A synchronous motor operates at rated voltage and frequency and has a load torque angle of 30°. If both the terminal voltage and frequency are reduced by 10%, then

the load torque angle increases

the load torque angle remains the same

the load torque angle increases

the load torque angle decreases

nothing can be said about the torque angle

3 phase synchronous motor needs:

3 phase AC supply as well as single phase AC supply

3 phase AC supply as well as DC supply

3 phase AC supply as well as single phase AC supply

A synchronous motor connected to infinite bus bars has at constant full load, 100% excitation and unity power factor (p.f.). On changing the excitation only, the armature current will have:

Leading p.f. with under excitation

Lagging p.f. with over excitation

Leading p.f. with over excitation

In synchronous motor, the load on the motor is increased and the rotor progressively tends to:

lead in phase by some angle but it still continues to run synchronously

fall back in phase by some angle but it still continues to run synchronously

Consider the table given:Constructional featureMachine typeMitigation (P) Damper bars (S) Induction motor (X) Hunting (Q) Skewed rotor slots (T) Transformer (Y) Magnetic locking (R) Compensation winding (U)Synchronous machine (Z) Armature reaction (V) DC machine The correct combination that relates the constructional feature, machine type and mitigation is

P - T - Y, Q - V - Z, R - S - X

P - V - X, Q - U - Z, R - T - Y

P - U - X, Q - S - Y, R - V - Z

P - T - Y, Q - V - Z, R - S - X

Before an alternator can be connected to an infinite bus, which of the following must be the same for the alternator and the infinite bus?

Voltage, frequency and phase sequence; and the phase difference between the alternator and infinite voltages must be zero

In an alternator, when the armature current increases, the terminal voltage drops due to:

Armature effective resistance, armature leakage reactance and armature reaction

During the operation of a connecting a busbar in parallel with an incoming alternator the:

Terminal voltage, speed and phase of the incoming alternator need not be same as that of the busbar

Terminal voltage and speed of the incoming alternator should be same as that of busbar, but the phase can be different

Terminal voltage and phase of the incoming alternator should be same as that of the busbar, but the speed can be different

Terminal voltage, speed and phase of the incoming alternator need not be same as that of the busbar

Terminal voltage, speed and phase of the incoming alternator should be same as that of the busbar

Damper bars in case of Salient Pole Rotors of hydro-alternators are usually inserted in pole faces to

Help improve the power factor of load

Strengthen the excitation current of the poles

Damp out the rotor oscillations during transient state owing to sudden change in load conditions

Help improve the power factor of load

Reduce the no-load current when load is thrown-off

In a synchronous machine, the magnitude of magnetising reactance is:

directly proportional to air gap length squared

directly proportional to air gap length

inversely proportional to air gap length

directly proportional to air gap length squared

A synchronous phase modifier, used for voltage control in a line

Is a rotating machine, driven by an induction motor

Always draws leading var from the bus

A 3-phase synchronous motor with constant excitation drives a certain load drawing electrical power from an infinite bus at leading power factor. If the shaft load decrease

The power angle increases while power factor decreases

The power angle decreases while power factor increases

Both power angle and power factor increase

Both power angle and power factor decrease

The power angle increases while power factor decreases

Let ϕ be the phase angle between the line current and line voltage, and α be the load angle. The per phase mechanical power developed by a synchronous motor is:

\(\frac{{{E_b}V}}{{{X_S}}}\sin \phi \)

\(\frac{{{E_b}V}}{{{X_S}}}\cos \alpha \)

\(\frac{{{E_b}V}}{{{X_s}}}\cos \phi \)

\(\frac{{{E_b}V}}{{{X_S}}}\sin \alpha \)

A three-phase synchronous motor connected to ac mains is running at full load and unity power factor. If its shaft load is reduced by half and field current is held constant then its new power factor will be

Dependent on machine parameters

In the method of synchronization of the alternator to the busbar, a synchroscope indicates correct speed when:

the pointer moves towards the left

the pointer moves towards the right

the pointer vibrates at the center

the pointer points vertically up

the pointer moves towards the left

In a 3-phase synchronous motor, If the direction of its field current is reversed

