Q.1 What is the primary principle of operation of a DC shunt motor?
Field winding is connected in series with the armature
Field winding is connected in parallel (shunt) with the armature
The motor uses permanent magnets for the stator
It operates on induction principle
Explanation - In a shunt motor, the field winding is connected across the armature terminals, providing a constant field flux independent of load current.
Correct answer is: Field winding is connected in parallel (shunt) with the armature
Q.2 Which type of electric motor is most commonly used in household appliances like washing machines and fans?
Synchronous motor
Universal motor
Stepper motor
Brushless DC motor
Explanation - Universal motors can run on either AC or DC supply and provide high speed with a compact size, making them ideal for appliances.
Correct answer is: Universal motor
Q.3 In an induction motor, the rotating magnetic field is produced by:
Permanent magnets on the rotor
Current flowing through the rotor bars
Three-phase AC currents in the stator winding
A commutator and brushes
Explanation - The stator of an induction motor is supplied with three-phase AC, which creates a rotating magnetic field that induces current in the rotor.
Correct answer is: Three-phase AC currents in the stator winding
Q.4 What does the term 'slip' refer to in an induction motor?
The difference between synchronous speed and rotor speed expressed as a percentage
The amount of torque produced by the motor
The voltage drop across the stator winding
The mechanical loss due to friction
Explanation - Slip is the relative speed difference between the rotating magnetic field and the rotor, required for torque production.
Correct answer is: The difference between synchronous speed and rotor speed expressed as a percentage
Q.5 Which motor type can maintain a constant speed irrespective of load variations, provided the supply frequency is constant?
DC series motor
Induction motor
Synchronous motor
Universal motor
Explanation - Synchronous motors rotate at a speed that is locked to the supply frequency (synchronous speed) and do not change with load.
Correct answer is: Synchronous motor
Q.6 The back‑EMF generated in a DC motor is proportional to:
Armature current
Flux per pole
Armature speed
Supply voltage
Explanation - Back‑EMF (E) = k * Φ * ω, where ω is the angular speed of the armature; thus it increases with speed.
Correct answer is: Armature speed
Q.7 Which of the following is NOT a typical advantage of a brushless DC (BLDC) motor?
Higher efficiency
No brush wear
Simpler controller circuitry
Low electromagnetic interference
Explanation - BLDC motors require electronic commutation and usually more complex control electronics compared to brushed DC motors.
Correct answer is: Simpler controller circuitry
Q.8 In a three‑phase induction motor, the number of poles determines:
The magnitude of the starting torque
The synchronous speed of the rotating field
The resistance of the stator winding
The motor’s efficiency
Explanation - Synchronous speed Ns = 120f / P, where f is supply frequency and P is the number of poles.
Correct answer is: The synchronous speed of the rotating field
Q.9 Which component in a DC motor is responsible for switching the current direction in the armature windings?
Commutator
Stator
Rotor
Capacitor
Explanation - The commutator, together with brushes, reverses current in the armature windings each half turn, maintaining torque direction.
Correct answer is: Commutator
Q.10 A motor rated at 5 kW, 400 V, 50 Hz, three‑phase, has a power factor of 0.85. What is its apparent power (kVA)?
5.88 kVA
5.00 kVA
5.88 kW
5.00 kW
Explanation - Apparent power S = P / pf = 5 kW / 0.85 ≈ 5.88 kVA.
Correct answer is: 5.88 kVA
Q.11 Which motor type typically requires a starter with a series resistor to limit starting current?
Synchronous motor
DC shunt motor
DC series motor
Induction motor
Explanation - A DC series motor has low resistance and can draw excessive current at start; a series resistor reduces this inrush.
Correct answer is: DC series motor
Q.12 The term 'torque ripple' in a stepper motor refers to:
Variation in torque output as the motor steps from one position to another
Increase in torque with rising temperature
Loss of torque due to magnetic saturation
Torque produced only at standstill
Explanation - Torque ripple is the periodic variation in torque caused by the discrete stepping action of the motor.
Correct answer is: Variation in torque output as the motor steps from one position to another
Q.13 In a permanent‑magnet synchronous motor (PMSM), the rotor magnetic field is produced by:
Field windings energized by DC
Permanent magnets mounted on the rotor
Induced currents in the rotor bars
External permanent magnets on the stator
Explanation - PMSMs use permanent magnets on the rotor to create a constant magnetic field, eliminating the need for field windings.
Correct answer is: Permanent magnets mounted on the rotor
Q.14 Which of the following is a common method to start a squirrel‑cage induction motor?
Direct on line (DOL)
Star‑Delta starter
Soft starter
All of the above
Explanation - Squirrel‑cage induction motors can be started directly, with a star‑delta starter, or using a soft starter to limit inrush current.
Correct answer is: All of the above
Q.15 The efficiency of an electric motor is defined as:
Output mechanical power divided by input electrical power
Output electrical power divided by input mechanical power
Torque multiplied by speed
Power factor multiplied by voltage
Explanation - Efficiency η = (Mechanical output power) / (Electrical input power) × 100%.
Correct answer is: Output mechanical power divided by input electrical power
Q.16 Which motor characteristic is most directly affected by increasing the number of poles?
Starting torque
Rated speed
Armature resistance
Efficiency
Explanation - More poles reduce synchronous speed (Ns = 120f / P), thus lowering the rated speed of the motor.
Correct answer is: Rated speed
Q.17 In a DC motor, which loss is associated with the resistance of the armature winding?
