Dear reader
thanks for reading like and subscribe and share
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
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.
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
(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
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
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.
