Q.1 What is torque?
Speed
Torque is the measure of rotational force
Electrical current
Voltage
Explanation - Torque is defined as the rotational equivalent of linear force, calculated as force multiplied by radius.
Correct answer is: Torque is the measure of rotational force
Q.2 Which component is primarily used to control the speed of a DC motor?
Resistor
Capacitor
Inductor
Diode
Explanation - A series resistor reduces armature voltage, thereby reducing speed.
Correct answer is: Resistor
Q.3 Increasing the supply voltage to a DC motor will generally have what effect on its speed?
Increase speed
Decrease speed
No change
Decrease torque only
Explanation - Higher voltage increases back EMF, causing the motor to accelerate until speed stabilizes.
Correct answer is: Increase speed
Q.4 In a DC motor, if the armature resistance increases, the speed will
Increase
Decrease
Remain unchanged
Increase torque only
Explanation - Higher resistance reduces current, lowering back EMF and causing the motor to run slower.
Correct answer is: Decrease
Q.5 Which device is commonly used to change the frequency supplied to an induction motor?
Transformer
Variable transformer
Inverter
Resistor
Explanation - An inverter (VFD) converts DC to AC with controllable frequency to regulate motor speed.
Correct answer is: Inverter
Q.6 The synchronous speed of a motor depends on the supply frequency and
Voltage
Armature resistance
Number of poles
Load torque
Explanation - Synchronous speed = (120 × f) / P, where f is frequency and P is pole count.
Correct answer is: Number of poles
Q.7 The torque constant (K_t) of a motor is proportional to which electrical parameter?
Speed
Voltage
Current
Resistance
Explanation - Torque = K_t × current; higher current generates more torque.
Correct answer is: Current
Q.8 Which fundamental law explains why a motor produces torque?
Ohm’s law
Faraday’s law
Lorentz force
Kirchhoff’s law
Explanation - Lorentz force (F = I × L × B) acts on current-carrying conductors in a magnetic field, producing torque.
Correct answer is: Lorentz force
Q.9 In a DC motor, back EMF is proportional to
Speed
Torque
Current
Resistance
Explanation - Back EMF (E_b) = K_e × ω; it opposes applied voltage and increases with speed.
Correct answer is: Speed
Q.10 What is the main function of a commutator in a DC motor?
Smooth torque
Switch current direction
Change voltage
Adjust speed
Explanation - The commutator and brushes reverse armature current to maintain continuous rotation.
Correct answer is: Switch current direction
Q.11 Which of the following is true for a brushed DC motor’s torque-speed characteristic?
Torque is constant with speed
Speed is independent of load
Torque decreases linearly with speed
Speed increases with load torque
Explanation - As speed rises, back EMF rises, reducing current and thus torque.
Correct answer is: Torque decreases linearly with speed
Q.12 In an induction motor, slip (s) is defined as
Speed difference between rotor and stator
Rotor speed divided by synchronous speed
Rotor speed minus synchronous speed
Ratio of synchronous speed to rotor speed
Explanation - Slip s = (n_s - n_r)/n_s; it represents how far the rotor lags behind the rotating magnetic field.
Correct answer is: Rotor speed minus synchronous speed
Q.13 What is the typical range of slip for a standard induction motor at full load?
0–1%
1–5%
5–10%
10–20%
Explanation - Most 3-phase induction motors operate with slip between 1–5% under normal loading.
Correct answer is: 1–5%
Q.14 Which control method adjusts the voltage to a DC motor for speed control?
Pulse Width Modulation (PWM)
Variable resistor (potentiometer)
Field weakening
Series resistor
Explanation - PWM rapidly switches the supply to vary the average voltage, effectively controlling speed.
Correct answer is: Pulse Width Modulation (PWM)
Q.15 The torque of an induction motor is maximized when the slip equals
Zero
Maximum possible slip
The slip corresponding to the motor’s maximum torque point
Slip is irrelevant for torque
Explanation - Maximum torque occurs at a particular slip, typically a few percent for squirrel-cage motors.
Correct answer is: The slip corresponding to the motor’s maximum torque point
Q.16 What type of controller is commonly used in variable frequency drives (VFDs) to regulate motor speed?
Open-loop
Closed-loop with tachometer feedback
On-off controller
Manual regulator
Explanation - Closed-loop VFDs use tachometer or encoder feedback to maintain desired speed accurately.
Correct answer is: Closed-loop with tachometer feedback
Q.17 In DC motor speed regulation, which component is used for field weakening to allow speeds above synchronous speed?
Armature resistor
Field winding
Shunt resistor
Field resistor in series
Explanation - Adding resistance to the field winding reduces field flux, permitting higher speeds.
Correct answer is: Field resistor in series
Q.18 Which equation represents the power factor (PF) of an induction motor?
PF = cos(φ) where φ is the phase angle
PF = sin(φ)
PF = tan(φ)
PF = 1
Explanation - Power factor is the cosine of the phase difference between voltage and current.
Correct answer is: PF = cos(φ) where φ is the phase angle
Q.19 A DC motor’s speed can be increased by
Increasing armature resistance
Decreasing supply voltage
Increasing field flux
Reducing field flux
Explanation - Lower field flux raises the back EMF coefficient, allowing higher speed for the same voltage.
Correct answer is: Reducing field flux
Q.20 Which of these is NOT a type of torque control for an induction motor?
Field-oriented control (FOC)
Direct torque control (DTC)
Voltage-frequency (V/f) control
Current-voltage (I/V) control
Explanation - I/V control is not a standard torque control strategy; V/f and FOC are common.
Correct answer is: Current-voltage (I/V) control
Q.21 In a VFD, why is the output frequency varied to control motor speed?
To change the motor’s mechanical load
To adjust the magnetic flux in the stator
To synchronize the rotor speed with the supply frequency
To increase the motor’s torque constant
Explanation - Motor speed in induction machines is directly proportional to supply frequency.
Correct answer is: To synchronize the rotor speed with the supply frequency
Q.22 The torque produced by a permanent magnet synchronous motor (PMSM) is directly proportional to
Rotor speed
Field flux density
Stator current
Supply voltage
Explanation - Torque = (P/2) * (Flux * Current * sin(φ)), where flux is constant for PMSM.
Correct answer is: Stator current
Q.23 Which statement about a DC motor’s speed-torque curve is true?
Torque is zero at no load
Speed is highest at full load
Torque decreases as speed increases
Speed is independent of load torque
Explanation - With increasing speed, back EMF rises, reducing armature current and thus torque.
Correct answer is: Torque decreases as speed increases
Q.24 Why is a tachometer used in motor speed control?
To measure electrical current
To provide feedback on actual speed
To increase motor torque
To adjust motor voltage
Explanation - A tachometer gives real-time speed data for closed-loop control.
Correct answer is: To provide feedback on actual speed
Q.25 What is the primary advantage of using a brushless DC (BLDC) motor over a brushed DC motor?
