A synchronous motor is running from an infinite bus of voltage V1 in steady-state, at about 50% of its rated load with a power-angle 1 between Vt and Ef (induced voltage). The load is suddenly decreased to 25%. Ef attains its new steady-state power angle 2 with Vt by initially

falling behind, followed by oscillation about

falling behind and making a complete rotation

advancing and making a complete rotation

falling behind, followed by oscillation about

advancing, followed by oscillation about

A single-phase transformer is feeding a lagging load. What will be the effect on t he voltage regulation of the transformer if (i) an inductor is connected in parallel to the load (ii) a capacitor is connected in parallel to the load The regulation will

decrease in case (i) but increase in case (ii).

increase in case (i) but decrease in case (ii).

decrease in case (i) but increase in case (ii).

A transformer has maximum efficiency at full load. Compare its all-day efficiency when operated (i) Full-load all the time (ii) Full-load half time and half load half time (iii) Full-load half time and no load half time Of these the correct answer is

Efficiency will be the same in all the three cases.

Efficiency will be highest in case (i) and lowest in case (ii).

Efficiency will be lowest in case (i) and highest in case (iii).

Efficiency will be highest in case (i) and lowest in case (iii).

ZPfC (zero power fact or characteristic) of a synchronous generator is obtained by performing

generator is loaded to give varying zero pf lagging current while its excitation is adjusted to give the rated terminal voltage.

generator is loaded to give varying zero pf leading current while its excitation is adjusted to give the rated terminal voltage.

generator is loaded to give rated zero pf leading current and its excitation is adjusted to vary the terminal voltage

generator is loaded to give rated zero pf lagging current and its excitation is adjusted to vary the terminal voltage.

In a generating synchronous machine carrying load (usual symbols are used)

E and V are in phase opposition

If a synchronous generator is rated 10 MVA,11 kV, 0.85 pf, 50 Hz, then the significance of 0.85 pf is that

load of pf higher than 0.85 cannot be supplied.

load of pf lower than 0.85 cannot be supplied.

the prime mover power rating is 8.5 MW.

load of pf higher than 0.85 cannot be supplied.

If the steam input to a turbogenerator connected to infinite busbars in increased, the generator will feed more

leading kVAr to busbars but its power angle will not change

real power to busbars and its power angle will increase

leading kVAr to busbars but its power angle will not change

real power to busbars and its power angle will decrease

lagging kVAr to busbars but its power angle will not change

If the excitation of a turbogenerator feeding lagging current to infinite busbars is increased, then

the generator would feed more leading kVAr

the generator would feed more lagging kVAr

the generator would feed more leading kVAr

Mmf of a sinusoidally distributed current sheet is

sinusoidal in phase with the current wave

triangular shifted from the current by 90

triangular in phase with the current wave

sinusoidal shifted from the current wave by 90

sinusoidal in phase with the current wave

In a salient pole synchronous machine, the airgap is

largest under the middle of the pole shoe and decrease outwards

uniform under the whole pole shoe

least under the middle of the pole shoe and increases outwards

largest under the middle of the pole shoe and decrease outwards

least at one end of the pole shoe and increases to the maximum value at the other end.

Two sinusoidally distributed rotating fields are created by the startor and rotor of a cylindrical magnetic structure. Two fields having same number of poles are rotating in the same direction at the same speed. The interaction of these fields will produce a torque proportional to the sine of the angle between

either of the fields and the resultant field and acts in a direction to increase this angle.

either of the fields and the resultant field and acts in a direction to reduce this angle.

either of the fields and the resultant field and acts in a direction to increase this angle.

the rotor field (but not the startor field) and the resultant field and acts in a direction to reduce this angle.

the rotor field (but not the startor field) and the resultant field and acts in a direction to increase this angle.

In a generating synchronous machine, the rotor field

leads the startor field and electromagnetic torque is developed in the direction of rotation of the rotor.

lags the startor field and electromagnetic torque is developed in a direction opposite to the direction of rotation of rotor.

lags the startor field and electromagnetic torque is developed in the direction of rotation of the rotor.

leads the startor field and electromagnetic torque is developed in the direction opposite to that of rotation of the rotor.

leads the startor field and electromagnetic torque is developed in the direction of rotation of the rotor.

