Q.1 Which component converts electrical energy into linear mechanical motion using a magnetic field?
Solenoid
Transformer
Capacitor
Inductor
Explanation - A solenoid creates a magnetic field when current flows, pulling a plunger to produce linear motion.
Correct answer is: Solenoid
Q.2 What is the primary function of a relay in a circuit?
To store electrical charge
To amplify a signal
To provide electrical isolation and control a high‑current load with a low‑current signal
To convert AC to DC
Explanation - A relay uses an electromagnet to operate a set of contacts, allowing a small control current to switch a larger load while keeping the circuits isolated.
Correct answer is: To provide electrical isolation and control a high‑current load with a low‑current signal
Q.3 In a DC motor, which part is responsible for generating the rotating magnetic field?
Stator
Commutator
Brushes
Armature
Explanation - The armature windings carry current that interacts with the stator’s magnetic field, producing torque and rotation.
Correct answer is: Armature
Q.4 Which type of electromechanical component is typically used to convert rotational motion into linear motion?
Stepper motor
Lead screw
Solenoid
Transformer
Explanation - A lead screw (or ball screw) converts rotary motion of a motor into precise linear displacement.
Correct answer is: Lead screw
Q.5 A contactor differs from a relay mainly because it:
Has a higher current rating and is used for motor loads
Operates only with AC voltage
Uses a solid‑state switch
Cannot be used for overload protection
Explanation - Contactors are designed for switching heavy currents, especially for motors, whereas relays are for lighter loads.
Correct answer is: Has a higher current rating and is used for motor loads
Q.6 What is the purpose of the brushes in a brushed DC motor?
To provide a permanent magnetic field
To supply current to the rotating armature
To cool the motor windings
To generate back‑EMF
Explanation - Brushes maintain electrical contact with the rotating commutator, delivering current to the armature windings.
Correct answer is: To supply current to the rotating armature
Q.7 Which electromechanical device is commonly used as a position feedback sensor in CNC machines?
Potentiometer
Hall‑effect sensor
Optical encoder
Thermistor
Explanation - Optical encoders provide precise digital pulses representing angular position, ideal for motion control.
Correct answer is: Optical encoder
Q.8 A stepper motor moves in discrete steps. What determines the step angle?
Number of poles on the rotor
Supply voltage
Number of stator phases
Number of teeth on the gear attached
Explanation - The step angle is calculated as 360° divided by the number of rotor teeth (or poles) multiplied by the number of phases.
Correct answer is: Number of poles on the rotor
Q.9 Which component provides overload protection in a motor starter circuit?
Contactor
Thermal overload relay
Fuse
Circuit breaker
Explanation - A thermal overload relay senses motor current and opens the circuit when the current exceeds a set value for a certain time.
Correct answer is: Thermal overload relay
Q.10 In an electromechanical buzzer, sound is produced by:
Rapidly moving a magnetic plunger in and out of a coil
Heating a resistor
Charging a capacitor
Vibrating a piezo crystal
Explanation - Electromechanical buzzers use a coil to drive a metal plunger, creating sound through mechanical vibration.
Correct answer is: Rapidly moving a magnetic plunger in and out of a coil
Q.11 What type of electromechanical actuator converts electrical energy directly into rotary motion without a motor?
Solenoid
Gear reducer
Linear actuator
Rotary vane actuator
Explanation - Rotary vane actuators use electromagnetic forces on a rotating vane to produce torque directly.
Correct answer is: Rotary vane actuator
Q.12 A DC generator differs from a DC motor primarily in:
The number of windings
The direction of power flow
The type of core material
The presence of brushes
Explanation - In a generator mechanical energy is converted to electrical energy, opposite of a motor.
Correct answer is: The direction of power flow
Q.13 Which parameter of a solenoid determines the force it can exert?
Core material permeability
Supply frequency
Number of turns and current
Resistance of the coil
Explanation - Force is proportional to the product of coil turns and current (ampere‑turns) and the magnetic circuit design.
Correct answer is: Number of turns and current
Q.14 Which electromechanical component is used to convert a small linear displacement into a larger linear motion?
Gear train
Hydraulic cylinder
Lead screw
Cam mechanism
Explanation - A lead screw translates rotation into linear motion with mechanical advantage, amplifying displacement.
Correct answer is: Lead screw
Q.15 A relay's coil is rated at 12 V DC, 100 mA. What is the coil's resistance?
0.12 Ω
1.2 Ω
12 Ω
120 Ω
Explanation - R = V / I = 12 V / 0.1 A = 120 Ω.
Correct answer is: 120 Ω
Q.16 In a three‑phase induction motor, what is the role of the squirrel‑cage rotor?
To provide a rotating magnetic field
To induce current and develop torque
To act as a permanent magnet
To supply power to the stator
Explanation - The rotating magnetic field induces currents in the squirrel‑cage bars, producing torque.
Correct answer is: To induce current and develop torque
Q.17 Which electromechanical component is commonly used in automotive applications for valve timing?
Stepper motor
Solenoid
Piezoelectric actuator
Linear variable differential transformer (LVDT)
Explanation - Solenoids are used to open/close engine valves and actuate fuel injectors.
Correct answer is: Solenoid
Q.18 A potentiometer is an example of a:
Variable resistor
Capacitor
Inductor
Transformer
Explanation - A potentiometer provides a continuously variable resistance, often used as a voltage divider.
Correct answer is: Variable resistor
Q.19 What is the main advantage of a solid‑state relay (SSR) over an electromechanical relay?
Higher current rating
Zero moving parts leading to longer life and faster switching
Ability to handle AC only
Lower cost
Explanation - SSRs use semiconductor devices, eliminating mechanical wear and providing rapid switching.
Correct answer is: Zero moving parts leading to longer life and faster switching
Q.20 In an AC induction motor, slip is defined as:
The difference between rotor speed and synchronous speed expressed as a percentage of synchronous speed
The phase shift between voltage and current
The voltage drop across the stator windings
The resistance of the rotor bars
Explanation - Slip = (Ns – Nr) / Ns, where Ns is synchronous speed and Nr is rotor speed.
