Q.1 Which type of motor is most commonly used in CNC machine spindles for high speed and precise position control?
Induction motor
Universal motor
Servo motor
Stepper motor
Explanation - Servo motors provide high torque at high speeds and can be precisely controlled via feedback, making them ideal for CNC spindles.
Correct answer is: Servo motor
Q.2 In a stepper motor, the angle of rotation per step is determined by:
Number of poles on the stator
Number of phases in the winding
Number of teeth on the rotor
Number of teeth on the stator
Explanation - The step angle equals 360° divided by the number of stator teeth (or steps) per revolution.
Correct answer is: Number of teeth on the stator
Q.3 A linear motor differs from a conventional rotary motor primarily in:
Its use of permanent magnets
The elimination of any rotating parts
Its reliance on a commutator
The type of winding used
Explanation - Linear motors produce direct linear motion without converting rotary motion, eliminating moving mechanical parts like shafts.
Correct answer is: The elimination of any rotating parts
Q.4 Which component in a brushless DC (BLDC) motor provides the electronic commutation?
Permanent magnet rotor
Hall‑effect sensor array
Carbon brushes
Slip rings
Explanation - Hall‑effect sensors detect rotor position and trigger the inverter to switch phases, achieving commutation without brushes.
Correct answer is: Hall‑effect sensor array
Q.5 Switched Reluctance Motors (SRM) are known for:
High efficiency at low speeds
Low torque ripple
Simple and rugged rotor construction
Use of permanent magnets
Explanation - SRM rotors consist of salient poles without windings or permanent magnets, making them robust and inexpensive.
Correct answer is: Simple and rugged rotor construction
Q.6 Which of the following is a typical application of a homopolar generator?
Powering electric vehicles
Providing high current pulses for electromagnetic forming
Driving household appliances
Supplying stable DC voltage for telecom
Explanation - Homopolar generators can deliver very high currents in short pulses, useful for processes like electromagnetic forming.
Correct answer is: Providing high current pulses for electromagnetic forming
Q.7 In an induction heating system, the heating frequency is usually:
Below 50 Hz
Between 20 kHz and 200 kHz
Between 1 MHz and 10 MHz
Above 100 MHz
Explanation - Industrial induction heating typically uses frequencies in the 20–200 kHz range to obtain suitable skin depth for metal heating.
Correct answer is: Between 20 kHz and 200 kHz
Q.8 An eddy‑current brake works on the principle of:
Magnetic hysteresis loss
Induced currents generating opposing magnetic fields
Friction between conductive plates
Thermal expansion of metal discs
Explanation - Relative motion between a conductor and a magnetic field induces eddy currents that produce forces opposing motion, thereby braking.
Correct answer is: Induced currents generating opposing magnetic fields
Q.9 Which type of special purpose machine uses a pair of permanent magnets and a moving coil to produce thrust without any contact?
Linear induction motor
Linear synchronous motor
Voice coil actuator
Electrostatic motor
Explanation - Voice coil actuators generate linear motion through the interaction of a permanent magnetic field and a current‑carrying coil, without contact.
Correct answer is: Voice coil actuator
Q.10 A flywheel energy storage system stores energy primarily as:
Magnetic field energy
Thermal energy
Kinetic energy of rotation
Electrostatic energy
Explanation - Flywheels store energy in the form of rotational kinetic energy, which can be recovered by decelerating the wheel.
Correct answer is: Kinetic energy of rotation
Q.11 Which of the following is NOT an advantage of a permanent‑magnet synchronous motor (PMSM) used as a special purpose machine?
High power density
Zero rotor losses
Simple control without sensors
Excellent torque control
Explanation - While PMSMs can be sensorless, accurate high‑speed operation often requires position sensors; thus, simple control without sensors is not always an advantage.
Correct answer is: Simple control without sensors
Q.12 In a magnetic levitation (maglev) train, the primary levitation force is produced by:
Hydraulic pistons
Electromagnetic repulsion
Aerodynamic lift
Mechanical springs
Explanation - Maglev trains use electromagnetic forces (either attractive or repulsive) to levitate the vehicle above the guideway, eliminating contact.
Correct answer is: Electromagnetic repulsion
Q.13 A piezoelectric actuator converts electrical energy into:
Magnetic flux
Thermal expansion
Mechanical displacement
Electrostatic force
Explanation - The piezoelectric effect causes certain crystals to deform (expand or contract) when an electric voltage is applied.
Correct answer is: Mechanical displacement
Q.14 Which of the following machines is specifically designed for high‑speed, high‑torque applications in electric vehicle traction?
Switched reluctance motor
Universal motor
Brushless DC (BLDC) motor
Stepper motor
Explanation - BLDC motors provide high efficiency, high torque density, and controllable speed, making them suitable for EV traction.
Correct answer is: Brushless DC (BLDC) motor
Q.15 The main disadvantage of a universal motor when used as a special purpose machine is:
Low starting torque
Heavy weight
Brush and commutator wear
Inability to run on DC
Explanation - Universal motors use brushes and a commutator, which wear out over time, limiting their lifespan in demanding applications.
Correct answer is: Brush and commutator wear
Q.16 In a synchronous reluctance motor, torque is produced by:
Interaction of permanent magnet fields
Salient rotor geometry causing reluctance variation
Induced currents in the rotor
Electrostatic attraction
Explanation - The rotor’s anisotropic magnetic reluctance creates a torque that aligns the rotor with the stator’s rotating magnetic field.
Correct answer is: Salient rotor geometry causing reluctance variation
Q.17 Which type of machine is most suitable for precise, repeatable incremental motion in 3‑D printers?
Servo motor
Stepper motor
Switched reluctance motor
Universal motor
Explanation - Stepper motors move in discrete steps, providing accurate positioning without the need for feedback encoders in many 3‑D printers.
Correct answer is: Stepper motor
Q.18 A linear induction motor (LIM) achieves thrust by:
Permanent magnet attraction
Eddy currents induced in a secondary conductive plate
Electrostatic attraction between plates
Mechanical gearing
Explanation - LIMs induce eddy currents in a conductive secondary (reaction plate), producing a Lorentz force that creates linear thrust.
