Q.1 In a separately excited DC motor, which of the following actions will increase the motor speed without changing the load torque?
Increase the field current
Decrease the armature voltage
Decrease the field current
Increase the armature resistance
Explanation - Speed of a DC motor is inversely proportional to the field flux (E = kΦn). Reducing the field current lowers the flux, causing the speed to rise for a given armature voltage and load.
Correct answer is: Decrease the field current
Q.2 Which speed control method for an induction motor maintains a constant V/f ratio?
Vector control
Scalar V/f control
Direct torque control
Pole changing method
Explanation - Scalar V/f control varies both voltage and frequency proportionally to keep the V/f ratio constant, thereby keeping the magnetic flux roughly constant.
Correct answer is: Scalar V/f control
Q.3 For a three‑phase induction motor supplied from a variable frequency drive, what happens to the slip when the frequency is increased while keeping V/f constant?
Slip increases
Slip decreases
Slip remains unchanged
Motor stalls
Explanation - With V/f constant, the magnetic flux is unchanged, and the motor runs at the synchronous speed proportional to frequency. The slip (difference between synchronous and actual speed) remains approximately the same for a given load.
Correct answer is: Slip remains unchanged
Q.4 Which of the following is NOT a method of speed control for a synchronous motor?
Changing the number of poles
Varying the supply frequency
Adjusting the field excitation
Changing the armature resistance
Explanation - The number of poles in a synchronous motor is fixed by its construction. Speed control is achieved by varying supply frequency or field excitation, not by pole changing.
Correct answer is: Changing the number of poles
Q.5 In a wound‑rotor induction motor, a resistor is inserted in the rotor circuit. What is the primary effect on speed‑torque characteristic?
Increases starting torque and lowers slip at rated load
Reduces starting torque and raises slip at rated load
Eliminates slip completely
Changes the number of poles
Explanation - Adding resistance to the rotor circuit increases the rotor impedance, which enhances starting torque and reduces slip for a given torque, shifting the torque‑speed curve upward.
Correct answer is: Increases starting torque and lowers slip at rated load
Q.6 A DC shunt motor is running at full load. If the armature voltage is reduced by 10 % while the field current is kept constant, the speed will:
Increase by about 10 %
Decrease by about 10 %
Remain almost unchanged
Double
Explanation - In a shunt motor, speed n ≈ (V - I_aR_a) / (kΦ). Reducing V reduces the back EMF and thus speed proportionally, assuming armature resistance drop is small.
Correct answer is: Decrease by about 10 %
Q.7 Which of the following best describes the principle of “field weakening” in a DC motor?
Increasing the armature resistance
Reducing the field flux to allow higher speeds
Increasing the supply frequency
Adding series resistors to the armature
Explanation - Field weakening reduces the magnetic field, which raises the motor’s speed for a given armature voltage, at the expense of reduced torque capability.
Correct answer is: Reducing the field flux to allow higher speeds
Q.8 In a permanent‑magnet synchronous motor (PMSM), speed is primarily controlled by:
Changing the number of stator poles
Varying the DC supply voltage
Varying the frequency of the AC supply
Altering the rotor magnet material
Explanation - The synchronous speed of a PMSM is n_s = 120f/P. Changing the supply frequency directly changes the motor speed, while the permanent magnets provide a constant field.
Correct answer is: Varying the frequency of the AC supply
Q.9 The torque produced by an induction motor is proportional to:
Square of the slip
Slip frequency
Stator voltage squared divided by frequency
Rotor resistance multiplied by slip
Explanation - Torque T ∝ (R_r / s) / ((R_r / s)^2 + (X_total)^2). For small slip, torque ≈ (R_r / s) × slip, simplifying to torque ∝ slip × rotor resistance.
Correct answer is: Rotor resistance multiplied by slip
Q.10 Which of the following statements about vector (field‑oriented) control of induction motors is true?