Correct answer is: The difference between rotor speed and synchronous speed expressed as a percentage of synchronous speed
Q.21 Which of the following electromechanical components is best suited for precise angular positioning in robotics?
Servo motor
Shaded‑pole motor
Universal motor
Reluctance motor
Explanation - Servo motors combine a motor, feedback sensor, and controller for accurate position control.
Correct answer is: Servo motor
Q.22 A reed switch operates based on:
Thermal expansion
Magnetic field movement of ferromagnetic reeds
Electrostatic attraction
Piezoelectric effect
Explanation - Reed switches consist of two ferromagnetic blades sealed in a glass tube that close when a magnetic field is applied.
Correct answer is: Magnetic field movement of ferromagnetic reeds
Q.23 What is the typical purpose of a snubber circuit across a relay contact?
To limit inrush current
To suppress voltage spikes caused by inductive loads
To increase contact bounce time
To provide a steady DC bias
Explanation - Snubber circuits (RC or diode) protect contacts from high transient voltages generated when an inductive load is switched off.
Correct answer is: To suppress voltage spikes caused by inductive loads
Q.24 In a brushed DC motor, what would happen if the brushes are worn out?
The motor speed would increase
The motor would produce more torque
The motor may fail to start or run erratically
The motor would become more efficient
Explanation - Worn brushes lose proper contact with the commutator, leading to intermittent current flow and poor performance.
Correct answer is: The motor may fail to start or run erratically
Q.25 Which electromechanical component is essentially a DC motor with built‑in position feedback?
Stepper motor
Servo motor
Linear actuator
Universal motor
Explanation - Servo motors incorporate a motor, sensor (often an encoder or resolver), and control circuitry for closed‑loop position control.
Correct answer is: Servo motor
Q.26 The back‑EMF in a DC motor is proportional to:
Armature resistance
Motor speed
Supply voltage
Number of brushes
Explanation - Back‑EMF = k × speed, where k is a motor constant; it opposes the applied voltage.
Correct answer is: Motor speed
Q.27 Which of the following is a key disadvantage of using a solenoid as a linear actuator in high‑speed applications?
Limited stroke length
High power consumption
Magnetic saturation leading to reduced force at high speed
Inability to operate in vacuum
Explanation - At high actuation speeds, the magnetic circuit can saturate, limiting the force generated by the solenoid.
Correct answer is: Magnetic saturation leading to reduced force at high speed
Q.28 A torque‑to‑weight ratio is an important metric for which electromechanical component?
Capacitor
Stepper motor
Transformer
Resistor
Explanation - Step motors are evaluated for how much torque they can deliver relative to their mass, especially in robotics.
Correct answer is: Stepper motor
Q.29 Which type of electromechanical switch is designed to handle very high currents (several kilo‑amps) in industrial power distribution?
Miniature relay
Contactors
Solid‑state relay
Bistable relay
Explanation - Contactors are built for high‑current applications such as motor starters and power distribution panels.
Correct answer is: Contactors
Q.30 In a linear variable differential transformer (LVDT), the output voltage is:
Directly proportional to displacement
Inversely proportional to displacement
Independent of displacement
Proportional to the square of displacement
Explanation - LVDT generates an AC voltage whose amplitude varies linearly with the core's position.
Correct answer is: Directly proportional to displacement
Q.31 Which electromechanical component is most suitable for a fail‑safe design where the device should stay off when power is lost?
Normally‑closed relay
Normally‑open relay
Solid‑state relay
Push‑button switch
Explanation - A normally‑closed (NC) relay opens when energized; loss of power keeps the circuit open (off).
Correct answer is: Normally‑closed relay
Q.32 What is the typical duty cycle rating for a standard electromechanical relay?
100 % continuous
50 % on / 50 % off
10 % on / 90 % off
Depends on coil voltage
Explanation - Most relays are rated for intermittent use; a 10 % duty cycle is common to avoid overheating.
Correct answer is: 10 % on / 90 % off
Q.33 In a three‑phase synchronous motor, the rotor is magnetized by:
Induction
Permanent magnets
DC excitation winding
Self‑excitation through slip rings
Explanation - Synchronous motors use a DC field winding on the rotor (or permanent magnets) to create a constant magnetic field.
Correct answer is: DC excitation winding
Q.34 What is the primary reason for using a flyback diode across a relay coil?
To increase coil resistance
To protect driving transistor from voltage spikes
To reduce coil inductance
To speed up coil de‑energization
Explanation - When the coil is turned off, the flyback diode provides a path for the inductive current, limiting voltage spikes.
Correct answer is: To protect driving transistor from voltage spikes
Q.35 A motor’s 'stall torque' refers to:
The torque at maximum speed
The torque when the motor is not moving
The torque at half‑load
The torque after the motor has been running for 10 minutes
Explanation - Stall torque is the maximum torque the motor can produce at zero speed.
Correct answer is: The torque when the motor is not moving
Q.36 Which electromechanical component is often used as a safety interlock in a machine door?
Solenoid
Reed switch
Potentiometer
Thermistor
Explanation - Reed switches can detect the presence of a magnetic field from a magnet on a door, providing a simple, reliable interlock.
Correct answer is: Reed switch
Q.37 In a gear motor, the gear reduction primarily affects:
Voltage rating
Current rating
Speed‑to‑torque ratio
Insulation resistance
Explanation - Gear reduction lowers speed while proportionally increasing torque.
Correct answer is: Speed‑to‑torque ratio
Q.38 A brushed DC motor with a commutator having 4 segments will have how many distinct electrical positions per electrical cycle?
2
4
8
16
Explanation - Each segment represents one electrical position; 4 segments give 4 distinct positions per cycle.
Correct answer is: 4
Q.39 What is the main function of a 'free‑wheel' or 'diode' in a DC motor driver circuit?
To limit the motor speed
To provide a path for inductive current when switching off
To increase motor torque
To convert DC to AC
Explanation - A free‑wheel diode (flyback diode) safely dissipates the energy stored in motor inductance.