Correct answer is: Eddy currents induced in a secondary conductive plate
Q.19 Which control method is commonly used to reduce torque ripple in stepper motors?
Microstepping
Field‑oriented control
Scalar V/f control
Direct torque control
Explanation - Microstepping divides each full step into smaller increments, smoothing current waveforms and reducing torque ripple.
Correct answer is: Microstepping
Q.20 In a BLDC motor, the back‑EMF waveform is typically:
Sinusoidal
Square
Triangular
Sawtooth
Explanation - Most BLDC motors are designed to produce a near‑sinusoidal back‑EMF, especially when driven with sinusoidal commutation for smooth torque.
Correct answer is: Sinusoidal
Q.21 A common method to start a switched reluctance motor is:
Applying DC voltage to the stator
Using an external starter motor
Closed‑loop torque control from zero speed
Pre‑magnetizing the rotor
Explanation - SRMs can be started directly from zero speed using a closed‑loop controller that sequentially energizes the phases to generate torque.
Correct answer is: Closed‑loop torque control from zero speed
Q.22 The term ‘torque density’ of a motor refers to:
Torque per unit volume
Torque per unit mass
Torque per unit cost
Torque per unit speed
Explanation - Torque density measures how much torque a motor can produce relative to its physical size, important for compact special‑purpose machines.
Correct answer is: Torque per unit volume
Q.23 Which type of machine is typically used as a high‑speed spindle in CNC milling machines?
Induction motor
Brushless DC motor
Synchronous reluctance motor
Switched reluctance motor
Explanation - BLDC motors provide high speed, low vibration, and excellent control, making them ideal for CNC spindles.
Correct answer is: Brushless DC motor
Q.24 In an induction heating coil, the depth of heating (skin depth) decreases with:
Higher frequency
Lower frequency
Higher conductivity of the workpiece
Lower magnetic permeability
Explanation - Skin depth is inversely proportional to the square root of frequency; higher frequencies confine currents to a thinner surface layer.
Correct answer is: Higher frequency
Q.25 A variable frequency drive (VFD) used with a special purpose induction motor primarily controls:
Voltage magnitude only
Frequency of supply voltage
Current waveform shape
Rotor position
Explanation - A VFD adjusts the output frequency (and usually voltage) to control the speed of an induction motor.
Correct answer is: Frequency of supply voltage
Q.26 Which material is commonly used for the core of high‑frequency transformers in special purpose machines?
Silicon steel
Ferrite
Aluminum
Copper
Explanation - Ferrite has low core losses at high frequencies, making it ideal for high‑frequency transformer cores.
Correct answer is: Ferrite
Q.27 A torque ripple in a motor can cause:
Reduced efficiency only
Noise, vibration, and position error
Higher thermal rating
Increased magnetic flux density
Explanation - Torque ripple leads to periodic variations in output torque, resulting in audible noise, mechanical vibration, and positioning inaccuracies.
Correct answer is: Noise, vibration, and position error
Q.28 In a voice coil actuator, the force produced is proportional to:
Square of the current
Current times magnetic flux density
Frequency of the supply
Voltage applied
Explanation - F = B·I·L (Lorentz force) – force is directly proportional to current and magnetic flux density.
Correct answer is: Current times magnetic flux density
Q.29 Which of the following is a typical application of a switched reluctance motor?
Electric aircraft propulsion
Household washing machines
High‑precision robot joints
Low‑cost HVAC compressors
Explanation - SRMs are robust and inexpensive, making them suitable for cost‑sensitive applications such as HVAC compressors.
Correct answer is: Low‑cost HVAC compressors
Q.30 The primary purpose of a damper winding in a synchronous machine used as a special purpose machine is:
To increase the output voltage
To provide starting torque
To suppress oscillations during transients
To reduce core losses
Explanation - Damper windings act like squirrel‑cage bars, damping oscillations and providing a path for induced currents during sudden changes.
Correct answer is: To suppress oscillations during transients
Q.31 Which type of motor is most often used in handheld power tools because of its high speed and compact size?
Induction motor
Universal motor
Stepper motor
Switched reluctance motor
Explanation - Universal motors can operate on AC or DC, run at high speeds, and are compact, making them ideal for handheld tools.
Correct answer is: Universal motor
Q.32 In a permanent‑magnet linear motor, the moving part is called:
Stator
Rotor
Platen
Armature
Explanation - In linear motors, the moving part (often carrying permanent magnets) is termed the platen, while the stationary part is the stator.
Correct answer is: Platen
Q.33 Which control technique is widely used for high‑performance torque control in BLDC motors?
Scalar V/f control
Field‑oriented control (FOC)
Direct torque control (DTC)
Open‑loop control
Explanation - FOC decouples torque and flux components, allowing precise torque control similar to that of AC induction motors.
Correct answer is: Field‑oriented control (FOC)
Q.34 A typical characteristic of a synchronous motor used as a magnetic bearing actuator is:
Zero cogging torque
High slip
Low inductance
Variable frequency operation
Explanation - Synchronous motors with smooth air gaps can be designed to have negligible cogging, essential for precise bearing control.
Correct answer is: Zero cogging torque
Q.35 What is the main reason for using a high‑permeability core material in a magnetic levitation (maglev) guideway?
To reduce weight
To increase electrical resistance
To concentrate magnetic flux
To improve thermal conductivity
Explanation - High‑permeability materials guide magnetic flux efficiently, strengthening the levitation force in maglev systems.
Correct answer is: To concentrate magnetic flux
Q.36 In a three‑phase induction motor used for a special purpose winding machine, the slip at rated load is typically:
0.5 % to 1 %
2 % to 5 %
10 % to 15 %
20 % to 25 %
Explanation - Standard induction motors operate with a slip of about 2–5 % at rated load, providing stable torque.
Correct answer is: 2 % to 5 %
Q.37 Which of the following is a key benefit of using a brushless DC motor in a UAV (unmanned aerial vehicle)?
Higher maintenance cost
Reduced electromagnetic interference
Presence of brushes for easy repair
Low efficiency at low speeds
Explanation - BLDC motors lack brushes, resulting in lower EMI and higher reliability, which are critical for UAVs.