Higher maintenance cost
Lower efficiency
Reduced electrical noise
No need for field winding
Explanation - BLDC motors eliminate brushes, reducing sparking and noise.
Correct answer is: Reduced electrical noise
Q.26 Which parameter is critical for achieving high torque in a DC motor at low speeds?
High field flux
Low armature resistance
High back EMF
Low supply voltage
Explanation - Strong field flux increases torque generation at low speeds.
Correct answer is: High field flux
Q.27 A variable frequency drive (VFD) that uses trapezoidal PWM is primarily used to control which aspect of a motor?
Mechanical load only
Electrical voltage only
Speed and torque
Temperature only
Explanation - Trapezoidal PWM shapes the voltage waveform to regulate both speed and torque.
Correct answer is: Speed and torque
Q.28 In the context of motor control, what does the term 'field weakening' refer to?
Increasing the magnetic field strength
Decreasing the magnetic field strength
Adding a field resistor in series with the armature
Shortening the field winding
Explanation - Field weakening reduces flux to allow speeds above the motor’s base speed.
Correct answer is: Decreasing the magnetic field strength
Q.29 Which of the following best describes the torque-speed characteristic of an induction motor?
A straight line through the origin
A hyperbolic curve
A parabolic curve
A constant torque across all speeds
Explanation - Induction motors exhibit a hyperbolic torque-speed curve due to slip variations.
Correct answer is: A hyperbolic curve
Q.30 A DC motor’s speed can be increased by reducing the armature voltage while maintaining the field current. This is called
Field weakening
Field strengthening
Armature boosting
Voltage stepping
Explanation - Lower armature voltage relative to field flux reduces speed.
Correct answer is: Field weakening
Q.31 Which parameter is most directly proportional to the torque produced by a DC motor?
Armature voltage
Field current
Armature current
Back EMF
Explanation - Torque depends on armature current and the magnetic field produced.
Correct answer is: Armature current
Q.32 What is the main function of a VFD’s internal inverter?
To convert AC to DC only
To convert DC to AC with adjustable frequency
To increase motor voltage
To decrease motor current
Explanation - The inverter provides the controlled AC waveform for motor speed regulation.
Correct answer is: To convert DC to AC with adjustable frequency
Q.33 Which of the following is a characteristic of a wound-rotor induction motor?
Fixed rotor resistance
Variable rotor resistance via external resistors
Permanent magnets in rotor
No rotor winding
Explanation - Wound-rotor motors allow external resistors to be connected to rotor windings for speed control.
Correct answer is: Variable rotor resistance via external resistors
Q.34 Which of the following best describes a 'stepper motor'?
A motor that runs continuously at a fixed speed
A motor that moves in discrete steps
An induction motor with variable speed
A DC motor with no commutator
Explanation - Stepper motors advance in incremental positions, useful for precise positioning.
Correct answer is: A motor that moves in discrete steps
Q.35 What is the typical response time for a DC motor when its armature voltage is suddenly increased?
Nanoseconds
Microseconds
Milliseconds
Seconds
Explanation - DC motors accelerate within milliseconds after voltage change.
Correct answer is: Milliseconds
Q.36 In an induction motor, what effect does increasing the supply voltage have on torque at a fixed speed?
Torque decreases
Torque increases
Torque unchanged
Torque becomes zero
Explanation - Higher voltage raises current and thus torque while maintaining the same slip.
Correct answer is: Torque increases
Q.37 Which of these is an advantage of using a brushless DC motor over a brushed DC motor?
Higher maintenance
Lower efficiency
Longer lifespan
Simpler control electronics
Explanation - Without brushes, BLDC motors have fewer wear points and last longer.
Correct answer is: Longer lifespan
Q.38 The 'base speed' of a motor is
The speed at zero load
The speed at full load
The synchronous speed
The speed when slip is zero
Explanation - Base speed refers to the synchronous speed corresponding to rated voltage and frequency.
Correct answer is: The synchronous speed
Q.39 Which of the following is a key benefit of using a variable frequency drive (VFD) in a conveyor system?
Increased noise
Higher energy consumption
Improved load handling
Reduced speed control
Explanation - VFDs allow smooth acceleration and deceleration, protecting the system from overload.
Correct answer is: Improved load handling
Q.40 A DC motor’s torque constant (K_t) is measured in
Nm/A
V/Hz
Ohms
Radians per second
Explanation - Torque constant relates torque output to armature current.
Correct answer is: Nm/A
Q.41 What is the primary purpose of the 'dead-time' in a PWM controller?
To allow the motor to cool
To prevent both high-side and low-side transistors from turning on simultaneously
To increase speed
To reduce voltage
Explanation - Dead-time ensures no overlap that could cause a short circuit.
Correct answer is: To prevent both high-side and low-side transistors from turning on simultaneously
Q.42 Which of these is NOT a method for controlling torque in a DC motor?
Armature voltage control
Field flux control
Speed feedback
Armature resistance change
Explanation - Speed feedback is used for speed control, not direct torque control.
Correct answer is: Speed feedback
Q.43 A DC motor operating at higher speeds exhibits what type of back EMF characteristic?
Non-linear
Linear with speed
Inverse linear
Constant
Explanation - Back EMF increases proportionally with speed.
Correct answer is: Linear with speed
Q.44 Which type of motor typically requires a V/f ratio control strategy?
Synchronous motor
Squirrel-cage induction motor
DC motor
Stepper motor
Explanation - V/f control maintains constant torque by keeping the ratio of voltage to frequency constant.
Correct answer is: Squirrel-cage induction motor
Q.45 The main reason for using a PWM controller in motor drives is
To increase the physical size of the motor
To reduce heat generation only
To provide efficient voltage control and reduce harmonic distortion
To simplify wiring
Explanation - PWM allows precise average voltage control with minimal energy loss.
Correct answer is: To provide efficient voltage control and reduce harmonic distortion
Q.46 The torque produced by an induction motor is maximum at a slip of approximately
0%
10-15%
30-35%
50%
Explanation - Maximum torque occurs near a small slip value, typically around 10–15% for many motors.
Correct answer is: 10-15%
Q.47 In a DC motor, increasing the field current while keeping armature voltage constant will
Increase speed
Decrease speed
Increase torque
Decrease torque
Explanation - Higher field flux enhances torque production without changing speed significantly.
Correct answer is: Increase torque
Q.48 Which of the following is a key advantage of brushless DC motors over brushed DC motors?
Higher maintenance
Reduced torque
Higher reliability
Lower efficiency
Explanation - BLDC motors have no brushes, reducing wear and increasing reliability.
Correct answer is: Higher reliability
Q.49 Which component in a VFD is responsible for converting DC to AC?
Rectifier
Inverter
Filter
Controller
Explanation - The inverter stage produces the AC waveform required by the motor.
Correct answer is: Inverter
Q.50 The term 'synchronous speed' refers to the speed of the
Rotor
Stator
Both rotor and stator magnetic field
The external load
Explanation - Synchronous speed is the speed at which the rotor would rotate if it were locked to the rotating magnetic field.