Correct answer is: To provide a path for inductive current when switching off
Q.40 Which of the following electromechanical components typically uses a permanent magnet rotor?
Induction motor
Universal motor
Brushless DC motor
Shaded‑pole motor
Explanation - Brushless DC motors have permanent magnet rotors and electronically switched stator windings.
Correct answer is: Brushless DC motor
Q.41 A 'latching relay' differs from a standard relay because:
It requires continuous power to maintain the contact position
It stays in its last position without continuous coil power
It can only switch AC loads
It has a built‑in transformer
Explanation - Latching relays use a bistable mechanism; a pulse changes state and it remains until the next pulse.
Correct answer is: It stays in its last position without continuous coil power
Q.42 The term 'contact bounce' in relays refers to:
Mechanical vibration that causes multiple rapid make‑break cycles when contacts close
Electrical arcing during switching
Thermal expansion of the contacts
Noise generated by the coil
Explanation - Contact bounce can cause false triggering; debouncing circuits are often used to eliminate it.
Correct answer is: Mechanical vibration that causes multiple rapid make‑break cycles when contacts close
Q.43 Which electromechanical device is commonly used to convert rotational motion into a reciprocating (back‑and‑forth) motion in engines?
Camshaft
Crankshaft
Gearbox
Flywheel
Explanation - A crankshaft transforms the circular motion of pistons into rotational motion and vice‑versa.
Correct answer is: Crankshaft
Q.44 In a DC motor, the relationship between voltage (V), speed (N), and back‑EMF (Eb) is:
V = Eb + I·R
V = Eb – I·R
V = Eb × I·R
V = Eb / I·R
Explanation - The applied voltage equals back‑EMF plus the voltage drop across the armature resistance (I·R).
Correct answer is: V = Eb + I·R
Q.45 Which type of sensor is used to detect angular position by generating two square‑wave outputs 90° out of phase?
Resolver
Hall‑effect sensor
Quadrature encoder
Potentiometer
Explanation - Quadrature encoders provide A and B channels offset by 90°, allowing direction detection.
Correct answer is: Quadrature encoder
Q.46 A 'thermostatic switch' operates based on:
Magnetic field strength
Temperature‑dependent expansion of a bimetallic strip
Light intensity
Acoustic vibrations
Explanation - The bimetallic strip bends with temperature changes, opening or closing contacts.
Correct answer is: Temperature‑dependent expansion of a bimetallic strip
Q.47 Which electromechanical component typically includes a spring return mechanism?
Solenoid
Relay
Stepper motor
Transformer
Explanation - Many solenoids have a spring that returns the plunger to its original position when the coil is de‑energized.
Correct answer is: Solenoid
Q.48 In a DC motor, if the supply voltage is increased while the load torque remains constant, what happens to the motor speed?
Speed decreases
Speed remains unchanged
Speed increases
Torque decreases
Explanation - Higher voltage raises back‑EMF, causing the motor to run faster for the same load.
Correct answer is: Speed increases
Q.49 Which electromechanical component is designed to produce a precise, repeatable linear displacement using a motor and a screw mechanism?
Linear actuator
Stepper motor
Solenoid
Relay
Explanation - Linear actuators combine a motor with a lead or ball screw to provide accurate linear motion.
Correct answer is: Linear actuator
Q.50 A 'Hall‑effect sensor' is commonly used in which type of electromechanical device?
Brushless DC motor for rotor position detection
Transformer for flux measurement
Relay for contact monitoring
Capacitor for voltage regulation
Explanation - Hall sensors detect magnetic fields from the rotor, providing commutation signals for BLDC motors.
Correct answer is: Brushless DC motor for rotor position detection
Q.51 The primary loss mechanism in an electromechanical relay when switching inductive loads is:
Contact erosion due to arcing
Heat generated in the coil
Magnetic saturation of the core
Capacitive coupling
Explanation - When breaking an inductive current, high voltage arcs across the contacts, causing wear.
Correct answer is: Contact erosion due to arcing
Q.52 In a pneumatic‑electric hybrid actuator, the electric component typically provides:
Air pressure
Mechanical linkage
Control of valve timing
Magnetic field generation
Explanation - Electric solenoids or motor‑driven valves regulate the flow of compressed air in hybrid actuators.
Correct answer is: Control of valve timing
Q.53 What is the effect of increasing the number of poles in a stepper motor while keeping the supply voltage constant?
Larger step angle
Smaller step angle
Higher torque at low speed
Reduced inductance only
Explanation - More poles mean more steps per revolution, decreasing the step angle.
Correct answer is: Smaller step angle
Q.54 Which electromechanical component is most appropriate for a high‑frequency (kHz) switching application?
Electromechanical relay
Solid‑state relay
Contactor
Mechanical switch
Explanation - SSRs have no moving parts and can switch at kilohertz frequencies, unlike electromechanical relays.
Correct answer is: Solid‑state relay
Q.55 In a three‑phase induction motor, the frequency of the induced current in the rotor is:
Equal to the supply frequency
Zero
Supply frequency minus slip frequency
Supply frequency times the number of poles
Explanation - Rotor frequency = s × f (where s is slip), i.e., supply frequency reduced by slip.
Correct answer is: Supply frequency minus slip frequency
Q.56 A 'cage' in a squirrel‑cage rotor is made of:
Copper or aluminum bars short‑circuited by end rings
Permanent magnets
Ferromagnetic laminations
Insulated windings
Explanation - The cage forms a closed conducting loop that allows induced currents to generate torque.
Correct answer is: Copper or aluminum bars short‑circuited by end rings
Q.57 When a relay coil is connected to a 24 V DC source and the coil resistance is 240 Ω, what is the coil current?
0.1 A
1 A
10 A
0.01 A
Explanation - I = V / R = 24 V / 240 Ω = 0.1 A.
Correct answer is: 0.1 A
Q.58 Which type of electromechanical actuator can provide both linear motion and force feedback without using a motor?
Pneumatic cylinder with a pressure sensor
Solenoid with a Hall sensor
Stepper motor with an encoder
Hydraulic pump
Explanation - The cylinder provides linear motion, while the pressure sensor offers force feedback.
