Q.1 What is the main purpose of the field winding in a synchronous motor?
To produce torque directly
To provide a rotating magnetic field
To supply the armature current
To reduce iron losses
Explanation - The field winding, when supplied with DC, creates a constant magnetic field that interacts with the rotating armature field, producing the rotating magnetic field required for synchronous operation.
Correct answer is: To provide a rotating magnetic field
Q.2 In a salient‑pole synchronous generator, which reactance is larger?
Direct‑axis reactance (Xd)
Quadrature‑axis reactance (Xq)
Synchronous reactance (Xs)
Armature resistance (Ra)
Explanation - Salient‑pole machines have a larger magnetic reluctance along the direct axis, making Xd > Xq.
Correct answer is: Direct‑axis reactance (Xd)
Q.3 The synchronous speed of a 4‑pole machine operating at 60 Hz is:
1800 rpm
1500 rpm
1200 rpm
900 rpm
Explanation - Ns = 120f / P = 120×60 / 4 = 1800 rpm.
Correct answer is: 1800 rpm
Q.4 Which of the following statements about a synchronous motor is FALSE?
It can operate at leading power factor.
It requires external excitation.
It can run at any speed lower than synchronous speed.
It has a constant speed for a given supply frequency.
Explanation - A synchronous motor runs only at synchronous speed; it cannot operate at a speed lower than synchronous speed without slipping, which does not occur in a true synchronous motor.
Correct answer is: It can run at any speed lower than synchronous speed.
Q.5 In a synchronous generator, what is the effect of increasing the field current while keeping the load constant?
Terminal voltage decreases
Terminal voltage remains unchanged
Terminal voltage increases
Armature current increases
Explanation - Increasing field current raises the internal generated emf (E), which raises the terminal voltage for a given load.
Correct answer is: Terminal voltage increases
Q.6 Which component is used to control the voltage of a synchronous generator?
Armature resistance
Load tap changer
Exciter
Stator winding
Explanation - The exciter provides the DC field current; varying it changes the internal emf and thus the terminal voltage.
Correct answer is: Exciter
Q.7 The power angle δ in a synchronous machine represents:
The angle between the stator and rotor mechanical axes
The phase difference between terminal voltage and induced emf
The lag between current and voltage in the armature
The slip of the machine
Explanation - δ is the angle between the internal generated voltage (E) and the terminal voltage (V). It determines the real power transferred.
Correct answer is: The phase difference between terminal voltage and induced emf
Q.8 A synchronous motor operates at a lagging power factor. Which of the following is true about its armature current?
Current leads the terminal voltage
Current lags the terminal voltage
Current is in phase with terminal voltage
Current magnitude is independent of power factor
Explanation - Lagging power factor means the current lags the voltage, indicating inductive load characteristics.
Correct answer is: Current lags the terminal voltage
Q.9 In a synchronous generator, the stability limit is primarily determined by:
Armature resistance
Synchronous reactance
Inertia of the rotor
Field winding resistance
Explanation - Rotor inertia determines how much kinetic energy is available to sustain the machine during disturbances, affecting the transient stability limit.
Correct answer is: Inertia of the rotor
Q.10 Which type of synchronous machine is best suited for hydro‑electric power plants?
Round‑rotor type
Salient‑pole type
Damper‑wound type
Reversible type
Explanation - Hydro‑electric generators usually operate at low speeds, where salient‑pole machines are advantageous due to their high pole numbers and simple construction.
Correct answer is: Salient‑pole type
Q.11 The term "pull‑out torque" for a synchronous motor refers to:
Maximum torque before the motor stalls
Torque at rated load
Torque when the motor is started
Torque at zero slip
Explanation - Pull‑out torque is the highest torque a synchronous motor can develop before losing synchronism.
Correct answer is: Maximum torque before the motor stalls
Q.12 In a synchronous motor, the damper winding is used to:
Provide additional field excitation
Reduce copper losses
Improve starting and damping of oscillations
Increase the number of poles
Explanation - Damper windings act like squirrel‑cage bars, allowing the motor to start as an induction motor and to damp transient oscillations.
Correct answer is: Improve starting and damping of oscillations
Q.13 If a synchronous generator is over‑excited, its power factor at the terminals will be:
Lagging
Leading
Unity
Zero
Explanation - Over‑excitation raises the internal emf above the terminal voltage, causing the current to lead the voltage (leading power factor).
Correct answer is: Leading
Q.14 The synchronous reactance Xs of a machine is given by:
Xd + Xq
Xd – Xq
Xd × Xq / (Xd + Xq)
Xd + Xd′
Explanation - Xs = (Xd × Xq) / (Xd + Xq), representing the combined effect of direct and quadrature axis reactances.
Correct answer is: Xd × Xq / (Xd + Xq)
Q.15 Which of the following is NOT an advantage of using a synchronous motor as a motor‑generator set?
High efficiency
Ability to control frequency
Self‑starting
Good voltage regulation
Explanation - Synchronous motors are not self‑starting; they require auxiliary means (e.g., damper windings or induction start) to reach synchronous speed.
Correct answer is: Self‑starting
Q.16 During a short circuit on the terminal of a synchronous generator, the initial fault current is primarily limited by:
Armature resistance
Synchronous reactance
Field winding resistance
Stator leakage inductance
Explanation - The synchronous reactance (Xd) dominates the impedance seen during a fault, limiting the initial fault current.
Correct answer is: Synchronous reactance
Q.17 The term "pole slip" in a synchronous motor refers to:
Difference between mechanical and electrical speed
Number of poles lost during operation
Slip caused by load variations
Angular displacement between stator and rotor fields
Explanation - Pole slip is the angular difference (δ) between the rotating magnetic field of the stator and the rotor field.
