# Basic Power Electronics Interview Questions : Set-3

Basic Power Electronics Interview Questions

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?

1. The input power factor is improved.
2. It prevents the output voltage from becoming negative.
3. The 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?

1. Natural commutation
2. 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 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?

1. Voltage commutated chopper
2. Current commutated chopper
3. 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. ie, 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.

### 17 Responses

1. muthu says:

good...need more questions

Thanks muthu...sure we will publish more Q & A

2. Soujanya says:

Excellent. still add more

Soujanya,

Thanks for your commets. Sure we will add more questions soon...

it was very usefull for me...thankyou a bunch for these questions...

4. mahesh says:

Really amazing I feel very happy to know all q and ans
Is any other website about complete description of electrical machines

5. mahesh says:

If any website please inform

6. praveen pandey says:

Its good to study all such basic power electronics question, thanks a lot

7. santhosh says:

superb........

8. sagar says:

Excellent we need more

9. kaushik patel says:

Sir
When freewheeling diode is active then why changin polarity?

10. MUHAMMED FASIL says:

Nice set of questions and answers. User friendly website. Add more questions about INVERTERS for HVDC applications

11. Ravi says:

Good Questions but still lot of improvment needed.

12. vijay says:

very nice helpful data thanks a lot of

13. h singh says:

Nice sir, we we require more questions on thyristor family and modulation

14. Krunal says:

Nice que and ans
I want to suggest few questions
- what is power electronics?
- what is the difference between scr and thyristors?
Hope that you will add some more questions!

15. Sampath says:

Thank u n it is very usefull ...