Basic Power Electronics Interview Questions -- 3 ( sai saikumar jn)

                            

Basic Power Electronics Interview Questions -- 3

     In the series of basic power electronics interview questions, we had added some more questions in this set-3. These basic power electronics interview questions are collected by considering that the candidate is Fresher and having no previous experience.

[Q] What are the advantages of free wheeling diode in rectifier circuit?
The input power factor is improved. It prevents the output voltage from becoming negative. Load current waveform is improved.

[Q] What is meant by commutation?
The process of changing the direction of current flow in a particular path of the circuit. It is used to turn off the SCR.

[Q] What are the types of commutation?
Natural commutation
Forced commutation

[Q] What is natural commutation?
The process of the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off is called as natural commutation.

[Q] What is natural commutation?
The process of the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off is called as natural commutation.

[Q] What is forced commutation?

The process of the current flowing through the thyristor is forced to become zero by external circuitry is called as forced commutation.

[Q] What are the types of commutation with respect to commutation process?
Voltage commutated chopper
Current commutated chopper
Load commutated chopper

[Q] What is meant by cyclo-converter?
It is also known as frequency changer. It converts input power at one frequency to output power at another frequency with one stage conversion.

[Q] What are the types of cyclo-converters?
Step up cyclo-converter
Step down cyclo-converter.

[Q] What is step down cyclo-converter?
It is the converter whose output frequency is less than the input frequency.

[Q] What is step up cyclo-converter ?
It is the converter whose output frequency is more than the input frequency.

[Q] What does the Voltmeter in AC mode show? Is it RMS value or peak value?
Multimeter in AC mode shows RMS value of the voltage or current. Also when it is DC mode it will show the RMS value only.

[Q] What is the necessity to use the special machines? 
General purpose motors (Induction motors, synchronous motors)  are neither precision speed nor precision position motors. For many automated systems require high precise speed and high precise positioning motors. In such cases special purpose motors like stepper motors, PMDC motors etc. are used.

[Q] What are the control strategies of chopper?
The control strategies of chopper are
1. Pulse width modulation PWM (Variable TON, Constant frequency)
2. Frequency modulation (Constant TON or TOFF, Variable frequency)
3. Current Limit Control (CLC)

[Q] What is delay angle or what is firing angle of phase controlled rectifier?
The delay angle is the angle at which thyristors are triggered after zero crossing. After zero crossing of supply voltage, one pair of thyristors is
forward biased. After delay angle(α) these SCRs are triggered.

[Q] What is Universal Motor?
It is defined as a motor which can be operated either on DC or single-phase AC supply at approximately the same speed and output.
The universal motor is built exactly like a series DC motor. But a series DC motor cannot be run as a universal motor, even though both motors look the same internally and externally.
We cannot use these motors in the industrial applications due to the low efficiency (25% -35%). It has high starting torque and a variable speed characteristic. It runs at dangerously high-speed on no load.

[Q] Give some examples of power electronics applications in the day-to-day life?
We can list a huge number of power electronics applications. Few of the applications which we can see in our daily life are 
UPS – Uninterruptible Power Supply
SMPS – Switch Mode Power Supply
Speed Control of Motors
ICU

[Q] What is meant by PMDC?
PMDC stands for Permanent Magnet DC Motor
A  Permanent Magnet DC Motor is similar to an ordinary dc shunt motor except that its field is provided by permanent magnets instead of salient-pole wound field structure.
There are three types of permanent magnets used for such motors namely;
(i) Alnico Magnets
(ii) Ceramic magnets
(iii) Rare-earth magnets
The major advantages are low noise, small size, high-efficiency, low manufacturing cost.

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                                                                                              -- Bellapuri saikumar
                                                                                                        ( sai saikumar jn)