lecture 3 (Induction machine).pdf

ECE-203
Introduction to Electrical Machines
Undergraduate Course for Electrical & Computer Engineering
Lecture 1
Department of Electrical &
Computer Engineering
Aksum University
Course Instructor Getnet/M

AC motor
 It is a type of electric motor in which alternating current
from a power source is fed through a primary winding and
induces a current in a secondary winding, with the parts
arranged so that the resulting magnetic field causes a
movable rotor to rotate with respect to a fixed stator.
 An electrical motor is such an electromechanical device
which converts electrical energy into a mechanical energy.
In case of three phase AC operation, most widely used
motor is Three phase induction motor as this type of
motor does not require any starting device or we can say
they are self starting induction motor.

Synchronous motor
 Synchronous Motors are principally used as
alternating current AC generators. They
supply the electric power used by all sectors
of modern societies:-
- industrial
-Commercial
-Agricultural
-domestic

Synchronous Motors
 Synchronous motors are sometimes
used as constant speed for reactive
power control in large power systems.
 Synchronous machines (Generator
performance for stand-alone and grid
aplications)

Induction Motor
 Induction motor is also called asynchronous
motor as it runs at a speed other than the
synchronous speed.
 Like any other electrical motor, induction
motor have two main parts namely rotor and
stator.

Induction Motor
Construction Of Induction Motor
1. Stator
 As the name indicates stator is a stationary
part of induction motor.
 A three phase supply is given to the stator of
induction motor.

cont….
 Stator of Three Phase Induction Motor The
stator of the three phase induction motor
consists of three main parts:
1. Stator frame
2. Stator core
3. Stator winding or field winding

Cont……
1. Stator Frame:
 It is the outer most part of the three phase
induction motor.
 Its main function is to support the stator core
and the field winding.
 It acts as a covering and provide protection
and mechanical strength to all the inner parts
of the machine.
 The frame is either made up of die cast or
fabricated steel.

Cont…..
 The frame of three phase induction motor
should be very strong and rigid as the air gap
length of three phase induction motor is very
small (Small mmf, Small Magnetizing
Current,Small flux, Lower Power Losses,
High PF, High VR) , otherwise rotor will not
remain concentric with stator which will give
rise to unbalanced magnetic pull.

Cont…..
2. Stator Core :
 The main function of the stator core is to
carry alternating flux.
 In order to reduce the eddy current losses
the stator core is laminated.
 This laminated type of structure is made up
of stamping which is about 0.4 to 0.5 mm
thick.
 All the stamping are stamped together to
form stator core, which is then housed in
stator frame.

Cont….
 The stampings are generally made up of
silicon steel, which reduces the hysteresis
loss.
Fig. stator core

Cont….
3. Stator Winding or Field Winding :
 The slots on the periphery of stator core of
the three phase induction motor carries three
phase windings.
 This three phase winding is supplied by three
phase ac supply.
 The three phases of the winding are
connected either in star or delta depending
upon which type of starting method is used.

Cont…..
 The squirrel cage motor is mostly started by
star-delta stater and hence the stator of
squirrel cage motor are delta connected.
 The slip ring three phase induction motor are
started by inserting resistances so, the stator
winding can be connected either in star or
delta.
 The winding wound on the stator of three
phase induction motor is also called field
winding and when this winding is excited by
three phase ac supply it produces rotating
magnetic field.

Cont……
Fig. Stator winding or field winding

Cont….
Rotor
 The rotor is a rotating part of induction motor.
The rotor is connected to the mechanical
load through the shaft.
 The rotor of the three phase induction motor
are further classified as- Squirrel cage rotor,
1. Slip ring rotor or wound rotor or phase
wound rotor.
2. Depending upon the type of rotor used the
three phase induction motor are classified as-

Cont…
1. Squirrel cage induction motor
2. Slip ring induction motor or wound
induction motor or phase wound induction
motor
1. Squirrel cage three phase induction motor:
 The rotor of the squirrel cage three phase
induction motor is cylindrical in shape and
have slots on its periphery.
 The squirrel cage rotor consists of aluminum,
brass or copper bars (copper bras rotor is
shown in the figure beside).

