Q.1 What is the main principle on which a three‑phase induction motor operates?
Electromagnetic induction
Electrostatic attraction
Magnetic hysteresis
Permanent magnet interaction
Explanation - A three‑phase induction motor works on Faraday’s law: a rotating magnetic field induces currents in the rotor, producing torque.
Correct answer is: Electromagnetic induction
Q.2 In a balanced three‑phase system, the synchronous speed (Ns) of an induction motor is given by Ns = 120f / P. What is Ns for a 4‑pole motor supplied at 50 Hz?
1500 rpm
3000 rpm
1200 rpm
1800 rpm
Explanation - Ns = 120 × 50 / 4 = 1500 rpm.
Correct answer is: 1500 rpm
Q.3 Which of the following statements about slip (s) in an induction motor is true?
Slip is zero at rated load
Slip is always positive for motoring operation
Slip can be negative during motoring
Slip is independent of load
Explanation - In motoring mode the rotor rotates slower than the synchronous speed, giving a positive slip.
Correct answer is: Slip is always positive for motoring operation
Q.4 The equivalent circuit of a three‑phase induction motor referred to the stator contains which component that represents rotor resistance referred to the stator?
R2'
R1
Xm
Rc
Explanation - R2' is the rotor resistance reflected to the stator side in the equivalent circuit.
Correct answer is: R2'
Q.5 What type of starting method supplies a reduced voltage to the stator winding of an induction motor?
Star‑Delta starter
Auto‑transformer starter
Soft‑starter
Direct‑on‑line starter
Explanation - An auto‑transformer starter reduces the voltage applied to the stator, limiting in‑rush current.
Correct answer is: Auto‑transformer starter
Q.6 A three‑phase induction motor has a rated torque of 120 Nm at 1500 rpm. What is the approximate rated power output?
9.4 kW
11.0 kW
7.5 kW
13.2 kW
Explanation - P = T·ω = 120 Nm × (2π·1500/60) ≈ 9.42 kW.
Correct answer is: 9.4 kW
Q.7 Which component in the equivalent circuit of an induction motor represents the magnetizing reactance?
Xm
R1
R2'
X1
Explanation - Xm models the magnetizing reactance of the motor core.
Correct answer is: Xm
Q.8 In a wound‑rotor induction motor, what is the purpose of the slip rings?
To provide a path for rotor currents to external circuits
To increase rotor resistance
To reduce stator losses
To balance the three phases
Explanation - Slip rings allow external resistance to be added to the rotor circuit for speed control.
Correct answer is: To provide a path for rotor currents to external circuits
Q.9 Which of the following is NOT a typical advantage of three‑phase induction motors over single‑phase motors?
Higher efficiency
Self‑starting
Higher power factor
Lower starting torque
Explanation - Three‑phase induction motors generally have higher starting torque than single‑phase motors.
Correct answer is: Lower starting torque
Q.10 If the rotor resistance of an induction motor is increased (while other parameters stay the same), what happens to the motor’s starting torque?
It increases
It decreases
It remains unchanged
It becomes zero
Explanation - Higher rotor resistance shifts the torque‑speed curve, giving a higher starting torque but lower efficiency.
Correct answer is: It increases
Q.11 Which of the following is true for the rotor of a squirrel‑cage induction motor?
It consists of laminated steel with copper or aluminum bars short‑circuited at both ends
It contains windings connected to external resistors via slip rings
It is made of permanent magnets
It has a separate DC excitation source
Explanation - The squirrel‑cage rotor is a simple construction of conductive bars shorted by end rings.
Correct answer is: It consists of laminated steel with copper or aluminum bars short‑circuited at both ends
Q.12 What is the typical slip value of a standard 4‑pole induction motor at full load?
0.5 %
5 %
20 %
50 %
Explanation - Standard induction motors operate with slip of about 2–5 % at rated load.
Correct answer is: 5 %
Q.13 The term ‘breakdown torque’ of an induction motor refers to:
The maximum torque that can be developed before the motor stalls
The torque at which the motor reaches synchronous speed
The torque at rated current
The torque when the motor is disconnected from the supply
Explanation - Breakdown torque is the peak torque on the torque‑speed curve; exceeding it causes the motor to stall.
Correct answer is: The maximum torque that can be developed before the motor stalls
Q.14 In a star‑connected three‑phase induction motor, the line voltage is 400 V. What is the phase voltage?
230 V
400 V
692 V
115 V
Explanation - Phase voltage Vph = Vline / √3 = 400 V / 1.732 ≈ 230 V.
Correct answer is: 230 V
Q.15 Which of the following starting methods gives the highest starting current reduction?
Star‑Delta starter
Auto‑transformer starter
Soft‑starter
Direct‑on‑line starter
Explanation - A soft‑starter controls voltage ramp‑up, reducing in‑rush current more than other methods.
Correct answer is: Soft‑starter
Q.16 For a three‑phase induction motor, the term ‘pull‑out torque’ is synonymous with:
Breakdown torque
Starting torque
Rated torque
No‑load torque
Explanation - Pull‑out torque is another name for the maximum torque before stall.
Correct answer is: Breakdown torque
Q.17 What is the effect on motor speed if the supply frequency is increased while the number of poles remains unchanged?
Speed increases
Speed decreases
Speed remains the same
Motor stalls
Explanation - Synchronous speed Ns = 120f / P; increasing f raises Ns.
Correct answer is: Speed increases
Q.18 Which component in the induction motor equivalent circuit represents core losses?
Rc
Xm
R1
X1
Explanation - Rc (or Rfe) models the iron (core) losses in the magnetic circuit.
Correct answer is: Rc
Q.19 A three‑phase induction motor is said to be ‘self‑starting’. What does this mean?
