Power electronics Viva voce or interview questions part-10

 Q-226 What are the two modes of operation of a TRIAC?

Ans *Junction gate operation

 *Remote gate operation

Q-227 What are merits of TRIAC?

Ans 1. TRIAC is a bi-directional device that is it conducts in both directions.

2. TRIAC turns-off when voltage is reversed.

3. Single gate controls conduction in both directions.

4. TRIAC’s with high voltages and current ratings are available.

Q-228 What are de-merits of TRIAC?

Ans *TRIAC’s are latching devices like SCR; hence they are not suitable for DC

power applications.

 *Gate has no control over the conduction once Triac is turned on.

*TRIAC’s have very small switching frequencies.

Q-229 When is the sensitivity of TRIAC greatest?

Ans The sensitivity of TRIAC is greatest in I quadrant (mode 1) when MT2 and gate

are positive with respect to MT 1 and it triggers for a low value of latching

current.

Q-230 When is the sensitivity of TRIAC low?

Ans The sensitivity is less in mode III (3rd quadrant) when MT 2 is negative and

positive gate current and it requires larger value of latching current to trigger.

Q-231 Why should we use TRIAC for bi-directional flow of current, instead we

can use an ordinary conductor (wire)?

Ans The ordinary conductor does not provide the facility to switch on or off that is,

we cannot control the flow of current. We have to manually switch on or off the

supply.

Q-232 How is the TRIAC different from RCT?

Ans The TRIAC is equivalent to two SCR’s connected in antiparallel where as the

RCT is a SCR and a diode in antiparallel direction.

Q-233 What happens when gate voltage is increased?

Ans The forward break over voltage starts decreasing.

Q-234 What is stepper motor?

Ans A stepper motor is one in which the motor rotates step by step in steps of 1.8

degree.

Q-235 Why stepper motor is also called digital motor?

Ans A stepper motor is driven by square pulses and hence does not require pulse

width modulation technique. It can be driven by simple digital circuits and hence

is called digital motor.

Q-236 How many steps are covered in one revolution?

Ans 200 steps with step angle of 1.8 degree.

Q-237 What are merits of stepper motor?

Ans It is easy to interface with digital controller, low cost solution for position

control and also zero feedback error in servo system.

Q-238 What are its applications?

Ans Teleprinters,typewriters,plotters,X ray machinery, valve control, servo

mechanism, photo printing and developing, film projectors ,cameras ,carburettor

adjusting, electronic gear box, blood analysers etc

Q-239 What is the advantage of stepper motor?

Ans The major advantage is that its speed can be varied by the digital input. We can

use a microprocessor to control the speed .It can also be rotated in steps. They

can be used for precision control of speed without using closed loop feedback.

Q-240 What is step angle?

Ans The angle through which the motor shaft rotates for each command pulse is

called the step angle. It can be 1.8, 2.5, 7.5 degrees.

Q-241 What is stepping frequency?

Ans F is the stepping frequency in pulses per second.

Q-242 What are the moving parts in stepper motor?

Ans The only moving part in stepper motor is rotor which has no winding,

commutatore, brushes.

Q-243 What are types of stepper motor?

Ans *permanent magnet motor.

*variable reluctance motor-when stator is excited ,the rotor is pulled into the

nearest

Minimum reluctance position.

Q-244 What does stepping motor consist of?

Ans *The indexer is a microprocessor capable of generating step pulses and

direction signals for the driver.

 *The driver (amplifier) converts the indexer command signals into the power

necessary to energise the motor windings.

Q-245 What are its advantages?

Ans The step motor is an electromechanical device that converts digital pulses into

mechanical shaft rotation. Advantages are low cost, high reliability, high torque

at low speeds and a simple, rugged construction that operates almost in any

environment.

Q-246 What are its disadvantages?

Ans The main disadvantage in using stepper motor is the resonance effect often

exhibited at low speeds and decreasing torque with increasing speed.

Q-247 What are inverters and what are its applications?

Ans DC to AC converters is known as inverters. The function of an inverter is to

change a DC input voltage into AC output voltage of desired magnitude and

frequency. Inverters are widely used in industrial applications like variable speed

AC motor drives, induction heating, stand-by power supplies and uninterrupted

power supplies.

Q-248 Why the circuit is called parallel inverter?

Ans The circuit is called parallel inverter because the commutating capacitor is in

parallel with the primary winding of the output transformer whose secondary is

fed to the load.

Q-249 What is the main classification of inverters?

Ans Inverters can be broadly classified into two types namely, Single-phase inverters

and three phase inverters. Each type can use controlled turn-on and controlled

turn-off devices (eg. BJT’s and MOSFET’s etc) or forced commutation thyristers

depending on application.

Q-250 What is VFI and CFI?

Ans An inverter is called a Voltage Fed Inverter (VFI) if the input voltage remains

constant, a Current Fed Inverter (CFI) if the input current is maintained constant.