Correct answer is: Reduced electromagnetic interference
Q.38 The term ‘cogging torque’ is most associated with:
Induction motors
Universal motors
Permanent‑magnet synchronous motors
Switched reluctance motors
Explanation - Cogging torque arises from the interaction between permanent magnet poles and stator teeth in PMSMs.
Correct answer is: Permanent‑magnet synchronous motors
Q.39 In an electrostatic motor, motion is produced by:
Magnetic attraction
Electric field attraction and repulsion
Thermal expansion
Hydraulic pressure
Explanation - Electrostatic motors use forces generated by electric fields between charged electrodes to produce motion.
Correct answer is: Electric field attraction and repulsion
Q.40 Which of the following is a common failure mode for a brushless DC motor used in a high‑speed spindle?
Commutator wear
Stator winding overheating
Brush arcing
Permanent magnet demagnetization
Explanation - High speeds generate significant copper losses; insufficient cooling can cause stator winding overheating.
Correct answer is: Stator winding overheating
Q.41 A synchronous motor equipped with a damper winding can be started like an:
Induction motor
Universal motor
Stepper motor
Switched reluctance motor
Explanation - The damper winding provides a path for induction currents, allowing the motor to start as an induction motor before locking into synchronism.
Correct answer is: Induction motor
Q.42 In a high‑frequency transformer for a special purpose power supply, the winding is usually made of:
Copper foil or litz wire
Aluminum bus bar
Thick copper rod
Silver plating
Explanation - Thin copper foil or litz wire reduces skin effect losses at high frequencies.
Correct answer is: Copper foil or litz wire
Q.43 Which control method is typically employed to reduce the torque ripple in a BLDC motor used for a camera gimbal?
Square‑wave commutation
Sinusoidal PWM commutation
Open‑loop voltage control
Direct torque control
Explanation - Sinusoidal PWM produces smoother current waveforms, minimizing torque ripple, which is essential for steady camera operation.
Correct answer is: Sinusoidal PWM commutation
Q.44 A magnetic gear (magnetically coupled gear) differs from a conventional gear by:
Using permanent magnets instead of teeth
Having higher friction losses
Requiring lubrication
Operating only at low speeds
Explanation - Magnetic gears transmit torque via magnetic fields, eliminating physical contact and wear.
Correct answer is: Using permanent magnets instead of teeth
Q.45 The term ‘back‑EMF constant (Kb)’ of a motor relates:
Torque to current
Voltage to speed
Power to efficiency
Flux density to air gap
Explanation - Kb expresses the proportionality between generated voltage (back‑EMF) and rotor speed.
Correct answer is: Voltage to speed
Q.46 In a linear synchronous motor (LSM) used for maglev, the synchronous speed is determined by:
Supply voltage amplitude
Supply frequency and pole pitch
Number of stator windings
Air gap length
Explanation - Linear synchronous speed v = 2·f·τ where f is frequency and τ is pole pitch.
Correct answer is: Supply frequency and pole pitch
Q.47 A common way to increase the power rating of a stepper motor without changing its size is to:
Increase the supply voltage
Reduce the number of phases
Add permanent magnets to the rotor
Use a larger air gap
Explanation - Higher supply voltage allows faster current rise in windings, increasing torque capability while keeping size constant.
Correct answer is: Increase the supply voltage
Q.48 Which of the following is NOT a typical feature of a special purpose electro‑hydraulic actuator?
High force density
Fast response time
Direct conversion of electrical energy to motion
Low power consumption at idle
Explanation - Electro‑hydraulic actuators convert electrical energy to hydraulic fluid pressure before motion, so conversion is indirect.
Correct answer is: Direct conversion of electrical energy to motion
Q.49 The main advantage of using a ferrite core in a high‑frequency transformer is:
Higher saturation flux density
Lower eddy‑current loss
Higher thermal conductivity
Lower cost than silicon steel
Explanation - Ferrite’s high electrical resistivity reduces eddy‑current losses at high frequencies.
Correct answer is: Lower eddy‑current loss
Q.50 In a magnetic bearing system, the control loop that keeps the rotor centered is usually:
Open‑loop voltage control
Closed‑loop current control
Scalar V/f control
Direct torque control
Explanation - Current control allows precise regulation of magnetic forces to maintain rotor position.
Correct answer is: Closed‑loop current control
Q.51 A common use of a switched reluctance generator in renewable energy is:
Wind turbine direct drive
Solar photovoltaic inverter
Hydroelectric turbine excitation
Battery charger
Explanation - SRGs are rugged, fault‑tolerant, and can operate at variable speeds, making them suitable for direct‑drive wind turbines.
Correct answer is: Wind turbine direct drive
Q.52 The purpose of a damper winding in a synchronous machine used as a special purpose motor is to:
Increase the output voltage
Provide starting torque and damp oscillations
Reduce core loss
Improve power factor
Explanation - Damper windings act like a squirrel‑cage, giving the machine start-up torque and damping during transients.
Correct answer is: Provide starting torque and damp oscillations
Q.53 Which type of motor is most appropriate for a high‑precision, low‑speed positioning stage in semiconductor lithography?
Stepper motor
Switched reluctance motor
Servo motor with high‑resolution encoder
Universal motor
Explanation - Servo systems with fine feedback provide the sub‑micron accuracy required in lithography tools.
Correct answer is: Servo motor with high‑resolution encoder
Q.54 In a permanent‑magnet linear motor, the term ‘synchronous speed’ is defined as:
Speed at which back‑EMF equals supply voltage
Speed at which the moving magnetic field and the permanent magnets are in phase
Maximum achievable speed
Speed at which slip is 100 %
Explanation - Synchronous speed occurs when the traveling magnetic wave matches the velocity of the magnet‑carrying platen.
Correct answer is: Speed at which the moving magnetic field and the permanent magnets are in phase
Q.55 A typical reason for using a high‑speed, low‑inertia motor in a robotic end‑effector is:
To increase the overall mass of the robot
To achieve fast response and reduce settling time
To simplify the control algorithm
To lower the manufacturing cost
Explanation - Low inertia allows rapid acceleration and deceleration, improving the dynamic performance of the robot.