Cont…..
 These aluminum, brass or copper bars are
called rotor conductors and are placed in the
slots on the periphery of the rotor.
 The rotor conductors are permanently
shorted by the copper or aluminum rings
called the end rings.
 In order to provide mechanical strength
these rotor conductor are braced to the end
ring and hence form a complete closed
circuit resembling like a cage and hence got
its name as "squirrel cage induction motor".

Cont…..
 The squirrel cage rotor winding is made
symmetrical.
 As the bars are permanently shorted by end
rings, the rotor resistance is very small and it
is not possible to add external resistance as
the bars are permanently shorted.

Cont…..
Advantages of squirrel cage induction
rotor:-
 Its construction is very simple and rugged.
 As there are no brushes and slip ring, these
motors requires less maintenance.
Applications:
 Squirrel cage induction motor is used in
lathes, drilling machine, fan, blower printing
machines e.t.c.

Cont……
2. Slip ring or wound three phase induction
motor :
 In this type of three phase induction motor
the rotor is wound for the same number of
poles as that of stator but it has less number
of slots and has less turns per phase of a
heavier conductor.
 The rotor also carries star or delta winding
similar to that of stator winding.
 The rotor consists of numbers of slots and
rotor winding are placed inside these slots.
The three end terminals are connected
together to form star connection.

Cont……
 The three ends of three phase windings are
permanently connected to these slip rings.
The external resistance can be easily
connected through the brushes and slip rings
and hence used for speed control and
improving the starting torque of three phase
induction motor.
 The brushes are used to carry current to and
from the rotor winding.

Cont…….
 These brushes are further connected to
three phase star connected resistances.
 At starting, the resistance are connected in
rotor circuit and is gradually cut out as the
rotor pick up its speed.
 When the motor is running the slip ring are
shorted by connecting a metal collar, which
connect all slip ring together and the brushes
are also removed.

Cont….
 In squirrel cage motors, the rotor bars are
short-circuited in the construction itself, so rotor
terminals are not available.
 But in slip ring motors, it is essential to bring
out the rotor terminals as it gives possibility
of inserting an additional resistance in the rotor
circuit.
 In the terminal box shown,only D, E, F
constitute the rotor terminals, assuming that
the other ends of it have already been
connected inside the machine. Sometimes, it
is desirable to bring out ends of each of the
phases of the rotor.

Cont…….
 Initially maximum resistance is in the circuit.
As motor gather speed, the resistance is
gradually cut-off.
 The operation may be manual or automatic.
 The starting torque is proportional to the rotor
resistance.
 Hence important advantage of this method
is not only the starting current is limited but
starting torque of the motor also gets
improved.

Cont…..
 Advantages of slip ring induction motor
- It has high starting torque and low starting
current.
- Possibility of adding additional resistance to
control speed.
Application:
- Slip ring induction motor are used where
high starting torque is required i.e in hoists,
cranes, elevator etc.

Cont….
Difference between Slip Ring and Squirrel Cage
Induction Motor

Induction Motor
Working prinicple of Induction motor
 An electrical motor is an electromechanical
device which converts electrical energy into a
mechanical energy.
 In case of three phase AC operation, most
widely used motor is Three phase induction
motor as this type of motor does not require
any starting device or we can say they are
self starting induction motor.

cont…..
 For better understanding the principle of
three phase induction motor, the basic
constructional feature of this motor must be
known to us.
 This Motor consists of two major parts:
1.Stator:
 Stator of three phase induction motor is
made up of numbers of slots to construct a 3
phase winding circuit which is connected to 3
phase AC source.

Cont…..
 The three phase winding are arranged in
such a manner in the slots that they produce
a rotating magnetic field after 3Ph. AC supply
is given to them.
2. Rotor:
 Rotor of three phase induction motor
consists of cylindrical laminated core with
parallel slots that can carry conductors.
 Conductors are heavy copper or aluminum
bars which fits in each slots & they are short
circuited by the end rings.

Cont….
 The slots are not exactly made parallel to the
axis of the shaft but are slotted a little
skewed because this arrangement reduces
magnetic humming noise & can avoid stalling
of motor.

