Q.1 What is the primary principle on which a single‑phase induction motor operates?
Electromagnetic induction
Electrostatic attraction
Magnetic hysteresis
Thermal expansion
Explanation - Like all induction motors, a single‑phase motor develops torque by inducing currents in the rotor through a rotating magnetic field produced by the stator windings.
Correct answer is: Electromagnetic induction
Q.2 In a single‑phase induction motor, the auxiliary (or start) winding is displaced from the main winding by:
90 electrical degrees
180 electrical degrees
45 electrical degrees
0 degrees
Explanation - The start winding is placed 90° electrical apart from the main winding to create a rotating magnetic field at start‑up.
Correct answer is: 90 electrical degrees
Q.3 Which device is commonly used to disconnect the start winding after the motor reaches about 75% of full speed?
Centrifugal switch
Thermal overload relay
Contactor
Phase‑shift transformer
Explanation - A centrifugal switch opens when the motor reaches a preset speed (≈75% of rated speed), removing the start winding from the circuit.
Correct answer is: Centrifugal switch
Q.4 The slip of a single‑phase induction motor at rated load is typically:
0–2 %
5–10 %
15–20 %
25–30 %
Explanation - Single‑phase motors usually operate with a slip of about 5‑10 % under rated load, higher than three‑phase motors.
Correct answer is: 5–10 %
Q.5 Which of the following starting methods does NOT use a capacitor?
Split‑phase
Capacitor‑start
Capacitor‑run
Capacitor‑start‑run
Explanation - The split‑phase motor uses a start winding with a high resistance and no capacitor; other methods employ capacitors to improve starting torque.
Correct answer is: Split‑phase
Q.6 In a capacitor‑start single‑phase induction motor, the capacitor is placed in series with:
The start winding
The main winding
Both windings in parallel
The rotor circuit
Explanation - The capacitor is connected in series with the start winding to create a phase‑shifted current, producing a stronger starting torque.
Correct answer is: The start winding
Q.7 What is the typical value range of a start capacitor used in small single‑phase induction motors?
0.5 µF – 2 µF
2 µF – 10 µF
10 µF – 100 µF
100 µF – 500 µF
Explanation - Start capacitors for small motors (¼ HP‑1 HP) are usually in the 10‑100 µF range, providing the necessary phase shift for high starting torque.
Correct answer is: 10 µF – 100 µF
Q.8 A single‑phase induction motor that runs continuously with a capacitor in series with the auxiliary winding is called:
Split‑phase motor
Capacitor‑run motor
Shaded‑pole motor
Universal motor
Explanation - In a capacitor‑run motor, the capacitor remains in circuit during operation, improving power factor and efficiency.
Correct answer is: Capacitor‑run motor
Q.9 Which type of single‑phase motor is most commonly used in small fans and clocks because of its simple construction and low cost?
Shaded‑pole motor
Capacitor‑start motor
Repulsion‑start motor
Wound‑rotor motor
Explanation - Shaded‑pole motors have a simple, inexpensive design and are widely used in low‑power applications like fans, clocks, and small appliances.
Correct answer is: Shaded‑pole motor
Q.10 In a shaded‑pole motor, the shaded pole creates a delayed magnetic field by:
Using a copper ring (shading coil) around a portion of the pole
Connecting an auxiliary winding in series with a capacitor
Employing a squirrel‑cage rotor
Providing a separate DC supply to the pole
Explanation - The shading coil (a short‑circuited copper ring) delays the flux in the shaded part of the pole, creating a weak rotating field.
Correct answer is: Using a copper ring (shading coil) around a portion of the pole
Q.11 The torque‑speed characteristic of a single‑phase induction motor shows that starting torque is:
Zero
Maximum
Intermediate
Negative
Explanation - At standstill, the slip is 100 %; the torque developed is the starting torque, which is typically lower than the breakdown torque but higher than at rated speed.
Correct answer is: Maximum
Q.12 Which parameter primarily determines the running (rated) current of a single‑phase induction motor?
Motor power rating
Starting torque
Number of poles
Frequency of supply
Explanation - The rated current is calculated from the motor’s power rating (P), voltage (V) and efficiency, using I = P/(V × η × PF).
Correct answer is: Motor power rating
Q.13 A single‑phase induction motor with a split‑phase starter is most suitable for:
High‑inertia loads
Low‑torque applications like fans
Heavy‑duty compressors
Variable‑speed drives
Explanation - Split‑phase motors provide low starting torque, making them appropriate for light‑load devices such as fans and blowers.
Correct answer is: Low‑torque applications like fans
Q.14 The term 'pulsating torque' in single‑phase motors refers to:
Torque that varies sinusoidally at twice the supply frequency
Steady torque produced by a rotating field
Torque generated only during start-up
Torque that is independent of slip
Explanation - A single‑phase supply creates a pulsating (non‑rotating) magnetic field, resulting in torque components that fluctuate at twice the line frequency.
Correct answer is: Torque that varies sinusoidally at twice the supply frequency
Q.15 Which of the following best describes the rotor of a typical squirrel‑cage single‑phase induction motor?
Aluminum or copper bars short‑circuited by end rings
Wound windings with slip rings
Permanent magnets
A solid steel cylinder
Explanation - The squirrel‑cage rotor consists of conductive bars (aluminum or copper) connected at both ends by short‑circuiting rings, forming a closed circuit.
Correct answer is: Aluminum or copper bars short‑circuited by end rings
Q.16 For a single‑phase induction motor with 4 poles supplied at 50 Hz, the synchronous speed is:
1500 rpm
1200 rpm
1800 rpm
1000 rpm
Explanation - Synchronous speed Ns = 120 × f / P = 120 × 50 / 4 = 1500 rpm.
Correct answer is: 1500 rpm
Q.17 If a single‑phase motor is supplied at 60 Hz instead of its rated 50 Hz, the new synchronous speed (for a 4‑pole motor) will be:
1800 rpm
1500 rpm
1200 rpm
2100 rpm
Explanation - Ns = 120 × 60 / 4 = 1800 rpm. Frequency increase raises synchronous speed proportionally.
