Oil acidity test

Answer: B



18. Open-circuited armature coils can lead to:

A) Smooth current flow

B) Increased torque

C) Uneven magnetic field

D) Balanced armature reaction

Answer: C





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7. Maintenance of Transformer Oil


19. Transformer oil is mainly used for:

A) Cooling and insulation

B) Only cooling

C) Increasing winding resistance

D) Increasing core losses

Answer: A



20. Which of the following indicates transformer oil deterioration?

A) High BDV value

B) Low acidity

C) Dark color and sludge

D) High insulation resistance

Answer: C



21. Filtering transformer oil helps to:

A) Remove magnetic flux

B) Remove moisture and impurities

C) Increase core efficiency

D) Reduce insulation

Answer: B





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8. Transformer Oil Tester – Acidity Test, BDV Test


22. BDV test of transformer oil measures:

A) Magnetic strength

B) Voltage at which breakdown occurs

C) Acid content

D) Current carrying capacity

Answer: B



23. In acidity testing of transformer oil, the measured quantity is:

A) pH value

B) Neutralization value (mg KOH per gram of oil)

C) Conductivity

D) Temperature rise

Answer: B



24. Low BDV value of transformer oil indicates:

A) High insulation strength

B) Oil is in good condition

C) Oil has absorbed moisture or dirt

D) Oil is free of impurities

Answer: C





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9. Earthing


25. The main purpose of earthing in electrical equipment is to:

A) Increase voltage

B) Provide a path for fault current

C) Improve load efficiency

D) Reduce harmonics

Answer: B



26. Equipment earthing ensures:

A) Protection of equipment only

B) Safety of both equipment and personnel

C) Only personnel safety

D) High voltage operation

Answer: B



27. The most common type of earthing used in substations is:

A) Rod earthing

B) Plate earthing

C) Chemical earthing

D) Any of the above based on soil condition

Answer: D





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10. Measurement of Earth Resistance


28. Earth resistance is measured using:

A) Megger

B) Tong tester

C) Earth tester

D) Growler

Answer: C



29. A high earth resistance value indicates:

A) Good earthing

B) Poor earthing

C) Neutral fault

D) Proper soil moisture

Answer: B



30. To improve earth resistance

, one can:

A) Reduce soil moisture

B) Add salt and charcoal around the electrode

C) Remove soil around the electrode

D) Reduce electrode size

Answer: B



UNIT III – Single Phase Transformers (10 MCQs)

  1. A 230/115 V transformer has a primary current of 4 A. The secondary current is:
    a) 2 A
    b) 4 A
    c) 8 A ✅
    d) 16 A

  2. In the EMF equation , doubling the frequency while keeping flux constant will:
    a) Halve the voltage
    b) Double the voltage ✅
    c) Keep voltage same
    d) Reduce voltage to zero

  3. A transformer has iron loss = 150 W and full-load copper loss = 150 W. Maximum efficiency occurs at:
    a) 100% load
    b) 70.7% load ✅
    c) 50% load
    d) Any load

  4. If secondary load is disconnected, the transformer operates at:
    a) Full load
    b) No-load ✅
    c) Half load
    d) Short circuit

  5. Which phasor diagram is used to find voltage regulation at different power factors?
    a) Open-circuit test diagram
    b) Load phasor diagram ✅
    c) Short-circuit test diagram
    d) Efficiency diagram

  6. Two single-phase transformers are connected in parallel. For equal load sharing, they must have:
    a) Same efficiency
    b) Same voltage ratio ✅
    c) Same kVA rating
    d) Same iron loss

  7. All-day efficiency is preferred over commercial efficiency when:
    a) Transformer operates continuously at full load
    b) Transformer operates with varying load ✅
    c) Transformer operates only at night
    d) Transformer operates for short durations

  8. In an ideal transformer, the power input is:
    a) Less than power output
    b) Equal to power output ✅
    c) Greater than power output
    d) Zero

  9. In an autotransformer, copper saving is higher when:
    a) Voltage ratio is low ✅
    b) Voltage ratio is high
    c) Load is small
    d) Load is purely resistive

  10. A transformer has full-load efficiency of 98%. If the output is 49 kW, the input is:
    a) 49 kW
    b) 50 kW ✅
    c) 51 kW
    d) 52 kW


UNIT IV – Three Phase Transformers (10 MCQs)