The winding of the motor will burn

The motor will run in the reverse direction

The motor continue to run in the same direction

A synchronous generator is feeding power to infinite bus bars at unity power factor. Its excitation is now increased. It will feed

More power at unity power factor

The same power but at a leading power factor

The same power but at a lagging power factor

More power at unity power factor

Less power at unity power factor

In the synchronous motor, the breakdown torque is ______.

directly proportional to the square of the applied voltage

inversely proportional to the square of the applied voltage

inversely proportional to applied voltage

directly proportional to applied voltage

directly proportional to the square of the applied voltage

517 + Mcqs in Synchronous Motors in Electrical Engineering Page 5 McqOptions

201.	A three-phase induction motor is supplied with a 400 V, 50 Hz AC supply and has a full load speed of 840 revolutions per minute. The number of poles with which the magnetic field produced by stator current rotates are:
A.	8
B.	2
C.	4
D.	6
Answer» E.

Discussion

202.	In alternator, during hunting when the speed becomes super synchronous, the damper bars develop
A.	reluctance torque
B.	pseudo-stationary torque
C.	eddy current torque
D.	induction generator torque
Answer» E.

Discussion

203.	A synchronous motor operates at rated voltage and frequency and has a load torque angle of 30°. If both the terminal voltage and frequency are reduced by 10%, then
A.	the load torque angle remains the same
B.	the load torque angle increases
C.	the load torque angle decreases
D.	nothing can be said about the torque angle
Answer» B. the load torque angle increases

Discussion

204.	3 phase synchronous motor needs:
A.	3 phase AC supply only
B.	3 phase AC supply as well as DC supply
C.	3 phase AC supply as well as single phase AC supply
D.	DC supply only
Answer» C. 3 phase AC supply as well as single phase AC supply

Discussion

205.	A synchronous motor connected to infinite bus bars has at constant full load, 100% excitation and unity power factor (p.f.). On changing the excitation only, the armature current will have:
A.	Leading p.f. with under excitation
B.	No change of p.f.
C.	Lagging p.f. with over excitation
D.	Leading p.f. with over excitation
Answer» E.

Discussion

206.	In a synchronous motor, the magnitude of back emf depends on ______.
A.	Speed of the motor
B.	D.C excitation only
C.	Load on the motor
D.	Both the speed and rotor flux
Answer» C. Load on the motor

Discussion

207.	For the following specifications of a synchronous motor, determine the ratio of no load speed to full load speed.200 kVA, 12 pole, 2200 V, 3 phase, 60 Hz
A.	2 : 3
B.	1 : 1
C.	3 : 5
D.	2 : 7
Answer» C. 3 : 5

Discussion

208.	The maximum torque which a synchronous motor will develop at rest for any angular position of the rotor, at rated stator supply voltage and frequency, is known as
A.	ocked-rotor torque
B.	ynchronous torque
C.	ull up torque
D.	eluctance torque
Answer» B. ynchronous torque

Discussion

209.	If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor
A.	ill become more
B.	ill become less
C.	ill remain unchanged
D.	one of the above.
Answer» C. ill remain unchanged

Discussion

210.	The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of
A.	ero degree
B.	wo degrees
C.	ive degrees
D.	en degrees
Answer» D. en degrees

Discussion

211.	A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will
A.	emain same
B.	o down
C.	mprove
D.	one of the above
Answer» D. one of the above

Discussion

212.	If in a synchronous motor, driving mechanical load and drawing current at lagging power factor from constant voltage supply, its field excitation is increased, then its power factor
A.	ecome more
B.	ecome less
C.	emain constant
D.	one of the above
Answer» C. emain constant

Discussion

213.	A three-phase cylindrical rotor synchronous generator has a synchronous reactance Xs and a negligible armature resistance. The magnitude of per phase terminal voltage is VA and the magnitude of per phase induced emf is EA. Considering the following two statements, P and Q,P: For any three-phase balanced leading load connected across the terminals of this synchronous generator, VA is always more than EAQ: For any three-phase balanced lagging load connected across the terminals of this synchronous generator, VA is always less than EAWhich of the following options is correct?
A.	P is false and Q is true
B.	P is true and Q is false
C.	P is false and Q is false
D.	P is true and Q is true
Answer» B. P is true and Q is false