Core loss
Copper loss
Friction loss
Windage loss
Explanation - Copper loss = I²R losses in the armature winding due to its resistance.
Correct answer is: Copper loss
Q.18 What is the function of a damper winding in a synchronous motor?
To provide starting torque
To improve power factor
To dampen oscillations during transient conditions
To increase speed
Explanation - Damper windings act like an induction motor rotor, providing damping during sudden load changes or start‑up.
Correct answer is: To dampen oscillations during transient conditions
Q.19 Which of the following motors typically operates at the highest speed?
DC series motor
Induction motor
Stepper motor
Synchronous motor
Explanation - DC series motors can achieve very high speeds under light load because flux decreases with reduced armature current.
Correct answer is: DC series motor
Q.20 The term 'breakdown torque' for an induction motor refers to:
Maximum torque the motor can develop before stalling
Torque at which the motor starts rotating
Torque at rated speed
Torque produced at zero slip
Explanation - Breakdown (or pull‑out) torque is the highest torque the motor can deliver without losing synchronism.
Correct answer is: Maximum torque the motor can develop before stalling
Q.21 In a three‑phase motor, the phase sequence ABC (or R‑Y‑B) determines:
The direction of rotation of the motor
The motor's efficiency
The amount of torque produced
The motor's thermal rating
Explanation - Reversing the phase sequence reverses the direction of the rotating magnetic field and thus motor rotation.
Correct answer is: The direction of rotation of the motor
Q.22 Which material is commonly used for the core of high‑efficiency electric motors due to its low hysteresis loss?
Silicon steel
Aluminum
Copper
Iron cast
Explanation - Silicon steel laminations reduce hysteresis and eddy‑current losses, improving motor efficiency.
Correct answer is: Silicon steel
Q.23 A motor rated at 230 V, 50 Hz, 2‑pole will have a synchronous speed of:
1500 rpm
3000 rpm
1200 rpm
1800 rpm
Explanation - Ns = 120f / P = 120×50 / 2 = 3000 rpm.
Correct answer is: 3000 rpm
Q.24 In a wound‑rotor induction motor, the rotor resistance can be varied during operation. What is the main purpose of this variation?
To control motor speed
To improve power factor
To increase starting torque
To reduce core loss
Explanation - Increasing rotor resistance raises slip at maximum torque, thereby enhancing starting torque.
Correct answer is: To increase starting torque
Q.25 Which of the following statements about a stepper motor is true?
It can run at any arbitrary speed without control circuitry.
It moves in discrete steps and can hold position without power.
It requires a commutator and brushes to operate.
Its speed is directly proportional to supply voltage only.
Explanation - Stepper motors move in fixed angular increments; many designs can hold a position when energized (though not without power).
Correct answer is: It moves in discrete steps and can hold position without power.
Q.26 What is the typical purpose of a capacitor in a single‑phase induction motor starter circuit?
To increase the motor's rated voltage
To provide a phase shift that creates a starting torque
To reduce the motor's current draw during steady‑state operation
To store mechanical energy
Explanation - The capacitor creates a phase‑shifted current in an auxiliary winding, generating a rotating field for starting.
Correct answer is: To provide a phase shift that creates a starting torque
Q.27 In a DC motor, the speed‑torque characteristic of a series‑wound motor is:
Linear and inversely proportional
Flat (constant speed) over a wide torque range
High starting torque with speed decreasing rapidly as load increases
Independent of load
Explanation - Series‑wound motors have flux proportional to current, giving high starting torque but speed falls sharply with load.
Correct answer is: High starting torque with speed decreasing rapidly as load increases
Q.28 Which control method is most commonly used for regulating speed of a three‑phase induction motor?
Varying the supply voltage only
Changing the supply frequency (VFD)
Adjusting the stator winding resistance
Altering the rotor geometry
Explanation - Variable Frequency Drives (VFDs) adjust supply frequency, thereby changing synchronous speed and motor speed.
Correct answer is: Changing the supply frequency (VFD)
Q.29 The term 'no‑load current' in a motor refers to:
Current drawn when the motor runs at rated load
Current drawn when the motor is running without any external mechanical load
Current required to start the motor
Current flowing only through the field winding
Explanation - No‑load current is the supply current when the motor rotates freely, primarily supplying magnetizing and friction losses.
Correct answer is: Current drawn when the motor is running without any external mechanical load
Q.30 For a motor with a rated voltage of 415 V (line‑line) three‑phase, what is the phase voltage?
415 V
240 V
240 V (line‑line)
120 V
Explanation - Phase voltage Vph = Vll / √3 = 415 V / 1.732 ≈ 240 V.
Correct answer is: 240 V
Q.31 Which type of motor inherently provides regenerative braking capability when operated with a suitable controller?
Universal motor
Synchronous motor
DC shunt motor
Induction motor
Explanation - When the armature is driven as a generator, a DC shunt motor can feed energy back to the supply, enabling regeneration.
Correct answer is: DC shunt motor
Q.32 What is the main reason for using laminated steel cores in motor construction?
To increase mechanical strength
To reduce eddy‑current losses
To improve thermal conductivity
To lower manufacturing cost
Explanation - Lamination breaks up conductive paths, thereby reducing circulating eddy currents induced in the core.
