Power electronics Viva voce or interview questions part-8

 Q-176 What is interring base resistance?

Ans Inter base resistance is the resistance between 2 bases

Q-177 What is intrinsic standoff ratio?

Ans Intrinsic standoff ratio=Rb1/(Rb1+Rb2). Its value ranges between .52 to .81.

Q-178 What is the width of the triggering pulse?

Ans TG=Rb1.C

Q-179 Why are the capacitors CIF and C used?

Ans Capacitor CIF is used to minimize the ripples and C is used for charging and

discharging so that the trigger is eventually formed.

Q-180 What is ac voltage controller?

Ans If a Thyristor switch is connected between ac supply and load, the power flow

can be controlled by varying the rms value of ac voltage applied to the load and

this type of power circuit is known as an ac voltage regulator

Q-181 What are the applications of ac voltage controllers?

Ans The most common applications of ac voltage controllers are: industrial heating,

on-load transformer tap changing, light controls, speed control of polyphase

induction motors and ac magnet controls.

Q-182 What do you mean by sequence control?

Ans The use of two or more stages voltages controllers in parallel for the regulation

of output voltage.

Q-183 Give the classification of ac voltage regulators.

Ans They are classified as:

 1.single phase controllers

 2.three phase controllers

Each type can be subdivided into unidirectional and bi-directional control.

Q-184 What are the two types of control?

Ans *on off control: Here Thyristor switches connect the load to the ac source for a

few cycles of input voltage and then disconnect it for another few cycles.

*phase angle control: Here Thyristor switches connect the load to the ac source

for a portion of each cycle of input voltage.

Q-185 Why are extra commutation components not required?

Ans The ac voltage controllers have main supply as input. The SCR’s in these

controllers are turned off by natural commutation. Hence extra commutation

components are not required. Therefore ac voltage controllers are simple and

easy to implement if SCR’s are used.

Q-186 What is the difference between cycloconverters and ac voltage controllers?

Ans In cycloconverters (ac to variable ac) frequency of output can be varied. In ac

voltage controller’s frequency of output is kept constant, just the output average

value is controlled (on and off times varied).

Q-187 What is diac firing circuit?

Ans A diac firing circuit consists of a diac that is used to generate trigger pulses for

the Thyristor diac can conduct in both directions and it does not have any control

terminal in the form of a gate.

Q-188 What are the merits and demerits of voltage controllers?

Ans The merits are that they are simple without commutation circuits, high efficiency

and less maintenance.

The demerits are that the load current is asymmetric (phase control) and hence

harmonics are present and intermittent supply of power in on-off control.

Q-189 Why is the trigger source for the two Thyristor isolated from each other in a

single-phase voltage controller?

Ans When one Thyristor is on, the other should be off. Both the Thyristor should not

conduct at a time.

Q-190 What is a full controlled rectifier?

Ans It is a two-quadrant ac to dc converter. It has 4 thyristors and hence all of them

can be controlled for rectification purpose. In a full converter the polarity of the

output voltage can be either positive or negative but the output current has only

one polarity.

Q-191 What is a semi converter?

Ans A semi converter is a one-quadrant converter and it has only one polarity of

output voltage and current.

Q-192 What is a dual converter?

Ans A dual converter can operate in all 4 quadrants and both output voltage and

current can be either positive or negative.

Q-193 How can we control the output voltage of a single-phase full converter?

Ans By varying the trigger angle.

Q-194 What is MCB?

Ans MCB-Miniature circuit breaker. This is used as switch, which opens or switches

off when the voltage or current is above the rated value of that of MCB.

Q-195 How many lines are there in single-phase system?

Ans Two lines- 1line

1neutral

Q-196 What is rectification mode and inversion mode?

Ans During the period alpha to 180° the input voltage Vs and input current Is are

positive and the power flows from supply to the load. The converter is said to be

operating in rectification mode. During the period 180 to 180°+alpha the input

voltage Vs and the input current Is positive and there will be reverse power flow

from load to supply. The converter is said to be operating in inversion mode.

Q-197 Where is full bridge converter used?

Ans It is mainly used for speed control of dc motors.

Q-198 What is the effect of adding free wheeling diode?

Ans Free wheeling action does not takes place in single-phase full converter

inherently as there are 4 thyristors and no diodes. From 180 to 180+alpha( ) free

wheeling diode starts conducting. It is more forward biased compared to T1 and

T2.Hence freewheeling diode conducts. The freewheeling diode is connected

across the output Vo. Hence Vo=0 during freewheeling. The energy stored in the

load inductance is circulated back to the load itself.

Q-199 What are choppers?

Ans A dc chopper converts directly from dc to dc and is also known as dc-dc

converter.

Q-200 What does a chopper consist of?

Ans It can be a power transistor, SCR, GTO, power MOSFET, IGBT or a switching

device.