Correct answer is: Angular displacement between stator and rotor fields
Q.18 For a 3‑phase, 4‑pole synchronous generator, the frequency of the induced emf is:
f = (P/2) × Ns
f = Ns / (2P)
f = (P × Ns) / 120
f = (P × Ns) / 60
Explanation - The induced frequency f = (P × Ns) / 120, where Ns is speed in rpm and P is the number of poles.
Correct answer is: f = (P × Ns) / 120
Q.19 If a synchronous motor is operating at a leading power factor, the armature reaction is:
Demagnetizing
Magnetizing
Neutral
Negligible
Explanation - A leading power factor causes the armature reaction to be magnetizing, adding to the main field.
Correct answer is: Magnetizing
Q.20 The main difference between a salient‑pole and a round‑rotor synchronous machine is:
Number of phases
Construction of the rotor
Type of excitation
Presence of damper windings
Explanation - Salient‑pole machines have protruding poles on the rotor, while round‑rotor machines have a smooth cylindrical rotor.
Correct answer is: Construction of the rotor
Q.21 In a synchronous generator, the term "voltage regulation" is defined as:
The ratio of load current to field current
The change in terminal voltage from no‑load to full‑load expressed as a percentage of full‑load voltage
The difference between generated emf and terminal voltage
The variation of frequency with load
Explanation - Voltage regulation = [(V_no‑load – V_full‑load) / V_full‑load] × 100%.
Correct answer is: The change in terminal voltage from no‑load to full‑load expressed as a percentage of full‑load voltage
Q.22 A synchronous motor supplied with a constant frequency will accelerate when:
The load torque is reduced
The field current is increased
The supply voltage is decreased
The number of poles is increased
Explanation - Reducing load torque reduces the opposing mechanical torque, allowing the motor to speed up to its synchronous speed (if not already there).
Correct answer is: The load torque is reduced
Q.23 Which of the following is true about the phasor diagram of a synchronous generator operating at unity power factor?
E, V, and I are all in phase
E leads V by δ, and I is in phase with V
E lags V by δ, and I leads V
E is perpendicular to V
Explanation - At unity PF, the current is in phase with terminal voltage V; the internal emf E leads V by the load angle δ.
Correct answer is: E leads V by δ, and I is in phase with V
Q.24 For a given synchronous machine, increasing the number of poles while keeping frequency constant will:
Increase synchronous speed
Decrease synchronous speed
Leave synchronous speed unchanged
Increase armature resistance
Explanation - Ns = 120f / P; increasing P reduces Ns.
Correct answer is: Decrease synchronous speed
Q.25 A synchronous motor is said to be “over‑excited”. Which of the following statements is correct?
It operates at lagging power factor
It operates at leading power factor
It cannot start without an external starter
Its terminal voltage is lower than the induced emf
Explanation - Over‑excitation raises the internal emf above terminal voltage, causing the current to lead the voltage (leading PF).
Correct answer is: It operates at leading power factor
Q.26 In the equivalent circuit of a synchronous generator, the internal generated emf (E) is in series with:
Armature resistance only
Synchronous reactance only
Armature resistance and synchronous reactance
Field resistance
Explanation - The internal emf E is followed by series armature resistance (Ra) and synchronous reactance (Xs) before reaching the terminal voltage.
Correct answer is: Armature resistance and synchronous reactance
Q.27 The effect of a high Xd/Xq ratio in a salient‑pole machine is:
Higher starting torque
Lower stability limit
Higher transient reactance
Reduced iron losses
Explanation - A high Xd compared to Xq indicates larger direct‑axis reactance, increasing the transient reactance and affecting stability.
Correct answer is: Higher transient reactance
Q.28 Which of the following is NOT a method of starting a synchronous motor?
Using damper windings
Using an auxiliary induction motor
Direct-on-line start
Using a variable frequency drive
Explanation - Direct‑on‑line starting would try to accelerate the rotor to synchronous speed instantly, which is not possible for a synchronous motor without assistance.
Correct answer is: Direct-on-line start
Q.29 The term "reactive power" (Q) supplied by a synchronous generator is mainly a function of:
Field current
Armature resistance
Mechanical input power
Load torque
Explanation - Changing the field current changes the internal emf and thus the amount of reactive power (leading or lagging) the generator supplies.
Correct answer is: Field current
Q.30 In a synchronous motor, if the load torque exceeds the pull‑out torque, the motor:
Accelerates to a higher speed
Falls out of synchronism
Operates at a lower power factor
Increases its field current automatically
Explanation - Exceeding pull‑out torque causes the rotor to lose synchronism with the rotating magnetic field.
Correct answer is: Falls out of synchronism
Q.31 The main advantage of a round‑rotor synchronous generator in steam turbine applications is:
Higher speed capability
Lower construction cost
Better voltage regulation
Simpler cooling system
Explanation - Round‑rotor machines can operate at high speeds, matching the high rotational speed of steam turbines.
Correct answer is: Higher speed capability
Q.32 Which of the following influences the synchronous speed of a machine?
Number of poles only
Supply frequency only
Both supply frequency and number of poles
Armature resistance
Explanation - Ns = 120f / P, so both frequency (f) and pole count (P) determine synchronous speed.
Correct answer is: Both supply frequency and number of poles
Q.33 In a synchronous motor, the term “field weakening” refers to:
Reducing the armature current
Reducing the field current
Increasing the number of poles
Increasing the supply frequency
Explanation - Field weakening lowers the magnetic flux, allowing the motor to operate at higher speeds (above synchronous speed) in some control schemes.