It can start without any external starting device
It starts with a permanent magnet rotor
It requires a DC supply for starting
It can start only under no‑load conditions
Explanation - The rotating magnetic field in a three‑phase supply produces starting torque automatically.
Correct answer is: It can start without any external starting device
Q.20 In the per‑phase equivalent circuit of an induction motor, why is the rotor resistance divided by the slip (R2'/s)?
Because induced rotor EMF is proportional to slip
Because slip represents the frequency of rotor currents
Because slip reduces stator resistance
Because rotor reactance is independent of slip
Explanation - Rotor frequency = s·f, so rotor resistance appears as R2'/s in the equivalent circuit.
Correct answer is: Because slip represents the frequency of rotor currents
Q.21 Which of the following is a characteristic of a wound‑rotor induction motor?
External resistors can be inserted into the rotor circuit
It has a squirrel‑cage rotor
It cannot be started with a star‑delta starter
It operates only on DC supply
Explanation - Wound‑rotor motors have slip rings that allow external resistance to be added for speed control.
Correct answer is: External resistors can be inserted into the rotor circuit
Q.22 The term ‘torque ripple’ in an induction motor refers to:
Fluctuation of torque during steady operation
Maximum torque capability
Torque at start-up
Torque loss due to friction
Explanation - Torque ripple is the periodic variation of torque caused by supply harmonics or magnetic saturation.
Correct answer is: Fluctuation of torque during steady operation
Q.23 If an induction motor is supplied with a 3‑phase 400 V, 50 Hz supply and has 6 poles, what is its synchronous speed?
1000 rpm
1500 rpm
3000 rpm
6000 rpm
Explanation - Ns = 120×50 / 6 = 1000 rpm.
Correct answer is: 1000 rpm
Q.24 Which starting method provides a soft voltage ramp to the motor but still uses the full line voltage after start?
Soft‑starter
Star‑Delta starter
Direct‑on‑line starter
Wound‑rotor starter
Explanation - A soft‑starter gradually increases voltage, then switches to full line voltage once the motor reaches near‑rated speed.
Correct answer is: Soft‑starter
Q.25 In a three‑phase induction motor, what is the phase relationship between the stator magnetic field and the rotor current?
Rotor current lags the rotating magnetic field by the slip angle
Rotor current leads the rotating magnetic field
Rotor current is in phase with the magnetic field
There is no fixed relationship
Explanation - The induced rotor EMF leads the rotor current, causing the current to lag the rotating field by an angle related to slip.
Correct answer is: Rotor current lags the rotating magnetic field by the slip angle
Q.26 What is the purpose of the damper winding in a synchronous motor that also functions as an induction motor during start‑up?
To provide starting torque like an induction motor
To increase rated speed
To reduce stator resistance
To supply DC excitation
Explanation - Damper windings act as a squirrel‑cage rotor, giving the synchronous motor induction‑type starting torque.
Correct answer is: To provide starting torque like an induction motor
Q.27 When an induction motor is operated at a voltage lower than its rated voltage, which of the following is true?
Both torque and speed decrease
Speed increases, torque remains constant
Torque increases, speed decreases
Speed remains unchanged, torque decreases
Explanation - Reduced voltage lowers magnetizing flux, decreasing both developed torque and speed.
Correct answer is: Both torque and speed decrease
Q.28 In the context of induction motor efficiency, which loss is most affected by increasing the rotor resistance?
Copper loss in the rotor
Core loss
Stator copper loss
Friction and windage loss
Explanation - Higher rotor resistance increases I²R losses in the rotor, raising rotor copper loss.
Correct answer is: Copper loss in the rotor
Q.29 What is the typical relationship between starting torque (Ts) and rated torque (Tr) for a standard squirrel‑cage induction motor?
Ts ≈ 1.5 Tr
Ts ≈ 0.5 Tr
Ts ≈ 3 Tr
Ts ≈ 0.1 Tr
Explanation - Standard squirrel‑cage motors usually have a starting torque about 1.5–2 times the rated torque.
Correct answer is: Ts ≈ 1.5 Tr
Q.30 Which of the following statements correctly describes slip at no‑load condition?
Slip is nearly zero
Slip equals rated slip
Slip is maximum
Slip is negative
Explanation - At no‑load the motor runs close to synchronous speed, so slip is very small.
Correct answer is: Slip is nearly zero
Q.31 A three‑phase induction motor has a power factor of 0.85 lagging at full load. Which component in its equivalent circuit primarily determines this power factor?
Magnetizing reactance (Xm)
Rotor resistance (R2')
Stator resistance (R1)
Core resistance (Rc)
Explanation - Xm determines the reactive part of the current, thus influencing the overall power factor.
Correct answer is: Magnetizing reactance (Xm)
Q.32 In a star‑delta starter, why is the motor first connected in star configuration?
To reduce the voltage per phase to √3 of the line voltage
To increase the starting torque
To double the line current
To eliminate the need for a contactor
Explanation - Star connection reduces phase voltage, limiting starting current and torque.
Correct answer is: To reduce the voltage per phase to √3 of the line voltage
Q.33 What is the effect on the torque–speed curve when the rotor resistance of a wound‑rotor motor is increased?
The peak torque shifts to a higher slip
The peak torque shifts to a lower slip
The curve becomes linear
No effect on the curve
Explanation - Higher rotor resistance moves the maximum torque point to higher slip values.
Correct answer is: The peak torque shifts to a higher slip
Q.34 Which of the following is the most common method for speed control of a three‑phase induction motor in industrial practice?
Variable Frequency Drive (VFD)
Resistor starting
Star‑Delta starter
Capacitor start
Explanation - VFDs vary supply frequency and voltage, providing efficient and precise speed control.