Correct answer is: Pneumatic cylinder with a pressure sensor
Q.59 A 'bi‑stable relay' maintains its state after the coil is de‑energized and requires:
Continuous power to stay in either state
A second pulse to change state
Mechanical latch
Thermal heating
Explanation - Bi‑stable relays have two stable positions; each pulse toggles between them.
Correct answer is: A second pulse to change state
Q.60 Which of the following best describes the function of a 'torque sensor' in an electromechanical system?
Measures rotational speed
Measures angular displacement
Measures the twisting force applied to a shaft
Measures temperature of the motor windings
Explanation - Torque sensors convert mechanical torque into an electrical signal for monitoring or control.
Correct answer is: Measures the twisting force applied to a shaft
Q.61 In a DC motor driver using PWM, why does increasing the duty cycle increase motor speed?
It reduces the back‑EMF
It raises the average voltage applied to the motor
It changes the motor winding resistance
It adds more magnetic poles
Explanation - Higher duty cycle means the motor sees a higher average voltage, resulting in higher speed.
Correct answer is: It raises the average voltage applied to the motor
Q.62 A 'variable reluctance sensor' is commonly used for:
Measuring temperature
Detecting position of ferrous teeth on a rotating shaft
Regulating voltage
Controlling current
Explanation - These sensors produce a voltage when magnetic flux changes due to passing ferrous teeth.
Correct answer is: Detecting position of ferrous teeth on a rotating shaft
Q.63 What is the typical effect of a high ambient temperature on the contact resistance of a relay?
Resistance decreases
Resistance increases
No effect
Resistance becomes zero
Explanation - Higher temperature raises metal resistance, slightly increasing contact resistance.
Correct answer is: Resistance increases
Q.64 In a three‑phase motor starter, the 'over‑current relay' is used to:
Start the motor
Detect phase loss
Protect the motor from excessive current
Control speed
Explanation - Over‑current relays open the circuit when current exceeds a preset value, safeguarding the motor.
Correct answer is: Protect the motor from excessive current
Q.65 Which electromechanical component is most commonly used to provide a rapid on/off signal in a digital circuit?
Relay
Transformer
Capacitor
Inductor
Explanation - Relays can switch quickly between open and closed states, suitable for digital control signals.
Correct answer is: Relay
Q.66 A 'brushless DC motor' requires which of the following for commutation?
Mechanical brushes
Electronic controller with position sensors
Permanent magnet rotor only
Variable capacitor
Explanation - Electronic commutation replaces brushes, using Hall sensors or back‑EMF detection.
Correct answer is: Electronic controller with position sensors
Q.67 What is the primary advantage of a 'planetary gear set' in an electromechanical drive?
High torque capacity in a compact size
Low noise operation
Simple construction
Low cost
Explanation - Planetary gears provide large reduction ratios and high torque density.
Correct answer is: High torque capacity in a compact size
Q.68 A 'linear variable differential transformer' (LVDT) output is typically:
DC voltage proportional to displacement
AC voltage whose amplitude varies with displacement
Digital pulses
Current proportional to speed
Explanation - LVDTs produce a sinusoidal output whose magnitude changes linearly with core position.
Correct answer is: AC voltage whose amplitude varies with displacement
Q.69 When a relay coil is energized, the magnetic field pulls the armature. Which material property of the core enhances this action?
High electrical conductivity
High magnetic permeability
High thermal conductivity
Low density
Explanation - A high‑µ core concentrates magnetic flux, increasing the force on the armature.
Correct answer is: High magnetic permeability
Q.70 A 'circuit breaker' differs from a relay in that:
It can be manually reset after tripping
It uses a coil to operate contacts
It is only for AC circuits
It does not provide isolation
Explanation - Circuit breakers are protective devices that trip on over‑current and can be reset; relays are control devices.
Correct answer is: It can be manually reset after tripping
Q.71 Which electromechanical component is typically used as a 'limit switch' in CNC machines to define travel boundaries?
Proximity sensor
Reed switch
Mechanical micro‑switch
Thermistor
Explanation - Micro‑limit switches provide reliable physical contact when a moving part reaches a preset position.
Correct answer is: Mechanical micro‑switch
Q.72 In a DC motor, if the armature resistance doubles while supply voltage remains the same, what happens to the stall current?
It doubles
It halves
It remains unchanged
It becomes zero
Explanation - Stall current I = V / R; doubling R halves the current.
Correct answer is: It halves
Q.73 Which of the following is a common failure mode of a solenoid when operated continuously at high temperature?
Loss of magnetic field due to core saturation
Coil insulation breakdown
Excessive contact bounce
Reduction of coil inductance
Explanation - High temperature can degrade insulation, leading to short circuits.
Correct answer is: Coil insulation breakdown
Q.74 In a brushed DC motor, the number of commutator segments directly influences:
The motor's inductance
The smoothness of torque
The voltage rating
The magnetic flux density
Explanation - More segments reduce torque ripple by providing more frequent commutation points.
Correct answer is: The smoothness of torque
Q.75 A 'piezoelectric actuator' produces motion by:
Heating a material
Changing shape of a crystal under electric field
Magnetizing a ferromagnetic core
Expanding a pneumatic chamber
Explanation - Piezoelectric materials deform when voltage is applied, creating precise small displacements.
Correct answer is: Changing shape of a crystal under electric field
Q.76 Which electromechanical component is typically used to convert high‑speed, low‑torque motor output into low‑speed, high‑torque output?
Gear reducer
Capacitor bank
Transformer
Inductor
Explanation - Gear reducers provide mechanical advantage, reducing speed and increasing torque.
Correct answer is: Gear reducer
Q.77 In a three‑phase induction motor starter, why is a star‑delta connection used during start‑up?
To reduce starting current
To increase starting torque
To provide phase‑shifted voltage
To eliminate slip
Explanation - Star connection reduces line voltage to the motor windings to about 58 % of line voltage, limiting start‑up current.