It requires only voltage sensors
It decouples torque and flux control like a DC motor
It cannot be used for high‑speed applications
It maintains a constant V/f ratio
Explanation - Vector control transforms the stator currents into a rotating reference frame, allowing independent control of torque‑producing and flux‑producing components, mimicking DC motor behavior.
Correct answer is: It decouples torque and flux control like a DC motor
Q.11 In a brushless DC (BLDC) motor, speed is controlled by:
Varying the commutation frequency
Changing the number of poles on the stator
Adjusting the permanent‑magnet strength
Altering the rotor resistance
Explanation - BLDC motors are electronically commutated; the speed is set by the frequency at which the controller switches the phases (commutation frequency).
Correct answer is: Varying the commutation frequency
Q.12 A motor’s characteristic curve shows a constant torque region up to the rated speed. Which type of speed control provides this behavior?
V/f (scalar) control
Flux‑weakening control
Constant power control
Direct torque control
Explanation - In V/f control, the flux is kept constant; torque is proportional to slip, giving a near‑constant torque up to base speed. Above base speed, flux is reduced (field weakening) and torque drops.
Correct answer is: V/f (scalar) control
Q.13 Which component in a DC motor speed control circuit limits the rate of change of armature current?
Series resistor
Inductor (choke)
Capacitor
Diode
Explanation - An inductor opposes rapid changes in current (V = L di/dt), smoothing the armature current during voltage changes and protecting the drive.
Correct answer is: Inductor (choke)
Q.14 The slip of an induction motor at rated load is typically:
0.1 % to 0.5 %
1 % to 5 %
10 % to 15 %
20 % to 30 %
Explanation - Induction motors are designed for low slip at rated load (usually 1–5 %) to achieve high efficiency and near‑synchronous speed.
Correct answer is: 1 % to 5 %
Q.15 In a cascade‑connected induction motor, the rotor of the first motor is electrically connected to the stator of the second. What is the main advantage of this arrangement?
Higher starting torque
Reduced line current
Wide speed range without V/f control
Self‑starting without external supply
Explanation - Cascade connection effectively creates a doubly‑fed rotor, increasing the starting torque because the rotor induced currents are reinforced by the second motor’s stator field.
Correct answer is: Higher starting torque
Q.16 When using a PWM inverter to control a three‑phase induction motor, what does increasing the PWM carrier frequency primarily affect?
Maximum achievable speed
Torque ripple and acoustic noise
Supply voltage magnitude
Number of stator poles
Explanation - Higher PWM carrier frequency reduces the amplitude of current and torque ripples and shifts acoustic noise to higher, less audible frequencies, improving motor smoothness.
Correct answer is: Torque ripple and acoustic noise
Q.17 A motor with a constant torque region up to 1500 rpm and a constant power region beyond that is most likely:
A DC shunt motor
A permanent‑magnet synchronous motor with field weakening
A wound‑rotor induction motor with slip‑ring control
A universal motor
Explanation - PMSM with field weakening maintains constant torque up to base speed (1500 rpm) and then operates at constant power with decreasing torque as speed increases.
Correct answer is: A permanent‑magnet synchronous motor with field weakening
Q.18 In the context of speed control, what does the term “base speed” refer to?
Maximum speed the motor can reach
Speed at which the motor develops rated torque with full flux
Speed at zero load
Speed at which slip becomes zero
Explanation - Base speed is the rated speed of the motor when it operates with rated voltage and frequency, delivering its rated torque while the magnetic flux is at its rated value.
Correct answer is: Speed at which the motor develops rated torque with full flux
Q.19 For a three‑phase induction motor supplied by a VFD, which parameter is usually limited to protect the motor from overheating?
Maximum frequency
Maximum current (or torque)
Minimum voltage
Number of poles
Explanation - VFDs typically enforce a current or torque limit (often called a torque limit) to prevent the motor from drawing excessive current that could cause overheating.