Synchronous reactance is

difference of armature leakage reactance and reactance equivalent of armature reaction.

same as armature leakage reactance.

reactance equivalent of armature reaction.

sum of armature leak age reactance and reactance equivalent of armature reaction.

As observed from the startor of a synchronous generator, the armature reaction mmf phasor

leads the armature current phasor by 90

lags behind the armature current phasor by 90 .

is in phase with the armature current phasor.

leads the armature current phasor by 90

in phase opposition to the armature current phasor.

A synchronous generator is feeding power to infinite busbars 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.

309 + Mcqs in Electrical machines miscellaneous in Electrical Machines Page 1 McqOptions

1.	A single phase 10 kVA, 50 Hz transformer with 1 kV primary winding draws 0.5 A and 55 W, at rated voltage and frequency, on no load. A second transformer has a core with all its linear dimensions 2 times the corresponding dimensions of the first transformer.The core material and lamination thickness are the same in both transformers. The primary windings of both the transformers have the same number of turns. If a rated voltage of 2 kV at 50 Hz is applied to the primary of the second transformer, then the no load current and power, respectively, are
A.	0.7 A, 77.8 W
B.	0.7 A, 155.6 W
C.	1 A, 110 W
D.	1 A, 220 W
Answer» C. 1 A, 110 W

Discussion

2.	A synchronous motor is running from an infinite bus of voltage V₁ in steady-state, at about 50% of its rated load with a power-angle ₁ between V_t and E_f (induced voltage). The load is suddenly decreased to 25%. E_f attains its new steady-state power angle ₂ with V_t by initially
A.	falling behind and making a complete rotation
B.	advancing and making a complete rotation
C.	falling behind, followed by oscillation about
D.	advancing, followed by oscillation about
Answer» C. falling behind, followed by oscillation about

Discussion

3.	A single-phase transformer is feeding a lagging load. What will be the effect on t he voltage regulation of the transformer if (i) an inductor is connected in parallel to the load (ii) a capacitor is connected in parallel to the load The regulation will
A.	decrease in both the cases.
B.	increase in both the cases.
C.	increase in case (i) but decrease in case (ii).
D.	decrease in case (i) but increase in case (ii).
Answer» D. decrease in case (i) but increase in case (ii).

Discussion

4.	A three phase, wound rotor induction motor is to be operated with slip energy recovery in the constant torque mode, when it delivers an output power P_o at slip s. Theoretically, maximum power available for recovery at the rotor terminals, is
A.	P
B.	P
C.	s
Answer» C. s

Discussion

5.	Two coupled dc series motors with constant torque load are changed over from series to parallel connection across a fixed volt age supply. As compared to the original speed the set speed
A.	becomes
B.	times
C.	becomes double
D.	becomes half
E.	remains same
Answer» C. becomes double

Discussion

6.	A dc series motor has linear magnetization and negligible armature resistance, the motor speed is where, T = load torque
A.	directly proportional to
B.	inversely proportional to
C.	directly proportional to T
D.	inversely proportional to T
Answer» C. directly proportional to T

Discussion

7.	A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is
A.	magnetizing
B.	demagnetizing
C.	cross-magnetizing
D.	ineffective
Answer» C. cross-magnetizing

Discussion

8.	A 400 V, 50 Hz, three phase balanced source supplies power to a star connected load whose rating is 12 3 k VA, 0.8 pf (lag). The rating (in kVAR) of the delta connected (capacitive) reactive power bank necessary to bring the pf to unity is
A.	28.78
B.	21.60
C.	16.60
D.	12.47
Answer» E.

Discussion

9.

A 3 phase, 10 kW, 400 V, 4 pole, 50 Hz, star connected induction motor draws 20 A on full load. Its no load and blocked rotor test data are given below :
No Load test : 400V 6 A 1002 W
Blocked Rotor test : 90 V 15 A 762 W

Neglecting copper loss in no Load test and core loss in Blocked Rotor test, estimate motor 's full load efficiency.

A.

76%

B.

81%

C.

82.4%

D.