Correct answer is: To reduce starting current
Q.78 Which component is used to detect the position of a rotating shaft by measuring changes in capacitance?
Capacitive encoder
Hall sensor
Optical encoder
Resolver
Explanation - Capacitive encoders vary capacitance with shaft position, converting it to an electrical signal.
Correct answer is: Capacitive encoder
Q.79 A 'thermal fuse' differs from a regular fuse because:
It resets automatically
It opens at a specific temperature, not current
It can handle higher currents
It is used only in AC circuits
Explanation - Thermal fuses cut off the circuit when a predefined temperature is reached.
Correct answer is: It opens at a specific temperature, not current
Q.80 When using a relay to switch a 240 V AC load, what rating should the contacts have at minimum?
250 V AC, 2 A
240 V AC, 5 A
120 V DC, 10 A
480 V AC, 1 A
Explanation - The contacts must be rated at least equal to the load voltage and current; a common safe margin is 5 A for 240 V AC.
Correct answer is: 240 V AC, 5 A
Q.81 A 'rotary encoder' that provides absolute position information is called:
Incremental encoder
Absolute encoder
Linear encoder
Differential encoder
Explanation - Absolute encoders encode the exact position in each reading, unlike incremental encoders which only give changes.
Correct answer is: Absolute encoder
Q.82 Which electromechanical device is typically used as a fast‑acting safety cutoff in high‑voltage power supplies?
Solid‑state relay
Electromechanical relay
Contactors
Circuit breaker
Explanation - SSRs can trip within microseconds, providing rapid isolation for high‑voltage circuits.
Correct answer is: Solid‑state relay
Q.83 In a DC motor, the constant that relates back‑EMF voltage to speed (in V/(rad/s)) is known as:
Torque constant (Kt)
Speed constant (Ke)
Resistance constant (R)
Inductance constant (L)
Explanation - Ke defines the proportionality between angular speed and generated voltage.
Correct answer is: Speed constant (Ke)
Q.84 A 'magnetic clutch' transmits torque between shafts using:
Friction pads
Magnetic fields
Hydraulic fluid
Gear teeth
Explanation - Magnetic clutches engage by generating a magnetic attraction between rotating elements.
Correct answer is: Magnetic fields
Q.85 When a relay is specified as 'DPDT', it means:
Double pole, double throw
Dual power, double tone
Direct polarity, dual trigger
Double pole, double tone
Explanation - DPDT relays have two separate circuits (poles) each with a normally open and normally closed contact (double throw).
Correct answer is: Double pole, double throw
Q.86 Which type of electromechanical motor is most suitable for high‑speed, low‑torque applications such as fans?
Shaded‑pole motor
Stepper motor
Servo motor
Gear motor
Explanation - Shaded‑pole motors are simple, inexpensive, and run at high speeds with modest torque, ideal for fans.
Correct answer is: Shaded‑pole motor
Q.87 A 'contactor' typically includes a built‑in:
Over‑current protection device
Thermal overload relay
Snubber network
None of the above
Explanation - While contactors may be paired with overload relays, the contactor itself does not contain these protection devices.
Correct answer is: None of the above
Q.88 Which electromechanical component uses a magnetic field to hold a latch in place until a release pulse is applied?
Solenoid latch
Reed switch
Thermostatic switch
Capacitor
Explanation - A solenoid latch keeps the plunger engaged magnetically and releases when the coil is de‑energized or reversed.
Correct answer is: Solenoid latch
Q.89 The main advantage of using a 'brushless DC motor' over a brushed motor in precision equipment is:
Higher torque at low speed
No brush wear, resulting in longer life and lower maintenance
Lower cost
Simpler control circuitry
Explanation - Brushless motors eliminate mechanical wear of brushes, offering higher reliability and precision.
Correct answer is: No brush wear, resulting in longer life and lower maintenance
Q.90 A 'variable frequency drive' (VFD) controls a motor by:
Changing the supply voltage amplitude
Varying the frequency of the supplied AC power
Altering the motor winding resistance
Switching between star and delta connections
Explanation - VFDs adjust frequency (and often voltage) to control motor speed and torque.
Correct answer is: Varying the frequency of the supplied AC power
Q.91 In a pneumatic‑electric hybrid system, the 'solenoid valve' performs which function?
Generates pneumatic pressure
Regulates airflow based on an electrical signal
Converts electrical energy to mechanical rotation
Measures pressure
Explanation - Solenoid valves open or close to control the flow of compressed air when energized.
Correct answer is: Regulates airflow based on an electrical signal
Q.92 Which of the following statements about 'contact bounce' is TRUE?
It can be completely eliminated by using a mechanical filter
It is undesirable because it may cause multiple unintended triggers
It increases the life of the contacts
It only occurs in solid‑state relays
Explanation - Contact bounce creates rapid make‑break cycles that can be misinterpreted by digital circuits.
Correct answer is: It is undesirable because it may cause multiple unintended triggers
Q.93 A 'torque motor' is a type of motor that:
Provides high speed with low torque
Delivers a constant torque over a wide speed range without a feedback sensor
Requires a gearbox for any useful output
Operates only on DC voltage
Explanation - Torque motors are designed for applications where torque, not speed, is the primary requirement.
Correct answer is: Delivers a constant torque over a wide speed range without a feedback sensor
Q.94 Which electromechanical component is typically used to detect the presence of a metal object without physical contact?
Inductive proximity sensor
Thermistor
Potentiometer
Reed switch
Explanation - Inductive sensors generate a field that changes when a metal object enters, indicating presence.
Correct answer is: Inductive proximity sensor
Q.95 When a DC motor is operated at a voltage lower than its rated voltage, what is the typical effect on its speed and torque?
Both speed and torque increase
Speed decreases, torque remains roughly the same at low loads
Speed remains constant, torque decreases
Both speed and torque remain unchanged
Explanation - Speed is roughly proportional to voltage; torque depends mainly on current, which may stay similar at light loads.