Correct answer is: Both rotor and stator magnetic field
Q.51 Which of the following is a typical application of a stepper motor?
High-speed conveyor
Automated camera slider
Wind turbine generator
Cooling fan
Explanation - Stepper motors provide precise positional control, ideal for camera sliders.
Correct answer is: Automated camera slider
Q.52 In DC motor speed regulation, what is the effect of adding a resistor in series with the armature?
Increases speed
Decreases speed
No effect
Increases torque only
Explanation - The resistor drops voltage across the armature, reducing effective voltage and speed.
Correct answer is: Decreases speed
Q.53 Which of the following is the most accurate description of torque in a synchronous motor?
Proportional to current only
Inversely proportional to speed
Dependent on field flux and stator current
Independent of field flux
Explanation - Torque = K_t * Φ * I_s where Φ is field flux and I_s is stator current.
Correct answer is: Dependent on field flux and stator current
Q.54 The main purpose of a soft start circuit in motor drives is to
Increase starting torque
Reduce inrush current and mechanical shock
Increase speed immediately
Shorten motor life
Explanation - Soft starters limit voltage during startup to protect the motor and mechanical system.
Correct answer is: Reduce inrush current and mechanical shock
Q.55 What is the main role of the 'filter' in a VFD?
Increase voltage
Smooth the DC link voltage
Reduce AC ripple from the rectifier
Provide mechanical support
Explanation - Filters smooth out the rectified DC supply before it feeds the inverter.
Correct answer is: Reduce AC ripple from the rectifier
Q.56 A DC motor’s torque is primarily determined by the
Armature resistance
Field winding current
Supply voltage
Back EMF
Explanation - Field flux produced by field winding interacts with armature current to generate torque.
Correct answer is: Field winding current
Q.57 Which of these statements correctly describes the torque-speed curve of a wound-rotor induction motor?
It is always linear
It is parabolic
It is hyperbolic and can be flattened by adding external resistance
It is flat at low speeds
Explanation - External resistors modify rotor circuit, changing the curve shape.
Correct answer is: It is hyperbolic and can be flattened by adding external resistance
Q.58 Why do variable frequency drives (VFDs) use a PWM technique for inverter output?
To reduce the number of components
To generate a sinusoidal waveform with adjustable frequency
To increase motor torque automatically
To avoid the need for rectifiers
Explanation - PWM allows synthesis of a near-sinusoidal output at any desired frequency.
Correct answer is: To generate a sinusoidal waveform with adjustable frequency
Q.59 What is the primary benefit of using a variable resistor in the field winding of a DC motor?
Increase speed beyond base speed
Control torque at constant speed
Simplify the control circuit
Reduce power consumption at all loads
Explanation - Adjusting field resistance modifies flux, which changes torque at a fixed speed.
Correct answer is: Control torque at constant speed
Q.60 Which parameter is most affected by a change in the supply voltage for a DC motor?
Back EMF
Armature current
Field flux
Speed
Explanation - Higher voltage increases armature current for a given load and resistance.
Correct answer is: Armature current
Q.61 Which type of motor is typically used in high-speed applications requiring precise speed control?
Synchronous motor
Induction motor
DC motor
Stepper motor
Explanation - Stepper motors can rotate in discrete steps with high precision.
Correct answer is: Stepper motor
Q.62 The torque constant (K_t) of a DC motor is typically expressed in units of
Volts per radian per second
Amps per volt
Newtons-meters per ampere
Radians per second per volt
Explanation - K_t converts armature current to torque output.
Correct answer is: Newtons-meters per ampere
Q.63 Which of these is a characteristic of a permanent magnet synchronous motor?
Requires external field winding
Has a high torque constant
Operates only at high speeds
Needs brushes for commutation
Explanation - Permanent magnets provide strong constant flux, enabling high torque per ampere.
Correct answer is: Has a high torque constant
Q.64 The main purpose of a 'dead-time' interval in a PWM inverter is to
Prevent overlap of high-side and low-side switches
Increase output voltage
Decrease switching losses
Improve motor efficiency
Explanation - Dead-time avoids shoot-through that would damage the inverter.
Correct answer is: Prevent overlap of high-side and low-side switches
Q.65 Which of the following is a disadvantage of using a DC motor for high-speed applications?
High efficiency
Large size
High maintenance due to brushes
Precise speed control
Explanation - Brushes wear out at high speeds, requiring regular maintenance.
Correct answer is: High maintenance due to brushes
Q.66 Which control method is used to regulate torque in an induction motor by directly manipulating the stator voltage vector?
V/f control
FOC (Field-Oriented Control)
Open-loop control
Speed feedback control
Explanation - FOC decouples torque and flux components by controlling voltage vectors.
Correct answer is: FOC (Field-Oriented Control)
Q.67 In a VFD, the 'DC link' is
The AC input side
The DC bus that supplies the inverter
The control logic unit
The output transformer
Explanation - The DC link holds DC voltage before it is inverted back to AC.
Correct answer is: The DC bus that supplies the inverter
Q.68 Which of these best describes the role of the 'gate driver' in an inverter?
Regulate voltage
Drive the MOSFET gate with appropriate signals
Control current flow
Provide mechanical support
Explanation - The gate driver ensures rapid switching of the inverter transistors.
Correct answer is: Drive the MOSFET gate with appropriate signals
Q.69 The 'soft starter' for a motor primarily reduces
Motor torque
Motor speed
Starting inrush current
Operating frequency
Explanation - Soft starters limit voltage during startup to reduce current surge.
Correct answer is: Starting inrush current
Q.70 A VFD can provide which of the following benefits for a conveyor belt system?
Constant load regardless of speed
Reduced energy consumption during acceleration and deceleration
Higher starting torque
Elimination of mechanical brakes
Explanation - VFDs enable smooth acceleration, saving energy and reducing wear.
Correct answer is: Reduced energy consumption during acceleration and deceleration
Q.71 In an induction motor, what is the primary cause of torque ripple?
Variable load
Harmonic voltage components
Rotor bar spacing
Synchronous speed
Explanation - Harmonics in supply create fluctuations in torque known as ripple.
Correct answer is: Harmonic voltage components
Q.72 Which type of motor typically uses a 'rotary transformer' to adjust flux for speed control?
Synchronous motor
Induction motor
DC motor
Stepper motor
Explanation - Rotary transformers change field flux for speed regulation.
Correct answer is: Synchronous motor
Q.73 A brushless DC motor uses which of the following for commutation?
Mechanical brushes
Electronic sensors
External commutators
Permanent magnets only
Explanation - Electronic controllers detect rotor position and switch windings accordingly.
Correct answer is: Electronic sensors
Q.74 Which of the following is a common application for a wound-rotor induction motor?
High-speed conveyors
High-torque electric vehicles
Large pumps and compressors
Small fans
Explanation - Wound-rotor motors provide variable speed and torque for large industrial machines.