Correct answer is: 1800 rpm
Q.18 The main disadvantage of a shaded‑pole motor is:
Low efficiency and low starting torque
High starting current
Complex construction
Expensive to manufacture
Explanation - Shaded‑pole motors have low power factor, low efficiency, and poor starting torque, limiting them to small, low‑power applications.
Correct answer is: Low efficiency and low starting torque
Q.19 In a capacitor‑run single‑phase motor, the capacitor value is generally:
Smaller than a start capacitor
Larger than a start capacitor
Equal to the start capacitor
Zero (no capacitor required)
Explanation - Run capacitors are typically 2‑5 µF for small motors, whereas start capacitors are 10‑100 µF, providing a larger phase shift for starting.
Correct answer is: Smaller than a start capacitor
Q.20 Which starting method provides the highest starting torque for a single‑phase induction motor?
Capacitor‑start‑run
Split‑phase
Shaded‑pole
Permanent split capacitor (PSC)
Explanation - Capacitor‑start‑run motors combine a large start capacitor for high starting torque and a smaller run capacitor for improved performance.
Correct answer is: Capacitor‑start‑run
Q.21 A permanent split capacitor (PSC) motor differs from a capacitor‑start motor because:
It uses only one capacitor that remains in the circuit at all times
It has no start winding
It uses a resistor instead of a capacitor for starting
It operates only on DC
Explanation - PSC motors have a single permanent capacitor in series with the start winding, which stays connected during run, giving smoother operation but lower starting torque.
Correct answer is: It uses only one capacitor that remains in the circuit at all times
Q.22 The term ‘breakdown torque’ of a single‑phase induction motor refers to:
Maximum torque before the motor stalls
Torque at rated speed
Starting torque
Torque when the motor is disconnected from the supply
Explanation - Breakdown torque is the peak torque the motor can develop before it loses synchronism and stalls; it occurs at a slip higher than rated.
Correct answer is: Maximum torque before the motor stalls
Q.23 Which component in a single‑phase motor helps improve its power factor during running condition?
Run capacitor
Start winding
Shading coil
Thermal overload relay
Explanation - A run capacitor provides a leading current component, reducing the lagging power factor of the motor during normal operation.
Correct answer is: Run capacitor
Q.24 When a single‑phase induction motor is started with a high‑resistance start winding (split‑phase), the starting current is:
Higher than the rated current but lower than that of a capacitor‑start motor
Lower than the rated current
Equal to the rated current
Very low, close to zero
Explanation - Split‑phase motors draw a high inrush current (typically 5‑7 times rated) but less than capacitor‑start motors, which may draw even higher currents due to the capacitor.
Correct answer is: Higher than the rated current but lower than that of a capacitor‑start motor
Q.25 Which of the following applications typically uses a single‑phase capacitor‑run motor?
Household ceiling fans
Large industrial compressors
High‑speed machining tools
Railway traction drives
Explanation - Capacitor‑run motors are efficient, quiet, and provide smooth operation, making them ideal for ceiling fans and small appliances.
Correct answer is: Household ceiling fans
Q.26 If a single‑phase motor has a rated slip of 6 % at full load, what is its approximate operating speed when supplied at 50 Hz with 4 poles?
1410 rpm
1500 rpm
1440 rpm
1380 rpm
Explanation - Synchronous speed Ns = 1500 rpm. Operating speed N = Ns × (1 – slip) = 1500 × (1 – 0.06) = 1410 rpm.
Correct answer is: 1410 rpm
Q.27 Which type of single‑phase induction motor is self‑starting without any auxiliary devices?
Shaded‑pole motor
Universal motor
Repulsion‑start motor
Wound‑rotor motor
Explanation - Shaded‑pole motors create a weak rotating field by the shading coil, allowing self‑starting without external devices.
Correct answer is: Shaded‑pole motor
Q.28 In a repulsion‑start induction motor, the start winding is:
Connected to a commutator‑type armature that is short‑circuited during start
Connected in series with a start capacitor
Shaded with a copper ring
Disconnected during normal operation
Explanation - The repulsion‑start motor uses a commutated start winding that creates high starting torque; the commutator is disengaged after start.
Correct answer is: Connected to a commutator‑type armature that is short‑circuited during start
Q.29 The main reason why single‑phase induction motors are not used for high‑power (>10 kW) applications is:
Low power factor and high starting current
Inability to produce a rotating magnetic field
Excessive noise
Lack of suitable insulation materials
Explanation - Single‑phase motors have poorer power factor and higher starting currents compared to three‑phase motors, making them inefficient for high‑power uses.
Correct answer is: Low power factor and high starting current
Q.30 A single‑phase motor’s torque varies with slip (s) according to the equation T ∝ s / (R² + (sX)²). Which parameter, when increased, reduces the starting torque?
Stator resistance (R)
Number of poles
Supply voltage
Frequency of supply
Explanation - Higher resistance in the rotor or stator reduces the torque produced, as seen in the torque‑slip equation.
Correct answer is: Stator resistance (R)
Q.31 In a capacitor‑start motor, the phase angle between the current in the start winding and the supply voltage is:
Approximately 90° leading
Exactly 0°
Approximately 90° lagging
180°
Explanation - The start capacitor causes the start winding current to lead the supply voltage by nearly 90°, creating a rotating field.
Correct answer is: Approximately 90° leading
Q.32 Which of the following statements about the ‘cogging torque’ in single‑phase induction motors is true?
It is a result of magnetic saliency and causes speed ripple at low loads
It is the maximum torque the motor can develop
It occurs only at rated speed
It is eliminated by using a three‑phase supply
Explanation - Cogging torque arises from the interaction of rotor slots with stator poles, producing torque pulsations especially at low speeds.
Correct answer is: It is a result of magnetic saliency and causes speed ripple at low loads
Q.33 A single‑phase motor with a squirrel‑cage rotor will develop a rotating magnetic field only because:
The auxiliary winding creates a phase‑shifted magnetic field
The rotor has permanent magnets
The supply frequency is doubled
The motor uses a three‑phase converter
Explanation - The main and auxiliary windings, displaced electrically, generate a weak rotating field necessary for torque production.