Correct answer is: Reducing the field current
Q.34 When a synchronous generator operates at a lagging power factor, the armature reaction is:
Demagnetizing
Magnetizing
Neutral
None of the above
Explanation - Lagging PF causes armature reaction that opposes the main field, reducing the net flux (demagnetizing effect).
Correct answer is: Demagnetizing
Q.35 The synchronous reactance of a round‑rotor machine is typically:
Higher than that of a salient‑pole machine
Lower than that of a salient‑pole machine
Equal to the direct‑axis reactance
Independent of rotor construction
Explanation - Round‑rotor machines have more uniform air‑gap, resulting in lower reactance compared to salient‑pole machines.
Correct answer is: Lower than that of a salient‑pole machine
Q.36 In a three‑phase synchronous machine, the line voltage V_L is related to the phase voltage V_Ph by:
V_L = V_Ph
V_L = √3 × V_Ph
V_L = V_Ph / √3
V_L = 3 × V_Ph
Explanation - For a balanced three‑phase system, line voltage = √3 times the phase voltage.
Correct answer is: V_L = √3 × V_Ph
Q.37 A synchronous motor can be made to operate at a leading power factor to:
Increase its speed
Improve voltage regulation of the supply system
Reduce field current requirement
Decrease the number of poles
Explanation - Leading PF operation supplies reactive power to the grid, helping maintain voltage levels.
Correct answer is: Improve voltage regulation of the supply system
Q.38 In the equivalent circuit of a synchronous motor, which element represents the effect of armature reaction?
Armature resistance Ra
Synchronous reactance Xs
Field resistance Rf
Capacitive reactance Xc
Explanation - Xs models the combined effect of leakage inductance and armature reaction.
Correct answer is: Synchronous reactance Xs
Q.39 If the frequency of the supply to a synchronous motor is increased, the synchronous speed:
Decreases
Increases
Remains unchanged
Depends on field current
Explanation - Ns = 120f / P, so higher frequency f raises Ns.
Correct answer is: Increases
Q.40 Which parameter of a synchronous generator is directly proportional to the induced emf (E)?
Armature resistance
Number of poles
Field flux
Load current
Explanation - E = 4.44 f N Φ K (per phase), where Φ is the flux per pole produced by the field winding.
Correct answer is: Field flux
Q.41 A synchronous motor operating at a leading power factor will have:
Higher terminal voltage than internal emf
Lower terminal voltage than internal emf
Terminal voltage equal to internal emf
Zero armature current
Explanation - Leading PF causes the armature reaction to be magnetizing, raising the terminal voltage above the internal emf.
Correct answer is: Higher terminal voltage than internal emf
Q.42 The purpose of a brushless exciter in a synchronous generator is to:
Eliminate the need for slip rings
Increase the number of poles
Provide three‑phase output directly
Reduce the machine’s weight
Explanation - A brushless exciter uses a small AC generator and a rectifier to supply DC to the main field without brushes or slip rings.
Correct answer is: Eliminate the need for slip rings
Q.43 In a synchronous motor, the term “locked‑rotor test” is used to determine:
Synchronous speed
Stator resistance and leakage reactance
Field winding resistance
Torque‑speed characteristic
Explanation - With the rotor locked, the test measures short‑circuit current to calculate Ra and Xs.
Correct answer is: Stator resistance and leakage reactance
Q.44 Which of the following statements about the induced emf in a synchronous machine is correct?
It is independent of speed
It is directly proportional to speed
It is inversely proportional to frequency
It depends only on armature resistance
Explanation - E = 4.44 f N Φ K; frequency f is proportional to speed for a given number of poles.
Correct answer is: It is directly proportional to speed
Q.45 A synchronous generator connected to a large grid will tend to operate at:
Leading power factor
Lagging power factor
Unity power factor
Variable power factor depending on load
Explanation - The generator’s PF follows the system’s overall load; it can be leading, lagging, or unity based on reactive power exchange with the grid.
Correct answer is: Variable power factor depending on load
Q.46 The torque produced by a synchronous motor is given by:
T = (3 V I cos δ) / ω_s
T = (3 E I sin δ) / ω_s
T = (V I sin δ) / ω_s
T = (E I cos δ) / ω_s
Explanation - Real power P = 3 E I sin δ; torque T = P / ω_s.
Correct answer is: T = (3 E I sin δ) / ω_s
Q.47 Which of the following is a typical use of a synchronous motor?
Variable‑speed fan drives
High‑speed grinding machines
Constant‑speed pumps
Electric vehicle propulsion
Explanation - Synchronous motors provide precise constant speed, ideal for pumps, compressors, and conveyors.
Correct answer is: Constant‑speed pumps
Q.48 In a synchronous generator, the term “short‑circuit ratio” (SCR) is defined as:
Ratio of rated voltage to short‑circuit current
Ratio of field current to armature current
Ratio of synchronous reactance to armature resistance
Ratio of rated apparent power to short‑circuit apparent power
Explanation - SCR = V_rated / (I_sc × Z_base); a higher SCR indicates lower reactance and better voltage regulation.
Correct answer is: Ratio of rated voltage to short‑circuit current
Q.49 The damper winding in a synchronous motor is analogous to:
A field winding
A squirrel‑cage rotor of an induction motor
A series capacitor
A permanent magnet
Explanation - Damper bars act like a cage, allowing the machine to start as an induction motor and providing damping.
Correct answer is: A squirrel‑cage rotor of an induction motor
Q.50 When a synchronous motor runs at a lagging power factor, the terminal voltage is:
Higher than the internal emf
Lower than the internal emf
Equal to the internal emf
Independent of the internal emf
Explanation - Lagging PF creates a demagnetizing armature reaction, reducing terminal voltage relative to internal emf.