85%

Answer» C. 82.4%

Discussion

10.	A 3 phase, 4 pole, 400 V, 50 Hz, star connected induction motor has following circuit parameters r₁ = 1.0 Ω , r₂' = 0.5 Ω x₁ = x₂' = 1.2 Ω, x_m = 35 Ω The starting torque when the motor is started direct- on-line is (use approximate equivalent circuit model)
A.	63.6 Nm
B.	74.3 Nm
C.	190.8 Nm
D.	222.9 Nm
Answer» C. 190.8 Nm

Discussion

11.	A synchronous generator is connected to an infinite bus with excitation voltage E_f = 1.3 pu. The generator has a synchronous reactance of 1.1 pu and is delivering real power (P) of 0.6 pu to the bus. Assume the infinite bus voltage to be 1.0 pu. Neglect stator resistance. The reactive power (Q) in pu supplied by the generator to the bus under this condition is _________.
A.	3
B.	1.12
C.	3.10
D.	0.10 to 0.12
Answer» E.

Discussion

12.	A transformer has maximum efficiency at full load. Compare its all-day efficiency when operated (i) Full-load all the time (ii) Full-load half time and half load half time (iii) Full-load half time and no load half time Of these the correct answer is
A.	Efficiency will be the same in all the three cases.
B.	Efficiency will be highest in case (i) and lowest in case (ii).
C.	Efficiency will be lowest in case (i) and highest in case (iii).
D.	Efficiency will be highest in case (i) and lowest in case (iii).
Answer» E.

Discussion

13.	The hysteresis eddy current losses of a single phase transformer working on 200 V, 50 Hz supply are P_h and P_e respectively. The per cent age decreases in these, when operated on a 160 V, 40 Hz supply are
A.	32, 36
B.	20, 36
C.	25, 20
D.	40, 80
Answer» C. 25, 20

Discussion

14.	A star connected 400V, 50Hz, 4 pole synchronous machine gave the following open circuit and short circuit test results : Open circuit test : V_oc = 400 V (rms, line-to-line) at field current, I_f = 2.3 A Short circuit test : I_sc = 10 A (rms, phase) at field current, I_f = 1.5 A The value of per phase synchronous impedance in Ω at rated voltage is _____________
A.	156
B.	15.06
C.	16
D.	5.06
Answer» C. 16

Discussion

15.	A shunt-connected DC motor operates at its rated terminal voltage. Its no-load speed is 200 radians/ second. At its rated torque of 500 Nm, its speed is 180 radian/second. The motor is used to directly drive a load whose load torque T_L depends on its rotational speed (in radians/second), such that T_L = 2.78 _T. Neglecting rotational losses, the steady-state speed (in radian/second) of the motor, when it drives this load is ______.
A.	179.86
B.	179
C.	86
D.	17
Answer» B. 179

Discussion

16.	Two single-phase transformers T₁ and T₂ each rated at 500 k VA are operated in parallel. Percentage impedances of T₁ & T₂ are (1 + j6) and (0.8 + j4.8) respectivel y. To share a load of 1000 k VA at 0.8 lagging power factor, the contribution of T₂ (in kVA) is _________.
A.	614
B.	525
C.	555
D.	None of these
Answer» D. None of these

Discussion

17.	A 400 V, 50 Hz, 4 pole, 1400 rpm, star connected squirrel cage induction motor has the following parameters referred to the stator R_r' = 1.0 Ω , X_s = X_r' = 1.5 Ω Neglect stator resistance and core and rotational losses of the motor. The motor is controlled from a 3-phase voltage source inverter with constant V/f control. The stator line-to-line voltage (rms) and frequency to obtain the maximum torque at starting will be
A.	20.6 V, 2.7 Hz
B.	133.3 V, 16.7 Hz
C.	266.6 V, 33.3 Hz
D.	323.3 V, 40.3 Hz
Answer» C. 266.6 V, 33.3 Hz

Discussion

18.	A dielectric slab with 500 mm 500 mm crosssection is 0.4 m long. The slab is subjected to a uniform electric field of E = 6a_x + 8a_y kV/mm. The relative permittivity of the dielectric material is equal to 2. The value of constant ₀ is 8.85 10¹² F/m. The energy stored in the dielectric in Joules is
A.	8.85 10
B.	8.85 10
C.	88.5
D.	885
Answer» C. 88.5

Discussion

19.	The flux density at a point in space is given by B = 4xa_x +2kya_y + 8a_z Wb/m². The value of Qconstant k must be equal to
A.	2
B.	0.5
C.	+0.5
D.	+2
Answer» B. 0.5