Correct answer is: Speed decreases, torque remains roughly the same at low loads
Q.96 A 'bistable relay' can hold its state without power because:
It uses a permanent magnet
Its contacts are mechanically latched
It contains a rechargeable battery
It has a built‑in capacitor
Explanation - Bistable relays have a mechanical latch that keeps contacts in position after the coil is de‑energized.
Correct answer is: Its contacts are mechanically latched
Q.97 The main reason for using a 'flyback diode' across a relay coil in a microcontroller circuit is to:
Increase the coil's magnetic field strength
Protect the microcontroller's I/O pin from voltage spikes
Reduce coil resistance
Enable PWM control of the coil
Explanation - When the coil is turned off, the inductive spike is clamped by the diode, preventing damage.
Correct answer is: Protect the microcontroller's I/O pin from voltage spikes
Q.98 In a stepper motor, the term 'microstepping' refers to:
Dividing each full step into smaller increments using current control
Increasing the number of poles on the rotor
Using a gear reduction to lower speed
Operating the motor at a lower voltage
Explanation - Microstepping varies the coil currents to achieve finer angular resolution than the motor’s native step angle.
Correct answer is: Dividing each full step into smaller increments using current control
Q.99 A 'Hall‑effect sensor' in a brushless DC motor provides:
Speed feedback only
Position information for commutation
Thermal protection
Voltage regulation
Explanation - Hall sensors detect rotor magnet positions, enabling the controller to switch phases at the correct times.
Correct answer is: Position information for commutation
Q.100 Which electromechanical component would you select to provide a quick, repeatable linear displacement of a few millimeters in a laboratory instrument?
Solenoid
Stepper motor with lead screw
Gear motor
Pneumatic cylinder
Explanation - Combining a stepper motor (precise step control) with a lead screw yields accurate, repeatable linear motion.
Correct answer is: Stepper motor with lead screw
Q.101 In an AC induction motor, the term 'slip' is expressed as a percentage. If the synchronous speed is 1800 rpm and the rotor runs at 1750 rpm, what is the slip?
2.78 %
5 %
0.5 %
10 %
Explanation - Slip = (Ns – Nr) / Ns × 100 = (1800 – 1750) / 1800 × 100 ≈ 2.78 %.
Correct answer is: 2.78 %
Q.102 Which of the following devices is specifically designed to convert rotary motion into intermittent linear motion?
Cam
Gearbox
Solenoid
Flywheel
Explanation - A cam converts continuous rotation into a prescribed pattern of linear displacement.
Correct answer is: Cam
Q.103 A 'magnetic encoder' provides position feedback by:
Measuring changes in magnetic field as a magnet rotates past sensors
Using optical slots and photodiodes
Counting pulses from a Hall sensor
Measuring resistance changes in a potentiometer
Explanation - Magnetic encoders detect angular position via magnetic field variations, often using Hall or magnetoresistive sensors.
Correct answer is: Measuring changes in magnetic field as a magnet rotates past sensors
Q.104 The main purpose of a 'snubber circuit' across a relay contact switching an inductive load is to:
Increase the switching speed
Reduce contact arcing by limiting dv/dt
Provide a path for DC current
Heat the contacts for better conductivity
Explanation - Snubbers (RC networks) absorb the voltage spike, decreasing the rate of voltage rise and preventing arcing.
Correct answer is: Reduce contact arcing by limiting dv/dt
Q.105 Which electromechanical component is most suitable for a high‑precision, low‑force application such as focusing a camera lens?
Piezoelectric actuator
Solenoid
Gear motor
Hydraulic cylinder
Explanation - Piezo actuators produce very small, precise displacements with high resolution, ideal for lens focusing.
Correct answer is: Piezoelectric actuator
Q.106 In a relay datasheet, the term 'contact rating' of 250 V AC, 10 A indicates:
Maximum voltage the coil can handle
Maximum voltage and current the contacts can safely switch
Minimum voltage required to operate the coil
Power dissipation of the relay
Explanation - Contact rating specifies the safe operating limits for the switching contacts.
Correct answer is: Maximum voltage and current the contacts can safely switch
Q.107 When a DC motor's armature windings are short‑circuited, what effect does this have on the motor's operation?
It runs faster
It stalls immediately
It behaves like a generator and creates a braking torque (dynamic braking)
It produces no back‑EMF
Explanation - Short‑circuited armature induces current that opposes rotation, providing a braking effect.
Correct answer is: It behaves like a generator and creates a braking torque (dynamic braking)
Q.108 A 'limit switch' in a motion control system is typically used to:
Measure speed
Detect the end of travel and stop the motion
Regulate voltage
Provide PWM control
Explanation - Limit switches sense when a moving part reaches a predefined position and halt further motion.
Correct answer is: Detect the end of travel and stop the motion
Q.109 Which electromechanical component uses a rotating magnetic field to induce torque without direct electrical connections to the rotor?
Induction motor
Brushed DC motor
Stepper motor
Solenoid
Explanation - The rotating magnetic field induces currents in the rotor (squirrel cage), producing torque.
Correct answer is: Induction motor
Q.110 If a relay coil is rated at 24 V DC and 50 mA, what is the minimum power supply rating (in watts) needed to safely drive the coil?
0.6 W
1.2 W
12 W
24 W
Explanation - Power = V × I = 24 V × 0.05 A = 1.2 W. A supply rated higher than this (e.g., 2 W) provides margin.
Correct answer is: 1.2 W
Q.111 A 'servo motor' typically includes which of the following components internally?
Motor, position sensor, and control electronics
Only a motor and gearbox
Motor and a thermistor
Motor and a flyback diode
Explanation - Servo systems integrate the motor, feedback sensor (encoder or resolver), and driver for closed‑loop control.
Correct answer is: Motor, position sensor, and control electronics
Q.112 Which of the following statements about a 'linear actuator' is FALSE?
It converts rotary motion to linear motion
It can be powered by electric, pneumatic, or hydraulic means
It inherently provides position feedback without additional sensors
It can be designed with a lead screw for precise movement
Explanation - While some linear actuators include integrated sensors, most require external devices for position feedback.
