PRN:

GOVERNMENT COLLEGE OF ENGINEERING, JALGAON

(An Autonomous Institute of Government of Maharashtra)
National Highway No.6, Jalgaon -425002

Name of Examination: Second Year B. Tech Examination: Winter 2019

Course Code and Course Name: ET255U Power Electronics
Maximum marks: 60 Duration: 3 hours

Instructions to Candidates:
1. All questions are compulsory. Do not write anything on question paper.
2. Illustrate your answer with suitable figures/sketches wherever necessary.
3. Additional supplement will not be provided.
4. Assume suitable additional data; if required.
5. Answer sheet should be written with blue ink only. Graph or diagram should be drawn with same
pen being used for writing paper or black HB pencil.
6. Figures to right indicate full marks.

Que 1 Solve any two sub-questions

a) Explain the UJT trigger method for controlling firing angle of an SCR. 6

When the voltage is applied, the capacitor starts charging through the variable
resistance. By varying the resistance value voltage across the capacitor get varied.
Once the capacitor voltage is equal to the peak value of the UJT, it starts conducting
and hence produce a pulse output till the voltage across the capacitor equal to the
valley voltage Vv of the UJT. This process repeats and produces a train of pulses at
base terminal 1.

The pulse output at the base terminal 1 is used to turn ON the SCR at predetermined
time intervals.
a) Draw & explain the characteristics of SCR. 6

The thyristor has three basic modes of operation, namely the reverse
blocking mode, forward blocking (off-state) mode and forward
conduction (on-state) mode.
Initially for the reverse blocking mode of the thyristor, the cathode is
made positive with respect to anode by supplying voltage E and the
gate to cathode supply voltage Es is detached initially by keeping
switch S open. For understanding this mode we should look into the
fourth quadrant where the thyristor is reverse biased.
Now considering the anode is positive with respect to the cathode,
with gate kept in open condition.
When the anode to cathode forward voltage is increased, with gate
circuit open, the reverse junction J2 will have an avalanche breakdown
at forward break over voltage VBO leading to thyristor turn on. Once
the thyristor is turned on we can see from the diagram for
characteristics of thyristor, that the point M at once shifts toward N
and then anywhere between N and K. Here NK represents the
forward conduction mode of the thyristor. In this mode of operation,
the thyristor conducts maximum current with minimum voltage drop,
this is known as the forward conduction forward conduction or the
turn on mode of the thyristor.

Explain the working of a 1 – Ф full bridge voltage source inverter with R load.
 c) 6

The full-bridge inverter is similar to the half bridge-inverter, but it has an additional leg
to connect the neutral point to the load.

To avoid shorting out the voltage source, S1+ and S1- cannot be on at the same time,
and S2+ and S2- also cannot be on at the same time. Any modulating technique used
for the full-bridge configuration should have either the top or the bottom switch of each
leg on at any given time. Due to the extra leg, the maximum amplitude of the output
waveform is Vi, and is twice as large as the maximum achievable output amplitude for
the half-bridge configuration.

Que 2 Solve any two sub-questions

6
a) Explain the working of 1-phase full wave controlled rectifier using 'RL' load.

3) Due to positive polarity of load voltage & load current, load inductance will store
energy. At wt=ϖ, input supply is equal to zero & after ϖ it becomes negative. But
inductance opposes any change through it. In order to maintain a constant load current
& also in same direction. A self induced emf appears across 'L' as shown. Due to this
induced voltage, SCR's T1 & T2 are forward bias in spite the negative supply voltage.
The load voltage is negative & equal to instantaneous ac supply voltage whereas load
current is positive. Thus, load acts as source & stored energy in inductance is returned
 back to the ac supply.

At wt=ϖ+α SCR's T3 & T4 are turned on & T1, T2 are reversed bias. Thus , process of
conduction is transferred from T1,T2 to T3,T4. Load voltage again becomes positive
& energy is stored in inductor T3, T4 conduct in negative half cycle from (ϖ+α) to 2ϖ
With positive load voltage & load current energy gets stored

At wt=2ϖ, input voltage passes through zero. Inductive load will try to oppose any
change in current if in order to maintain load current constant & in the same direction.
Induced emf is positive & maintains conducting SCR's T3 & T4 with reverse polarity
also. Thus VL is negative & equal to instantaneous ac supply voltage. Whereas load
current continues to be positive. Thus load acts as source & stored energy in inductance
is returned back to ac supplAt wt=α or 2ϖ+α, T3 & T4 are commutated and T1,T2 are
turned on.In positive half cycle of applied ac signal, SCR's T1 & T2 are forward bias &
can be turned on at an angle α. Load voltage is equal to positive instantaneous ac
supply voltage. The load current is positive, ripple free, constant and equal to Io.
6
b)
Explain the working of 3-phase half controlled bridge rectifier using 'R' load.