Discussion

20.	The parameters measured for a 220V/110V, 50 Hz, single-phase transformer are : Self inductance of primary winding = 45 mH Self inductance of secondary winding = 30 mH Mutual inductance between primary and secondary windings = 20 mH Using the above parameters, the leakage (L_l1 , L_l2) and magnetizing (L_m) inductances as referred to primary side in the equivalent circuit respectively, are
A.	5mH, 20mH and 40mH
B.	5mH, 80mH and 40mH
C.	25mH, 10mH and 20mH
D.	45mH, 30mH and 20mH
Answer» C. 25mH, 10mH and 20mH

Discussion

21.	A 20-pole alternator is having 180 identical stator slots with 6 conductors in each slot. All the coils of a phase are in series. If the coils are connected to realize single-phase winding, the generated voltage is V₁. If the coils are reconnected to realize three-phase star -connected winding, the generated phase voltage is V₂. Assuming full pitch, single-layer winding, the ratio V₁ / V₂ is
A.
B.	2
Answer» E.

Discussion

22.	If hysteresis loop of a magnetic material has an area of 5 cm² with the scales given as 1 cm = 2 AT and 1 cm = 50 mWb. At 50 Hz, then total hysteresis loss is _______ W .
A.	2.5 W
B.	5 W
C.	15 W
D.	25 W
Answer» E.

Discussion

23.	The area of the hysteresis loop of a given magnetic material is 50 cm² with the two axs scaled with the two axs scaled as 1 cm = 20 AT and 1 cm = 50 mWb. For 50 Hz frequency, the total hysteresis loss is _______ kW .
A.	2.5 kW
B.	25 kW
C.	5 kW
D.	0.25 kW
Answer» B. 25 kW

Discussion

24.	If a 10 pole 25 Hz alternator is directly coupled to and is driven by a 60 Hz synchronous motor, then number of poles in a synchronous motor are (a)
A.	48
B.	12
C.	24
D.	none of these
Answer» D. none of these

Discussion

25.	Fifth harmonic component of the induced voltage in 3-phase ac generator can be entirely eliminated by using a winding pitch of
A.	2/3
B.	4/5
C.	5/6
D.	6/7
Answer» C. 5/6

Discussion

26.	For a three-phase induction motor input power on no load is W_o = 600 watts and windage and friction losses is w_f = 183 watts. The startor copper loss will be equal to _______ W .
A.	183 W
B.	81 W
C.	123 W
D.	18 W
Answer» B. 81 W

Discussion

27.	For a p-pole machine, the relation between electrical ( _e) and mechanical ( _m) degree is given by
A.
B.	= p.
C.
D.	=
Answer» E.

Discussion

28.	A 250 V dc shunt machine has armature circuit resistance of 0.6 Ω. and field circuit resistance of 125 Ω. The machine is connected to 250 V supply mains. The motor is operated as a generator and then as a motor separately. The line current of the machine in both the cases is 50 A. The ratio of the speed as a generator to the speed as a motor is ___.
A.	1.22 to 1.32
B.	1
C.	2
D.	1.44
Answer» B. 1

Discussion

29.	A single phase 10 kVA, 50 Hz transformer with 1 kV primary winding draws 0.5 A and 55 W, at rated voltage and frequency, on no load. A second transformer has a core with all its linear dimensions
A.	0.7 A, 77.8 W
B.	0.7 A, 155.6 W
C.	1 A, 110 W
D.	1 A, 220 W
Answer» C. 1 A, 110 W

Discussion

30.	A 220 V, 15 kW, 1000 rpm shunt motor with armature resistance of 0.25 Ω , has a rated line current of 68 A and a rated field current of 2.2 A. The change in field flux required to obtain a speed of 1600 rpm while drawing a line current of 52.8 A and a field current of 1.8 A is
A.	18.18% increase
B.	18.18% decrease
C.	36.36% increase
D.	36.36% decrease
Answer» E.

Discussion

31.	A 300 kVA, 3300/400 V, three-phase transformer has its primary and secondary windings connected in delta for a dummy load test, for circulating full-load current, the magnitude of the injected voltage in open delta of HV windings is 360 V. The leakage impedance in per -unit system is _______
A.	364
B.	0.0064
C.	64
D.	0.0364
Answer» E.

