Electrical Troubleshooting MCQ Quiz — Part 2 of 5

Motors & Starters | 50 Practice Questions (Q51–Q100 of 250) | For ITI Electrician, Wireman, Diploma Electrical/EEE and industrial electrical maintenance trainees

Topic Focus: Motors & Starters
Covers induction motor theory, DOL/star-delta starting, overload relays (including IEC 60947-4-1 trip classes), single-phasing, bearing faults, megger/insulation-resistance testing and insulation classes.
This is Part 2 of a 5-part, 250-question Electrical Troubleshooting practice series covering real-world fault-diagnosis scenarios.

Question Palette (tap any number to jump)

Source basis: Standard industrial electrical maintenance principles and troubleshooting methods as taught in ITI/Diploma electrical curricula, with reference to recognised practice such as IS 3043 (earthing), IS 732 (wiring colour code) and IEC 60947-4-1 (overload relay trip classes) where applicable. Prepared for practice and skill-building purposes; always follow your equipment manufacturer's manual and workplace safety procedures.

About this practice set

This page is Part 2 of 5 in the Electrical Troubleshooting MCQ Quiz series. It contains 50 multiple-choice questions (Q51 to Q100 of the 250-question set) on Motors & Starters. The questions are written for ITI, diploma and B.Tech trainees and for anyone preparing for a maintenance or fault-finding interview. Each question has four options; the quiz above checks your answer and shows an explanation, and the full answer key is printed below so you can also revise without running the quiz.

How to use this quiz

Attempt the questions in one sitting without searching for answers.
Read the explanation shown after each answer, even when you were right.
Write down the questions you missed and the reason in your own words.
Open the answer key below to revise those questions.
Retake the quiz after a day or two and compare your score.

Sample of what is covered

The basic working principle of a three-phase induction motor relies on:
The main purpose of a star-delta starter is to:
The typical starting (inrush) current of a standard induction motor, compared to its full load running current, is approximately:
If a running three-phase motor experiences single-phasing, the most likely consequence is:
A megger (insulation resistance tester) is primarily used to:
To reverse the direction of rotation of a three-phase induction motor, a technician should:
Voltage unbalance across the three phases supplying an induction motor is particularly harmful because it:
Shaft misalignment between a motor and its driven equipment (pump, gearbox, etc.) primarily causes:

Answer key with explanations (50 questions)

Q1. The basic working principle of a three-phase induction motor relies on:

A. A rotating magnetic field in the stator inducing current in the rotor by electromagnetic induction ✓ Correct answer

B. Direct electrical connections between stator and rotor windings

C. Permanent magnets fixed on the rotor

D. A commutator switching current direction in the rotor

Explanation: The three stator windings, fed by three-phase AC, create a rotating magnetic field that induces EMF and current in the rotor bars, producing torque by Faraday's law.

Q2. The synchronous speed of a three-phase induction motor is calculated using the formula:

A. Ns = 120f / P ✓ Correct answer

B. Ns = f / 120P

C. Ns = P / 120f

D. Ns = 120P / f

Explanation: Synchronous speed in RPM equals 120 times the supply frequency divided by the number of poles of the motor.

Q3. Slip in an induction motor is defined as:

A. The relative difference between synchronous speed and actual rotor speed, expressed as a fraction of synchronous speed ✓ Correct answer

B. The difference between line voltage and phase voltage

C. The angle between stator and rotor windings

D. The ratio of starting current to full load current

Explanation: Slip = (Ns - N) / Ns, representing how much the rotor lags behind the rotating magnetic field, which is essential for inducing rotor current.

Q4. Why does an induction motor never actually run at exact synchronous speed?

A. Because relative motion between the rotating field and rotor is required to induce the rotor EMF and current that produce torque ✓ Correct answer

B. Because the stator windings are designed to prevent it

C. Because the supply frequency is unstable

D. Because the rotor bars are non-magnetic

Explanation: If the rotor reached synchronous speed there would be no relative motion between it and the rotating field, so no EMF would be induced and torque would drop to zero, causing the motor to slow down.

Q5. What is the key construction difference between a squirrel cage rotor and a slip ring (wound) rotor?