Correct answer is: To achieve fast response and reduce settling time
Q.56 The primary loss mechanism in a high‑frequency transformer core at 200 kHz is:
Hysteresis loss
Eddy‑current loss
Copper loss
Mechanical vibration loss
Explanation - At high frequencies, eddy‑current losses dominate because they increase with the square of frequency.
Correct answer is: Eddy‑current loss
Q.57 Which of the following is a benefit of using a permanent‑magnet generator in a wind turbine over an induction generator?
Higher slip at rated speed
Lower cost
Higher efficiency and lower weight
Ability to operate without a power electronics converter
Explanation - PMSGs have higher efficiency due to reduced copper losses and are lighter because they lack rotor windings.
Correct answer is: Higher efficiency and lower weight
Q.58 In a magnetic levitation system, the stability of the levitated object is often achieved using:
Passive permanent magnet arrangement only
Active feedback control of electromagnets
Hydraulic dampers
Mechanical springs
Explanation - Active control adjusts the electromagnetic forces in real time to maintain stable levitation.
Correct answer is: Active feedback control of electromagnets
Q.59 Which of the following is the main reason to use a three‑phase inverter instead of a six‑step (trapezoidal) drive for a BLDC motor in a precision application?
To reduce the number of power devices
To obtain smoother current waveforms and lower torque ripple
To increase the back‑EMF amplitude
To simplify the control algorithm
Explanation - A three‑phase sinusoidal PWM inverter provides smoother currents, reducing torque ripple and noise.
Correct answer is: To obtain smoother current waveforms and lower torque ripple
Q.60 A typical application of an electro‑static linear motor is:
High‑speed train propulsion
Micro‑positioning stages in microscopy
Large‑scale conveyor belts
Industrial welding
Explanation - Electro‑static motors provide very fine, low‑force motion suitable for micro‑positioning tasks.
Correct answer is: Micro‑positioning stages in microscopy
Q.61 In a switched reluctance motor, the torque is generated because:
Permanent magnets attract the stator poles
Reluctance is minimized when the rotor aligns with the energized stator pole
Induced currents in the rotor create a magnetic field
Electrostatic forces pull the rotor
Explanation - SRM torque arises from the tendency of the rotor to move to a position of lowest magnetic reluctance.
Correct answer is: Reluctance is minimized when the rotor aligns with the energized stator pole
Q.62 The main disadvantage of a permanent‑magnet motor in a high‑temperature environment is:
Increased copper losses
Demagnetization of the permanent magnets
Higher slip
Reduced inductance
Explanation - High temperatures can reduce the magnetic flux density of permanent magnets, decreasing motor performance.
Correct answer is: Demagnetization of the permanent magnets
Q.63 Which method is commonly used to improve the thermal performance of a high‑power BLDC motor?
Increasing the number of poles
Embedding cooling channels in the stator
Using higher‑resistance winding wire
Reducing the number of phases
Explanation - Direct liquid cooling of the stator removes heat efficiently from high‑power motors.
Correct answer is: Embedding cooling channels in the stator
Q.64 In a stepper motor, the term ‘microstepping’ refers to:
Dividing a full step into finer steps by controlling current amplitudes
Using a higher supply voltage
Changing the number of poles
Operating the motor in reverse direction
Explanation - Microstepping modulates winding currents to achieve fractional steps, increasing resolution and smoothness.
Correct answer is: Dividing a full step into finer steps by controlling current amplitudes
Q.65 A typical use of an eddy‑current brake in a special purpose machine is:
Providing continuous torque in a motor
Actively accelerating a rotating shaft
Providing non‑contact, wear‑free braking in amusement rides
Generating electrical power
Explanation - Eddy‑current brakes offer smooth, maintenance‑free deceleration ideal for rides and some industrial machines.
Correct answer is: Providing non‑contact, wear‑free braking in amusement rides
Q.66 The torque constant (Kt) of a motor is numerically equal to:
The back‑EMF constant (Kb) when expressed in SI units
The resistance of the winding
The inductance of the winding
The slip of the motor
Explanation - In SI units, Kt (Nm/A) = Kb (V·s/rad).
Correct answer is: The back‑EMF constant (Kb) when expressed in SI units
Q.67 Which of the following is a primary advantage of using a three‑phase permanent‑magnet linear motor for a high‑speed maglev train?
Lower construction cost
Higher thrust per unit length
Reduced magnetic field strength
Simpler control electronics
Explanation - Three‑phase PMS linear motors can generate high thrust densities, essential for high‑speed maglev propulsion.
Correct answer is: Higher thrust per unit length
Q.68 In a flywheel energy storage system, the maximum stored kinetic energy is proportional to:
The square of the rotational speed
The cube of the flywheel radius
The mass of the flywheel only
The square of the flywheel diameter
Explanation - Kinetic energy = ½ J ω², where ω is angular speed; thus energy varies with the square of speed.
Correct answer is: The square of the rotational speed
Q.69 A common method for reducing cogging torque in a permanent‑magnet motor is to:
Increase the air gap
Skew the rotor or stator slots
Decrease the number of poles
Use higher supply voltage
Explanation - Skewing distributes the magnetic interaction over a range of positions, smoothing out cogging.
Correct answer is: Skew the rotor or stator slots
Q.70 Which type of special purpose machine typically uses a combination of permanent magnets and a wound field to achieve high torque at low speeds?
Switched reluctance motor
Hybrid stepper motor
Hybrid synchronous motor (PM + wound field)
Universal motor
Explanation - Hybrid synchronous motors combine permanent magnets with a field winding for enhanced low‑speed torque.
Correct answer is: Hybrid synchronous motor (PM + wound field)
Q.71 The main cause of heat generation in a high‑speed BLDC motor is:
Core hysteresis loss
Copper (I²R) loss in the windings
Friction in the brushes
Eddy‑current loss in the rotor
Explanation - At high speeds, winding resistance losses dominate because there are no brushes and core losses are relatively low.
