BLDC Motor Question & Answers October 12, 2026 By Wat Electrical Brushless DC (BLDC) motors are widely used in modern electrical and electronic systems because of their efficiency, reliability, and ability to provide precise speed control. They operate using electronic commutation instead of the brushes and mechanical commutator found in conventional brushed DC motors. Understanding how these motors work involves learning about their construction, operating principles, control methods, and performance characteristics. This article presents 100 multiple-choice questions (MCQs) on BLDC motors, covering basic concepts, motor drives, commutation, speed and torque calculations, advanced control techniques, and troubleshooting. Each question includes the correct answer, a hint, and an explanation to help readers understand the concepts and strengthen their technical knowledge. 1). What is the full form of BLDC? Brushless Linear DC Brushless Direct Current Bipolar Loop Drive Control Brushless Digital Controller None Hint 2). What is the primary purpose of electronic commutation in a BLDC motor? To increase winding resistance To eliminate the need for a rotor To switch phase currents in the correct sequence To convert AC directly into mechanical energy without switching None Hint 3). Which component generally produces the rotor's magnetic field in a BLDC motor? Permanent magnets Carbon brushes Slip rings Mechanical commutator segments None Hint 4). Where are the main armature windings usually located in a conventional BLDC motor? On the shaft On the stator On the bearings On the outer casing only None Hint 5). Which component is eliminated in a BLDC motor compared with a conventional brushed DC motor? Bearings Shaft Mechanical brushes and commutator Stator None Hint 6). What is the most common number of phases in a conventional three-phase BLDC motor? One Two Three Six None Hint 7). Which supply is commonly used to power a BLDC motor drive? A DC source connected through an electronic inverter A DC source connected directly to every winding without switching A mechanical source only A constant magnetic field with no electrical supply None Hint 8). What is the primary function of the stator in a BLDC motor? To store electrical charge To produce a controlled magnetic field through its windings To mechanically switch brushes To eliminate back EMF None Hint 9). Which statement about a BLDC motor is correct? It always requires brushes to start. It cannot operate from a battery. It uses electronic commutation. Its rotor must always contain windings. None Hint 10). What is the primary source of torque in a BLDC motor? Friction between the shaft and bearings Interaction between stator magnetic fields and rotor permanent magnets Air resistance Heat generated in the windings None Hint 11). Which feature is a typical advantage of a BLDC motor? High brush-maintenance requirements Low efficiency in all operating conditions Reduced mechanical wear from the absence of brushes No need for a controller in any application None Hint 12). What is back electromotive force (back EMF)? Voltage induced by motor rotation that opposes the applied voltage Voltage generated only when the motor is stationary Voltage across a disconnected capacitor only A mechanical force acting on the shaft None Hint 13). What happens to back EMF as the rotational speed increases, assuming constant flux? It generally increases It always becomes zero It remains constant at every speed It changes only with bearing size None Hint 14). Which unit is used to measure motor torque? Watt Volt Newton-metre Ampere-hour None Hint 15). Which application commonly uses BLDC motors? Computer cooling fans Only steam turbines Only manually operated switches Passive resistors None Hint 16). What is the function of permanent magnets in a BLDC motor? To provide rotor excitation To reduce the DC bus voltage To operate the gate driver To cool the stator None Hint 17). What does the term “inrunner” mean? The stator rotates inside the rotor. The rotor rotates inside the stator. The motor has no rotor. The shaft remains stationary in every design. None Hint 18). Which arrangement is characteristic of an outrunner BLDC motor? The outer rotor rotates around an inner stator. The rotor has no magnets. The stator rotates freely around the shaft. The motor has no air gap. None Hint 19). What is the purpose of the air gap between the rotor and stator? To allow mechanical clearance while enabling magnetic interaction To prevent all magnetic flux from crossing the gap To eliminate the need for bearings To create electrical contact between the rotor and stator None Hint 20). Which material is commonly used for the magnetic core of a BLDC