Correct answer is: Variable Frequency Drive (VFD)
Q.35 When the supply frequency is reduced to 40 Hz while keeping the number of poles unchanged, what happens to the motor’s synchronous speed?
It decreases proportionally
It increases proportionally
It stays the same
The motor stalls
Explanation - Ns = 120f / P, so decreasing f lowers Ns linearly.
Correct answer is: It decreases proportionally
Q.36 In a three‑phase induction motor, the term ‘slip frequency’ refers to:
The frequency of rotor currents
The frequency of the stator supply
The frequency of core losses
The frequency of mechanical vibrations
Explanation - Rotor frequency = slip × supply frequency.
Correct answer is: The frequency of rotor currents
Q.37 Which of the following is NOT a typical cause of reduced efficiency in induction motors?
High rotor resistance
Improper cooling
Balanced three‑phase supply
Excessive stray load losses
Explanation - A balanced supply is ideal; the other factors increase losses and reduce efficiency.
Correct answer is: Balanced three‑phase supply
Q.38 The purpose of the fan attached to the rotor shaft of many induction motors is to:
Provide cooling to the motor windings
Increase starting torque
Reduce slip
Generate additional power
Explanation - The fan creates airflow to remove heat from stator and rotor windings.
Correct answer is: Provide cooling to the motor windings
Q.39 For a 3‑phase induction motor rated at 7.5 kW, 415 V, 50 Hz, 4‑pole, what is the approximate rated current per phase (star connection)?
12 A
18 A
25 A
30 A
Explanation - Apparent power S = 7500 VA, line current I_L = S / (√3·V) ≈ 10.4 A; per‑phase current in star = I_L ≈ 12 A (allowing for PF).
Correct answer is: 12 A
Q.40 Which phenomenon explains why a three‑phase induction motor produces a rotating magnetic field rather than a pulsating one?
Phase displacement of 120° between phases
Use of laminated cores
Presence of a squirrel‑cage rotor
High supply voltage
Explanation - The 120° phase shift creates a smoothly rotating resultant magnetic field.
Correct answer is: Phase displacement of 120° between phases
Q.41 When an induction motor is operated with a variable frequency drive, which of the following must be adjusted to maintain constant flux?
Voltage (V) must be varied proportionally with frequency (f)
Stator resistance must be increased
Rotor resistance must be decreased
Number of poles must be changed
Explanation - V/f control keeps magnetic flux approximately constant across frequency changes.
Correct answer is: Voltage (V) must be varied proportionally with frequency (f)
Q.42 If an induction motor’s slip is 2 % at rated load, what is its rotor speed when supplied at 60 Hz with 4 poles?
1470 rpm
1500 rpm
1480 rpm
1450 rpm
Explanation - Ns = 1500 rpm; rotor speed = Ns × (1 – s) = 1500 × 0.98 = 1470 rpm.
Correct answer is: 1470 rpm
Q.43 Which type of loss in an induction motor is directly proportional to the square of the supply voltage?
Stator copper loss
Core loss
Friction loss
Windage loss
Explanation - I²R loss in the stator winding increases with the square of voltage (since current ∝ V).
Correct answer is: Stator copper loss
Q.44 A three‑phase induction motor has a rated speed of 1450 rpm and a synchronous speed of 1500 rpm. What is its rated slip?
3.33 %
5 %
2 %
10 %
Explanation - s = (Ns – Nr) / Ns = (1500‑1450)/1500 = 0.0333 = 3.33 %.
Correct answer is: 3.33 %
Q.45 Which of the following statements is true about a capacitor‑start induction motor?
It uses a capacitor to create a phase shift for higher starting torque
It has a wound rotor with external resistance
It operates only on single‑phase supply
It does not require a capacitor for starting
Explanation - The start capacitor creates a leading current in the auxiliary winding, improving starting torque.
Correct answer is: It uses a capacitor to create a phase shift for higher starting torque
Q.46 In the context of induction motors, the term ‘skin effect’ refers to:
The tendency of alternating current to flow near the surface of conductors at high frequencies
The magnetic saturation of the core
The increase of slip with load
The formation of eddy currents in the rotor
Explanation - Skin effect raises effective resistance of conductors at high frequencies, affecting losses.
Correct answer is: The tendency of alternating current to flow near the surface of conductors at high frequencies
Q.47 What is the usual purpose of inserting a reactor (inductor) in series with the supply of a large induction motor?
To limit the in‑rush current during start‑up
To increase the motor speed
To improve power factor
To provide a ground fault path
Explanation - The reactor adds inductive reactance, reducing the instantaneous current at start.
Correct answer is: To limit the in‑rush current during start‑up
Q.48 If the number of poles in an induction motor is doubled while keeping the supply frequency constant, what happens to its synchronous speed?
It halves
It doubles
It remains the same
It becomes zero
Explanation - Ns = 120f / P; doubling P reduces Ns by a factor of two.
Correct answer is: It halves
Q.49 Which component of the induction motor equivalent circuit is responsible for representing the leakage flux that does not contribute to torque production?
Leakage reactance (X1 or X2')
Magnetizing reactance (Xm)
Core resistance (Rc)
Stator resistance (R1)
Explanation - Leakage reactances model flux that links only one winding (stator or rotor) and does not produce torque.
Correct answer is: Leakage reactance (X1 or X2')
Q.50 In a three‑phase induction motor, why is the stator winding usually distributed rather than concentrated?
To reduce harmonic content and improve sinusoidal MMF
To increase starting torque dramatically
To simplify manufacturing
To enable operation at very low frequencies
Explanation - Distributed windings produce a smoother rotating magnetic field with less harmonic distortion.
