Electrical Machines Test - 4 - PDF Flipbook
Electrical Machines Test - 4
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Electrical
Machines
Test-04Solutions
ELECTRICAL MACHINES
1. Which of the following methods are suitable for the speed
control of squirrel cage induction motors?
1. Voltage control
2. Rotor resistance control
3. Frequency control
4. Pole changing method
Select the correct answer using the codes given below:
a) 2, 3 and 4
b) 1, 3 and 4
c) 1, 2 and 3
d) 2 and 4
Answer: (b)
Solution:
In squirrel cage induction motor rotor winding is not accessible
so rotor resistance control of speed is not possible.
2. The direction of rotation of a d.c shunt motor can be reversed by
interchanging
a) the armature terminals only.
b) either field or armature terminals.
c) the supply terminals.
d) the field terminals only.
Answer: (a)
1
Solution:
T = kϕIa
ϕ ∝ If (Field)
so, T ∝ If and T ∝ Ia
The direction of rotation of a d.c shunt motor can be reversed by
interchanging the armature terminals only because field has a
very high inductance. If shunt field connection is interchanged
then high voltage can be induced which can damage the field
winding.
3. For a distribution sub-station power transformer, operating at
normal voltage and frequency, and with usual 24-hour urban
domestic load-cycle, let the commercial efficiency be ηc, and the
all-day efficiency be ηa. Under the operating conditions stated
above, which one of the following is correct?
a) ηc = ηa
b) ηc > ηa
c) ηc < ηa
d) ηc and ηa cannot be compared, as they have different units.
Answer: (c)
Solution:
A distribution transformer is designed to have low core losses,
as primary is energized even if load on secondary is very less, so
maximum commercial efficiency of transformer is losses than
all day efficiency.
2
4. A 3-phase squirrel cage induction motor has a full load
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 points is
a) less than 0.4
b) greater than 0.6
c) in the range of 0.8 ± 0.1
d) none of the above
Answer: (d)
Solution:
Neglecting stator core losses and copper losses, stator input =
rotor input = Tωs. Mechanical power developed = Mechanical
output = Tωr (if friction and windage losses are treated as a part
of output).
Then efficiency = Tωr = ωr
Tωs ωs
At a slip of 0.6, ωs−ωr = 1 − ωr = 0.6 and ωr = 0.4
ωs ωs ωs
The efficiency should be 0.4. Considering the stator losses
which have been neglected, efficiency at a slip of 0.6 < 0.4.
5. A lap wound dc generator having 250 armature conductors runs
at 1200 rpm. If the generated emf is 200 V then the operating
flux of the dc generator is
a) 0.08 wb
b) 0.04 wb
c) 0.06 wb
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d) 0.02 wb
Answer: (b)
Solution:
Emf, E =
60
Given, N = 1200 rpm, Z = 250, E = 200V
For lap winding, A = P
Hence, 200 = ϕ×1200×250
60
ϕ = 0.04 wb
6. A 2 kVA transformer has iron loss of 150 W and full load
copper loss of 250 W. The maximum efficiency of the
transformer will occur when the total loss is:
a) 500 W
b) 400 W
c) 300 W
d) 275 W
Answer: (c)
Solution:
For maximum efficiency Pcu = Pi
i.e., at ,
Total loss = Pi + Pcu
= Pi + Pi = 2Pi
∴ Total loss = 300W
4
7. The speed/torque characteristics of a universal motor is similar
to which one of the following?
a) DC Shunt motor
b) DC level compound motor
c) DC series motor
d) 3-phase induction motor
Answer: (c)
Solution:
Universal motor for both AC and DC It is a series motor with
high starting torque and high speed
8. A 4 pole, 1200 rpm DC lap wound generator has 1520
conductors. If the flux per poles is 0.01 weber, the emf of
generator is
a) 608 volts
b) 304 volts
c) 152 volts
d) 76 volts
Answer: (b)
Solution:
The emf generated, E = P∅NZ
60A
P = 4, ϕ = 0.01 Weber
A = P = 4 (for lap)
N = 1200 rpm, Z = 1520
∴ Emf, E = 4×0.01×1200×1520 = 304 volts
60×4
5
9. In order to control the speed of a slip ring 3-phase induction
motor through injected voltage in its rotor circuit, this voltage
and the rotor voltage should essentially be
a) In same phase
b) In quadrature
c) In phase opposition
d) Of same frequency
Answer: (d)
Solution:
Injected emf voltage should be of slip frequency which is same
as rotor voltage frequency.
