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4. A field excitation of 20 A in a certain alternator
results in an armature current of 400 A in short circuit and a terminal voltage
of 2000 V on open circuit. The magnitude of the internal voltage drop
within the machine at a load current of 200 A is
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Previous gate question Topic Electrical
Machines
1. The slip of an induction motor normally does not depend on
(A) rotor speed (B) synchronous speed
(C) shaft torque (D) core-loss component
Ans: Option (D) is correct.
ns = synchronous speed
n = rotor speed
Thus, slip depend on synchronous speed and the rotor speed. Also,
torque
increases with increasing slip up to a maximum value and then
decreases.
Slip does not depend on core/loss component
2. A 4 point starter is used to start and control the speed of a
(A) dc shunt motor with armature resistance control
(B) dc shunt motor with field weakening control
(C) dc series motor
(D) dc compound motor
ans: Option (A) is correct.
3.A three phase, salient pole synchronous motor is connected to an
infinite bus. It is operated at no load a normal excitation. The field
excitation of the motor is first reduced to zero and then increased in reverse
direction gradually. Then the armature current.
(A) Increases continuously
(B) First increases and then decreases steeply
(C) First decreases and then increases steeply
(D) Remains constant
Ans: Option (B) is correct.
The Back emf will go to zero when field is reduced, so Current
input will be increased. But when Field increases (though in reverse
direction) the back emf will cause the current to reduce.
(A) 1 V (B) 10 V
(C) 100 V (D) 1000 V
Ans Option (D) is correct.
Given field excitation of = 20 A
Armature current = 400 A
Short circuit and terminal voltage = 200 V
On open circuit, load current = 200 A
So, Internal resistance = 2000/400 =5 Ω
Internal vol. drop = 5x 200
= 1000 V
5. Distributed winding and short chording employed in AC machines
will result in
(A) increase in emf and reduction in harmonics
(B) reduction in emf and increase in harmonics
(C) increase in both emf and harmonics
(D) reduction in both emf and harmonics
Ans Option (D) is correct Distributed winding and short
chording employed in AC machine will result in reduction of emf and harmonics
6. In a stepper motor, the detent torque means
(A) minimum of the static torque with the phase winding excited
(B) maximum of the static torque with the phase winding excited
(C) minimum of the static torque with the phase winding unexcited
(D) maximum of the static torque with the phase winding unexcited
Ans Option (D) is correct.
Detent torque/Restraining toque: The residual magnetism in the
permanent magnetic material produced. The detent torque is defined as the
maximum load torque that can be applied to the shaft of an unexcited motor
without causing continuous rotation. In case the motor is unexcited.
7. The speed of rotation of stator magnetic field with respect to
rotor structure will be
(A) 90 rpm in the direction of rotation
(B) 90 rpm in the opposite direction of rotation
(C) 1500 rpm in the direction of rotation
(D) 1500 rpm in the opposite direction of rotation
Ans Option (A) is correct.
Given 3-φ, 440 V, 50 Hz, 4-Pole slip ring motor
Motor is coupled to 220 V
Ns =1500 rpm ,
Relative speed = 1500 − 1410
= 90 rpm in the direction of rotation.
8.In a transformer, zero voltage regulation at full load is
(A) not possible
(B) possible at unity power factor load
(C) possible at leading power factor load
(D) possible at lagging power factor load
Ans : Option (C) is correct.
In transformer zero voltage regulation at full load gives leading
power factor.
9. The dc motor, which can provide zero speed regulation at full
load without any controller is
(A) series
(B) shunt
(C) cumulative compound
(D) differential compound
Option (B) is correct.
10. 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,
with field current held constant, its new power factor will be
(A) unity
(B) leading
(C) lagging
(D) dependent on machine parameters
Ans : Option (B) is correct. When the 3-φ synchronous motor running
at full load and unity power
factor and shaft load is reduced half but field current is constant
then it
gives leading power factor
11. A three-phase, three-stack, variable reluctance step motor has
20 poles on each rotor and stator stack. The step angle of this step motor is
(A) 3c (B) 6c
(C) 9c (D) 18c
Ans : Option (B) is correct. Given 3-φ, 3-stack
Variable reluctance step motor has 20-poles
Step angle = (360)/ (3x20)= 6
12. In transformers, which of the following statements is valid ?
(A) In an open circuit test, copper losses are obtained while in
short circuit
test, core losses are obtained
(B) In an open circuit test, current is drawn at high power factor
(C) In a short circuit test, current is drawn at zero power factor
(D) In an open circuit test, current is drawn at low power factor
ans : Option (D) is correct.