Correct answer is: To reduce harmonic content and improve sinusoidal MMF
Q.51 Which of the following is a disadvantage of using a star‑delta starter?
Reduced starting torque
Higher starting current
Complex control circuitry
Inability to start at low voltage
Explanation - Star connection reduces voltage per phase, consequently lowering starting torque.
Correct answer is: Reduced starting torque
Q.52 A three‑phase induction motor is connected in delta and supplied with 400 V line voltage. What is the phase voltage?
400 V
230 V
692 V
115 V
Explanation - In delta connection, line voltage equals phase voltage.
Correct answer is: 400 V
Q.53 Which of the following describes the ‘pull‑in torque’ of an induction motor?
The minimum torque needed to start the motor from rest
The maximum torque at rated load
The torque at no‑load speed
The torque when the motor is stalled
Explanation - Pull‑in torque is the lowest torque at which the motor can start and accelerate to running speed.
Correct answer is: The minimum torque needed to start the motor from rest
Q.54 What is the main advantage of using a variable frequency drive (VFD) over a star‑delta starter for speed control?
Precise speed regulation across a wide range
Lower cost
Simpler wiring
Higher starting torque
Explanation - VFDs can vary frequency and voltage continuously, offering fine speed control.
Correct answer is: Precise speed regulation across a wide range
Q.55 In an induction motor, the term ‘core loss’ comprises:
Hysteresis loss and eddy‑current loss in the iron core
Stator copper loss
Rotor copper loss
Friction and windage loss
Explanation - Core loss is the sum of magnetic hysteresis and eddy‑current losses in the laminated steel.
Correct answer is: Hysteresis loss and eddy‑current loss in the iron core
Q.56 If a three‑phase induction motor’s slip is 0.02 and the supply frequency is 60 Hz, what is the frequency of the rotor currents?
1.2 Hz
0.02 Hz
60 Hz
58.8 Hz
Explanation - Rotor frequency = slip × supply frequency = 0.02 × 60 = 1.2 Hz.
Correct answer is: 1.2 Hz
Q.57 Which of the following is true for a motor supplied with a three‑phase, 50 Hz, 6‑pole system?
Synchronous speed = 1000 rpm
Synchronous speed = 1500 rpm
Synchronous speed = 2000 rpm
Synchronous speed = 2500 rpm
Explanation - Ns = 120 × 50 / 6 = 1000 rpm.
Correct answer is: Synchronous speed = 1000 rpm
Q.58 In the design of an induction motor, increasing the air‑gap length generally leads to:
Higher magnetizing current and lower efficiency
Higher torque
Lower slip
Reduced rotor temperature
Explanation - A larger air gap reduces magnetic coupling, requiring more magnetizing current and reducing efficiency.
Correct answer is: Higher magnetizing current and lower efficiency
Q.59 Which of the following starting methods is most suitable for a high‑inertia load such as a crusher?
Soft‑starter
Direct‑on‑line starter
Star‑Delta starter
Capacitor start
Explanation - Soft‑starters provide controlled voltage ramp, reducing mechanical stress on high‑inertia loads.
Correct answer is: Soft‑starter
Q.60 The torque produced by an induction motor is proportional to which of the following?
The product of stator flux and rotor current
The square of the supply voltage only
The frequency of the supply only
The stator resistance
Explanation - Developed torque = k·Φ·I_r, where Φ is the air‑gap flux and I_r is rotor current.
Correct answer is: The product of stator flux and rotor current
Q.61 For a three‑phase induction motor with a rated power of 15 kW, a power factor of 0.9 lagging, and a line voltage of 400 V, what is the approximate line current?
30 A
25 A
20 A
35 A
Explanation - Apparent power S = P / PF = 15/0.9 ≈ 16.7 kVA; I = S/(√3·V) ≈ 16.7×10³/(1.732×400) ≈ 24 A; rounding to nearest typical value gives ~30 A.
Correct answer is: 30 A
Q.62 Which of the following best describes the effect of harmonics on induction motor performance?
They increase iron losses and cause extra heating
They improve torque smoothness
They reduce starting current
They have no effect on motor operation
Explanation - Harmonics produce additional eddy currents and hysteresis losses in the core, raising temperature.
Correct answer is: They increase iron losses and cause extra heating
Q.63 A three‑phase induction motor runs at 1455 rpm under load. If the supply frequency is increased to 60 Hz while keeping the motor’s number of poles unchanged, the new approximate speed will be:
1746 rpm
1500 rpm
1600 rpm
1800 rpm
Explanation - Original Ns = 1500 rpm (50 Hz, 4‑pole). New Ns = 120×60/4 = 1800 rpm. Assuming same slip (3 %), speed ≈ 1800×0.97 ≈ 1746 rpm.
Correct answer is: 1746 rpm
Q.64 In a wound‑rotor induction motor, the external resistors are connected to the rotor circuit through:
Slip rings and brushes
Direct coupling to the shaft
Stator windings
Permanent magnets
Explanation - Slip rings provide access to the rotor windings, allowing external resistance insertion.
Correct answer is: Slip rings and brushes
Q.65 Which of the following statements is true about the ‘no‑load current’ of a three‑phase induction motor?
It is primarily magnetizing current
It equals the rated current
It is zero
It is equal to the starting current
Explanation - At no load, only a small current is needed to produce the rotating magnetic field, largely reactive (magnetizing).
Correct answer is: It is primarily magnetizing current
Q.66 When a three‑phase induction motor is started with a direct‑on‑line (DOL) starter, the starting current is approximately:
5–7 times the rated current
Equal to the rated current
0.5 times the rated current
10 times the rated current
Explanation - DOL starting draws a high in‑rush current, typically 5–7× the motor’s full‑load current.
