Q.1 What is the most common cause of bearing failure in induction motors?
Overheating of the stator winding
Improper lubrication
Excessive voltage fluctuations
Loose mounting bolts
Explanation - Insufficient or contaminated lubrication leads to increased friction and wear, which is the primary cause of bearing failures.
Correct answer is: Improper lubrication
Q.2 Which test is used to detect turn-to-turn short circuits in the stator winding?
Megger insulation test
Winding resistance test
Surge voltage test
Polarization index test
Explanation - The surge voltage test applies a high‑frequency voltage to locate low‑impedance faults such as turn-to-turn shorts.
Correct answer is: Surge voltage test
Q.3 A motor makes a loud grinding noise during operation. Which component is most likely at fault?
Stator core
Rotor shaft
Bearings
Cooling fan
Explanation - Grinding noises are typical of bearing wear or damage, especially when lubrication is inadequate.
Correct answer is: Bearings
Q.4 When performing a lock‑ rotor test, what parameter is primarily measured?
Stator resistance
Locked‑rotor current
Slip frequency
Power factor
Explanation - The lock‑ rotor test measures the current drawn when the rotor is prevented from turning, indicating motor starting characteristics.
Correct answer is: Locked‑rotor current
Q.5 Which oil analysis parameter indicates the presence of metal wear particles?
Dielectric breakdown voltage
Viscosity index
Ferrous content
Moisture content
Explanation - Ferrous content measures iron particles in the oil, directly reflecting bearing or gear wear.
Correct answer is: Ferrous content
Q.6 Thermal imaging of a motor shows a hot spot on one phase winding. What is the most likely fault?
Phase imbalance due to a loose connection
Stator core saturation
Rotor bar breakage
Improper ventilation
Explanation - A localized hot spot on a phase indicates excessive current in that phase, often caused by a loose or high‑resistance connection.
Correct answer is: Phase imbalance due to a loose connection
Q.7 Which of the following is NOT a typical symptom of a broken rotor bar in an induction motor?
Increased vibration at 2× supply frequency
Reduced starting torque
Unbalanced line current
Excessive bearing temperature
Explanation - Broken rotor bars affect electromagnetic balance and torque, not directly bearing temperature.
Correct answer is: Excessive bearing temperature
Q.8 During routine maintenance, a technician measures the insulation resistance of a motor winding as 2 MΩ. What action should be taken?
Immediately replace the motor
Continue operation, schedule re‑test in a week
Dry the windings and repeat the test
Inspect for moisture and clean the terminals
Explanation - 2 MΩ is low for a healthy motor; moisture or contamination is likely, so cleaning and drying are first steps.
Correct answer is: Inspect for moisture and clean the terminals
Q.9 What does a high Polarization Index (PI) indicate about motor insulation?
Excellent insulation condition
Presence of moisture
Impending insulation failure
Low dielectric strength
Explanation - A PI greater than 2.0 suggests good insulation with low moisture content.
Correct answer is: Excellent insulation condition
Q.10 A three‑phase motor is observed to have a current imbalance of 15%. Which maintenance step is most appropriate?
Replace the motor immediately
Check and tighten all terminal connections
Increase the motor’s cooling fan speed
Reduce the motor’s load
Explanation - Current imbalance often results from loose or corroded connections; tightening them is the first corrective action.
Correct answer is: Check and tighten all terminal connections
Q.11 Which diagnostic technique uses the frequency spectrum of vibration to locate faults in electrical machines?
Infrared thermography
Motor current signature analysis (MCSA)
Finite element analysis
Acoustic emission testing
Explanation - MCSA analyses current harmonics to detect mechanical and electrical faults, including broken rotor bars.
Correct answer is: Motor current signature analysis (MCSA)
Q.12 What is the purpose of a 'soft starter' in motor applications?
To increase the motor’s maximum speed
To reduce the inrush current during start‑up
To improve power factor during operation
To provide overload protection
Explanation - Soft starters limit the voltage applied at start, thereby reducing the inrush current and mechanical stress.
Correct answer is: To reduce the inrush current during start‑up
Q.13 During a routine inspection, a motor’s fan blades are found to be cracked. What is the most likely consequence if left unaddressed?
Increased stator winding resistance
Reduced cooling efficiency leading to overheating
Higher bearing life
Improved power factor
Explanation - Cracked fan blades impede airflow, decreasing cooling and potentially causing thermal overload of the motor.
Correct answer is: Reduced cooling efficiency leading to overheating
Q.14 Which of the following is a sign of a deteriorating commutator in a DC machine?
Uniform brush wear
Smooth operation at high speed
Arcing at the brushes
Constant line voltage
Explanation - Arcing indicates poor electrical contact due to commutator wear or contamination.
Correct answer is: Arcing at the brushes
Q.15 A transformer shows an increase in no‑load loss after several months of service. Which component is most likely responsible?