Correct answer is: Copper (I²R) loss in the windings
Q.72 In a magnetic levitation system using electrodynamic suspension (EDS), levitation is achieved by:
Attractive forces between permanent magnets and rails
Repulsive forces generated by induced currents in conductive tracks
Hydraulic pressure
Mechanical springs
Explanation - EDS uses the repulsion between moving magnets and induced currents in the guideway to levitate the vehicle.
Correct answer is: Repulsive forces generated by induced currents in conductive tracks
Q.73 Which of the following control strategies is most suitable for a direct‑drive wind turbine using a permanent‑magnet synchronous generator?
Scalar V/f control
Field‑oriented control with rotor position sensor
Open‑loop voltage control
Current‑feedforward control only
Explanation - FOC with rotor position sensing provides optimal torque and efficient operation across varying wind speeds.
Correct answer is: Field‑oriented control with rotor position sensor
Q.74 A key advantage of using a brushless DC motor in a robotic arm joint is:
Lower cost than a stepper motor
Absence of brushes leading to higher reliability and lower maintenance
Higher cogging torque for better holding
Simpler control without sensors
Explanation - Brushless design eliminates brush wear, increasing lifespan and reducing maintenance for robotic joints.
Correct answer is: Absence of brushes leading to higher reliability and lower maintenance
Q.75 In a high‑frequency induction heater, the workpiece is heated primarily due to:
Hysteresis loss in the workpiece
Eddy‑current loss in the workpiece
Radiation from the coil
Conduction from the coil
Explanation - Rapidly changing magnetic fields induce eddy currents that dissipate energy as heat within the conductive workpiece.
Correct answer is: Eddy‑current loss in the workpiece
Q.76 Which type of motor is most appropriate for a high‑precision, low‑speed camera pan‑tilt platform?
Stepper motor
Switched reluctance motor
Servo motor with encoder feedback
Universal motor
Explanation - Servo systems with precise feedback enable smooth, accurate positioning at low speeds required for camera platforms.
Correct answer is: Servo motor with encoder feedback
Q.77 The main function of the ‘air gap’ in a magnetic bearing is to:
Provide lubrication
Allow magnetic flux to pass while permitting movement
Reduce electrical resistance
Increase mechanical friction
Explanation - The air gap separates the rotor and stator, enabling magnetic forces to act while allowing the rotor to levitate.
Correct answer is: Allow magnetic flux to pass while permitting movement
Q.78 In a hybrid stepper motor (permanent‑magnet + wound rotor), the advantage over a conventional permanent‑magnet stepper is:
Higher step angle
Lower torque at low speed
Higher holding torque
Simpler driver electronics
Explanation - The wound rotor adds magnetic flux, increasing the motor’s holding torque compared to a pure PM stepper.
Correct answer is: Higher holding torque
Q.79 Which of the following is a typical frequency range for a linear synchronous motor used in maglev transportation?
50 Hz – 60 Hz
400 Hz – 800 Hz
2 kHz – 5 kHz
10 kHz – 20 kHz
Explanation - Maglev LSMs commonly operate in the low‑kHz range to achieve high thrust while keeping core losses manageable.
Correct answer is: 2 kHz – 5 kHz
Q.80 A primary reason to select a switched reluctance motor for a high‑temperature environment is:
Its permanent magnets are heat‑resistant
It has no permanent magnets, avoiding demagnetization issues
It operates at high slip
It requires less cooling
Explanation - SRMs are magnet‑free, so they are not affected by high temperatures that would demagnetize permanent‑magnet motors.
Correct answer is: It has no permanent magnets, avoiding demagnetization issues
Q.81 In an induction motor, the term ‘slip’ is defined as:
Difference between mechanical speed and synchronous speed expressed as a percentage of synchronous speed
Phase difference between voltage and current
Loss of magnetic flux
Decrease in resistance with temperature
Explanation - Slip = (Ns – Nm)/Ns, where Ns is synchronous speed and Nm is mechanical speed.
Correct answer is: Difference between mechanical speed and synchronous speed expressed as a percentage of synchronous speed
Q.82 Which of the following is the most suitable motor for a high‑speed, low‑torque application such as a small UAV propeller?
Switched reluctance motor
Brushless DC motor
Stepper motor
Universal motor
Explanation - BLDC motors provide high RPM, good efficiency, and low weight, fitting small UAV propeller needs.
Correct answer is: Brushless DC motor
Q.83 The term ‘pole pitch’ in a linear motor refers to:
The distance between two adjacent stator teeth
The number of magnetic poles per unit length
The length of one magnetic pole pair
The width of the air gap
Explanation - Pole pitch is the spatial length covering one north and one south pole in the linear motor.
Correct answer is: The length of one magnetic pole pair
Q.84 In a magnetic bearing, the active control loop typically measures:
Rotor temperature
Rotor position
Supply voltage
Air pressure
Explanation - Sensors detect the rotor's position to adjust electromagnetic forces and keep it centered.
Correct answer is: Rotor position
Q.85 Which type of motor is commonly used in a high‑precision CNC lathe for the main spindle?
Induction motor with VFD
Brushless DC motor
Synchronous reluctance motor
Switched reluctance motor
Explanation - BLDC spindles deliver high speed, low vibration, and precise control required for CNC lathe operations.
Correct answer is: Brushless DC motor
Q.86 A common method to achieve sensorless operation of a BLDC motor is:
Measuring back‑EMF in the un‑powered phase
Using Hall‑effect sensors
Employing a resolver
Installing a rotary encoder
Explanation - Back‑EMF detection provides rotor position information without physical sensors.
Correct answer is: Measuring back‑EMF in the un‑powered phase
Q.87 Which characteristic is typically higher in a switched reluctance motor compared with a permanent‑magnet motor of the same size?
Torque density
Efficiency at low speed
Power factor
Rotor inertia
Explanation - SRMs can achieve higher torque per unit volume due to robust rotor design and high magnetic flux density in salient poles.
Correct answer is: Torque density
Q.88 In an induction heating coil, the skin depth in the workpiece is primarily a function of:
Coil diameter
Supply voltage magnitude
Frequency of the supply current
Number of coil turns
Explanation - Skin depth δ = √(2/ (ω μ σ)), inversely proportional to the square root of frequency.
