GATE Guru by Kreatryx_ Electrical Machines Demystified.pdf

9/29/2015

GATE Guru by Kreatryx: Electrical Machines Demystified

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Saturday, 23 August 2014

Electrical Machines Demystified

Blog by Kreatryx

Hi Guys, I sincerely hope that your preparation is on track. In this post I will discuss on how to tackle the most important subject of Electrical Engineering which is Electrical Machines and believe me when I say that by the end of this post you will understand how to cover the machines course in the least possible time. Electrical Machines is the most important topic for Electrical Engineers not only from GATE perspective but also for understanding as you are not a complete Electrical Engineer unless you have a good grasp of Machines concepts. In GATE exam if they focus on core subjects then Machines will easily have weight‐age of more than 10 marks. The problem with machines is not the complexity of concepts but rather the impact of the problem on one's mind. People usually presume that machines' question would be tough and this presumption is based on the language of the question generally which is more complex than the question itself. First we will cover each topic and the concepts required for GATE from that topic and then I will tell you some problem solving tips: Transformers The important concepts from GATE perspective are equivalent circuit of Transformer and the concepts of Voltage Regulation and efficiency. Generally the biggest confusion in the Voltage Regulation Concepts is whether it should be calculated with respect to No‐load Voltage or Full‐Load Voltage, I would suggest that you calculate both as either of them may be used and you mark the option based on that but if question is numerical type we generally calculate with respect to rated voltage. Also, one of the most commonly asked concepts is the core losses which are Hysteresis and Eddy Current Losses so practice few problems on that where we are given total core losses and we need to calculate individual Hysteresis and Eddy Current Losses. In case of three phase transformers, if you can understand the phasor diagrams and naming then it is very good else you can leave that as question is very rarely asked but you need to practice numericals based on three phase transformers. The next concept is the auto‐transformer which I believe is the easiest as entire question just depends on the ratio of low voltage to high voltage rating of auto‐transformer and we can scale each and every quantity of two winding transformer based on that. So totally we have to remember 4‐5 concepts for Transformers. DC Machines This Chapter totally revolves around 4 equations which are two performance equations one each for generator and motor and the expressions for Torque and EMF. The most commonly asked questions are based on speed control of DC Motor and we must always remember that whenever we want to calculate speed we need to calculate the back emf and then take the ratio of two emfs to compute the ratio of two speeds. But the complication arise when this question is based on DC Drives and either the speed is controlled via rectifier or an chopper so for that you need to remember the final expression for each type of rectifier and also buck‐ converter and if you can do that then the problem is merely reduced to playing along with 4‐5 equations. But most probably since drives is not mentioned in this year's course will not be asked in GATE exam so you can leave such problems. Synchronous Machines The first concept in this chapter that you need to remember is the pitch factor and distribution factor as sometimes questions may be asked on the ratio of emfs induced when a 3‐phase winding is re‐wounded as a single phase and the ratio of KVA Rating of the two winding.  Also we must remember the concept of Armature Reaction that whether this effect is magnetizing, de‐magnetizing or cross‐magnetizing based on the power factor at which the machine operates. The next concept is that of Synchronous Impedance that comes from Open Circuit Characteristics and Short Circuit Characteristics. Other than that this chapter is mostly based on two equations which are Power‐Angle Equation and the emf‐ terminal voltage relation which comes from equivalent circuit of the machine and you need to solve these two equation to calculate any parameter that is asked.

