Electrical Engineering - Electrical Machines
Exam Duration: 45 Mins Total Questions : 30
The primary coil of three phase transformer is connected in star The secondary coil is connected in delta If suffixes 1 are connected to 2 then the phase difference between the phase and neutral on the delta side compared to phase-neutral on the star side is
- (a)
\(-30^{ 0 }\)
- (b)
\(+30^{ 0 }\)
- (c)
\(+90^{ 0 }\)
- (d)
\(-90^{ 0 }\)
A 2400/240 V,200 KVA single-phase transformer has a core loss of 1.8 KVA single-phase transformer has a core loss of 1.8 KW at rated voltage.Its equivalent resistance is 1.1% Then the transformer efficiency at 0.9 PF and on full load is
- (a)
97.57%
- (b)
98.05%
- (c)
97.82%
- (d)
95.56%
Consider the following statements :
For a level compounded speed the series field mmf must effectively compensate------------
1.Armature reaction mmf
2.Armature resistance voltage drop
3.Brush contact voltage drop
of these statements correct statements is/are
- (a)
Only 2
- (b)
1 and 2
- (c)
1 and 3
- (d)
1,2 and 3
Match List I with List II and select the correct answer using the codes given below the lists
List I | List II |
P.Single-phase induction motor | 1.Rotor resistance starting |
Q.3-phase slip-ring induction motor | 2.Not self starting |
R.3-phase squirrel cage induction motor | 3.Autotransformer starting |
- (a)
P Q R 2 1 3 - (b)
P Q R 1 2 3 - (c)
P Q R 2 3 1 - (d)
P Q R 3 1 2
Slip test is performed to determine
- (a)
slip
- (b)
direct axis reactance and quadrature axis
- (c)
positive sequence reactance and negative sequence reactance
- (d)
sub-transient reactance
A 6-pole, 50Hz wound rotor induction motor when supplied at the rated voltage and frequency with slip-ring open-circuited, developed a voltage of 10V between any two two rings.If the rotor is direction of stator field, the frequency of voltage across slip-rings will be
- (a)
Zero
- (b)
50Hz
- (c)
100Hz
- (d)
200Hz
Consider the following statements:
The synchronous generators used in thermal power stations have
1. cylindrical rotor.
2. slip rings and brushes.
3. laminated rotor.
4. stator slots in multiples of 6.
5. 3-phase star-connected stator windings.
Of these statements, the correct statements are
- (a)
1, 2, 3 and 4
- (b)
1, 2, 3 and 5
- (c)
3, 4 and 5
- (d)
1, 2, 4 and 5
In a 3-phase star-connected generator, if the fundamental and third harmonic rms voltages in each phase are respectively V1 and V3 the line voltage of the generator wiII be
- (a)
\(\sqrt{3(V_1^2+V_3^2}\)
- (b)
\(\sqrt3 V_1\)
- (c)
\(\sqrt3\left(V_1+{V_3\over3}\right)\)
- (d)
None of these
Match List I(type of single-phase motor) with List II (type of appliance) and select the correct answer using the codes given below the lists
List I | List II |
---|---|
P.Permanent magnet type | 1.Rocket |
Q.Shaded pole | 2.Refrigerator compressor |
R.Universal | 3.Sewing machine |
S.Capacitor-shunt, capacitor-run induction motor | 4.Photocopying machine |
- (a)
P Q R S 1 2 3 4 - (b)
P Q R S 1 2 4 3 - (c)
P Q R S 4 1 3 2 - (d)
P Q R S 1 4 3 2
Two transformers, of different kVA rating working in parallel, share the load in proportion to their rating when their
- (a)
per unit leakage impedances on the same kVA base are the same
- (b)
per unit leakage impedance on their respective rating are equal
- (c)
ohmic values of the leakage impedances are inversely proportional to their rating
- (d)
ohmic values of the magnetising reactances are the same
Two transformers of identical voltage but of different capacities are operating in parallel.For satisfactory load sharing
- (a)
impedances must be equal
- (b)
per unit impedances on their own rating must be equal
- (c)
per unit impedances and X/R ratios must be equal
- (d)
impedances and X/R ratios must be equal
The hysteresis and eddy current losses of a single phase transformer working on 200V, 50Hz supply are Ph and Pe respectively.The percentage decrease in these, when operated on a 160V, 40Hz supply are
- (a)
32, 36
- (b)
20, 36
- (c)
25, 20
- (d)
40, 80
Which three-phase connection can be used in a transformer to introduce a phase difference of 30° between its output and corresponding input line voltages?
