Students of First Semester B.E of RGPV University Subject BE 104 Unit III Rotating Electrical machines

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READING MATERIAL FOR B.E. STUDENTS
OF RGPV AFFILIATED ENGINEERING COLLEGES
SUBJECT BASIC ELECTRICAL AND ELECTRONICS ENGG
Professor MD Dutt
Addl General Manager (Retd)
BHARAT HEAVY ELECTRICALS LIMITED
Professor(Retd) in EX Department
Bansal Institute of Science and Technology
KOKTA ANANAD NAGAR BHOPAL
Presently Head of The Department ( EX)
Shri Ram College Of Technology
Thuakheda BHOPAL
Sub Code BE 104 Subject Basic Electrical & Electronics
UNIT III Rotating Electrical Machines

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RGPV Syllabus
BE 104 BASIC ELECTRICAL & ELECTRONICS ENGINEERING
UNIT III
ROTATING ELECTRICAL MACHINES
Construction details of DC machines, induction machine and synchronous
machine. Working principle of 3 ph induction motor, EMF equation of 3 ph
induction motor. Concept of slip in 3 ph induction motor. Explanation of torque
slip characteristic of 3 ph induction motor . Classification of series DC Motor and
DC Generator.
INDEX
S No Topic Page
1 Construction details of DC machines 3,4,5,6
2 Construction details of 3ph induction motor 6,7,8,9
3 Construction details of Synchronous machines 10,11
4 Working principle of 3 ph induction motor reaction 12,13,14
5 EMF equation of 3 ph induction motor 14,15
6 Concept of slip in 3 ph induction motor 15,16
7 Starting Torque of 3ph induction motor; 16,17
8 Explanation of torque slip characteristic of 3 ph induction
motor
18,19
9 Series excited DC motor and Generators 20,21,22,23
10 References 24

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Construction Details of DC Machines
There are four main parts of a DC machine
1)Field Magnet
2)Armature
3) Commutator
4)Brush and Brush gear
Field System:- The purpose of field system is to create a uniform magnetic field
within which armature rotates. It consists of following four parts.
a) Yoke or frame
b) Pole cores
c) Pole Shoes
d) Magnetizing coils
Cylindrical yoke or frame is used which acts as frame and carries the
magnetic flux produced by the poles. Poles are used to carry coils of
insulated wires carrying the exciting current. The pole shoes acts as support
to the coils and spread out flux over the armature periphery more uniformly.
The magnetizing coils is to provide number of ampere turns of excitation required
to give the proper flux through the armature to induce the desired potential
difference.
ARMATURE:- It is the rotating part of the DC machine and is built up in a
cylindrical drum. The purpose of armature is to rotate the conductors in the
uniform magnetic field , It consists of coils insulated wires wound around a iron

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core and is so arranged that the electric current are induced in these wires when
armature is rotated in the magnetic field. The armature core is made from high
permeability silicon sheet steel stampings.
A small airgap exists so that armature can rotate freely without rubbing or touching
poles. Armatures are LAP or WAVE wound.
COMMUTATOR:- The commutator is a form of switch (rotating) placed
between the armature and the external circuit so arranged that the input is fed
(incase of motor) and the output is taken out (in case of generator) through
commutator by brushes and brushgear. Two important functions it is doing in case
of DC machine.
1) It connects the rotating armature conductors to the external circuit through
brushes.
2) It converts the alternating alternating current induced in the armature
conductors into unidirectional current to the external load circuit in
generating action, where as it converts the alternating torque into
unidirectional torque in motor action.
The commutator is of cylindrical shape and is made up of wedge shaped
hard drawn copper segments of mica. These segments are insulated from
each other by thin sheet of mica . The segments are held together by two

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Vee rings that fits into the groove cut into the segments. Each armature coil
is connected to the commutator segments through riser.
BRUSHES :- The brushes usually made from carbon are pressed upon the
commutator and from the connecting link between armature winding and
external circuit. They are made from carbon because is conducting material
and at the same time in powdered form provides lubrication effect on the
commutator surface. The brushes are held in brush holders and brushgear on
commutator.
END HOUSINGS:- End housings are attached to the ends of the main
frame and supports bearings. The commutator side supports brushgear
assembly. Where as NCE side supports only bearing.
BEARINGS :- The ball and roller bearings are fitted in the end housings.
The function of bearing is to reduce the friction between the rotating part
and stationary part.

