Analysis of a 3-phase Induction Motor.
As-Salaam-Alaikum, Welcome to my blogger "World of Knowledge".Here we discuss the topic analysis of 3 Phase Induction Motor.The main idea of this article is to get familiar with the working,
construction and about each and every part of our electrical machine element
which is 3 phase induction motor.
Stator core:
Stator Frame:
It is the outer 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 it provides protection and
mechanical strength to all the inner parts of the induction motor. The frame is
either made up of die-cast or fabricated steel. The frame of three phase
induction motor should be strong and rigid as the air gap length of three phase
induction motor is very small. Otherwise, the rotor will not remain concentric
with the stator, which will give rise to an unbalanced magnetic pull it is the outer part of the three phase induction motor.
It
is the part of the motor which will be in a rotation to give mechanical output
for a given amount of electrical energy. The rated output of the motor is
mentioned on the nameplate in horsepower. It consists of a shaft,
short-circuited copper/aluminum bars, and a core.
Points to be discuss in this Article are given below :
Types of Motor:
Squirrel Cage
Induction Motor Construction:
A
squirrel cage induction motor consists of the following parts:
- Abstract
- Introduction
- What is Motor
- Types of Motor
- What is Squirrel Cage Induction Motor
- Squirrel Cage Induction Motor Construction
- Stator
- Stator core
- Stator Frame
- Stator Winding or Field Winding
- Rotor
- Fan
- Bearings
- Rotor Types:
- Squirrel Cage Rotor
- Features of Squirrel Cage Rotor
- Wound Type Rotor or Slip Ring Type Rotor
- Features of Wound Type Rotor or Slip Ring Type Rotor
- Basic Working Principle of an Induction Motor
- Slip
- Torque Slip Characteristics of Three Phase Induction Motor
- Braking Mode
- Motoring Mode
- Generating Mode
- Methods for the Speed Controlling of the induction Motor
- Controlling Supply Voltage
- Changing the number of Stator Poles:
- Adding Rheostat in Stator Circuit
- Adding External Resistance on Rotor Side
- Application of Induction Motor
Abstract:
In this article we take a 3-phase induction motor and study its working and construction. Also we later disassemble it to get to know about each of its components and then we assemble it again so that we learn it properly that how induction motor can be assembled.
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| Figure 1 |
Introduction:
An electric motor is a device
which converts an electrical energy into mechanical energy. This mechanical
energy then can be supplied by various types of loads. The motor can operate on
DC as well as on AC single phase and three phase a.c. supply are called a.c
motor. As a.c. supply is commonly available, the a.c. motor is very popularly
used in practice.
Types of Motor:
1) DC motors
a) Shunt
motor
b) Separately
Excited motor
c) Series
motor
d) Permanent
magnet DC motor (PMDC)
e) Compound
motor
2) AC Motors
a) Induction
motor
i) Squirrel
Cage induction motor
ii) Wound
rotor type induction motor
b) Synchronous
motor
A 3 phase squirrel cage induction motor is a type of three phase
induction motor. It is called a ‘squirrel cage’ motor because the rotor inside
of it – known as a ‘squirrel cage rotor’ – looks like a squirrel cage.
Construction of squirrel cag induction motor is
very simple; slip rings and brush assembly are absent.
Squirrel Cage
Induction Motor Construction:
- Stator
- Rotor
- Fan
- Bearings
 |
| Figure 2 |
Stator:
It consists of a 3 phase winding with a core and metal housing. Windings are such placed that they are electrically and mechanically 120o apart from in space. The winding is mounted on the laminated iron core to provide low reluctance path for generated flux by AC currents.
As its name indicates stator is a stationary part of induction motor. A stator winding is placed in the stator of induction motor and the three phase supply is given to it. Stator is made up of number of stampings in which different slots are cut to receive 3 phase winding circuit which is connected to 3 phase AC supply. The three phase windings are arranged in such a manner in the slots that they produce a rotating magnetic field after AC supply is given to them.
The windings are wound for a definite number of poles depending upon the speed requirement, as speed is inversely proportional to the number of poles, given by the formula:
Ns= 120f/p
Where,
Ns= Synchronous Speed
f = Frequency
P = No. of Poles
 |
| Figure 3 |
Stator Frame:
The slots
on the periphery of the stator core of the three-phase induction motor carry
three phase windings. We apply three phase ac supply to this three-phase
winding. The three phases of the winding are connected either in star or delta
depending upon which type of starting method we use. We start the squirrel cage
motor mostly with star-delta starter and hence the stator of squirrel cage
motor is delta connected. We start the slip ring three-phase induction motor by
inserting resistances so; the stator winding of slip ring induction
motor 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 a rotating magnetic
field.