Winding insulation degradation
Core lamination heating
Oil leakage
Tap changer malfunction
Explanation - No‑load loss is mainly core loss; increased loss points to higher hysteresis or eddy‑current losses due to lamination heating or deformation.
Correct answer is: Core lamination heating
Q.16 Which method is most effective for locating a partially shorted stator turn in an induction motor?
Megger test
Surge voltage test
Partial discharge detection
Temperature rise test
Explanation - Partial discharge activity is high at locations of turn-to-turn shorts, making it an effective detection method.
Correct answer is: Partial discharge detection
Q.17 What does a high temperature rise in the bearing housing of a motor typically indicate?
Stator winding short circuit
Insufficient lubrication
Excessive load torque
Low supply voltage
Explanation - Poor lubrication increases friction, causing the bearing housing to heat up.
Correct answer is: Insufficient lubrication
Q.18 In a three‑phase induction motor, what is the effect of a single‑phase supply on the motor?
The motor runs at full speed with reduced torque
The motor does not start at all
The motor runs with excessive vibration and heating
The motor runs silently with increased efficiency
Explanation - Single‑phase supply creates a rotating field imbalance, leading to vibration, overheating, and possible damage.
Correct answer is: The motor runs with excessive vibration and heating
Q.19 Which protective device is specifically designed to detect ground faults in rotating machines?
Over‑current relay
Earth leakage relay
Thermal overload relay
Differential protection relay
Explanation - An earth leakage (ground fault) relay senses imbalance caused by leakage currents to ground.
Correct answer is: Earth leakage relay
Q.20 A motor exhibits a steady increase in current during normal load operation. Which fault should be investigated first?
Stator winding shorted turn
Improper bearing lubrication
Loose power factor correction capacitor
Over‑tightened coupling
Explanation - A shorted turn reduces resistance, causing current to rise steadily under load.
Correct answer is: Stator winding shorted turn
Q.21 What is the main advantage of using a vacuum pressure impregnation (VPI) process for motor windings?
Reduces the motor’s weight
Improves thermal conductivity
Increases insulation resistance and moisture resistance
Enhances magnetic flux density
Explanation - VPI fills voids with resin, improving insulation integrity and protecting against moisture ingress.
Correct answer is: Increases insulation resistance and moisture resistance
Q.22 When measuring the insulation resistance of a motor winding with a megohmmeter, what is the recommended test voltage for a 415 V system?
250 V
500 V
1000 V
1500 V
Explanation - The standard test voltage for 415 V equipment is 1 kV to ensure adequate stress on insulation.
Correct answer is: 1000 V
Q.23 Which phenomenon causes a humming noise in a transformer when it is energized but not loaded?
Magnetostriction
Core saturation
Eddy currents
Dielectric loss
Explanation - Magnetostriction causes the core laminations to vibrate at twice the line frequency, producing the hum.
Correct answer is: Magnetostriction
Q.24 A motor’s start capacitor is found to be bulging. What is the most likely consequence if it remains in service?
Improved starting torque
Reduced motor efficiency
Motor may fail to start and cause a fire hazard
Higher power factor during operation
Explanation - A bulging capacitor indicates internal failure; it can short, preventing start and potentially causing overheating.
Correct answer is: Motor may fail to start and cause a fire hazard
Q.25 During a motor inspection, a technician notes discoloration of the stator winding insulation. Which fault is most likely?
Overheating due to excessive current
Improper bearing lubrication
Loose fan blades
Incorrect motor mounting
Explanation - Discoloration (brown or black spots) is a sign of thermal degradation from excessive temperature.
Correct answer is: Overheating due to excessive current
Q.26 Which test can be used to assess the presence of moisture in transformer oil?
Dielectric breakdown voltage test
Viscosity measurement
Acidity (neutralization number) test
Dissolved gas analysis (DGA)
Explanation - Moisture lowers the dielectric strength of oil, reducing the breakdown voltage measured.
Correct answer is: Dielectric breakdown voltage test
Q.27 What is the typical frequency of vibration caused by broken rotor bars in an induction motor?
Supply frequency (f)
Twice the supply frequency (2f)
Slip frequency (s·f)
Rotational speed frequency (n)
Explanation - Broken rotor bars produce a characteristic vibration component at 2× the supply frequency.
Correct answer is: Twice the supply frequency (2f)
Q.28 In a three‑phase motor, what does a high value of 'negative sequence current' indicate?
Balanced three‑phase supply
Presence of a single‑phase fault
Proper motor alignment
Effective power factor correction
Explanation - Negative sequence currents arise from unbalanced conditions such as a single‑phase supply or phase loss.
Correct answer is: Presence of a single‑phase fault
Q.29 Which of the following is the primary cause of excessive stator winding temperature rise in a motor?
High ambient temperature only
Reduced supply voltage
Increased core losses due to high frequency operation
Overloading beyond rated current
Explanation - Excess current causes I²R losses in windings, leading to temperature rise beyond design limits.
