Electrical Machines
Exam Duration: 45 Mins Total Questions : 30
If a sinusoidal voltage source is connected to a power transformer its no load current would be
- (a)
Sinusoidal and lagging the voltage by \(90^{ 0 }\)
- (b)
Sinsoidal and lagging the voltage by less than \(90^{ 0 }\)
- (c)
rich in third harmonic and its fundamentals would lag the voltage by \(90^{ 0 }\)
- (d)
rich in third harmonic and its fundamental would lag the voltage by less than \(90^{ 0 }\)
Consider the following statements:
At starting the field excitation of a DC shunt motor is kept at its maximum value to reduce
1.sparkling of brushes
2.acceleration time
3.starting current
4.voltage dip in supply
- (a)
1 and 2
- (b)
2,3 and 4
- (c)
1,3 and 4
- (d)
1,2,3 and 4
As compared to the use of a single series DC motor for electric traction for a given starting time t, the series-parallel control using two similar motors with time t/2 for each series and parallel operation would give a saving in starting energy of
- (a)
100%
- (b)
50%
- (c)
25%
- (d)
10%
When circuit test on a transformer is performed at 25 V and 50 Hz, the drawn current \(I_{ 1 }\) is at lagging power factor angle of \(\Phi \) If the test is performed at 25 V and 25 Hz and the drawn current \(I_{ 2 }\) is at a lagging power factor angle of \(\Phi _{ 2 }\) , then
- (a)
\(I_{ 2 }>I_{ 1 }\) and \(\Phi _{ 2 }<\Phi _{ 1 }\)
- (b)
\(I_{ 2 }<I_{ 1 }\) and \(\Phi _{ 2 }<\Phi _{ 1 }\)
- (c)
\(I_{ 2 }>I_{ 1 }\) and \(\Phi _{ 2 }>\Phi _{ 1 }\)
- (d)
\(I_{ 2 }<I_{ 1 }\) and \(\Phi _{ 2 }>\Phi _{ 1 }\)
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
In a synchronous machine, damper windings are used to
- (a)
help in starting as a motor
- (b)
run it as an induction motor
- (c)
help in starting as a motor and to reduce hunting
- (d)
increase efficiency
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
A 1 kVa, 230V/100V, single-phase, 5Hz transformer having negligible winding resistance and leakage inductance is operating under saturation, while 250 V, 50 Hz sinusoidal supply is connected to the high voltage winding. A resistive load is connected to the low voltage winding which draws rated current. Which one of the following quantities will not be sinusoidal?
- (a)
Voltage induced across the low voltage winding
- (b)
Core flux
- (c)
Load current
- (d)
Current drawn from the source
A cumulative compounded long shunt motor is driving a load at rated torque and rated speed. If the series field is shunted by a resistance equal to the resistance of the series field, keeping the torque constant,
- (a)
the armature current increases
- (b)
the motor speed increases
- (c)
the armature current decreases
- (d)
the motor speed decreases
A differentially compounded DC motor with interpoles and with brushes on the neutral axis is to be driven as a generator in the same direction with the same polarity of the terminal voltage.Then, it will be a
- (a)
cumulatively compounded generator but the interpole coil connections are to be reversed
- (b)
cumulatively compounded generator without reversing the interpole coil connections
- (c)
differentially compounded generator without reversing the interpole coil connections
- (d)
differentially compounded generator but the interpole coil connections are to be reversed
The torque speed characteristic of a repulsion motor resembles which of the following DC motor characteristic?
- (a)
Separately excited
- (b)
Shunt
- (c)
Series
- (d)
Compound
The compensating ~inding in a DC machine is located
- (a)
in armature slots for compensation of the armature reaction
- (b)
on commutating poles for improving the commutation
- (c)
on pole shoes for avoiding the flashover at the commutator surface
- (d)
on pole shoes to avoid the sparking at the brushes
Figure shows the magnetization curves of an alternator at rated armature current, unity power factor and also at no load. The magnetization curve for rated armature current, 0.8 power factor leading is given by
- (a)
curve A
- (b)
curve B
- (c)
curve C
- (d)
curve D
A 400 V, 15 kW, 4-poles, 50 Hz Y-connected induction motor has full load siip of 4%.The output torque of the machine at full load is
- (a)
1.66 N-m
- (b)
95.50 -rn
- (c)
99.47 N-m
- (d)
624.73 N-m
Unbalanced supply voltage given to a 3-phase, delta connected induction motor wi II cause
- (a)
zero sequence currents
- (b)
less heating of the rotor
- (c)
negative sequence component current
- (d)
All of the above
A 3-phase squirrel cage induction motor has a full efficiency of 0.8 and a maximum efficiency of 0.9. It is operated at a slip of 0.6 by applying a reduced voltage. The efficiency of the motor at this operating point is
- (a)
less than 0.4
- (b)
grater than 0.6
- (c)
in the range of 0.8 ± 0.1
- (d)
None of the above
No load test on a 3-phase induction motor was conducted at different supply voltages and a plot of input power versus voltage was drawn. This curve was extra-plotted to intersect the Y-axis.This intersection point yields
- (a)
Core loss
- (b)
Stator copper loss
- (c)
Stray load loss
- (d)
Friction and windage loss
On the torque-speed curve of induction motor shown in figure, four points of operation are marked as W, X, Y and Z. Which one of them represents the operation at a slip greater than 1?