Correct answer is: Large pumps and compressors
Q.75 In a DC motor, the relationship between back EMF (E_b) and speed (ω) is
E_b ∝ 1/ω
E_b ∝ ω^2
E_b ∝ ω
E_b is independent of ω
Explanation - Back EMF increases linearly with speed.
Correct answer is: E_b ∝ ω
Q.76 Which of these statements best describes a 'soft start' function?
Rapidly ramps up speed to full load
Gradually increases voltage to limit current
Keeps voltage constant during startup
Reduces motor temperature by 50%
Explanation - Soft start limits the initial current surge for protection.
Correct answer is: Gradually increases voltage to limit current
Q.77 What is the main advantage of using a VFD with a 'phase-locked loop (PLL)' controller?
Simpler hardware
Better speed regulation under varying loads
Lower manufacturing cost
Higher mechanical stress
Explanation - PLL synchronizes inverter output to a reference frequency, improving control.
Correct answer is: Better speed regulation under varying loads
Q.78 Which parameter of an induction motor is directly proportional to the rotor resistance?
Torque constant
Slip
Efficiency
Speed
Explanation - Increasing rotor resistance raises slip for a given load.
Correct answer is: Slip
Q.79 The 'power triangle' used in AC circuits primarily helps to calculate
Voltage only
Current only
Real, reactive, and apparent power
Only reactive power
Explanation - The triangle represents relationships among P, Q, and S.
Correct answer is: Real, reactive, and apparent power
Q.80 In a DC motor, what happens to torque if the armature current is doubled?
Torque quadruples
Torque doubles
Torque remains the same
Torque halves
Explanation - Torque is directly proportional to armature current.
Correct answer is: Torque doubles
Q.81 Which of the following is true for a synchronous motor’s slip?
Slip is always zero
Slip can be positive or negative
Slip is equal to the load torque
Slip is constant across speeds
Explanation - Synchronous motors operate at synchronous speed, so slip is zero.
Correct answer is: Slip is always zero
Q.82 A variable frequency drive (VFD) typically uses a 'PWM' strategy to generate an AC waveform at what frequency range?
0.1 Hz to 1 Hz
1 Hz to 10 Hz
10 Hz to 100 Hz
20 Hz to 2000 Hz
Explanation - Industrial drives operate over a wide frequency range to control speed.
Correct answer is: 20 Hz to 2000 Hz
Q.83 The torque of an induction motor is affected by which of the following?
Supply voltage
Rotor resistance
Shaft speed
All of the above
Explanation - Torque depends on voltage, slip (related to rotor resistance), and speed.
Correct answer is: All of the above
Q.84 What is the typical characteristic of a 'field-oriented control' (FOC) scheme?
It requires no feedback
It decouples torque and flux control
It is only suitable for DC motors
It increases harmonic distortion
Explanation - FOC provides independent control of torque and flux, improving performance.
Correct answer is: It decouples torque and flux control
Q.85 Which of the following is a typical application of a permanent magnet synchronous motor?
High-speed drives
Low-speed heavy-duty pumps
Small fans
Electric vehicles
Explanation - PMSMs offer high torque density and efficiency suitable for EVs.
Correct answer is: Electric vehicles
Q.86 In a DC motor, the back EMF constant (K_e) is usually equal to
The torque constant
The inverse of the torque constant
The square of the torque constant
Zero
Explanation - For many DC motors, K_e equals K_t in appropriate units.
Correct answer is: The torque constant
Q.87 The 'soft-start' feature in a VFD primarily helps to
Increase motor speed rapidly
Reduce electromagnetic interference
Prevent mechanical shock and reduce inrush current
Increase efficiency by 10%
Explanation - Soft-start limits voltage ramp to protect equipment.
Correct answer is: Prevent mechanical shock and reduce inrush current
Q.88 Which of these is a characteristic of a stepper motor’s torque?
Constant at all speeds
Zero when not energized
Proportional to speed
Independent of current
Explanation - Without current, a stepper motor holds position but produces no torque.
Correct answer is: Zero when not energized
Q.89 In an induction motor, increasing the supply voltage while keeping the frequency constant results in
Higher torque
Lower torque
Same torque
No effect
Explanation - Higher voltage raises current, thus increasing torque.
Correct answer is: Higher torque
Q.90 The 'dead-time' in a PWM inverter is essential to
Reduce the output frequency
Avoid shoot-through of power devices
Increase motor speed
Decrease power losses
Explanation - Dead-time ensures that no two devices conduct simultaneously.
Correct answer is: Avoid shoot-through of power devices
Q.91 Which of the following best explains the relationship between the torque constant K_t and the back EMF constant K_e in a DC motor?
K_t = K_e
K_t = 1/K_e
K_t = K_e^2
They are unrelated
Explanation - In SI units, the torque and back EMF constants are numerically equal.
Correct answer is: K_t = K_e
Q.92 What is a primary advantage of using a VFD for a conveyor belt system?
Eliminate the need for a motor
Reduce energy consumption during variable load
Increase starting torque dramatically
Eliminate mechanical wear completely
Explanation - VFDs allow precise speed control, saving energy during non-full-load operation.
Correct answer is: Reduce energy consumption during variable load
Q.93 Which of the following is an example of a 'soft-start' control strategy?
Direct application of full voltage
Gradual ramp-up of voltage
Constant voltage with no current limit
None of the above
Explanation - Soft start slowly increases voltage to limit inrush current.
Correct answer is: Gradual ramp-up of voltage
Q.94 A brushless DC motor uses which of the following to detect rotor position?
Hall effect sensors
Mechanical switches
No sensors needed
Magnetic bearings
Explanation - Hall sensors detect magnetic fields for electronic commutation.
Correct answer is: Hall effect sensors
Q.95 In an induction motor, the maximum torque occurs at a slip of approximately
0%
5–10%
30–40%
70–80%
Explanation - Maximum torque is reached at a small slip value.
Correct answer is: 5–10%
Q.96 Which type of inverter is typically used in a variable frequency drive?
H-bridge inverter
Three-phase inverter
Single-phase inverter
All of the above
Explanation - Industrial drives use three-phase inverters to supply motors.
Correct answer is: Three-phase inverter
Q.97 The 'field weakening' technique in a DC motor allows the motor to
Increase torque only
Increase speed above base speed
Decrease speed below base speed
Maintain constant speed at all loads
Explanation - Reducing field flux allows the motor to run faster.
Correct answer is: Increase speed above base speed
Q.98 A 'soft starter' for a motor typically uses
Variable transformers
Electronic control of voltage
Mechanical brakes
Manual switches
Explanation - Soft starters regulate voltage digitally.
Correct answer is: Electronic control of voltage
Q.99 The torque produced by a DC motor is most directly proportional to
Armature voltage
Field flux
Armature current
Speed
Explanation - Torque = K_t × I_a, where I_a is armature current.
Correct answer is: Armature current
Q.100 In a VFD, the 'filter' component serves primarily to
Increase voltage
Smooth the rectified DC supply
Control frequency
Provide mechanical support
Explanation - The filter reduces ripple in the DC link before inversion.
