West Bengal Institute of Technology (WBIT) 2009-5th Sem B.Tech Electrical Engineering WBUT, ELECTRIC MACHINES-II, . - Question Paper

CS/B.Tech(EE)/SEM-5/EE-501/2009-10
2009
ELECTRIC MACHINES-II

Time Allotted : three Hours Full Marks : 70

The figures in the margin indicate full marks.
Candidates are needed to provide answers in their own words as far as practicable.


GROUP-A
(Multiple option kind Questions)
1. select the accurate options for any ten of the subsequent : [10 X 1=10]

i) In a double revolving field theory , the slip of the forward motor is S, then the slip coils are used to
a) 2S b) S c) 2-S d) S-2

ii) In a shaded motor, shading coils are used to
a) decrease winding losses
b) decrease friction losses
c) produce rotating magnetic field
d) protect against sparking

iii) A universal motor is 1 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.

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

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 auxiliary winding
c) reversing the supply leads
d) either (a) or (b)

vi) A capacitor begin and capacitor run induction motor is supplied from
a) single-phase supply b) 2-phase supply c) 3-phase supply d) none of these.

vii) The electrical angle, ß ranging from adjacent slots in a 4-pole alternation with 36 slots is in electrical degree
a) 9° b) 10° c) 20° d) 90°

viii) Which of the subsequent motors will provide relatively high starting torque?
a) capacitor begin motor b) capacitor run motor c) split phase motor d) shaded pole motor

ix) In signal-phase repulsion motor, power factor is
a) always leading b) always unit c) always lagging d) none of these.

x) Induction generator runs at
a) super synchronous speed b) sub-synchronous speed c) synchronous speed d) none of these.

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.

GROUP-B
(Short ans kind Questions)
ans any 3 of the subsequent. [3 X five = 15]

2. discuss the double revolving field theory as applied to a single-phase motor.

3. discuss why in a universal motor, the performance of the machine is better under d.c. than in a.c. operation.

4. What is hunting in an alternator? discuss how hunting can be minimized?

5. discuss the operating principle of a stepper motor.

6. Why are synchronous motors not self-starting?

GROUP-C
(Long ans kind Questions)
ans any 3 of the subsequent. [3 X 15 = 45]

7. a) define the armature reaction of an alternator motor for zero p.f. lagging, one zero p.f. leading and unity p.f. load.

b) define any synchronizing method of an alternator with infinite bus. [7+8=15]

8. a) Draw & discuss the phasor diagram of salient pole alternator supplying full-load lagging power current. Show that the power output per phase is provided by
P= (EV/Xd ) sin d + (V^2/2)[1/Xq - 1/Xd] sin 2d

b) A 150 kVA, 4000 V, 50 Hz, 3-phase star-connected alternator has effective armature resistance of 0.3 ohm. The field current of 40 A produces short-circuit current of 200 A and an open circuit e.m.f of 1080 V (line value). compute the full-load regulation at 0.8 p.f. lagging and 0.8 p.f. leading. [8+7=15]

9. a) draw and discuss phasor diagram of a single-phase series motor.

b) elaborate uncompensated & compensated single-phase series commutator motors? Develop their performance formula. [7+8=15]

10. a) discuss why a single phase single winding induction motor produces no starting torque.

b) Why are high speeds often desirable in the operation of universal motors? What limits the speed?

c) Why is shaded pole motor recommended over a resistance begin for the identical application?

d) The resistance and total inductance of a single phase fractional horse power series motor are 30 ohm and 0.5 H respectively. It draws 0.8 A current and runs at 2000 r.p.m when connected to a 250 V d.c. power supply. compute the speed and power factor when connected to a 250 V, 50 Hz supply and takes needed for getting 2000 r.p.m with a.c supply? presume resistance and reactance remains constant. [3+4+3+5=15]

11. Write short notes on any 3 of the subsequent : [3 x five = 15]

a) Stepper motor and "micro stepping".

b) Brushless DC motor.

c) Servomotor.

d) Induction regulator.

e) Resolver.

Earning:   Approval pending.