Correct answer is: Frequency of the supply current
Q.89 A common application of a permanent‑magnet synchronous generator (PMSG) in renewable energy is:
Wind turbine direct drive
Hydroelectric turbine excitation
Solar panel power conversion
Battery charger
Explanation - PMSGs are used in direct‑drive wind turbines for their high efficiency and low maintenance.
Correct answer is: Wind turbine direct drive
Q.90 In a hybrid stepper motor, the term ‘detent torque’ refers to:
Torque produced when the motor is powered
Torque due to magnetic attraction when the motor is un‑powered
Torque generated by the winding current
Torque at maximum speed
Explanation - Detent torque is the residual holding torque that keeps the rotor in position without current.
Correct answer is: Torque due to magnetic attraction when the motor is un‑powered
Q.91 Which of the following best describes the operating principle of a linear induction motor (LIM)?
Moving permanent magnets interact with a stationary stator
A rotating magnetic field induces currents in a secondary conductive plate, producing thrust
Electrostatic forces pull the mover
Mechanical gears convert rotary motion to linear motion
Explanation - LIMs generate a traveling magnetic field that induces eddy currents in the secondary, creating a thrust force.
Correct answer is: A rotating magnetic field induces currents in a secondary conductive plate, producing thrust
Q.92 In a magnetic levitation system that uses superconducting magnets, the primary benefit is:
Reduced power consumption due to zero resistance
Simpler control electronics
Higher operating temperature
Lower magnetic field strength
Explanation - Superconductors carry current without resistance, allowing strong magnetic fields with minimal power loss.
Correct answer is: Reduced power consumption due to zero resistance
Q.93 The main reason to use a three‑phase inverter with sinusoidal PWM for a BLDC motor in a high‑precision motion system is:
To increase the back‑EMF amplitude
To reduce acoustic noise and torque ripple
To simplify the gate driver design
To operate the motor at higher voltage
Explanation - Sinusoidal PWM creates smoother current waveforms, lowering torque ripple and audible noise, which is essential for precision.
Correct answer is: To reduce acoustic noise and torque ripple
Q.94 Which of the following is a characteristic advantage of a permanent‑magnet linear motor over a traditional belt‑driven linear system?
Lower initial cost
Zero mechanical wear and maintenance
Higher friction
Simpler alignment requirements
Explanation - Because motion is generated by magnetic forces without contact, there is no wear, leading to lower maintenance.
Correct answer is: Zero mechanical wear and maintenance
Q.95 In a high‑speed BLDC motor, the maximum achievable speed is primarily limited by:
Back‑EMF approaching the supply voltage
Number of poles
Core saturation
Air gap length
Explanation - As speed increases, back‑EMF rises; when it equals the supply voltage, current (and thus torque) falls to zero.
Correct answer is: Back‑EMF approaching the supply voltage
Q.96 Which type of special purpose machine is typically used for rapid, high‑precision cutting of thin metal sheets in the aerospace industry?
Induction heating coil
Eddy‑current brake
Linear induction motor
Electro‑static discharge machine
Explanation - Linear induction motors can provide high thrust and precise, contact‑less motion for cutting operations.
Correct answer is: Linear induction motor
Q.97 A key advantage of using a permanent‑magnet synchronous motor (PMSM) in a robotic joint is:
High cogging torque for stable holding
Low torque density
High power‑to‑weight ratio
Requirement of a large air gap
Explanation - PMSMs deliver high torque in a compact, lightweight package, ideal for robotic joints.
Correct answer is: High power‑to‑weight ratio
Q.98 Which parameter primarily determines the size of a flywheel needed to store a given amount of energy?
Flywheel radius
Flywheel material density
Flywheel rotational speed
Air gap length
Explanation - Stored kinetic energy varies with the square of angular velocity; increasing speed reduces required mass/size.
Correct answer is: Flywheel rotational speed
Q.99 In a magnetic bearing, the term ‘active magnetic bearing’ (AMB) indicates that:
The bearing uses permanent magnets only
The magnetic forces are controlled in real time by an electronic feedback system
The bearing is mechanically locked
The system operates at cryogenic temperatures
Explanation - Active magnetic bearings rely on sensors and actuators to adjust magnetic forces and maintain stability.
Correct answer is: The magnetic forces are controlled in real time by an electronic feedback system
Q.100 Which of the following is a typical characteristic of a switched reluctance generator used in a small wind turbine?
High inertia
Low fault tolerance
Capability to operate without a separate excitation source
Requirement for permanent magnets
Explanation - SR generators produce their own magnetic field through rotor saliency, eliminating the need for an external exciter.
Correct answer is: Capability to operate without a separate excitation source
Q.101 The primary function of the 'damper winding' in a synchronous motor used as a special purpose machine is to:
Increase the motor's rated voltage
Provide a path for induced currents during transients, reducing oscillations
Reduce the motor's weight
Improve the motor's thermal conductivity
Explanation - Damper windings act like a squirrel‑cage, damping speed oscillations and supplying start‑up torque.
Correct answer is: Provide a path for induced currents during transients, reducing oscillations
Q.102 In a high‑speed spindle using a BLDC motor, the main reason for selecting a three‑phase winding instead of a single‑phase is:
To reduce the number of power switches
To achieve smoother torque and lower ripple
To increase the motor's weight
To simplify the control algorithm
Explanation - Three‑phase windings produce a rotating magnetic field, yielding smoother torque compared with single‑phase designs.
Correct answer is: To achieve smoother torque and lower ripple
Q.103 A common method for detecting the position of a rotor in a sensorless BLDC motor at low speeds is:
Back‑EMF zero‑crossing detection
Hall‑effect sensor readout
Optical encoder feedback
Resolver measurement
Explanation - At low speeds, back‑EMF amplitude is small; detecting its zero crossings can infer rotor position without sensors.
Correct answer is: Back‑EMF zero‑crossing detection
Q.104 In a magnetic levitation (maglev) system employing electrodynamic suspension, the levitation force is proportional to:
Current squared and velocity of the magnets
Supply voltage only
Air gap length only
Number of permanent magnets
Explanation - In EDS, levitation force depends on induced currents, which are proportional to both magnetic field strength (current) and relative velocity.
