WBUT Exam Papers Electric Machines – II B Tech 5th Sem Dec 2008

WBUT Exam Papers Electric Machines – II

B Tech 5th  Sem Dec 2008

 

Time Allotted I 3 Hours

Full Marks : 70

me figures in the margin indicate full marks.

Candidates are reauired in

qmrea to gwe their answers in their own words

as far as practicable.

GROUP – A ( Multiple Choice Type Questions )

1 • Choose the correct alternatives for any ten of the following :

i) A 230 V dc srrif^                                  •

– cries motor is connected to a 230 V ac

supply.                1‘                                                                                   ‘

a)              The motor will vibrate violently

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

c)              The motor will not run tO I he fuse will be blown.

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.

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 r2 and in ohms are respectively

, a) 2. 3-464                            b) 3-464, 2

c) 4, 6-928                             d) 6-928, 4.

iv)         When a 3-phase synchronous mtor 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.

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

r        .                                   ‘                 * ,

a)                                                                                     0 b) 1

c)                                                                                      2 d) 3.

003                                             2

vi)         Induction generator runs at

a)           supersynchronous speed

b)          sub-synchronous speed

c)           synchronous speed

d)          none of these.

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

, axis synchronous reactance,

a) Xq = Xd                               b) Xq > Xd

c) Xq<Xd                                 d) Xq = 0.

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.

ix)        A synchronous motor is operating on no load at unity p.f. If the field currcnt 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.

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

b)           36°

d) none of these.

xi)         In brushless dc motor we have

a)          no mechanical commutator

b)          no brushes

c)           no arcing

d)          all of these.

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.

GROUP -B ( Short Answer Type Questions )

Answer any three of the following. 3×5= 15

  1. With appropriate circuit and connection diagrams, explain the 2 bright and 1 dark” lamp method of synchronizing a 3-phase alternator to the grid or to another existing alternator. Is a 1 bright and 2 dark’ lamp method possible ? Justify your answer in brief.  4+1
  2. What is a synchronous condenser ? Explain its operation and utility with phasor diagram.                                                     1+4
  3. Describe the principle of operation of an induction generator. Can it be classified as a synchronous generator or as an asynchronous generator ?                                                                    4 + ]
  4. Explain the operation of a variable-reluctance type stepper motor. Why does it fall out of step if the pulsing rate of logic controller is increased ?                                                                   3 + 2
  5. With appropriate circuit diagram explain the operation of a biushless D.C. motor. Can it run without motor position feedback ? 4+1
  6. GROUP -c ( Long Answer Type Questions )
  7. Answer any three of the following. 3 x 15 = 45

Itor halQ H2‘                          yC°nne<** “.V-hronous

reactance o^0• 2^2 *and ^ T 8ynCh

Th. , .                         and 2 2 Per Phase respectively

he input ,s 800 kW at rated voltage and the induced

m ( llnc vaIue ) is 2500 V. Evaluate

i)            the line current *

ii)          the power factor

in) the motor speed (if it is a 6-pole machine ) and

IV) I e readings of the two wattmeters if the power is measured by 2-wattmeter method

Draw the par-phase phasor diagram at this condition.

found *? wVi^amPer Wlndlng and ^ WhlCh ^chineVu explain its working’   WUhln machin^ ^

a) What is a switched reluctance motor ? Describe to

basic operating principle with regard to a 4-phase

po e stator and 6-pole rotor radial ilux ( conventional) o.K. motor, t”                                                                              .

b) r, ;vh:stepang,c in ihe ^—and what

Whici the T TerVa‘ ‘ in mCChaniCal de«rees ) after

switched on sequential^ Tnc^ “T ^ ^ at a time ? Y‘ ” C3Se °ne Phase excited

0 What is the nature of the torque-speed characteristic ? What are the application areas of this motor ?                                     3

lus^Thby? DerlVe the volta^e equation and .   ic ro e of the term reluctance’. Write down the

expression for torque developed.

Write short notes on any three of the following :

a) A universal motor

Linear induction motor

Salient pole synchronous machine and its particular applicability in synchronous motors.

A.C. servo motor.

Starting from the expression of sinusoidal supply current, mathematically derive the two revolving field theory model and the equivalent circuit of a single phase induction motor.                                           0

What are the different methods of starting a single phase induction motor ? With the help of appropriate circuit diagram and phasor diagram, explain the principle of capacitor start-capactitor run I-phase induction motor. Briefly indicate how to design the value of capacitor to be used.     2 + 3 + 2

What may happen if one of the 3-lines of a 3-phase cage lotor induction motor suddenly gets disconneted ? Justify your answer.   p

A 1-0 MVA, 11 kV, 3-phase, ^-connected alternator has an effective Resistance of 20 per phase. The characteristics on open-circuit and with full load current at zero power factor and core losses ( on open

Field current (A)

Open ckt Voltage, V (line value)

Core loss, kW

Saturation curve, zerop.f., V

40

0

50

7000

7-5

110

12500

16-6

8500

140

13750

22-4

10500

180

15000

33-5

12400

5003

7

[ Turn over

 

Deduce by the zero power-factor method

i)                   the percentage regulation for full load at a p.f. of 0-8 lag and

ii)                 the efficiency at this load, given that the field circuit has a resistance of 0-5 Q and that the mechanical and additional losses add up to 10 kW. Draw the phasor diagram.

b) Define regulation of an alternator. Name the different

methods of determination of regulation of an alternator.

Which of these methods is known optimistic’ and why ?

Briefly describe the method only.

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