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GATE Guru by Kreatryx: Electrical Machines Demystified

Sometimes, question may be asked based on the torque angle of a Salient‐Pole Machine so we need to remember a direct formula for that and the last concept is power‐factor correction using Synchronous Motor. 

exam and now let's get straight to business.Now I will be tackling the qu... Blog Archive

►  2015 (9) ▼  2014 (29) ►  December (4) ►  November (2) ►  October (3) ►  September (1) ▼  August (4) Induction Machines This is the only machine whose functioning you need to understand well as mostly the questions are based upon the concept of slip and for that you need to understand various things like speed of rotor and speed of rotor magnetic field and stuff like that. The new concept that is now a days appearing and is non‐trivial is the concept of Inverted Induction Motor where supply is connected to the rotor and stator terminals are connected to load there also you need to understand the speeds of all field and rotor speed also. Equivalent Circuit is not much important here.Probably the most important concept in this chapter is the torque‐ slip characteristics and conditions for maximum toque and starting torque and question may be based on ratio of these. Also remember the formulas for Air‐Gap Power and rotor copper loss. The last concept is the starting of Induction Motor by Auto‐Transformer or Star‐Delta Starter which you need to study.  This is the topic from which will have highest weight‐age among all machines. Important Note Please understand the power‐flow diagram for each machine as that will be used for calculating efficiency and the concepts for Single Phase Induction Motor and other special Machines are not much important but you can remember the step size for stepper motor.

How to constantly improve yourself? Electrical Machines Demystified GATE‐2015 Notification 5 techniques to enhance your memory ►  July (2) ►  June (5) ►  May (2) ►  April (6)

Problem‐Solving Tips Practice more and more problem to develop the skill where while reading the problem you will easily understand the equations that will be involved while solving the problems. Also before solving any problem you need to devote 30 seconds to carefully read and understand the problem as most of the time mistake is committed due to not reading the question properly. If you want concise yet sufficiently detailed study material for GATE preparation, click on the link below: https://www.kreatryx.com/plan‐and‐pricing#tab2

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Also, last year's most successful program for GATE last month's revision Krash 2016 will be launched soon. Click on GATE preparation portal the link below to know more: Sign Up for Free https://www.kreatryx.com/gate‐courses#krash If you like the article please share it with your friends and let me know how was the article in comments and do revert to me with comments in case you have any query. Posted by ankit goyal at 20:32

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Labels: AIR‐1, Ankit Goyal, books, GATE, GATE‐2015, GATE‐2016, IES, machines, Online, power, Subject Tips

25 comments: Rajarshi Bhattacharya 23 August 2014 at 21:14 Sir, thank you once again for this brief yet illustrative article. I wanted to know about the inverted induction motor concept that you mentioned above. I haven't found any questions based on that, so could you suggest where I can get the concept? Same goes for synch machines ratio of emfs induced when a 3‐ phase winding is re‐wounded as a single phase and the ratio of KVA Rating of the two winding. And you mentioned about dc machines using drives...in these problems, sometimes inductance is given in armature circuit. How do we approach it then? Please reply. Thank you again! Reply Replies ankit goyal

23 August 2014 at 21:22

http://blog.kreatryx.com/2014/08/electrical­machines­demystified.html

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GATE Guru by Kreatryx: Electrical Machines Demystified The inverted Induction Motor concept you can refer from IES Conventional Paper‐2 Solutions here: http://www.kreatryx.com/ This concept was asked in GATE‐2008. The concepts about distribution factor you can refer from any book. If inductance is given in armature circuit we generally neglect that.

Unknown 3 February 2015 at 18:04 This comment has been removed by the author.

Unknown 3 February 2015 at 18:05 I could not find the IES Conventional Paper‐2 solution on http://www.kreatryx.com/ site sir. Also there was a question on GATE‐2014(Code 2) on the concept of inverted induction motor where frequency at slip ring and commutator brushes were asked.I saw your solution but could not get the concept. Can you please help me out. Reply

Sushant Gautam 23 August 2014 at 22:55 sir in DC machines like shunt motor when we reduce Vt (half ) then sometimes in numericals we take flux constant or half; It is toomuch confusing ,Plz Guide Reply Replies ankit goyal

30 November 2014 at 18:24

If Vt is halved in shunt machine then flux will be halved,it will be constant for a separately excited machine. Reply

Sanjay 24 August 2014 at 03:09 Very nice sir. Please give one for signals and systems too. Reply Replies ankit goyal