Correct answer is: Current squared and velocity of the magnets
Q.105 Which of the following is a typical design goal when using a permanent‑magnet linear motor for a high‑precision positioning stage?
Maximize cogging torque
Minimize magnetic thrust ripple
Increase air gap size
Reduce winding resistance at the expense of size
Explanation - Reducing thrust ripple improves positioning accuracy and smoothness in precision stages.
Correct answer is: Minimize magnetic thrust ripple
Q.106 The main advantage of a switched reluctance motor for an oil‑pump in a deep‑sea submersible is:
High efficiency at low speeds
No permanent magnets, avoiding demagnetization under high pressure
Low torque ripple
Built‑in position sensors
Explanation - SRMs are magnet‑free, making them robust in harsh high‑pressure environments where permanent magnets could be compromised.
Correct answer is: No permanent magnets, avoiding demagnetization under high pressure
Q.107 In an induction heating system, the term ‘Q‑factor’ of the work coil primarily influences:
The frequency of operation
The efficiency of power transfer to the workpiece
The mechanical strength of the coil
The temperature rating of the power supply
Explanation - A high Q‑factor indicates lower resistive losses in the coil, improving efficiency of energy delivery to the workpiece.
Correct answer is: The efficiency of power transfer to the workpiece
Q.108 Which type of motor is most suitable for a high‑torque, low‑speed drive in an electric forklift?
Switched reluctance motor
Brushless DC motor
Universal motor
Stepper motor
Explanation - SRMs provide high torque at low speeds, are robust, and can be designed for high power densities, fitting forklift requirements.
Correct answer is: Switched reluctance motor
Q.109 A key limitation of using a permanent‑magnet motor for a high‑temperature industrial furnace drive is:
Low torque at low speeds
Loss of magnetic flux due to demagnetization at high temperatures
Excessive size
High slip
Explanation - Permanent magnets can lose magnetization when exposed to elevated temperatures, reducing motor performance.
Correct answer is: Loss of magnetic flux due to demagnetization at high temperatures
Q.110 In a magnetic bearing, the term ‘stiffness’ refers to:
The maximum current the bearing can handle
The relationship between magnetic force and rotor displacement
The thermal resistance of the bearing
The magnetic material’s coercivity
Explanation - Stiffness is the derivative of force with respect to displacement, indicating how strongly the bearing resists motion.
Correct answer is: The relationship between magnetic force and rotor displacement
Q.111 Which of the following is a primary benefit of using a flywheel for short‑term energy storage in a power grid?
Very high energy density compared to batteries
Instantaneous power delivery and absorption
Zero maintenance requirements
Ability to store energy for months without loss
Explanation - Flywheels can rapidly absorb and release kinetic energy, making them ideal for grid frequency regulation.
Correct answer is: Instantaneous power delivery and absorption
Q.112 A hybrid stepper motor combines permanent magnets with a wound rotor to achieve:
Higher step angle
Higher holding torque and better detent torque
Lower inductance
Simpler driver circuitry
Explanation - The wound rotor adds magnetic flux, increasing both holding and detent torques compared with pure PM stepper motors.
Correct answer is: Higher holding torque and better detent torque
Q.113 Which type of special purpose machine is commonly employed for non‑contact material handling in clean‑room environments?
Eddy‑current brake
Magnetic levitation transport system
Induction heating coil
Universal motor
Explanation - Maglev transport offers frictionless, contaminant‑free movement, ideal for clean‑room handling of delicate components.
Correct answer is: Magnetic levitation transport system
Q.114 In a three‑phase BLDC motor, the back‑EMF waveform is ideally:
Square
Triangular
Sinusoidal
Sawtooth
Explanation - A sinusoidal back‑EMF leads to smoother torque and is the target for high‑performance BLDC designs.
Correct answer is: Sinusoidal
Q.115 Which of the following is the main advantage of using a permanent‑magnet synchronous generator (PMSG) over an induction generator in offshore wind turbines?
Lower cost
Higher efficiency and reduced weight
Ability to operate without power electronics
Simpler cooling requirements
Explanation - PMSGs have higher efficiency due to eliminated rotor copper losses and are lighter, beneficial for offshore installation.
Correct answer is: Higher efficiency and reduced weight
Q.116 A key design consideration for a linear motor used in a high‑speed maglev train is:
Minimizing the air gap to increase thrust
Maximizing cogging torque for stability
Using low‑frequency operation to reduce core losses
Employing high‑resistance windings
Explanation - A small air gap enhances magnetic coupling, increasing thrust per unit length, critical for high‑speed maglev propulsion.
Correct answer is: Minimizing the air gap to increase thrust
Q.117 In a permanent‑magnet synchronous motor, the back‑EMF constant (Kb) and torque constant (Kt) are related by:
Kb = Kt × 2π
Kb = Kt / 2π
Kb = Kt (identical in SI units)
Kb = √(Kt)
Explanation - When expressed in SI units (V·s/rad for Kb and Nm/A for Kt), the two constants are numerically equal.
Correct answer is: Kb = Kt (identical in SI units)
Q.118 Which of the following is a typical application of an eddy‑current dynamometer?
Measuring torque of high‑speed electric motors
Providing propulsion for marine vessels
Generating electrical power from wind
Actuating valve position in pipelines
Explanation - Eddy‑current dynamometers provide a non‑contact method to measure torque and speed of rotating shafts.
Correct answer is: Measuring torque of high‑speed electric motors
Q.119 A common method to increase the efficiency of a switched reluctance motor is to:
Increase the number of poles
Add permanent magnets to the rotor
Use soft magnetic composite (SMC) materials for the rotor
Reduce the stator winding resistance
Explanation - Lower winding resistance reduces I²R losses, directly improving motor efficiency.
Correct answer is: Reduce the stator winding resistance
Q.120 In a magnetic levitation system, the term ‘passive magnetic bearing’ refers to:
A bearing that uses permanent magnets only, without active control
A bearing that requires hydraulic fluid
A bearing that uses electromagnets with real‑time feedback
A bearing that relies on mechanical springs
Explanation - Passive magnetic bearings rely on static magnetic fields from permanent magnets to support the load, without active regulation.