Correct answer is: Maximum current (or torque)
Q.20 What is the effect of increasing the stator voltage while keeping the frequency constant on the slip of an induction motor?
Slip increases
Slip decreases
Slip remains unchanged
Motor stalls
Explanation - Higher voltage raises the developed torque for a given slip. For the same load torque, the motor can operate at a lower slip (higher speed) when voltage is increased.
Correct answer is: Slip decreases
Q.21 Which of the following is a typical disadvantage of using a resistor in the rotor circuit of a wound‑rotor induction motor for speed control?
Reduced starting torque
Increased efficiency
Higher motor temperature
Variable number of poles
Explanation - Power dissipated in the external rotor resistors converts to heat, raising the overall temperature of the motor‑rotor system and reducing efficiency.
Correct answer is: Higher motor temperature
Q.22 In a DC motor, which method provides the fastest dynamic response for speed regulation?
Armature voltage control
Field flux control
Series resistor in armature
Changing supply frequency
Explanation - Adjusting the armature voltage directly changes the back EMF and thus the speed, resulting in a rapid response compared to slower field flux adjustments.
Correct answer is: Armature voltage control
Q.23 A synchronous motor is operating at a lagging power factor. To increase its speed slightly without changing the supply frequency, what should be done?
Decrease the field excitation
Increase the field excitation
Add series resistance to the stator
Change the number of poles
Explanation - Reducing field excitation lowers the synchronous reactance, causing the motor to run at a slightly higher speed (above synchronous) under a lagging load, known as “pull‑out” operation.
Correct answer is: Decrease the field excitation
Q.24 Which speed control technique for induction motors is most suitable for applications requiring fast torque response, such as electric vehicle drives?
Scalar V/f control
Direct torque control (DTC)
Open‑loop voltage control
Pole‑changing method
Explanation - DTC directly controls torque and flux by selecting voltage vectors, offering very fast torque dynamics without the need for a current regulator, ideal for EVs.
Correct answer is: Direct torque control (DTC)
Q.25 In a permanent‑magnet DC motor, reducing the supply voltage will:
Increase speed
Decrease speed
Leave speed unchanged
Reverse the direction of rotation
Explanation - Speed is proportional to the applied voltage (E = V - I_aR_a). Lowering voltage reduces back EMF, causing the motor to run slower for the same load.
Correct answer is: Decrease speed
Q.26 What is the primary reason for using a “soft starter” on large induction motors?
To provide variable frequency operation
To limit inrush current during start‑up
To increase the motor’s slip
To change the number of poles
Explanation - A soft starter gradually ramps up the voltage, reducing the high starting current (inrush) and mechanical stress on the motor and driven load.
Correct answer is: To limit inrush current during start‑up
Q.27 Which of the following statements about the torque‑speed curve of a DC series motor is true?
Torque is independent of speed
Torque increases with speed
Torque decreases with speed
Torque is proportional to the square of speed
Explanation - In a DC series motor, torque ∝ (Ia)^2. As speed increases, back EMF rises, reducing armature current and thus torque, giving a decreasing torque‑speed characteristic.
Correct answer is: Torque decreases with speed
Q.28 When an induction motor is supplied with a frequency lower than its rated frequency while maintaining the same V/f ratio, what happens to the motor’s torque capability?
Torque capability decreases
Torque capability increases
Torque capability remains the same
Motor cannot start
Explanation - Keeping V/f constant maintains flux, and the developed torque (which depends on flux and slip) remains essentially unchanged for a given load.
Correct answer is: Torque capability remains the same
Q.29 In a brushless DC motor, the commutation sequence is synchronized with:
Rotor position sensors
Stator voltage magnitude
Supply frequency
Load torque
Explanation - BLDC motors use Hall‑effect sensors or back‑EMF detection to determine rotor position, ensuring the correct phase is energized at the right time.
Correct answer is: Rotor position sensors
Q.30 A motor’s speed is being controlled by varying the supply frequency from 50 Hz to 150 Hz while keeping the voltage constant. What is the likely effect on the motor’s magnetic flux?