stator? Laminated electrical steel Pure rubber Wood Plastic foam None Hint 21). Why are stator cores laminated? To increase mechanical friction To reduce eddy-current losses To eliminate copper resistance To remove the need for insulation None Hint 22). Which material is commonly used in high-performance permanent magnets for BLDC motors? Neodymium-iron-boron Pure aluminium Brass Graphite alone None Hint 23). What is the purpose of the motor shaft? To transmit mechanical power To generate PWM signals To measure winding resistance automatically To perform electronic commutation None Hint 24). What is the main function of motor bearings? To generate the rotating magnetic field To support the shaft and permit rotation To switch stator currents To increase electrical resistance None Hint 25). What does the number of motor poles primarily influence? The relationship between electrical frequency and mechanical speed The colour of the motor casing The resistance of the battery terminals only The number of bearings required in every design None Hint BLDC Motors MCQs for Exams 26). What is the most common commutation method for a conventional trapezoidal three-phase BLDC motor? Six-step commutation Mechanical brush commutation Single-step commutation No commutation None Hint 27). In conventional six-step BLDC commutation, how many phases are typically energized at one time? One Two Three in every sector Zero in every sector None Hint 28). How many distinct commutation states occur during one electrical cycle of six-step commutation? Two Three Six Twelve None Hint 29). What is the primary purpose of Hall-effect sensors in a sensored BLDC motor? To measure rotor position To measure winding insulation thickness To cool the rotor To increase the battery capacity None Hint 30). How many Hall sensors are commonly used in a three-phase, six-step BLDC drive? One hundred Three Eight Zero in every sensored motor None Hint 31). What is a typical property of the Hall signals in a conventional three-Hall-sensor BLDC motor? They are digital signals that change with rotor position. They always remain at zero volts. They directly provide the motor's mechanical output power. They are identical to the three phase currents. None Hint 32). What is sensorless BLDC control? Control without any electronic switching Control that estimates rotor position without a dedicated rotor-position sensor Control that uses only mechanical brushes Control without measuring or regulating anything None Hint 33). Which signal is commonly used for sensorless six-step BLDC commutation? Back EMF of the floating phase Ambient light intensity Shaft paint colour Bearing lubricant level alone None Hint 34). Why can sensorless BLDC motors have difficulty starting from rest? Back EMF is absent at zero speed. Permanent magnets stop producing flux at zero speed. The motor shaft disappears at zero speed. PWM cannot operate at low speed. None Hint 35). What is the meaning of a back-EMF zero crossing in sensorless six-step control? The floating-phase back EMF crosses the reference level, commonly the virtual neutral All motor currents become permanently zero The battery becomes fully charged The rotor stops immediately None Hint 36). What is the usual purpose of a rotor-alignment sequence during sensorless startup? To establish a known approximate rotor position To increase the number of motor poles To eliminate all winding resistance To permanently lock the rotor None Hint 37). What is the primary purpose of the floating phase in conventional two-phase-conduction sensorless control? To provide a phase-voltage signal for back-EMF detection To carry the full DC bus current continuously To replace the inverter To short all three motor terminals None Hint 38). What does incorrect commutation timing often cause? Reduced torque and increased current or noise Guaranteed maximum efficiency Zero electrical losses Infinite motor speed None Hint 39). Which statement about Hall-sensor spacing is correct? It must suit the motor's magnetic and commutation design. It must always be zero degrees. It is unrelated to rotor-position detection. All motors require the sensors to be mounted on the shaft. None Hint 40). What is the main difference between sensored and sensorless BLDC control? Sensored control uses rotor-position sensors; sensorless control estimates rotor position. Sensored control uses no electricity. Sensorless control never requires a controller. Sensored motors cannot rotate continuously. None Hint 41). What is the main function of a three-phase inverter in a BLDC drive? Convert DC bus power into controlled phase voltages and currents Convert shaft rotation into compressed air Eliminate the permanent magnets