Discussion

214.	A synchronous motor runs at 600 rpm, which of the following case is true?
A.	P = 10, f = 60 Hz
B.	P = 12, f = 60 Hz
C.	P = 8, f = 50 Hz
D.	P = 12, f = 50 Hz
Answer» C. P = 8, f = 50 Hz

Discussion

215.	If the field of a Synchronous motor is under-excited the power factor will be
A.	Lagging
B.	Leading
C.	Unity
D.	More than unity
Answer» B. Leading

Discussion

216.	In synchronous motor, the load on the motor is increased and the rotor progressively tends to:
A.	lead in phase by some angle but it still continues to run synchronously
B.	rise in speed
C.	fall back in phase by some angle but it still continues to run synchronously
D.	fall back in speed
Answer» D. fall back in speed

Discussion

217.	An alternator on open-circuit generates 360 V at 60 Hz when the field current is 3.6 A. Neglecting saturation, determine the open-circuit EMF when the frequency is 40 Hz and the field current is 2.4 A.
A.	110 V
B.	140 V
C.	210 V
D.	160 V
Answer» E.

Discussion

218.	In synchronous motors, if the motor is provided with over excitation, then the power factor becomes ________.
A.	leading
B.	zero
C.	unity
D.	lagging
Answer» B. zero

Discussion

219.	Consider the table given:Constructional featureMachine typeMitigation (P) Damper bars (S) Induction motor (X) Hunting (Q) Skewed rotor slots (T) Transformer (Y) Magnetic locking (R) Compensation winding (U)Synchronous machine (Z) Armature reaction (V) DC machine The correct combination that relates the constructional feature, machine type and mitigation is
A.	P - V - X, Q - U - Z, R - T - Y
B.	P - U - X, Q - S - Y, R - V - Z
C.	P - T - Y, Q - V - Z, R - S - X
D.	P- U - X, Q - V - Y, R - T - Z
Answer» C. P - T - Y, Q - V - Z, R - S - X

Discussion

220.	Before an alternator can be connected to an infinite bus, which of the following must be the same for the alternator and the infinite bus?
A.	Only frequency
B.	Voltage, frequency and phase sequence; and the phase difference between the alternator and infinite voltages must be zero
C.	Only voltage
D.	Only phase sequence
Answer» C. Only voltage

Discussion

221.	In an alternator, when the armature current increases, the terminal voltage drops due to:
A.	Armature reaction
B.	Armature effective resistance, armature leakage reactance and armature reaction
C.	Armature effective resistance
D.	Armature leakage reactance
Answer» C. Armature effective resistance

Discussion

222.	During the operation of a connecting a busbar in parallel with an incoming alternator the:
A.	Terminal voltage and speed of the incoming alternator should be same as that of busbar, but the phase can be different
B.	Terminal voltage and phase of the incoming alternator should be same as that of the busbar, but the speed can be different
C.	Terminal voltage, speed and phase of the incoming alternator need not be same as that of the busbar
D.	Terminal voltage, speed and phase of the incoming alternator should be same as that of the busbar
Answer» C. Terminal voltage, speed and phase of the incoming alternator need not be same as that of the busbar

Discussion

223.	If the excitation of the synchronous generator fails, it acts as a/an
A.	Synchronous generator
B.	Synchronous motor
C.	Induction motor
D.	Induction generator
Answer» E.

Discussion

224.	From no-load to full-load, synchronous motors give:
A.	Constant speed
B.	Variable speed
C.	Gradually increasing speed
D.	Gradually decreasing speed
Answer» B. Variable speed

Discussion

225.	A synchronous generator connected to infinite bus is supplying half of the maximum power which is 1 pu. The synchronizing power coefficient is
A.	0.5 pu
B.	1 pu
C.	0 pu
D.	0.866 pu
Answer» E.