Correct answer is: Lower than the internal emf
Q.51 The “pull‑in torque” of a synchronous motor is:
The torque required to start the motor from rest
The maximum torque the motor can develop during acceleration
The torque at which the motor loses synchronism
The torque at rated load
Explanation - Pull‑in torque is the minimum torque that must be applied (or developed via starting aids) to bring the motor up to synchronous speed.
Correct answer is: The torque required to start the motor from rest
Q.52 In a synchronous generator, the armature reaction is said to be "cross‑magnetizing" when the load is:
Unity power factor
Lagging power factor
Leading power factor
Zero power factor
Explanation - At unity PF, the armature reaction is orthogonal to the main field, causing a cross‑magnetizing effect.
Correct answer is: Unity power factor
Q.53 The main cause of "pole slipping" in a synchronous motor is:
Excessive field current
Load torque exceeding pull‑out torque
Insufficient supply frequency
High armature resistance
Explanation - When torque demand exceeds pull‑out torque, the rotor cannot maintain synchronism and slips.
Correct answer is: Load torque exceeding pull‑out torque
Q.54 When a synchronous generator is operated at a leading power factor, the stator current:
Lags the terminal voltage
Leads the terminal voltage
Is in phase with terminal voltage
Is zero
Explanation - Leading PF means current leads voltage.
Correct answer is: Leads the terminal voltage
Q.55 In a synchronous machine, the term "direct‑axis" refers to:
The axis aligned with the rotor’s magnetic field
The axis perpendicular to the rotor’s magnetic field
The stator winding axis
The axis of maximum leakage inductance
Explanation - Direct‑axis (d‑axis) is the axis of the main field; quadrature‑axis (q‑axis) is perpendicular to it.
Correct answer is: The axis aligned with the rotor’s magnetic field
Q.56 If a synchronous motor’s field winding is short‑circuited, the motor will:
Run at a higher speed
Run at a lower speed
Become an induction motor
Stop immediately
Explanation - Short‑circuited field eliminates the DC field, so the rotor behaves like a cage, and the machine operates as an induction motor.
Correct answer is: Become an induction motor
Q.57 The synchronous speed of a 6‑pole machine supplied at 50 Hz is:
600 rpm
1000 rpm
1500 rpm
2000 rpm
Explanation - Ns = 120 f / P = 120 × 50 / 6 = 1000 rpm.
Correct answer is: 1000 rpm
Q.58 Which parameter of a synchronous machine determines its ability to withstand short circuits?
Armature resistance
Synchronous reactance
Field resistance
Number of poles
Explanation - Higher Xs limits the fault current during a short circuit.
Correct answer is: Synchronous reactance
Q.59 The “reactive power” supplied by a synchronous generator is most directly controlled by:
Changing the load resistance
Changing the field excitation
Changing the armature resistance
Changing the supply frequency
Explanation - Adjusting field current changes the internal emf, thereby controlling the reactive power output.
Correct answer is: Changing the field excitation
Q.60 A synchronous motor that operates at a lagging power factor will:
Supply reactive power to the grid
Absorb reactive power from the grid
Operate at leading power factor automatically
Have zero reactive power exchange
Explanation - Lagging PF indicates inductive operation, drawing reactive power from the supply.
Correct answer is: Absorb reactive power from the grid
Q.61 The term “stator winding distribution factor” (Kd) influences:
The magnitude of the induced emf
The number of poles
The field winding resistance
The mechanical losses
Explanation - Kd accounts for the effect of distributed windings on the generated voltage.
Correct answer is: The magnitude of the induced emf
Q.62 Which of the following is true for a synchronous motor operating at unity power factor?
Armature reaction is purely demagnetizing
Armature reaction is purely magnetizing
Armature reaction is cross‑magnetizing
Armature reaction is zero
Explanation - At unity PF, the armature reaction is perpendicular to the main field, resulting in a cross‑magnetizing effect.
Correct answer is: Armature reaction is cross‑magnetizing
Q.63 The frequency of the induced emf in the rotor of a synchronous machine is equal to:
Supply frequency
Half the supply frequency
Double the supply frequency
Zero (DC)
Explanation - The rotor experiences the rotating magnetic field at the same frequency as the stator supply.
Correct answer is: Supply frequency
Q.64 In a synchronous generator, which condition will cause the terminal voltage to rise above the rated value?
Heavy load at lagging power factor
No load condition
Light load at leading power factor
Short circuit on the terminals
Explanation - Leading PF supplies reactive power, raising terminal voltage above rated.
Correct answer is: Light load at leading power factor
Q.65 The term "synchronizing torque" in a synchronous motor refers to:
Torque required to start the motor from rest
Torque that aligns the rotor with the rotating field
Torque developed at rated load
Torque lost due to slip
Explanation - Synchronizing torque is produced by the interaction of the rotor field with the stator rotating field, pulling the rotor into synchronism.
Correct answer is: Torque that aligns the rotor with the rotating field
Q.66 In a salient‑pole synchronous generator, the direct‑axis inductance Ld is:
Larger than quadrature‑axis inductance Lq
Equal to Lq
Smaller than Lq
Independent of rotor geometry
Explanation - Salient poles cause higher reluctance along the direct axis, resulting in higher Ld.
Correct answer is: Larger than quadrature‑axis inductance Lq
Q.67 A synchronous motor can be used as a starter for an induction motor because:
It can operate at variable speed
It can supply high starting torque
Its damper winding provides induction‑type starting
It has a lower cost
Explanation - Damper windings act like a squirrel‑cage, allowing the synchronous motor to start as an induction motor.