Discussion

32.	A 220 V, 60 Hz single-phase transformer has hysteresis loss of 340 watts and eddy current loss of 120 watts. If the transformer is operated from, 230 V, 50 Hz supply mains, then total core-loss, assuming steinmetz s constant equal to 1.6 will be _______ w .
A.	119.3 watt
B.	539.3 watt
C.	51.3 watt
D.	None of the above
Answer» C. 51.3 watt

Discussion

33.	A 400 / 100 V, 5kVA, single-phase two winding transformer is converted into an autotransformer to supply 400 volts from a 500 V voltage source. When tested as a two winding transformer at rated load and 0.85 p.f. lagging, efficiency is 0.95, than its efficiency as auto-transformer at rated-load and 0.8 p.f. lagging will be _______ % .
A.	98.95%
B.	45%
C.	38.42%
D.	9.995%
Answer» B. 45%

Discussion

34.	The self inductance of the primary winding of a single phase, 50 Hz, transformer is 800 mH, and that of the secondary winding is 600 mH. The mutual inductance between these two windings i s 480 mH. The secondar y winding of this transformer is short circuited and the primary winding is connected to a 50 Hz, single phase, sinusoidal voltage source. The current flowing in both the winding is less than their respective rated currents. The resistance of both windings can be neglected. In this connection, what is the effective inductance (in mH) seen by the source?
A.	416
B.	440
C.	200
D.	920
Answer» B. 440

Discussion

35.	An induction motor having full load torque of 60 Nm when delta-connected develops a starting torque of 120 Nm. For the same supply voltage, if motor is changed to star -connection, then starting torque developed will be _______ Nm .
A.	40 Nm
B.	80 Nm
C.	150 Nm
D.	All of the above
Answer» B. 80 Nm

Discussion

36.	A 6-pole lap-connected dc generator with 480 conductors has armature resistance of 0.06 ohm. If the conductors are reconnected to form a wave winding, other things remaining unchanged, the value of the armature resistance will be _______ ohm .
A.	18 ohm
B.	54 ohm
C.	0.54 ohm
D.	5.4 ohm
Answer» D. 5.4 ohm

Discussion

37.	ZPfC (zero power fact or characteristic) of a synchronous generator is obtained by performing
A.	generator is loaded to give varying zero pf lagging current while its excitation is adjusted to give the rated terminal voltage.
B.	generator is loaded to give varying zero pf leading current while its excitation is adjusted to give the rated terminal voltage.
C.	generator is loaded to give rated zero pf leading current and its excitation is adjusted to vary the terminal voltage
D.	generator is loaded to give rated zero pf lagging current and its excitation is adjusted to vary the terminal voltage.
Answer» E.

Discussion

38.	In a generating synchronous machine carrying load (usual symbols are used)
A.	E leads V by angle
B.	E lags V by angle
C.	E and V are in phase
D.	E and V are in phase opposition
Answer» B. E lags V by angle

Discussion

39.	If a synchronous generator is rated 10 MVA,11 kV, 0.85 pf, 50 Hz, then the significance of 0.85 pf is that
A.	load of pf lower than 0.85 cannot be supplied.
B.	the prime mover power rating is 8.5 MW.
C.	load of pf higher than 0.85 cannot be supplied.
D.	it is not of any significance.
Answer» C. load of pf higher than 0.85 cannot be supplied.

Discussion

40.	If the steam input to a turbogenerator connected to infinite busbars in increased, the generator will feed more
A.	real power to busbars and its power angle will increase
B.	leading kVAr to busbars but its power angle will not change
C.	real power to busbars and its power angle will decrease
D.	lagging kVAr to busbars but its power angle will not change
Answer» B. leading kVAr to busbars but its power angle will not change

Discussion

41.	If the excitation of a turbogenerator feeding lagging current to infinite busbars is increased, then
A.	the generator would feed more lagging kVAr
B.	the generator would feed more leading kVAr
C.	the generator would feed more leading kVAr
D.	there would be no change
Answer» B. the generator would feed more leading kVAr

Discussion

42.	Mmf of a sinusoidally distributed current sheet is
A.	triangular shifted from the current by 90
B.	triangular in phase with the current wave
C.	sinusoidal shifted from the current wave by 90
D.	sinusoidal in phase with the current wave
Answer» D. sinusoidal in phase with the current wave

Discussion

43.	In a non-salient pole synchronous machine, distribution of field mmf around the air gap is a
A.	sinusoidal wave
B.	rectangular wave
C.	stepped triangular wave
D.	flat topped stepped wave
Answer» E.