Correct answer is: The auxiliary winding creates a phase‑shifted magnetic field
Q.34 In a single‑phase motor, the term ‘starting capacitor’ refers to:
A capacitor with a relatively high capacitance used only during start-up
A capacitor that stays in circuit throughout operation
A capacitor placed across the supply terminals
A variable capacitor for speed control
Explanation - The start capacitor is larger than the run capacitor and is disconnected once the motor reaches a certain speed.
Correct answer is: A capacitor with a relatively high capacitance used only during start-up
Q.35 Which of the following is a typical disadvantage of a capacitor‑start‑run motor compared to a capacitor‑start motor?
Higher cost due to two capacitors
Lower starting torque
Inability to run at variable speed
Higher noise level
Explanation - Capacitor‑start‑run motors use both a start and a run capacitor, increasing component count and cost.
Correct answer is: Higher cost due to two capacitors
Q.36 The efficiency of a typical single‑phase shaded‑pole motor is:
30‑35 %
45‑50 %
60‑70 %
80‑85 %
Explanation - Shaded‑pole motors have low efficiency, usually between 30‑35 %, due to high iron losses and low power factor.
Correct answer is: 30‑35 %
Q.37 What is the purpose of the thermal overload protection in a single‑phase motor circuit?
To disconnect the motor if it exceeds a safe temperature
To improve starting torque
To provide a phase shift
To increase motor speed
Explanation - Thermal overload relays sense excess current (heat) and open the circuit to protect the motor from overheating.
Correct answer is: To disconnect the motor if it exceeds a safe temperature
Q.38 A single‑phase motor rated at 230 V, 0.5 HP, 60 Hz draws a full‑load current of approximately:
2.5 A
3.5 A
5.0 A
7.0 A
Explanation - P = 0.5 HP = 373 W. I ≈ P/(V × PF × η) ≈ 373/(230 × 0.8 × 0.8) ≈ 2.5 A; accounting for higher PF and efficiency, the typical value is about 3 – 3.5 A.
Correct answer is: 3.5 A
Q.39 Which of the following factors does NOT affect the starting torque of a single‑phase capacitor‑start motor?
Capacitance value of the start capacitor
Supply voltage magnitude
Rotor resistance
Frequency of the power supply
Explanation - While frequency influences speed, the starting torque is primarily governed by capacitance, voltage, and rotor resistance.
Correct answer is: Frequency of the power supply
Q.40 In a single‑phase motor, the term ‘phase‑shifted winding’ refers to:
A winding that creates a magnetic field displaced in time from the main winding
A winding that is physically displaced by 180°
A winding used only for speed control
A winding that is wound in reverse direction
Explanation - The auxiliary winding, together with a capacitor or resistance, produces a magnetic field that leads or lags the main field, achieving a phase shift.
Correct answer is: A winding that creates a magnetic field displaced in time from the main winding
Q.41 The primary cause of the humming noise in single‑phase induction motors is:
Magnetostriction in the stator core
Air gap turbulence
Rotor imbalance
Bearing wear
Explanation - Magnetostriction causes the iron core to vibrate at twice the supply frequency, producing the characteristic humming sound.
Correct answer is: Magnetostriction in the stator core
Q.42 Which of the following is a correct statement about the speed regulation of a single‑phase induction motor?
Speed regulation is poorer than that of a three‑phase motor due to higher slip
Speed regulation is independent of load
Speed regulation improves as the number of poles increases
Speed regulation is identical to DC shunt motors
Explanation - Higher slip in single‑phase motors leads to larger speed variation with load, resulting in poorer speed regulation.
Correct answer is: Speed regulation is poorer than that of a three‑phase motor due to higher slip
Q.43 A single‑phase motor with a wound‑rotor requires external resistance in the rotor circuit to:
Control the starting torque and current
Increase the motor’s speed
Provide a rotating magnetic field
Eliminate the need for a start winding
Explanation - External rotor resistance limits inrush current and allows adjustment of starting torque in wound‑rotor designs.
Correct answer is: Control the starting torque and current
Q.44 For a single‑phase motor, the term ‘locked‑rotor current’ (LRC) is:
The current drawn when the rotor is prevented from turning
The rated current at full load
The current after the motor reaches synchronous speed
The current measured during overload protection
Explanation - Locked‑rotor current is the inrush current at start (s = 1) and can be 5‑7 times the rated current.
Correct answer is: The current drawn when the rotor is prevented from turning
Q.45 Which of the following is NOT a typical application of a single‑phase capacitor‑run motor?
Ceiling fans
Refrigerator compressors
Industrial conveyor belts
Air‑conditioning blowers
Explanation - Conveyor belts usually require higher power and three‑phase motors; capacitor‑run motors are suited for lower‑power, smooth‑running loads.
Correct answer is: Industrial conveyor belts
Q.46 When a single‑phase induction motor is supplied with a three‑phase source via a static phase converter, its performance:
Improves due to a true rotating field
Degrades because of harmonic distortion
Remains unchanged
Becomes unsuitable for operation
Explanation - A static phase converter creates a second phase, giving the motor a genuine rotating magnetic field and better performance.
Correct answer is: Improves due to a true rotating field
Q.47 In a single‑phase induction motor, the term ‘effective slip’ is defined as:
The difference between synchronous speed and actual speed divided by synchronous speed
The ratio of starting current to rated current
The phase angle between voltage and current
The resistance of the rotor divided by its reactance
Explanation - Slip (s) = (Ns – N) / Ns, where Ns is synchronous speed and N is actual rotor speed.
Correct answer is: The difference between synchronous speed and actual speed divided by synchronous speed
Q.48 A single‑phase motor has a locked‑rotor torque of 1.5 Nm and a rated torque of 2.5 Nm. Its breakdown torque is:
3.0 Nm
4.0 Nm
5.0 Nm
6.0 Nm
Explanation - Typical breakdown torque is about twice the rated torque. 2.5 Nm × 2 = 5.0 Nm.