Rotor:
The rotor core
is laminated to avoid power loss from eddy currents and hysteresis. Conductors
are skewed to prevent cogging during starting operation and gives better
transformation ratio between stator and rotor.
 |
| Figure 4 |
Fan:
A fan is
attached to the back side of the rotor to provide heat exchange, and hence it
maintains the temperature of the motor under a limit.
 |
| Figure 5 |
Bearings:
Bearings are
provided as the base for rotor motion, and the bearings keep the smooth
rotation of the motor.
 |
| Figure 6 |
Rotor Types:
Induction
Motor Rotor is of Two Types:
1. Squirrel
Cage Rotor
2. Wound
Type Rotor or Slip Ring Type Rotor
Squirrel Cage Rotor:
In this type of rotor, the rotor winding consists of conductors, in form of copper or aluminum bars embedded in
semi-closed slots of a laminated rotor core. To facilitate a closed path in the
rotor circuit, both sides of the rotor bars are short-circuited by end rings.
Features of Squirrel Cage Rotor:
This type of rotor does not have
a definite number of poles, but the same number of stator poles will be induced
in the rotor automatically through induction. Hence for a squirrel cage rotor
The squirrel cage rotor has very
low leakage reactance as it does contain any winding on the rotor and it
results in low starting torque and maximum running torque.
As we know that for increasing
the value of stating torque we have to increase the value of rotor resistance
and to increase it we have to insert a resistance in series with rotor winding,
but in case of squirrel cage rotor we cannot insert it as its rotor bars are
short-circuited by end ring from both sides. Thus we can say that the squirrel
cage rotor produces good running performance but poor starting performance.
 |
| Figure 7 |
This type of rotor is also made
up of laminated cold rolled grain oriented silicon steel to reduce eddy current
loss and hysteresis loss. The rotor winding is distributed as well short
pitched to get a sinusoidal emf output.
The
operation of an induction motor is not possible with an unequal number of
stator and rotor poles, and this type of rotor does not respond automatically
to change in a number of stator poles. Hence the number of rotor poles must be
made equal to the number of stator poles.
Features of Wound Type Rotor or Slip Ring Type Rotor:
The
major difference between squirrel cage rotor and wound type rotor is the
presence of a slip ring in wound type rotor hence it is also called a slip ring
rotor. The three terminals of star connected rotor windings are brought out and
are connected to the external resistors through the slip ring.
Slip
rings are made up of a high resistance material such as phosphorous bronze or
brass. Brush contacts are used for making the connection of the rotor winding
with the external circuit, Brushes are made up of carbon or copper material,
but carbon is preferred due to its self-lubricating property. So friction
losses are less with carbon brushes.
To
improve the starting torque, external resistors are used. This external resistor
also limits the starting current drawn by the motor at the time of starting.
Hence power factor improves.
 |
| Figure 8 |
Basic Working Principle of an Induction
Motor:
In a DC motor,
supply is needed to be given for the stator winding as well as the rotor
winding. But in an induction motor only the stator winding is fed with an AC
supply.
Alternating
flux is produced around the stator winding due to AC supply. This alternating
flux revolves with synchronous speed. The revolving flux is called as "Rotating
Magnetic Field" (RMF).
The relative
speed between stator RMF and rotor conductors causes an induced emf in the
rotor conductors, according to the Faraday's law of electromagnetic induction.
The rotor conductors are short circuited, and hence rotor current is produced
due to induced emf. That is why such motors are called as induction motors.
Now, induced
current in rotor will also produce alternating flux around it. This rotor flux
lags behind the stator flux. The direction of induced rotor current, according
to Lenz's law, is such that it will tend to oppose the cause of its production.
As the cause of
production of rotor current is the relative velocity between rotating stator
flux and the rotor, the rotor will try to catch up with the stator RMF. Thus
the rotor rotates in the same direction as that of stator flux to minimize the
relative velocity. However, the rotor never succeeds in catching up the
synchronous speed. This is the basic working principle of induction motor of
either type, single phase of 3 phase.
The difference
between the synchronous speed (Ns) and actual speed (N) of the rotor is called
as slip.
Rotor tries to
catch up the synchronous speed of the stator field, and hence it rotates. But
in practice, rotor never succeeds in catching up. If rotor catches up the
stator speed, there won’t be any relative speed between the stator flux and the
rotor, hence no induced rotor current and no torque production to maintain the
rotation. However, this won't stop the motor, the rotor will slow down due to loss
of torque, and the torque will again be exerted due to relative speed. That is
why the rotor rotates at speed which is always less the synchronous speed.
