Q.1 What type of current is used in a three‑phase AC induction motor to create a rotating magnetic field?
DC current
Sine‑wave AC current
Square‑wave AC current
Rectangular DC current
Explanation - A sinusoidal AC supply is used to produce a rotating magnetic field in the stator, which induces torque in an induction motor.
Correct answer is: Sine‑wave AC current
Q.2 Which component is NOT typically part of a Variable Frequency Drive (VFD) used for vector control?
Inverter
Controller
Rectifier
Load bank
Explanation - A load bank is a testing device; the main components of a VFD are rectifier, inverter, and controller.
Correct answer is: Load bank
Q.3 What is the purpose of the stator current reference vector in vector control?
To control motor speed
To maintain torque balance
To regulate voltage
To synchronize the rotor field
Explanation - The stator current reference vector is split into torque‑producing and flux‑producing components to balance torque generation.
Correct answer is: To maintain torque balance
Q.4 Which of the following is an advantage of vector control over scalar (V/f) control?
Higher torque ripple
Simpler controller design
Better dynamic performance
Lower power factor
Explanation - Vector control provides independent control of torque and flux, resulting in faster dynamic response and improved performance.
Correct answer is: Better dynamic performance
Q.5 In a Field‑Oriented Control (FOC) scheme, the d‑axis is aligned to:
The stator voltage vector
The rotor flux vector
The rotor speed vector
The load torque vector
Explanation - FOC aligns the d‑axis with the rotor flux to decouple torque and flux control.
Correct answer is: The rotor flux vector
Q.6 Which transform is used to convert three‑phase currents into d‑q components?
Laplace Transform
Clarke Transform
Fourier Transform
Z‑Transform
Explanation - The Clarke transform maps three‑phase currents into a two‑axis stationary reference frame (α‑β), which is then transformed into d‑q via Park transform.
Correct answer is: Clarke Transform
Q.7 The Park transform is applied to:
Convert DC to AC
Map α‑β to d‑q axis
Linearize the motor equations
Filter noise from sensor signals
Explanation - Park transform rotates the stationary α‑β frame into a rotating d‑q frame aligned with the rotor field.
Correct answer is: Map α‑β to d‑q axis
Q.8 What is the main role of the PI controller in vector control?
Estimate rotor speed
Generate PWM signals
Regulate d‑axis and q‑axis currents
Measure voltage drop
Explanation - PI controllers in the d‑ and q‑axes maintain the desired current references by adjusting inverter voltages.
Correct answer is: Regulate d‑axis and q‑axis currents
Q.9 Which equation represents the torque produced by a three‑phase induction motor in vector control?
T = k_e * (i_d + i_q)
T = k_t * (i_q * Φ_d)
T = k_t * (i_d * Φ_q)
T = k_t * (i_q * Φ_q)
Explanation - Torque in vector control is proportional to the product of q‑axis current and d‑axis flux.
Correct answer is: T = k_t * (i_q * Φ_d)
Q.10 Which of the following sensors is NOT required for a sensorless vector control algorithm?
Hall‑effect sensor
Encoder
Speed sensor
Current sensor
Explanation - Sensorless techniques estimate rotor position from current measurements; an encoder is a sensor-based method.
Correct answer is: Encoder
Q.11 The back‑EMF of a motor is used in sensorless control to estimate which of the following?
Stator resistance
Rotor speed
Load torque
Supply voltage
Explanation - Back‑EMF is proportional to speed; it can be used to estimate rotor speed in sensorless control.
Correct answer is: Rotor speed
Q.12 During a torque‑increase command, the q‑axis current reference changes while the d‑axis reference:
Increases
Decreases
Remains constant
Flips sign
Explanation - The d‑axis current is kept at zero for maximum torque per amp; only q‑axis is adjusted to change torque.
Correct answer is: Remains constant
Q.13 Which of the following is a common fault detected by a vector‑controlled VFD?
Short‑circuit in the power supply
Over‑current in the inverter
Incorrect harmonic content
Excessive torque ripple
Explanation - Over‑current detection protects the inverter from damage; other faults may also be detected but over‑current is typical.