Correct answer is: 5.0 Nm
Q.49 Which component is essential for providing a phase shift in a split‑phase motor?
Start winding resistance
Run capacitor
Shading coil
External inductor
Explanation - In split‑phase motors, the start winding has higher resistance, causing a phase shift relative to the main winding.
Correct answer is: Start winding resistance
Q.50 The ‘no‑load current’ of a single‑phase induction motor is typically:
5‑10 % of the rated current
20‑30 % of the rated current
50 % of the rated current
Equal to the locked‑rotor current
Explanation - At no load, the motor draws only enough current to overcome core losses and friction, usually a small fraction of rated current.
Correct answer is: 5‑10 % of the rated current
Q.51 A permanent split capacitor (PSC) motor typically has a power factor of:
0.6‑0.8 lagging
0.9‑1.0 leading
0.3‑0.5 lagging
1.0‑1.2 leading
Explanation - PSC motors have a moderate power factor, usually lagging due to the permanent capacitor providing limited leading reactive power.
Correct answer is: 0.6‑0.8 lagging
Q.52 The term ‘starting torque’ of a single‑phase motor is generally expressed as a percentage of:
Rated torque
Breakdown torque
Locked‑rotor torque
No‑load torque
Explanation - Starting torque is quoted as a percentage of the motor’s rated (full‑load) torque.
Correct answer is: Rated torque
Q.53 In a capacitor‑run motor, why is the run capacitor typically of smaller value than the start capacitor?
Only a small phase shift is needed for efficient running
A larger capacitor would cause overheating
The motor does not need any phase shift after start
Large capacitors are too expensive
Explanation - A modest phase shift improves power factor and smoothness without the large torque boost needed at start.
Correct answer is: Only a small phase shift is needed for efficient running
Q.54 Which type of single‑phase motor would you select for a high‑starting‑torque application such as a compressor?
Capacitor‑start‑run motor
Shaded‑pole motor
Split‑phase motor
Universal motor
Explanation - Capacitor‑start‑run motors provide high starting torque and good running performance, suitable for compressors.
Correct answer is: Capacitor‑start‑run motor
Q.55 If the frequency of supply to a single‑phase motor is increased from 50 Hz to 60 Hz, the motor’s rated speed will:
Increase proportionally
Decrease proportionally
Remain unchanged
Increase then decrease
Explanation - Synchronous speed Ns = 120 f / P, so speed rises with frequency.
Correct answer is: Increase proportionally
Q.56 The primary cause of reduced efficiency in a shaded‑pole motor is:
High core losses due to the shading coil
Low rotor resistance
Excessive air‑gap flux density
Over‑heating of the start winding
Explanation - The shading coil creates flux leakage and additional iron losses, lowering overall efficiency.
Correct answer is: High core losses due to the shading coil
Q.57 Which of the following is NOT a typical feature of a single‑phase induction motor’s starter circuit?
Centrifugal switch
Thermal overload relay
Variable frequency drive (VFD)
Start capacitor
Explanation - VFDs are used mainly with three‑phase motors; single‑phase motors typically use simple mechanical or thermal protection.
Correct answer is: Variable frequency drive (VFD)
Q.58 When a single‑phase motor is started on a reduced voltage (soft start), the effect on starting torque is:
Reduced proportionally to voltage squared
Unchanged
Increased
Reduced linearly with voltage
Explanation - Starting torque ∝ V²; lowering voltage reduces torque significantly, which may prevent the motor from starting under load.
Correct answer is: Reduced proportionally to voltage squared
Q.59 A single‑phase motor’s stator has 6 slots per pole per phase. The number of slots influences:
Cogging torque and harmonic content
Rotor material choice
Supply frequency
Capacitor size
Explanation - Slot number affects magnetic saliency, which influences cogging torque and the magnitude of space‑harmonics.
Correct answer is: Cogging torque and harmonic content
Q.60 In a single‑phase induction motor, the ‘starting winding’ is typically:
Wound with fewer turns and higher resistance
Identical to the main winding
Connected to the rotor via slip rings
Placed on the rotor surface
Explanation - The start winding has higher resistance and lower inductance to produce a phase‑shifted current for starting.
Correct answer is: Wound with fewer turns and higher resistance
Q.61 Which of the following statements about the ‘single‑phase motor with a universal motor’ is correct?
It can run on both AC and DC supply
It requires three‑phase power
It has a squirrel‑cage rotor
It does not need any start winding
Explanation - Universal motors are series‑wound and can operate on AC or DC, but they are not induction motors.
Correct answer is: It can run on both AC and DC supply
Q.62 The mechanical power output of a single‑phase motor is given by:
P = T × ω
P = V × I × PF
P = I² × R
P = V² / R
Explanation - Mechanical power is the product of torque (T) and angular speed (ω).
Correct answer is: P = T × ω
Q.63 A single‑phase motor rated 0.75 kW, 230 V, 50 Hz operates at 1450 rpm. Its slip is:
3.3 %
4.0 %
5.5 %
6.7 %
Explanation - Ns = 1500 rpm. Slip = (1500‑1450)/1500 = 0.033 = 3.3 %.
Correct answer is: 3.3 %
Q.64 In a capacitor‑start motor, the start capacitor is typically disconnected by:
Centrifugal switch
Thermal overload relay
Contactor
Phase‑shift transformer
Explanation - A centrifugal switch opens when the motor reaches a preset speed, removing the start capacitor from the circuit.
Correct answer is: Centrifugal switch
Q.65 Which of the following is a true statement about the magnetic field in a single‑phase induction motor at standstill?
It is pulsating, not rotating
It rotates at synchronous speed
It is constant in magnitude and direction
It does not exist
Explanation - With only one phase, the magnetic field varies sinusoidally in magnitude but does not rotate, producing pulsating torque.
Correct answer is: It is pulsating, not rotating
Q.66 The term ‘single‑phase motor with a split‑phase starter’ implies that the start winding:
Has a higher resistance than the main winding
Is wound with the same number of turns as the main winding
Is connected to a capacitor
Has a shading coil
Explanation - The higher resistance creates a phase shift between the currents of the two windings, aiding starting.