Mechanically switch the motor windings None Hint 42). Which semiconductor device is commonly used in low- and medium-voltage BLDC drives? MOSFET Mechanical relay as the sole high-frequency switch Incandescent lamp Fuse as a switching controller None Hint 43). What does PWM stand for? Power Winding Method Pulse Width Modulation Permanent Wave Magnetism Phase Wire Measurement None Hint 44). What does PWM duty cycle represent? The percentage of a switching period for which the signal is on The number of motor bearings The permanent-magnet temperature only The number of rotor poles None Hint 45). An ideal PWM switch applies 24 V during its on-time and 0 V during its off-time. What is the average voltage at a 50% duty cycle? 0 V 6 V 12 V 24 V None Hint 46). What is the primary purpose of a gate driver? To provide appropriate control signals and drive capability to power-switch gates To measure shaft torque mechanically To replace all inverter transistors To increase the motor's pole count None Hint 47). What is dead time in a half-bridge inverter? A delay between turning off one switch and turning on its complementary switch A period when the motor must remain permanently stopped The total time required to manufacture a motor A period when the battery is disconnected during every revolution None Hint 48). What can happen if shoot-through occurs in an inverter? Excessive current may damage power switches Motor efficiency becomes 100% The rotor gains additional permanent magnets Winding resistance becomes zero None Hint 49). What is the function of a DC-link capacitor in a BLDC drive? To help stabilize the DC bus and supply transient current To detect rotor position directly in all drives To provide mechanical support to the shaft To replace all phase windings None Hint 50). What is the main purpose of current sensing in a BLDC controller? To monitor and regulate current and support protection To determine the colour of the rotor To eliminate all switching losses To change the number of phases None Hint BLDC Motors MCQs for Interviews 51). What does an overcurrent protection circuit do? Detects excessive current and limits or interrupts operation Increases current indefinitely Disables all temperature measurements Guarantees that bearings never wear out None Hint 52). Which microcontroller peripheral is particularly useful for BLDC control? PWM timer Audio speaker output alone Keyboard backlight controller alone Real-time clock alone None Hint 53). What is the purpose of complementary PWM outputs in an inverter? To control the upper and lower switches of a bridge leg To power two unrelated motors from one signal in all cases To replace the DC bus capacitor To measure motor insulation resistance None Hint 54). What is a major disadvantage of an excessively high PWM switching frequency? Increased switching losses Guaranteed elimination of all heat Guaranteed zero electromagnetic interference Automatic improvement in every operating condition None Hint 55). Why is proper PCB layout important in a BLDC controller? It reduces parasitic inductance, noise coupling, and unwanted voltage spikes. It determines the motor's permanent-magnet material automatically. It eliminates the need for current protection. It makes all gate drivers unnecessary. None Hint 56). What is the approximate relationship between mechanical power, torque, and angular velocity? P=Tω P=T/ω P=T+ω P=T-ω None Hint 57). A motor produces 2 N•m of torque at 100 rad/s. What is its mechanical output power? 20 W 50 W 200 W 500 W None Hint 58). Which quantity is directly proportional to motor current in a simplified BLDC torque model? Electromagnetic torque Permanent-magnet mass Bearing diameter Shaft length alone None Hint 59). What does the torque constant K_t describe? Torque produced per unit current under a defined convention Resistance per unit shaft length Temperature per bearing Battery capacity per hour of operation None Hint 60). What does the back-EMF constant describe? Induced voltage as a function of rotational speed Shaft friction as a function of bearing diameter Rotor mass as a function of current Insulation thickness as a function of frequency None Hint 61). A BLDC motor has a speed constant of 500 rpm/V. What is its approximate ideal no-load speed at 12 V? 500 rpm 1,000 rpm 6,000 rpm 12,000 rpm None Hint 62). What happens to motor current when mechanical load increases, assuming the speed controller attempts to maintain speed? It generally increases. It always becomes zero It cannot change. It becomes independent of torque. None Hint 63). What is the SI unit of angular velocity? Ohm Rad/s Tesla Coulomb None Hint 64). A motor rotates at 3,000 rpm. What is its rotational frequency in revolutions per second? 