A. A squirrel cage rotor has short-circuited bars with no external connections, while a slip ring rotor has windings brought out via slip rings for external resistance control ✓ Correct answer

B. A squirrel cage rotor always runs on DC supply

C. A slip ring rotor has no bearings

D. A squirrel cage rotor requires brushes for operation

Explanation: Squirrel cage rotors are simple, rugged, and maintenance-free with permanently shorted bars, whereas slip ring rotors allow external resistance to be added to the rotor circuit via brushes and slip rings for starting torque control.

Q6. A Direct-On-Line (DOL) starter is most suitable for:

A. Relatively small motors where the high starting current does not cause unacceptable voltage dips or mechanical shock ✓ Correct answer

B. Only very large motors above 100 HP

C. Motors that must start with zero inrush current

D. Motors operating exclusively on DC supply

Explanation: DOL starting connects the motor directly to full supply voltage, drawing high inrush current, so it is generally limited to smaller motors where this surge is acceptable to the supply system.

Q7. The main purpose of a star-delta starter is to:

A. Reduce the starting current drawn by the motor by initially connecting the windings in star before switching to delta ✓ Correct answer

B. Increase the starting torque of the motor beyond DOL levels

C. Convert a single-phase motor into a three-phase motor

D. Permanently run the motor in star connection at all times

Explanation: Starting in star reduces the voltage across each winding to 1/√3 of line voltage, cutting starting current substantially before the motor transitions to full delta running connection.

Q8. Compared to DOL starting, a star-delta starter typically reduces the line starting current to approximately:

A. One-third (1/3) of the DOL starting current ✓ Correct answer

B. Two-thirds (2/3) of the DOL starting current

C. The same as DOL starting current

D. Ten times the DOL starting current

Explanation: Since starting current is proportional to the square of the voltage applied per winding, and star gives 1/√3 voltage, the resulting line current works out to about 1/3 of the direct-on-line value.

Q9. Compared to DOL starting, the starting torque produced by a star-delta starter during the star phase is approximately:

A. One-third (1/3) of the DOL starting torque ✓ Correct answer

B. Equal to the DOL starting torque

C. Three times the DOL starting torque

D. Double the DOL starting torque

Explanation: Torque is proportional to the square of the applied voltage, so the reduced per-winding voltage in star connection also reduces available starting torque to about a third of DOL.

Q10. In a star-delta starter, the timer's main function is to:

A. Control the time delay before switching the motor connection from star to delta after starting ✓ Correct answer

B. Measure the total running hours of the motor

C. Trip the overload relay after a fixed time

D. Control the motor's rotational direction

Explanation: The timer allows the motor to accelerate in the reduced-current star mode for a set period before automatically switching contactors to connect the motor in delta for full running.

Q11. Star-delta starting can only be used on motors that are:

A. Designed to run normally in delta connection at the supply line voltage (i.e., dual voltage rated, e.g. 415/240V) ✓ Correct answer

B. Designed only for DC operation

C. Single-phase induction motors

D. Rated for star connection as their only operating mode

Explanation: The motor nameplate must show delta connection matches the supply line voltage, since the motor runs permanently in delta after the starting sequence completes.

Q12. A major drawback of high inrush starting current in large motors on a weak supply network is:

A. It can cause a significant voltage dip, affecting other connected equipment ✓ Correct answer

B. It permanently increases the supply frequency

C. It automatically improves the power factor

D. It reduces the resistance of the supply cables

Explanation: A large sudden current draw causes a voltage drop across the source impedance, which can dim lights and disturb other sensitive loads on the same network.

Q13. The typical starting (inrush) current of a standard induction motor, compared to its full load running current, is approximately:

A. 5 to 7 times the full load current ✓ Correct answer

B. Equal to the full load current

C. Less than the full load current

D. 50 times the full load current

Explanation: At standstill, an induction motor behaves like a short-circuited transformer, drawing a high inrush current typically 5 to 7 times its rated full load current until it accelerates.

Q14. The primary purpose of an overload relay in a motor starter is to:

A. Protect the motor from sustained overcurrent that could cause winding damage due to overheating ✓ Correct answer

B. Provide short-circuit protection against extremely high fault currents

C. Reverse the direction of motor rotation

D. Reduce the starting current of the motor

Explanation: Overload relays monitor motor current continuously and disconnect the motor if it draws excessive current for too long, protecting the windings from thermal damage, while fuses/MCCBs handle short-circuit faults.