Correct answer is: Over‑current in the inverter
Q.14 A common technique to reduce torque ripple in vector control is to:
Increase the switching frequency
Use a higher stator voltage
Decrease the d‑axis current
Shorten the pulse width modulation period
Explanation - Higher switching frequency reduces harmonic distortion, thus reducing torque ripple.
Correct answer is: Increase the switching frequency
Q.15 In a vector‑controlled motor, what does the term 'saturated flux' refer to?
When flux linkage equals supply voltage
When the magnetic core reaches its saturation limit
When the rotor speed is constant
When back‑EMF equals stator voltage
Explanation - Saturation occurs when increasing current no longer increases flux due to core material limits.
Correct answer is: When the magnetic core reaches its saturation limit
Q.16 Which of the following best describes the 'dead zone' in sensorless vector control?
A voltage range where the inverter cannot switch
A frequency range where back‑EMF is too low for detection
A speed range where torque is zero
A region where the motor overheats
Explanation - At very low speeds, back‑EMF is too small to estimate rotor position accurately, creating a dead zone.
Correct answer is: A frequency range where back‑EMF is too low for detection
Q.17 The torque constant \(K_t\) of a motor is related to which physical quantity?
Stator resistance
Flux linkage per ampere
Rotor inertia
Back‑EMF constant
Explanation - Torque constant is proportional to the magnetic flux produced per unit current.
Correct answer is: Flux linkage per ampere
Q.18 Which component is responsible for converting AC input to DC in a VFD?
Inverter
Rectifier
Controller
Filter
Explanation - The rectifier converts the AC supply to DC for the inverter.
Correct answer is: Rectifier
Q.19 In vector control, the q‑axis current is primarily responsible for:
Generating torque
Producing flux
Controlling speed
Stabilizing voltage
Explanation - The q‑axis current produces torque in the motor.
Correct answer is: Generating torque
Q.20 Which of these is NOT a requirement for implementing a sensorless vector control algorithm?
High‑speed microcontroller
Accurate current measurements
Direct measurement of rotor position
Knowledge of motor parameters
Explanation - Sensorless control avoids direct rotor position measurement, estimating it from currents.
Correct answer is: Direct measurement of rotor position
Q.21 Why is the Park transform used in vector control?
To filter noise from the voltage
To simplify the motor equations by rotating the reference frame
To convert DC to AC
To estimate the load torque
Explanation - Park transform aligns the reference frame with the magnetic field, simplifying control.
Correct answer is: To simplify the motor equations by rotating the reference frame
Q.22 Which of the following statements about vector control is correct?
It requires no knowledge of motor parameters.
It is identical to scalar control in performance.
It decouples torque and flux control.
It cannot be implemented in an inverter.
Explanation - Vector control allows independent control of torque and flux.
Correct answer is: It decouples torque and flux control.
Q.23 In a VFD, the inverter uses which type of switching device?
Diodes
Transistors (IGBT/SiC)
Resistors
Capacitors
Explanation - IGBTs or SiC MOSFETs are used to switch DC to AC at high speeds.
Correct answer is: Transistors (IGBT/SiC)
Q.24 Which of the following is a key benefit of vector control for AC induction motors?
Simpler hardware
Lower torque control accuracy
Higher starting torque
Reduced motor speed
Explanation - Vector control can provide higher starting torque by controlling stator current direction.
Correct answer is: Higher starting torque
Q.25 What is the main purpose of the zero‑sequence current component in vector control?
To generate torque
To monitor harmonic distortion
To improve voltage regulation
To increase speed
Explanation - Zero‑sequence current indicates unbalanced loads and harmonics.
Correct answer is: To monitor harmonic distortion
Q.26 During sensorless vector control, the estimated rotor position is typically derived from:
Voltage phase shift
Current phase shift
Mechanical encoder
Temperature sensor
Explanation - The phase shift between voltage and current gives rotor position in sensorless methods.
Correct answer is: Current phase shift
Q.27 Which control variable is adjusted by the PI controller to regulate the motor torque?
Stator voltage amplitude
Rotor flux linkage
q‑axis current magnitude
d‑axis voltage frequency
Explanation - Torque is directly proportional to q‑axis current, which the PI controller regulates.