  1. In Δ–Y connection, the line-to-line voltage ratio is:
    a) Equal to turns ratio
    b) √3 × turns ratio ✅
    c) Turns ratio / √3
    d) Always unity

  2. For parallel operation of three-phase transformers, they must have:
    a) Same vector group ✅
    b) Same kVA rating
    c) Same copper loss
    d) Same efficiency

  3. The function of a conservator tank is to:
    a) Cool the oil
    b) Allow expansion of oil ✅
    c) Increase efficiency
    d) Remove harmonics

  4. Buchholz relay operates due to:
    a) External short circuit
    b) Internal gas formation ✅
    c) Overvoltage
    d) Overload

  5. ONAN cooling stands for:
    a) Oil Natural, Air Natural ✅
    b) Oil Natural, Air Forced
    c) Oil Forced, Air Forced
    d) Oil Forced, Air Natural

  6. In Δ–Δ connection, if one transformer fails, the system can operate as:
    a) Open delta ✅
    b) Star connection
    c) Double delta
    d) Zig-zag

  7. The breather in a transformer is filled with:
    a) Charcoal
    b) Silica gel ✅
    c) Transformer oil
    d) Copper oxide

  8. Which connection eliminates third harmonics from line voltage?
    a) Y–Y
    b) Δ–Δ ✅
    c) Δ–Y
    d) Y–Δ

  9. On-load tap changers are used for:
    a) Varying frequency
    b) Adjusting voltage under load ✅
    c) Fault detection
    d) Cooling

  10. If a transformer has Δ–Y connection, the phase shift between primary and secondary is:
    a) 0°
    b) 30° ✅
    c) 60°
    d) 90°


If you want, I can now prepare short numerical MCQs for both units so they test calculation skills alongside theory. That will make the set even stronger for an "apply" level exam.






 Got it — you want 20 MCQ questions each from Unit III (Single Phase Transformers) and Unit IV (Three Phase Transformers), focusing on Understand and Apply level questions, so they’re not just pure memory recall but require some interpretation, concept linking, or application of formulas.


I’ll create them in a clear Q&A format with 4 options each and mark the correct answer.

We’ll keep them syllabus-aligned and mix conceptual understanding with problem-oriented ones.



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UNIT III – Single Phase Transformers (20 MCQs)


1. The EMF equation of a single-phase transformer is given by:

a) ✅

b) 

c) 

d) 