Three phase half controlled bridge converters & fully controlled bridge converters are
used extensively in industrial applications up to about 15kW of output power.
Features of 3-phase controlled rectifiers are
• Operate from 3 phase ac supply voltage.
• They provide higher dc output voltage and higher dc output power.
• Higher output voltage ripple frequency.
• Filtering requirements are simplified for smoothing out load voltage and load current
In the case of a three-phase half wave controlled rectifier with resistive load, the
thyristor T1 is triggered at ωt=(30°+α) and T1 conducts upto ωt=180°=&pron; radians.
When the phase supply voltage decreases to zero at , the load current falls to zero and
the thyristor T1 turns off. Thus T1 conducts from ωt=(30° + α) to (180°).
c) 6
Explain comparison between controlled and uncontrolled rectifiers.
The uncontrolled rectifier has only diodes but the controlled rectifier has SCR.
The controlled rectifier needs a triggering circuit whereas the uncontrolled
rectifier does not need a triggering circuit. In an uncontrolled rectifier, we can
 not control the output whereas, in a controlled rectifier, we can control the
output. By varying the firing angle, we can change the output in a controlled
rectifier and there is no firing or triggering concept in the uncontrolled rectifier.

Que 3 Solve any two sub-questions

a) Explain the step-up DC-DC converter with neat circuit diagram & waveforms 6
A boost converter (step-up converter) is a DC-to-DC power converter that steps up
voltage (while stepping down current) from its input (supply) to its output (load). It is a
class of switched-mode power supply (SMPS) containing at least two semiconductors
(a diode and a transistor) and at least one energy storage element: a capacitor, inductor,
or the two in combination. To reduce voltage ripple, filters made of capacitors
(sometimes in combination with inductors) are normally added to such a converter's
output (load-side filter) and input.

When a boost converter operates in continuous mode, the current through the inductor
never falls to zero.

b) Explain basic chopper operation with the duty cycle control strategy. 6
In DC-DC converters, the average output voltage is controlled by varying the alpha (α)
value. This is achieved by varying the Duty Cycle of the switching pulses.
Duty cycle ( Duty Cycle is the ratio of ‘On Time’ to ‘Time Period of a pulse’) can be
varied usually in 2 ways: Time Ratio Control: the time ratio (i.e. the duty cycle ratio
Ton/T) is varied. This kind of control can be achieved using 2 ways: Pulse Width
Modulation (PWM) and Frequency Modulation Control (FMC)
And second way as Current Limit Control

c) What are commutation methods of SCR. Explain natural commutation. 6

The turn OFF process of an SCR is called Commutation. Commutation
means transfer of current from one circuit to another.

Natural Commutation
In natural commutation, the source of commutation voltage is the supply
source itself. If the SCR is connected to an AC supply, at every end of
the positive half cycle, the anode current naturally becomes zero (due to
the alternating nature of the AC Supply). As the current in the circuit
goes through the natural zero, a reverse voltage is applied immediately
across the SCR (due to the negative half cycle). These conditions turn
 OFF the SCR.

Que 4 Solve all sub-questions

a) Discuss the PWM switching scheme for the inverter. 6

In this technique, the time period is kept constant, but the ‘On Time’ or the ‘OFF
Time’ is varied. Using this, the duty cycle ratio can be varied. Since the ON time or the
‘pulse width’ is getting changed in this method, so it is popularly known as Pulse width
modulation.

The term duty cycle describes the proportion of 'on' time to the regular interval or
'period' of time; a low duty cycle corresponds to low power, because the power is off
for most of the time. Duty cycle is expressed in percent, 100% being fully on.

What is the necessity of an UPS? Draw & explain the block diagram of UPS.
b) It is an electrical device, gives emergency power to various loads when the input power
typically fails. A UPS fluctuates from an emergency power system in that it will
deliver near-instantaneous safety from i/p power interruptions by providing energy 6
stored in batteries, super capacitors. The run time of battery for most UPS is relatively
short but enough to start a standby power source.
The main purpose of a UPS is to provide a protection to the equipments like
computers, electrical equipment, computer and data centers when there is a power
disruption. This device keeps a computer running for a few minutes after a power
disruption and protects the data in the computer.
Que 5 Solve all sub-questions
6
Explain DIAC and TRIAC with symbols and characteristics
a)

A Triac device comprises of two thyristors that are connected in opposite direction but
in parallel but, it is controlled by the same gate. Triac is a 2-dimensional thyristor
which is activated on both halves of the i/p AC cycle using + Ve or -Ve gate pulses.
The three terminals of the Triac are MT1; MT2 & gate terminal (G). Generating pulses
are applied between MT1 and gate terminals. The ‘G’ current to switch 100A from
triac is not more than 50mA or so.
The DIAC is a bi-directional semiconductor switch that can be switched on in both
polarities. The full form of the name DIAC is diode alternating current. Diac is
connected back to back using two zener diodes and the main application of this DIAC
is, it is widely used to help even activating of a TRIAC when used in AC switches,
dimmer applications and starter circuits for florescent lamps.

Explain Series inverter

In series inverter, the commutating elements L and C are connected in series with the
load. This constitutes a series RLC resonant circuit. The Two SCRs are used to
produce the halves (positive and negative half cycle) in the output. In the first half of
 the output currents when SCR T1 is triggered it will allow the current to flow through
L1, and load, and C2 thus charging. The capacitor C1 which is already charged at these
instant discharges through SCR1, L1 and the Load. Hence 50% of the current is drawn
from the input source and 50% from the capacitor. Similarly in the second half of the
output current C1 will be charged and C2 will discharge through the load, L2
b) and SCR2, Again 50% of the load current is obtained from the DC input source and
rest from the capacitor. The SCRs T1 and T2 are alternatively fired to get AC voltage 6
and current.