- (a)
Stat-star
- (b)
Star-delta
- (c)
Delta-delta
- (d)
Delta-zigzag
In order to keep the magnetization current at the same level as that for normal 50 Hz supply, at 25 Hz the supply voltage should be
- (a)
230V
- (b)
460V
- (c)
115V
- (d)
65V
Skew is used in induction motors in order to reduce torque due to
- (a)
time harmonics
- (b)
space harmonics
- (c)
slot harmonics
- (d)
reverse rotating fields
A 6-pole 50 Hz induction motor rotating at 1400 rpm is in mode
- (a)
generating
- (b)
breaking
- (c)
motoring
- (d)
None of these
If a 400 V, 50 Hz, star connected, 3-phase squirrel cage induction motor is operated from a 400 V, 75 Hz supply, the torque that the motor can now provide while drawing rated current from the supply?
- (a)
Reduces
- (b)
Increases
- (c)
Remain the same
- (d)
Increase or reduces, depending upon the rotor resistance
A 500 MV A, 11 kV synchronous generator has 0.2 pu synchronous reactance. The per unit synchronous reactance on the base values of 100 MVA and 22 kV is
- (a)
0.16
- (b)
0.01
- (c)
4.0
- (d)
0.25
It is desirable to eliminate 5th harmonic voltage from the phase voltage of an alternator.The coils should be short-pitched by an electrical angle of
- (a)
300
- (b)
360
- (c)
720
- (d)
180
For a given stepper motor, which of the following torque has the highest numerical value?
- (a)
Detent torque
- (b)
Pull-in torque
- (c)
Pull-out torque
- (d)
Holding torque
A 50 Hz transformer having equal hysteresis and eddy current losses at rated excitation is operated at 45 Hz at 90% of its rated voltage. Compared to rated operating point, the core losses under this condition
- (a)
reduce by 10%
- (b)
reduce by 19%
- (c)
reduce by 14.5 %
- (d)
remain unchanged
A DC series motor driving an electric train faces constant power load. It is running at rated speed and rated voltage. If the speed has to be brought down to 0.25 pu the supply voltage has to be approximately brought down to
- (a)
0.75pu
- (b)
0.5pu
- (c)
0.25pu
- (d)
0.125pu
The armature resistance of a permanent magnet DC motor is 0.8 n. At no load, the motor draws 1.5 A from a supply voltage of 25 V and runs at 1500 rpm. The efficiency of the motor while it is operating on load at 1500 rpm drawing a current of 3.5 A form the same source will be
- (a)
48.0%
- (b)
57.1%
- (c)
59.2%
- (d)
88.8%
Two magnetic poles revolve around a stationary armature carrying two coils (C1 -C'1 , C2 -C'2) as shown in figure.Consider the instant when the poles are in a position as shown. Identify the correct statement regarding the polarity of the induced emf at this instant in coil sides Cl and C2•
- (a)
⊙ in c1, no emf in c2
- (b)
⊗ in c1, no emf in c2
- (c)
⊙ in c2, no emf in c1
- (d)
⊗ in c2, no emf in c1
A 50 kW DC shunt motor is loaded to draw rated armature current at any given speed. When driven
(i) an half the rated speed by armature voltage control and ..................
(ii) at 1.5 times the rated speed by field control, the respective output powers delivered by the motor are approximately ............
- (a)
25 kW in (i) and 75 kW in (ii)
- (b)
25 kW in (i) and 50 kW in (ii)
- (c)
50 kW in (i) and 75 kW in (ii)
- (d)
50 kW in (i) and 50 kW in (ii)
A 220 V DC machine supplies 20 A at 200 V as a generator.The armature resistance is 0.2 Ω. Now, if the machine is operated as a motor at same terminal voltage and current but with the flux increased by 10%, then ratio of motor speed to generator speed is
- (a)
0.87
- (b)
0.95
- (c)
0.96
- (d)
1.06
A single-phase, 2000 V alternator has armature resistance and reactance of 0.8 nand 4.94 n respectively.The voltage regulation of the alternator at 100 A load at 0.8 leading power factor is
- (a)
7%
- (b)
-8.7%
- (c)
14%
- (d)
zero
The flux per pole in a synchronous motor with the field circuit on and the stator disconnected from the supply is found to be 25 mWb. When the stator is connected to the rated supply with the field excitation unchanged, the flux per pole in the machine is found to be 20 mWb while the motor is running on no load. Assuming no load losses to be zero, the no load current drawn by the motor from the supply
- (a)
lags the supply voltage
- (b)
leads the supply voltage
- (c)
is in phase with the supply voltage
- (d)
is zero
A 100 kVA, 6.6 kV, 3-phase star connected cylindrical pole synchronous generator has a synchronous reactance of 20 Ω. Neglect the armature resistance and consider operation at full load and unity power factor.The power (or torque) angle is close to
- (a)
13.90
- (b)
18.30
- (c)
24.60
- (d)
33.00
A 100 kVA, 11 kV/400 V, 3-phase transformer has its maximum possible efficiency of 98%, when it delivers 80 kVA at unity pf and rated voltage. Maximum possible voltage regulation of the transformer is 4%. The rated kVA output at 0.8 pf lagging.
- (a)
2.3%
- (b)
3.3%
- (c)
4.4%
- (d)
5.5%