MCQ For Electrical Eng.-3



1.      The power angle characteristic of a machine-infinite bus system is P= 2 sin  (pu).
The initial operating angle is 60 deg., inertia constant H= 5 sec. System frequency is 50 Hz. The angular frequency of oscillation following small perturbation will be
a.       sqrt(31.4) rad/sec
b.      sqrt(15.7) rad/sec
c.       sqrt(62.8) rad/sec
Ans.: (a)

2.      2.        In an interconnected power system ,the frequency of electro-mechanical modes of oscillation lies in the range
(a) 0.5-2.5 Hz
(b) 1-10 Hz
(c) 30-60 Hz

Ans.: (a) Small  and large perturbations

3.      3.         If  the sending end and receiving end voltages  for a 3-phase transmission  line are each 33kV(line), and if the reactance of the line is 13 ohms per phase, the maximum power transmitted per phase will be

a.       a.        60 MW
b.      b.       30 MW
c.       c.        29 MW
d.      d.       28 MW

Ans. d

4.      4.        The torque angle corresponding to the steady-state stability limit of a salient-pole alternator is

a.       a.        Less than 90 deg.
b.      b.       Greater than 90 deg.
c.       c.        Equal to 90 deg.

Ans. a

5.      5.        When the alternator stalls (near the stability limit) the armature current is

a.       a.        Less than the rated value
b.      b.       Greater than the rated value
c.       c.        Equal to the rated value.

Ans. a










INDUCTION  MOTOR

  1. The speed at which the rotating magnetic field produced by stator currents rotates in an induction motor is:

(A)  Synchronous speed

(B) Rotor speed
(C) Greater than synchronous speed

Ans.  (A)

2.  A 3-phasde 50 Hz induction motor runs at speed of 940 RPM; the speed of the rotating magnetic field will be:


(A) 940 RPM
(B) 1000RPM
(C) 1050 RPM
(D) 1100 RPM

Ans.  (B)

  1. The no-load speed of an induction motor is of the order of

(A) 8%
(B) 4%
      (C) 2%
(D) 6%

Ans.  (C)
                                                                                                                                      

  1. Iron losses in the rotor of a 3-phase induction motor are negligible, because

(A)    Frequency of rotor emf is too low
(B)    Flux linking the rotor is of constant magnitude
(C)    Flux density in the rotor parts is too low

Ans: (A)

  1. Full-load copper losses in a 3-phase 50 Hz 4-pole induction motor running at 1455 rpm are 300 W. The rotor input is
 (A)    5 kW
       (B)    10 kW
(C)    20 kW
(D)    50 kW

Ans.  (B)

  1. The no-load current of a 3-phase induction motor in terms of its full-load current is of the order of
(A)    5%
(B)    10%
(C)    25%
(D)    50%
 Ans.  (C)

  1. The power factor of a 3-phase induction motor at no load is approximately

(A)   0.2
(B)    0.7
(C)    0.85
(D)    1

Ans: (A)

8.The maximum torque in a 3-phase induction motor occurs at a slip

(A)       R2/ X2
(B)       X2/ R2
(C)      R22/ X2
(D)      R22/ X22

Ans: (A)

9. For high starting torque, the most 3-phase induction motor is

(A)    Squirrel-cage type
(B)    Slip-ring type
(C)    Deep bar squirrel-cage type
(D)    Double-cage induction motor

Ans.  (B)

9.      10.     The starting torque in a single-phase induction motor is
 (A)   High
(B)    Low
                  (C)    Zero
(D)    Very low

Ans.  (C)

11.     The efficiency of a single-phase induction motor as compared to a 3-phase induction motor rating is
(A)   Lower
(B)    Higher
(C)    Equal

Ans: (A) 
10.  12.     The starting torque of a 3-phase induction motor can be increased by increasing
                                            a       the rotor reactance

b      the rotor resistance

c       the stator resistance

Ans. B

11.  13.  The maximum starting torque in the induction motor is developed when the rotor reactance is

a)      a)      half of  the rotor resistance
b)      b)       twice of the rotor resistance
c)      c)       equal to the  rotor resistance

Ans. C

12.  14.  A change of 55 in the supply voltage of an induction motor will produce  a change of approximately

a)      a)      5%
b)      b)       7%
c)      c)       10%
d)     d)       25%
in the rotor torque.
Ans. D

13.  15.     The transformation ratio of an induction motor can be found by

a)      a)       blocked rotor test
b)      b)       open circuit test
c)      c)       slip test
d)     d)       stator resistance test

14.  16.     The crawling in the induction motor is caused by

a)      a)       improper design of the machine
b)      b)       low supply voltage
c)      c)       high loads
d)     d)       harmonics developed in the motor.

Ans. D

15.  17.     Which one of the following motors is most suitable for signalling devices and timers?

a)      a)       dc series motor
b)      b)       ac series motor
c)      c)       induction motor
d)     d)       reluctance motor

Ans. D








Synchronous motors

  1. The damper winding in the synchronous moor is often used to

a)       Prevent hunting only
b)       Prevent hunting & provide the starting toque
c)       Maintain synchronism
d)       provide the starting torque only.
Ans. b
  1. Which of the following statements is true?
Short-pitched coils in the alternators are used to

a)       reduce the size of the machine
b)       reduce the harmonics or to eliminate the harmonics from the generated emf
c)       provide accurate phase difference of 120 degrees  between phases
d)       reduce copper losses.

Ans. b

  1. The armature flux of an alternator  at unity power factor load  will be

a)       demagnetizing
b)       of square waveform
c)       cross-magnetizing

Ans. c

  1. The armature reaction of an alternator  will be completely magnetizing  when
load power factor  is
a)       unity
b)       zero lagging
c)       zero leading
d)       0.8 lagging

Ans. c

  1. If two alternators are running in parallel and he voltage of one machine is suddenly increased
a)       the machine winding will burn
b)       both machines will stop
c)       synchronizing torque will be produced to restore synchronism

Ans. c

  1. If the  input  to the prime mover of an alternator ( in a system of three alternators) is kept constant, and the excitation is changed, then

  1. the reactive component of the output is changed
  2. the active component of the output is changed
  3. the power factor of the load remains  the same
  4. (a)and (b)  occur simultaneously.
 Ans. a



  1. The coupling angle or load angle of a synchronous motor is defined as the angle between the

a)      a)       rotor and the stator poles  of same polarity
b)      b)       rotor and the stator poles  of opposite  polarity
c)      c)       rotor and stator teeth

Ans. b



TRANSIENT STABILITY -OBJECTIVE TYPE QUESTIONS
  1. The critical clearing angle of a given power system for a certain fault is
  1. proportional to the Inertia Constant M
  2. proportional to the Inertia Constant M
  3. independent of M.
Ans.: ©
  1. Equal area criterion of stability is applicable to
  1. a machine infinite bus system only
  2. both to a machine-infinite bus and a two-machine system
  3. a multi-machine system.
Ans.: (b)
  1. In a two-machine power system, machine A delivers power to machine B. A 3-phase fault occurs at the terminals of machine A. Initial acceleration of machine A is
(a) positive
(b) zero
  1. negative
Ans.: (a)
  1. Which one of the following enhances the transient stability of a system the most
  1. proper choice of make and break capabilities of the circuit breakers
  2. installation of 3-pole auto-reclose circuit breakers
  3. installation of single pole auto-reclose circuit breakers
Ans.: (c )
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