Correct answer is: q‑axis current magnitude
Q.28 The term 'dead zone' in sensorless control refers to:
A period where the inverter cannot switch on
A speed range where back‑EMF is too small to detect rotor position
A voltage drop across the inverter
An overload protection feature
Explanation - At low speeds, back‑EMF is negligible, making rotor position estimation difficult.
Correct answer is: A speed range where back‑EMF is too small to detect rotor position
Q.29 In a VFD, the inverter typically operates at:
50 Hz
60 Hz
100 kHz
1 Hz
Explanation - High‑frequency switching (tens to hundreds of kHz) is used to approximate a continuous sine wave.
Correct answer is: 100 kHz
Q.30 Which of these is an advantage of vector control over scalar control?
Easier tuning
Slower response to load changes
Higher torque ripple
Better dynamic performance
Explanation - Vector control can respond quickly to changes because torque and flux are independently controlled.
Correct answer is: Better dynamic performance
Q.31 What type of waveform does the inverter generate to drive the AC motor in vector control?
Square wave
Triangular wave
Pulse‑width modulated sine wave
Rectangular wave
Explanation - PWM approximates a sine wave by varying the width of pulses.
Correct answer is: Pulse‑width modulated sine wave
Q.32 Which of the following is a typical error in vector control systems?
Incorrect voltage regulator settings
Incorrect current sensor calibration
Over‑driving the load
All of the above
Explanation - All listed issues can lead to errors in vector control.
Correct answer is: All of the above
Q.33 In vector control, the d‑axis current is often set to:
Zero
Maximum
Half of the rated current
Negative of the q‑axis current
Explanation - Setting d‑axis current to zero maximizes torque per amp.
Correct answer is: Zero
Q.34 The back‑EMF constant \(K_e\) of a motor is numerically equal to which of the following?
Torque constant \(K_t\)
Voltage constant
Current constant
Speed constant
Explanation - In SI units, \(K_e\) and \(K_t\) have the same magnitude.
Correct answer is: Torque constant \(K_t\)
Q.35 Which type of control algorithm uses both speed and torque references to calculate current references?
Scalar control
Vector control
Direct torque control (DTC)
Speed‑to‑torque control
Explanation - Vector control maps speed and torque to d‑q current references.
Correct answer is: Vector control
Q.36 Which of the following best describes the role of the inverter in a VFD?
Converts AC to DC
Converts DC to AC with controllable frequency
Monitors motor temperature
Provides power factor correction
Explanation - The inverter produces a variable‑frequency AC waveform to drive the motor.
Correct answer is: Converts DC to AC with controllable frequency
Q.37 Why is PWM switching frequency chosen to be higher than the motor frequency in VFDs?
To reduce harmonic distortion
To increase torque ripple
To save power
To simplify the controller
Explanation - Higher switching reduces the relative effect of harmonics.
Correct answer is: To reduce harmonic distortion
Q.38 Which parameter must be known accurately for effective vector control?
Stator resistance
Motor length
Ambient temperature
Battery voltage
Explanation - Stator resistance is needed for flux and current estimation.
Correct answer is: Stator resistance
Q.39 In sensorless vector control, the rotor position is often estimated using:
Back‑EMF zero‑crossing detection
Mechanical encoder
Hall sensors
Current sensor phase shift
Explanation - Phase shift between current and voltage signals indicates rotor position.
Correct answer is: Current sensor phase shift
Q.40 Which of the following best explains the concept of 'field weakening' in vector control?
Increasing d‑axis flux to raise torque
Reducing d‑axis flux to allow higher speed
Eliminating q‑axis current
Increasing rotor resistance
Explanation - Field weakening reduces flux so that the motor can run faster than base speed.
Correct answer is: Reducing d‑axis flux to allow higher speed
Q.41 What is the main purpose of the zero‑sequence current measurement in a 3‑phase VFD?
To detect phase imbalances
To increase torque
To reduce inverter size
To control speed
Explanation - Zero‑sequence currents indicate imbalance among the phases.
Correct answer is: To detect phase imbalances
Q.42 In vector control, which axis is responsible for torque production?
d‑axis
q‑axis
α‑axis
β‑axis
Explanation - The q‑axis current produces torque.
Correct answer is: q‑axis
Q.43 Which of the following is a typical fault detected by a VFD in sensorless operation?