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SHAFT :- The shaft is made of steel having maximum breaking strength.
The shaft is used to transfer mechanical power from or to the machine. The
rotating parts like armature core, cooling fan etc are keyed to the shaft
CONSTRUCTION OF INDUCTION MOTOR:- A three phase induction
motor consists mainly two parts namely stator and rotor.
Stator It is the stationary part of the motor. It consists of following parts
i) Outer frame
ii) Stator core
iii) Stator winding
Outer Frame:- It is the outer body of the motor. Its function is to support
stator core and protects the inner parts of the machine. For small machines it
is casted and for large machines it is fabricated. It also supports the motor to
be placed on the foundation through feet’s at the bottom.
Stator Core:- The stator core is to carry the alternating field which is
produced hysteresis and eddy losses.. The core is made from high grade
silicon sheet steel. The stampings are assembled under hydraulic pressure

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and are keyed to the frame. Each lamination is insulated with a thin layer of
varnish. The thickness of lamination varies from 0.3mm to 0.5mm .Slots are
punched on the inner periphery of the stampings
Stator Winding:- The stator core carries three phase winding which is
supplied from three phase supply. Six terminals of winding ( two each per
phase) are connected to terminal box of the motor. The stator winding are
wound for definite number of poles depends upon the required speed.
Ns = 120f/P
The three phase winding can be connected in star or delta externally.
Through starter.
ROTOR:- It is the rotating part of the motor. There are two type of rotors
for induction motor.
1) Squirrel Cage Rotor:- Most of the induction motors are of this type of
rotor because of simple and rugged construction of rotor. In cage
construction, copper bars or aluminum bars are placed, the rotor bars are
almost placed parallel to the shaft. The rotor conductors are short
circuited with short circuiting rings made up similar material of rotor bar.
This resembles as squirrel cage. The slots in the rotor stampings are semi
closed or closed type. The use of semi closed or closed slots is for
reducing the magnetizing current.

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1) WOUND ROTOR :- The rotor is wound with an insulating winding
similar to that of stator except that the number of slots are smaller and
fewer turns per phase of heavier conductors are used. A large number of
turns increases secondary voltage and reduces the current that flows
through the sliprings. Based on secondary voltage the insulation of rotor
winding is decided. The voltage of rotor and current influences the value
of rotor resistance to be put across the slip rings.
The rotor is wound for the same number of poles as that of stator. The
finish terminals are connected in star and are connected to the three
phosphor Bronze sliprings, which is mounted on the shaft. The rotor
current is carried to the external resistance through brushes mounted on
the three sliprings.
Since sliprings are used that is why this type of motor is called as
SLIPRING MOTORS.

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CONSTRUCTION OF SYNCHRONOUS MOTOR:- The synchronous
motor essentially consists of two parts mainly the armature ( stator) and field
magnet system ( rotor).
STATOR : - The armature is an iron ring formed of laminations of special
magnetic material ( silicon sheet steel) . It is having slots on the inner
periphery to accommodate armature conductors and is known as stator. The
whole structure is held in a cast iron or fabricated frame. The field rotates in
between the stator, flux of rotating field cuts the stator core continuously and
causes eddy current losses in the core. The laminations are insulated from
each other by thin layer of varnish.
ROTOR:- Similar to DC field system the rotor field system of synchronous
machine is excited by DC 125 250V DC supply from exciter which is
mounted on the same shaft. Rotors are of two type
1) Salient pole type rotor
2) Smooth cylindrical rotor
The rotor of this type is used entirely for low speed alternators. These type
of machines are called projected pole type machines. The poles are made
from lamination punched from silicon sheet steel and joined together by pole
rivets. The each lamination is insulated by this layer varnish. The damper