Discussion

44.	In a salient pole synchronous machine, the airgap is
A.	uniform under the whole pole shoe
B.	least under the middle of the pole shoe and increases outwards
C.	largest under the middle of the pole shoe and decrease outwards
D.	least at one end of the pole shoe and increases to the maximum value at the other end.
Answer» C. largest under the middle of the pole shoe and decrease outwards

Discussion

45.	Two sinusoidally distributed rotating fields are created by the startor and rotor of a cylindrical magnetic structure. Two fields having same number of poles are rotating in the same direction at the same speed. The interaction of these fields will produce a torque proportional to the sine of the angle between
A.	either of the fields and the resultant field and acts in a direction to reduce this angle.
B.	either of the fields and the resultant field and acts in a direction to increase this angle.
C.	the rotor field (but not the startor field) and the resultant field and acts in a direction to reduce this angle.
D.	the rotor field (but not the startor field) and the resultant field and acts in a direction to increase this angle.
Answer» B. either of the fields and the resultant field and acts in a direction to increase this angle.

Discussion

46.	In a generating synchronous machine, the rotor field
A.	lags the startor field and electromagnetic torque is developed in a direction opposite to the direction of rotation of rotor.
B.	lags the startor field and electromagnetic torque is developed in the direction of rotation of the rotor.
C.	leads the startor field and electromagnetic torque is developed in the direction opposite to that of rotation of the rotor.
D.	leads the startor field and electromagnetic torque is developed in the direction of rotation of the rotor.
Answer» D. leads the startor field and electromagnetic torque is developed in the direction of rotation of the rotor.

Discussion

47.	Synchronous reactance is
A.	difference of armature leakage reactance and reactance equivalent of armature reaction.
B.	same as armature leakage reactance.
C.	reactance equivalent of armature reaction.
D.	sum of armature leak age reactance and reactance equivalent of armature reaction.
Answer» E.

Discussion

48.	In a synchronous machine, the voltage induced by armature reaction flux acts like the voltage drop in
A.	inductive reactance
B.	resistance
C.	capacitive reactance
D.	inductive impedance
Answer» B. resistance

Discussion

49.	As observed from the startor of a synchronous generator, the armature reaction mmf phasor
A.	lags behind the armature current phasor by 90 .
B.	is in phase with the armature current phasor.
C.	leads the armature current phasor by 90
D.	in phase opposition to the armature current phasor.
Answer» C. leads the armature current phasor by 90

Discussion

50.	A synchronous generator is feeding power to infinite busbars 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

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The flux density at a point in space is given by B = 4xax +2kyay + 8az Wb/m2. The value of Qconstant k must be equal to

If hysteresis loop of a magnetic material has an area of 5 cm2 with the scales given as 1 cm = 2 AT and 1 cm = 50 mWb. At 50 Hz, then total hysteresis loss is _______ W .

The area of the hysteresis loop of a given magnetic material is 50 cm2 with the two axs scaled with the two axs scaled as 1 cm = 20 AT and 1 cm = 50 mWb. For 50 Hz frequency, the total hysteresis loss is _______ kW .

If a 10 pole 25 Hz alternator is directly coupled to and is driven by a 60 Hz synchronous motor, then number of poles in a synchronous motor are(a)

Fifth harmonic component of the induced voltage in 3-phase ac generator can be entirely eliminated by using a winding pitch of

For a three-phase induction motor input power on no load is Wo = 600 watts and windage and friction losses is wf = 183 watts. The startor copper loss will be equal to _______ W .

For a p-pole machine, the relation between electrical ( e) and mechanical ( m) degree is given by

A single phase 10 kVA, 50 Hz transformer with 1 kV primary winding draws 0.5 A and 55 W, at rated voltage and frequency, on no load. A second transformer has a core with all its linear dimensions

A 220 V, 15 kW, 1000 rpm shunt motor with armature resistance of 0.25 Ω , has a rated line current of 68 A and a rated field current of 2.2 A. The change in field flux required to obtain a speed of 1600 rpm while drawing a line current of 52.8 A and a field current of 1.8 A is

A 300 kVA, 3300/400 V, three-phase transformer has its primary and secondary windings connected in delta for a dummy load test, for circulating full-load current, the magnitude of the injected voltage in open delta of HV windings is 360 V. The leakage impedance in per -unit system is _______

A 220 V, 60 Hz single-phase transformer has hysteresis loss of 340 watts and eddy current loss of 120 watts. If the transformer is operated from, 230 V, 50 Hz supply mains, then total core-loss, assuming steinmetz s constant equal to 1.6 will be _______ w .