Correct answer is: 5–7 times the rated current
Q.67 The term ‘locked‑rotor torque’ (LRT) refers to:
Torque produced when the rotor is stationary
Torque at rated speed
Torque at synchronous speed
Torque during overload
Explanation - LRT is the torque developed when the rotor cannot turn (locked) at start.
Correct answer is: Torque produced when the rotor is stationary
Q.68 Which of the following is a key reason for using a three‑phase supply for induction motors rather than a single‑phase supply?
Three‑phase supplies produce a rotating magnetic field without additional capacitors
Single‑phase supplies are cheaper
Three‑phase motors are smaller in size
Single‑phase motors have higher efficiency
Explanation - Three‑phase currents inherently create a smooth rotating field, enabling self‑starting.
Correct answer is: Three‑phase supplies produce a rotating magnetic field without additional capacitors
Q.69 If the rotor resistance of a wound‑rotor motor is increased, what happens to its efficiency at rated load?
Efficiency decreases
Efficiency increases
Efficiency remains unchanged
Efficiency becomes infinite
Explanation - Higher rotor resistance raises I²R losses, reducing overall efficiency.
Correct answer is: Efficiency decreases
Q.70 In the per‑phase equivalent circuit of an induction motor, why is the magnetizing reactance (Xm) placed in parallel with the core loss resistance (Rc)?
Both represent the magnetizing branch of the motor
Because they are both series elements
To model rotor slip
To simplify calculations
Explanation - Xm models reactive magnetizing flux, while Rc models resistive core losses; both are part of the magnetizing branch.
Correct answer is: Both represent the magnetizing branch of the motor
Q.71 Which of the following starting methods provides the highest starting torque?
Capacitor‑start
Star‑Delta
Direct‑on‑line
Soft‑starter
Explanation - The start capacitor creates a large phase shift, giving high starting torque for single‑phase motors.
Correct answer is: Capacitor‑start
Q.72 When an induction motor operates at a slip of 0.04, what fraction of the synchronous speed does the rotor actually rotate at?
96 %
4 %
50 %
104 %
Explanation - Rotor speed = (1 – s) × Ns = 0.96 Ns, i.e., 96 % of synchronous speed.
Correct answer is: 96 %
Q.73 In a three‑phase induction motor, which loss component is most affected by operating at a lower supply voltage while keeping frequency constant?
Stator copper loss
Core loss
Friction loss
Windage loss
Explanation - Lower voltage reduces current, thus reducing I²R losses in the stator windings.
Correct answer is: Stator copper loss
Q.74 Which of the following best explains why induction motors are widely used in industrial applications?
Simple construction, robustness, and low cost
Ability to run on DC power
Very high starting torque without any control devices
Self‑regulating speed without any external devices
Explanation - Induction motors have a rugged, inexpensive design, making them ideal for many applications.
Correct answer is: Simple construction, robustness, and low cost
Q.75 If a 3‑phase induction motor rated at 5 kW, 415 V (line), 50 Hz, 4‑pole runs at 1470 rpm, what is its slip?
2 %
5 %
0 %
10 %
Explanation - Ns = 1500 rpm; slip = (1500‑1470)/1500 = 0.02 = 2 %.
Correct answer is: 2 %
Q.76 The function of the ‘start winding’ in a single‑phase induction motor is to:
Create a phase shift for starting torque
Provide the main torque during normal operation
Reduce the motor’s efficiency
Increase the motor’s synchronous speed
Explanation - The start winding, often with a capacitor, produces a second phase to start the motor.
Correct answer is: Create a phase shift for starting torque
Q.77 In a three‑phase induction motor, the term ‘asynchronous’ refers to:
The rotor rotates at a speed different from the synchronous speed
The motor runs on DC supply
The stator windings are not balanced
The motor has a permanent magnet rotor
Explanation - ‘Asynchronous’ indicates that rotor speed is not equal to the rotating magnetic field speed.
Correct answer is: The rotor rotates at a speed different from the synchronous speed
Q.78 Which of the following is a typical characteristic of a high‑efficiency (IE3) induction motor compared to a standard IE1 motor?
Lower core and copper losses
Higher starting current
Lower rated speed
Larger physical size
Explanation - IE3 motors are designed with better materials and designs to reduce losses and improve efficiency.
Correct answer is: Lower core and copper losses
Q.79 When a three‑phase induction motor is connected to a 400 V, 50 Hz supply, what is the frequency of the induced EMF in the rotor?
s × 50 Hz
50 Hz
100 Hz
0 Hz
Explanation - Rotor EMF frequency equals slip (s) multiplied by the supply frequency.
Correct answer is: s × 50 Hz
Q.80 A wound‑rotor induction motor can be used as a generator. Which condition must be met for generation?
Rotor speed must exceed synchronous speed
Supply frequency must be reduced to zero
Slip must be zero
Stator must be short‑circuited
Explanation - When the rotor runs above synchronous speed, the machine acts as an induction generator, delivering power to the grid.
Correct answer is: Rotor speed must exceed synchronous speed
Q.81 Which of the following is a typical method for reducing the sound level of a large induction motor?
Using vibration isolation mounts
Increasing supply voltage
Reducing the number of poles
Operating at a higher slip
Explanation - Mechanical isolation reduces transmitted vibrations and acoustic noise.
Correct answer is: Using vibration isolation mounts
Q.82 In the torque–speed characteristic of an induction motor, the region where torque decreases with increasing speed is called:
The stable operating region
The unstable region
The breakdown region
The pull‑in region
Explanation - In the falling‑torque part, a small increase in speed reduces torque, potentially leading to instability.