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windings are provided at the pole shoes for avoiding hunting. The pole faces
are so shaped that airgap is minimum at centre and increases from the pole
centre for the sinusoidal flux so that the induced EMF is sinusoidal. The end
of the field windings are connected through sliprings to a DC source. They
have following special features:-
i) Salient pole field structure has large diameter and short shaft lengths
ii) The pole shoes cover about ⅔ of pole pitch
iii) These are employed in HYDRO turbine or diesel engines, where RPM is
low ( 100rpm to 325 rpm)
SMOOTH CYLINDRICAL ROTOR
The rotor of this type is used in very high speed alternators. ( Steam
Turbine) To reduce the peripherals velocity the diameter of this type of rotor
is small and the axial length is increased. Such rotor normally have two or
four poles. It consists of steel forgings with radial slots in which field copper
,usually strips are placed. The coils are held by steel or bronze wedges and
coil ends are fastened by metal strips. This type of rotor have uniform air
gap. For getting sinusoidal EMF slots are shapes machined in the rotor
forging.
i) Less windage loss
ii) Very high operating speed ( 3000rpm)
iii) Robust construction and noiseless operation.

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WORKING PRINCIPAL OF 3 PH INDUCTION MOTOR:- In a
induction motor there is no electrical connection to the rotor, but the currents
are induced in the rotor circuit and therefore , the same condition exists as in
the case of DC motor. The rotor conductors carry current in the stator
magnetic field and thereby have force exerted upon them tending to move
them at right angle to the field.
When the stator winding of 3ph induction motor is connected to a 3ph
supply, a rotating field is established which rotates at synchronous speed.
The direction of rotation of the magnetic field will depend upon the phase
sequence of the stator current. The direction of magnetic field can be
reversed by reversing the phase sequence of 3ph supply. This can be done by
interchanging any two leads of 3ph supply. The number of poles of the
revolving field will be same as the number of poles for each phase of stator
winding is wound. The speed at which the field is rotating is called
synchronous speed.
Ns = 120f
P
Here P is number of Poles, and f is supply frequency
As magnetic field sweeps across the rotor conductors, and EMF is induced
in these conductors, ( as similar to the transformers). Since the rotor circuit
is either shorted or closed through some external resistance, the induced
EMF due to revolving field causes current to flow in rotor conductors.
A section of induction motor stator and rotor, with the magnetic field
assumed to be rotating in a clockwise direction and with rotor stationary, as
at starting.
The relative motion of the rotor with respect to the stator field is
anticlockwise. Now by the effect of combined field and LHS rule . the rotor

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conductor experiences a force tending to move the rotor conductor to the
right, one half cycle late, the stator field direction will be reversed, the rotor
current will also be reversed, so the force on the rotor is still same. Likewise
rotor conductors under stator poles will have a force exerted upon them, all
tending to turn the rotor in the clockwise direction. If the developed torque is
great enough to overcome the resisting torque of the load, the rotor will
accelerate in the clockwise direction or in the same direction of the stator
field.
When the rotor is stationary and about to start, the frequency of the induced
EMF in the rotor is equal to the supply fed to the stator. Because of relative
motion is at synchronous speed . As the rotor picks up the speed, the relative
motion between rotor and the synchronously rotating magnetic field
becomes less and the frequency induced in the rotor decreases. The
magnitude of rotor induced EMF, induced rotor current so that the torque
developed depends upon the relative motion. In case of relative motion is
zero the rotor runs at synchronous speed. There will be no induced EMF, no
current, no torque, Thus we can say that the induction motor cannot run at
synchronous speed.
Induction motor at no load will have speed very near to the synchronous
speed, therefore EMF in the rotor will also be very small. The small EMF
will produce little rotor current producing a torque just sufficient to
overcome the losses due to friction and maintain the rotor in motion. As