Correct answer is: To reduce eddy‑current losses
Q.33 In a three‑phase induction motor, the rotor speed is always:
Equal to synchronous speed
Higher than synchronous speed
Lower than synchronous speed
Independent of supply frequency
Explanation - The rotor must lag the rotating field (slip > 0) to develop torque, so its speed is slightly less than synchronous speed.
Correct answer is: Lower than synchronous speed
Q.34 Which of the following motors is most suitable for precise position control without feedback sensors?
Brushless DC motor
Stepper motor
Synchronous motor
Universal motor
Explanation - Stepper motors move in fixed increments, allowing open‑loop control of position without encoders.
Correct answer is: Stepper motor
Q.35 A motor with a high starting current is typically started using a:
Direct‑on‑line (DOL) starter
Star‑Delta starter
Soft starter
All of the above
Explanation - All three methods can be used to mitigate high inrush currents, though they employ different techniques.
Correct answer is: All of the above
Q.36 The term 'hysteresis loss' in an electric motor is associated with:
Changing magnetic field in the core material
Resistance of the winding conductors
Friction between moving parts
Air resistance on the rotor
Explanation - Hysteresis loss occurs due to the reversal of magnetization in the core during each AC cycle.
Correct answer is: Changing magnetic field in the core material
Q.37 What is the primary advantage of a three‑phase motor over a single‑phase motor?
Lower cost
Simpler wiring
Higher starting torque and smoother operation
Ability to run on DC supply
Explanation - Three‑phase supply creates a continuously rotating magnetic field, giving higher torque and less vibration.
Correct answer is: Higher starting torque and smoother operation
Q.38 In a DC motor, increasing the field flux while keeping armature voltage constant will:
Increase speed
Decrease speed
Have no effect on speed
Increase armature current only
Explanation - Speed N = (V - IaRa) / (kΦ); increasing Φ reduces speed for a given voltage.
Correct answer is: Decrease speed
Q.39 Which type of motor is commonly used in electric vehicle (EV) propulsion due to its high power density and efficiency?
Universal motor
DC series motor
Brushless DC (BLDC) motor
Induction motor
Explanation - BLDC motors provide high torque, efficiency, and low maintenance, making them ideal for EVs.
Correct answer is: Brushless DC (BLDC) motor
Q.40 The term 'back‑EMF constant' (Kb) for a motor is expressed in which units?
V·s/rad
Nm/A
A/V
Hz/V
Explanation - Kb relates the generated voltage to angular speed: E = Kb·ω (V = V·s/rad × rad/s).
Correct answer is: V·s/rad
Q.41 In a wound‑field DC motor, if the field resistance is increased, the motor will:
Run faster
Run slower
Produce more torque at the same speed
Have no change in speed
Explanation - Higher field resistance reduces field current, lowering flux, which increases speed for a constant armature voltage.
Correct answer is: Run faster
Q.42 What is the usual purpose of an encoder attached to an electric motor?
To reduce electrical noise
To provide feedback on position or speed
To increase the motor's torque
To cool the motor
Explanation - Encoders generate pulses corresponding to rotation, enabling precise control of speed and position.
Correct answer is: To provide feedback on position or speed
Q.43 A motor with a low slip at rated load is typically:
An induction motor
A synchronous motor
A DC series motor
A stepper motor
Explanation - Induction motors operate with small slip (typically 1‑5%) at rated load to develop torque.
Correct answer is: An induction motor
Q.44 Which of the following is NOT a typical cause of motor overheating?
Overloading the motor
Insufficient ventilation
High ambient temperature
Increasing supply frequency
Explanation - Higher frequency can actually reduce current for a given power, while overload, poor ventilation, and high ambient temperature directly raise temperature.
Correct answer is: Increasing supply frequency
Q.45 In a three‑phase induction motor, the term 'rotor cage' refers to:
The laminated iron core
The set of conductors short‑circuited by end rings
The cooling fan attached to the shaft
The protective housing
Explanation - The squirrel‑cage rotor consists of conductive bars (the cage) connected at both ends by rings.
Correct answer is: The set of conductors short‑circuited by end rings
Q.46 For a motor with a torque constant Kt of 0.1 Nm/A, what armature current is required to produce 5 Nm of torque?
0.5 A
5 A
50 A
0.02 A
Explanation - Torque = Kt × I ⇒ I = Torque / Kt = 5 Nm / 0.1 Nm/A = 50 A.
Correct answer is: 50 A
Q.47 Which motor type is inherently self‑starting when connected to a three‑phase supply?
DC shunt motor
Synchronous motor without damper winding
Induction motor
Universal motor
Explanation - Induction motors develop starting torque directly from the rotating magnetic field of a three‑phase supply.
Correct answer is: Induction motor
Q.48 A motor’s 'rated torque' is defined as:
The maximum torque it can ever produce
The torque at which the motor runs at its rated speed and power
The torque when the motor is stalled
The torque at zero speed
Explanation - Rated torque corresponds to the continuous torque the motor can deliver at its nominal operating point.
Correct answer is: The torque at which the motor runs at its rated speed and power
Q.49 In a permanent‑magnet DC motor, if the armature resistance is doubled while keeping voltage constant, the no‑load speed will:
Increase
Decrease slightly
Remain the same
Drop to zero
Explanation - At no‑load, armature current is very small, so voltage drop across resistance is negligible; speed is essentially unchanged.
