Roll No. …………………..
L i n g a ya ’ s U n i ve r s i t y, F a r i d a b a d
M . Te c h ( P a rt - Ti m e) – E l e ct ri c a l E n g g .
Te r m - I I I E x am i n at i o n – M a y, 2 0 1 0
S o l i d St a t e C o n t r ol l e r of Dr i ve s ( E L - 50 6 )
[Time: 3 Hours]
[Max. Marks: 100]
Before answering the question, candidate should ensure that they have
been supplied the correct and complete question paper. No complaint
in this regard, will be entertained after examination.
Note: Question No. 1 Section – A is compulsory. Attempt any two
questions from Section – B and any two questions from Section C. In
all attempt five questions.
Section – A
Q-1
(i) When two identical SCRs are placed back to back in series
with a load and if each is fired at 90o, a D.C. voltmeter across the load
read
(2x10=20)
(a)
2
x peak voltage
(b) zero

1
(c)
x peak voltage

(d) none of these
(ii) Most of SCRS can be turned off by voltage reversal during negative
half cycle of the A.C. supply for
(a) all frequencies
(c) frequencies upto 30 KHz
(b) frequencies upto 300 Hz
(d) frequencies upto 300 KHz
(iii) The effect of source inductance on the performance of single-phase
and three phase full converter is to
(a) reduce the ripples in the load current
(b) make discontinuers current as continuous
(c) reduce the output voltage
(d) increase the load voltage
1
(iv) The total number of SCRs conducting simultaneously in a 3-phase
full converter with overlap considered has the sequence of
(a) 3, 3, 2, 2
(b) 3, 3, 3,2
(c) 3, 2, 3, 2
(d) 2, 2, 2, 3
(v) High switching losses occur in a _______________ inverter as
compared to other inverters.
(a) three phase, six step
(c) current source
(b) square wave PWM
(d) 120o – mode voltage source
(vi) The phenomenon of commutation overlap occurs in the
(a) Input circuit commutated inverter
(b) CSI
(c) MC Murray Inverter
(d) LCI
(vii) The torque insulations in a line commutated, three-phase
cycloconverter will be ____ those in a three phase six step VSI.
(a) an order smaller than
(b) an order larger than
(c) much larger than
(d) nearly equal to
(viii) Not maintaining the relation  p +  N = 180o in a simultaneously
controlled cycle converter leads to
(a) short circuiting of AC source (b) high voltage transients
(c) high current transients
(d) high DC circulating current
(ix) During the regenerative breaking operations of a chopper-based
DC drive, energy transfer takes place:
(a) from the load at high voltage to the source at low voltage
(b) from the source at high voltage to the load at low voltage
(c) from the source at low voltage to the load at high voltage
(d) from the load at low voltage to the source at high voltage
(x) In the angle control loop of the field control scheme using a current
source inverter, reversible switching occurs only in the ____________
range
(a) low-speed (b) medium-speed (c) high-speed (d) very high speed
2
Section – B
Q-2. (a) What is cycloconverter? Explain the working of 3  to 1 step
down cycloconverter for resistive as well as inductive load. Draw neat
and clean diagram.
(10)
(b) Draw and explain the general block diagram of a thyristor trigger
circuit.
(10)
Q-3. (a) Explain the continuous conduction and non continuous
conduction mode of class A chopper.
(10)
(b) Interface DC motor to microprocessor. Draw the interfacing and also
write the sub routine programme to control the speed of DC motor. (10)
Q-4. (a) Explain the operation of single phase capacitor current source
inverter with resistive load.
(10)
(b) Describe the different modules of a typical closed loop brushless
DC drive system.
(10)
Section – C
Q-5. A three phase to single phase cycloconverter which is fed by a
three phase dual converter has input and output frequencies of 50 Hz
and 16.6 Hz. Its load consists of R=2 and L-30 mH. If the transformer
secondary rating is 108 A, determine the (a) input displacement factor
(b) distortion factor (c) fundamental component of converter input
current (d) KVA rating of the transformer
(e) maximum and half
cycle average current ratings of the thyristors (f) RMS current ratings of
thyristor.
(20)
Q-6. Explain the open loop and close loop schemes for the control of
induction motor.
(20)
Q-7. Explain the working of 3 bridge inverter in 180o conduction mode.
Draw neat and clean diagram.
(20)
Q-8. A single phase fully controlled thyristor bridge converter supplies a
load consisting of R, L and Vc. The inductance L in the circuit is so
large that the output current may be considered to be virtually constant.
Assume the SCR to be ideal with following data:
3
RMS supply voltage = 220v, load resistance = 0.5, output current I
dc= 10A, Determine:
(i) firing angle  if Ec=135v
(ii)  if Ec = -145
(iii) Which source (Ac or Dc) is supplying power in (a) and (b)
(iv) Draw the load voltage waveform for (a) and (b)
4
(20)