Discussion

226.	A 400 kVA, 11 KV star connected alternator has resistance of 5 ohm per phase. At half load, total short circuit load loss will be:
A.	2582.6 W
B.	1365.8 W
C.	1652.9 W
D.	None of the abvoe
Answer» D. None of the abvoe

Discussion

227.	Damper bars in case of Salient Pole Rotors of hydro-alternators are usually inserted in pole faces to
A.	Strengthen the excitation current of the poles
B.	Damp out the rotor oscillations during transient state owing to sudden change in load conditions
C.	Help improve the power factor of load
D.	Reduce the no-load current when load is thrown-off
Answer» C. Help improve the power factor of load

Discussion

228.	In the design process of an alternator, it is desired that the number of poles be four-pole, three-phase and two-layer winding. Which of the following number of slots is NOT desirable for the design?
A.	12
B.	24
C.	48
D.	32
Answer» E.

Discussion

229.	In a synchronous machine, the magnitude of magnetising reactance is:
A.	directly proportional to air gap length
B.	inversely proportional to air gap length
C.	directly proportional to air gap length squared
D.	constant
Answer» C. directly proportional to air gap length squared

Discussion

230.	In alternators, the ratio of the vector sum of the induced EMFs per coil to the arithmetic sum of the induced EMFs per coil is defined as:
A.	power factor
B.	pitch factor
C.	pole pitch
D.	form factor
Answer» C. pole pitch

Discussion

231.	A 3 phase synchronous motor, connected to an infinite bus, operating at a leading pf with a constant load torque, if excitation is increased (δ is load angle and ϕ power factor angle)
A.	Both δ and ϕ decreases
B.	δ and ϕ both increases
C.	δ increases but ϕ decreases
D.	δ decreases but ϕ increases
Answer» E.

Discussion

232.	A synchronous phase modifier, used for voltage control in a line
A.	Is a static device
B.	Is a rotating machine, driven by an induction motor
C.	Always draws leading var from the bus
D.	None of these
Answer» D. None of these

Discussion

233.	A 75 kW, three-phase star-connected 50 Hz, 440 V cylindrical rotor synchronous motor operates at the rated condition with 0.8 power factor leading. The motor efficiency excluding field and starter losses is 95% and Xs = 2.5 Ω. Calculate the mechanical power developed.
A.	780 W
B.	7 KW
C.	78950 W
D.	79 W
Answer» D. 79 W

Discussion

234.	A constant speed motor is
A.	D.C. shunt motor
B.	Synchronous motor
C.	Induction motor
D.	D.C. compound motor
Answer» C. Induction motor

Discussion

235.	In case of synchronous motor, which of the following are true?a. Synchronous motor is not self-startingb. If the rotor poles are excited by field current and the stator terminals are connected to AC supply, motor vibrates.c. Variable frequency supply is used to start synchronous motor.d. By using frequency converter, a synchronous motor can be brought from standstill to its desired speed.
A.	Only a and b are true
B.	Only b and c are true
C.	All are true
D.	None of the above
Answer» D. None of the above

Discussion

236.	A 3-phase synchronous motor with constant excitation drives a certain load drawing electrical power from an infinite bus at leading power factor. If the shaft load decrease
A.	The power angle decreases while power factor increases
B.	Both power angle and power factor increase
C.	Both power angle and power factor decrease
D.	The power angle increases while power factor decreases
Answer» D. The power angle increases while power factor decreases

Discussion

237.	A three-phase, six-pole, star-connected alternator has the following specifications:Flux per pole is 0.1 Wb54 slots in statorDouble layer windingEach coil has 8 turnsCoil is chorded by 1 slot.Find the no-load phase voltage in the alternator running at 1200 rpm. Assume distribution and pitch factors are unity.
A.	1.92 kV
B.	1.82 kV
C.	1.72 kV
D.	2.2 kV
Answer» B. 1.82 kV

Discussion

238.	For Modern alternator, what will be the specific value of SCR?
A.	3/2
B.	1/6
C.	1/2
D.	3/5
Answer» D. 3/5

Discussion

239.	Let ϕ be the phase angle between the line current and line voltage, and α be the load angle. The per phase mechanical power developed by a synchronous motor is:
A.	\(\frac{{{E_b}V}}{{{X_S}}}\sin \phi \)
B.	\(\frac{{{E_b}V}}{{{X_S}}}\cos \alpha \)
C.	\(\frac{{{E_b}V}}{{{X_s}}}\cos \phi \)
D.	\(\frac{{{E_b}V}}{{{X_S}}}\sin \alpha \)
Answer» E.