5 Hz 25 Hz 50 Hz 100 Hz None Hint 65). A motor has four pole pairs and rotates at 3,000 rpm. What is its electrical frequency? 25 Hz 50 Hz 100 Hz 200 Hz None Hint 66). What is the electrical frequency of a two-pole-pair motor running at 1,500 rpm? 25 Hz 50 Hz 100 Hz 150 Hz None Hint 67). If motor speed doubles while flux remains approximately constant, what happens to back EMF? It approximately doubles. It becomes zero. It is always halved. It remains unchanged. None Hint 68). Which equation relates input electrical power, output mechanical power, and efficiency? η=P_out/P_in η=P_in/P_out η=P_in+P_out η=P_in-P_out None Hint 69). A motor takes 500 W of electrical input power and delivers 400 W of mechanical power. What is its efficiency? 20% 40% 80% 125% None Hint 70). Which losses commonly contribute to BLDC motor heating? Winding copper losses and magnetic-core losses Useful shaft power alone Battery capacity alone Permanent-magnet colour alone None Hint 71). What is the main objective of closed-loop speed control? Maintain the commanded speed despite load disturbances Keep the motor current permanently at zero Eliminate all feedback signals Increase the rotor's pole count during operation None Hint 72). What is the role of a PI controller in BLDC speed control? To reduce speed error using proportional and integral actions To mechanically align the rotor To replace the motor inverter To eliminate permanent magnets None Hint 73). What is field-oriented control (FOC)? A method that controls motor current components in a rotating reference frame A method that uses mechanical brushes to switch phases A technique for cooling permanent magnets only A control method that cannot regulate torque None Hint 74). Which control method is particularly associated with conventional trapezoidal-back-EMF BLDC operation? Six-step commutation Mechanical commutation by brushes Open-circuit operation only Passive cooling control None Hint 75). Which motor-drive method generally provides smooth torque and low torque ripple when properly implemented? Well-tuned sinusoidal current control or FOC Random switching without rotor-position information Permanent short-circuiting of the windings Uncontrolled phase energization None Hint BLDC Motors MCQs for Quiz 76). What is torque ripple? Variation in torque during motor rotation A permanent increase in battery voltage The total number of stator slots A measure of insulation resistance None Hint 77). What is cogging torque? Torque variation caused by interaction between permanent magnets and stator-slot geometry Torque produced only by a gearbox Torque caused exclusively by brush friction The torque required to charge a battery None Hint 78). Why is current-loop bandwidth important in a motor controller? It affects how quickly current and torque respond to commands and disturbances. It determines the colour of the stator insulation. It fixes the motor's physical diameter. It eliminates the need for a power supply. None Hint 79). What is regenerative braking in a BLDC motor drive? Converting mechanical energy into electrical energy and returning some of it to the supply Dissipating all energy through mechanical friction only Increasing shaft speed without any energy input Disabling the inverter permanently None Hint 80). Which condition is important for safe regenerative braking into a battery? The battery and power electronics must be able to accept the charging current and voltage. The battery must be disconnected from all protection circuits. The inverter must be permanently shorted. The motor must have mechanical brushes. None Hint 81). What is the main purpose of maximum torque per ampere (MTPA) control? Produce a required torque with minimum current magnitude Maximize current regardless of torque Eliminate all rotor-position estimation Operate without a motor controller None Hint 82). What is field weakening in a permanent-magnet motor drive? A technique that permits higher speed by reducing effective flux linkage or opposing the magnet flux with current A method for removing the rotor A method that always increases torque at every speed A technique for increasing the number of motor phases None Hint 83). What is the purpose of a speed ramp in a BLDC controller? To change the speed command gradually To force the motor instantly to maximum speed To permanently disable current regulation To change the motor's magnet polarity mechanically None Hint 84). Why can a BLDC motor produce less torque at very high speeds? Voltage and current limits can restrict torque-producing current. Bearings automatically create more permanent magnets. The rotor stops having magnetic properties at every high speed. Copper