Hello everybody, in the last class we had

started our discussion on induction machines or the induction motors. Induction motors

are very popular; something like 80 percent of the prime movers all over the world use

induction motors for various applications so it is important that we learn about the

induction machine. So we began our discussion by an initial comparison of the induction

motor with the 3 phase transformers. In the case of 3 phase transformers we had

the energy movement from the electrical side into the magnetic domain and then back again

into the electrical domain in the secondary. In the case of the induction motor it is also

similar in the sense that you have a primary side coil called the stator, the secondary

side coil called the rotor plus there is also another domain called the mechanical domain

so you have the energy which is moving from the electrical domain into the magnetic domain

and you can tap energy in the electrical domain or in the mechanical domain or both depending

upon the type of the motor. So, in the induction motor you have two main

categories the squirrel cage induction motor where the rotor is shorted you cannot take

any electrical energy from the rotor so all the electrical energy can be taken only from

the mechanical domain. So you have the electrical domain magnetic mechanical domain as the path

for the energy and in the case of the other category the wound rotor induction motor or

the slip ring induction motor you have the electrical domain, energy enters the machine

goes into the mechanical domain and the energy can be tapped out either in the electrical

domain of the rotor or the mechanical domain of the rotor. And all the working principles

operating principles of the transformers are valid even in the case of the induction motor. We have the Faraday's law of electromagnetic

e is equal to Bℓv which induces the voltage on the rotor conductors and thereby a current

in the rotor conductors by virtue of the relative motion of the rotor conductors with respect

to the field inside the machine. And because there is a current in the rotor conductors

and there is a field in the machine, by virtue of the Lorentz law that is force is equal

to BIL there is a force which is induced on the rotor conductors and the accumulated force

on all the rotor conductors creates a torque and which makes the rotor move. So this is

how we get the mechanical motion. We will look into the details of that much later.

But the important concept now is that how do we bring about a relative motion between

the field in the motor and the rotor conductors. Now, to achieve that we use the 3 phase source.

So in the 3 phase source we have the three sources wherein they have the same effective

amplitudes however they are 120 degrees apart in time. Now applying one more constraint

that is these the currents from these three sources flow through coils in the stator of

the motor wherein the coils are physically located in space 120 degrees apart, a combination

of these two produces a rotating magnetic field. So let us now see how this is created.

First we shall have the rotor represented as a nice circle here and let me have ……….. the

circle is not as nice but let us into the circle and let us have the 3 phase currents

flow through three poles electromagnetic poles located 120 degrees spatially apart. So let us say the A phase current flows through

a A phase this is let us say flows through A phase poles set and let me indicate the

pole the salient pole in this manner so you have one pole pair per phase as shown

here then let us have 120 degrees apart one more set of poles which are like this along

this axis as shown, now this is 120 degrees apart 120 degrees. Now along this also I am

going to have some fixed stationary poles pole pairs of course, you cannot have single

poles; you should always have a south and a north pole. Now one more pole set which

is placed 240 degrees with respect to the A axis and this is 240 degrees with respect

to A axis 240 degrees, this is the C axis and of course the red one is the B. So on

the C axis also we are going to place the stationary poles as shown here shown in this

manner you see. Now we can of course connect all these into

the same structure and that structure is called the stator. So all these are connected into

the same structure and that structure is called the stator so we have the stator like this

as shown with salient poles projected inward and we have the rotor in blue. Now let us

say we have the 3 phases and let me take the let me take the A phase and the A phase you

have a coil as shown here on the stator and internally the same current whatever is entering

here let me call this one as I A from the A phase source. Internally this coil is then

brought in and connected wound around the other part of the pole to get the south pole

and taken out. And then we have the B phase so I am going to take the B phase part wound

wind it around the pole here then bring it to the other side and then take out in this

direction. So we have I B which will flow here and then I C so we will take I C which

is going to flow like that taken all through like this and goes out. Now the other ends are joined together. So

let me join this end, this end and this end and that is called the neutral. So, as the

other ends of the coil are joined together that is the neutral so you have the other

open end through which the sources are connected and you have the I A I B I C flowing. So I

