Aryabhatta Knowledge University
Sem. iv 2013 Time 3hr Full marks: 70
ELECTRICAL MACHINE
-ii
Attempt any five questions in which question no 1 is
compulsory
1.
Choose the correct option (any seven) :
(a)
Three –phase alternators are invariably
connected in star so as to
(i)
Reduce the size of stator conductors used
(ii)
Increase the terminal voltage
(iii)
Reduce magnetic losses
(iv)
All of the above
(b)
The phase sequence of a three-phase alternator
will reverse ,if
(i)
The field current is reversed keeping the
direction of rotation same
(ii)
The field current remains the same but direction
of rotation is reversed
(iii)
The field current is reversed and the number of
poles is doubled
(iv)
The number of poles is doubled without reversing
the fields current
(c)
The regulation of an alternator of an alternator
is given as
(i)
100(V-E˳)/V
(ii)
100(E˳-V)/V
(iii)
100V/E˳
(iv)
100(E-V)/E˳
(d)
In the measurement of Xd ,Xq
(in ohms), following data are obtained by the slip test on salient pole
machine:
Id max=10A ,
Id min=6.5A ,Vd
max=30 V ,Vd min=25 V
Which one of the following is correct?
(i)
Xd=3Ω ,
Xq =3.86Ω
(ii)
Xd=4.165Ω , Xq=2.5Ω
(iii)
Xd=3
Ω , Xq=2.5Ω
(iv)
Xq=4.16Ω ,
Xq=3.86Ω
(e)
In which of the following is reluctance power is
developed?
(i)
Salient pole alternator
(ii)
Non salient pole alternator
(iii)
Squirrel cage induction motor
(iv)
Transformer
(f)
Which motor can conveniently operate at lagging
as well as leading power factor?
(i)
Squirrel-cage induction motor
(ii)
Wound-rotor induction motor
(iii)
Synchronous motor
(iv)
Dc shunt motor
(g)
An inverted V curve of synchronous motor shows
the variation of
(i)
Power factor and DC excitation at constant load
(ii)
Supply voltage and field current at constant
excitation
(iii)
Power factor and supply voltage during hunting supply
voltage and excitation current at constant load
(h)
A single-phase induction motor is running at N
r.p.m. Its synchronous speed is Ns .if its slip with respect to
speed is s, what is the slip with respect to the backward field?
(i)
S
(ii)
–s
(iii)
(1-s)
(iv) (2-s)
(i)
The torque-speed characteristic of two- phase
induction motor is largely affected by
(i)
Voltage
(ii)
Speed
(iii)
X/R ratio
(iv) Supply
frequency
(j)
In a.c. series motors, armature coils are
usually connected to commutator
(i)
Through resistor
(ii)
Through inductors
(iii)
through capacitors
(iv)
solidly
2.
(a) draw the following phasor diagram of a 3
phase synchronous machine:
(i)
Type: non salient pole , mode generator ,
pf-lagging
(ii)
Type : salient pole ,mode motor ,pf –leading
(iii)
Type : non –salient pole,mode motor ,pf- leading
(b) a 3 phase ,10 kVA ,400 V ,star connected
alternator supplies the rated load at 0.8 p-f
lagging. If the armature resistance is 0.5 ohm and synchronous reactance
is 10 ohms , find the torque angle and voltage regulation.
3.
(a) from the phasor diagram of a salient –pole
alternator working at a leading pf , but with pf angle θ less than load angle δ
,obtain the following relation:
(i)
Tan(δ-θ)=
[ia Xq –Vt sinθ]/[Vtcosθ+Ia
ra ]
(ii)
Ef =Vt cosδ+Ia
ra cos(δ-θ)+Id Xd
(b)
Find excitation voltage of an alternator when
delivering rated output at 0.8 pf lagging and at a rated voltage. For this
alternator ,Xd =1discuss the effects.25p.u. and Xq=1.00
p.u.
4.
Show that for alternators running in parallel
,the divison load between them is governed mainly by the speed- load
characteristics of their prime movers
5.
(a) discus the effects of field current on
synchronous motor power factor with the
help of
phasor
diagram