Zero‑sequence current exceeds threshold
Rotor speed is too high
Over‑voltage on DC bus
All of the above
Explanation - All these faults can be detected and protected against.
Correct answer is: All of the above
Q.44 In a vector‑controlled induction motor, the torque is proportional to which variable?
Stator flux linkage
Rotor speed
q‑axis current
Stator voltage magnitude
Explanation - Torque is directly proportional to the product of q‑axis current and d‑axis flux.
Correct answer is: q‑axis current
Q.45 Which of the following best describes the difference between vector control and scalar (V/f) control?
Vector control uses only speed feedback
Scalar control controls torque directly
Vector control decouples torque and flux
Scalar control requires a sensorless algorithm
Explanation - Vector control allows independent control of torque and flux; scalar control does not.
Correct answer is: Vector control decouples torque and flux
Q.46 A VFD typically uses which type of power electronic converter to produce AC output?
Rectifier
Inverter
Chopper
Flyback transformer
Explanation - An inverter switches DC to produce a variable‑frequency AC output.
Correct answer is: Inverter
Q.47 Which parameter is most affected by temperature in a VFD?
Switching frequency
DC bus voltage
Current sensor offset
Stator resistance
Explanation - Stator resistance increases with temperature, affecting flux estimation.
Correct answer is: Stator resistance
Q.48 What is the primary function of the Hall sensor in a sensorless VFD?
Measure back‑EMF
Detect rotor position indirectly
Measure stator temperature
Control switching frequency
Explanation - Hall sensors can provide a coarse rotor position measurement used by the controller.
Correct answer is: Detect rotor position indirectly
Q.49 Which control strategy is used to adjust the inverter voltage vector magnitude and angle in vector control?
Proportional control
Integral control
Vector‑field control
Feedforward control
Explanation - Vector‑field control adjusts the voltage vector to achieve desired current components.
Correct answer is: Vector‑field control
Q.50 Which of the following best explains 'flux weakening' in vector control?
Increasing d‑axis flux to increase torque
Reducing d‑axis flux to increase speed beyond base speed
Eliminating q‑axis current
Increasing rotor resistance
Explanation - Flux weakening reduces magnetic flux to allow higher operating speeds.
Correct answer is: Reducing d‑axis flux to increase speed beyond base speed
Q.51 In vector control, what role does the PI controller play in the d‑axis loop?
Adjusts stator voltage to maintain zero d‑axis current
Adjusts rotor speed to match load torque
Regulates motor temperature
Controls PWM duty cycle
Explanation - The d‑axis PI controller maintains the d‑axis current at zero or a desired value.
Correct answer is: Adjusts stator voltage to maintain zero d‑axis current
Q.52 What is the purpose of the 'zero‑sequence filter' in a VFD?
Reduce harmonic distortion
Eliminate DC offset
Smooth the PWM signal
Measure rotor position
Explanation - Zero‑sequence filter removes the DC component of zero‑sequence currents to reduce harmonics.
Correct answer is: Reduce harmonic distortion
Q.53 Which of these is a common method for speed estimation in sensorless vector control?
Using a tachometer
Back‑EMF zero‑crossing detection
Mechanical encoder
Infrared sensor
Explanation - Back‑EMF zero‑crossing provides a convenient speed estimate.
Correct answer is: Back‑EMF zero‑crossing detection
Q.54 The torque equation in vector control can be written as:
T = k_e * I_d
T = k_t * I_q * Φ_d
T = k_t * I_d * Φ_q
T = k_e * I_q * Φ_q
Explanation - Torque is proportional to q‑axis current and d‑axis flux linkage.
Correct answer is: T = k_t * I_q * Φ_d
Q.55 Why is field‑oriented control often preferred in industrial motor drives?
Simpler implementation
Lower cost
Better dynamic performance and efficiency
Fewer components required
Explanation - FOC provides precise control of torque and flux, improving performance.
Correct answer is: Better dynamic performance and efficiency
Q.56 In a VFD, the inverter switching devices are typically:
Diodes
IGBTs or MOSFETs
Transistors (BJT)
Resistors
Explanation - IGBTs or MOSFETs are used for high‑speed, high‑power switching.