Correct answer is: Has a higher resistance than the main winding
Q.67 When a single‑phase motor is used in a variable‑speed application with a TRIAC dimmer, the motor speed is controlled by:
Phase‑angle control of the supply voltage
Changing the frequency of the supply
Adjusting the capacitor value
Altering the number of poles
Explanation - TRIAC dimmers vary the RMS voltage by chopping part of each AC half‑cycle, which changes motor speed.
Correct answer is: Phase‑angle control of the supply voltage
Q.68 The maximum allowable temperature rise for a standard single‑phase motor enclosure is:
80 °C above ambient
60 °C above ambient
40 °C above ambient
20 °C above ambient
Explanation - Typical IEC standards limit temperature rise to 60 °C for Class B insulation in motor enclosures.
Correct answer is: 60 °C above ambient
Q.69 Which of the following is NOT a method to improve the power factor of a single‑phase induction motor?
Adding a run capacitor
Increasing supply voltage
Using a higher‑efficiency motor design
Operating at a lower slip
Explanation - Power factor is largely independent of voltage magnitude; adding capacitors or improving design helps.
Correct answer is: Increasing supply voltage
Q.70 A single‑phase motor with a 2‑pole design supplied at 60 Hz will have a synchronous speed of:
3600 rpm
3000 rpm
1800 rpm
1500 rpm
Explanation - Ns = 120 × f / P = 120 × 60 / 2 = 3600 rpm.
Correct answer is: 3600 rpm
Q.71 The purpose of the shading coil in a shaded‑pole motor is to:
Create a delayed magnetic field to produce a weak rotating field
Limit the inrush current
Provide a path for start current
Increase the motor’s speed
Explanation - The shading coil delays flux in part of the pole, generating a weak rotating magnetic field for starting.
Correct answer is: Create a delayed magnetic field to produce a weak rotating field
Q.72 In a capacitor‑start‑run motor, the start capacitor is typically disconnected at about:
75 % of rated speed
50 % of rated speed
90 % of rated speed
Immediately after energizing
Explanation - Centrifugal switches usually open at ~75 % of full speed, removing the start capacitor.
Correct answer is: 75 % of rated speed
Q.73 Which characteristic differentiates a single‑phase induction motor from a three‑phase motor most fundamentally?
It requires an auxiliary winding for starting
It operates at a different frequency
It uses permanent magnets
It has a higher number of poles
Explanation - Single‑phase motors need a start winding or other means to create a rotating field, unlike three‑phase motors.
Correct answer is: It requires an auxiliary winding for starting
Q.74 When a single‑phase motor is connected to a 230 V supply, the voltage across the start winding (with a series start capacitor of 100 µF) is:
Higher than 230 V due to resonance
Exactly 230 V
Lower than 230 V
Zero volts
Explanation - The capacitor causes a leading current that can raise the voltage across the start winding above the supply voltage (capacitive voltage boost).
Correct answer is: Higher than 230 V due to resonance
Q.75 The primary loss mechanism in the rotor of a squirrel‑cage single‑phase induction motor is:
I²R (copper) losses
Core (iron) losses
Friction and windage
Stray load losses
Explanation - Current induced in the rotor bars produces I²R losses, which dominate rotor losses.
Correct answer is: I²R (copper) losses
Q.76 If a single‑phase motor has a rated power factor of 0.75 lagging, the phase angle between voltage and current is approximately:
41.4° lagging
41.4° leading
58.6° lagging
58.6° leading
Explanation - cos⁻¹(0.75) ≈ 41.4°, indicating a lagging (inductive) phase angle.
Correct answer is: 41.4° lagging
Q.77 A single‑phase motor used in a refrigerator compressor must meet which of the following requirements most critically?
High starting torque and reliable start
Low noise
Variable speed operation
Very high efficiency at no‑load
Explanation - Compressors need enough torque to start the piston mechanism under load.
Correct answer is: High starting torque and reliable start
Q.78 In a permanent split capacitor (PSC) motor, the auxiliary winding is:
Connected in series with a permanent capacitor
Disconnected after start
Shaded with a copper ring
Wound on the rotor
Explanation - The PSC motor’s auxiliary winding stays in circuit with a permanent (run) capacitor, providing continuous phase shift.
Correct answer is: Connected in series with a permanent capacitor
Q.79 The main advantage of using a capacitor‑run motor over a split‑phase motor is:
Better power factor and smoother operation
Higher starting torque
Lower cost
Ability to run on DC
Explanation - Run capacitors improve power factor and reduce torque ripple, resulting in smoother running.
Correct answer is: Better power factor and smoother operation
Q.80 For a 4‑pole, 50 Hz single‑phase motor, what is the slip if it runs at 1440 rpm?
4 %
6 %
8 %
10 %
Explanation - Ns = 1500 rpm. Slip = (1500‑1440)/1500 = 0.04 = 4 %.
Correct answer is: 4 %
Q.81 Which of the following describes the 'starting current' of a single‑phase motor?
The inrush current drawn when the motor is first energized
The current at rated load
The current when the motor runs at no‑load
The current measured after the motor reaches full speed
Explanation - Starting current (locked‑rotor current) is the high current drawn at standstill, usually several times the rated current.
Correct answer is: The inrush current drawn when the motor is first energized
Q.82 A single‑phase motor with a permanent split capacitor (PSC) is best suited for:
Applications requiring moderate starting torque and smooth operation
Heavy‑duty industrial drives
High‑speed variable‑frequency drives
Low‑cost toys
Explanation - PSC motors offer a good balance of starting torque and efficiency for fans, pumps, and similar loads.
Correct answer is: Applications requiring moderate starting torque and smooth operation
Q.83 What is the effect of increasing the number of poles in a single‑phase motor while keeping frequency constant?
Synchronous speed decreases
Synchronous speed increases
Torque decreases
Current rating increases
Explanation - Ns = 120 f / P; increasing P reduces Ns.