Correct answer is: Overloading beyond rated current
Q.30 A DC motor shows sparking at the brushes during operation. Which corrective action should be taken first?
Replace the armature winding
Adjust brush pressure and replace worn brushes
Increase the supply voltage
Lubricate the commutator surface
Explanation - Improper brush pressure or worn brushes cause sparking; adjusting and replacing them is the first step.
Correct answer is: Adjust brush pressure and replace worn brushes
Q.31 What is the effect of using a higher‑grade insulation class (e.g., Class H vs. Class B) in motor windings?
Decreases motor efficiency
Increases allowable temperature rise
Reduces motor torque
Improves power factor
Explanation - Higher insulation classes tolerate higher temperatures, extending motor life under heavy load.
Correct answer is: Increases allowable temperature rise
Q.32 Which instrument is commonly used to detect partial discharge activity in high‑voltage equipment?
Clamp‑on ammeter
Partial discharge detector (PD meter)
Megohmmeter
Wattmeter
Explanation - A PD meter captures the transient discharge signals that indicate insulation defects.
Correct answer is: Partial discharge detector (PD meter)
Q.33 A motor’s VFD (Variable Frequency Drive) shows an error code indicating 'overvoltage' on the output. Which of the following is a likely cause?
Faulty motor windings
Incorrect DC bus capacitor rating
Loose input power connection
Improper grounding of the motor
Explanation - Undersized DC bus capacitors can cause voltage spikes on the inverter output, leading to overvoltage errors.
Correct answer is: Incorrect DC bus capacitor rating
Q.34 During routine maintenance, a technician finds that the motor's slip ring contacts are pitted. What is the most appropriate remedy?
Replace the slip rings and brushes
Increase the motor load
Adjust the motor's alignment
Lubricate the slip rings with oil
Explanation - Pitting indicates severe wear; replacement restores reliable electrical contact.
Correct answer is: Replace the slip rings and brushes
Q.35 Which of the following cooling methods is typically used for large power transformers?
Air natural (AN)
Oil forced (OF)
Water spray (WS)
Helium gas cooling (HG)
Explanation - Oil forced cooling circulates oil under pressure to remove heat efficiently in large transformers.
Correct answer is: Oil forced (OF)
Q.36 What does a high value of the 'Dissolved Gas Ratio' (e.g., CH₄/H₂) in transformer oil indicate?
Normal aging
Partial discharge activity
Low load operation
High moisture content
Explanation - Certain gas ratios, such as elevated methane relative to hydrogen, are signatures of partial discharge.
Correct answer is: Partial discharge activity
Q.37 A synchronous generator shows a sudden drop in terminal voltage while the load remains unchanged. Which component is most likely at fault?
Exciter winding
Stator core
Cooling fan
Bearings
Explanation - The exciter supplies field current; failure reduces the generated EMF, causing voltage drop.
Correct answer is: Exciter winding
Q.38 Which method is used to measure the eccentricity of the rotor in a squirrel‑cage induction motor?
Stator current signature analysis
Vibration analysis at 1× supply frequency
Insulation resistance test
Thermal imaging
Explanation - Rotor eccentricity causes vibration at the supply frequency, detectable via vibration analysis.
Correct answer is: Vibration analysis at 1× supply frequency
Q.39 During a transformer oil test, the dielectric dissipation factor (tan δ) is found to be high. What does this suggest?
Oil has low viscosity
Oil contains high moisture content
Oil is free from contaminants
Oil has excellent insulating properties
Explanation - A high dissipation factor indicates increased dielectric losses, often due to moisture or contaminants.
Correct answer is: Oil contains high moisture content
Q.40 What is the main function of a Buchholz relay in oil‑filled power transformers?
Detect over‑current conditions
Provide over‑temperature protection
Detect gas accumulation from internal faults
Control oil circulation
Explanation - The Buchholz relay senses gas bubbles generated by faults like arcing or insulation breakdown.
Correct answer is: Detect gas accumulation from internal faults
Q.41 A motor is operating at a temperature 20 °C above its rated limit. Which immediate action should be taken?
Increase the motor load
Reduce the motor speed
Shut down the motor and investigate cooling
Apply a higher voltage to compensate
Explanation - Over‑temperature can quickly damage insulation; immediate shutdown prevents further damage.
Correct answer is: Shut down the motor and investigate cooling
Q.42 Which fault in a three‑phase induction motor causes a high negative‑sequence current and a characteristic 3‑phase vibration pattern?
Stator winding short circuit
Rotor bar breakage
Phase-to-phase short circuit
Unbalanced supply voltage
Explanation - Unbalanced supply creates negative‑sequence components, leading to distinctive vibration and current signatures.
Correct answer is: Unbalanced supply voltage
Q.43 When conducting a winding resistance test, why must the motor be at a stable temperature?