A is flowing through the A phase pole this is stationary fixed and I B is flowing through

the B phase pole that is also stationary and fixed in space 120 degrees shifted with respect

to the A phase pole and I C is flowing through the C phase pole in the C phase axis and that

is also shifted 240 degrees with respect to the A phase this one in this manner. So this

is a star connected induction motor where the stator coils are star connected with the

currents I A I B I C flowing in this manner. So we have the 3 phase currents 120 degrees

electrical in time that is when this is at zero this is negative, this is also negative

like in a 3 phase current so the amplitudes here along these three axes vary according

to the time. Let us see what happens to the amplitudes of the flux. So as a current flows here we have we have

a A phase flux in this direction, we have a B phase flux in this direction and we have

a C phase flux in this direction with the amplitudes changing according to the instance

of time. So now let us see, if we apply a 3 phase sinusoidal

waveform how does it look. So we generate a 3 phase sinusoidal waveform by the method

that we followed before; just draw this triangular wave shape then connect the tops with curves

like this as shown, connect the bottoms of the vertices with curved lines like this and

you have your 3 phase wave shape with respect to time. Now let me have the let me have the axis the

vertical axis at this point the zero point here. So blue is the A phase and let me call

that as I a, 120 degrees phase shifted would be our B phase and I will call that as……

I will use the red colour for that so that is our B phase current I b effective

value and then the C phase current is given by the violet so it comes in this fashion

which is 240; you see that it is now cutting the zero and going positive 240 degrees with

respect to the A phase. So these are the 3 phases I c. Now just we will look at some

crucial critical points okay. So let us have point of interest. Let us say

some of these points would be points of interest. Now I could also take a point along this,

now let us take this and this is also another point of interest and we have and let us and

so on every 60 degrees. Now let us say when I take a section at that

point how does the how does the resultant vector look like or there is going to be a

flux along the A phase axis at every instant of time B phase axis, C phase axis. Now if

we take at that point of time at this point of time so we have A phase current is maximum

positive direction so I will put it as A phase is maximum positive direction, B phase is

half negative so this was B phase so B phase negative and half, C phase is also negative

and half C phase negative is this direction and half so you have a resultant which is

having…….. let me draw a circle so the resultant is having a direction in

this that is these two add up and then you have a resultant in this direction. Now if we consider this point now let us say

or let us take this point here, now here you have B as maximum so B as maximum positive

so I will put it as maximum positive here like that, A is negative half A is negative

half and you also have C which is negative half and C negative half, you see that the

axes directions are always the same that is the mechanical axis directions are always

the same but only that it is positive and negative so which means now the resultant

is the resultant flux is now shifted and it is shifted like that. And then if we take

a point here, now you will see that I c is maximum positive so you have maximum positive

here I a is negative half I a is negative half I b is negative half and that is negative

so therefore you will have you will have a flux which is in this direction.

So you see that the resultant flux as time progresses it was in this direction then it

has rotated in that direction so we have taken some crucial points in between to show that

the flux positions is changing. So if we smoothly go along the time you will see that there

would be a smooth rotation along this circle and then you will see that the rotating magnetic

field has been created. So this is this vector which we are seeing here is rotating smoothly

around the circle and that is the rotating magnetic field. So once we have the rotating magnetic field

which is rotating inside the machine in a circular fashion so the magnetic field here

is in space it keeps rotating if we apply 120 degrees electrical phase shifted sign

waves to the three coils which are also 120 degrees phase shifted in the mechanical that

is in the stationary spatial coordinates. Now as this is rotating and if we have a rotor

conductor this magnetic field which is rotating is cutting this rotor conductor and as this

is cutting the rotor conductor there is a current which is going to flow through the

rotor conductor that is I. There is a length of the conductor L, this magnetic field is

having density B so by the Lorentz law you have a force which is BIL that gets reduced

which will push the conductor in an orthogonal fashion that is the direction in which the

current is flowing and the direction of the field in this case and then you will see the

third finger will give you the, the middle finger will give you the direction of the

force. So this will cause the conductors to move and it will try to catch up with the

field which is already rotating. Initially when the rotors are not moving the

field is rotating at whatever the applied frequency and there is a very large relative

speed between the rotating magnetic field and the stationary rotor conductors and therefore

that is going to induce a very large current in the rotors which is going to give you a

huge torque and then the rotor starts rotating and tries to catch up with the magnetic field

