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Power Electronics: Nightmare for Electrical Engineers

Hi Guys,

I am writing this post to let you all know, how to tackle Power Electronics subject as it is considered as one of the most tricky subject in GATE. Most of the Electrical Engineers that I have come across have a fear of this subject and that fear stems from incorrect strategy employed to study this subject. The weightage of this subject in GATE exam generally varies between 8 to 10 marks. The questions that are asked from this subject are not very trivial and hence students generally find questions from this topic difficult.
Now I will explain to you how to get good grip on Power Electronics.
Power Electronics
First of all be clear with the fact that in power electronics we have to deal with a few concepts only. It has pretty less syllabus as compared to other subjects and has got good weightage. In this study plan, I strictly suggest you own the following textbooks. PS Bimbhra, Rashid and Power Electronics pdf notes which are available in NPTEL website.

Power Electronic Devices:

In this chapter, the first thing that you have to study is the concept of switches. Under this concept you have to understand the concept of ideal and practical switch. Also, you need to understand how to compute the losses in switch such as Conduction Loss and Switching power Loss. Then you have to study about the ideal characteristics of power electronic devices. This will enable you to understand how to determine the characteristics of combination of multiple devices.
Then, you have to start from Power Diode and understand the concept of Reverse Recovery Characteristics which is further carried over to SCR as well. SCR is the most important topic in this chapter and first, you have to understand the construction and working of SCR and the role of Gate signal. You also have to understand the characteristics of the device i.e the three modes: Forward Conduction Mode, reverse Blocking Mode and Forward Blocking Mode. You also have to study about terms associated with SCR such as Latching Current and Holding Current.
The other topics that you have to study include: Triggering methods of SCR, Protection Techniques employed in SCR and Series and Parallel Connection of SCR. The Triggering Circuits such as UJT circuit are not important from GATE point of view.

In other devices, you have to get a basic idea about GTO, RCT, TRIAC, Power BJT, Power MOSFET, IGBT etc.
These topics are very well given in Power Electronics Book by PS Bimbhra.
 

Phase Controlled Converters:

In rectifiers you have to study about both Single Phase and Three Phase Rectifiers. You have to be cautious in understanding the difference between Half-Wave, Full-Wave and Half-Controlled, Fully Controlled rectifiers. In Single Phase half wave rectifiers you have to study about the nature of different loads such as R-Load, RL-Load, L-Load, RE Load and RLE Load. Based on this you can understand how the output voltage waveform changes when the load on system is changed. In Single Phase full wave rectifier, if you study about Highly inductive Load and Purely Resistive Load then it would be sufficient. But make sure to cover both fully controlled and half controlled converter. Also, understand how to draw the waveforms so that if circuit is modified in GATE exam, you can easily draw the waveform and derive the answer.
In case of three phase converters, you have to study about three phase half wave and three phase full wave converters and again you should focus only on two types of loads i.e. resistive Load and Highly Inductive load. Also, remember the concept of Discontinuous Conduction in three phase converters and understand the firing angle boundary between Continuous and Discontinuous Conduction.
For rectifier, Bimbhra is more than sufficient and one must be cautious about Fourier analysis of every rectifier. Understand the difference between ripple-free load and normal load. 90% of questions will be based on ripple-free load and in that case, if you have all the formulae on fingertips, you can fetch maximum marks. Coming to battery charging problems in rectifiers go through the NPTEL notes & few problems are also there in the textbook.

Choppers:

In Choppers, you need to understand the full operation of Step-Down Chopper and Buck Converter. There are three converters that are included in GATE course which are Buck Converter, Boost Converter and Buck-Boost Converter. In all these three converters you need to understand the general methods of determining certain parameters such as Average Output Voltage, Average Inductor Current, Ripple In Inductor Current, Ripple in Load Voltage etc. These parameters are determined from the basic Network Analysis and concepts of Volt-Sec Balance and Ampere-sec Balance. 
The next important concept that you have to study is the concept of Thyristor Commutation Techniques such as Class-A Commutation, Class-B Commutation or Current Commutation, Class-C or Complementary Commutation, Class-D or Voltage Commutation. It would be good if you can understand the working of Commutation Circuits and if it is not possible then you need to remember the expressions of Maximum Thyristor Current and the Circuit Turn-Off Time.
You can also study about different types of choppers such as Type-A Chopper Type-B chopper etc. The main focus of this chapter should be on solving more problems using the basic concepts rather than memorizing the expressions involved.
 

Inverters:

In case of Inverters, you have to study about the Single Phase and Three phase Inverters. In single Phase Inverters you have to study about Half Bridge and Full Bridge Inverters. In each type of Inverter you have to understand, how the load current waveform gets affected. You can memorize the Fourier Series for the Output Voltage of each of these configurations.
In Three Phase Inverter, there are two conduction modes which are 120 degree and 1180-degree conduction modes. In each of the modes, you should remember the RMS value of Phase Voltages, Line Voltages and RMS Values of their Fundamental Components. You can also take a look at the basic working of Current Source Inverter and its variation in Auto Sequential Commutated Inverter (ASCI).
Then comes the most important topic now a days which is Pulse Width Modulation (PWM). Earlier the most important PWM technique was Single Pulse Modulation (SPM), so you can memorize the RMS voltage output of this technique. Also, try and memorize the Fourier Series of Output Voltage of this technique as many questions have been asked on this in the past. The next modulation technique is Multiple Pulse Modulation (MPM) but questions have not been asked from it. Now a days the most popular technique is Sinusoidal Pulse Width Modulation (SPWM) and here you need to remember a basic expression for peak value of Fundamental component of Output Voltage.
 
 

Cyclo Converters and AC Voltage Regulators:

This topic has now been removed from GATE curriculum and if you want to prepare it for other exams you can prepare about Half Wave Regulator, Full Wave regulator and their RMS values of output voltages. You can also study ON/OFF Control in case of AC Voltage Regulator.

In Cyclo converters, you have to study the implementation of Step-Down and Step-Up Cyclo converter. You also have to remember the number of thyristors required to implement each of these.
 

AC and DC Drives:

This topic has now been removed from GATE curriculum and if you want to prepare it for other exams you can prepare about control of the speed of DC Motor using Rectifiers and Choppers. If you remember the expressions for the average output voltage of rectifiers and Choppers well as well the basic performance equations of DC Motors then this topic is pretty easy.

For those who feel Power Electronics as tough. I would suggest that you dedicate 12-15 days when preparing it for the first time and revise at least twice and you can gain the command over this subject. Practice as many as problems. Go for discussions. While solving a problem you may see some option as fitting your calculations and you may be tempted to go for it to save time but please do not do that. 

So, guys if you found this post helpful for your GATE preparation please share this with your friends and also comment with your queries.

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