Correct answer is: The unstable region
Q.83 Which of the following statements about the ‘air‑gap power’ (P_ag) in an induction motor is correct?
P_ag is the power transferred across the air gap from stator to rotor
P_ag equals the output mechanical power
P_ag is the same as the input electrical power
P_ag does not depend on slip
Explanation - Air‑gap power is the electrical power converted to mechanical form in the rotor, before rotor copper losses are subtracted.
Correct answer is: P_ag is the power transferred across the air gap from stator to rotor
Q.84 A three‑phase induction motor is rated at 0.75 kW, 380 V, 50 Hz, 4‑pole. What is the approximate rated current per phase (star connection)?
1.5 A
2.3 A
3.0 A
4.5 A
Explanation - Apparent power S = 0.75 kW / 0.8 ≈ 0.94 kVA; I_L = S/(√3·V) ≈ 0.94×10³/(1.732×380) ≈ 1.4 A; per‑phase current ≈ 2.3 A considering PF.
Correct answer is: 2.3 A
Q.85 When an induction motor is started with a star‑delta starter, the motor operates in star for a few seconds. What is the main purpose of this period?
To allow the motor to reach a speed where torque is sufficient for delta connection
To heat the stator windings
To synchronize the rotor with the supply
To discharge the start capacitor
Explanation - After acceleration in star, the motor switches to delta where full voltage provides higher torque.
Correct answer is: To allow the motor to reach a speed where torque is sufficient for delta connection
Q.86 In the context of induction motors, the term ‘slip ring motor’ is synonymous with:
Wound‑rotor motor
Squirrel‑cage motor
Permanent‑magnet motor
Capacitor‑start motor
Explanation - Slip‑ring motors have wound rotors connected via slip rings for external resistance control.
Correct answer is: Wound‑rotor motor
Q.87 If the frequency of the supply to an induction motor is reduced while the number of poles remains the same, which of the following is NOT affected?
Synchronous speed
Torque
Frequency of rotor currents
Number of poles
Explanation - Number of poles is a physical property of the motor and does not change with supply frequency.
Correct answer is: Number of poles
Q.88 Which of the following best describes the effect of increasing the stator supply voltage on the torque of an induction motor (all other conditions constant)?
Torque increases roughly with the square of the voltage
Torque decreases linearly
Torque remains unchanged
Torque becomes zero
Explanation - Developed torque is proportional to the square of the air‑gap flux, which is proportional to voltage.
Correct answer is: Torque increases roughly with the square of the voltage
Q.89 A three‑phase induction motor has a rated torque of 50 Nm at 1450 rpm. What is its rated mechanical power output?
4.8 kW
5.2 kW
6.0 kW
7.1 kW
Explanation - P = T·ω = 50 Nm × (2π·1450/60) ≈ 4.8 kW.
Correct answer is: 4.8 kW
Q.90 Which of the following starting methods is most suitable for a motor driving a pump that requires a soft start to avoid water hammer?
Soft‑starter
Direct‑on‑line starter
Star‑Delta starter
Capacitor start
Explanation - Soft‑starters gradually increase voltage, preventing sudden pressure surges in pump systems.
Correct answer is: Soft‑starter
Q.91 In a three‑phase induction motor, the term ‘skin effect’ becomes more significant at:
Higher frequencies
Lower frequencies
Zero slip
Higher torque
Explanation - Skin effect intensifies as frequency rises, concentrating current near conductor surfaces.
Correct answer is: Higher frequencies
Q.92 Which of the following is a typical application of a wound‑rotor induction motor?
Cranes where speed control is needed
Small household fans
High‑speed compressors
Battery chargers
Explanation - Wound‑rotor motors allow external resistance control, suitable for variable‑speed heavy‑load applications like cranes.
Correct answer is: Cranes where speed control is needed
Q.93 For a 3‑phase induction motor with 2 poles operating at 60 Hz, what is the synchronous speed?
3600 rpm
3000 rpm
1800 rpm
1200 rpm
Explanation - Ns = 120×60/2 = 3600 rpm.
Correct answer is: 3600 rpm
Q.94 Which of the following is a direct consequence of operating an induction motor at a very low slip (near‑synchronous speed)?
Reduced rotor copper losses
Higher starting torque
Increased stator current
Higher vibration
Explanation - At low slip, rotor frequency and thus rotor I²R losses are small.
Correct answer is: Reduced rotor copper losses
Q.95 When a three‑phase induction motor is started with a variable frequency drive at 30 Hz (instead of 50 Hz), what happens to the motor’s torque if voltage is kept constant?
Torque decreases
Torque increases
Torque remains the same
Torque becomes zero
Explanation - With constant voltage, lowering frequency reduces flux, leading to lower developed torque.
Correct answer is: Torque decreases
Q.96 Which of the following is NOT a loss component in an induction motor?
Stray load loss
Mechanical friction loss
Thermal convection loss
Core loss
Explanation - Thermal convection is part of cooling, not a direct motor loss term.
Correct answer is: Thermal convection loss
Q.97 A 3‑phase squirrel‑cage induction motor has a rated current of 12 A per phase at 400 V line voltage (star connection). What is the approximate apparent power rating?
8.3 kVA
10 kVA
5.2 kVA
12 kVA
Explanation - S = √3·V·I = 1.732×400×12 ≈ 8.3 kVA.
Correct answer is: 8.3 kVA
Q.98 Which of the following control devices is commonly used to protect an induction motor from overload?
Thermal overload relay
Capacitor bank
Soft‑starter
Variable frequency drive
Explanation - A thermal overload relay trips when motor current exceeds a preset value for a set time.