Correct answer is: Smooth the rectified DC supply
Q.101 Which of the following is a key feature of a 'synchronous motor'?
Requires external field winding
Runs at a speed independent of load
Has no brushes
All of the above
Explanation - Synchronous motors have external field windings, brushless construction, and run at synchronous speed.
Correct answer is: All of the above
Q.102 In a DC motor, the back EMF is equal to
Applied voltage minus armature voltage
Applied voltage plus armature current
Applied voltage minus armature current
Applied voltage minus armature voltage drop
Explanation - E_b = V_applied - I_a * R_a.
Correct answer is: Applied voltage minus armature voltage drop
Q.103 The main function of a 'tachometer' in motor control systems is to provide
Current measurement
Speed feedback
Voltage regulation
Temperature monitoring
Explanation - Tachometers give real-time speed data for closed-loop control.
Correct answer is: Speed feedback
Q.104 What is the relationship between the torque constant and back EMF constant in a DC motor?
They are inversely related
They are equal in magnitude
The torque constant is twice the back EMF constant
They are unrelated
Explanation - In SI units, K_t = K_e.
Correct answer is: They are equal in magnitude
Q.105 Which of the following statements about a VFD is FALSE?
It can change motor speed
It requires a transformer
It can reduce energy consumption
It can provide torque control
Explanation - VFDs typically use a rectifier/inverter; transformers are not always needed.
Correct answer is: It requires a transformer
Q.106 Which of these is an advantage of using a brushless DC motor?
High maintenance
Low efficiency
High reliability
Limited speed control
Explanation - Without brushes, BLDC motors have fewer wear points and longer life.
Correct answer is: High reliability
Q.107 The torque constant (K_t) of a motor is measured in
Volts per radian
Amperes per volt
Newton-meters per ampere
Radians per second
Explanation - K_t relates torque output to armature current.
Correct answer is: Newton-meters per ampere
Q.108 What type of control is commonly used for speed regulation in a DC motor?
Series resistor
Field weakening
Constant torque
Open-loop only
Explanation - Adding a series resistor limits voltage to the armature, controlling speed.
Correct answer is: Series resistor
Q.109 In an induction motor, the torque is maximum when
Slip is zero
Slip is moderate
Slip is high
Slip is irrelevant
Explanation - Maximum torque occurs at a slip typically around 5–10%.
Correct answer is: Slip is moderate
Q.110 Which of the following devices is essential for converting DC to AC in a VFD?
Rectifier
Inverter
Filter
Transformer
Explanation - The inverter stage creates the AC output for the motor.
Correct answer is: Inverter
Q.111 Which of the following is a typical use of a 'soft starter' in industry?
Increase motor speed instantly
Reduce start-up inrush current
Maintain constant speed at all times
Eliminate the need for a VFD
Explanation - Soft starters limit voltage ramp to protect equipment.
Correct answer is: Reduce start-up inrush current
Q.112 The 'field weakening' technique in a DC motor involves
Increasing field flux
Decreasing field flux
Adding armature resistance
Adding external power supply
Explanation - Reducing field flux allows the motor to run faster.
Correct answer is: Decreasing field flux
Q.113 Which of the following best describes a stepper motor’s operating principle?
Continuous rotation at a fixed speed
Discrete steps based on pulse sequence
Variable frequency drive
Field-oriented control
Explanation - Stepper motors move in defined steps controlled by pulse timing.
Correct answer is: Discrete steps based on pulse sequence
Q.114 In a VFD, the 'gate driver' is responsible for
Providing high voltage to the motor
Driving the MOSFET gates
Filtering the DC link
Sensing the rotor position
Explanation - Gate drivers control transistor switching in the inverter.
Correct answer is: Driving the MOSFET gates
Q.115 Which of the following is true about the torque-speed curve of an induction motor?
It is linear
It is hyperbolic
It is flat
It is quadratic
Explanation - The torque-speed relationship follows a hyperbola due to slip.
Correct answer is: It is hyperbolic
Q.116 In a DC motor, the back EMF constant (K_e) is numerically equal to
The torque constant
The inverse of the torque constant
The square of the torque constant
Zero
Explanation - In SI units, K_e = K_t.
Correct answer is: The torque constant
Q.117 The main benefit of using a variable frequency drive in a conveyor system is to
Eliminate the need for a motor
Reduce energy consumption during load changes
Increase starting torque drastically
Simplify the control wiring
Explanation - VFDs adjust speed to match load, saving energy.
Correct answer is: Reduce energy consumption during load changes
Q.118 Which of the following components in a DC motor is responsible for converting mechanical motion back to electrical energy?
Commutator
Field winding
Armature winding
Back EMF generator
Explanation - Back EMF is the generated voltage opposing the applied voltage.
Correct answer is: Back EMF generator
Q.119 In a brushless DC motor, the 'commutation' is achieved by
Mechanical brushes
Electronic control of windings
A permanent magnet rotor
A synchronous field
Explanation - Electronic commutation switches currents in the windings.
Correct answer is: Electronic control of windings
Q.120 Which of the following is a key advantage of using a wound-rotor induction motor?
Simplicity
Variable speed control with external resistors
Low cost
High starting torque only
Explanation - External resistors modify rotor impedance for speed control.
Correct answer is: Variable speed control with external resistors
Q.121 In a DC motor, the torque is proportional to
Back EMF
Armature voltage
Armature current
Supply voltage
Explanation - Torque = K_t × I_a.
Correct answer is: Armature current
Q.122 Which of these is a characteristic of a synchronous motor’s slip?
Non-zero at all times
Always zero
Variable with load
Dependent on field winding
Explanation - Synchronous motors operate at synchronous speed, so slip = 0.
Correct answer is: Always zero
Q.123 The main purpose of a 'soft-start' device in a motor drive is to
Increase speed instantly
Reduce mechanical shock and inrush current
Keep the motor at a constant speed
Eliminate the need for a VFD
Explanation - Soft-start gradually increases voltage, protecting equipment.
Correct answer is: Reduce mechanical shock and inrush current
Q.124 Which of the following is a common application for a stepper motor?
High-speed conveyor
Precision positioning
High torque drills
Electric vehicles
Explanation - Stepper motors are ideal for accurate positioning tasks.
Correct answer is: Precision positioning
Q.125 In a VFD, the 'PWM' strategy is used to
Reduce the motor’s voltage
Control the output voltage and frequency
Increase the motor’s torque directly
Generate a DC output
Explanation - PWM synthesizes the desired AC waveform.
Correct answer is: Control the output voltage and frequency
Q.126 The torque constant (K_t) and back EMF constant (K_e) of a DC motor are
Equal in magnitude
Inversely proportional
Unrelated
Always zero
Explanation - K_t and K_e are numerically equal in SI units.
Correct answer is: Equal in magnitude
Q.127 Which of these is NOT a typical use of a VFD in industrial applications?