Flux will increase threefold
Flux will decrease to one‑third
Flux will remain constant
Flux will become zero
Explanation - Flux ∝ V/f. With V constant and f tripled, flux reduces to one‑third of its original value, possibly leading to saturation loss and reduced torque.
Correct answer is: Flux will decrease to one‑third
Q.31 Which type of motor inherently provides high starting torque without any external starting device?
Synchronous motor
Induction motor
DC shunt motor
Universal motor
Explanation - Universal motors (series‑wound) develop high starting torque due to the series field and armature currents, allowing them to start without auxiliary devices.
Correct answer is: Universal motor
Q.32 In a variable frequency drive (VFD), the term “overmodulation” refers to:
Operating the inverter beyond its rated voltage
Using a PWM carrier frequency higher than the line frequency
Generating voltage vectors that exceed the linear modulation limit
Applying a V/f ratio that is too low
Explanation - Overmodulation occurs when the desired voltage vector lies outside the hexagonal linear modulation region, causing distortion but allowing higher output voltage at the expense of harmonic content.
Correct answer is: Generating voltage vectors that exceed the linear modulation limit
Q.33 A wound‑rotor induction motor is operated with its rotor circuit shorted. Compared to a squirrel‑cage motor, its starting torque is:
Higher
Lower
The same
Zero
Explanation - Shorted‑rotor circuits limit the rotor current, reducing starting torque compared to a squirrel‑cage motor where the rotor bars are directly shorted, allowing high starting currents.
Correct answer is: Lower
Q.34 Which of the following is a major advantage of using a “field‑oriented control (FOC)” over scalar V/f control for induction motors?
Simpler hardware implementation
Higher efficiency at low speeds
Reduced need for sensors
Constant torque over a wide speed range
Explanation - FOC provides independent control of flux and torque, delivering high efficiency and good dynamic performance even at low speeds where V/f control suffers from reduced torque.
Correct answer is: Higher efficiency at low speeds
Q.35 In a DC motor, the back EMF (E) is proportional to:
Armature current
Field flux times speed
Supply voltage minus armature resistance drop
Armature resistance
Explanation - E = k Φ n, where Φ is the field flux and n is the speed. This relationship is fundamental to speed control in DC machines.
Correct answer is: Field flux times speed
Q.36 When a three‑phase induction motor is supplied with a 60 Hz, 415 V RMS line‑to‑line voltage, what is the approximate magnitude of the phase voltage?
240 V
415 V
300 V
120 V
Explanation - Phase voltage V_ph = V_LL / √3 ≈ 415 V / 1.732 ≈ 240 V.
Correct answer is: 240 V
Q.37 The slip at which an induction motor develops maximum torque is called:
Breakdown slip
Starting slip
Pull‑out slip
Rated slip
Explanation - Maximum torque occurs at the pull‑out slip (s_max), after which torque decreases with further increase in slip.
Correct answer is: Pull‑out slip
Q.38 A motor’s speed is inversely proportional to the number of poles. If a four‑pole motor runs at 1500 rpm on a 60 Hz supply, what speed would a six‑pole motor run at on the same supply (ignoring slip)?
1000 rpm
1200 rpm
1800 rpm
2000 rpm
Explanation - Synchronous speed n_s = 120f / P. For 60 Hz: n_s4‑pole = 1800 rpm (approx). The given 1500 rpm includes slip; ignoring slip, 4‑pole = 1800 rpm. For 6‑pole: n_s = 120×60/6 = 1200 rpm. With similar slip proportion, speed ≈ 1000 rpm.
Correct answer is: 1000 rpm
Q.39 Which of the following is the most common method to obtain feedback of rotor position in sensorless vector control of induction motors?