Correct answer is: Its damper winding provides induction‑type starting
Q.68 For a synchronous machine, the per‑unit (p.u.) system is used to:
Simplify calculations by normalizing values
Increase the actual voltage levels
Measure mechanical speed directly
Determine the number of poles
Explanation - Per‑unit normalization makes analysis of different machines easier by scaling quantities to common bases.
Correct answer is: Simplify calculations by normalizing values
Q.69 The main cause of heating in the field winding of a synchronous motor is:
Armature reaction
Copper losses (I²R) in the field winding
Core losses
Friction losses
Explanation - Current flowing in the field winding produces I²R losses, generating heat.
Correct answer is: Copper losses (I²R) in the field winding
Q.70 A synchronous generator that is over‑excited will supply:
Lagging reactive power
Leading reactive power
No reactive power
Only active power
Explanation - Over‑excitation raises internal emf, causing the current to lead voltage, thus supplying reactive power.
Correct answer is: Leading reactive power
Q.71 Which of the following best describes the relationship between synchronous speed and the number of poles?
Speed increases with more poles
Speed decreases with more poles
Speed is independent of pole count
Speed is directly proportional to the square of pole count
Explanation - Ns = 120f / P; increasing P reduces Ns.
Correct answer is: Speed decreases with more poles
Q.72 In a synchronous motor, the term "field weakening" is primarily used to:
Increase torque at low speed
Allow operation above synchronous speed
Reduce armature current
Improve power factor
Explanation - Field weakening reduces magnetic flux, enabling higher speeds in certain control strategies.
Correct answer is: Allow operation above synchronous speed
Q.73 The main advantage of a brushless exciter over a traditional brush‑type exciter is:
Higher output power
Lower maintenance and longer life
Simpler control circuitry
Reduced voltage regulation
Explanation - Eliminating brushes removes wear points, reducing maintenance requirements.
Correct answer is: Lower maintenance and longer life
Q.74 In a synchronous machine, the term "pole pitch" refers to:
The angular distance between two adjacent poles
The distance between stator and rotor
The physical length of a pole
The number of slots per pole
Explanation - Pole pitch is the angular span of one pole pair in electrical degrees.
Correct answer is: The angular distance between two adjacent poles
Q.75 When a synchronous motor runs at a leading power factor, the armature reaction is:
Demagnetizing
Magnetizing
Cross‑magnetizing
Zero
Explanation - Leading PF produces a magnetizing effect, adding to the main field flux.
Correct answer is: Magnetizing
Q.76 The term “synchronous speed” is also known as:
Mechanical speed
Electrical speed
Slip speed
Rotor speed
Explanation - Synchronous speed is the speed of the rotating magnetic field (electrical speed).
Correct answer is: Electrical speed
Q.77 Which of the following devices is commonly used to limit the field current in a synchronous generator?
Voltage regulator
Current transformer
Field reactor (inductor)
Load tap changer
Explanation - A field reactor adds reactance to the field circuit, limiting field current.
Correct answer is: Field reactor (inductor)
Q.78 In a synchronous motor, the torque developed is proportional to:
sin δ
cos δ
tan δ
δ²
Explanation - Torque (and real power) is proportional to sin δ, where δ is the load angle.
Correct answer is: sin δ
Q.79 A synchronous machine with a high short‑circuit ratio (SCR) generally exhibits:
High synchronous reactance
Low synchronous reactance
High armature resistance
Low field resistance
Explanation - High SCR indicates the machine can deliver high currents during a short circuit, implying low Xs.
Correct answer is: Low synchronous reactance
Q.80 Which of the following is true about a salient‑pole synchronous motor’s torque ripple?
It is independent of rotor position
It is higher than in a round‑rotor machine
It is lower than in a round‑rotor machine
It is eliminated by damper windings
Explanation - Salient poles cause variation in reactance with rotor position, leading to higher torque ripple.
Correct answer is: It is higher than in a round‑rotor machine
Q.81 In a synchronous generator, the term “open‑circuit characteristic” (OCC) relates terminal voltage to:
Load current
Field current
Armature resistance
Supply frequency
Explanation - OCC is a plot of generated emf (or terminal voltage) versus field current at no load.
Correct answer is: Field current
Q.82 When a synchronous motor is operating at a lagging power factor, the terminal voltage tends to:
Rise above the internal emf
Fall below the internal emf
Remain equal to the internal emf
Become independent of load
Explanation - Lagging PF causes demagnetizing armature reaction, reducing terminal voltage.
Correct answer is: Fall below the internal emf
Q.83 The main purpose of a load tap changer in a synchronous generator is to:
Adjust the frequency
Regulate terminal voltage under varying load
Control field current
Change the number of poles
Explanation - Tap changers modify turns ratio to maintain voltage as load varies.
Correct answer is: Regulate terminal voltage under varying load
Q.84 In the phasor diagram of a synchronous motor, the angle δ (load angle) is measured between:
Stator voltage and current
Internal emf (E) and terminal voltage (V)
Field current and armature current
Armature resistance and reactance
Explanation - δ is the angular displacement between E and V.
Correct answer is: Internal emf (E) and terminal voltage (V)
Q.85 If the synchronous reactance of a machine is increased, the short‑circuit current will:
Increase
Decrease
Remain unchanged
Become zero
Explanation - Higher reactance provides greater impedance, limiting fault current.
Correct answer is: Decrease
Q.86 Which of the following is a characteristic of a round‑rotor synchronous generator?