Q15. A common thermal overload relay operates on the principle of:

A. A bimetallic strip bending due to heat generated by excess current, which trips a mechanism ✓ Correct answer

B. A magnetic field instantly opening a contact at any current above rated

C. A timer counting down a fixed duration regardless of current level

D. A capacitor discharging to trigger a trip

Explanation: Current passing through heater elements warms a bimetallic strip, which bends as it heats and mechanically trips the relay contacts once it deflects enough, mimicking motor thermal behavior.

Q16. An overload relay for a motor should normally be set to a current value:

A. Equal to the motor's rated full load current as given on its nameplate ✓ Correct answer

B. Double the motor's rated full load current

C. Equal to the motor's starting (locked rotor) current

D. Half the supply transformer's rated current

Explanation: Setting the overload close to the nameplate full load current ensures the motor is protected from sustained overload while allowing normal rated operation without nuisance tripping.

Q17. Overload relay trip Class 10 (per IEC 60947-4-1) means the relay is designed to trip within approximately:

A. 10 seconds at 6 times (600%) the relay's rated current ✓ Correct answer

B. 10 minutes at rated current

C. 10 seconds at rated current only

D. 10 hours at locked rotor current

Explanation: Trip class defines the maximum time the relay allows before tripping at 600% of set current; Class 10 trips fastest (within 10 s), suited for standard-starting motors, while Class 20 and 30 allow longer starting times for higher-inertia loads.

Q18. Single-phasing of a three-phase motor refers to the condition where:

A. One of the three supply phases is lost while the motor continues to be energized on the remaining two phases ✓ Correct answer

B. The motor is intentionally run on single-phase supply by design

C. All three phases fail simultaneously

D. The motor's speed exactly matches synchronous speed

Explanation: Single-phasing occurs when one incoming phase is interrupted (blown fuse, loose connection, tripped pole) while the motor remains connected to the other two, causing severe unbalance.

Q19. If a running three-phase motor experiences single-phasing, the most likely consequence is:

A. The remaining windings draw excessive current trying to compensate, causing overheating and possible winding burnout ✓ Correct answer

B. The motor speeds up beyond synchronous speed

C. The motor automatically switches to DC operation

D. The power factor immediately becomes unity

Explanation: With one phase lost, the motor tries to maintain output using the remaining two phases, drawing much higher current in them and generating excess heat that can quickly damage the windings.

Q20. A common real-world cause of single-phasing in an industrial motor circuit is:

A. A blown fuse or a loose/burnt terminal connection on one incoming phase ✓ Correct answer

B. The motor being correctly earthed

C. The overload relay being set too high

D. Using a star-delta starter instead of DOL

Explanation: A blown fuse, a tripped single pole, or a loose/corroded terminal on one line is a very common field cause of single-phasing in three-phase circuits.

Q21. Which protective device is specifically used to detect and prevent motor damage from single-phasing conditions?

A. A single-phase preventer / phase failure relay ✓ Correct answer

B. A capacitor bank

C. A step-down transformer

D. A soft starter's bypass contactor

Explanation: Phase failure (single-phasing preventer) relays continuously monitor all three phases and trip the motor circuit immediately if any phase is lost, preventing prolonged unbalanced operation.

Q22. Which of the following is NOT a typical cause of motor overheating?

A. Correct earthing of the motor frame ✓ Correct answer

B. Sustained overload beyond rated capacity

C. Blocked or dirty cooling fan/ventilation

D. Running on significantly unbalanced supply voltage

Explanation: Correct earthing is a safety measure and has no effect on motor thermal performance; overload, blocked ventilation, and voltage unbalance are all genuine causes of overheating.

Q23. Unusual noise and excessive vibration in a running motor are most often early symptoms of:

A. Bearing wear or failure ✓ Correct answer

B. Correctly balanced power factor

C. Proper lubrication

D. Normal healthy operation

Explanation: Worn or damaged bearings lose their smooth rolling action, producing grinding noise and vibration well before complete mechanical failure occurs.