2. A transformer has primary turns = 500, secondary turns = 100. If primary voltage is 230 V, the secondary voltage is:

a) 23 V ✅

b) 46 V

c) 115 V

d) 460 V



3. The main reason for laminating the core of a transformer is to:

a) Reduce hysteresis loss

b) Reduce eddy current loss ✅

c) Reduce copper loss

d) Reduce flux leakage



4. In an ideal transformer, the power output is:

a) Greater than input power

b) Equal to input power ✅

c) Less than input power

d) Zero



5. The all-day efficiency of a transformer is calculated using:

a) Constant load test

b) Ratio of output kWh to input kWh ✅

c) Load test only

d) OC and SC tests



6. If a transformer is operated at frequency less than rated, keeping V constant:

a) Core loss decreases

b) Flux increases ✅

c) Copper loss decreases

d) Efficiency increases



7. Which phasor diagram is used for studying power factor variation in load?

a) No-load phasor diagram

b) Load phasor diagram ✅

c) OC test diagram

d) SC test diagram



8. The condition for maximum efficiency in a transformer is:

a) Copper loss = Iron loss ✅

b) Copper loss > Iron loss

c) Copper loss < Iron loss

d) Copper loss = 0



9. Parallel operation of single-phase transformers requires:

a) Equal efficiency

b) Same voltage ratio ✅

c) Same kVA rating

d) Equal resistance only



10. Voltage regulation of a transformer at leading power factor can be:

a) Positive

b) Negative ✅

c) Zero only

d) Always positive



11. Which test is performed to determine iron losses?

a) Short-circuit test

b) Open-circuit test ✅

c) Load test

d) Polarity test



12. A 1000/200 V transformer has full load copper loss = 400 W, iron loss = 200 W. Max efficiency occurs at load:

a) 100%

b) 70.7% ✅

c) 50%

d) 25%



13. Which of these is NOT an application of a single-phase transformer?

a) Welding machine

b) Distribution system

c) Induction motor starting ✅

d) Voltage regulation in power system



14. In an ideal transformer, the ratio of primary current to secondary current is:

a) Equal to turns ratio

b) Inverse of turns ratio ✅

c) Proportional to load

d) Always unity



15. An autotransformer compared to two-winding transformer of same rating is:

a) Heavier

b) Lighter ✅

c) Same weight

d) Inefficient



16. The copper loss in a transformer is proportional to:

a) Load current ✅

b) Load current squared

c) Voltage

d) Frequency



17. If secondary load is disconnected, the transformer is on:

a) Full-load

b) No-load ✅

c) Half-load

d) Short-circuit



18. In a transformer, leakage flux is minimized by:

a) Using laminated core

b) Winding placement close together ✅

c) Increasing turns

d) Increasing air gap



19. The SC test on a transformer gives:

a) Iron loss

b) Copper loss ✅

c) Efficiency

d) Voltage regulation



20. In an ideal transformer, if input is 500 W, output is:

a) 400 W

b) 500 W ✅

c) 600 W

d) Zero





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UNIT IV – Three Phase Transformers (20 MCQs)


1. Which connection of a 3-phase transformer is suitable for small HV step-down transformers?

a) Δ–Δ

b) Δ–Y ✅

c) Y–Δ

d) Y–Y



2. In a Δ–Y connection, the line voltage ratio is:

a) √3 × Turns ratio ✅

b) Turns ratio / √3

c) Equal to turns ratio

d) Always unity



3. The vector group of a transformer defines:

a) Winding resistance

b) Phase displacement ✅

c) Copper loss

d) Cooling method



4. For parallel operation of 3-phase transformers, it is essential to have:

a) Same impedance ratio ✅

b) Same kVA rating only

c) Equal copper losses

d) Equal efficiency



5. Which cooling method uses forced oil and forced air?

a) ONAN

b) ONAF

c) OFAF ✅

d) OFAN



6. Buchholz relay is used for:

a) Detecting internal faults ✅

b) Cooling

c) Voltage regulation

d) Switching



7. The function of a conservator tank in transformer is to:

a) Store oil expansion ✅

b) Store magnetic flux

c) Prevent overload

d) Reduce voltage drop



8. Which connection eliminates third harmonics in the line voltage?

a) Y–Y

b) Δ–Δ ✅

c) Δ–Y

d) Y–Δ



9. A three-phase transformer can be constructed by using:

a) Three single-phase units ✅

b) Two single-phase units

c) Only one core

d) Auto transformer only



10. An explosion vent in transformer is meant for:

a) Cooling improvement

b) Pressure release ✅

c) Voltage control

d) Preventing harmonics



11. On-load tap changer is used for:

a) Changing turns ratio under load ✅

b) No-load voltage variation

c) Fault protection

d) Cooling



12. Grouping of transformers for parallel operation ensures:

a) Correct phase sequence ✅

b) Higher efficiency

c) Less copper loss

d) Reduced voltage



13. Which connection is preferred for industrial loads requiring high starting torque?

a) Y–Y

b) Δ–Δ ✅

c) Y–Δ

d) Δ–Y



14. Cooling arrangement using natural oil circulation and forced air is:

a) ONAN

b) ONAF ✅

c) OFAF

d) OFWF



15. A transformer with Y–Δ connection has a phase shift of:

a) 0°

b) 30° ✅

c) 60°

d) 90°



16. The breather in a transformer contains:

a) Charcoal

b) Silica gel ✅

c) Oil

d) Mercury



17. In Δ–Δ connection, if one transformer fails, the system can run in:

a) Open delta ✅

b) Star connection

c) Double delta

d) V connection



18. ONAN cooling in transformer means:

a) Oil Natural, Air Natural ✅

b) Oil Natural, Air Forced

c) Oil Forced, Air Forced

d) Oil Forced, Air Natural



19. Which is NOT a type of 3-phase transformer connection?

a) Δ–Δ

b) Y–Y

c) Δ–Z ✅

d) Y–Δ



20. A 3-phase transformer with Δ–Y connection is used for:

a) Step-up applications ✅

b) Step-

down applications only

c) Auto transformer operation

d) Testing





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If you want, I can also prepare 10 Two-Marks and 10 Three-Marks application-based questions for these units so you have a full exam set. That way, you’ll have both MCQs and short-answer formats ready for assessments.


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