30 November 2014 at 18:42

Check out earlier blog post on Signals and Systems. Reply

Pankaj Kumar 24 August 2014 at 08:12 very fine sir....this analysis definetly help us. Reply

Kartikeya Tripathi 30 August 2014 at 05:21 thank you sir, sir , please write on POWER SYSTEM asap. because i feel that this subject is more conceptual than others.so please guide to make it strong. Reply Replies ankit goyal

30 November 2014 at 18:42

Power System is already done, you can check it out in Subject Tips Section.

http://blog.kreatryx.com/2014/08/electrical­machines­demystified.html

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GATE Guru by Kreatryx: Electrical Machines Demystified Reply

Anonymous 17 October 2014 at 01:17 ankit bhaiyaaa,,thanku so much for this post,i have some doubt m listing here pls tell these all are important or not.. (1)transformer‐all day effiency,three winding transformer,parallel operation. (2)DC machine‐compound connection,ampere turn magnetising and demagnetising, (3)3‐phase I.M.‐ no load test,blocked rotor test,speed control by pole changing and by slip changing, (4)3‐phase syn machine‐slip test,v and inverted v‐curve, (5)1‐phase I.M.‐double field theory,T‐S curve,equv. ckt.,,and concept of rotating machines...thanking you(VIKRANT) Reply Replies ankit goyal

30 November 2014 at 18:45

I am listing down the important topics from your list: (1) all day efficiency, parallel operation (2) None of the above (3) No‐Load and Blocked Rotor Tests (4) None of the above (5) T‐s Curve Reply

vidhya 6 November 2014 at 01:08 Thanks for the information... I really love your blog posts... specially those on PCB Engineering Services Reply

Anonymous 17 December 2014 at 04:11 Sir from where to study induction motor circle diagram as recently some questions are being asked from it. Reply Replies ankit goyal

17 December 2014 at 05:02

Can you please point to me the Questions based on Circle Diagrams as i haven't had an opportunity to have a look at them. Reply

Anonymous 28 December 2014 at 05:31 Hi, can you please suggest from where we can get the worked‐out solutions for the numericals present at the end of the chapters ‐ Electrical Machinery ‐ P S Bimbhra Reply Replies ankit goyal

8 January 2015 at 02:55

Sorry they are not available Reply

debanshu 12 January 2015 at 02:25 sir please provide the solution to the machine kuestion synchronous machines type A question no‐3...how is the ratio coming 1:1.3???the machine is short pitched by two coils so the angle should be 180*(2/6)..so chording factor should be cos(60/2)= cos 30=0.866....please tell me where is the mistake Reply http://blog.kreatryx.com/2014/08/electrical­machines­demystified.html

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GATE Guru by Kreatryx: Electrical Machines Demystified Reply

Anonymous 13 February 2015 at 21:20 sir, i just want to know about number of questions asked from different subjects in IES conventional and objective exam. I searched a lot on net for this but I am not able to find this. If u can, will u provide me the details of questions asked from different subjects in IES for previous 5 years. Reply

siddharth nayak 25 March 2015 at 16:29 Sir, is theodre wildy sufficent for electric machines and w d stevenson for ps Reply

Anonymous 7 April 2015 at 06:47 Sir, The chapter given in the book P.S.bimbhra of electromechanical energy conversion principles is necessary to study? Reply

Anonymous 30 May 2015 at 00:19 Sir, Thank you for providing valuable information for us... but still a little confusion arises How deep should we study a subject?? For example if we consider a DC machine:commutation,compensating winding as many doubts arise when I refer Nagrath and Kothari book... Are topics like these can be neglected entirely or should have a glance over them?? Reply

Anonymous 30 May 2015 at 01:12 Sir... A small query about transformers.... you have mentioned hysteresis and eddy current losses.... does this include the testing of transformer( OC and SC)??? And I am following nagrath and kothari and getting confused with some concepts.... is it good to follow that book?? Reply

DURGESH MANESHWAR 24 September 2015 at 23:07 which is best book for machines? Reply

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