Q24. A common cause of premature motor bearing failure is:

A. Insufficient or contaminated lubrication combined with shaft misalignment ✓ Correct answer

B. Using the correct grease at recommended intervals

C. Operating the motor within its rated load

D. Proper alignment between motor and driven equipment

Explanation: Poor lubrication practice and misalignment place abnormal stress on bearings, accelerating wear, pitting, and eventual failure.

Q25. A megger (insulation resistance tester) is primarily used to:

A. Measure the insulation resistance between windings and the motor frame/earth ✓ Correct answer

B. Measure the exact running speed of the motor

C. Measure the motor's power factor directly

D. Measure the vibration amplitude of the rotor

Explanation: A megger applies a DC test voltage and measures the resulting leakage current to calculate insulation resistance between windings and earth, indicating insulation health.

Q26. For routine insulation resistance testing of a standard low-voltage (below 1 kV) motor winding, a commonly used megger test voltage is:

A. 500 V DC ✓ Correct answer

B. 50 V DC

C. 50,000 V DC

D. 5 V DC

Explanation: 500 V DC is a widely used standard test voltage for insulation resistance testing of low-voltage motor windings without stressing the insulation beyond its rating.

Q27. A commonly applied rule of thumb for minimum acceptable insulation resistance of a motor winding is:

A. 1 megohm per kV of rated voltage, with an absolute minimum of about 1 megohm ✓ Correct answer

B. Exactly 1 ohm regardless of voltage rating

C. 1000 megohms regardless of voltage rating

D. Zero ohms for a healthy motor

Explanation: A widely used guideline states minimum acceptable insulation resistance should be at least 1 MΩ per kV of the equipment's rated voltage, ensuring adequate insulation health margin.

Q28. Before performing an insulation resistance (megger) test on a motor, it is essential to:

A. De-energize and fully disconnect the motor from the supply and discharge any residual charge ✓ Correct answer

B. Keep the motor running at reduced speed

C. Increase the supply voltage to the motor

D. Short the earth terminal to the phase terminal

Explanation: Testing with the motor still connected to a live supply is dangerous and can give false readings, damage test equipment, or cause electric shock, so isolation and discharge are mandatory first steps.

Q29. The Polarization Index (PI) of a motor winding is calculated as the ratio of:

A. The 10-minute insulation resistance reading to the 1-minute reading ✓ Correct answer

B. The 1-minute reading to the 10-minute reading

C. The starting current to the running current

D. The winding resistance to the insulation resistance

Explanation: PI compares how insulation resistance builds up over time under sustained DC test voltage; a healthy, dry winding shows resistance increasing with time, giving a PI greater than 1.

Q30. A motor with insulation Class F has a maximum allowable winding temperature (including ambient and temperature rise) of approximately:

A. 155°C ✓ Correct answer

B. 60°C

C. 500°C

D. 40°C

Explanation: Class F insulation is rated for a maximum hot-spot temperature of about 155°C, higher than Class B (130°C) but lower than Class H (180°C), per IEC insulation classification.

Q31. To reverse the direction of rotation of a three-phase induction motor, a technician should:

A. Interchange the connections of any two of the three supply phases to the motor ✓ Correct answer

B. Interchange the neutral and earth connections

C. Reduce the supply voltage by half

D. Swap the star and delta connections

Explanation: Swapping any two of the three phase connections reverses the direction of the rotating magnetic field, which reverses the motor's direction of rotation.

Q32. In a single-phase capacitor-start induction motor, the starting capacitor's main function is to:

A. Create a phase shift between the start and run windings to produce starting torque ✓ Correct answer

B. Store backup energy in case of power failure

C. Improve the motor's insulation resistance

D. Act as the motor's earth connection

Explanation: The capacitor introduces a phase difference between the currents in the start and main windings, creating a rotating magnetic field effect needed to start a single-phase motor, since it has no inherent rotating field at standstill.

Q33. In a capacitor-start single-phase motor, the centrifugal switch's function is to:

A. Disconnect the starting winding/capacitor once the motor reaches a set percentage of running speed ✓ Correct answer

B. Continuously vary the motor's speed

C. Provide overload protection

D. Reverse the motor's direction automatically

Explanation: Once the rotor reaches roughly 75-80% of rated speed, the centrifugal switch opens to remove the starting winding from the circuit, since it is not designed for continuous duty.