10. A dc shunt generator is delivering 500 A at 220 V. The shunt
field current is 10 A. The armature resistance is 0.01 A. The
stray power is 5000 W. The efficiency of the generator is
a) 91.09 %
b) 95.82 %
c) 95.64 %
d) 91.82 %
Answer: (d)
Solution:
Armature current, Ia = Ish + IL
= (10 + 500) A
= 510A
Poutput = 500 × 220
= 110000 watt = 110 kW
6
Shunt field copper loss = 220 × 10
= 2200 watt
= 2.2 kW
Armature copper loss = (510)2 × 0.01 = 2601 watt
Stray power loss = 5000 watt = 5 kW
Efficiency of the generator is
= 110000 × 100%
110000+[2200+2601+5000]
= 91.818% ≈ 91.82%
11. List-I lists different applications and List-II lists motors for
these applications. Match the application with the most suitable
motor and choose the right combination among the choices
given these after
List - I
A. Food mixer
B. Cassette tape recorder
C. Domestic water pump
D. Escalator
List - II
1. Permanent magnet dc motor
2. Single-phase induction motor
3. Universal motor
4. Three-phase induction motor
5. DC series motor
6. Stepper motor
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Codes:
ABCD
a) 3 6 4 5
b) 1 3 2 4
c) 3 1 2 4
d) 3 2 1 4
Answer: (c)
Solution:
Applications of universal motor are: Vacuum cleaner, drink and
food mixers, portable drills and domestic sewing machine.
Applications of permanent magnet dc motor are:
Air conditioner blowers, electric fuel pumps, toy industries,
cassette tape recorders. Single phase induction motors are used
for domestic water pumps. Three phase induction motor are
used for escalators.
12. Which motor has widest variety of methods for speed control?
a) DC shunt motor
b) Synchronous motor
c) Slip-ring induction motor
d) Schrage motor
Answer: (a)
Solution:
DC shunt motor has the best speed regulation i.e., for the
variation in load the speed change is minimum. Hence, we can
have a wide range of speed in the shunt motor applications.
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Speed regulation = −
This speed regulation is low in case of shunt motor.
13. In a 3-phase induction motor, if Pg is the power transferred
across the air gap and s is the slip, then the ratio of motor copper
loss to the mechanical power developed is given by
a) s
1−s
b) s
c) 1
s
d) 1−s
s
Answer: (a)
Solution:
Rotor copper loss (RCL) = I22r2
Pm = I22r2 (1−s)
s
⇒ RCL = s
Pm 1−s
9
14. A dc shunt motor drives a load at rated speed and supply
voltage. If both the load and voltage are halved, the speed of the
motor will be
a) almost doubled
b) almost halved
c) rated speed
d) slightly less than rated speed
Answer: (d)
15. Rotational losses in electrical machines consist of
a) friction and windage losses
b) stator core, friction and windage losses
c) rotor core, friction and windage losses
d) stray load losses and friction and windage losses
Answer: (b)
16. Which one of the following 3-phase induction motor starting
methods will give the highest starting torque?
a) Auto transformer starter
b) Direct on-line starting
c) Reactor starter
d) Star delta starter
Answer: (b)
Solution:
Torque ∝ V2
For DOL applied voltage is maximum whereas in all other
method reduced voltage is applied to reduce starting current.
10
17. In a shaded pole squirrel cage induction motor the flux in the
shaded part always
a) lead the flux in the unshaded pole segment
b) is in phase with the flux in the unshaded pole segment
c) lags the flux in the unshaded pole segment
d) none of these
Answer: (c)
18. When two identical transformer are available, then the best
method to find their efficiency under load condition is
a) short-circuit test
b) back to back test
c) open-circuit test
d) phase sequence test
Answer: (b)
19. Match List-I (Motors) with List-II (Applications) and select
the correct answer:
List-I
A. 1-phase induction motor
B. Stepper motor
C. Universal motor
D. Brushless dc motor
List-II
1. Fast acting servo drive
2. Hand drilling machine
3. Computer numerical control
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4. Domestic pump
Codes:
A B CD
a) 3 4 1 2
b) 3 4 2 1
c) 4 3 1 2
d) 4 3 2 1
Answer: (d)
Solution:
Domestic pump: 1-ϕ induction motor
Universal motor: Hand drill motor
Stepper motor: Computer numerical control
Brush less dc motor: Servo drive
20. What will happen if a single phase supply is connected to an
ordinary dc series motor?
a) Not run, at all
b) Run, but not satisfactorily
c) Armature burnt out
d) Field winding burnt out
Answer: (c)
21. The stator referred resistance in the equivalent circuit of an
induction motor, representing mechanical output is
a) r11
s
b) r21 �1s − 1�
12
c) r2
s
d) r12 �1s − 1�
Answer: (b)
22. Consider the following statements for a single phase hysteresis
motor:
1. Torque is constant from standstill up to synchronous speech.
2. Rotor is provided with narrow slots embedded with bars.
3. Torque at starting is somewhat more than that at synchronous
speed Ns.
4. As the name suggests, only hysteresis torque is produced
from zero speed right up to Ns.
