Electrical Machine-II Previous Year solved Paper Makaut-(wbut) 5th sem Electrical Engineering-

Makaut-(wbut) 5th sem Electrical Engineering

Electrical Machine-II

Question-2009

MCQ

i)In a double Revolving field Theory ,The slip of the forward motor is S, then the slip of the backward motor is

a) 2S b) S

c) 2–S d) S–2.

Ans-(c)

ii) In a shaded pole motor, shading coils are used to

a) Reduce winding losses

b) Reduce friction losses

c) Produce rotating magnitic field

d) Protect against sparking.

Ans-(c)

iii) A universal motor is one which has

a) constant speed

b) constant output

c) capability of operating both on a.c. and d.c. with comparable performance

d) maximum efficiency.

Ans-(c)

iv) The maximum possible speed at which an alternator can be driven to generate 50 Hz and 4000 V is

a) 4000 r.p.m. b) 3600 r.p.m.

c) 3000 r.p.m. d) 1500 r.p.m.

Ans-(c)

v) The direction of rotation of single-phase induction motor can be reversed by

a) reversing the leads of the main winding 

b) reversing the leads of the auxilliary winding

c) reversing the supply leads

d) either (a) or (b).

Ans-(d)

vi) A capacitor start and capacitor run induction motor is supplied from

a) single-phase supply  b) 2-phase supply

c) 3-phase supply d) none of these.

Ans-(a)

vii) The electrical angle, β between adjacent slots in  a 4-pole alternation with 36 slots is in electrial degree

a) 9˚ b) 10˚

c) 20˚ d) 90˚.

Ans-(a)

viii) Which of the following motors will give relatively high starting torque.

a) capacitor start motor b) capacitor run motor

c) split phase motor  d) shaded pole motor.

Ans-(a)

ix) In single-phase repulsion motor, power factor is

a) always leading  b) always unit

c) always lagging  d) none of these.

Ans-

x) Induction generator runs at

a) supersynchronous speed

b) sub-synchronous speed

c) synchronous speed

d) none of these.

Ans-(a)

xi) When the synchronous motor runs at synchronous speed, the voltage induced in the damper winding is

a) maximum  b) minimum

c) zero d) none of these.

Ans-(c)

Electrical Machine-II

Question-2010

MCQ

1)In  an  alternator  the  voltage  generated  per  phase  is proportional to

a)  number of turns per coil

b)  flux per pole

c)  frequency of wave form 

d)  all of these.

Ans-(d)

2)In which single phase motor, the rotor has no teeth or winding ?

a)  Split phase motor

b)  Reluctance motor

c)  Hysteresis motor

d)  Universal motor. 

Ans-(c)

iii)  In a synchronous motor, damper winding is provided in order to

a)  stabilize rotor motion 

b)  suppress rotor oscillation

c)  develope necessary starting torque 

d)  both (b) & (d).

Ans-(d)

iv)  The  frequency  of  voltage  generated  by  an  alternator having 4 poles and rotating at 1800 r.p.m. is

a)  60 Hz  b)  7200 Hz

c)  120 Hz d)  450 Hz.

Ans-(a)

v)  The winding of a 4-pole alternator having 36 slots and a coil span of 1 to 8 is shor pitched by

a)  140˚  b)  80˚

c)  20˚  d)  40˚.

Ans-(d)

vi)  For parallel operation, a.c. poly-phase alternator must have the same

a)  speed  b)  voltage rating

c)  KVA rating  d)  excitation.

Ans-(b)

vii)  The power factor of an alternator is determined by

a)  speed  b)  load

c)  excitation d)  prime mover. 

Ans-(b)

viii)  At lagging load, armature reaction in an alternator is

a)  cross-magnetizing  b)  demanetizing

c)  non-effective  d)  magnetizing.

Ans-(b)

ix)  A 3-phase induction motor, while supplying a constant load, has the fuse of one line suddenly blown off. The motor will run as a single phase induction motor with line current nearly increased to 

a)√3times    b)  3 times

c)  5 times  d)  6 times.

Ans-(a)

x)  A capacitor start and run single phase Induction motor is basically a

a)  single phase induction motor 

b)  two-phase induction motor

c)  three-phase induction motor 

d)  single phase reluctance motor.

Ans-(a)

xi)  The direction of rotation of a Single phase induction motor can be reversed  by

a)  reversing the leads of main winding 

b)  reversing the leads of auxiliary winding

c)  reversing the supply leads 

d)  reversing the leads of either main or auxiliary winding. 

Ans-(b)

Electrical Machine-II

Question-2011

MCQ

1)A 230 V dc series motor is connected to a 230 V ac supply.

a) The motor will vibrate violently 

b) The motor will run with less efficiency and more sparking

c) The motor will not run 

d) The fuse will be blown.