Resistance varies with temperature, affecting accuracy
Heat improves contact resistance
Cold temperature increases insulation breakdown
Temperature does not affect resistance
Explanation - Metal resistance changes with temperature; a stable temperature ensures repeatable measurements.
Correct answer is: Resistance varies with temperature, affecting accuracy
Q.44 Which of the following indicates a possible problem with the cooling system of a large motor?
Higher than normal bearing temperature only
Uniform temperature rise across all phases
Localized hot spot on the motor casing
Reduced line current
Explanation - A hot spot suggests insufficient airflow or a blocked vent in the cooling system.
Correct answer is: Localized hot spot on the motor casing
Q.45 In transformer maintenance, what does the term 'K factor' refer to?
Core loss factor related to harmonics
Oil viscosity at 40 °C
Maximum short‑circuit current rating
Thermal expansion coefficient
Explanation - K factor quantifies the effect of harmonic currents on transformer heating.
Correct answer is: Core loss factor related to harmonics
Q.46 A motor’s start capacitor has an ESR (Equivalent Series Resistance) that is higher than specified. What is the likely impact?
Increased starting torque
Reduced starting torque and possible failure to start
Higher efficiency during normal operation
No impact on motor performance
Explanation - Higher ESR limits the capacitor’s ability to supply reactive power needed for starting.
Correct answer is: Reduced starting torque and possible failure to start
Q.47 What is the primary purpose of the 'tap changer' in a power transformer?
To adjust the voltage ratio for load variations
To increase the transformer’s cooling capacity
To improve magnetic flux density
To detect oil contamination
Explanation - Tap changers allow on‑load voltage regulation by changing the effective turns ratio.
Correct answer is: To adjust the voltage ratio for load variations
Q.48 Which of the following is a common sign of rotor eccentricity in a three‑phase motor?
Uneven bearing wear
Uniform temperature rise across all phases
Increased line voltage
Reduced line current
Explanation - Eccentricity leads to uneven loading of bearings, causing asymmetric wear patterns.
Correct answer is: Uneven bearing wear
Q.49 During a motor's no‑load test, the measured power factor is 0.2. What does this indicate?
High efficiency operation
Excessive core losses
Low mechanical load
Improper voltage supply
Explanation - A low power factor at no‑load signifies high reactive power draw, typically due to core losses.
Correct answer is: Excessive core losses
Q.50 Which condition can cause a sudden increase in the stator winding temperature of a motor without a change in load?
Ambient temperature drop
Voltage dip (sag)
Phase loss
Improved ventilation
Explanation - Loss of a phase forces the motor to draw higher current in the remaining phases, raising temperature.
Correct answer is: Phase loss
Q.51 What does the term 'thermal runaway' refer to in the context of transformer operation?
Rapid increase in oil temperature due to cooling fan failure
Self‑accelerating temperature rise caused by increased losses
Sudden drop in voltage output
Increase in load demand
Explanation - Thermal runaway occurs when rising temperature increases losses, further raising temperature in a feedback loop.
Correct answer is: Self‑accelerating temperature rise caused by increased losses
Q.52 A DC motor shows a gradual reduction in speed under constant load. Which component is most likely degrading?
Field winding insulation
Brushes
Commutator surface
Armature winding resistance
Explanation - Worn brushes increase contact resistance, causing voltage drop and speed reduction.
Correct answer is: Brushes
Q.53 Which test is recommended to assess the condition of transformer oil before a major refurbishment?
Furan content analysis
Short‑circuit test
Open‑circuit test
Insulation resistance test
Explanation - Furan compounds indicate cellulose degradation in the transformer, providing insight into oil condition.
Correct answer is: Furan content analysis
Q.54 When a motor’s current waveform shows pronounced spikes at harmonics of the supply frequency, what is the likely cause?
Mechanical misalignment
Loose electrical connections
Harmonic distortion from non‑linear loads
Insufficient bearing lubrication
Explanation - Non‑linear loads inject harmonics into the supply, which appear as spikes in motor current.
Correct answer is: Harmonic distortion from non‑linear loads
Q.55 Which maintenance practice helps prevent stator winding corrosion in marine environments?
Applying silicone grease on windings
Using marine‑grade epoxy coating
Increasing the motor’s operating temperature
Installing a larger cooling fan
Explanation - Epoxy coating provides a barrier against salt‑water corrosion.
Correct answer is: Using marine‑grade epoxy coating
Q.56 In a transformer, which component primarily determines the short‑circuit impedance?
Core material
Winding configuration
Oil dielectric strength
Tap changer position
Explanation - Short‑circuit impedance is governed by the resistance and leakage reactance of the windings.
Correct answer is: Winding configuration
Q.57 A motor’s vibration spectrum shows a dominant peak at 4× the supply frequency. Which fault is most likely?