An induction motor having full load torque of 60 Nm when delta-connected develops a starting torque of 120 Nm. For the same supply voltage, if motor is changed to star -connection, then starting torque developed will be _______ Nm .

A 6-pole lap-connected dc generator with 480 conductors has armature resistance of 0.06 ohm. If the conductors are reconnected to form a wave winding, other things remaining unchanged, the value of the armature resistance will be _______ ohm .

ZPfC (zero power fact or characteristic) of a synchronous generator is obtained by performing

In a generating synchronous machine carrying load (usual symbols are used)

If a synchronous generator is rated 10 MVA,11 kV, 0.85 pf, 50 Hz, then the significance of 0.85 pf is that

If the steam input to a turbogenerator connected to infinite busbars in increased, the generator will feed more

If the excitation of a turbogenerator feeding lagging current to infinite busbars is increased, then

Mmf of a sinusoidally distributed current sheet is

In a non-salient pole synchronous machine, distribution of field mmf around the air gap is a

In a salient pole synchronous machine, the airgap is

In a generating synchronous machine, the rotor field

Synchronous reactance is

In a synchronous machine, the voltage induced by armature reaction flux acts like the voltage drop in

As observed from the startor of a synchronous generator, the armature reaction mmf phasor

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

A synchronous motor is running from an infinite bus of voltage V₁ in steady-state, at about 50% of its rated load with a power-angle ₁ between V_t and E_f (induced voltage). The load is suddenly decreased to 25%. E_f attains its new steady-state power angle ₂ with V_t by initially

A single-phase transformer is feeding a lagging load. What will be the effect on t he voltage regulation of the transformer if
(i) an inductor is connected in parallel to the load
(ii) a capacitor is connected in parallel to the load The regulation will

A three phase, wound rotor induction motor is to be operated with slip energy recovery in the constant torque mode, when it delivers an output power P_o at slip s. Theoretically, maximum power available for recovery at the rotor terminals, is

A dc series motor has linear magnetization and negligible armature resistance, the motor speed is
where, T = load torque

A synchronous generator is feeding a zero power factor (lagging) load at rated current.
The armature reaction is

A 3 phase, 4 pole, 400 V, 50 Hz, star connected induction motor has following circuit parameters
r₁ = 1.0 Ω , r₂' = 0.5 Ω
x₁ = x₂' = 1.2 Ω, x_m = 35 Ω
The starting torque when the motor is started direct- on-line is (use approximate equivalent circuit model)

A transformer has maximum efficiency at full load. Compare its all-day efficiency when operated
(i) Full-load all the time
(ii) Full-load half time and half load half time
(iii) Full-load half time and no load half time Of these the correct answer is

The hysteresis eddy current losses of a single phase transformer working on 200 V, 50 Hz supply are P_h and P_e respectively. The per cent age decreases in these, when operated on a 160 V, 40 Hz supply are

Two single-phase transformers T₁ and T₂ each rated at 500 k VA are operated in parallel. Percentage impedances of T₁ & T₂ are (1 + j6) and (0.8 + j4.8) respectivel y. To share a load of 1000 k VA at 0.8 lagging power factor, the contribution of T₂ (in kVA) is _________.

The flux density at a point in space is given by B = 4xa_x +2kya_y + 8a_z Wb/m². The value of Qconstant k must be equal to

If hysteresis loop of a magnetic material has an area of 5 cm² with the scales given as 1 cm = 2 AT and 1 cm = 50 mWb. At 50 Hz, then total hysteresis loss is _______ W .

The area of the hysteresis loop of a given magnetic material is 50 cm² with the two axs scaled with the two axs scaled as 1 cm = 20 AT and 1 cm = 50 mWb. For 50 Hz frequency, the total hysteresis loss is _______ kW .

If a 10 pole 25 Hz alternator is directly coupled to and is driven by a 60 Hz synchronous motor, then number of poles in a synchronous motor are
(a)

For a three-phase induction motor input power on no load is W_o = 600 watts and windage and friction losses is w_f = 183 watts. The startor copper loss will be equal to _______ W .

For a p-pole machine, the relation between electrical ( _e) and mechanical ( _m) degree is given by