- (a)
W
- (b)
X
- (c)
Y
- (d)
Z
A stand alone engine driven synchronous generator is feeding a partly inductive load. A capacitor is now connected across the load to completely nullify the inductive current. For this operating condition
- (a)
the field current and fuel input have to be reduced
- (b)
the field current and fuel input have to be increased
- (c)
the field current has to be increased and fuel input left unaltered
- (d)
the field current has to be reduced and fuel input left unaltered
Figure shows a Δ-Y connected 3-phase distribution transformer used to step down the voltage from 11000V to line.It has two switches S1 and S2.Under normal conditions S1 is closed and S2.Under certain special conditions S1 is open and S2 is closed.In such a case the magnitude of the voltage across the LV terminals a and c is
- (a)
240V
- (b)
480V
- (c)
415V
- (d)
Zero
A permanent magnet DC commutator motor has a no load speed of 6000 rpm when connected to a 120 V DC supply. The armature resistance is 2.50Ω and other losses may be neglected.The speed of the motor with supply voltage of 60 V developing a torque 0.5 Nm, is
- (a)
3000rpm
- (b)
2673rpm
- (c)
2836rpm
- (d)
5346rpm
Determine the correctness or otherwise of the following assertion A and the reason R.
Assertion A Under Vif control of induction motor, the maximum value of the developed torque remains constant over a wide range of speed in the subsynchronous region.
Reason R The magnetic flux is maintained almost constant at the rated value by keeping the ratio Vif constant over the considered speed range
- (a)
Both A and R are true and R is the correct reason for A
- (b)
Both A and R are true but R is not the correct reason for A
- (c)
Both A and Rare false
- (d)
A is true but R is false
A three-phase squirrel cage induction motor has a starting torque of 150% and a maximum torque 300% with respect to rated torque at rated voltage and rated frequency. Neglect the stator resistance and rotational losses.The value of slip for maximum torque is
- (a)
13.48%
- (b)
16.24%
- (c)
18.92%
- (d)
26.79%
A 230 V, 50 Hz, 4-pole, single-phase induction motor is rotating in the clockwise (forward) direction at a speed of 1425 rpm. If the rotor resistance at standstill is 7.8 Ω then, the effective
rotor resistance in the backward branch of the equivalent circuit will be
- (a)
2Ω
- (b)
4Ω
- (c)
78Ω
- (d)
156Ω
A 220 V, 50 Hz, single-phase induction motor has the following connection diagram and winding orientations shown. MM' is the axis of the main stator winding (M,M2) and AA' is that of the auxiliary winding (A,A2).Directions of the winding axis indicate direction of flux when currents in the windings are in the directions shown. Parameters of each winding are indicated. When switch 5 is closed, the motor
- (a)
rotates clockwise
- (b)
rotates anti-clockwise
- (c)
does not rotate
- (d)
rotates momehtarily and comes to a halt
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
A 100 kVA, 415 V (line) star-connected synchronous machine generates rated open-circuit voltage of 415 V at a field current of 15 A.The short-circuit armature current at a field current of lOA is equal to the rated armature current. The per unit saturated synchronous reactance is
- (a)
1.731
- (b)
1.5
- (c)
0.666
- (d)
0.577
A single-phase alternator has a synchronous reactance of 2Ω and negligible resistance.If it supplies 10 A to a purely load at 200 V, the generated emf will be and the regulation will be.......
- (a)
180 V -10%
- (b)
180 V, +10%
- (c)
100 V -10%
- (d)
None of these
The figure above shows coils 1 and 2 with dot markings as shown, having 4000 and 6000 turns respectively. Both coils have a rated current of 25 A. Coil 1 is excited with single-phase, 400 V, 50 Hz supply.
The coils are to be connected to obtain a single-phase, 400/100 V, autotransformer to drive a load of 10 kVA. Which of the options given should be exercised to realize the required autotransformer?
- (a)
Connect A and 0; Common B
- (b)
Connect Band 0; Common C
- (c)
Connect A and C; Common B
- (d)
Connect A and C; Common B