and the relative speed starts decreasing. So, as the relative speed starts decreasing

the induced current in the rotor also starts decreasing thereby reducing the torque to

a steady-state value. Now suppose let us say that the rotor has

caught up with the magnetic field which means the magnetic field and the rotor conductors

are rotating at the same speed this means there is no relative speed between the magnetic

field and the rotor conductors and therefore there is no cutting effect of the magnetic

field and therefore there is no induction of the current in the rotor and as a consequence

there is no current in the rotor and therefore there is no force for the rotors to move and

therefore it will again lag behind. So the rotors will never catch up, there will always

be a slight difference between the magnetic field rotation speed and the speed at which

the rotor conductors rotate and this difference is called the slip this difference in speed

is called the slip. So if the magnetic field is rotating at a

speed let us call n s called the synchronous speed synchronous speed in rpm and the rotor

conductors are rotating at speed n m this is the mechanical speed of the rotor mechanical

speed of the rotor in rpm then slip s is given by n s minus n m by n s and it is a value

which ranges from 0 to 1. We shall come back to the concept of the slip

a bit later again we shall revisit that. But now we should try to understand what happens

if we put more number of poles. We saw in the picture here that there is one pole set

there is one pole pair per phase. Now, for every phase you could have more than one pole

pair. In such a case what happens to the speed what happens to the synchronous speed. Let me this explain the concept with respect

to just one phase that is the A phase, what happens. So let us say we have

let us say we have the rotor here and we have one set of fields and that is let us say the

A field. So let us explain with respect to just this A field. So we have the A field

like that with coils wound and all those things. Now imagine that inside inside in the rotor

imagine there is an imaginary dumbbell there is an imaginary dumbbell, so one is

the north pole and then the other is the south pole. So as the coils here are fixed the pole

here is fixed this dumbbell does not rotate it only goes shrinks and expands. Let me first make a copy of this because I

will be using this later, copy. Now let us see what happens to

this dumbbell as time progresses and we have a sign wave which is applied to the coils.

So let us say this is I a and this is apply to the coils here and then we have also the

coils like that, this is connected to the neutral and this is connected to this and

you have a passing I a to that. Okay So at this point I a is 0, the flux intensity at

that instant is going to be 0 because N I is going to be 0 at that instant so the mmf

which is at at this instant is also going to be 0. So the mmf is going to follow more

or less this curve because it is N I so what happens to the dumbbell inside; so the dumbbell

inside which represents the magnitude of the flux will will be residing here so there is

no amplitude because it is 0. Now as the amplitude starts rising so let

us say at some point here the corresponding flux here would be somewhere would result

in a dumbbell which is of this size north and south and then at some points here the

peak amplitude the dumbbell would have length end and you will have the maximum peak amplitude

of the dumbbell like that. And as it starts coming down the

dumbbell shrinks and then at this point again comes to 0, at this point it would again come

to 0 and then further when the current starts going negative what happens is………

now the dumbbell sorry dumbbell reverses polarity that is you will have a dumbbell which is

half the size, now this is south and this is north note the change in polarity and then

a corresponding value negative peak it would reach the negative maximum. So what would

happen is that you would have a dumbbell which would reach the negative maximum that is this

is south pole and north pole. So like that you will see the dumbbell which is swinging

along that axis depending upon the amplitude of the current. Now let us take just the two crucial points

that is this point this and this point that is the positive max and the negative max so

at these two points we will see that the dumbbell size or the length will be the same maximum

except that during the positive max it is going to be it is going to be north on this

side, south on this side and during the negative max the dumbbell length is going to be the

same except that it is going to be south on this side and north on this side. That is

in a half cycle in a half cycle the dumbbell has rotated and shifted by 180 degrees. So

inside if we take the imaginary dumbbell which is on the rotor rotating it means it means

that the flux mmf inside has rotated 180 degrees for a half cycle traversal of I a and it would

have rotated full 360 degrees for a complete full cycle traversal or two half cycle traversal

of A that is for a two pole system that is…… this is two pole pole 1 pole 2 so you have

a two pole per phase so you have two poles per phase system. Now in this two pole two

pole per phase system 360 degrees is traversed by the dumbbell by the mmf dumbbell in two

half cycles of the two half cycles of the I a or the current wave. So now let us say the current wave has a frequency

f that is this current wave here has a time period of T and the frequency f is equal to

