M.Sc. (Semester IV) (CBCS) Examination Nov/Dec-2018
Electronics Science
POWER ELECTRONICS
Time: 2½ Hours Max. Marks: 70
Instructions: Q.1 and Q.2 are compulsory
Answer any three questions from Q.3 to Q.7.
All questions carry equal marks.
Use of nonprogrammable calculator is allowed.
Q.1 Select the most correct alternative 14
For obtaining regulated dc power supply device is used.
SMPS Inverter
Rectifier Cycloconverter
In a three-phase half wave rectifier, usually the primary side of the
transformer is delta connected because
it has no neutral connection
we can get greater output voltage
it provides a path for the triplen harmonics
it provides better temperature stability
In the principle of phase control
the load is on for some cycles and off for some cycles
control is achieved by adjusting the firing angle of the devices
control is achieved by adjusting the number of on off cycles
control cannot be achieved
The type of commutation in which the pulse to turn the SCR is obtained by
separate voltage source is
class B commutation class C commutation
class D commutation class E commutation
Voltage clamping device is used in protect thyristor from
over voltage Over current
dv/dt di/dt
The is used in SMPS circuits where the DC output
voltage needs to be lower than the DC input voltage.
Buck Cuk
Boost Buck-Boost
A thryistor requires trigger.
gate current anode current
cathode current gate voltage
In case of class A type commutation or load commutation with a low value
of
L is connected across R
L-C is connected across R
L is connected in series with R
L-C connected in series with R
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SMPS are based on the principle of
Delay control Integral control
Chopper MOSFET
10) Inverter converts
AC to DC DC to AC
DC to DC AC to AC
11) To protect a thyristor from high di dt conditions, a is used.
Fuse Inductor
Snubber circuit Voltage clamping device
12) dv/dt protection is provided to the SCR by
connecting a capacitor in parallel with the load
connecting an inductor in series with the load
connecting a capacitor resister in parallel with the device
connecting an inductor resister in parallel with the device
13) The device that does not have a gate terminal is called
Diac Triac
SCR FET
14) Which is not an advantage of ac voltage controllers?
High efficiency
Harmonics in the supply current
Less maintenance
Both a and c
Q.2 Answer the following.
Bridge controlled rectifier. 05
Cuk regulator. 05
Differentiate the natural and forced commutations. 04
Q.3 Explain overvoltage and over current protection of a thyristor in detail. 10
What is a dv/dt limit? 04
Q.4 Describe the working principle of buck-boost converter with necessary
circuit and waveforms.
10
What is meant by chopper? 04
Q.5 Explain with neat diagrams, the class D and class E commutations of a
thryistor.
10
With a neat diagram, explain the action of freewheeling diode. 04
Q.6 Draw and explain the single phase full controlled bridge rectifier with R
load.
10
Define chopper classify them. 04
Q.7 Draw and explain three phase half wave controller operation with R load. 08
Explain the need for AC voltage controllers. What is an on-off controls 06