Large direct‑axis reactance
High pole count
Operation at low speeds
Uniform air gap
Explanation - Round‑rotor machines have a uniform air gap, leading to lower reactance and suitability for high‑speed applications.
Correct answer is: Uniform air gap
Q.87 The effect of increasing the load on a synchronous motor operating at constant excitation is:
Decrease in terminal voltage
Increase in terminal voltage
No change in terminal voltage
Increase in field current automatically
Explanation - With constant excitation, increased load draws more current, causing a larger voltage drop across reactance.
Correct answer is: Decrease in terminal voltage
Q.88 The term “pole slip” is synonymous with:
Slip in induction machines
Load angle δ
Frequency deviation
Mechanical vibration
Explanation - Pole slip describes the angular displacement between the rotating magnetic field and rotor field, i.e., the load angle.
Correct answer is: Load angle δ
Q.89 For a synchronous machine, the power factor at which the armature reaction is purely cross‑magnetizing is:
0° (unity)
±90°
±45°
±30°
Explanation - At unity PF, the armature current is in phase with voltage, causing a 90° phase shift between current and flux, leading to cross‑magnetizing effect.
Correct answer is: 0° (unity)
Q.90 A synchronous motor’s pull‑out torque is directly proportional to:
Field current
Armature resistance
Supply frequency
Number of poles
Explanation - Higher field current increases internal emf, raising the maximum torque that can be developed before loss of synchronism.
Correct answer is: Field current
Q.91 In a synchronous generator, the term "reactive power output" is often controlled to:
Maintain system frequency
Maintain system voltage
Reduce active power losses
Increase mechanical shaft speed
Explanation - Reactive power exchange regulates voltage levels on the power system.
Correct answer is: Maintain system voltage
Q.92 Which of the following statements about damper windings is FALSE?
They provide starting torque.
They reduce oscillations during transients.
They increase the synchronous reactance.
They are short‑circuited conductors on the rotor.
Explanation - Damper windings actually help damp oscillations and provide starting torque; they do not increase Xs.
Correct answer is: They increase the synchronous reactance.
Q.93 A synchronous generator delivering 1 p.u. voltage with a short‑circuit ratio (SCR) of 5 will have a per‑unit synchronous reactance of approximately:
0.20 p.u.
0.25 p.u.
0.33 p.u.
0.40 p.u.
Explanation - SCR = 1 / Xs (p.u.), so Xs ≈ 1 / 5 = 0.20 p.u.
Correct answer is: 0.20 p.u.
Q.94 Which of the following is NOT a typical method of controlling voltage in a synchronous generator?
Adjusting field current
Changing stator winding turns
Using a load tap changer
Varying the prime‑mover speed
Explanation - Stator turns are fixed; voltage is controlled by field excitation, tap changers, or speed variations.
Correct answer is: Changing stator winding turns
Q.95 In a three‑phase synchronous motor, the line current is:
√3 times the phase current
One‑third of the phase current
Equal to the phase current
Twice the phase current
Explanation - For a balanced Y‑connected system, line current = √3 × phase current.
Correct answer is: √3 times the phase current
Q.96 The purpose of the “saturation curve” for a synchronous machine is to:
Show the relationship between terminal voltage and load current
Show the relationship between field current and generated emf
Show the variation of armature resistance with temperature
Show the dependence of frequency on speed
Explanation - The saturation (or open‑circuit) curve plots generated emf versus field current, reflecting magnetic saturation.
Correct answer is: Show the relationship between field current and generated emf
Q.97 When a synchronous motor is operating at a lagging power factor, the current phasor relative to the voltage phasor is:
Leading
Lagging
In phase
Perpendicular
Explanation - Lagging PF means current lags voltage.
Correct answer is: Lagging
Q.98 A synchronous machine’s “pole pitch” in electrical degrees for a 4‑pole machine is:
30°
45°
60°
90°
Explanation - Pole pitch = 180° / (P/2) = 180° / 2 = 90° electrical for a 4‑pole machine.
Correct answer is: 90°
Q.99 Which of the following effects is primarily responsible for the torque ripple in a salient‑pole synchronous motor?
Variation of armature resistance with position
Variation of synchronous reactance with rotor position
Fluctuation of supply frequency
Changes in field current
Explanation - Salient poles cause Xd and Xq to vary with rotor angle, leading to torque ripple.
Correct answer is: Variation of synchronous reactance with rotor position
Q.100 In a synchronous generator, the term “reactive power” is measured in:
Watts (W)
Volt‑amps (VA)
Volt‑amps reactive (VAR)
Amperes (A)
Explanation - Reactive power is expressed in VAR (or kVAR).
Correct answer is: Volt‑amps reactive (VAR)
Q.101 The main disadvantage of operating a synchronous motor at a leading power factor is:
Increased heating of the field winding
Reduced efficiency
Risk of over‑voltage on the supply system
Higher mechanical losses
Explanation - Leading PF supplies reactive power, which can raise system voltage beyond safe limits.
Correct answer is: Risk of over‑voltage on the supply system
Q.102 A synchronous generator’s internal generated emf (E) is directly proportional to:
Armature resistance
Supply frequency
Field flux per pole
Load current
Explanation - E ∝ Φ (field flux) and frequency; for a given frequency, E ∝ Φ.
Correct answer is: Field flux per pole
Q.103 Which method is used to test the short‑circuit reactance (Xs) of a synchronous machine?
Open‑circuit test
Locked‑rotor test
Short‑circuit test
No‑load test
Explanation - Short‑circuit test measures voltage and current under shorted terminals to calculate Xs.