1 by T. So in our case f is equal to 50 hertz because it is a 50 hertz mains in our country

so f is the frequency of the sign wave. So it makes full 360 degrees, dumbbell makes

full 360 degrees or let us say 1 revolution in T seconds or 1 by f seconds. So in 60 seconds

for a minute in 60 seconds that is 1 minute how many revolutions does it make. So it is

60 by 1 by f or we should say 60 f is the revolutions per minute and this is your n

s okay this is your n s. This is for one pole pair one pole pair or per phase or should

we say there are two poles per phase. So if we use to use this notation that is instead

of saying pole pair rather than poles then we say it is 60 f by pole pair. If it is two

poles then let us say P is the number of poles number of poles per phase then n s will be

60 into f let us say into 2 by 2 this should be for two poles so that you still have the

same result, this will be 120 f by 2 where two indicates number of poles number of poles. And in general if there are P number of poles

n s is equal to 120 f by P where P is the number of poles per phase. What does it say?

As the number of poles this is always an even number because you cannot have single poles,

this means that as the frequency increases the synchronous frequency increases as the

number of poles increases the synchronous frequency decreases. Let us see how this happens. Now of course I will explain it again only

with respect to the A phase. Let me have this figure. So here we have just one pole pair

or two poles here P is equal to 2. Now let me have two more poles in the A phase all

A phase and the windings are so the windings are so

done such that the north and south poles alternate; all the currents flowing through this and

then this and then this and then this; they are all I a I a I a I a and I a is flowing

through all these poles. Then there are four sets of other B phase which are tilted and

shifted 120 that is 120 degrees axis and then another four sets for the C phase so they

are shifted and placed in such a manner meaning just to indicate…….. so we have let us

say the rotor so let me say this is the A pole set that is at the A phase and the B

phase let us say is shifted 120 degrees, you have one here see like that and like that

and the C pole set is again shifted 120 degrees and it will be in this manner. Only it is

minus 120 degrees the winding sense is just changed from clockwise to anti-clockwise to

obtain the 3 phases. So anyway let me explain with respect to only

one phase to get to give you the idea of how the synchronous frequency changes with the

number of poles. Now we have four poles. So at any given point of time we will be having

two dumbbells imaginary dumbbells which are like that. So we have these two imaginary

dumbbells and let us say this is north, this is south,

this is south so north south north south it alternates throughout the whole circumference. Now let us have the

let us have the I a current also this is I a current at this point. let me copy this

whole portion, copy and let me also paste it and have a duplicate here. Let me shift

this above, make some space so that you will be able to include this here. So now let us see what happens at these two

critical points that is one point is the positive max and the negative max so what is happening.

Now let us say at this point okay I a is positive max, let us say this is the position I a is

positive max. Now you have I a is negative max. So, between the positive max and the

negative max there should be a 180 degrees that is the north and south poles should interchange.

So here we have the north north south south so that is at the positive max. At the negative

max north and south poles should interchange so we shall interchange the north and the

south poles here and see the north and south so south

becomes this and this becomes north so which means there is a movement of……… between

this and this it looks as though the mmf has rotated by 90 degrees between this and this

the mmf has rotated by a 90 degrees mechanical. But however, here you have one complete half

cycle T by 2 that is one half cycle. So one half cycle makes the mmf rotate by 90 degrees

and to get 260 degrees you need four half cycles or two cycles to complete one complete

revolution of this so which means if there are two pole pairs

that is n s we saw was 60 f if there are two pole pairs that is if there is one pole pair

it is just 60 f by 1, if there are two pole pairs it has become 1 has become…….. two

cycles of this one is needed to make one complete mechanical revolution so the mechanical speed

the mech one complete the mechanical synchronous speed is 60 f by 2 and if there are three

pole pairs it becomes 60 f by 3. Now if you want express in terms of number

of poles rather than pole pairs then you have to multiply this and this by 2 which means

you are talking of 120 f by two poles in the earlier case and then in the case of this

four pole machine n s is equal to 120 f by four poles per phase and so on. So if you

have p poles it becomes 120 f by p poles so you see that it actually slows down the system