Correct answer is: It inherently provides position feedback without additional sensors
Q.113 When selecting a relay for switching a motor with a high inrush current, which specification is most critical?
Coil voltage
Contact material (e.g., silver‑cadmium)
Maximum switching frequency
Contact rating for inrush current
Explanation - Motors draw high inrush currents at start; the relay contacts must be rated to handle those peaks safely.
Correct answer is: Contact rating for inrush current
Q.114 In a PWM‑controlled DC motor driver, why is a low‑pass filter sometimes placed on the motor terminals?
To smooth the voltage and reduce audible noise
To increase the motor's back‑EMF
To protect the motor from over‑voltage
To provide regenerative braking
Explanation - A low‑pass filter reduces voltage ripple, minimizing torque ripple and audible whining.
Correct answer is: To smooth the voltage and reduce audible noise
Q.115 A 'gearless motor' (also known as a direct‑drive motor) is advantageous because:
It eliminates mechanical losses and backlash associated with gears
It provides higher torque than geared motors
It is cheaper to manufacture
It can only operate at low speeds
Explanation - Direct‑drive systems remove gearboxes, improving precision and reducing maintenance.
Correct answer is: It eliminates mechanical losses and backlash associated with gears
Q.116 Which electromechanical component is used to generate a high‑frequency acoustic signal for distance measurement?
Ultrasonic transducer
Solenoid
Relay
Potentiometer
Explanation - Ultrasonic transducers convert electrical pulses into high‑frequency sound waves and receive the echoes.
Correct answer is: Ultrasonic transducer
Q.117 A 'magnetic proximity sensor' can detect metal objects without contact because:
It uses a light beam
It relies on changes in inductance caused by the metal's presence
It measures temperature differences
It generates sound waves
Explanation - The sensor's magnetic field is altered when a conductive object approaches, changing the output signal.
Correct answer is: It relies on changes in inductance caused by the metal's presence
Q.118 Which electromechanical device is commonly used to provide a quick disconnect in high‑voltage power distribution?
Contactor
Relay
Circuit breaker
Switch
Explanation - Circuit breakers can rapidly interrupt high‑voltage circuits and provide over‑current protection.
Correct answer is: Circuit breaker
Q.119 In a servo system, the term 'settling time' refers to:
The time required for the motor to reach maximum speed
The time taken for the system to reach and stay within a specified error band after a command
The time needed to charge the motor windings
The time between successive control pulses
Explanation - Settling time measures how quickly a servo reaches its final position within an acceptable tolerance.
Correct answer is: The time taken for the system to reach and stay within a specified error band after a command
Q.120 A 'linear variable resistor' (LVR) is essentially a:
Potentiometer that provides a variable resistance proportional to linear displacement
Capacitor that changes value with position
Inductor that varies with distance
Diode array
Explanation - LVRs act like a sliding potentiometer, changing resistance as the slider moves.
Correct answer is: Potentiometer that provides a variable resistance proportional to linear displacement
Q.121 When a stepper motor is driven in full‑step mode, how many phases are energized at any given time?
One
Two
Three
All phases
Explanation - Full‑step operation typically energizes two adjacent phases to create the magnetic field.
Correct answer is: Two
Q.122 A 'torque sensor' in an electromechanical system commonly uses which principle to measure torque?
Strain gauge deformation
Magnetic field variation
Capacitive change
Thermal expansion
Explanation - Strain gauges attached to a torque‑transmitting element change resistance proportional to applied torque.
Correct answer is: Strain gauge deformation
Q.123 Which electromechanical component typically includes a 'normally open' (NO) contact that closes when the coil is energized?
Relay
Capacitor
Inductor
Transformer
Explanation - Standard relays have NO contacts that move to the closed position when the coil creates a magnetic field.
Correct answer is: Relay
Q.124 A 'magnetically latching solenoid' differs from a regular solenoid because:
It requires continuous power to stay latched
It stays latched after power is removed until a reverse pulse is applied
It operates only on AC voltage
It has built‑in position sensors
Explanation - Magnetic latching solenoids retain the plunger position magnetically without continuous power.
Correct answer is: It stays latched after power is removed until a reverse pulse is applied
Q.125 Which component is used to provide a controlled, repeatable force in a testing rig without using a motor?
Pneumatic cylinder with pressure regulator
Solenoid
Stepper motor
Transformer
Explanation - By regulating air pressure, a pneumatic cylinder can apply a known force directly.
Correct answer is: Pneumatic cylinder with pressure regulator
Q.126 In a DC motor, the relationship between torque (T) and armature current (I) is:
T = k × I (directly proportional)
T = V / I
T = I² × R
T = V × I
Explanation - Torque constant (k) relates torque linearly to armature current.
Correct answer is: T = k × I (directly proportional)
Q.127 A 'bimetallic strip' in a thermostat works because:
Two metals expand at different rates when heated, causing the strip to bend
It generates a magnetic field when heated
It changes resistance with temperature
It emits light when heated
Explanation - Differential expansion creates curvature, opening or closing contacts.
Correct answer is: Two metals expand at different rates when heated, causing the strip to bend
Q.128 Which electromechanical component would you use to convert rotational motion into a reciprocating linear motion for a pump piston?
Crank‑and‑connecting‑rod mechanism
Gearbox
Solenoid
Cam
Explanation - A crank converts continuous rotation into back‑and‑forth motion, ideal for piston pumps.
Correct answer is: Crank‑and‑connecting‑rod mechanism
Q.129 A 'soft starter' for an AC motor primarily reduces:
Starting current
Operating voltage
Motor speed
Thermal resistance
Explanation - Soft starters ramp up voltage gradually, limiting the high inrush current typical of direct‑on‑line starts.
Correct answer is: Starting current
Q.130 When a relay coil is left energized for a prolonged period, the primary failure mode is:
Contact bounce
Coil overheating
Magnetic saturation
Increased inductance
Explanation - Continuous current causes the coil to dissipate power as heat, potentially damaging insulation.
Correct answer is: Coil overheating
Q.131 Which electromechanical device can be used as a simple binary (on/off) sensor by detecting the presence of a magnetic field?