Speed control of motors
Torque control of induction motors
Eliminating the need for a motor
Energy savings during variable loads
Explanation - VFDs control motors but do not replace them.
Correct answer is: Eliminating the need for a motor
Q.128 In an induction motor, increasing the supply voltage while keeping frequency constant results in
Higher torque
Lower torque
Same torque
No effect on torque
Explanation - Higher voltage increases current, thereby increasing torque.
Correct answer is: Higher torque
Q.129 The main function of a 'gate driver' in a VFD is to
Supply DC power
Switch the inverter’s MOSFETs
Filter AC ripple
Measure motor speed
Explanation - Gate drivers provide the control signals to the MOSFETs.
Correct answer is: Switch the inverter’s MOSFETs
Q.130 Which of the following is an advantage of using a brushless DC motor?
High maintenance
Low efficiency
High reliability and low maintenance
Limited speed control
Explanation - BLDC motors lack brushes, reducing wear and maintenance.
Correct answer is: High reliability and low maintenance
Q.131 In a DC motor, the relationship between speed and back EMF is
Inverse
Direct proportional
Quadratic
Independent
Explanation - Back EMF increases linearly with speed.
Correct answer is: Direct proportional
Q.132 The primary purpose of a VFD in an electric motor system is to
Increase the motor’s voltage
Provide variable speed and torque control
Reduce the motor’s size
Eliminate the need for a controller
Explanation - VFDs allow precise speed and torque control for various applications.
Correct answer is: Provide variable speed and torque control
Q.133 Which of the following statements is true about the torque constant of a DC motor?
It is equal to the back EMF constant
It is unrelated to the motor’s current
It is constant only at no load
It is always zero
Explanation - In SI units, the torque constant equals the back EMF constant.
Correct answer is: It is equal to the back EMF constant
Q.134 In a VFD, the 'PWM' technique is used to produce an AC output that is
A perfect sine wave
A square wave
A triangular wave
A variable frequency waveform
Explanation - PWM synthesizes a near-sinusoidal output at desired frequency.
Correct answer is: A variable frequency waveform
Q.135 Which of the following is an example of a field-oriented control technique?
Direct torque control
V/f control
Field-oriented control (FOC)
Open-loop control
Explanation - FOC decouples torque and flux for improved performance.
Correct answer is: Field-oriented control (FOC)
Q.136 In a DC motor, if the armature voltage is increased while keeping the field flux constant, the speed will
Increase
Decrease
Remain the same
Become zero
Explanation - Higher voltage raises the back EMF and speed until a new equilibrium is reached.
Correct answer is: Increase
Q.137 Which of the following devices is used to reduce the voltage supplied to the motor for speed control?
Variable resistor
Variable transformer
Both a and b
None of the above
Explanation - Both a series resistor and a variable transformer can reduce voltage.
Correct answer is: Both a and b
Q.138 The 'field weakening' technique allows a DC motor to run at speeds
Below its base speed
At its base speed only
Above its base speed
Only at no load
Explanation - Reducing field flux increases speed beyond base speed.
Correct answer is: Above its base speed
Q.139 In a variable frequency drive, the 'inverter' stage is responsible for
Converting AC to DC
Filtering the DC link
Converting DC to AC
Providing mechanical support
Explanation - The inverter creates the variable frequency AC output.
Correct answer is: Converting DC to AC
Q.140 Which of the following best describes the torque-speed relationship in a synchronous motor?
Hyperbolic
Linear
Constant torque across speeds
Parabolic
Explanation - Synchronous motors have a relatively linear torque-speed characteristic at constant field flux.
Correct answer is: Linear
Q.141 What is the primary purpose of a 'soft starter' in motor applications?
Instantly increase motor speed
Reduce inrush current and mechanical shock
Maintain constant torque at all speeds
Eliminate the need for a controller
Explanation - Soft starters gradually ramp up voltage to protect equipment.
Correct answer is: Reduce inrush current and mechanical shock
Q.142 In a brushless DC motor, the 'commutation' process is performed by
Mechanical brushes
Electronic controllers
Synchronous field windings
Permanent magnets
Explanation - Electronic commutation switches currents in the windings.
Correct answer is: Electronic controllers
Q.143 Which of the following is a key advantage of a wound-rotor induction motor?
Simple construction
Variable speed control using external resistors
Low cost
High efficiency only at full load
Explanation - External resistors modify rotor impedance for speed adjustment.
Correct answer is: Variable speed control using external resistors
Q.144 What is the main benefit of using a variable frequency drive (VFD) in a conveyor system?
Eliminate the need for a motor
Increase motor speed dramatically
Reduce energy consumption during load changes
Simplify the mechanical design
Explanation - VFDs adjust motor speed to match load, saving energy.
Correct answer is: Reduce energy consumption during load changes
Q.145 In a DC motor, the torque constant (K_t) is defined as
Torque per volt
Torque per ampere
Voltage per torque
Current per torque
Explanation - K_t is the proportionality constant between armature current and torque.
Correct answer is: Torque per ampere
Q.146 Which of the following statements about the torque constant (K_t) is FALSE?
It is equal to the back EMF constant
It is a property of the motor’s construction
It is independent of load
It is measured in Nm/A
Explanation - K_t is a motor constant and does not depend on load; it is defined by construction.
Correct answer is: It is independent of load
Q.147 The 'gate driver' in a VFD is primarily used to
Switch the inverter MOSFETs
Filter the DC link
Generate the AC waveform
Sense rotor position
Explanation - Gate drivers provide the drive signals to the MOSFETs in the inverter.
Correct answer is: Switch the inverter MOSFETs
Q.148 Which of the following is a characteristic of a permanent magnet synchronous motor (PMSM)?
Requires external field winding
Has a fixed torque constant
Operates at variable speed with field weakening
Needs brushes for commutation
Explanation - PMSMs can vary speed by adjusting field flux while maintaining high torque.
Correct answer is: Operates at variable speed with field weakening
Q.149 What is the purpose of the 'filter' in a VFD’s DC link?
Increase voltage
Smooth DC voltage ripple
Control frequency
Provide mechanical support
Explanation - The filter removes ripple before the inverter stage.
Correct answer is: Smooth DC voltage ripple
Q.150 Which of the following best describes a 'soft-start' in a motor drive?
Instantly applying full voltage
Gradual increase in voltage to limit current
Constant voltage with no current limit
A type of VFD
Explanation - Soft-start reduces inrush current by ramping voltage.
Correct answer is: Gradual increase in voltage to limit current
Q.151 Which of the following statements about a wound-rotor induction motor is TRUE?
It has no external resistors
It can provide variable speed control via external resistors
It operates only at synchronous speed
It cannot be used for high-torque applications
Explanation - External resistors adjust rotor impedance for speed control.
Correct answer is: It can provide variable speed control via external resistors
Q.152 Which of the following devices is used to convert DC to AC in a VFD?
Transformer
Rectifier
Inverter
Filter
Explanation - The inverter stage produces the AC output for the motor.