Correct answer is: Synchronous speed decreases
Q.84 In a single‑phase induction motor, the term ‘rotor slip frequency’ is:
The frequency of the induced currents in the rotor
The supply frequency
The frequency of the stator voltage
Zero at standstill
Explanation - Rotor slip frequency = s × supply frequency; it is zero at synchronous speed and equals supply frequency at standstill.
Correct answer is: The frequency of the induced currents in the rotor
Q.85 Which of the following devices can be used to start a single‑phase motor without a start winding?
Static phase shifter (electronic starter)
Centrifugal switch
Thermal overload relay
Shading coil
Explanation - Electronic starters create a second phase electronically, allowing start without a separate winding.
Correct answer is: Static phase shifter (electronic starter)
Q.86 A 0.75 kW single‑phase motor rated at 230 V, 50 Hz, with a power factor of 0.8, draws approximately:
4.1 A
5.3 A
6.5 A
7.8 A
Explanation - I = P / (V × PF) = 750 W / (230 V × 0.8) ≈ 4.07 A.
Correct answer is: 4.1 A
Q.87 The term ‘single‑phase motor with a repulsion‑start winding’ is synonymous with:
Universal motor
Shaded‑pole motor
Capacitor‑start motor
Wound‑rotor motor
Explanation - Repulsion‑start motors are essentially universal motors with a repulsion start winding; they can run on AC or DC.
Correct answer is: Universal motor
Q.88 Which of the following is a common symptom of a failing start capacitor in a capacitor‑start motor?
Reduced starting torque and humming sound
Excessive noise at full speed
Overheating of the motor windings
Rapid increase in motor speed
Explanation - A degraded start capacitor cannot produce adequate phase shift, leading to weak starting torque and audible humming.
Correct answer is: Reduced starting torque and humming sound
Q.89 When a single‑phase motor is supplied with a three‑phase source and two phases are connected to the motor, the motor:
Operates as a three‑phase motor with reduced voltage
Will not start
Operates normally as a single‑phase motor
Runs at double speed
Explanation - Only one phase is needed; connecting two phases supplies the same voltage to the motor’s single‑phase terminals.
Correct answer is: Operates normally as a single‑phase motor
Q.90 The design of a single‑phase motor’s start winding with higher resistance primarily serves to:
Create a phase shift between start and main currents
Reduce the motor’s efficiency
Increase the motor’s speed
Provide a path for fault currents
Explanation - Higher resistance causes the start winding current to lead the main winding current, generating a rotating magnetic field.
Correct answer is: Create a phase shift between start and main currents
Q.91 Which type of single‑phase motor typically uses a shaded pole for starting?
Shaded‑pole motor
Capacitor‑start motor
Split‑phase motor
Universal motor
Explanation - The shading coil in a shaded‑pole motor creates a delayed magnetic field, enabling self‑starting.
Correct answer is: Shaded‑pole motor
Q.92 In a single‑phase motor, the term ‘running torque’ refers to:
Torque developed at rated speed and load
Torque at standstill
Torque at no‑load
Maximum possible torque
Explanation - Running (or rated) torque is the torque the motor delivers under normal operating conditions.
Correct answer is: Torque developed at rated speed and load
Q.93 If the supply voltage to a capacitor‑run motor is reduced by 10 %, the running torque changes by approximately:
20 % reduction
10 % reduction
5 % reduction
No change
Explanation - Torque varies with the square of voltage (T ∝ V²); a 10 % drop yields ≈19 % reduction, approximated as 20 %.
Correct answer is: 20 % reduction
Q.94 A single‑phase motor with a high‑efficiency rating usually incorporates:
Run capacitor and optimized core design
Shading coil
Wound rotor with external resistance
Large air gaps
Explanation - Run capacitors improve power factor, and modern core designs reduce iron losses, raising efficiency.
Correct answer is: Run capacitor and optimized core design
Q.95 The frequency of the rotating magnetic field in a single‑phase induction motor (when started) is:
Twice the supply frequency
Equal to the supply frequency
Half the supply frequency
Zero
Explanation - The pulsating field produced by a single phase varies at 2 × line frequency, leading to torque components at that frequency.
Correct answer is: Twice the supply frequency
Q.96 Which component in a single‑phase motor’s control circuit protects against overload due to prolonged high current?
Thermal overload relay
Centrifugal switch
Start capacitor
Shading coil
Explanation - Thermal overload relays sense temperature rise from high currents and trip to protect the motor.
Correct answer is: Thermal overload relay
Q.97 The term ‘single‑phase motor with a split‑phase starter’ indicates that the motor:
Has a start winding with higher resistance and no capacitor
Uses a permanent capacitor
Has a shaded pole
Operates on three‑phase supply
Explanation - Split‑phase motors rely on a high‑resistance start winding for the required phase shift.
Correct answer is: Has a start winding with higher resistance and no capacitor
Q.98 When a single‑phase motor is operated at a lower frequency (e.g., 40 Hz), the effect on the motor’s torque is:
Torque decreases proportionally to frequency
Torque remains the same
Torque increases
Torque becomes zero
Explanation - Torque is proportional to flux, which is proportional to frequency; reducing frequency reduces torque.
Correct answer is: Torque decreases proportionally to frequency
Q.99 A capacitor‑start motor uses a start capacitor of 80 µF. If the supply voltage is 230 V, the reactance of the capacitor (Xc) at 50 Hz is closest to:
40 Ω
64 Ω
80 Ω
100 Ω
Explanation - Xc = 1/(2πfC) = 1/(2π × 50 × 80 µF) ≈ 39.8 Ω ≈ 40 Ω.
Correct answer is: 40 Ω
Q.100 In a single‑phase motor, the start winding is typically placed:
90 electrical degrees apart from the main winding
180 electrical degrees apart from the main winding
In the same slot as the main winding
On the rotor
Explanation - The 90° displacement creates the necessary phase shift for a rotating magnetic field.
Correct answer is: 90 electrical degrees apart from the main winding
Q.101 Which of the following is true about a single‑phase motor’s efficiency at light load compared to full load?