Correct answer is: Short‑circuit test
Q.104 A synchronous motor with a constant field current and increasing load torque will experience:
Decrease in load angle δ
Increase in load angle δ
No change in δ
Decrease in terminal voltage
Explanation - Higher torque requires a larger δ to maintain power balance (P ∝ sin δ).
Correct answer is: Increase in load angle δ
Q.105 In a synchronous generator, the terminal voltage at no load is mainly determined by:
Armature resistance
Field excitation
Load current
Supply frequency
Explanation - With no load, V ≈ E, which depends on field current (excitation).
Correct answer is: Field excitation
Q.106 The term “reactive power capability curve” of a synchronous generator shows:
Active power vs. speed
Reactive power vs. terminal voltage
Reactive power vs. field current
Reactive power limits at various terminal voltages
Explanation - Capability curves illustrate the permissible ranges of active and reactive power for safe operation.
Correct answer is: Reactive power limits at various terminal voltages
Q.107 When a synchronous motor is supplied with a three‑phase balanced supply at 50 Hz and has 6 poles, the number of electrical cycles per mechanical revolution is:
2
3
4
6
Explanation - Number of electrical cycles per rev = P/2 = 6/2 = 3.
Correct answer is: 3
Q.108 A synchronous motor operating at a lagging power factor will have its terminal voltage:
Higher than internal emf
Lower than internal emf
Equal to internal emf
Independent of power factor
Explanation - Lagging PF causes demagnetizing armature reaction, reducing terminal voltage.
Correct answer is: Lower than internal emf
Q.109 The primary reason for using a brushless exciter in large hydro‑electric generators is:
To increase the generated voltage
To eliminate brush wear and reduce maintenance
To simplify the control system
To lower the machine’s inertia
Explanation - Brushless exciters avoid mechanical contacts, extending service life and reducing upkeep.
Correct answer is: To eliminate brush wear and reduce maintenance
Q.110 The per‑unit value of synchronous reactance Xs for a machine with an SCR of 2 is:
0.2 p.u.
0.5 p.u.
1.0 p.u.
2.0 p.u.
Explanation - Xs = 1 / SCR = 1 / 2 = 0.5 p.u.
Correct answer is: 0.5 p.u.
Q.111 In a synchronous motor, the torque produced at a given load angle δ is proportional to which quantity?
E × V × sin δ
E × I × cos δ
V² / Xₛ
I² Rₐ
Explanation - Real power P = √3 E V sin δ; torque T = P / ωₛ, thus proportional to E V sin δ.
Correct answer is: E × V × sin δ
Q.112 The main advantage of a synchronous motor over an induction motor for constant‑speed applications is:
Higher starting torque
Self‑starting capability
Precise constant speed irrespective of load
Lower cost
Explanation - Synchronous speed is fixed by supply frequency; the motor maintains this speed regardless of load.
Correct answer is: Precise constant speed irrespective of load
Q.113 When a synchronous generator operates at a lagging power factor, the reactive power flow is:
From generator to grid
From grid to generator
Zero
Alternating direction
Explanation - Lagging PF means the generator absorbs reactive power (absorbs VAR) from the system.
Correct answer is: From generator to grid
Q.114 In the equivalent circuit of a synchronous motor, the term Xd′ (d‑axis transient reactance) is used to model:
Steady‑state operation
Transient response after a sudden change
Short‑circuit conditions
No‑load condition
Explanation - Xd′ represents the reactance seen during transient events (e.g., faults).
Correct answer is: Transient response after a sudden change
Q.115 If a synchronous motor’s field current is increased while running at the same load, the terminal voltage will:
Decrease
Increase
Remain unchanged
Become zero
Explanation - Higher field current raises internal emf, which raises terminal voltage for the same load.
Correct answer is: Increase
Q.116 A synchronous generator connected to an infinite bus will have its terminal voltage:
Controlled by field current
Fixed by the bus voltage
Varied by armature resistance
Independent of the grid
Explanation - An infinite bus maintains a constant voltage; the generator’s terminal voltage follows it.
Correct answer is: Fixed by the bus voltage
Q.117 The primary purpose of the damper winding in a synchronous motor is to:
Provide field excitation
Increase synchronous reactance
Assist in starting and damping oscillations
Reduce core losses
Explanation - Damper windings allow induction‑type starting and suppress rotor oscillations.
Correct answer is: Assist in starting and damping oscillations
Q.118 In a synchronous generator, the term "open‑circuit characteristic" (OCC) is plotted as:
Terminal voltage vs. armature current
Generated emf vs. field current
Terminal voltage vs. load power factor
Frequency vs. speed
Explanation - OCC shows how the generated voltage varies with field current at no load.
Correct answer is: Generated emf vs. field current
Q.119 The main cause of voltage rise in a synchronous generator when the load is switched off is:
Decrease in field current
Increase in armature resistance
Reduction of armature reaction (leading)
Increase in mechanical speed
Explanation - With light or no load, the generator may operate at leading PF, causing a voltage rise.
Correct answer is: Reduction of armature reaction (leading)
Q.120 For a 4‑pole synchronous motor supplied at 60 Hz, the electrical frequency per mechanical revolution is:
60 Hz
30 Hz
120 Hz
15 Hz
Explanation - Electrical cycles per rev = P/2 = 2; thus f_e = 60 Hz / 2 = 30 Hz.
Correct answer is: 30 Hz
Q.121 Which of the following best describes the relationship between field current and reactive power output in a synchronous generator?
Directly proportional
Inversely proportional
Independent of each other
Quadratically related
Explanation - Increasing field current raises internal emf, thereby increasing reactive power capability.