Correct answer is: Inverter
Q.153 The torque constant K_t of a DC motor is given by the formula
K_t = V / ω
K_t = I / T
K_t = T / I
K_t = R × I
Explanation - Torque constant is torque divided by armature current.
Correct answer is: K_t = T / I
Q.154 In a VFD, the 'PWM' strategy is used to generate an AC output that is
A perfect sine wave
A square wave
A variable frequency waveform
A DC waveform
Explanation - PWM synthesizes a near-sinusoidal output at desired frequency.
Correct answer is: A variable frequency waveform
Q.155 Which of the following best describes a 'soft-start' device?
A device that instantly increases speed
A device that limits starting current by ramping voltage
A device that eliminates the need for a VFD
A device that maintains constant speed
Explanation - Soft starters protect motors by gradually raising voltage.
Correct answer is: A device that limits starting current by ramping voltage
Q.156 What is the main advantage of using a VFD in industrial motor applications?
Increased speed only
Reduced energy consumption and improved speed control
Elimination of motor maintenance
Simplified mechanical design
Explanation - VFDs adjust speed and torque, saving energy and enhancing control.
Correct answer is: Reduced energy consumption and improved speed control
Q.157 Which of the following best describes the torque-speed curve of a wound-rotor induction motor?
Hyperbolic
Linear
Parabolic
Flat
Explanation - The torque-speed relationship follows a hyperbola due to slip.
Correct answer is: Hyperbolic
Q.158 In a VFD, the 'inverter' stage converts the DC link voltage into
DC voltage
AC voltage at variable frequency
Mechanical energy
No output
Explanation - The inverter provides the AC supply to the motor.
Correct answer is: AC voltage at variable frequency
Q.159 Which of the following is a key feature of a permanent magnet synchronous motor (PMSM)?
Requires a field winding
Has high torque density
Uses brushes
Cannot operate at variable speed
Explanation - PMSMs provide high torque per unit size due to permanent magnets.
Correct answer is: Has high torque density
Q.160 What is the purpose of a 'soft-start' device in a motor application?
Increase speed instantly
Reduce mechanical shock and inrush current
Maintain constant torque at all speeds
Eliminate the need for a controller
Explanation - Soft starters mitigate inrush current and mechanical shock.
Correct answer is: Reduce mechanical shock and inrush current
Q.161 Which of the following statements about a brushless DC motor is FALSE?
It has no brushes
It uses electronic commutation
It requires a field winding
It can operate at high speeds
Explanation - BLDC motors use permanent magnets for the field; no field winding is needed.
Correct answer is: It requires a field winding
Q.162 In a DC motor, increasing the armature resistance will
Increase speed
Decrease speed
Not affect speed
Decrease torque only
Explanation - Higher resistance reduces current, lowering back EMF and speed.
Correct answer is: Decrease speed
Q.163 Which of the following is a characteristic of a synchronous motor’s torque-speed relationship?
Hyperbolic
Linear
Parabolic
Flat
Explanation - Synchronous motors exhibit a relatively linear torque-speed curve at constant field flux.
Correct answer is: Linear
Q.164 The torque constant K_t of a DC motor is equal to the
Back EMF constant
Voltage constant
Current constant
Resistance constant
Explanation - In SI units, K_t numerically equals K_e.
Correct answer is: Back EMF constant
Q.165 In a VFD, the 'gate driver' is used to
Filter the AC output
Switch the MOSFET gates
Convert AC to DC
Measure motor speed
Explanation - Gate drivers provide the necessary drive signals to the MOSFETs.
Correct answer is: Switch the MOSFET gates
Q.166 Which of the following best describes a 'soft start' device?
Instantly applies full voltage
Gradually increases voltage to reduce inrush current
Provides constant torque
Eliminates the need for a motor
Explanation - Soft starters protect equipment by ramping voltage.
Correct answer is: Gradually increases voltage to reduce inrush current
Q.167 Which of the following is NOT a typical use of a VFD?
Speed control of induction motors
Torque control
Providing mechanical power directly
Energy savings in variable load applications
Explanation - VFDs control motor speed, they do not provide mechanical power themselves.
Correct answer is: Providing mechanical power directly
Q.168 In a DC motor, the back EMF is proportional to
Speed
Torque
Current
Resistance
Explanation - Back EMF increases linearly with rotor speed.
Correct answer is: Speed
Q.169 Which of the following best describes the torque-speed curve of a permanent magnet synchronous motor (PMSM)?
Hyperbolic
Linear with field weakening
Flat
Parabolic
Explanation - PMSMs exhibit a relatively linear torque-speed curve that can be extended by field weakening.
Correct answer is: Linear with field weakening
Q.170 Which of the following is a typical feature of a wound-rotor induction motor?
No external resistors
Variable speed via external resistors
Fixed speed only
High efficiency at no load
Explanation - External resistors can be connected to rotor windings for speed control.
Correct answer is: Variable speed via external resistors
Q.171 In a DC motor, the torque constant K_t is given by the equation
K_t = V / I
K_t = T / I
K_t = I / T
K_t = R × I
Explanation - Torque constant is torque per ampere.
Correct answer is: K_t = T / I
Q.172 Which of the following best describes a 'soft start' device?
Instantly applies full voltage
Gradually increases voltage to limit current
Provides a constant torque
Eliminates the need for a controller
Explanation - Soft starters reduce inrush current by ramping voltage.
Correct answer is: Gradually increases voltage to limit current
Q.173 Which of the following is a key advantage of using a brushless DC motor?
High maintenance
Low reliability
High reliability and low maintenance
Limited speed control
Explanation - BLDC motors lack brushes, reducing wear and maintenance costs.
Correct answer is: High reliability and low maintenance
Q.174 What is the primary function of a 'tachometer' in a motor control system?
Measure voltage
Measure current
Provide speed feedback
Measure temperature
Explanation - Tachometers give real-time speed data for closed-loop control.
Correct answer is: Provide speed feedback
Q.175 In a DC motor, increasing the applied voltage while keeping the field flux constant leads to
Increased torque only
Increased speed only
Increased torque and speed
No change in speed or torque
Explanation - Higher voltage raises both current and back EMF, increasing torque and speed.
Correct answer is: Increased torque and speed
Q.176 Which of the following devices is typically used to provide a variable frequency output in a VFD?
Variable resistor
Inverter
Rectifier
Transformer
Explanation - The inverter creates the variable frequency AC output.
Correct answer is: Inverter
Q.177 In a VFD, the 'PWM' strategy is used to produce an AC output that is
A perfect sine wave
A square wave
A variable frequency waveform
A DC waveform
Explanation - PWM synthesizes a near-sinusoidal output at the desired frequency.
Correct answer is: A variable frequency waveform
Q.178 Which of the following best describes the torque-speed relationship of a wound-rotor induction motor?
Hyperbolic
Linear
Parabolic
Flat
Explanation - The torque-speed relationship follows a hyperbola due to slip.