if you have more number of poles. So the n s the synchronous speed of the flux

is 120 f by P; here P is number of poles per phase so be careful about this. Now here you

can also have 60 f by P p where P p is number of pole pairs per phase because these are

various formulas that you will find in different literature it is better you know the nomenclature

and P p is always P by 2 number of poles by 2 and n s this is in rpm this is in rpm. Now the synchronous speed is also expressed

in radians per second because radians per second is the SI system of units so you should

know how it is converted. So in this SI system if you say synchronous speed is given as omega

s is the synchronous speed synchronous speed in radians per second, so to convert something

to radians per second that is 60 let us use 60 f by P p this is in rpm and we want to

convert in radians per second it becomes 2 pi by 60. So this we saw while doing the DC

machines; conversion from rpm to radians per second, you multiply by a fact of 2 pi by

60 and therefore you have 2 pi f by P p and this is the applied frequency. So omega synchronous is equal to omega applied

by P p. Or if you want to express it in terms of this one it is 2 omega applied by the number

of poles. So this is also something that you should know how to express it in radians radian

terms. Now let us just have a small example from

the book. Okay Let us say we have a 3 phase induction motor 3 phase induction motor which

has 20 poles per phase and it is operated from and operated from a 50 hertz 3 phase

source. Now what is the synchronous speed. So we know that n s in rpm is given by 120

applied frequency by P where P is the number of poles per phase. So this will be 120 into

50 divided by 20 so that is 300 rpm revolutions per minute. Now we have also defined the slip. the slip

which is s is given by n s minus n m that is the rotor speed mechanical speed by n s

and this is something always less than 1 because n s is always greater than n m; it could also

be written as omega s in radians per second, omega m in radians per second divided by omega

s where this is in rpm and this is in radians per second. So this is the slip. Let us take

a simple exercise on this from the book. So we have a 6 pole induction motor and this

is excited by a 3 phase 60 hertz source, the full load rpm is 1140 revolutions per minute,

calculate the slip. So first let us say what is n s. n s is 120 f by P which is given by

120 into 60 divided by 6 poles which is equal to 1200 rpm 1200 rpm. Now slip s equals 1200

minus the full load motor rpm divided by 1200 and this is equal to 60 by 1200 and in terms

of percentage this is equal to 0.05 and this is something like 5 percent in terms of percentage.

So this is how one calculates the slip. Now there is one important thing that you

should know. We saw that there are machines with different number of poles; you have the

2 pole machine, the 4 pole machine, the 6 pole machine, 8 pole machine, 20 pole machine

and so on for different applications. What is the maximum speed that one can achieve

that is maximum synchronous speed that one can achieve for a given frequency source and

that is at 2 pole. That is if we have just two poles per phase that is the maximum frequency

that you could get which means n s is equal to 120 f by 2 or which is 60 f is the maximum

synchronous synchronous n o u s frequency for a machine that is operated or exited from

a f hertz source. So in India f is 50 hertz so therefore the

maximum synchronous speed can be 60 into 50 which is equal to 3000 rpm and many of the

machines which are there in the market are 4 pole machines 4 pole machines which means

the n s typically of all these 4 pole machines in our country you will see there are 1500

rpm which will be on the name plate, so keep that in mind. Now there is one thing that I have to show

you and that is a picture an exploded picture of an induction motor taken from one of the

textbooks that I have referred and I have also introduced during the introduction class. So let us see this picture. So you see that

this is a picture of an induction motor that is broken apart that is split apart this called

the exploded view of the induction motor. Now look at the various parts. Now let me

move the cursor here on this part, this is the rotor, this is the shaft and this is the

rotor and the rotor is die cast it is aluminium die cast and it is shorted at both the ends. You see this aluminium ring here shorts all

the coils all the coils are shorted and that is why it looks like a………. they say

it is a squirrel cage. So this is a rotor and then inside here you see some coils or

windings and that is the stator windings which form the various poles the A phase poles,

the B phase poles, C phase poles so on which are wound on the circumference of this stationary

system and that is called the stator. So this whole coil is wound on the stationary system.