Cont…
 When three phase supply is connected
across stator windings then a rotating flux is
produced in stator which rotates clockwise at
synchronous speed giving
 Ns = 120f/p
 When the rotor of 3-phase motor is placed in
air gap of stator, a emf is induced in rotor
conductors.
 For squirrel cage rotor, conductors are
shorted together and the four terminal
wound rotor conductors are connected with
additional resistance between slip rings.

Cont…
 The induced emf causes a current to flow in
these shorted conductors which produces a
flux.
 Due to reaction of this flux with stator flux a
torque is produced and rotor rotates in same
direction of rotating flux with a speed less than
synchronous speed.
 Slip and Effect of Slip on Rotor
Parameters

Cont…
 The rotating field revolves with the speed of
synchronism(), and if the rotor conductors
were to revolve at the same speed there
would not be any torque.
 Hence, there is a difference between rotor
and rotating field speeds.

Cont….
 The rotor speed is less than the rotating field
speed and the difference in speed is known
as the slip of motor. Generally, slip lies
between zero to five percent.
 Slip(s) = Ns-Nr
Ns
 where Ns is synchronous speed of rotating
field in stator and Nr is speed of rotor (motor
speed)
or

Cont…
 Rotor emf frequency = s × frequency of
applied voltage to stator = sf
 The induced emf in rotor also depends on
slip.
 When rotor is stationary the emf induced in
rotor is

Cont….
 where n2 and n1 are number of turns in rotor
and stator windings and E1 is applied voltage
in stator.
 Now for moving rotor, rotor emf becomes
sE2.
 Important thing to remember here
is that both E1 and E2 remain fixed by above
relation even at different slip or load values.

Cont…
 The rotor resistance (R2) is constant with
change in slip unless external resistance is
added or there is a change in temperature.
 Rotor reactance at slip one (standstill) is X2
or X2 = 2p f L2
 where f is frequency of rotor emf at standfill
and L2 is inductance of rotor winding.
 Now moving rotor frequency becomes sf, so
Rotor reactance = 2p sf L2 = s X2
So, rotor impedance at motion

Cont…
Example 3.1
A 460V,100hp,60HZ, Four-pole induction
machine delivers rated out put power at a slip
of 0.05.
Determine
e. Rotor induced voltage at the operating
speed, if the stator-to-rotor turns ratio is 1:0.5

Cont…
Production of rotating field & rotor rotation
 A rotating magnetic field is a magnetic field
that has moving polarities in which its
opposite poles rotate about a central point or
axis.
 Ideally the rotation changes direction at a
constant angular rate.
 This is a key principle in the operation of the
alternating-current motor.
 Rotating magnetic fields are often utilized for
electromechanical applications such as
induction motors and electric generators.

Cont…
How to produce a rotating magnetic field in
a three-phase stator?
 How can the stator magnetic field be made to
rotate?
 The fundamental principle of ac machine
operation is that if “a three-phase set of
currents, each of equal magnitude and
differing in phase by 120°, flows in a three-
phase winding, then it will produce a rotating
magnetic field of constant magnitude.”

Cont…
Rotor rotation
 The rotor is a moving component of an
electromagnetic system in the electric motor,
electric generator, or alternator.
 Its rotation is due to the interaction between
the windings and magnetic fields which
produces a torque around the rotor's axis.

Cont…
 Rotor bar voltage
 The rotating magnetic field induces a voltage
in the rotor bars as it passes over them.
 This equation applies to induced voltage in
the rotor bars.

Cont….
 Torque in rotor
 A torque is produced by the force produced
through the interactions of the magnetic field
and current as expressed by the given:

Cont…
Induction motor as a generalized transformer
 The difference is that the normal transformer is an
alternating flux transformer while induction motor is
rotating flux transformer.
 The normal transformer has no air gap as against
this an induction motor has distinct air gap between
its stator and rotor.
 In an alternating flux transformer the
frequency of induced emf and current in
primary and secondary is always same.

Cont…
 However in the induction motor frequency of
emf and current on the stator side remains
same but frequency of rotor emf and current
depends on the slip and slip depends on load
on the motor.
 And last difference is that in case of the
alternating flux transformer the entire energy
present in the secondary circuit, is in the
electrical form.
 As against this, in an induction motor part of
its energy in the rotor circuit is in electrical
form and the remaining part is converted into
mechanical form.