Correct answer is: IGBTs or MOSFETs
Q.57 Which of the following is NOT a benefit of vector control?
Higher torque density
Simplified control algorithm
Better dynamic response
Independent flux control
Explanation - Vector control is more complex than scalar control.
Correct answer is: Simplified control algorithm
Q.58 In vector control, what is the effect of increasing the d‑axis current from zero?
Increases torque
Decreases torque
Has no effect
Changes motor speed
Explanation - Increasing d‑axis current reduces the effective flux, decreasing torque per amp.
Correct answer is: Decreases torque
Q.59 The term 'PM' in a Permanent Magnet (PM) synchronous motor refers to:
Power Management
Permanent Magnet
Phase Modulation
Pulse Modulation
Explanation - PM indicates the motor uses permanent magnets for flux generation.
Correct answer is: Permanent Magnet
Q.60 Which component in a VFD is responsible for filtering the DC bus voltage?
Rectifier
Inverter
LC filter
Transformer
Explanation - An LC filter smooths the rectified DC voltage before it reaches the inverter.
Correct answer is: LC filter
Q.61 The motor torque is proportional to the product of which two variables in vector control?
Stator voltage and rotor speed
d‑axis current and q‑axis voltage
q‑axis current and d‑axis flux
Rotor speed and stator resistance
Explanation - Torque ∝ i_q * Φ_d.
Correct answer is: q‑axis current and d‑axis flux
Q.62 In a VFD, what does the 'DC bus' refer to?
The AC input line
The DC link between rectifier and inverter
The output AC lines
The control circuit
Explanation - The DC bus is the link that supplies DC to the inverter.
Correct answer is: The DC link between rectifier and inverter
Q.63 Which of the following is a typical parameter that needs to be estimated in sensorless vector control?
Stator resistance
Rotor inertia
Motor temperature
Capacitor voltage
Explanation - Stator resistance is required for accurate flux and current estimation.
Correct answer is: Stator resistance
Q.64 In vector control, which of the following statements about the q‑axis current is true?
It is used to control speed only.
It is used to control torque only.
It is used to control flux only.
It controls both torque and speed simultaneously.
Explanation - The q‑axis current directly generates torque.
Correct answer is: It is used to control torque only.
Q.65 Which type of inverter topology is most common in VFDs?
Three‑phase bridge inverter
Half‑bridge inverter
Full‑bridge inverter
Three‑phase forward inverter
Explanation - A three‑phase bridge inverter provides three AC outputs for motor drive.
Correct answer is: Three‑phase bridge inverter
Q.66 The 'd‑axis' in vector control represents:
The torque‑producing axis
The flux‑producing axis
The speed axis
The voltage axis
Explanation - d‑axis current produces magnetic flux; q‑axis produces torque.
Correct answer is: The flux‑producing axis
Q.67 What is the main reason for using a sensorless approach in vector control?
Lower cost and complexity
Higher accuracy
Simpler hardware
Better torque ripple control
Explanation - Sensorless eliminates expensive sensors, reducing cost and complexity.
Correct answer is: Lower cost and complexity
Q.68 Which of the following is a typical output of a PI controller in vector control?
Reference current
Reference voltage
Reference torque
Reference speed
Explanation - The PI controller outputs a voltage reference that drives the desired current.
Correct answer is: Reference current
Q.69 What is the effect of increasing the inverter switching frequency?
Increases torque ripple
Reduces harmonic distortion
Lowers efficiency
Increases motor size
Explanation - Higher switching frequency spreads harmonic energy into higher frequencies, reducing low‑order harmonics.
Correct answer is: Reduces harmonic distortion
Q.70 Which of the following is NOT an advantage of vector control?
Precise torque control
Lower torque ripple
Simpler controller design
Better dynamic response
Explanation - Vector control requires complex coordinate transformations and PI loops.
Correct answer is: Simpler controller design
Q.71 The torque constant \(K_t\) is expressed in units of:
N·m/A
W/A
V/A
Hz/A
Explanation - Torque constant converts current to torque, so units are Newton‑meter per ampere.
Correct answer is: N·m/A
Q.72 In vector control, the term 'rotor flux linkage' refers to:
Magnetic flux in the rotor core
Flux produced by stator current
Voltage induced in the stator
Current flowing in the rotor
Explanation - Rotor flux linkage is the magnetic flux that links the rotor windings.