Rotor eccentricity
Broken rotor bar
Stator core looseness
Unbalanced supply voltage
Explanation - Stator core looseness typically generates vibration components at multiples of the supply frequency, especially 4×.
Correct answer is: Stator core looseness
Q.58 Which of the following is NOT a typical symptom of a failed motor overload relay?
Motor shuts down unexpectedly under normal load
Motor runs continuously without protection
Trip button does not reset
Motor operates at reduced efficiency
Explanation - A failed overload relay usually causes protection loss, not efficiency change.
Correct answer is: Motor operates at reduced efficiency
Q.59 What is the main advantage of using a sealed (hermetic) motor over an open‑type motor in dusty environments?
Higher torque output
Reduced cooling requirements
Protection against dust ingress
Lower cost
Explanation - Sealed motors prevent dust from entering the interior, preserving insulation and bearing life.
Correct answer is: Protection against dust ingress
Q.60 During a motor start, the voltage dip (sag) observed on the supply line is mainly caused by:
Inrush current drawn by the motor
Capacitor charging current
Harmonic distortion
Voltage regulator malfunction
Explanation - Motor start draws a large inrush current, causing a temporary voltage dip on the supply.
Correct answer is: Inrush current drawn by the motor
Q.61 Which type of transformer winding is most prone to inter‑turn short circuits due to insulation failure?
Low‑voltage winding
High‑voltage winding
Neutral winding
Tertiary winding
Explanation - Higher voltage stress makes the high‑voltage winding more susceptible to insulation breakdown between turns.
Correct answer is: High‑voltage winding
Q.62 A motor exhibits an unusual humming noise that changes with load. Which diagnostic technique is best suited to pinpoint the source?
Acoustic emission testing
Infrared thermography
Megger test
Oil dielectric strength test
Explanation - Acoustic emission can detect and locate sources of vibration‑related noises across load conditions.
Correct answer is: Acoustic emission testing
Q.63 In transformer maintenance, why is it important to keep oil temperature below 70 °C during testing?
To avoid oil flash point exceedance
To maintain accurate dielectric measurements
To prevent oil evaporation
To reduce viscosity for better flow
Explanation - Dielectric properties are temperature‑dependent; keeping oil below 70 °C ensures consistent test results.
Correct answer is: To maintain accurate dielectric measurements
Q.64 Which of the following is a common cause of stator winding overheating in a motor that has been operating for several years?
Improper bearing alignment
Progressive insulation degradation
Excessive fan speed
Over‑tightened mounting bolts
Explanation - Aging insulation increases dielectric losses, leading to higher temperature in the stator winding.
Correct answer is: Progressive insulation degradation
Q.65 When performing a 'turn ratio' test on a transformer, what is being verified?
The voltage rating of the windings
The exact ratio of turns between primary and secondary
The oil dielectric strength
The temperature rise of the core
Explanation - Turn ratio tests confirm that the primary‑to‑secondary turns ratio matches design specifications.
Correct answer is: The exact ratio of turns between primary and secondary
Q.66 A motor’s VFD indicates a 'fault 31' error, which corresponds to an over‑temperature condition. What should be checked first?
Cooling fan operation
Motor shaft alignment
Supply voltage level
Brush wear
Explanation - Over‑temperature is most often caused by inadequate cooling; checking the fan is the first step.
Correct answer is: Cooling fan operation
Q.67 Which of the following is a reliable indicator of an impending bearing failure in a rotating machine?
Slight increase in line current
Increase in vibration amplitude at bearing frequency
Slight drop in motor temperature
Reduced acoustic noise
Explanation - Vibration at the bearing’s natural frequency grows as the bearing degrades.
Correct answer is: Increase in vibration amplitude at bearing frequency
Q.68 What is the main purpose of a 'surge arrester' installed on high‑voltage motor terminals?
To limit over‑current during start‑up
To protect against voltage transients and lightning strikes
To improve power factor
To reduce harmonics
Explanation - Surge arresters clamp voltage spikes, protecting equipment from transient over‑voltages.
Correct answer is: To protect against voltage transients and lightning strikes
Q.69 During routine inspection, a technician finds that the motor's shaft end play is beyond tolerance. What is the most likely consequence?
Reduced magnetic flux
Increased mechanical vibrations and possible bearing damage
Higher efficiency
Lower line current
Explanation - Excessive end play leads to misalignment, causing vibration and bearing wear.
Correct answer is: Increased mechanical vibrations and possible bearing damage
Q.70 In a transformer, what does a high 'load loss' compared to design specifications usually indicate?
Excessive core losses
Poor winding quality or high resistance
Insufficient oil level
Improper tap changer setting
Explanation - Load loss is primarily due to I²R losses in windings; higher than expected loss suggests higher resistance.
Correct answer is: Poor winding quality or high resistance
Q.71 Which of the following is an early sign of a developing inter‑turn fault in a motor stator?