resistance becomes exactly zero. None Hint 85). What is the main purpose of a motor's thermal protection system? Prevent damaging overheating of the motor or drive Increase current without limits Eliminate the need for insulation Guarantee maximum speed at every load None Hint 86). A BLDC motor does not start, but its supply voltage is correct. Which is a plausible cause? Faulty Hall sensors or incorrect commutation signals A perfectly functioning controller in every possible case An ideal bearing with no friction Correct wiring under all circumstances None Hint 87). A BLDC motor vibrates and makes unusual noise during operation. What should be checked first? Commutation timing, phase connections, and mechanical alignment Only the colour of the power cable Only the room lighting The number of buttons on the controller None Hint 88). What can cause a BLDC motor to overheat under excessive load? High winding current and increased copper losses Zero current under all conditions A complete absence of electrical losses A reduction in all mechanical forces to zero None Hint 89). Which equation describes resistive copper loss in a winding? P=I^2 R P=V/I P=R/I^2 P=I/R None Hint 90). Why is current limiting especially important when a BLDC rotor is stalled? The motor may draw excessive current because back EMF is very low. The rotor produces unlimited electrical power. The battery voltage always becomes zero. Winding resistance disappears. None Hint 91). Which measurement is useful for checking a motor winding for an open circuit? Winding continuity or resistance Room brightness Shaft colour Fan blade diameter alone None Hint 92). Why should winding resistance measurements be interpreted carefully in a low-resistance BLDC motor? Lead resistance and instrument accuracy can significantly affect readings. Resistance cannot be measured in any winding. Resistance always equals zero. The rotor speed determines every resistance reading exactly. None Hint 93). What is a likely result of incorrect motor phase connections in a sensored BLDC system? Poor starting, vibration, or failure to rotate correctly Guaranteed maximum efficiency Automatic correction of every wiring fault Permanent elimination of back EMF None Hint 94). Which application is particularly suited to a compact, high-speed BLDC motor? Computer cooling fan Passive structural beam Fixed building wall Unpowered mechanical bracket None Hint 95). Why are BLDC motors widely used in drones? They can provide high power-to-weight ratio and precise electronic speed control. They operate without any electrical energy. They require mechanical brushes for every commutation event. They cannot change rotational speed. None Hint 96). In an electric vehicle, what is one major benefit of a suitable BLDC or permanent-magnet motor drive? Efficient traction and electronically controlled torque No need for a battery or energy source Guaranteed zero maintenance for the entire vehicle Elimination of all drivetrain losses None Hint 97). What is the primary purpose of an electronic speed controller (ESC) for a BLDC motor? Control power switching and motor operation Act as the motor's mechanical bearing Replace the permanent magnets Convert motor torque directly into temperature readings None Hint 98). Which condition is most likely to shorten the life of BLDC motor insulation? Excessive winding temperature over time Correct operation within the rated temperature range Proper current limiting Appropriate cooling and installation None Hint 99). Which statement about BLDC motor efficiency is correct? Efficiency depends on the operating point, motor design, and controller losses. Efficiency is always 100%. Efficiency is independent of load and speed. Every BLDC motor has exactly the same efficiency. None Hint 100). Which combination is most important for reliable BLDC motor operation? Correct motor selection, suitable commutation, current protection, and thermal management Maximum current at all times Random phase switching and no feedback Removing all protective circuitry None Hint A clear understanding of BLDC motors requires more than knowing their basic operating principle. It also involves understanding how electronic controllers manage phase switching, how speed and torque are related, and how factors such as current, temperature, and commutation timing affect motor performance. The 100 questions in this article provide an opportunity to review these concepts and identify areas that may need further study. Working through the questions and explanations can help students prepare for examinations, support practical learning, and build a stronger foundation for understanding BLDC motor applications and control systems. Time's up