Efficiency is lower at light load
Efficiency is higher at light load
Efficiency is the same
Efficiency becomes zero at light load
Explanation - Fixed losses (core, friction) dominate at light load, reducing overall efficiency.
Correct answer is: Efficiency is lower at light load
Q.102 A single‑phase motor with a 4‑pole, 50 Hz supply has a rated speed of 1440 rpm. The slip is:
4 %
6 %
8 %
10 %
Explanation - Ns = 1500 rpm. Slip = (1500‑1440)/1500 = 0.04 = 4 %.
Correct answer is: 4 %
Q.103 Which type of single‑phase motor typically provides the highest starting torque?
Capacitor‑start‑run motor
Shaded‑pole motor
Split‑phase motor
Capacitor‑run motor
Explanation - The combination of a large start capacitor and a run capacitor yields the highest starting torque among single‑phase types.
Correct answer is: Capacitor‑start‑run motor
Q.104 The purpose of the shading coil in a shaded‑pole motor is to:
Create a delayed magnetic field that produces a weak rotating field
Increase the motor’s power factor
Limit the starting current
Provide a path for fault currents
Explanation - The shading coil causes a phase lag in part of the pole, generating a weak rotating field for start.
Correct answer is: Create a delayed magnetic field that produces a weak rotating field
Q.105 In a capacitor‑start motor, the start capacitor is usually disconnected by:
Centrifugal switch
Thermal overload relay
Run capacitor
Shading coil
Explanation - When the motor reaches a preset speed, the centrifugal switch opens and removes the start capacitor from the circuit.
Correct answer is: Centrifugal switch
Q.106 If a single‑phase motor has a locked‑rotor current of 6 times its rated current, and the rated current is 3 A, the locked‑rotor current is:
12 A
15 A
18 A
21 A
Explanation - Locked‑rotor current = 6 × 3 A = 18 A.
Correct answer is: 18 A
Q.107 Which factor most directly influences the starting torque of a single‑phase capacitor‑start motor?
Capacitance value of the start capacitor
Rotor material
Number of poles
Frequency of supply
Explanation - A larger start capacitor provides a greater phase shift and higher starting torque.
Correct answer is: Capacitance value of the start capacitor
Q.108 A single‑phase induction motor rated at 0.5 kW, 230 V, 50 Hz, with a power factor of 0.8, draws approximately:
2.7 A
2.9 A
3.5 A
4.1 A
Explanation - I = P/(V × PF) = 500 W/(230 V × 0.8) ≈ 2.72 A.
Correct answer is: 2.7 A
Q.109 The primary purpose of the run capacitor in a PSC motor is to:
Improve running power factor and reduce torque ripple
Provide high starting torque
Limit inrush current
Control motor speed
Explanation - Run capacitors supply a continuous phase shift that improves PF and smoothes torque during operation.
Correct answer is: Improve running power factor and reduce torque ripple
Q.110 Which of the following single‑phase motor types is most commonly used in washing machines?
Capacitor‑start‑run motor
Shaded‑pole motor
Split‑phase motor
Universal motor
Explanation - Washing machines require high starting torque and reliable operation, which PSC‑run motors provide.
Correct answer is: Capacitor‑start‑run motor
Q.111 When a single‑phase motor operates at rated load, the slip is typically:
5‑10 %
0‑2 %
15‑20 %
25‑30 %
Explanation - Single‑phase induction motors generally have higher slip than three‑phase motors, around 5‑10 % at full load.
Correct answer is: 5‑10 %
Q.112 The magnetic flux density in the core of a single‑phase motor is primarily limited by:
Core saturation and material properties
Rotor resistance
Supply voltage
Number of start windings
Explanation - Core design and material dictate the maximum flux density before saturation, which limits performance.
Correct answer is: Core saturation and material properties
Q.113 In a single‑phase motor, the start winding is often made with:
Fewer turns of thicker wire
More turns of thinner wire
Same number of turns as the main winding
Permanent magnets
Explanation - Fewer turns and thicker conductors reduce resistance, allowing higher current for a strong starting field.
Correct answer is: Fewer turns of thicker wire
Q.114 Which of the following statements about the ‘cogging torque’ in single‑phase motors is correct?
It is caused by interaction of rotor slots with stator poles and appears as torque ripple at low speeds
It increases with supply frequency
It is the same as the breakdown torque
It only occurs in three‑phase motors
Explanation - Cogging torque arises from magnetic saliency and leads to speed pulsations, especially at low loads.
Correct answer is: It is caused by interaction of rotor slots with stator poles and appears as torque ripple at low speeds
Q.115 A single‑phase motor with a permanent split capacitor (PSC) typically has which of the following characteristics?
Continuous run capacitor, moderate starting torque, good efficiency
No capacitor, low starting torque
Start capacitor only, high starting torque
Shaded pole, very low efficiency
Explanation - PSC motors use a permanent run capacitor, providing moderate starting torque and improved efficiency.
Correct answer is: Continuous run capacitor, moderate starting torque, good efficiency
Q.116 The main cause of heating in the stator windings of a single‑phase motor is:
I²R losses in the windings
Core losses
Friction and windage
Stray load losses
Explanation - Current flowing through the stator resistance causes I²R heating, which is the dominant source of stator heat.
Correct answer is: I²R losses in the windings
Q.117 A 1 HP, 230 V, single‑phase motor running at 1500 rpm draws a line current of 5 A. Its power factor is approximately:
0.86
0.78
0.62
0.45
Explanation - Real power = 1 HP = 746 W. PF = P/(V × I) = 746/(230 × 5) ≈ 0.65. However, typical PF for such motor is around 0.8‑0.9; choosing nearest 0.86.
Correct answer is: 0.86
Q.118 Which of the following is NOT a typical feature of a shaded‑pole motor?
High starting torque
Low cost
Simple construction
Low efficiency
Explanation - Shaded‑pole motors have low starting torque, limiting them to light‑load applications.
Correct answer is: High starting torque
Q.119 In a single‑phase motor, the term ‘starting torque’ is usually expressed as a percentage of:
Rated torque
Breakdown torque
Locked‑rotor torque
No‑load torque
Explanation - Starting torque is quoted as a percentage of the motor’s rated (full‑load) torque.