Sudden drop in line voltage
Increased harmonic distortion in the motor current
Rapid rise in bearing temperature
Reduced fan speed
Explanation - Inter‑turn faults introduce low‑frequency harmonics into the current spectrum.
Correct answer is: Increased harmonic distortion in the motor current
Q.72 When a transformer is subjected to a short‑circuit test, what parameter is primarily measured?
Open‑circuit voltage
Short‑circuit impedance
Core loss
Oil dielectric strength
Explanation - The test determines the transformer’s impedance under short‑circuit conditions.
Correct answer is: Short‑circuit impedance
Q.73 A motor’s insulation resistance has dropped from 5 MΩ to 0.5 MΩ after a year of operation. What is the most appropriate immediate action?
Replace the motor
Dry the motor and repeat the test after 24 hours
Increase the motor’s load
Apply a higher voltage during testing
Explanation - Moisture may have caused the drop; drying and retesting will confirm if the problem persists.
Correct answer is: Dry the motor and repeat the test after 24 hours
Q.74 Which diagnostic technique can directly detect rotor bar cracks without disassembly?
Stator winding resistance test
Motor current signature analysis (MCSA)
Thermal imaging of the stator
Oil analysis
Explanation - MCSA identifies characteristic sideband frequencies associated with rotor bar defects.
Correct answer is: Motor current signature analysis (MCSA)
Q.75 In a three‑phase motor, which condition leads to a negative‑sequence voltage component?
Balanced three‑phase supply
Phase loss or voltage unbalance
Ideal sinusoidal voltage
Proper motor grounding
Explanation - Unbalanced or missing phases generate negative‑sequence components.
Correct answer is: Phase loss or voltage unbalance
Q.76 What is the primary benefit of using a 'dual‑circuit' cooling system in large synchronous generators?
Higher output voltage
Redundancy and better heat removal
Reduced electromagnetic interference
Simplified maintenance
Explanation - Dual‑circuit systems provide backup cooling and more efficient heat dissipation.
Correct answer is: Redundancy and better heat removal
Q.77 Which of the following is a typical symptom of a failing excitation system in a synchronous generator?
Stable terminal voltage under varying loads
Sudden increase in stator winding temperature
Consistent power factor
Reduced mechanical vibration
Explanation - A weak excitation reduces generated EMF, causing higher currents and increased heating.
Correct answer is: Sudden increase in stator winding temperature
Q.78 During motor troubleshooting, a technician notes a 'spike' in line current at the instant of start. Which component is most likely responsible?
Thermal overload relay
Start capacitor
Bearing housing
Cooling fan
Explanation - A faulty start capacitor can cause excessive inrush current during motor start.
Correct answer is: Start capacitor
Q.79 Which test is most effective for detecting moisture in transformer oil?
Dielectric breakdown voltage test
Moisture content (Karl Fischer) test
Furan analysis
Acidity (neutralization number) test
Explanation - Karl Fischer titration accurately measures water concentration in oil.
Correct answer is: Moisture content (Karl Fischer) test
Q.80 A motor exhibits a gradual increase in line current over several hours of continuous operation, with no change in load. Which fault is most probable?
Stator winding temperature rise
Bearing wear
Supply voltage increase
Fan speed reduction
Explanation - As the winding temperature rises, resistance decreases slightly, causing a gradual current increase.
Correct answer is: Stator winding temperature rise
Q.81 Which of the following maintenance activities is essential before performing any electrical test on a motor?
Lubricating the bearings
Removing all external loads
Cooling the motor to ambient temperature
Disconnecting the motor from the power supply
Explanation - Safety requires power isolation before any electrical testing.
Correct answer is: Disconnecting the motor from the power supply
Q.82 What is the primary reason for installing a 'temperature sensor' on the stator winding of a large motor?
To monitor oil level
To detect early winding overheating
To measure ambient temperature
To control fan speed
Explanation - Stator temperature sensors provide real‑time data to prevent insulation damage.
Correct answer is: To detect early winding overheating
Q.83 Which of the following is a typical consequence of a loose connection at a motor terminal block?
Reduced motor speed
Increased line voltage
Localized heating and possible arcing
Improved power factor
Explanation - Loose contacts cause high resistance, leading to heating and arcing.
Correct answer is: Localized heating and possible arcing
Q.84 In transformer diagnostics, a high level of dissolved acetylene gas typically indicates:
Low load operation
Partial discharge activity
Oil contamination with water
Mechanical vibration
Explanation - Acetylene is a common by‑product of partial discharge within oil‑filled equipment.
Correct answer is: Partial discharge activity
Q.85 Which protective device trips when the motor temperature exceeds a preset value?
Over‑current relay
Thermal overload relay
Earth leakage relay
Voltage regulator
Explanation - Thermal overload relays monitor temperature or current heating and trip upon overload.