Ans-(b)

ii) In a poly-phase A.C. machine "short-pitching" of coils leads to

a) high terminal voltage b) higher efficiency

c) higher power factor  d) better induced emf.

Ans-

iii) When the main winding of a 1-phase induction motor is connected to 220 V, 50 Hz source, it takes a current of 50 L – 60˚A at standstill. Neglecting stator leakage impedance and magnetizing current, the rotor circuit parameters r 2 and x 2 in ohms are respectively

a) 2, 3·464 b) 3·464, 2

c) 4, 6·928 d) 6·928, 4.

Ans-(a)

iv) When a 3-phase synchronous motor is  running at synchronous speed, the damper winding produces

a) damping torque 

b) eddy current torque

c) torque aiding the developed torque

d) no torque.

Ans-(d)

v) Number of slip-rings required  in  a  conventional synchronous machine is

a) 0 b) 1

c) 2 d) 3.

Ans-(c)

vi) Induction generator runs at

a) supersynchronous speed

b) sub-synchronous speed

c) synchronous speed

d) none of these.

Ans-(b)

vii) In  a  salient  pole synchronous machine, where X d = d-axis synchronous reactance, Xq = quadrature axis synchronous reactance,

a) Xq= Xd

b) Xq> Xd

c) Xq< Xd

d) Xq= 0.

Ans-(c)

viii) The backward rotor slip in a single-phase induction motor is equal to

a) 1 – s b) s

c) 2 – s d) s/2.

Ans-(c)

ix) A synchronous motor is operating on no load at unity p.f. If  the field  current is  increased,  the p.f. will become

a) leading and current will decrease 

b) lagging and current will increase

c) lagging and current will decrease

d) leading and current will increase.

Ans-(a)

x) It is desirable to eliminate 5th harmonic voltage from the phase voltage of an alternator. The coils should be short pitched by an electrical angle of

a) 30˚ b) 36˚

c) 18˚ d) none of these.

Ans-(b)

xi) In brushless dc motor we have

a) no mechanical commutator

b) no brushes

c) no arcing

d) all of these.

Ans-

xii) If the prime mover of the alternator supplying load to an infinite bus is suddenly shut down, then it will

a) stop

b) continue to run as an alternator

c) continue to run as a synchronous motor in the reverse direction

d) continue to run as a synchronous motor in the same direction.

Ans-(d)

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Electrical Machine-II

Question-2014

MCQ

i)) A capacitor start and run single phase induction motor is basically a

a) single phase induction motor b) two phase induction motor

c) three phase induction motor d) single phase reluctance motor

Ans-A

2) The direction of rotation of a single phase induction motor can be reversed by

a) reversing the leads of main winding

b) reversing the leads of auxiliary winding

c) reversing the supply leads

d) reversing the leads of with main or auxiliary winding

Ans-B

3) Usually the starting torque available in various split phase induction motors are the following ascending order

a) capacitor split resistor-reactor split, resistor split

b) resistor split, resistor-reactor split, capacitor split

c) resistor-reactor split, resistor split, capacitor split

d) resistor split, capacitor split, resistor-reactor split

Ans-B

4) Which one of the statements is true regarding speed and power factor of a single phase ac series motor?

a) both increase with increase in load torque

b) both decrease with increase in load torque

c) the former increases and the latter decreases with increase in load torque

d) the former decreases and the latter increases with increase in load torque

Ans-B

5) A compensating winding in a single phase series motor

a) reduces reactance drop and improves commutation

b) reduces reactance only

c) reduces reactance drop but retards commutation

d) improves commutation only

Ans-D

6) In a synchronous generator operating at zero power factor lagging, the effect of armature reaction is

a) magnetizing

b) demagnetizing c) cross magnetizing

d) both magnetizing and cross magnetizing

Ans-A

7) zero power factor characteristic for Potier diagram can be obtained by loading the alternator using

a) lamp load b) synchronous motor

c) water load d) dc motor

Ans-A

8) The max mum possible speeds in rpm at which an alternator can be driven to generates at 60Hz and 50Hz are respectively

a)2000, 2400

c) 2400, 2000

b) 3000, 3600

d) 3600, 3000

Ans-D

9) The power factor of an alternator under short circuit conditions is almost near

a) zero leading b) zero lagging c) unity

d) zero leading or zero lagging depending upon the type of alternator

Ans-A

10) Potier reactance of an alternator is almost the same as

a) field winding resistance b) total armature reactance

c) leakage reactance of field winding d) armature leakage reactance

Ans-D

11) A three phase synchronous generator is operating at constant load while the excitation is adjusted to give unity pf current. If the excitation is now increased, the pf will

a) become leading b) become lagging

c) remain at unity d) become zero

Ans-B

12) A three phase synchronous motor is operating on no load at unity power factor If the field current is now increased, the power factor and armature current will

a) leading and the current will decrease

b) lagging and the current will increase

c) lagging and the current will decrease

d) leading and the current will increase

Ans-A