5. Rotor material possesses very wide hysteresis loop.
Which of the statements given above are correct?
a) 1, 2 and 5
b) 3 and 5
c) 1 and 4
d) 2, 3 and 5
Answer: (b)
Solution:
• Rotor of hysteresis motor is made up of a smooth cylinder of
magnetically hard chrome, cobalt, steel. There is not slot in it.
• B-H curve has wide hysteresis loop.
• At starting Tst = Due to hysteresis and eddy current in rotor.
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But at synchronous speed eddy current is zero.
23. An induction motor is fed from a balanced three-phase supply
at rated voltage and frequency through a bank of three single
phase transformers connected in delta-delta. One unit of the
bank develops fault and is removed. Then
a) single phasing will occur and the machine fails to start
b) single phasing will not occur but the motor terminal voltages
will become unbalanced and the machine can be loaded to the
extent of 57.7% of its rating
c) the machine can be loaded to the extent of 57.7% of its rating
with balanced supply at its terminals
d) the machine can be loaded to the extent of 66 ⅔%with
balanced supply at its terminals
Answer: (c)
Solution:
T1, T2, T3: Transformer bank (turns ratio = 1 assumed for each
transformer.
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1 = = Rated secondary current of each transformer
√3
All three transformers connected in delta Fig.1
An inspiration of the figures shows that even with only two
single-phase transformers in the circuit, the induction motor
receives the same balanced 3-phase supply as it did with three
single-phase transformers.
But figure 2 shows that the induction motor can draw a line
current of I/√3 only when 2 transformers are being used (in
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order to avoid overloading the transformers) while it can receive
a line current of I A when all three transformers are being used.
Without overloading transformers, [VA rating with open-
delta/VA rating with normal delta-delta operation]
= 1 = 57.7%
√3
24. In autotransformer, the number of turns in primary winding is
210 and in secondary winding is 140. If the input current is 60
A, the currents in output and in common winding are
respectively.
a) 40 A, 20 A
b) 40 A, 100 A
c) 90 A, 30 A
d) 90 A, 150 A
Answer: (c)
Solution:
∵ 1 = 1 = 210 = 2
2 2 140 1
∵ 1 = 60
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∴ 2 = 1 × 210
140
= 60 × 210 = 90
140
So, I (common winding current) 90 – 60 = 30A
25. Match List-I (Motors) with List-II (Applications) and select
the correct answer using the codes given below the lists:
List-I
A. Hysteresis motor
B. Shaded-pole motor
C. Reluctance motor
D. Universal motor
List-II
1. Photocopying machine
2. Photograph turntable
3. Electric Shaver
4. Record player
Codes:
A B CD
a) 4 3 2 1
b) 2 1 4 3
c) 4 1 2 3
d) 2 3 4 1
Answer: (c)
17
Solution:
• Hysteresis motor : Extremely quiet operation
Eg: record Player, electric clocks
• Reluctance motor: Constant speed drive e.g. recorder
instrument, phonograph turn tables
• Universal motor: Electric shaver
• Shaded pole motor: Projector, photocopying machine
26. If the field circuit resistance of a dc shunt generator exceeds its
critical value, then generator
a) fails to build up
b) builds up very high voltage
c) exceeds its current capacity
d) produces power beyond its rating
Answer: (a)
27. The iron loss per unit frequency in a ferromagnetic core, when
plotted against frequency, is a
a) Straight line with positive slope
b) Straight line with negative slope
c) Parabola
d) Constant
Answer: (a)
Solution:
Iron loss given by Pi = af + bf2; Where a, b is constant > 0
Now, = + …….(i)
Equation (i) represents straight line with positive slope.
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28. Among single-phase induction motors which one of the
following motors has the least starting torque in per unit terms?
a) Split-phase motor
b) Capacitor motor
c) Capacitor start motor
d) Shaded-pole motor
Answer: (d)
Solution:
Least Tst: Shaded pole motor
Highest Tst: Capacitor motor
29. In a transformer the exciting current has two component
magnetizing constant and core loss component. Neglecting
leakage impedance drop,
a) both of them lag the impressed voltage by 90°
b) both of them are in phase with impressed voltage
c) the former lags the impressed voltage by 90° while the latter
is in phase with impressed voltage
d) the former is in phase with the impressed voltage while the
latter lags the impressed voltage by 90°
Answer: (c)
30. The load characteristic of dc shunt generator is determined by
a) the voltage drops in armature resistance.
b) the voltage drops due to armature reaction, voltage drop due
to decreased field current and voltage drop in armature
resistance.
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c) the voltage drops due to armature reaction and voltage drop
in armature resistance.
d) the voltage drops due to armature reaction, voltage drop due
to decreased field current and voltage drops in armature
resistance and field resistance.
Answer: (b)
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