Correct answer is: Remain the same
Q.50 The 'torque‑speed curve' of a DC shunt motor is approximately:
Horizontal line (constant torque)
Vertical line (constant speed)
Hyperbolic (inverse relationship)
Linear (direct relationship)
Explanation - For a shunt motor, speed ≈ (V – IaRa) / (kΦ) and torque ∝ Ia; thus speed decreases as torque (and Ia) increases, forming a hyperbola.
Correct answer is: Hyperbolic (inverse relationship)
Q.51 Which of the following best describes 'core loss' in electric motors?
Losses due to resistance of windings
Losses due to magnetic hysteresis and eddy currents in the core
Losses caused by friction in bearings
Losses due to windage
Explanation - Core loss comprises hysteresis loss (magnetic domain reversal) and eddy‑current loss (induced circulating currents).
Correct answer is: Losses due to magnetic hysteresis and eddy currents in the core
Q.52 When a three‑phase induction motor is supplied with a reduced voltage but the same frequency, the resulting effect is:
Higher starting torque
Reduced slip and higher efficiency
Increased slip and reduced torque
No change in performance
Explanation - Reduced voltage lowers the air‑gap flux, decreasing torque; the motor must slip more to develop the required torque.
Correct answer is: Increased slip and reduced torque
Q.53 Which motor type typically uses a commutator made of copper segments?
Induction motor
Synchronous motor
Brushless DC motor
DC motor
Explanation - DC motors have a segmented copper commutator that works with brushes to reverse current direction.
Correct answer is: DC motor
Q.54 The primary reason for using a star (Y) connection for the stator of a large induction motor at start is:
To increase the starting torque
To reduce the starting current
To double the motor speed
To eliminate slip
Explanation - In star connection, each phase voltage is reduced to 1/√3 of line voltage, lowering the inrush current.
Correct answer is: To reduce the starting current
Q.55 A motor with a high power factor (close to 1) indicates:
Low efficiency
Low reactive power draw
High starting torque
Large mechanical losses
Explanation - Power factor = real power / apparent power; a value near 1 means the motor consumes little reactive (imaginary) power.
Correct answer is: Low reactive power draw
Q.56 In a three‑phase synchronous motor, the rotor rotates at:
Synchronous speed regardless of load
Slightly less than synchronous speed
Slightly more than synchronous speed
Variable speed depending on load
Explanation - By definition, a synchronous motor runs at the same speed as the rotating magnetic field.
Correct answer is: Synchronous speed regardless of load
Q.57 Which motor is most commonly used in applications requiring high torque at low speed, such as conveyors?
Stepper motor
DC series motor
Induction motor with VFD
Universal motor
Explanation - An induction motor driven by a VFD can provide high torque at low speeds with efficient operation.
Correct answer is: Induction motor with VFD
Q.58 What is the typical effect of increasing the supply frequency on the speed of an induction motor (assuming voltage is proportionally increased)?
Speed decreases
Speed remains unchanged
Speed increases
Motor stalls
Explanation - Synchronous speed Ns = 120f / P; increasing frequency raises Ns, and the motor speed follows (with constant slip).
Correct answer is: Speed increases
Q.59 In a DC motor, the term 'armature reaction' refers to:
The heating of the armature due to current
The distortion of the main field flux by armature current
The mechanical reaction of the shaft
The commutation process
Explanation - Armature reaction is the effect of the armature's magnetic field on the distribution of the main field, potentially causing flux weakening or distortion.
Correct answer is: The distortion of the main field flux by armature current
Q.60 A motor rated at 150 A, 400 V, three‑phase, with a power factor of 0.9, consumes how much real power (kW)?
93.5 kW
104 kW
117 kW
130 kW
Explanation - Apparent power S = √3·V·I = 1.732·400·150 ≈ 103.9 kVA. Real power P = S·pf = 103.9·0.9 ≈ 93.5 kW.
Correct answer is: 93.5 kW
Q.61 Which of the following statements about a wound‑rotor induction motor is true?
It has permanent magnets on the rotor
Its rotor windings are connected directly to the supply
External resistors can be connected to the rotor circuit to control torque and speed
It cannot be started directly on line
Explanation - Wound‑rotor motors have slip rings allowing external resistors to be added to the rotor circuit.
Correct answer is: External resistors can be connected to the rotor circuit to control torque and speed
Q.62 The main function of a cooling fan attached to the rotor shaft of an induction motor is to:
Increase torque
Reduce electrical losses
Provide air flow for cooling
Improve power factor
Explanation - The fan moves air over the motor's surface, dissipating heat generated during operation.
Correct answer is: Provide air flow for cooling
Q.63 A motor with a high 'starting torque' is essential for:
Running at constant speed
Starting heavy loads such as compressors
Maintaining low noise levels
Reducing power consumption
Explanation - High starting torque overcomes the inertia of heavy loads during start‑up.
Correct answer is: Starting heavy loads such as compressors
Q.64 In a three‑phase motor, the term 'line‑to‑line voltage' refers to:
Voltage measured between any two phases
Voltage between a phase and neutral
Voltage across the motor winding only
Supply voltage after conversion to DC
Explanation - Line‑to‑line voltage is the potential difference between any two of the three-phase conductors.
Correct answer is: Voltage measured between any two phases
Q.65 If the number of poles in an induction motor is increased from 2 to 4 while keeping supply frequency constant, the synchronous speed will:
Double
Remain the same
Halve
Increase by 25 %
Explanation - Ns = 120f / P; doubling P from 2 to 4 halves Ns.