Correct answer is: Hyperbolic
Q.179 What is the main purpose of a 'soft-start' device in motor applications?
Instantly increase speed
Reduce inrush current and mechanical shock
Maintain constant torque
Eliminate the need for a controller
Explanation - Soft starters protect motors by gradually raising voltage.
Correct answer is: Reduce inrush current and mechanical shock
Q.180 Which of the following is a characteristic of a brushless DC motor?
Requires brushes
Uses electronic commutation
Has no magnetic field
Cannot operate at high speed
Explanation - BLDC motors switch currents electronically without brushes.
Correct answer is: Uses electronic commutation
Q.181 What does the 'field weakening' technique in a DC motor enable the motor to do?
Run at higher speeds above base speed
Run at lower speeds below base speed
Operate at the same speed regardless of load
Eliminate the need for a field winding
Explanation - Reducing field flux allows the motor to run faster.
Correct answer is: Run at higher speeds above base speed
Q.182 In a DC motor, the back EMF (E_b) is calculated as
E_b = V_applied - I_a × R_a
E_b = I_a × R_a - V_applied
E_b = V_applied + I_a × R_a
E_b = V_applied × I_a
Explanation - Back EMF equals applied voltage minus voltage drop across armature resistance.
Correct answer is: E_b = V_applied - I_a × R_a
Q.183 Which of the following best describes a 'soft-start' device?
Instantly applies full voltage
Gradually ramps voltage to limit current
Provides constant torque
Eliminates the need for a controller
Explanation - Soft starters mitigate inrush current by ramping voltage.
Correct answer is: Gradually ramps voltage to limit current
Q.184 Which of the following devices is used to reduce the voltage supplied to a motor for speed control?
Variable resistor
Variable transformer
Both a and b
None of the above
Explanation - Both a series resistor and a variable transformer can reduce voltage.
Correct answer is: Both a and b
Q.185 In a brushless DC motor, the commutation is performed by
Mechanical brushes
Electronic controllers
Permanent magnets
Synchronous field windings
Explanation - Electronic commutation switches the currents in the windings.
Correct answer is: Electronic controllers
Q.186 Which of the following best describes the torque-speed curve of a permanent magnet synchronous motor (PMSM)?
Hyperbolic
Linear with field weakening
Flat
Parabolic
Explanation - PMSMs exhibit a relatively linear torque-speed curve that can be extended by field weakening.
Correct answer is: Linear with field weakening
Q.187 Which of the following is a typical application of a stepper motor?
High-speed conveyors
Precision positioning
High torque drills
Electric vehicles
Explanation - Stepper motors are ideal for accurate positioning tasks.
Correct answer is: Precision positioning
Q.188 In a VFD, the 'inverter' stage is responsible for
Converting AC to DC
Filtering the DC link
Converting DC to AC
Providing mechanical support
Explanation - The inverter creates the variable frequency AC output.
Correct answer is: Converting DC to AC
Q.189 What is the main benefit of using a VFD in a conveyor system?
Eliminate the need for a motor
Reduce energy consumption during load changes
Increase motor speed dramatically
Simplify the mechanical design
Explanation - VFDs adjust motor speed to match load, saving energy.
Correct answer is: Reduce energy consumption during load changes
Q.190 Which of the following best describes a 'soft-start' device?
Instantly applies full voltage
Gradually increases voltage to limit current
Provides constant torque
Eliminates the need for a controller
Explanation - Soft starters protect equipment by ramping voltage.
Correct answer is: Gradually increases voltage to limit current
Q.191 Which of the following is a key advantage of a brushless DC motor?
High maintenance
Low reliability
High reliability and low maintenance
Limited speed control
Explanation - BLDC motors lack brushes, reducing wear and maintenance.
Correct answer is: High reliability and low maintenance
Q.192 In a DC motor, the back EMF constant (K_e) is numerically equal to
The torque constant
The inverse of the torque constant
The square of the torque constant
Zero
Explanation - In SI units, K_e = K_t.
Correct answer is: The torque constant
Q.193 Which of the following is a typical use of a VFD in industrial applications?
Speed control of motors
Torque control of induction motors
Providing mechanical power directly
Energy savings in variable load applications
Explanation - VFDs adjust speed for energy efficiency.
Correct answer is: Energy savings in variable load applications
Q.194 What is the main purpose of a 'soft-start' device in a motor drive?
Instantly increase speed
Reduce mechanical shock and inrush current
Maintain constant torque
Eliminate the need for a controller
Explanation - Soft-start protects equipment by gradually raising voltage.
Correct answer is: Reduce mechanical shock and inrush current
Q.195 Which of the following is a key feature of a wound-rotor induction motor?
No external resistors
Variable speed via external resistors
Fixed speed only
High efficiency at no load
Explanation - External resistors modify rotor impedance for speed control.
Correct answer is: Variable speed via external resistors
Q.196 Which of the following best describes the torque-speed relationship of a synchronous motor?
Hyperbolic
Linear
Parabolic
Flat
Explanation - Synchronous motors have a relatively linear torque-speed curve at constant field flux.
Correct answer is: Linear
Q.197 Which of the following is a characteristic of a brushless DC motor?
Requires a field winding
Uses electronic commutation
Has a high maintenance requirement
Operates only at low speeds
Explanation - BLDC motors switch currents electronically, no brushes required.
Correct answer is: Uses electronic commutation
Q.198 What is the main function of a 'tachometer' in motor control systems?
Measure voltage
Measure current
Provide speed feedback
Measure temperature
Explanation - Tachometers provide real-time speed data for closed-loop control.
Correct answer is: Provide speed feedback
Q.199 In a DC motor, the torque constant K_t is given by the equation
K_t = V / I
K_t = I / T
K_t = T / I
K_t = R × I
Explanation - Torque constant is torque divided by armature current.
Correct answer is: K_t = T / I
Q.200 Which of the following statements about the torque constant K_t is FALSE?
It is equal to the back EMF constant
It is a property of the motor’s construction
It is independent of load
It is measured in Nm/A
Explanation - K_t is a motor constant and does not depend on load; it is defined by construction.
Correct answer is: It is independent of load
Q.201 In a VFD, the 'gate driver' is primarily used to
Switch the inverter MOSFETs
Filter the DC link
Generate the AC waveform
Sense rotor position
Explanation - Gate drivers provide the drive signals to the MOSFETs.
Correct answer is: Switch the inverter MOSFETs
Q.202 Which of the following best describes a 'soft start' device?
Instantly applies full voltage
Gradually increases voltage to limit current
Provides constant torque
Eliminates the need for a controller
Explanation - Soft starters mitigate inrush current by ramping voltage.
Correct answer is: Gradually increases voltage to limit current
Q.203 What is the main advantage of using a brushless DC motor?
High maintenance
Low reliability
High reliability and low maintenance
Limited speed control
Explanation - BLDC motors lack brushes, reducing wear and maintenance.
Correct answer is: High reliability and low maintenance