Discussion

240.	A three-phase synchronous motor connected to ac mains is running at full load and unity power factor. If its shaft load is reduced by half and field current is held constant then its new power factor will be
A.	Unity
B.	Leading
C.	Lagging
D.	Dependent on machine parameters
Answer» C. Lagging

Discussion

241.	Droop control is related to:
A.	Frequency control of generator
B.	Voltage control of generator
C.	Current control of generator
D.	Impedance control of generator
Answer» B. Voltage control of generator

Discussion

242.	During the short-circuit test of a synchronous machine, which of the following are accounted for in the short-circuit test loss?(i), I2R loss in the armature winding due to the flow of short-circuiting (AC) current(ii) Local core loss caused by armature leakage flux(iii) Core loss due to resultant air-gap flux.(iv) Windage and friction loss
A.	(i), (ii), (iii) and (iv)
B.	Only (i), (ii) and (iii)
C.	Only (ii), (iii) and (iv)
D.	Only (i), (ii) and (iv)
Answer» B. Only (i), (ii) and (iii)

Discussion

243.	Due to which one of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current?
A.	High field current
B.	Low short circuit ratio
C.	High core losses
D.	Low field current
Answer» E.

Discussion

244.	In the method of synchronization of the alternator to the busbar, a synchroscope indicates correct speed when:
A.	the pointer moves towards the right
B.	the pointer vibrates at the center
C.	the pointer points vertically up
D.	the pointer moves towards the left
Answer» D. the pointer moves towards the left

Discussion

245.	In a 3-phase synchronous motor, If the direction of its field current is reversed
A.	The winding of the motor will burn
B.	The motor will stop
C.	The motor will run in the reverse direction
D.	The motor continue to run in the same direction
Answer» E.

Discussion

246.	An alternator’s operating principle is quite similar to that of:
A.	A.C. Generator
B.	D.C. Generator
C.	A.C. Inverter
D.	A.C. Stabilizer
Answer» C. A.C. Inverter

Discussion

247.	A synchronous generator is feeding power to infinite bus bars at unity power factor. Its excitation is now increased. It will feed
A.	The same power but at a leading power factor
B.	The same power but at a lagging power factor
C.	More power at unity power factor
D.	Less power at unity power factor
Answer» C. More power at unity power factor

Discussion

248.	How much load current will a 900 MW star-connected three phase alternator with a voltage of 5 kV per phase deliver when full load is supplied at 0.6 lagging power factor?
A.	1 kA
B.	1000 kA
C.	100 kA
D.	10 kA
Answer» D. 10 kA

Discussion

249.	In the synchronous motor, the breakdown torque is ______.
A.	inversely proportional to the square of the applied voltage
B.	inversely proportional to applied voltage
C.	directly proportional to applied voltage
D.	directly proportional to the square of the applied voltage
Answer» D. directly proportional to the square of the applied voltage

Discussion

250.	A salient pole alternator has Xd = 1.4 pu and Xq = 1pu and Ra = 0 pu. If the alternator develops rated power at UPF and at rated voltage what is the power angle?
A.	45°
B.	60°
C.	30°
D.	55°
Answer» B. 60°

Discussion

Explore topic-wise MCQs in Electrical Engineering.

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In synchronous motor, the load on the motor is increased and the rotor progressively tends to:

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A synchronous generator connected to infinite bus is supplying half of the maximum power which is 1 pu. The synchronizing power coefficient is

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Damper bars in case of Salient Pole Rotors of hydro-alternators are usually inserted in pole faces to

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In a synchronous machine, the magnitude of magnetising reactance is:

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A synchronous phase modifier, used for voltage control in a line

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A 3-phase synchronous motor with constant excitation drives a certain load drawing electrical power from an infinite bus at leading power factor. If the shaft load decrease

For Modern alternator, what will be the specific value of SCR?

Let ϕ be the phase angle between the line current and line voltage, and α be the load angle. The per phase mechanical power developed by a synchronous motor is:

A three-phase synchronous motor connected to ac mains is running at full load and unity power factor. If its shaft load is reduced by half and field current is held constant then its new power factor will be

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In a 3-phase synchronous motor, If the direction of its field current is reversed

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