Correct answer is: Halve
Q.66 Which motor is most suitable for applications requiring precise speed control over a wide range without using a VFD?
DC shunt motor
Synchronous motor
Induction motor
Universal motor
Explanation - By varying the armature voltage, a DC shunt motor can provide smooth speed control over a wide range.
Correct answer is: DC shunt motor
Q.67 The term 'reactance' in an AC motor refers to:
Resistance to DC current
Opposition to change of current due to inductance and capacitance
Mechanical friction
Heat generated in the windings
Explanation - Reactance X = ωL (inductive) or 1/(ωC) (capacitive) and represents the AC opposition of inductors and capacitors.
Correct answer is: Opposition to change of current due to inductance and capacitance
Q.68 In a BLDC motor, the electronic commutation is typically performed based on:
Mechanical brushes
Hall‑effect sensors or back‑EMF detection
Rotating commutator rings
Fixed frequency AC supply
Explanation - BLDC motors use position sensors (Hall) or back‑EMF to determine rotor position for electronic switching.
Correct answer is: Hall‑effect sensors or back‑EMF detection
Q.69 What is the typical effect of increasing the supply voltage to an induction motor while keeping frequency constant?
Decrease torque
Increase slip
Increase torque capability
Reduce motor speed
Explanation - Higher voltage raises the air‑gap flux, which enhances torque production.
Correct answer is: Increase torque capability
Q.70 A motor that can operate directly from both AC and DC supplies is called a:
Universal motor
Induction motor
Synchronous motor
Stepper motor
Explanation - Universal motors are series‑wound DC motors designed to run on AC as well, due to the commutation action.
Correct answer is: Universal motor
Q.71 The term 'skin effect' in motor windings is most significant at:
Low frequencies
High frequencies
DC operation
Medium frequencies
Explanation - Skin effect causes current to concentrate near the surface of conductors, increasing AC resistance at high frequencies.
Correct answer is: High frequencies
Q.72 Which type of motor is commonly used in printers and CNC machines for precise, high‑speed positioning?
Induction motor
Stepper motor
Universal motor
Synchronous motor
Explanation - Stepper motors provide accurate incremental movement suitable for positioning tasks.
Correct answer is: Stepper motor
Q.73 When a motor is said to be 'over‑fluxed', it means:
The field current is too low
The magnetic flux in the core is higher than design value
The motor is running at too high a speed
The armature resistance is too high
Explanation - Over‑fluxing increases core losses and can cause saturation, reducing efficiency.
Correct answer is: The magnetic flux in the core is higher than design value
Q.74 The 'starting current' of a typical induction motor is usually:
1–2 times the rated current
3–7 times the rated current
Equal to the rated current
Less than the rated current
Explanation - Induction motors draw a large inrush current at start due to low slip and high magnetizing current.
Correct answer is: 3–7 times the rated current
Q.75 In a three‑phase motor, the term 'balanced load' means:
All phases have equal voltage magnitude and are 120° apart
One phase is loaded more than the others
The motor is running at half speed
The motor is supplied with DC
Explanation - A balanced three‑phase system has equal magnitude and phase displacement, resulting in smooth operation.
Correct answer is: All phases have equal voltage magnitude and are 120° apart
Q.76 Which motor type is most appropriate for an application that requires continuous rotation at a fixed speed synchronized with the grid frequency?
Induction motor
Synchronous motor
DC series motor
Stepper motor
Explanation - Synchronous motors lock to the supply frequency, providing constant speed regardless of load.
Correct answer is: Synchronous motor
Q.77 A motor’s efficiency can be improved by:
Increasing the stator resistance
Operating at a higher slip
Using higher quality magnetic materials
Running the motor at overload
Explanation - Better magnetic materials reduce core losses, increasing overall efficiency.
Correct answer is: Using higher quality magnetic materials
Q.78 In a DC motor, the term 'commutation' refers to:
Changing the direction of current in the armature windings
Increasing the magnetic field strength
Cooling the motor
Adjusting the supply voltage
Explanation - Commutation ensures that the torque-producing current is always in the correct direction relative to the magnetic field.
Correct answer is: Changing the direction of current in the armature windings
Q.79 The typical rated speed of a 4‑pole, 60 Hz induction motor is:
1200 rpm
1800 rpm
3000 rpm
3600 rpm
Explanation - Ns = 120f / P = 120×60 / 4 = 1800 rpm.
Correct answer is: 1800 rpm
Q.80 Which of the following statements about 'torque density' is correct?
It is the torque per unit mass of the motor
It is the torque per unit volume of the motor
It is the torque per unit current
It is the torque per unit voltage
Explanation - Torque density measures how much torque a motor can produce relative to its size (volume).
Correct answer is: It is the torque per unit volume of the motor
Q.81 In a motor drive system, a 'soft starter' primarily reduces:
Steady‑state current
Starting inrush current
Motor speed
Torque ripple
Explanation - A soft starter gradually ramps up voltage, limiting the high initial current draw.
Correct answer is: Starting inrush current
Q.82 A motor's 'breakdown torque' is typically higher than its 'rated torque' by:
10–20 %
30–50 %
70–100 %
It is always the same
Explanation - Breakdown torque is usually about 1.3 to 1.5 times the rated torque, providing a safety margin.
Correct answer is: 30–50 %
Q.83 Which of the following is a major advantage of using a permanent‑magnet motor over a wound‑field motor?