Q34. IE3 (Premium Efficiency) motors compared to standard IE1 motors are designed primarily to:

A. Reduce energy losses and operating cost by improving overall motor efficiency ✓ Correct answer

B. Increase the motor's starting current substantially

C. Eliminate the need for an overload relay

D. Reduce the motor's rated power output

Explanation: IEC efficiency classes (IE1 to IE4) rank motors by their energy loss performance; higher IE classes use better materials and design to cut losses and reduce lifetime running costs.

Q35. If supply voltage drops significantly while a motor drives a constant mechanical load, the motor's current will typically:

A. Increase, since torque and current must rise to compensate for reduced voltage at the same load ✓ Correct answer

B. Decrease proportionally with voltage

C. Remain completely unchanged

D. Drop to zero immediately

Explanation: Since motor torque is roughly proportional to voltage squared, a voltage drop reduces available torque at a given slip, so slip increases and current rises to keep delivering the required load torque.

Q36. Running an induction motor continuously at significantly below its rated voltage typically leads to:

A. Increased current draw, higher heating, and reduced starting torque ✓ Correct answer

B. Improved efficiency and cooler operation

C. No change in operating characteristics

D. Automatically corrected power factor

Explanation: Undervoltage causes the motor to draw more current for the same output torque, increasing I²R heating losses while also reducing the torque available for starting and acceleration.

Q37. Voltage unbalance across the three phases supplying an induction motor is particularly harmful because it:

A. Causes a disproportionately large increase in current in one or more phases, leading to localized overheating ✓ Correct answer

B. Has no measurable effect on motor operation

C. Always improves the motor's power factor

D. Increases the motor's insulation resistance

Explanation: Even a small percentage voltage unbalance can cause a much larger percentage current unbalance (often 6-10 times), heavily overheating one winding and shortening motor life.

Q38. A three-phase motor that hums loudly but fails to start when switched on is most likely suffering from:

A. Single-phasing, a mechanically jammed shaft, or an open winding preventing torque development ✓ Correct answer

B. A perfectly balanced three-phase supply

C. An overload relay set too high

D. Excess insulation resistance

Explanation: A motor that hums without rotating typically has power on some but not all windings (single-phasing/open winding) or is mechanically locked, so it cannot develop starting torque.

Q39. A locked rotor test on an induction motor is performed to determine:

A. The starting current and torque characteristics of the motor with the rotor held stationary ✓ Correct answer

B. The no-load speed of the motor

C. The motor's insulation resistance at rated voltage

D. The exact bearing temperature under normal running

Explanation: With the rotor mechanically locked and reduced voltage applied, this test reveals the motor's starting current and torque behavior, used for design verification and protection setting.

Q40. A no-load test on an induction motor is primarily used to determine:

A. Core losses and magnetizing current of the motor ✓ Correct answer

B. The motor's full load starting torque

C. The insulation resistance of the stator winding

D. The exact locked rotor current

Explanation: Running the motor unloaded reveals the fixed losses (core/iron losses and friction/windage) plus the magnetizing current, useful for equivalent circuit parameter calculation.

Q41. In a typical DOL starter panel, the combination of a contactor and an overload relay serves to:

A. Switch the motor on/off (contactor) while continuously monitoring current for overload protection (overload relay) ✓ Correct answer

B. Provide only short-circuit protection with no overload function

C. Convert AC supply to DC for the motor

D. Replace the need for any circuit breaker upstream

Explanation: The contactor handles the frequent making/breaking of motor current under normal switching duty, while the overload relay senses sustained excess current and signals a trip to protect the motor.

Q42. In a star-delta transition, proper interlocking ensures that:

A. The star contactor fully opens before the delta contactor closes, preventing a short circuit across the supply ✓ Correct answer

B. Both star and delta contactors remain closed together permanently

C. The delta contactor closes before the star contactor opens

D. The overload relay is bypassed during transition

Explanation: If both contactors were closed simultaneously, the motor windings would be connected in a way that creates a direct short circuit across two phases, so correct sequencing/interlocking is essential.