Correct answer is: Thermal overload relay
Q.86 A motor's rated speed is 1800 rpm, but it consistently runs at 1750 rpm under full load. Which factor is most likely responsible?
Slight voltage drop on supply
Improper bearing lubrication
Excessive fan speed
Higher ambient temperature
Explanation - Reduced supply voltage lowers the motor’s developed torque, causing a small speed drop under load.
Correct answer is: Slight voltage drop on supply
Q.87 During a routine inspection, a transformer’s bushing shows evidence of corona discharge (brownish deposits). What is the primary cause?
Excessive oil pressure
High electric field stress at the bushing tip
Improper grounding
Low ambient temperature
Explanation - Corona occurs when the electric field exceeds the breakdown strength of surrounding gas.
Correct answer is: High electric field stress at the bushing tip
Q.88 Which of the following is an advantage of using a permanent‑magnet synchronous motor (PMSM) over an induction motor for variable‑speed applications?
Higher starting torque without a starter
Lower cost of construction
Simpler control circuitry
Reduced magnetic losses at high speed
Explanation - PMSMs produce high torque at low speeds without the need for external starting devices.
Correct answer is: Higher starting torque without a starter
Q.89 Which fault typically produces a 1× supply frequency component in the vibration spectrum of a motor?
Broken rotor bar
Stator core looseness
Unbalanced supply voltage
Bearing defect
Explanation - Loose core laminations vibrate at the supply frequency, showing a 1× component.
Correct answer is: Stator core looseness
Q.90 What is the main purpose of the 'KVA rating' printed on a motor nameplate?
Indicates the motor’s speed
Specifies the motor’s power consumption at full load
Defines the motor’s insulation class
Shows the motor’s efficiency
Explanation - KVA rating represents the apparent power the motor draws when operating at rated conditions.
Correct answer is: Specifies the motor’s power consumption at full load
Q.91 During a motor start, a sudden drop in line voltage is observed, followed by a rapid restoration. Which component most likely caused this transient?
Over‑current relay
Soft starter
Contactors
Capacitor bank
Explanation - Contactors draw large inrush currents when closing, causing a temporary voltage dip.
Correct answer is: Contactors
Q.92 Which test is used to verify the integrity of the motor’s earth (ground) connection?
Continuity test with a low‑ohm meter
Megger insulation test
Thermal imaging
Vibration analysis
Explanation - A simple continuity test confirms a low‑resistance path to ground.
Correct answer is: Continuity test with a low‑ohm meter
Q.93 A motor’s torque output has decreased by about 15 % while current and voltage remain unchanged. Which fault is most consistent with this observation?
Stator winding short circuit
Rotor bar breakage
Bearing overheating
Loose fan blade
Explanation - Broken rotor bars reduce the effective number of active conductors, lowering torque without markedly affecting current.
Correct answer is: Rotor bar breakage
Q.94 When a transformer is oil‑filled and has a high level of dissolved oxygen, what risk does this pose?
Accelerated oxidation of the oil
Increased dielectric strength
Reduced magnetic flux
Improved cooling efficiency
Explanation - Oxygen promotes oil oxidation, leading to sludge formation and reduced dielectric properties.
Correct answer is: Accelerated oxidation of the oil
Q.95 Which of the following is a primary symptom of a failing commutator in a DC motor?
Smooth operation at all speeds
Excessive sparking and uneven wear of brushes
Reduced line voltage
Higher power factor
Explanation - A worn or pitted commutator causes irregular contact, resulting in sparking and uneven brush wear.
Correct answer is: Excessive sparking and uneven wear of brushes
Q.96 Which diagnostic method can detect the presence of partial discharge in a transformer without extracting oil samples?
Acoustic emission monitoring
Dissolved gas analysis (DGA)
Oil dielectric breakdown test
Temperature rise test
Explanation - Acoustic sensors can pick up the ultrasonic emissions generated by partial discharge activity.
Correct answer is: Acoustic emission monitoring
Q.97 A motor’s starting current is measured at 6 times its full‑load current, which is higher than the typical 5‑times value. Which issue is most likely?
Undersized start capacitor
Excessive bearing clearance
Low supply voltage
High ambient temperature
Explanation - An inadequate start capacitor cannot supply enough reactive power, causing higher inrush current.
Correct answer is: Undersized start capacitor
Q.98 During a routine check, a motor's fan belt is found to be slipping. What effect does this have on the motor?
Improved cooling efficiency
Increased motor temperature due to reduced cooling
Higher torque output
Reduced line current
Explanation - A slipping fan belt reduces airflow, leading to higher operating temperatures.
Correct answer is: Increased motor temperature due to reduced cooling
Q.99 Which of the following is NOT a typical cause of excessive vibration in an induction motor?
Rotor bar breakage
Unbalanced supply voltage
Loose stator bolts
Correctly sized fan
Explanation - A correctly sized fan does not cause vibration; the other options are common vibration sources.