Reed switch
Thermistor
Potentiometer
Capacitor
Explanation - Reed switches close when a magnetic field is present, acting as a binary sensor.
Correct answer is: Reed switch
Q.132 In a servo motor, the term 'backlash' refers to:
Delay between command and motion
Play or lost motion between mechanical components, often gear teeth
Electrical noise in the control signal
Heat generated by the motor
Explanation - Backlash is the angular gap that must be taken up before motion is transmitted, affecting precision.
Correct answer is: Play or lost motion between mechanical components, often gear teeth
Q.133 A 'magnetic brake' works by:
Applying friction pads
Generating a magnetic field that opposes motion of a metal disc
Using hydraulic pressure
Engaging a mechanical latch
Explanation - Eddy currents induced in a conductive disc by a magnetic field create a retarding torque.
Correct answer is: Generating a magnetic field that opposes motion of a metal disc
Q.134 When selecting a relay for a high‑frequency switching application (several kHz), which specification is most critical?
Coil voltage
Contact bounce time
Maximum switching frequency (contact life)
Physical size
Explanation - High‑frequency operation demands contacts rated for rapid actuation and long life under those conditions.
Correct answer is: Maximum switching frequency (contact life)
Q.135 A 'piezo buzzer' produces sound by:
Heating a resistor
Vibrating a piezoelectric crystal when voltage is applied
Moving a magnetic plunger
Flashing an LED
Explanation - Piezo buzzers use the rapid expansion and contraction of a crystal to generate audible sound.
Correct answer is: Vibrating a piezoelectric crystal when voltage is applied
Q.136 Which electromechanical component is used to sense angular position by detecting changes in magnetic reluctance?
Variable reluctance sensor
Hall‑effect sensor
Potentiometer
Capacitive sensor
Explanation - Variable reluctance sensors produce voltage pulses as a ferromagnetic tooth passes a coil, indicating position.
Correct answer is: Variable reluctance sensor
Q.137 A 'linear actuator' driven by a screw mechanism typically has a resolution of:
Micrometers to sub‑micrometers
Centimeters
Meters
Inches only
Explanation - Fine lead screws or ball screws can achieve very high positional resolution.
Correct answer is: Micrometers to sub‑micrometers
Q.138 In a three‑phase motor starter, the purpose of a 'star‑delta' transition is to:
Reduce starting current while allowing full voltage during run
Increase starting torque
Provide overload protection
Switch the motor between AC and DC operation
Explanation - Starting in star reduces voltage to each winding, limiting current; after acceleration, the motor switches to delta for full voltage.
Correct answer is: Reduce starting current while allowing full voltage during run
Q.139 A 'magnetic encoder' that provides absolute position typically uses:
Multiple magnetic poles with a Gray‑code pattern
Single pole detection
Optical slots
Resistive ladder
Explanation - Absolute magnetic encoders encode position using a unique pattern of magnetic fields.
Correct answer is: Multiple magnetic poles with a Gray‑code pattern
Q.140 Which electromechanical component is best suited for a high‑precision, high‑speed rotational positioning system in a telescope?
Stepper motor with microstepping
Brushless DC motor with encoder feedback
Solenoid
Shaded‑pole motor
Explanation - BLDC motors with high‑resolution encoders provide fast, precise, low‑vibration motion suitable for telescopes.
Correct answer is: Brushless DC motor with encoder feedback
Q.141 The 'holding torque' of a stepper motor is:
The torque the motor can produce while rotating at rated speed
The torque the motor can resist when powered but not moving
The maximum torque at stall
The torque after the motor is turned off
Explanation - Holding torque is the static torque that keeps the shaft stationary when the motor is energized.
Correct answer is: The torque the motor can resist when powered but not moving
Q.142 A 'linear variable differential transformer' (LVDT) requires an external circuit to:
Convert its AC output to a DC voltage proportional to displacement
Increase its frequency
Provide mechanical motion
Generate magnetic flux
Explanation - Signal conditioning circuitry demodulates the LVDT's AC output into a usable DC signal.
Correct answer is: Convert its AC output to a DC voltage proportional to displacement
Q.143 Which electromechanical component is most appropriate for a safety interlock that must stay open when power is lost?
Normally‑closed (NC) relay
Normally‑open (NO) relay
Solid‑state relay
Latching relay
Explanation - An NC relay opens its contacts when energized; loss of power leaves contacts open (off).
Correct answer is: Normally‑closed (NC) relay
Q.144 When a DC motor is driven by a PWM signal with a 25 % duty cycle, the average voltage applied to the motor is:
25 % of the supply voltage
75 % of the supply voltage
Equal to the supply voltage
Zero volts
Explanation - Average voltage = Duty cycle × Supply voltage; 0.25 × V_supply.
Correct answer is: 25 % of the supply voltage
Q.145 A 'gear motor' combines a motor with a gearbox to:
Increase speed and reduce torque
Decrease speed and increase torque
Eliminate the need for a motor
Provide direct linear motion
Explanation - Gear reduction trades speed for higher torque output.
Correct answer is: Decrease speed and increase torque
Q.146 Which electromechanical component is commonly used as a 'dead‑man' switch in industrial machinery?
Push‑button normally closed
Limit switch
Reed switch
Thermostatic switch
Explanation - A normally‑closed push button releases (opens) when released, stopping the machine if the operator lets go.
Correct answer is: Push‑button normally closed
Q.147 In an electromechanical system, the term 'actuation force' refers to:
The voltage applied to the coil
The magnetic field strength
The mechanical force generated by the device when activated
The current flowing through the contacts
Explanation - Actuation force is the output mechanical force produced by components like solenoids or actuators.
Correct answer is: The mechanical force generated by the device when activated
Q.148 A 'rotary solenoid' differs from a linear solenoid because it:
Produces rotational motion instead of linear motion
Requires AC power
Has a lower magnetic field
Uses hydraulic fluid
Explanation - Rotary solenoids are designed to twist a shaft rather than push/pull a plunger.
Correct answer is: Produces rotational motion instead of linear motion