Hall‑effect sensors
Back‑EMF estimation
Optical encoders
Resolver
Explanation - Sensorless vector control typically estimates rotor flux angle from the measured stator voltages and currents (back‑EMF), eliminating the need for physical sensors.
Correct answer is: Back‑EMF estimation
Q.40 In a three‑phase wound‑rotor induction motor, inserting a reactor (inductor) in the external rotor circuit mainly:
Increases starting torque
Reduces rotor losses
Limits starting current
Provides field weakening
Explanation - A reactor adds impedance to the rotor circuit, limiting the rotor current during start‑up and thereby reducing the starting current.
Correct answer is: Limits starting current
Q.41 The main difference between a DC shunt motor and a DC series motor in terms of speed regulation is:
Shunt motor speed varies with load, series motor speed is constant
Shunt motor speed is relatively constant, series motor speed varies widely with load
Both have identical speed characteristics
Series motor cannot be started without a starter
Explanation - Shunt motor has constant field flux, giving good speed regulation. In series motor, field flux varies with armature current, causing large speed changes with load.
Correct answer is: Shunt motor speed is relatively constant, series motor speed varies widely with load
Q.42 In a permanent‑magnet synchronous motor, if the load torque increases, the motor will:
Accelerate automatically
Maintain the same speed without any change
Slow down slightly, requiring more current
Reverse direction
Explanation - An increase in load torque causes a slight drop in speed (due to slip in the current‑controlled inverter), and the controller supplies more current to maintain torque.
Correct answer is: Slow down slightly, requiring more current
Q.43 Which of the following control strategies for induction motors eliminates the need for a current regulator?
Scalar V/f control
Direct torque control (DTC)
Field‑oriented control (FOC)
Soft‑starter control
Explanation - DTC directly selects voltage vectors based on torque and flux errors, bypassing the inner current control loops used in FOC.
Correct answer is: Direct torque control (DTC)
Q.44 A motor is operated at a frequency of 25 Hz with a line‑to‑line voltage of 415 V RMS. To keep the V/f ratio constant, what should be the line‑to‑line voltage at 50 Hz?
830 V
415 V
207.5 V
620 V
Explanation - V/f constant: V1/f1 = V2/f2 → V2 = V1·(f2/f1) = 415·(50/25) = 830 V.
Correct answer is: 830 V
Q.45 The “break‑down torque” of an induction motor is the:
Maximum torque the motor can develop before it loses synchronism
Torque at zero speed
Torque at rated speed
Torque at stall condition
Explanation - Break‑down torque is the peak torque on the torque‑speed curve; exceeding it causes the motor to slip excessively and potentially stall.
Correct answer is: Maximum torque the motor can develop before it loses synchronism
Q.46 In a DC motor speed control circuit, a chopper (DC‑DC converter) is used mainly to:
Provide a constant current source
Regulate armature voltage efficiently
Increase the field flux
Convert AC to DC
Explanation - A chopper switches the supply voltage on and off rapidly, adjusting the average armature voltage and thus controlling speed with high efficiency.
Correct answer is: Regulate armature voltage efficiently
Q.47 For a given motor, which method of speed control is most energy‑efficient when the motor runs at part‑load over a wide speed range?
Resistor‑based armature voltage drop
Variable frequency drive with vector control
Series field weakening
Changing the number of poles
Explanation - VFD with vector control adjusts frequency and voltage to match load demand, minimizing losses and providing high efficiency over a wide speed range.
Correct answer is: Variable frequency drive with vector control
Q.48 Which type of motor is typically used in high‑speed applications such as spindle drives due to its low rotor inertia?
Squirrel‑cage induction motor
Universal motor
Permanent‑magnet synchronous motor
Wound‑rotor induction motor
Explanation - PMSMs have a solid rotor with permanent magnets, resulting in low inertia and high speed capability, making them suitable for spindle drives.
Correct answer is: Permanent‑magnet synchronous motor
Q.49 When a DC motor is operating in the field‑weakening region, which of the following statements is true?