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mechanical load is applied on the motor shaft, it must slowdown, as the rotor
slows down the relative speed between the magnetic field and rotor is
increased , This results in greater rotor current and greater torque. Thus as
the load increases the motor slows down until the relative motion between
rotor and rotating magnetic field is just sufficient to result in development of
the torque necessary for particular load.
EMF EQUATION OF 3 PH INDUCTION MOTOR
Cross sectional view of a three phase induction motor is shown in figure.
The stator is supplied from a 3ph supply. The rotor is wound 3ph and for the
same number of poles as that of stator.
The rotor is short circuited, Neglecting stator resistance and leakage
reactance being negligibly small we get
Terminal Voltage V Per phase = Stator induced EMF per Phase E
V = E = 4.44 Kw1N1 Ø r f
Kw1 = stator winding factor
N1 = Number of series turn per phase
Ø r = Resultant air gap flux per pole
f = frequency of supply in Hz
The resultant airgap flux Ø r per pole is constant and is related to the
supply voltage V in view of assumption made. The MMF Fr with associated

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flux density Br , which causes for generating Ø r, rotates at synchronous
speed because it is associated with the 3 ph balanced supply to the stator.
Due to the relative speed between Br and the rotor, an EMF is induced in
the rotor winding which causes a current to flow in rotor conductors. The
torque is developed due to interaction between Br and rotor currents torque
so developed tends to turn the rotor in the direction of Br, so as to reduce the
relative speed. Thus the motor is self starting and rotor attends steady speed
N ( where N<Ns) depending upon the load coupled to it. In case rotor runs
at Ns, There would be no induced EMF and no rotor currents in conductors,
no field, hence no torque because relative speed between Br and rotor is
zero.
CONCEPT OF SLIP IN 3PH INDUCTION MOTOR The speed of
polyphase induction motor is always be less than the synchronous speed, as
load increase the speed of rotor decreases. The difference between the speed
of the stator field known as synchronous speed Ns and actual speed of the
rotor N is known as SLIP and is defined by s .Though the SLIP can be
expressed in rpm or in radians per second, but usually it is expressed in
fraction or percentage of synchronous speed.
s = Syn Speed - Rotor Speed
Syn Speed
= Ns – N
Ns
% slip = Ns – N X 100
Ns
At normal load slip of induction motor is between 2 to 5 percent. At no-load
the slip is very small 0.5%. As the load is applied, the natural effect of the
load torque ids to cause the rotor to slow down. As it does so, increases and
with it increase the current and torque increase until the driving torque of the
machine balances with retarding torque of the load.
FREQUENCY OF ROTOR

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Rotor emf frequency f' = Relative speed
120/P
Ns – N = sNs = s 120f/P
f' = Ns –N
120/P
We know and /replacing Ns –N with s120f/P
We get f' = s 120 f P = s f
P 120
EXPLANATION OF TORQUE /SLIP CHARACTERISTICS OF
INDUCTION MOTOR.
Rotor Torque :- The torque is a induction motor is produced due to the
interaction of rotor and stator fields. The in induction motor is:-
i) Proportional to rotor current I2
ii) Stator flux ϕ
iii) Proportional to the rotor power factor Cos ϕ₂
Torque α I2 ϕ Cos ϕ₂
E2 α ϕ
T = K I2 E2Cos ϕ₂
Under running condition
I2 = sE2
√ R₂² + s² X₂²
Cos ϕ 2 = R2
√ R₂² + s² X₂²

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Putting the value of I2 andCos ϕ 2
We get
T = K sE2 X E2 X R2
√ R₂² + s² X₂² √ R₂² + s² X₂²
T = K s E2² R2
R₂² + s² X₂²
This is the torque under running condition
STARTING TORQUE:- At the start the rotor is stationary and s=1
Tst = K E2² R2
R₂² + X₂²
Since the supply voltage is constant the flux also will be constant and E2 will be
constant
Let K1 = K E2²
Tst = K1 R2√
R₂² + X₂²
We know that R₂² + X₂² = Z²
So Tst == K1 R2
Z²

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CONDITION FOR MAXIMUM TORQUE
T = K s E2² R2
√ R₂² + s² X₂²
This will be maximum when s R2 or R2
R₂² + s² X₂² R₂²/s + s X₂² is maximum
R2/√s - √sX₂ = 0
Or R2/ X₂ = s
Putting the value of s we get
Tmax = ( K E2² R2 ) R2/ X₂
R₂² + R₂²
So Tmax = (K E2² R2 ²)
2 R2 ² X₂
Tmax = K E2²
X₂
From this equation it is obvious that
i) Maximum torque is independent of rotor resistance
ii) The slip at which the maximum torque occurs is generated rotor
resistance and rotor reactance becomes equal, this can be easily done in
the case of slip ring motors.
iii) Maximum torque various inversely with standstill rotor reactance. It is
kept minimum by placing the rotor bars near the rotor periphery.
iv) Maximum torque varies directly square of supply voltage.