Correct answer is: Magnetic flux in the rotor core
Q.73 What is the main purpose of the Hall sensor in a vector‑controlled VFD?
To measure temperature
To detect rotor position
To sense load torque
To provide voltage reference
Explanation - Hall sensors detect changes in magnetic field for rotor position sensing.
Correct answer is: To detect rotor position
Q.74 Which of the following is an example of a torque‑controlled motor application?
High‑speed fans
Conveyor belts
Pumps
All of the above
Explanation - Torque control is essential in many industrial drives.
Correct answer is: All of the above
Q.75 Which component in a VFD is responsible for generating the PWM signals?
Rectifier
Controller
Filter
Inverter
Explanation - The controller creates the PWM signals based on current references.
Correct answer is: Controller
Q.76 The 'α‑β' axes are used in which transform?
Laplace transform
Fourier transform
Clarke transform
Z‑transform
Explanation - Clarke transform maps three‑phase currents into a two‑axis stationary frame α‑β.
Correct answer is: Clarke transform
Q.77 Which of the following best describes the role of the PI controller in the q‑axis loop?
Maintain d‑axis current at zero
Generate torque reference
Maintain q‑axis current at the reference value
Adjust switching frequency
Explanation - The q‑axis PI controller keeps the q‑current at its desired value to control torque.
Correct answer is: Maintain q‑axis current at the reference value
Q.78 Which of the following is a characteristic of a permanent‑magnet synchronous motor (PMSM)?
Requires field winding
Flux is produced by external magnetic field
Has no rotor windings
Uses rotor resistance to produce flux
Explanation - PMSM uses permanent magnets to generate magnetic flux.
Correct answer is: Flux is produced by external magnetic field
Q.79 What is the purpose of the zero‑sequence current filter in a VFD?
Reduce DC offset in the inverter bus
Eliminate unbalanced phase currents
Smooth the PWM signal
Increase motor torque
Explanation - Zero‑sequence filter removes the zero‑sequence component of unbalanced currents.
Correct answer is: Eliminate unbalanced phase currents
Q.80 Which of the following is true about the d‑axis current in field‑oriented control?
It should be kept zero for maximum torque per amp
It is used to regulate speed
It is always higher than q‑axis current
It does not influence torque
Explanation - Setting d‑axis current to zero maximizes torque efficiency.
Correct answer is: It should be kept zero for maximum torque per amp
Q.81 Which of the following is NOT part of a typical VFD power stage?
Rectifier
Inverter
Controller
Transformer
Explanation - Most VFDs use a transformer for isolation but not as a core power stage component.
Correct answer is: Transformer
Q.82 What is the main effect of increasing the q‑axis current while keeping d‑axis current at zero?
Decrease torque
Increase torque
No change in torque
Decrease speed
Explanation - q‑axis current directly increases torque.
Correct answer is: Increase torque
Q.83 Which of the following is a typical fault protection feature in a VFD?
Over‑temperature protection
Under‑frequency protection
Over‑current protection
All of the above
Explanation - VFDs incorporate multiple protection features.
Correct answer is: All of the above
Q.84 In a vector‑controlled AC motor, which quantity is proportional to the angular velocity?
Back‑EMF
Stator resistance
d‑axis current
Voltage magnitude
Explanation - Back‑EMF is directly proportional to motor speed.
Correct answer is: Back‑EMF
Q.85 Which of the following best explains 'field weakening' in an induction motor?
Increasing rotor flux to increase torque
Reducing rotor flux to allow operation beyond base speed
Eliminating q‑axis current
Increasing stator resistance
Explanation - Field weakening reduces flux to increase speed beyond rated frequency.
Correct answer is: Reducing rotor flux to allow operation beyond base speed
Q.86 Which of the following is a direct benefit of using sensorless control?
Reduced mechanical wear
Simplified installation
Higher torque output
Lower voltage requirement
Explanation - Removing sensors simplifies the drive assembly.
Correct answer is: Simplified installation
Q.87 What is the primary function of the current sensor in a VFD?