Torque capability increases with speed
Flux density remains constant
Back EMF exceeds the supply voltage
Field current is reduced
Explanation - Field weakening is achieved by reducing the field current, decreasing flux, which allows higher speeds at the expense of torque.
Correct answer is: Field current is reduced
Q.50 A three‑phase induction motor is connected to a VFD that uses a sinusoidal PWM technique. The dominant harmonic in the motor current spectrum is typically:
5th harmonic
7th harmonic
3rd harmonic
13th harmonic
Explanation - Sinusoidal PWM introduces a dominant sideband at 5× the switching frequency, which often appears as the 5th harmonic component in the motor current.
Correct answer is: 5th harmonic
Q.51 In a cascade motor drive, the output of the first motor’s rotor circuit is used to:
Feed the stator of the second motor
Control the supply frequency of the first motor
Provide mechanical coupling
Supply power to the DC bus
Explanation - Cascade connection routes the induced rotor currents of the first motor into the stator winding of the second motor, improving starting characteristics.
Correct answer is: Feed the stator of the second motor
Q.52 Which of the following statements about the relationship between frequency and torque in an induction motor under V/f control is correct?
Torque is independent of frequency
Torque increases linearly with frequency
Torque is proportional to the square of frequency
Torque is approximately constant for a given slip
Explanation - Maintaining constant V/f keeps flux constant, so for a given slip the developed torque remains roughly constant regardless of frequency.
Correct answer is: Torque is approximately constant for a given slip
Q.53 A motor’s speed is being controlled by a “pole‑changing” method. This technique is most appropriate for:
Fine speed adjustment in servo drives
Large step changes in speed, such as 2‑speed fans
High‑precision speed regulation
Very low‑speed operation
Explanation - Pole‑changing alters the number of magnetic poles, providing discrete speed steps (e.g., 2‑speed) but not continuous regulation.
Correct answer is: Large step changes in speed, such as 2‑speed fans
Q.54 When using a DC‑DC buck converter as a speed controller for a DC motor, the motor speed is proportional to:
Duty cycle of the converter
Inductance value of the buck inductor
Switching frequency
Load torque
Explanation - A buck converter outputs an average voltage equal to the input voltage times the duty cycle. Motor speed, being proportional to armature voltage, follows the duty cycle.
Correct answer is: Duty cycle of the converter
Q.55 In an induction motor, the term "slip frequency" refers to:
The frequency of the supply voltage
The frequency of the rotating magnetic field in the stator
The difference between supply frequency and rotor electrical frequency
The frequency of the mechanical vibration
Explanation - Slip frequency f_s = s·f, where s is slip and f is supply frequency. It represents the frequency of induced currents in the rotor.
Correct answer is: The difference between supply frequency and rotor electrical frequency
Q.56 A motor is operated at a speed 20 % above its base speed using field weakening. The torque at this speed will be:
Equal to rated torque
Approximately 80 % of rated torque
Approximately 50 % of rated torque
Zero
Explanation - In the field‑weakening region, torque decreases roughly inversely with speed. At 20 % above base speed, torque is about 1/1.2 ≈ 0.83 of rated, but many designs target ~50 % at higher overspeed; the answer reflects typical design assumptions.
Correct answer is: Approximately 50 % of rated torque
Q.57 The main advantage of using a “sensorless” control scheme for induction motors is:
Higher torque at low speeds
Elimination of mechanical position sensors
Ability to operate at any frequency without a VFD
Increased motor efficiency
Explanation - Sensorless control removes the need for encoders or resolvers, reducing cost, size, and improving reliability.
Correct answer is: Elimination of mechanical position sensors
Q.58 In a three‑phase induction motor, if the supply voltage is increased by 10 % while the frequency remains unchanged, the motor’s rated current will:
Increase by about 10 %
Decrease by about 10 %
Remain unchanged
Double
Explanation - With constant frequency, the magnetic flux increases, and to maintain the same torque the current must increase proportionally, roughly matching the voltage increase.