In transformer, in open circuit test, current is drawn at low power
factor but
in short circuit test current drawn at high power factor.
13.For a single phase capacitor start induction motor which of the
following statements is valid ?
(A) The capacitor is used for power factor improvement
(B) The direction of rotation can be changed by reversing the main
winding
terminals
(C) The direction of rotation cannot be changed
(D) The direction of rotation can be changed by interchanging the
supply
terminals
Ans: Option (B) is correct.
A single-phase capacitor start induction motor. It has cage rotor
and its
stator has two windings. The two windings are displaced 90c in
space. The direction of rotation can
be changed by reversing the main winding terminals.
14. In a DC machine, which of the following statements is true ?
(A) Compensating winding is used for neutralizing armature
reaction while
interpole winding is used for producing residual flux
(B) Compensating winding is used for neutralizing armature
reaction while
interpole winding is used for improving commutation
(C) Compensating winding is used for improving commutation while
interpole winding is used for neutralizing armature reaction
(D) Compensation winding is used for improving commutation while
interpole winding is used for producing residual flux
Ans: Option (B) is correct.
In DC motor, compensating winding is used for neutralizing armature
reactance while interpole winding is used for improving
commutation.
Interpoles generate voltage necessary to neutralize the e.m.f of
self induction
in the armature coils undergoing commutation. Interpoles have a
polarity
opposite to that of main pole in the direction of rotation of
armature.
15.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
Option (B) is correct.
A synchronous generator is feeding a zero power factor(lagging)
load at
rated current then the armature reaction is demagnetizing.
16. In relation to the synchronous machines, which on of the
following statements
is false ?
(A) In salient pole machines, the direct-axis synchronous
reactance is greater
than the quadrature-axis synchronous reactance.
(B) The damper bars help the synchronous motor self start.
(C) Short circuit ratio is the ratio of the field current required
to produces
the rated voltage on open circuit to the rated armature current.
(D) The V-cure of a synchronous motor represents the variation in
the
armature current with field excitation, at a given output power.
Ans: Option (C) is correct.
In synchronous machine, when the armature terminal are shorted the
field
current should first be decreased to zero before started the
alternator.
In open circuit the synchronous machine runs at rated synchronous
speed.
The field current is gradually increased in steps.
The short circuit ratio is the ratio of field current required to
produced the
rated voltage on open to the rated armature current
17. For a given stepper motor, the following torque has the highest
numerical value
(A) Detent torque (B) Pull-in torque
(C) Pull-out torque (D) Holding torque
Option (C) is correct.
Stepper motor is rotated in steps, when the supply is connected
then the
torque is produced in it. The higher value of torque is pull out
torque and
less torque when the torque is pull in torque.
18. The following motor definitely has a permanent magnet rotor
(A) DC commutator motor (B) Brushless dc motor
(C) Stepper motor (D) Reluctance motor
MCQ 4.84 The type of single-phase induction motor having the
highest power factor
at full load is
(A) shaded pole type (B) split-phase type
(C) capacitor-start type (D) capacitor-run type
Option (C) is correct.
The stepper motor has the permanent magnet rotor and stator has
made of
windings, it’s connected to the supply
19.A single-phase induction motor with only the main winding excited
would
exhibit the following response at synchronous speed
(A) Rotor current is zero
(B) Rotor current is non-zero and is at slip frequency
(C) Forward and backward rotaling fields are equal
(D) Forward rotating field is more than the backward rotating
field
Ans: Option (D) is correct.
Given that: 1-φ induction motor main winding excited then the
rotating
field of motor changes, the forward rotating field of motor is
greater then
the back ward rotating field.
20. The core flux of a practical transformer with a resistive load
(A) is strictly constant with load changes
(B) increases linearly with load
(C) increases as the square root of the load
(D) decreases with increased load
Option (A) is correct.
We know that in case of practical transformer with resistive load,
the core
flux is strictly constant with variation of load.
21. In the protection of transformers, harmonic restraint is used to
guard against
(A) magnetizing inrush current (B) unbalanced operation
(C) lightning (D) switching over-voltages
Option (A) is correct.
In transformer protection, harmonic restraint is used to guard
against
“Magnetizing inroush current”.
22. In case of an armature controlled separately excited dc motor
drive with
closed-loop speed control, an inner current loop is useful because
it
(A) limits the speed of the motor to a safe value
(B) helps in improving the drive energy efficiency
(C) limits the peak current of the motor to the permissible value
(D) reduces the steady state speed error
Option (C) is correct.
For armature controlled separately excited dc motor drive with
closed loop
speed control. We use inner current loop because inner current loop
limits
the peak current of motor to the permissible value.