 In general, an induction motor can be treated
as a generalized transformer as shown in the
Figure (above). In this, the slip ring induction
motor with star connected stator and rotor is
shown.
 So if E1= Stator emf per phase in volts.
E2= Rotor induced emf per phase
in volts at start when motor is at standstill.
 Then according to general transformer there
exists a fixed relation between E1 and E2
called transformer ratio.
 At start when N =0, s =1 and we get,

Cont…
Equivalent circuit of Induction motor and its
analysis
 As in the case of a transformer, the secondary (rotor)
parameters of the induction motor can be transferred
in to the primary (stator) side and vice versa to draw
its equivalent circuit.
 In this derivation, only machines with symmetric
poly-phase windings excited by balanced poly-phase
voltages are considered.
 As in many other discussions of poly-phase devices,
it is helpful to think of three-phase machines as
being Y-connected, so that currents are always line
values and voltages are always line-to-neutral
values.

Cont…
 In this case, we can derive the equivalent
circuit for one phase, with the understanding
that the voltages and currents in the
remaining phases can be found simply by an
appropriate phase shift of those of the
phases under study.

Cont…
 When shifting impedance or resistance from
secondary (or rotor) to primary (or stator), the
resistance or impedance should be multiplied
by K2; where K is the voltage ratio ES/Er
whereas current should be multiplied by the
inverse of the voltage ratio (i.e. 1/K).
 First, consider conditions in the stator.

Cont…
 The synchronously-rotating air-gap flux wave
which is created in the stator generates
balanced poly-phase counter emfs in the
phases of the stator.
 The emf induced in thestat or differs from the
terminal (applied) voltage by the voltage drop
in the stator leakage impedance Z1 = R1 + j
X1.
 Thus

Cont…
 Where:
- V1 is line to neutral terminal voltage of the
stator
- E1 line to neutral induced voltage in the
stator
- I1 is stator current
- R1 is stator resistance, and X1 is stator
leakage reactance

Cont…
 The resultant air-gap flux is created by the
combined mmf's of the stator and rotor
currents.
 Just as in the case of a transformer, the
stator current can be resolved into two
components: a load component and an
exciting (magnetizing) component.

Cont…
 The load component I2 produces an mmf that
corresponds to the mmf of the rotor current.
 The exciting component Iφ is the additional
stator current required to create the resultant
air-gap flux and is a function of the emf E1.
Fig. Stator equivalent circuit of Induction motor

Cont…
 The exciting current can be resolved into a
core-loss component Ic inphase with E1 and a
magnetizing component Im lagging E1 by 900.
These are represented as a shunt branch in
the equivalent circuit, formed by core loss
resistance Rc and a magnetizing reactance
Xm in parallel connected across E1, as shown
in figure (above).

Cont…
 Both Rc and Xm are usually determined at
rated stator frequency and for a value of E1
close to the expected operating value; they
are then assumed to remain constant for the
small departures of E1 associated with
normal operation of the motor.
 For the complete equivalent circuit of the
induction machine, the rotor can be
represented by its equivalent impedance
referred to the primary (stator) side.

Cont…
 Here we will adopt the concept which is
especially useful in modeling squirrel cage
rotors.
 It states that the performance of the rotor as
seen from the stator terminals would be
unchanged, if the rotor were to be replaced
by an equivalent rotor with a poly-phase
winding with the same number of phases and
turns as the stator but producing the same
mmf air gap flux as the actual rotor.

Cont…
 Where:
- Z2 is the impedance of the rotor referred
to the stator
- I2 is the rotor current referred to the stator
- K is transformation ratio (effective turns
ratio between the stator and the actual rotor
winding)

Cont…
Due to the relative motion between the
rotor and the stator the frequency of the
rotor changes since the rotor speed is
relatively less than the synchronous
speed. Hence, the slip frequency
leakage impedance of the rotor referred
to the rotor will be

Cont…
 Where:
- R2 is rotor resistance referred to the stator
- sX2 is rotor leakage reactance at slip
frequency referred to the stator
- Note that X2 is the rotor reactance at the
stator frequency.
- Finally, consider that the resultant flux
wave induces both the slip-frequency emf
induced in the referred rotor E2 and the stator
counter emf E1.