This yoke, this is called the yoke and that is the stator that is anchored on to the ground

through these foot plates and that does not know…….. so that is why we say that once

the poles are fixed in space they are fixed in space they do not rotate and what makes

the flux to rotate is basically by virtue of the 3 phase source that you supply which

are 120 degrees apart. And this rotor is inserted into it and then you have the end phasing

plates. These are the end plates here, this is one end plate and this is one end plate

that is fitted in through these holes and then there is bearing which you see also the

bearing here which is fixed to this thing and that will cause a full smooth rotation

of the shaft here. And here you could also see…. beyond this face plate there is something

mounted on the shaft and that is a fan and beyond that there is another enclosure a covering

for the fan, so this fan is to provide cooling for the machine that is this is an air cooled

enclosed air cooled induction machine. So this is how the various parts of the induction

motor would look like this is a squirrel cage induction motor. Now all these windings here are brought out;

there are three so all these windings are clubbed together and brought out as three

terminal points here A phase, B phase, C phase and also the neutral; you saw that we connected

all neutrals together for one or other end of the coil of the various phases. The neutral

and A B and C phases are brought out as terminal inside this box here through which the connection

to the external source is done. okay so this is how an induction motor a typical induction

motor would look like for a given rating. Of course for various ratings the sizes will

be different. So this is for all for now for this particular class. We shall continue in

the next class with more details on the slip, the formulas, the torque speed characteristics

and the equivalent circuits and so on and so forth. Thank you for now.

@22.10 in the lecture, the notations for the Fleming's left hand rule mentions force being represented by the middle finger, but conventionally the middle finger represents the current direction in this rule. Can someone explain this to me?

bohot hard bohot tyt

Thank you so much. It's been of great help.

35:28 i could not understand how 60/(1/f) introduced?

grt help thanks sir

first time…feel very gud……in the induction machine….sir u are……lord for me…very very…thanku sir

Sir please answer that in these here if two phase are connected here so that there is two dample of same polarity like south and south , North and north but in one cycle there is change of polarity will maintain . Induction motor introduction 40:33 minute.

super sir ……please upload more videos from ap

Talented sir

But the video quality is very poor.

For someone like me who has a basic understanding of network theory and who wants to know about induction motor , I can say that you won't find a better lecture than this.

it's an amazing video… the speed is perfect if one wants to take running notes, if not .. well… I used the right arrow to fast-forward the video when he was drawing 😛

thank you!

So if I built an induction motor and I wanted it to have the most torque possible I would need to make the rotor chok full of copper conductors.

22:00 Is the direction of Current I is incorrect? From Faraday's law, the induced EMF should make the current out of the paper direction, For an Anticlockwise rotating B field.

Good explanation sirr thank u

Sir please do more and more videos sirrr

I am from iit and i find this teaching far better.

Lodaaa

Lodaaa

Lodaaa

Lodaaa

you THANK YOU

and my professor go to hell 🙂

Awesome teaching sir. But ive a doubt. How can current be induced in a stationary conductor

the best elec induction motor lecture. tqvm!

It's really good for everyone hand's of you sir

Isnt it possible to increase the frequency of the line using power electronics?

It was great, thank you very much proff.

loved the way he call Ns as 'anus' 😂😂

anyway very nice video

Sir,every topic was precisely explained .Thanks a lot !

sir please make a lecture of 1 phase induction motor

Thank You sir

sir , a million times thank you..and the rotating magnetic field explanation

was really awesome

speak in hindi

I completed be with first class and I think know I am started learning.thank you sir you made the concepts so much easy to understand.

sir please explain me how to you are draw result vector in video at phase A,B,C at time 17:51

in 28.56 magnetic field direction due to stator field is shown wrong.

i have an induction motor. i want to replace/adapt the impeller to a bigger one therefore i want to extend the length of the shaft. my question is: is it possible to weld another 4cm piece to the shaft in order to extend it? please any comments will be appreciated.

may we get single phase induction motor lecture?

sir Hindi mai expan ho sakata hai kya

Exceptional teaching skills . Thanks a lot Sir!

Sir u really have good drawing skills.

Perfect illustrations and explanation! Thanks a lot!

Nice explanation…cleared many doubts 🙂

Thank you very much for the time…. But the drawings are not clear to see

why only cos is used for MMF and current sir?

thank you nptelhrd

My head just blew up when he drew the three-phase waveform that way. So beautiful =)

what software is that?