Correct answer is: Thermal overload relay
Q.99 The term ‘pole pitch’ in an induction motor refers to:
The distance between two adjacent magnetic poles
The number of poles per phase
The voltage per pole
The frequency of the supply
Explanation - Pole pitch is the mechanical distance (or number of slots) between two consecutive poles.
Correct answer is: The distance between two adjacent magnetic poles
Q.100 Which of the following statements is true about the efficiency of an induction motor at light load?
Efficiency is lower than at rated load
Efficiency is highest at light load
Efficiency is independent of load
Efficiency becomes zero
Explanation - At light load, fixed losses dominate, reducing overall efficiency.
Correct answer is: Efficiency is lower than at rated load
Q.101 In a three‑phase induction motor, the 'air‑gap' is the space between:
Stator core and rotor core
Stator winding and stator core
Rotor bars and end rings
Shaft and bearing
Explanation - The air‑gap separates the stationary stator core from the rotating rotor core.
Correct answer is: Stator core and rotor core
Q.102 A 3‑phase induction motor is supplied at 400 V, 50 Hz, 4‑pole. If the motor is rewound for 6‑pole operation while keeping the supply unchanged, what will be the new synchronous speed?
1000 rpm
1500 rpm
500 rpm
2000 rpm
Explanation - Ns = 120×50/6 = 1000 rpm.
Correct answer is: 1000 rpm
Q.103 Which of the following is the most common method for measuring slip in a running induction motor?
Measuring rotor speed and calculating slip from synchronous speed
Measuring stator voltage
Measuring line current
Measuring core temperature
Explanation - Slip is defined as (Ns‑Nr)/Ns; thus rotor speed measurement is required.
Correct answer is: Measuring rotor speed and calculating slip from synchronous speed
Q.104 What is the typical range of starting torque (as a multiple of rated torque) for a standard squirrel‑cage induction motor?
1.5–2.5 times rated torque
0.5–0.8 times rated torque
3–4 times rated torque
0.1–0.2 times rated torque
Explanation - Standard squirrel‑cage motors normally develop starting torque about 1.5 to 2.5 times the rated torque.
Correct answer is: 1.5–2.5 times rated torque
Q.105 Which component of the per‑phase equivalent circuit determines the shape of the torque‑speed curve at low slips?
Rotor resistance (R2')
Stator resistance (R1)
Magnetizing reactance (Xm)
Core loss resistance (Rc)
Explanation - Rotor resistance controls the torque produced at low slip values.
Correct answer is: Rotor resistance (R2')
Q.106 In a variable frequency drive, why is a diode bridge often placed at the output before the inverter stage?
To convert the AC supply to DC for the inverter
To increase the supply voltage
To provide isolation between phases
To generate a rotating magnetic field
Explanation - The diode bridge rectifies the AC mains to DC, which the inverter then switches to variable frequency AC.
Correct answer is: To convert the AC supply to DC for the inverter
Q.107 A three‑phase induction motor has a rated speed of 1470 rpm and a synchronous speed of 1500 rpm. If the motor is supplied with a 60 Hz source (same number of poles), what will be the new rated speed, assuming slip remains the same?
1764 rpm
1500 rpm
1800 rpm
1900 rpm
Explanation - Original slip = 30/1500 = 0.02. New Ns = 120×60/4 = 1800 rpm. New rated speed = Ns×(1‑s) = 1800×0.98 = 1764 rpm.
Correct answer is: 1764 rpm
Q.108 Which of the following best describes the effect of increasing the number of poles in an induction motor while keeping frequency constant?
Synchronous speed decreases
Torque increases proportionally
Current increases linearly
Efficiency improves automatically
Explanation - Ns = 120f/P; increasing P reduces Ns.
Correct answer is: Synchronous speed decreases
Q.109 In a three‑phase induction motor, the term ‘locked‑rotor current’ (LRC) is approximately:
6–8 times the rated current
Equal to the rated current
0.5 times the rated current
12–14 times the rated current
Explanation - Locked‑rotor current is typically 6–8 times the full‑load current for induction motors.
Correct answer is: 6–8 times the rated current
Q.110 Which starting method is specifically designed for single‑phase induction motors?
Capacitor start
Star‑Delta starter
Direct‑on‑line starter
Variable frequency drive
Explanation - Capacitor‑start methods create a phase shift in single‑phase motors for higher starting torque.
Correct answer is: Capacitor start
Q.111 The efficiency of an induction motor can be improved by:
Using higher quality steel laminations to reduce core loss
Increasing the air‑gap length
Operating at a very low slip
Adding more rotor bars
Explanation - Better lamination reduces hysteresis and eddy‑current losses, improving efficiency.
Correct answer is: Using higher quality steel laminations to reduce core loss
Q.112 What is the main reason why a three‑phase induction motor does not require brushes?
The rotor is a short‑circuited squirrel cage
The stator supplies DC current
The motor runs at synchronous speed
Brushes are only needed for DC machines
Explanation - Squirrel‑cage rotors have bars and end rings forming a closed circuit, eliminating the need for brushes.
Correct answer is: The rotor is a short‑circuited squirrel cage
Q.113 In a VFD-controlled induction motor, if the voltage is kept constant while frequency is increased, what happens to the motor flux?
Flux decreases
Flux remains constant
Flux increases
Flux becomes zero
Explanation - Flux ∝ V/f; constant V with higher f reduces flux.
Correct answer is: Flux decreases
Q.114 Which of the following is the correct expression for the mechanical output power (Pout) of an induction motor?
Pout = (1‑s) × Pag
Pout = Pag + rotor copper loss
Pout = s × Pag
Pout = Pag – stator copper loss
Explanation - Mechanical output is the air‑gap power minus rotor copper loss; rotor loss = s×Pag, thus Pout = Pag−s×Pag = (1‑s)×Pag.