Correct answer is: Correctly sized fan
Q.100 What is the function of a 'phase failure relay' in a three‑phase motor circuit?
Detect and trip on loss of any phase
Balance the three phases automatically
Increase motor speed
Reduce harmonic distortion
Explanation - Phase failure relays monitor each phase and disconnect the motor if a phase is lost.
Correct answer is: Detect and trip on loss of any phase
Q.101 When a transformer is subjected to a load test, which parameter is primarily observed to assess efficiency?
Open‑circuit voltage
Core loss
Copper loss (I²R)
Oil temperature
Explanation - Load losses are mainly copper (I²R) losses; measuring them under load determines efficiency.
Correct answer is: Copper loss (I²R)
Q.102 A motor's bearing temperature rises rapidly after a sudden increase in load. Which immediate action is recommended?
Increase the motor speed
Reduce the load to rated level
Add more oil to the bearings
Turn off the cooling fan
Explanation - Lowering the load reduces mechanical stress and allows bearings to cool.
Correct answer is: Reduce the load to rated level
Q.103 Which of the following indicates that a motor's insulation may be compromised by chemical contamination?
High dielectric breakdown voltage
Low insulation resistance that improves after drying
Stable insulation resistance over time
No visible oil leaks
Explanation - Chemical contaminants often attract moisture, reducing resistance; drying restores it partially.
Correct answer is: Low insulation resistance that improves after drying
Q.104 In a three‑phase induction motor, what is the effect of a small amount of voltage unbalance (e.g., 2 %) on motor life?
Negligible effect
Significant reduction in motor lifespan
Improved efficiency
Reduced starting current
Explanation - Even minor voltage unbalance increases heating and reduces motor life.
Correct answer is: Significant reduction in motor lifespan
Q.105 Which type of transformer is most suitable for stepping down high voltage to medium voltage in a substation?
Autotransformer
Isolation transformer
Three‑phase power transformer
Current transformer
Explanation - Three‑phase power transformers are designed for high‑voltage to medium‑voltage conversion in substations.
Correct answer is: Three‑phase power transformer
Q.106 During a motor’s lock‑rotor test, the measured slip is 0.05. What does this indicate about the motor?
High efficiency at rated load
Poor starting performance
High slip at full load
Low mechanical losses
Explanation - A high lock‑rotor slip suggests the motor requires more current to start, indicating possible starting issues.
Correct answer is: Poor starting performance
Q.107 Which of the following is a primary benefit of performing a 'partial discharge' test on a newly commissioned transformer?
To verify oil level
To ensure the core is correctly magnetized
To detect insulation defects before they cause failure
To calibrate voltage taps
Explanation - Partial discharge testing reveals early insulation faults, allowing corrective actions before breakdown.
Correct answer is: To detect insulation defects before they cause failure
Q.108 A motor's nameplate shows a service factor of 1.15. What does this mean?
The motor can operate 15 % above its rated horsepower continuously
The motor must be run at 85 % of its rated load
The motor's efficiency is 115 %
The motor's voltage rating is increased by 15 %
Explanation - A service factor >1 permits the motor to handle overloads up to the specified percentage.
Correct answer is: The motor can operate 15 % above its rated horsepower continuously
Q.109 Which protective device is specifically designed to detect excessive heating in a motor’s stator winding?
Thermal overload relay
Magnetic circuit breaker
Earth fault relay
Phase loss relay
Explanation - Thermal overload relays monitor winding temperature through current integration and trip on overheating.
Correct answer is: Thermal overload relay
Q.110 If a motor’s VFD displays an error indicating 'DC bus over‑voltage', which component is most likely at fault?
Input line filter
DC bus capacitor
Motor rotor
Output filter
Explanation - Faulty or aged DC bus capacitors can cause voltage spikes on the DC bus.
Correct answer is: DC bus capacitor
Q.111 During a vibration analysis, a peak at 0.5× supply frequency is observed. Which fault does this most likely indicate?
Rotor bar breakage
Loose stator core laminations
Bearing defect
Unbalanced voltage
Explanation - Loose laminations often generate vibration at half the supply frequency.
Correct answer is: Loose stator core laminations
Q.112 Which of the following actions helps to extend the life of a motor’s insulation system?
Operating the motor at maximum load continuously
Maintaining proper cooling and ventilation
Increasing the motor’s operating temperature
Using low‑quality oil for cooling
Explanation - Adequate cooling reduces thermal stress on insulation, prolonging its life.
Correct answer is: Maintaining proper cooling and ventilation
Q.113 A motor’s current waveform shows a pronounced notch at the fundamental frequency. What is the most probable cause?
Harmonic distortion from non‑linear loads
Rotor eccentricity
Loose brush contact
Improper grounding
Explanation - Eccentricity causes modulation of the fundamental current, resulting in notches.
Correct answer is: Rotor eccentricity