Higher starting current
No field winding losses
Adjustable field strength
Simpler speed control
Explanation - Permanent magnets provide constant flux without requiring energized field windings, eliminating related losses.
Correct answer is: No field winding losses
Q.84 When a three‑phase motor is supplied with a phase‑to‑phase voltage of 400 V and draws a line current of 10 A, what is its apparent power (kVA)?
6.93 kVA
7.00 kVA
6.93 MVA
7.00 MVA
Explanation - S = √3·V·I = 1.732·400·10 ≈ 6,928 VA = 6.93 kVA.
Correct answer is: 6.93 kVA
Q.85 Which type of motor uses a rotor that is physically short‑circuited, eliminating the need for slip rings?
Wound‑rotor induction motor
Synchronous motor
Squirrel‑cage induction motor
Universal motor
Explanation - The rotor bars are electrically shorted by end rings, forming a cage.
Correct answer is: Squirrel‑cage induction motor
Q.86 In a DC motor, the torque produced is proportional to:
Armature current only
Flux only
Product of armature current and flux
Supply voltage
Explanation - Torque T = k·Φ·Ia, where Φ is flux and Ia is armature current.
Correct answer is: Product of armature current and flux
Q.87 The primary reason for using three‑phase power in industrial motor drives is:
To reduce the number of conductors
To provide a rotating magnetic field without a commutator
To increase voltage levels
To simplify control circuitry
Explanation - Three‑phase supply creates a naturally rotating magnetic field, eliminating the need for brushes and commutators.
Correct answer is: To provide a rotating magnetic field without a commutator
Q.88 If a motor's slip is 2 % at rated load and its synchronous speed is 1800 rpm, what is its rotor speed?
1764 rpm
1800 rpm
1820 rpm
1750 rpm
Explanation - Rotor speed N = Ns (1 – slip) = 1800 × (1 – 0.02) = 1764 rpm.
Correct answer is: 1764 rpm
Q.89 A motor that needs to maintain a precise speed regardless of load variations is best served by:
DC series motor
Induction motor without control
Synchronous motor
Universal motor
Explanation - Synchronous motors lock to the supply frequency, keeping speed constant irrespective of load.
Correct answer is: Synchronous motor
Q.90 Which of the following is the most common method to achieve speed control in a DC motor?
Varying the supply frequency
Changing the number of poles
Varying the armature voltage
Altering the rotor mass
Explanation - Changing the armature voltage directly changes motor speed in a DC motor.
Correct answer is: Varying the armature voltage
Q.91 In an induction motor, the 'air‑gap' refers to:
The clearance between the stator and rotor laminations
The space between the motor and its housing
The distance between the brushes and commutator
The gap in the power supply
Explanation - The air‑gap is the small physical separation where magnetic flux passes from stator to rotor.
Correct answer is: The clearance between the stator and rotor laminations
Q.92 Which motor characteristic is directly affected by increasing the number of slots in the stator winding?
Torque ripple
Rated speed
Power factor
Thermal capacity
Explanation - More slots can reduce torque ripple and improve smoothness of operation.
Correct answer is: Torque ripple
Q.93 A motor rated at 7.5 kW, 400 V, 50 Hz, three‑phase, with a power factor of 0.8, draws how many amps per phase?
13.5 A
27.1 A
30.9 A
54.3 A
Explanation - Real power P = √3·V·I·pf → I = P / (√3·V·pf) = 7500 / (1.732·400·0.8) ≈ 27.1 A.
Correct answer is: 27.1 A
Q.94 The term 'field weakening' in a DC motor is used to:
Increase torque at low speeds
Reduce speed at high loads
Increase speed above the base speed by reducing flux
Improve commutation
Explanation - Weakening the field reduces flux, allowing higher speeds for a given voltage.
Correct answer is: Increase speed above the base speed by reducing flux
Q.95 Which of the following devices is used to protect a motor from excessive current during start‑up?
Thermostat
Fuse
Inverter
Soft starter
Explanation - A soft starter limits inrush current by ramping up voltage, protecting the motor during start‑up.
Correct answer is: Soft starter
Q.96 In a three‑phase induction motor, the term 'locked‑rotor current' (LRC) refers to:
Current when the motor is running at rated speed
Current when the rotor is prevented from turning
Current drawn during normal operation
Current after the motor reaches full load
Explanation - LRC is the high current drawn when the motor is energized but the rotor is mechanically locked.
Correct answer is: Current when the rotor is prevented from turning
Q.97 Which motor is most appropriate for a robotic joint that must hold position without power?
Stepper motor with holding torque
Induction motor
Universal motor
DC shunt motor
Explanation - When energized, stepper motors can produce holding torque to maintain position; some designs retain torque even when power is removed via detent torque, but generally power is required.
Correct answer is: Stepper motor with holding torque
Q.98 If a motor’s torque constant Kt is 0.2 Nm/A, what is the back‑EMF constant (Kb) in V·s/rad, assuming SI units consistency?
0.2 V·s/rad
5 V·s/rad
0.05 V·s/rad
2 V·s/rad
Explanation - In SI units, Kt (Nm/A) equals Kb (V·s/rad) for a motor; thus Kb = 0.2 V·s/rad.
Correct answer is: 0.2 V·s/rad
Q.99 A motor with a 'single‑phase' supply cannot produce a rotating magnetic field directly because:
It has only two poles
The supply frequency is too low
A single phase produces an alternating but not rotating field
It lacks a commutator
Explanation - A single‑phase supply creates an alternating field that oscillates, not rotates; auxiliary windings or capacitors are needed to create a phase shift.