Correct answer is: Pout = (1‑s) × Pag
Q.115 Which type of induction motor is most commonly used for high‑speed applications such as compressors?
Squirrel‑cage motor
Wound‑rotor motor
Capacitor‑start motor
Slip‑ring motor
Explanation - Squirrel‑cage motors are rugged and can operate at high speeds with simple construction.
Correct answer is: Squirrel‑cage motor
Q.116 When an induction motor is operated at a slip of 0.05, the rotor frequency is:
5 % of the supply frequency
Equal to the supply frequency
95 % of the supply frequency
Zero
Explanation - Rotor frequency = s × supply frequency = 0.05 × f.
Correct answer is: 5 % of the supply frequency
Q.117 Which of the following statements is true about the relationship between torque and slip for an induction motor?
Torque increases with slip up to the breakdown torque, then decreases
Torque is independent of slip
Torque decreases linearly with slip
Torque is maximum at zero slip
Explanation - The torque‑slip curve rises to a peak (breakdown torque) and then falls as slip continues to increase.
Correct answer is: Torque increases with slip up to the breakdown torque, then decreases
Q.118 For a 3‑phase, 6‑pole induction motor supplied at 50 Hz, what is the frequency of the rotating magnetic field?
50 Hz
100 Hz
25 Hz
150 Hz
Explanation - The rotating magnetic field frequency equals the supply frequency, independent of pole number.
Correct answer is: 50 Hz
Q.119 A three‑phase induction motor with a rated current of 10 A is connected to a 400 V line. If the motor is supplied via a star‑delta starter, what is the approximate line current during the star‑starting period?
5.8 A
10 A
17.3 A
20 A
Explanation - In star, phase voltage = Vline/√3, reducing current to roughly 1/√3 of the full‑load current: 10 A/√3 ≈ 5.8 A.
Correct answer is: 5.8 A
Q.120 Which of the following is the primary cause of the 'cogging' phenomenon in induction motors with salient pole rotors?
Torque ripple due to magnetic saliency
Excessive rotor resistance
Low supply voltage
High slip values
Explanation - Salient poles cause periodic variations in reluctance, leading to torque ripple (cogging).
Correct answer is: Torque ripple due to magnetic saliency
Q.121 In a three‑phase induction motor, the term ‘double‑frequency voltage’ refers to:
Voltage component at twice the supply frequency caused by harmonic distortion
Voltage applied to the rotor
Voltage across the stator windings
Voltage generated by the motor when operating as a generator
Explanation - Harmonics can produce voltage components at multiples of the fundamental frequency.
Correct answer is: Voltage component at twice the supply frequency caused by harmonic distortion
Q.122 If a three‑phase induction motor has a rated power of 22 kW, a power factor of 0.85, and a line voltage of 690 V, what is the approximate line current?
23 A
30 A
38 A
45 A
Explanation - Apparent power S = 22 kW/0.85 ≈ 25.9 kVA; I = S/(√3·V) ≈ 25.9×10³/(1.732×690) ≈ 21.7 A ≈ 23 A.
Correct answer is: 23 A
Q.123 Which of the following is a typical reason for using a variable frequency drive on a pump motor?
To match pump output to varying flow demand
To increase motor starting current
To simplify motor construction
To eliminate the need for bearings
Explanation - VFDs allow precise speed control, adjusting pump flow to process requirements.
Correct answer is: To match pump output to varying flow demand
Q.124 What is the effect on motor torque if the supply frequency is doubled while maintaining a constant V/f ratio?
Torque remains approximately the same
Torque doubles
Torque halves
Torque becomes zero
Explanation - Keeping V/f constant maintains flux, so torque (which depends on flux) stays roughly unchanged.
Correct answer is: Torque remains approximately the same
Q.125 In a 3‑phase induction motor, the term ‘starting current’ is typically:
5‑7 times the rated current
Equal to the rated current
Half the rated current
Zero
Explanation - Induction motors draw a high in‑rush current at start, usually 5‑7× the rated current.
Correct answer is: 5‑7 times the rated current
Q.126 Which of the following is a disadvantage of using a soft‑starter compared to a VFD?
Limited speed control range
Higher in‑rush current
Complex wiring
Higher cost
Explanation - Soft‑starters mainly limit start current and cannot provide the broad speed variation a VFD offers.
Correct answer is: Limited speed control range
Q.127 When a three‑phase induction motor is operated at a very high slip (near 1), what is the approximate condition of the rotor?
Rotor is almost stationary (locked)
Rotor runs at synchronous speed
Rotor runs faster than synchronous speed
Rotor experiences no losses
Explanation - Slip close to 1 means the rotor speed is near zero relative to synchronous speed.
Correct answer is: Rotor is almost stationary (locked)
Q.128 Which parameter of an induction motor is most directly influenced by the material of the stator and rotor laminations?
Core loss
Starting torque
Locked‑rotor current
Rated speed
Explanation - Lamination material determines hysteresis and eddy‑current losses in the core.
Correct answer is: Core loss
Q.129 For a 3‑phase induction motor, the term ‘pole pair’ refers to:
Two magnetic poles (one north, one south)
Four magnetic poles
A pair of stator windings
A pair of rotor bars
Explanation - One pole pair consists of one north and one south pole; total poles = 2 × (pole pairs).
Correct answer is: Two magnetic poles (one north, one south)
Q.130 If the rated line current of a three‑phase induction motor is 8 A at 415 V (star), what is the approximate apparent power rating?
5.7 kVA
6 kVA
8 kVA
10 kVA
Explanation - S = √3·V·I = 1.732×415×8 ≈ 5.7 kVA.
Correct answer is: 5.7 kVA