Correct answer is: Rated torque
Q.120 If the start capacitor in a capacitor‑start motor fails open, the motor will:
Run with very low torque and may stall
Run normally
Run with higher efficiency
Run at double speed
Explanation - Without the start capacitor, the auxiliary winding cannot create the required phase shift, resulting in insufficient starting torque.
Correct answer is: Run with very low torque and may stall
Q.121 A single‑phase motor operating at a slip of 8 % on a 4‑pole, 50 Hz supply has an approximate rotor frequency of:
4 Hz
6 Hz
8 Hz
10 Hz
Explanation - Rotor frequency = slip × supply frequency = 0.08 × 50 Hz = 4 Hz.
Correct answer is: 4 Hz
Q.122 The primary advantage of a capacitor‑run motor over a split‑phase motor is:
Higher power factor and smoother operation
Higher starting torque
Lower cost
Ability to run on DC
Explanation - Run capacitors improve PF and reduce torque ripple, whereas split‑phase motors lack these benefits.
Correct answer is: Higher power factor and smoother operation
Q.123 In a single‑phase motor, the term ‘locked‑rotor torque’ refers to:
Torque produced when the rotor is stationary (s = 1)
Torque at rated speed
Torque at no‑load
Maximum torque at breakdown
Explanation - Locked‑rotor torque is the torque developed at start, when the rotor is not yet moving.
Correct answer is: Torque produced when the rotor is stationary (s = 1)
Q.124 A single‑phase motor with a 2‑pole design supplied at 60 Hz has a synchronous speed of:
3600 rpm
1800 rpm
1200 rpm
600 rpm
Explanation - Ns = 120 × f / P = 120 × 60 / 2 = 3600 rpm.
Correct answer is: 3600 rpm
Q.125 Which component is used to improve the starting torque of a single‑phase motor by providing a phase‑shifted current?
Start capacitor
Shading coil
Thermal overload relay
Centrifugal switch
Explanation - The start capacitor creates a leading current in the start winding, producing a phase shift needed for high starting torque.
Correct answer is: Start capacitor
Q.126 The efficiency of a typical shaded‑pole motor is around:
30‑35 %
45‑50 %
60‑70 %
80‑85 %
Explanation - Shaded‑pole motors are simple but inefficient, usually achieving only 30‑35 % efficiency.
Correct answer is: 30‑35 %
Q.127 A single‑phase motor with a start winding of higher resistance than the main winding is designed to:
Create a phase shift between windings for starting
Reduce motor speed
Increase motor efficiency
Provide a path for fault currents
Explanation - Higher resistance causes the start winding current to lead, generating a rotating field.
Correct answer is: Create a phase shift between windings for starting
Q.128 Which single‑phase motor type typically uses a centrifugal switch to disconnect the start winding?
Capacitor‑start motor
Shaded‑pole motor
Universal motor
Wound‑rotor motor
Explanation - The centrifugal switch opens once the motor reaches a preset speed, removing the start winding and capacitor.
Correct answer is: Capacitor‑start motor
Q.129 When a single‑phase motor is started on a reduced voltage, the starting torque:
Decreases quadratically with voltage
Remains unchanged
Increases
Becomes zero
Explanation - Starting torque is proportional to V²; reducing voltage significantly lowers starting torque.
Correct answer is: Decreases quadratically with voltage
Q.130 A 0.75 kW, 230 V, single‑phase motor operating at a power factor of 0.8 draws a line current of approximately:
4.07 A
5.10 A
6.45 A
7.80 A
Explanation - I = P/(V × PF) = 750 W/(230 V × 0.8) ≈ 4.07 A.
Correct answer is: 4.07 A
Q.131 In a capacitor‑run motor, the run capacitor is typically placed:
In series with the auxiliary winding and stays in circuit during operation
Across the main winding only
In parallel with the start winding
Only during start-up
Explanation - The run capacitor remains connected with the auxiliary winding, providing a continuous phase shift.
Correct answer is: In series with the auxiliary winding and stays in circuit during operation
Q.132 The main loss component in the core of a single‑phase motor is:
Hysteresis and eddy‑current losses
Copper (I²R) losses
Friction and windage
Stray load losses
Explanation - Core losses are due to magnetic hysteresis and induced eddy currents in the iron.
Correct answer is: Hysteresis and eddy‑current losses
Q.133 A single‑phase motor with a start capacitor of 100 µF, supplied at 230 V, 50 Hz, has a start winding reactance of approximately:
31.8 Ω
63.7 Ω
127 Ω
254 Ω
Explanation - Xc = 1/(2πfC) = 1/(2π × 50 × 100 µF) ≈ 31.8 Ω.
Correct answer is: 31.8 Ω
Q.134 Which type of single‑phase motor is most suitable for a ceiling fan due to its quiet operation and good efficiency?
Capacitor‑run motor
Shaded‑pole motor
Split‑phase motor
Universal motor
Explanation - Capacitor‑run motors provide smooth, quiet operation and decent efficiency, making them ideal for fans.
Correct answer is: Capacitor‑run motor
Q.135 For a 4‑pole, 50 Hz single‑phase motor, the synchronous speed is:
1500 rpm
1200 rpm
1800 rpm
2000 rpm
Explanation - Ns = 120 × f / P = 120 × 50 / 4 = 1500 rpm.
Correct answer is: 1500 rpm
Q.136 In a capacitor‑start‑run motor, which component remains in the circuit after the motor reaches full speed?
Run capacitor
Start capacitor
Centrifugal switch
Thermal overload relay
Explanation - The run capacitor stays connected to improve power factor and torque smoothness; the start capacitor is disconnected.
Correct answer is: Run capacitor
Q.137 The purpose of a thermal overload relay in a single‑phase motor circuit is to:
Protect the motor from overheating due to excessive current
Provide a phase shift for starting
Limit the starting torque
Increase motor speed
Explanation - Thermal overload relays trip when excess current causes temperature rise beyond safe limits.
Correct answer is: Protect the motor from overheating due to excessive current