Correct answer is: A single phase produces an alternating but not rotating field
Q.100 Which of the following is NOT a typical method of starting a single‑phase induction motor?
Capacitor start
Split‑phase start
Star‑Delta start
Shaded‑pole start
Explanation - Star‑Delta start is used for three‑phase motors; single‑phase motors use capacitor or split‑phase methods.
Correct answer is: Star‑Delta start
Q.101 The 'speed‑torque curve' of an induction motor typically shows:
Speed increasing linearly with torque
Speed decreasing slightly as torque increases
Speed constant regardless of torque
Speed dropping sharply to zero at rated torque
Explanation - Induction motor speed falls slightly with increasing torque due to slip increase.
Correct answer is: Speed decreasing slightly as torque increases
Q.102 What is the main cause of 'cogging torque' in a permanent‑magnet motor?
Variations in winding resistance
Magnetic interaction between permanent magnets and stator slots
Uneven air‑gap clearance
Improper commutation
Explanation - Cogging torque arises from the tendency of permanent magnets to align with stator teeth, causing torque ripple at low speeds.
Correct answer is: Magnetic interaction between permanent magnets and stator slots
Q.103 In a DC motor, if the field resistance is increased while armature voltage remains constant, the motor will:
Accelerate
Decelerate
Maintain the same speed
Stall
Explanation - Higher field resistance reduces field current, lowering flux, which increases speed for a given armature voltage.
Correct answer is: Accelerate
Q.104 Which parameter primarily determines the thermal rating of an electric motor?
Number of poles
Size of the cooling fan
Current carrying capacity of the windings
Supply frequency
Explanation - Thermal rating is limited by how much current the windings can carry without overheating.
Correct answer is: Current carrying capacity of the windings
Q.105 A motor with a 'direct‑on‑line' (DOL) starter will:
Start with reduced voltage
Start with full line voltage
Start with a star‑Delta configuration
Start using a soft starter
Explanation - DOL directly connects the motor to the supply, applying full voltage at start.
Correct answer is: Start with full line voltage
Q.106 The 'synchronizing torque' in a synchronous motor is produced by:
Induced currents in the rotor
Permanent magnets on the rotor
Interaction between the rotating field and the rotor's field
Mechanical coupling to the load
Explanation - Synchronizing torque arises from the interaction of stator rotating field with the rotor's magnetic field, pulling the rotor into synchronism.
Correct answer is: Interaction between the rotating field and the rotor's field
Q.107 Which of the following is a typical symptom of a motor with a worn brush set?
Excessive vibration
Reduced back‑EMF
Sparking and arcing at the commutator
Higher power factor
Explanation - Worn brushes can cause poor contact, leading to sparking and increased maintenance.
Correct answer is: Sparking and arcing at the commutator
Q.108 In a three‑phase induction motor, the term 'pole pitch' refers to:
The distance between adjacent stator teeth
The angular distance between two consecutive poles
The width of the air‑gap
The number of turns in the winding
Explanation - Pole pitch is the mechanical angle (or number of slots) between adjacent magnetic poles.
Correct answer is: The angular distance between two consecutive poles
Q.109 A motor that requires a high starting torque and can operate at a constant speed regardless of load is best served by:
DC series motor
Synchronous motor
Induction motor with VFD
Universal motor
Explanation - Synchronous motors provide constant speed; additional starting methods can supply high starting torque.
Correct answer is: Synchronous motor
Q.110 The term 'skin depth' in conductors is inversely proportional to:
Square root of frequency
Frequency
Resistivity
Magnetic permeability
Explanation - Skin depth δ = √(2ρ / (ωμ)), decreasing with √frequency.
Correct answer is: Square root of frequency
Q.111 Which motor type is most commonly used in HVAC fans due to its reliability and low cost?
Universal motor
Synchronous motor
Induction motor
Stepper motor
Explanation - Induction motors are rugged, low‑maintenance, and inexpensive, making them ideal for HVAC fan drives.
Correct answer is: Induction motor
Q.112 When a motor's 'power factor' is less than 1, it indicates that:
The motor is operating at overload
The motor draws reactive power in addition to real power
The motor has high efficiency
The motor is running at no‑load
Explanation - Power factor = cos(φ); values less than 1 mean a phase difference between voltage and current, indicating reactive power.
Correct answer is: The motor draws reactive power in addition to real power
Q.113 In a brushless DC (BLDC) motor, the stator typically contains:
Permanent magnets
Wound field coils
Commutator segments
Capacitive plates
Explanation - BLDC motors have a wound stator that creates a rotating magnetic field, while the rotor carries permanent magnets.
Correct answer is: Wound field coils
Q.114 A motor that is rated for continuous operation at 5 kW but can tolerate 7 kW for short periods is said to have:
Overload capability
Under‑load capability
No‑load capability
Breakdown capability
Explanation - Overload capability permits temporary operation above rated power for limited times.
Correct answer is: Overload capability
Q.115 Which of the following is a disadvantage of using a wound‑field DC motor compared to a permanent‑magnet motor?
Higher efficiency
Lower torque density
Need for field excitation power
Inability to operate at high speeds
Explanation - Wound‑field motors require additional power to energize the field windings, reducing overall efficiency.