Correct answer is: A bearing that uses permanent magnets only, without active control
Q.121 Which of the following is NOT an advantage of using a brushless DC motor for an electric aircraft propulsion system?
Higher reliability due to lack of brushes
Reduced electromagnetic interference
Higher weight compared with induction motors
Better efficiency across a wide speed range
Explanation - BLDC motors are generally lighter than comparable induction motors, which is a benefit, not a disadvantage.
Correct answer is: Higher weight compared with induction motors
Q.122 In a linear synchronous motor, the term ‘synchronous speed’ is given by:
v = 2·f·τ
v = f/τ
v = τ/f
v = 2·π·f·L
Explanation - Synchronous linear speed v equals twice the product of frequency f and pole pitch τ.
Correct answer is: v = 2·f·τ
Q.123 A typical reason to use a permanent‑magnet linear motor in a semiconductor wafer stepper system is:
To provide extremely high thrust at low speed
To achieve ultra‑smooth, vibration‑free motion
To reduce the required power supply voltage
To allow operation in vacuum without cooling
Explanation - Linear motors provide contact‑less, precise motion with minimal vibration, essential for wafer positioning.
Correct answer is: To achieve ultra‑smooth, vibration‑free motion
Q.124 In a high‑speed rotor of a BLDC motor, which loss mechanism becomes dominant as speed increases?
Core hysteresis loss
Eddy‑current loss in the permanent magnets
Friction loss in bearings
Windage loss
Explanation - At high rpm, aerodynamic drag (windage) becomes the primary loss source.
Correct answer is: Windage loss
Q.125 Which of the following special purpose machines is most suited for non‑contact braking of a high‑speed rotor in a turbo‑generator?
Eddy‑current brake
Friction brake
Hydraulic brake
Mechanical clutch
Explanation - Eddy‑current brakes provide smooth, wear‑free deceleration ideal for high‑speed rotating machinery.
Correct answer is: Eddy‑current brake
Q.126 A major advantage of a permanent‑magnet synchronous generator (PMSG) in a direct‑drive wind turbine is:
Elimination of a gearbox
Higher slip compared with induction generators
Requirement of an external excitation system
Lower efficiency at low wind speeds
Explanation - PMSGs can be directly coupled to the turbine rotor, removing the gearbox and improving reliability.
Correct answer is: Elimination of a gearbox
Q.127 In a magnetic bearing system, the term ‘active control bandwidth’ refers to:
The frequency range over which the control system can effectively respond to disturbances
The physical width of the bearing housing
The maximum current the electromagnets can handle
The temperature range of operation
Explanation - Bandwidth determines how quickly the bearing can counteract position errors and vibrations.
Correct answer is: The frequency range over which the control system can effectively respond to disturbances
Q.128 Which of the following is a typical application of a piezoelectric linear actuator?
Heavy‑load industrial conveyor
Fine positioning in scanning electron microscopes
High‑speed train propulsion
Large‑scale robotic arm
Explanation - Piezoelectric actuators provide nanometer‑scale motion, ideal for precision instrumentation like SEMs.
Correct answer is: Fine positioning in scanning electron microscopes
Q.129 The primary reason a permanent‑magnet motor can achieve a higher power density than an induction motor of the same size is:
Absence of rotor windings reduces copper losses
Higher slip at rated speed
Use of higher frequency supply
Larger air gap
Explanation - Without rotor windings, permanent‑magnet motors eliminate rotor I²R losses, allowing more compact, higher‑density designs.
Correct answer is: Absence of rotor windings reduces copper losses
Q.130 In a switched reluctance motor, torque ripple can be minimized by:
Increasing the number of rotor poles
Skewing the stator slots
Using permanent magnets on the rotor
Operating at high slip
Explanation - Skewing spreads the torque production over a larger angular range, reducing ripple.
Correct answer is: Skewing the stator slots
Q.131 A magnetic bearing can be categorized as ‘self‑stabilizing’ when:
It uses permanent magnets only
Its magnetic configuration inherently provides restoring forces without active control
It requires a feedback controller for stability
It operates only at cryogenic temperatures
Explanation - Self‑stabilizing designs rely on magnetic geometry that naturally pushes the rotor back to the centered position.
Correct answer is: Its magnetic configuration inherently provides restoring forces without active control
Q.132 Which of the following machines is typically used for high‑speed, high‑precision sheet metal cutting in automotive manufacturing?
Linear induction motor
Stepper motor
Switched reluctance motor
Universal motor
Explanation - LIMs provide fast, contact‑less motion ideal for high‑speed sheet metal processing.
Correct answer is: Linear induction motor
Q.133 In a brushless DC motor, the term ‘commutation’ refers to:
Switching the supply voltage polarity manually
Changing the direction of the magnetic field in the stator windings
Rotating the permanent magnets
Adjusting the air gap length
Explanation - Electronic commutation alternates current in the windings to keep the torque-producing magnetic field aligned with the rotor.
Correct answer is: Changing the direction of the magnetic field in the stator windings
Q.134 Which of the following is a main advantage of using a permanent‑magnet linear motor in a high‑speed pick‑and‑place robot?
Lower cost compared with servo drives
Zero mechanical wear and high repeatability
Ability to operate at very low voltages
Simplified cooling system
Explanation - Linear motors provide contact‑less motion, leading to minimal wear and excellent positioning repeatability.
Correct answer is: Zero mechanical wear and high repeatability
Q.135 The term ‘skin effect’ in a high‑frequency transformer primarily affects:
Core losses
Winding resistance
Magnetic flux density
Air gap length
Explanation - At high frequencies, current concentrates near the surface of conductors, increasing effective resistance.
Correct answer is: Winding resistance
Q.136 In a magnetic levitation system employing electromagnetic attraction (EMS), the levitation force is generated by:
Repulsive forces between moving magnets
Attractive forces between electromagnets and permanent magnets
Hydraulic pressure
Mechanical springs
Explanation - EMS uses controlled electromagnetic attraction to pull the vehicle toward the guideway, balancing the gap.
Correct answer is: Attractive forces between electromagnets and permanent magnets