Correct answer is: Directly proportional
Q.122 When a synchronous motor operates at a leading power factor, the terminal voltage is:
Higher than the internal emf
Lower than the internal emf
Equal to the internal emf
Zero
Explanation - Leading PF causes magnetizing armature reaction, raising terminal voltage above internal emf.
Correct answer is: Higher than the internal emf
Q.123 The term “synchronous speed” of a machine is:
The speed at which the rotor lags the stator field
The speed of the rotating magnetic field produced by the stator
The maximum speed the rotor can achieve
The speed at which the machine produces maximum torque
Explanation - Synchronous speed is the speed of the stator's rotating magnetic field, given by Ns = 120f / P.
Correct answer is: The speed of the rotating magnetic field produced by the stator
Q.124 In a synchronous motor, the pull‑in torque is generally:
Higher than pull‑out torque
Equal to pull‑out torque
Lower than pull‑out torque
Independent of field current
Explanation - Pull‑in torque (starting) is usually less than the maximum torque the motor can develop (pull‑out).
Correct answer is: Lower than pull‑out torque
Q.125 If a synchronous generator is operating at a lagging power factor, the terminal voltage will tend to:
Rise above rated
Fall below rated
Stay equal to rated
Become zero
Explanation - Lagging PF causes demagnetizing armature reaction, reducing terminal voltage.
Correct answer is: Fall below rated
Q.126 The main effect of increasing the number of poles in a synchronous machine while keeping frequency constant is:
Higher synchronous speed
Lower synchronous speed
Higher voltage
Lower voltage
Explanation - Ns = 120f / P; more poles → lower Ns.
Correct answer is: Lower synchronous speed
Q.127 In a synchronous motor, the term “magnetizing current” refers to:
Current that produces torque
Current that produces the magnetic field in the stator
Current that flows in the field winding
Current that causes core losses
Explanation - Magnetizing current creates the rotating magnetic field in the stator windings.
Correct answer is: Current that produces the magnetic field in the stator
Q.128 The synchronous reactance Xs of a salient‑pole machine is mainly affected by:
Armature resistance
Air‑gap length variation with rotor position
Number of stator slots
Supply frequency
Explanation - Salient poles cause non‑uniform air gap, influencing Xs.
Correct answer is: Air‑gap length variation with rotor position
Q.129 Which of the following is a typical reason for using a synchronous motor in a compressor application?
Variable speed operation
High starting torque and constant speed
Low cost
Ability to operate without field excitation
Explanation - Compressors require steady speed and high torque, which synchronous motors provide.
Correct answer is: High starting torque and constant speed
Q.130 The voltage regulation of a synchronous generator improves when:
Synchronous reactance increases
Field current is reduced
Load is increased
Synchronous reactance decreases
Explanation - Lower Xs reduces voltage drop under load, improving regulation.
Correct answer is: Synchronous reactance decreases
Q.131 In a synchronous motor, the term “critical clearing time” refers to:
Maximum time for a fault to be cleared without losing synchronism
Time required for the motor to start
Time for the field winding to reach steady state
Time for the rotor to reach synchronous speed
Explanation - Critical clearing time is the limit beyond which the rotor cannot regain synchronism after a fault.
Correct answer is: Maximum time for a fault to be cleared without losing synchronism
Q.132 A 3‑phase synchronous motor with a line voltage of 400 V (line‑line) has a phase voltage of:
400 V
230 V
692 V
138 V
Explanation - Phase voltage V_Ph = V_LL / √3 = 400 / 1.732 ≈ 230 V.
Correct answer is: 230 V
Q.133 When a synchronous motor is over‑excited, the armature reaction is:
Demagnetizing
Magnetizing
Cross‑magnetizing
Zero
Explanation - Over‑excitation raises field strength, causing armature reaction to add to the main flux (magnetizing).
Correct answer is: Magnetizing
Q.134 The term "pole‑pair number" (p) for a 8‑pole machine is:
4
8
2
16
Explanation - Pole‑pair number p = P/2; for 8 poles, p = 4.
Correct answer is: 4
Q.135 In a synchronous generator, increasing the load at constant excitation typically causes:
Terminal voltage to rise
Terminal voltage to drop
No change in terminal voltage
Field current to increase automatically
Explanation - Higher load current causes larger voltage drop across Xs, reducing terminal voltage.
Correct answer is: Terminal voltage to drop
Q.136 The main advantage of using a salient‑pole rotor in low‑speed applications is:
Higher mechanical strength
Lower cost at high speeds
Ability to have many poles without increasing size
Reduced magnetic saturation
Explanation - Salient poles allow high pole numbers, suitable for low‑speed, high‑torque machines.
Correct answer is: Ability to have many poles without increasing size
Q.137 In a synchronous motor, the term “field weakening” is employed to:
Increase torque at low speeds
Enable operation above synchronous speed
Reduce copper losses
Improve power factor
Explanation - Reducing field flux permits the motor to run at speeds slightly above synchronous in certain vector‑control schemes.
Correct answer is: Enable operation above synchronous speed
Q.138 Which parameter directly influences the short‑circuit current of a synchronous generator?
Armature resistance
Synchronous reactance
Field resistance
Number of poles
Explanation - Short‑circuit current I_sc = V / Xs; thus Xs determines the magnitude of I_sc.
Correct answer is: Synchronous reactance
Q.139 In a synchronous motor, the term “pull‑out torque” corresponds to:
Maximum torque before loss of synchronism
Torque at rated load
Starting torque
Torque at zero load
Explanation - Pull‑out torque is the highest stable torque the motor can develop without slipping out of sync.
Correct answer is: Maximum torque before loss of synchronism