Torque Slip Characteristics of Three Phase Induction Motor:
The torque slip
curve for an induction motor gives us the information about the variation of
torque with the slip. The slip is defined as the ratio of difference of
synchronous speed and actual rotor speed to the synchronous speed of the
machine. The variation of slip can be obtained with the variation of speed that
is when speed varies the slip will also vary and the torque corresponding to that
speed will also vary.
The curve can
be described in three modes of operation:
 |
| Figure 9 |
Braking Mode:
In the Braking
mode, the two leads or the polarity of the supply voltage is changed so that
the motor starts to rotate in the reverse direction and as a result the motor
stops. This method of braking is known as plugging. This method is used when it
is required to stop the motor within a very short period of time. The kinetic
energy stored in the revolving load is dissipated as heat. Also, motor is still
receiving power from the stator which is also dissipated as heat. So as a
result of which motor develops enormous heat energy. For this stator is
disconnected from the supply before motor enters the braking mode.
Motoring Mode:
In this mode of
operation, supply is given to the stator sides and the motor always rotates
below the synchronous speed. The induction motor torque varies from zero to
full load torque as the slip varies. The slip varies from zero to one. It is
zero at no load and one at standstill. From the curve it is seen that the
torque is directly proportional to the slip.
That is, more
is the slip, more will be the torque produced and vice-versa. The linear
relationship simplifies the calculation of motor parameter to great extent.
Generating Mode:
In this mode of
operation induction motor runs above the synchronous speed and it should be
driven by a prime mover. The stator winding is connected to a three phase
supply in which it supplies electrical energy. Actually, in this case, the
torque and slip both are negative so the motor receives mechanical energy and
delivers electrical energy. Induction motor is not much used as generator
because it requires reactive power for its operation.
That is,
reactive power should be supplied from outside and if it runs below the
synchronous speed by any means, it consumes electrical energy rather than
giving it at the output. So, as far as possible, induction generators are
generally avoided.
Methods for the Speed Controlling of the induction
Motor:
The speed control
of three phase induction motor from stator side is further classified as:
- V / f Control or Frequency Control.
- Changing the number of Stator Poles.
- Controlling Supply Voltage.
- Adding Rheostat in the Stator Circuit.
The speed
controls of three phase induction motor from rotor side are further classified
as:
- Adding external resistance on rotor side.
- Cascade control method.
- Injecting slip frequency emf into rotor side.
Controlling Supply Voltage:
The torque produced by running three phase induction motor is given by
In low slip
region (sX2) is very small as compared to R2. So, it can be
neglected. Since rotor resistance, R2 is constant so the equation of torque
further reduces to:
We know that
rotor induced e.m.f. E2 ∝ V. So,
T ∝ sV2
The equation
above clears that if we decrease supply voltage torque will also decrease. But
for supplying the same load, the torque must remain the same, and it is only
possible if we increase the slip and if the slip increases the motor will run
at a reduced speed. This method of speed control is rarely used because a small
change in speed requires a large reduction in voltage, and hence the current
drawn by motor increases, which cause overheating of the induction motor.
Changing the number of Stator Poles:
The stator
poles can be changed by two methods.
Adding Rheostat in Stator Circuit:
In this method
of speed control of three phase induction motor rheostat is added in the stator
circuit due to this voltage gets dropped .In case of three phase induction
motor torque produced is given by T ∝ sV22. If we
decrease supply voltage torque will also decrease. But for supplying the same
load, the torque must remain the same and it is only possible if we increase
the slip and if the slip increase motor will run reduced speed.
Speed Control from Rotor Side
Adding External Resistance on Rotor Side:
In this method
of speed control of three phase induction motor external resistance are added
on rotor side. The equation of torque for three phase induction motor is:
The three-phase
induction motor operates in a low slip region. In low slip region term (sX2)
becomes very small as compared to R2. So, it can be neglected, And also E2 is
constant. So the equation of torque after simplification becomes:
Now if we
increase rotor resistance, R2 torque decreases but to supply the same load
torque must remain constant. So, we increase slip, which will further result in
the decrease in rotor speed. Thus by adding additional resistance in the rotor
circuit, we can decrease the speed of the three-phase induction motor. The main
advantage of this method is that with an addition of external resistance
starting torque increases but this method of speed control of three phase.
There are a lot
of applications of a 3 phase induction motor:
- Electric Train Engine
- Cooling Fans used to cool large machines like alternators etc.
- Chimneys at Power Plants
- Printing Machines
- Rolling Mills
- High Speed Vacuum
- Cleaner
- Drilling Machine
- Cranes
- Crushers
- Pumps
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