To measure motor speed
To provide current feedback for PI control
To monitor temperature
To adjust switching frequency
Explanation - Current sensor feedback is used by the PI controllers in vector control.
Correct answer is: To provide current feedback for PI control
Q.88 Which of the following is a key parameter for the accurate estimation of flux in sensorless control?
Motor temperature
Stator resistance
Inverter switching frequency
Ambient humidity
Explanation - Stator resistance must be known for accurate flux estimation.
Correct answer is: Stator resistance
Q.89 In vector control, the reference current for the d‑axis is commonly set to:
Maximum value
Zero
Negative of the q‑axis reference
Half of the rated current
Explanation - Zero d‑axis current maximizes torque per amp.
Correct answer is: Zero
Q.90 What is the primary advantage of using an IGBT over a MOSFET in a VFD inverter?
Higher voltage rating
Lower switching losses
Lower cost
Higher frequency operation
Explanation - IGBTs can handle higher voltages than MOSFETs.
Correct answer is: Higher voltage rating
Q.91 Which of the following best describes the role of the Clarke transform in vector control?
Converts stator currents to a rotating reference frame
Converts stator currents to a stationary two‑axis frame
Calculates back‑EMF
Filters noise from the voltage signals
Explanation - Clarke transform maps 3‑phase to α‑β stationary frame.
Correct answer is: Converts stator currents to a stationary two‑axis frame
Q.92 Which of the following is a characteristic of a vector‑controlled motor drive?
Torque and flux are coupled
Torque and flux can be controlled independently
Speed cannot be controlled accurately
Voltage is fixed
Explanation - Vector control decouples torque and flux.
Correct answer is: Torque and flux can be controlled independently
Q.93 What is the typical frequency range of the switching frequency in a VFD?
50–60 Hz
500–1000 Hz
10–20 kHz
1–2 MHz
Explanation - Switching frequencies are typically in the tens of kHz.
Correct answer is: 10–20 kHz
Q.94 Which of the following is NOT a typical control objective in vector‑controlled drives?
Speed regulation
Torque control
Temperature management
Flux control
Explanation - Temperature is monitored, but not directly controlled.
Correct answer is: Temperature management
Q.95 What does the 'q‑axis' current reference represent in vector control?
Flux production
Torque production
Speed regulation
Voltage regulation
Explanation - q‑axis current directly influences torque.
Correct answer is: Torque production
Q.96 Which of the following is a direct result of field weakening?
Higher torque per amp
Lower motor speed
Reduced back‑EMF
Higher operating speed beyond base speed
Explanation - Field weakening reduces flux to allow higher speed.
Correct answer is: Higher operating speed beyond base speed
Q.97 What type of waveform is produced by a VFD to drive an induction motor?
Pure sine wave
Square wave
Pulse‑width modulated sine wave
Triangular wave
Explanation - PWM approximates a sine wave.
Correct answer is: Pulse‑width modulated sine wave
Q.98 Which of the following is a benefit of using a sensorless algorithm in vector control?
Lower cost
Simpler hardware
Higher accuracy at low speed
All of the above
Explanation - Sensorless reduces cost and complexity, but may lose accuracy at low speed.
Correct answer is: All of the above
Q.99 Which of the following parameters is directly proportional to motor speed in a vector‑controlled system?
Back‑EMF magnitude
Stator resistance
d‑axis current
Voltage magnitude
Explanation - Back‑EMF increases with speed.
Correct answer is: Back‑EMF magnitude
Q.100 In a VFD, the 'DC bus' voltage is typically controlled by:
A transformer
An inductor and capacitor filter
A regulator circuit
A voltage divider
Explanation - The LC filter smooths the DC voltage.
Correct answer is: An inductor and capacitor filter
Q.101 Which component in a VFD is responsible for converting AC input to DC output?
Inverter
Rectifier
Filter
Controller
Explanation - The rectifier performs AC to DC conversion.
Correct answer is: Rectifier
Q.102 In a vector‑controlled drive, the torque is determined by the product of:
Stator voltage and speed
q‑axis current and d‑axis flux
d‑axis current and q‑axis flux
Back‑EMF and voltage
Explanation - Torque ∝ i_q * Φ_d.
Correct answer is: q‑axis current and d‑axis flux