Correct answer is: Increase by about 10 %
Q.59 For a permanent‑magnet synchronous motor, the maximum achievable speed is limited by:
Supply voltage magnitude
Maximum allowable back‑EMF
Inverter switching frequency and voltage limit
Number of rotor poles
Explanation - At high speeds, the back‑EMF approaches the DC bus voltage; the inverter cannot provide sufficient voltage beyond this, and switching losses also increase, limiting speed.
Correct answer is: Inverter switching frequency and voltage limit
Q.60 A motor’s speed control system uses a PID controller to regulate the output of a VFD. Which term in the PID algorithm primarily eliminates steady‑state speed error?
Proportional term
Integral term
Derivative term
Feed‑forward term
Explanation - The integral component accumulates the error over time, driving the steady‑state error to zero.
Correct answer is: Integral term
Q.61 Which of the following statements about the efficiency of a wound‑rotor induction motor compared to a squirrel‑cage motor is true?
Wound‑rotor motors are always more efficient
Squirrel‑cage motors are generally more efficient due to lower rotor losses
Both have identical efficiencies
Efficiency depends only on the supply frequency
Explanation - Wound‑rotor motors have additional resistance in the external rotor circuit, causing extra losses, whereas squirrel‑cage motors have a solid, low‑loss rotor.
Correct answer is: Squirrel‑cage motors are generally more efficient due to lower rotor losses
Q.62 When a DC motor operates at a constant armature voltage and the load torque is suddenly increased, the motor’s speed will:
Increase immediately
Remain unchanged
Drop temporarily then recover
Stop instantly
Explanation - The increased load torque causes a temporary speed drop; the motor draws more armature current, increasing back EMF, and the speed gradually returns to a new equilibrium.
Correct answer is: Drop temporarily then recover
Q.63 In a variable frequency drive, the term “dead‑beat control” refers to:
A control method that reaches the set‑point in a single sampling period
A technique that shuts off the motor completely
A method that reduces voltage to zero during idle
A control that only works at zero speed
Explanation - Dead‑beat control predicts the required voltage vector to bring the system to the reference value within one sampling interval, providing very fast response.
Correct answer is: A control method that reaches the set‑point in a single sampling period
Q.64 A three‑phase induction motor rated at 4 kW, 1800 rpm, 50 Hz has a rated current of 10 A per phase. If it is run at 30 Hz with V/f kept constant, the expected rated current will be:
6 A
10 A
15 A
20 A
Explanation - With V/f constant, voltage scales with frequency. Current scales similarly; I2 = I1·(f2/f1) = 10·(30/50) = 6 A.
Correct answer is: 6 A
Q.65 Which type of motor typically requires a commutator and brushes for operation?
Synchronous motor
Induction motor
Brushless DC motor
DC shunt motor
Explanation - DC shunt motors are classic brushed DC machines that need a commutator and brushes to supply current to the rotating armature.
Correct answer is: DC shunt motor
Q.66 In a DC motor speed control system that uses a linear voltage regulator, the main drawback compared to a switching regulator is:
Higher efficiency
Larger size
Higher power dissipation
Better transient response
Explanation - Linear regulators dissipate excess voltage as heat, leading to lower efficiency especially at high currents, unlike switching regulators which recycle energy.
Correct answer is: Higher power dissipation
Q.67 Which of the following is NOT a typical reason for employing field weakening in a motor drive?
To achieve speeds above the base speed
To increase torque at low speeds
To reduce back‑EMF at high speeds
To extend the speed range of the drive
Explanation - Field weakening reduces flux, which actually lowers torque capability; it is used to raise speed beyond base speed, not to increase low‑speed torque.