Correct answer is: Need for field excitation power
Q.116 In a motor control circuit, a 'contactor' is primarily used to:
Regulate speed
Provide overload protection
Switch the motor on and off under control of a pilot device
Measure motor temperature
Explanation - Contactors are electromechanical switches that control power to motors.
Correct answer is: Switch the motor on and off under control of a pilot device
Q.117 The main reason for using a star‑delta starter is to:
Reduce starting current
Increase starting torque
Eliminate slip
Provide variable speed
Explanation - Star connection reduces voltage applied during start, cutting inrush current; after acceleration, the motor switches to delta for full voltage.
Correct answer is: Reduce starting current
Q.118 If a motor operates at a speed of 1500 rpm with a slip of 2 % on a 60 Hz supply, what is its synchronous speed?
1500 rpm
1530 rpm
1562 rpm
1600 rpm
Explanation - Slip = (Ns – N)/Ns = 0.02 → Ns = N / (1 – 0.02) = 1500 / 0.98 ≈ 1530 rpm.
Correct answer is: 1530 rpm
Q.119 A motor rated at 0.75 kW, 230 V, single‑phase, has a current draw of 5 A. Its power factor is approximately:
0.65
0.75
0.85
0.95
Explanation - Apparent power S = V·I = 230·5 = 1150 VA = 1.15 kVA. Power factor pf = P/S = 0.75/1.15 ≈ 0.65.
Correct answer is: 0.65
Q.120 In a permanent‑magnet synchronous motor, the rotor speed will:
Always be lower than the supply frequency‑derived synchronous speed
Always match the synchronous speed of the supply
Vary proportionally with load torque
Increase indefinitely with load
Explanation - By definition, a synchronous motor's rotor speed is locked to the rotating magnetic field frequency.
Correct answer is: Always match the synchronous speed of the supply
Q.121 Which type of loss in an electric motor increases with the square of the frequency?
Copper loss
Core loss
Friction loss
Windage loss
Explanation - Core loss (hysteresis + eddy current) rises with frequency, especially the eddy‑current component which is proportional to f².
Correct answer is: Core loss
Q.122 A motor that requires an external DC supply to its field winding in order to start is a:
Shunt motor
Series motor
Compound motor
Synchronous motor
Explanation - Synchronous motors need DC excitation of the rotor field to lock with the rotating field.
Correct answer is: Synchronous motor
Q.123 The torque produced by a three‑phase induction motor is primarily a result of:
Interaction between the stator's rotating magnetic field and induced rotor currents
Permanent magnets on the rotor
Commutation by brushes
Mechanical gearing
Explanation - Torque arises from Lorentz forces on currents induced in the rotor by the stator's rotating field.
Correct answer is: Interaction between the stator's rotating magnetic field and induced rotor currents
Q.124 The main advantage of using a vector (field‑oriented) control for an induction motor is:
Simplified hardware
Direct torque control with fast response
Reduced motor size
Higher slip
Explanation - Vector control decouples torque and flux, enabling precise and rapid torque control similar to DC motors.
Correct answer is: Direct torque control with fast response
Q.125 A motor's 'rated voltage' is:
The maximum voltage it can ever withstand
The voltage at which it delivers its rated power continuously
The voltage required for starting only
The voltage needed to produce no-load speed
Explanation - Rated voltage is the nominal supply voltage for which the motor is designed to operate at rated conditions.
Correct answer is: The voltage at which it delivers its rated power continuously
Q.126 In a DC motor, the 'armature reaction' tends to:
Increase the main field flux
Distort and weaken the main field flux
Reduce the armature resistance
Increase commutation voltage
Explanation - Armature reaction creates a magnetic field that opposes the main field, causing flux weakening and distortion.
Correct answer is: Distort and weaken the main field flux
Q.127 A motor with a low 'starting torque' is unsuitable for:
Fan drives
Pump applications
Conveyor belts with heavy loads
Blowers
Explanation - Heavy‑load conveyors require high starting torque to overcome inertia and load.
Correct answer is: Conveyor belts with heavy loads
Q.128 Which component is used to limit the voltage applied to a motor during start‑up in a soft starter?
Resistor
Thyristor (SCR) or triac
Transformer
Capacitor
Explanation - Solid‑state devices such as SCRs or triacs gradually increase voltage, controlling inrush current.
Correct answer is: Thyristor (SCR) or triac
Q.129 If an induction motor has a rated slip of 5 % at full load, what is the approximate slip at no‑load?
5 %
0 %
2 %
1 %
Explanation - At no‑load, slip is very small, typically around 1 % or less.
Correct answer is: 1 %
Q.130 A motor that can operate both as a generator and a motor when the shaft is driven is called a:
Reversible motor
Regenerative motor
Bidirectional motor
Dynamic motor
Explanation - When driven, the motor can act as a generator, feeding power back to the supply (regeneration).
Correct answer is: Regenerative motor
Q.131 Which method is commonly used to improve the power factor of an induction motor?
Installing a capacitor bank
Increasing the supply voltage
Using a larger motor
Reducing the supply frequency
Explanation - Capacitor banks provide leading reactive power, improving overall power factor.
Correct answer is: Installing a capacitor bank
Q.132 The term 'overload protection' in motor circuits is designed to:
Prevent the motor from starting
Protect the motor from excessive current for extended periods
Increase the motor's efficiency
Reduce the motor's speed
Explanation - Overload devices trip when current exceeds safe limits for a set time, safeguarding the motor.
Correct answer is: Protect the motor from excessive current for extended periods