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TORQUE SPEED, TORQUE SLIP CURVES
1) When the speed is synchronous i.e slip is zero , the torque is also zero
so that the torque slip curve starts from 0
2) When the speed is very near to the synchronous speed Ns i.e slip is
very small , the value of sx2 is very small and is negligible in
comparison with rotor resistance, therefore T is proportional to slip s,
and is straight line
3) As the slip increases i.e the speed drops with increase in load, torque
increases reaches maximum when s = R2/X2 , The maximum torque is
known as pullout torque or breakdown torque and the slip is known as
breakdown slip Sb .
4) When further in increase in slip the speed drops due to increase in
load beyond the point of maximum torque the torque begins to
decrease, The reason is that motor slows down and eventually stops.

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CLASSIFICATION OF SELF EXCITED DC MOTORS AND
GENERATORS
A DC motor or generator whose field winding is supplied itself is
called self excited DC machine
In a self excited DC machine the field coils are connected in parallel
with armature winding .
Self excited DC motor are classified as follows:-
i) Shunt excited DC motor
ii) Series excited DC motor
iii) Compound DC motor
Shunt excited DC motor :- In a shunt excited DC motor the
field coil winding is connected parallel to the armature winding
Ish = V/ Rsh Ia = Il – Ish
Eb = V – Ia Ra -2Vb
SERIES EXCITED DC MOTOR
In case of series excited DC motor the line current passes through series field
winding and also armature current.
Ia = Il = Ise Eb = V – Ia(Ra +Rse) -2 Vb

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COMPOUND DC MOTOR:- There are two type of compound DC motors
i) Cumulative compound motor
ii) Differential compound motor
In the compound wound DC motor the field is produced by the shunt
field as well as series field. Generally shunt field is stronger than
series field. When the series field assists the shunt field it is called
cumulative compound wound DC motor. When series field opposes
the shunt field the motor is called differentially wound DC motor.
Cumulative compound dc motor Differential compound DC motor
SELF EXCITED DC GENERATORS
There are following type of self excited generators.
a) Shunt excited DC generator
b) Series excited DC generator
c) Compound DC generator

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Series Wound DC Generator :- In series wound Dc generator full line current or
armature current flows from the series winding. The series field current Ia = Il = Ise
V = Eg – Ia(Ra +Rse) -2 Vb Power developed Eg Ia power output = V Ia
Shunt wound DC generator The field winding is connected in parallel
to the armature winding
Ish = V/ Rsh Ia = Il + Ish
V = Eg – Ia Ra -2Vb
COMPOUND WOUND DC GENERATOR A compound generator may be called
short shunt or long shunt Short shunt compound

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Ise = Il Ish = V + IlRse = Eg -IaRa
Rsh Rse
Ish + Il = Ia
V = Eg – IaRa – Il Rse -2Vb
Power developed =Eg Ia
Power output = V Il
LONG SHUNT
Ish = V/Rsh
Ise = Ia= Il + Ish
V = Eg – IaRa – IseRse -2Vb
= Eg - Ia(Ra +Rse) -2Vb
Power Developed EgIa
Power output V Il
LONG SHUNT

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References
1. Basic electrical and Electronics Engineering By Pankaj Swarnakar and
Shiv Shankar Mishra Tech India publication
2. Electrical & Electronics Engineering By RK Chaturvedi and SK Sahdev
Dhanpatrai Publication.
3. Electrical & Electronics Engineering By JB Gupta KATSON Books
4. Basic Electrical Engineering by Vincent Deltoro
5. Basic Electrical Engineering by De an Sen TMH Publication
6.