Cont…
 If it were not for the effect of speed, these
voltages would be equal in magnitude since
the referred rotor winding has the same
number of turns per phase as the stator
winding.
 However, because the relative speed of the
flux wave with respect to the rotor is s times
its speed with respect to the stator, the
relation between these emfsis

Cont…
 Therefore, Z2 which is the impedance of the
stationary rotor will appear across the
terminals of the stator equivalent circuit
shown in figure (above).
 The combined single phase equivalent
circuitof the induction motor is shown infigure
(below).
 As it can be seen in the figure, the combined
effect of the shaft load and rotor resistance
appeared as a reflected resistance.

 Cont…
Fig. single phase equivalent circuit of poly-phase induction motor
The current in the reflected rotor impedance equals the load component I2 of stator
current, and the voltage across this impedance equals the stator voltage E1.

Cont…
 Note that when rotor currents and voltages
are reflected into the stator, their frequency is
also changed to stator frequency.
 All rotor electrical phenomena, when viewed
from the stator, become stator-frequency
phenomena, because the stator winding
simply sees mmf and flux waves traveling at
synchronous speed.

Cont…
Fig. Rearranged single phase equivalent circuit

Cont…
Analysis of equivalent circuit of three
phase induction motor
 The single phase equivalent circuit of the
above two figures can be used to determine
a wide variety of steady state performance
characteristics of three phase induction
machines.
 These include variations of current, speed
and losses as the load-torque requirements
change as well as the starting torque, and
the maximum torque.

Cont…
 The power flow shown in figure (below)
shows the input power, the power losses, air
gap power, power converted to mechanical
and output power in induction motor.
Fig. power flow in induction motor

Cont…
 Mechanical losses occur at the bearing and
brush friction loss occurs in wound rotor
induction motor.
 These losses are zero at start and with
increase in speed these losses increases.
 In three phase induction motor the speed
usually remains constant.
 Hence these losses almost remains constant.

Cont…
Induction motor operating as a generator
 When an induction motor runs faster than its
synchronous speed, it is said to be acting as a
generator.
 Hence, it converts the mechanical energy it receives
in to electrical energy which will be released by the
stator.

Cont…
 As soon as the speed of the squirrel cage
motor which is driven by a petrol engine
shown in figure (below) exceeds its
synchronous speed, it starts delivering active
power to the three phase line connected to it.
 However, for creating its own magnetic field,
it absorbs reactive power from the line to
which it is connected and its direction is
opposite to active power.

Cont…
Fig. (a) squirrel cage motor driven
by a petrol engine (b) energy
released by a stator

Cont…
 The active power is directly proportional to
the slip above the synchronous speed.
 The reactive power required by the machine
can also be supplied by a group of capacitors
connected across its terminals as shown in
figure ( below).
 This arrangement can be used to supply a
three phase load without using an external
source.
 The frequency generated is slightly less than
that corresponding to the speed of rotation.

Cont…
Fig.Power flow and connection of
shunt capacitors

Cont…
 Induction generator is not a self-excited
machine.
 Therefore, when running as a generator, the
machine takes reactive power from the AC
power line and supplies active power back
into the line.
 Reactive power is needed for producing
rotating magnetic field.
 The active power supplied back in the line is
proportional to slip above the synchronous
speed.

Cont…
Self-excited induction generator
 It is clear that, an induction machine needs
reactive power for excitation, regardless
whether it is operating as a generator or a
motor.
 When an induction generator is connected
to a grid, it takes reactive power from the
grid.
 But what if we want to use an induction
generator to supply a load without using an
external source (e.g. grid)?

Cont…
 An electric grid is a network of synchronized
power providers and consumers that are
connected by transmission and distribution
lines and operated by one or more control
centers.
 When most people talk about the power
"grid," they're referring to the transmission
system for electricity.

Cont…
 A capacitor bank can be connected across
the stator terminals to supply reactive power
to the machine as well as to the load.
 the rotor is rotated at an enough speed, a
small voltage is generated across the stator
terminals due to residual magnetism.
 Hence, capacitor bank be must be large
enough to supply the reactive power
normally drawn by the motor.

Cont…
Torque and power using Thevenin’s
theorem
 The Thevenin equivalent of the equivalent
circuit to the right of the parallel branch
(included):

lecture 3 (Induction machine).pdf

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