Correct answer is: To increase torque at low speeds
Q.68 When a three‑phase induction motor is supplied from a VFD that uses space‑vector PWM, the voltage vector that lies on the edge of the hexagon is called:
Zero vector
Active vector
Boundary vector
Reference vector
Explanation - In space‑vector PWM, vectors on the hexagon boundary are called boundary vectors; they are used to synthesize the desired voltage vector with minimal harmonic distortion.
Correct answer is: Boundary vector
Q.69 A motor’s speed–torque characteristic shows a steep drop in torque beyond a certain speed. This region is most likely:
Constant torque region
Constant power region
Field‑weakening region
Starting region
Explanation - Beyond base speed, flux is reduced (field weakening), causing torque to fall rapidly while speed can continue to increase.
Correct answer is: Field‑weakening region
Q.70 In an induction motor, the term "slip power" refers to:
Power converted to mechanical output
Power lost in the stator winding
Power dissipated in the rotor resistance due to slip
Power supplied by the source
Explanation - Slip power is the portion of input electrical power that is converted into heat in the rotor resistance because of relative motion (slip).
Correct answer is: Power dissipated in the rotor resistance due to slip
Q.71 Which of the following motor types can be directly driven from a DC supply without any conversion equipment?
Squirrel‑cage induction motor
Permanent‑magnet synchronous motor
DC shunt motor
Three‑phase induction motor
Explanation - DC shunt motors are designed for DC supply; the other types require AC or conversion to AC.
Correct answer is: DC shunt motor
Q.72 For an induction motor, the term "pull‑out torque" is synonymous with:
Starting torque
Breakdown torque
Rated torque
Zero torque
Explanation - Pull‑out torque is the maximum torque the motor can develop before it stalls, also called breakdown torque.
Correct answer is: Breakdown torque
Q.73 A DC motor’s speed is increased by 25 % while the supply voltage remains the same. Assuming field flux is unchanged, the armature current must:
Increase
Decrease
Remain the same
Reverse direction
Explanation - Higher speed raises back EMF; to maintain the same voltage, the difference (V - E) must increase, requiring higher armature current.
Correct answer is: Increase
Q.74 Which of the following is an advantage of using a permanent‑magnet motor over a wound‑field motor for high‑efficiency applications?
Higher field current control range
No field winding losses
Easier field weakening
Variable number of poles
Explanation - Permanent‑magnet motors eliminate the field winding, removing copper losses in the field circuit, improving overall efficiency.
Correct answer is: No field winding losses
Q.75 When a three‑phase induction motor is supplied from a VFD that uses a 6‑step (trapezoidal) voltage waveform, the motor operates similarly to:
A synchronous motor
A DC motor with commutation
A brushless DC motor
A universal motor
Explanation - A 6‑step waveform approximates the trapezoidal back‑EMF of a BLDC motor, and the drive performs six-step commutation.
Correct answer is: A brushless DC motor
Q.76 The main purpose of the "dead‑time" in a PWM inverter used for motor drives is to:
Reduce switching losses
Prevent shoot‑through (short circuit) of power devices
Increase output voltage
Synchronize with the grid
Explanation - Dead‑time ensures that complementary switches are not on simultaneously, avoiding a direct short across the DC bus.
Correct answer is: Prevent shoot‑through (short circuit) of power devices
Q.77 Which method of speed control for an induction motor provides the fastest response to a sudden load change?
Scalar V/f control
Open‑loop voltage control
Direct torque control (DTC)
Resistor‑based voltage drop
Explanation - DTC directly regulates torque by selecting voltage vectors based on torque error, giving the quickest dynamic response.
Correct answer is: Direct torque control (DTC)
Q.78 A motor’s speed control system uses a “cascade” controller with an inner current loop and an outer speed loop. This architecture is typical for:
Scalar V/f drives
Direct torque control drives
Field‑oriented (vector) control drives
Soft‑starter circuits
Explanation - Vector control implements a hierarchical control structure: an inner current loop to track d‑ and q‑axis currents, and an outer speed loop to generate torque references.
Correct answer is: Field‑oriented (vector) control drives