Q43. Shaft misalignment between a motor and its driven equipment (pump, gearbox, etc.) primarily causes:

A. Increased vibration and accelerated bearing wear on both machines ✓ Correct answer

B. Improved insulation resistance of the motor windings

C. A reduction in motor starting current

D. Automatic correction of the power factor

Explanation: Misalignment introduces abnormal radial and axial forces on the bearings of both coupled machines, causing excess vibration, heat, and premature bearing failure.

Q44. Vibration analysis is used in motor predictive maintenance mainly to:

A. Detect developing mechanical problems like bearing wear, imbalance, or misalignment before they cause failure ✓ Correct answer

B. Measure the motor's insulation resistance

C. Directly measure the motor's power factor

D. Replace the need for overload protection

Explanation: Characteristic vibration frequency patterns can reveal specific developing faults such as bearing defects, rotor imbalance, or misalignment well before a catastrophic breakdown occurs.

Q45. The squirrel cage rotor is widely preferred in industrial induction motors mainly because it is:

A. Simple, rugged, low-maintenance, and requires no external electrical connections or brushes ✓ Correct answer

B. The only rotor type capable of variable speed operation

C. Cheaper but far less reliable than a wound rotor

D. Only suitable for single-phase applications

Explanation: With no brushes, slip rings, or external rotor circuit to maintain, squirrel cage rotors offer high reliability and low maintenance, making them the standard choice for most industrial applications.

Q46. Motor rewinding becomes necessary when:

A. The winding insulation has failed or degraded beyond repair, often due to overheating or moisture ingress ✓ Correct answer

B. The motor's overload relay trips occasionally under genuine overload

C. The motor's terminal box cover is slightly loose

D. The motor bearings need routine greasing

Explanation: Once insulation breaks down (from thermal aging, moisture, or a fault), the winding can no longer safely hold voltage, requiring the coils to be removed and replaced (rewound).

Q47. In a Totally Enclosed Fan Cooled (TEFC) motor, the external fan mounted on the shaft primarily serves to:

A. Blow cooling air across the finned motor frame to dissipate heat from the enclosed windings ✓ Correct answer

B. Generate additional electrical output

C. Cool only the terminal box

D. Act as a backup bearing

Explanation: Since a TEFC motor's internal windings are sealed from the environment, an external shaft-mounted fan forces air over the ribbed outer frame to carry away heat generated inside.

Q48. An IP55 rating marked on a motor nameplate indicates that the motor enclosure provides:

A. Protection against dust ingress and low-pressure water jets from any direction ✓ Correct answer

B. Complete submersion protection underwater indefinitely

C. No protection against dust or water at all

D. Protection against dust only, with no water protection

Explanation: In the IP (Ingress Protection) code, the first digit '5' means dust-protected (limited ingress, no harmful deposit) and the second digit '5' means protected against water jets from any direction.

Q49. An anti-condensation space heater is sometimes fitted inside idle standby motors mainly to:

A. Prevent moisture condensation inside the windings during long periods of non-operation ✓ Correct answer

B. Increase the motor's running speed when started

C. Replace the need for insulation resistance testing

D. Reduce the motor's starting current when it is eventually started

Explanation: When a motor sits idle, especially in humid environments, internal temperature can drop below the dew point and moisture condenses on windings; a small heater keeps the interior slightly warm to prevent this.

Q50. Compared to grease lubrication, a key characteristic of oil-lubricated motor bearings is that they:

A. Are typically used in larger or higher-speed machines and require a defined oil level/circulation system ✓ Correct answer

B. Never need to be checked or topped up

C. Cannot be used in any industrial motor

D. Provide no lubrication benefit at all

Explanation: Oil lubrication is common in larger, higher-speed, or specially designed bearings, needing proper oil level monitoring and sometimes circulation, whereas grease is simpler and used in most standard motors.

Frequently asked questions

Who is this Electrical Troubleshooting quiz for?

It is for ITI, diploma and B.Tech students, apprentices and working technicians who want to test fault-finding knowledge. The explanations are written to teach the reason, not just the answer.

How should I use the answer key?

Attempt the quiz first without looking. Then open the answer key, read the explanation for every question you missed, and retake the part after a day to see whether the explanation stuck.

Do I need to finish all five parts?

No, but the parts build on one another: fundamentals first, then components, then circuits and real-world scenarios. Part 5 is the best test of whether you can apply the earlier parts to a live fault.

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